Computational study of pyrylium cation–water complexes: hydrogen bonds, resonance effects, and aromaticity

Computational study of pyrylium cation–water complexes:

hydrogen bonds, resonance effects, and aromaticity

Renato L.T. Parreira, Sergio E. Galembeck*

Departamento de Quımica, Faculdade de Filosofia, Ciencias e Letras de Ribeirao Preto, Universidade de Sao Paulo,

Avenida dos Bandeirantes 3900, 14040-901 Ribeirao Preto, SP, Brazil

Received 18 August 2005; received in revised form 14 November 2005; accepted 15 November 2005

Available online 3 February 2006

Abstract

Hydrogen bonds formed between the pyrylium cation and water were characterized by means of geometric, energetic and electronic parameters

through calculations done with the B3LYP/6-31CG(d,p) method. The wavefunctions were analyzed by the Natural Bond Orbitals (NBO), Natural

Steric Analysis (NSA), Natural Resonance Theory (NRT), and Atoms in Molecules (AIM) methods. The energy decomposition method proposed

by Xantheas was employed. The vibrational frequencies and the intensity of the C–H stretching bands were studied. The Nucleus Independent

Chemical Shifts (NICS) and Harmonic Oscillator Model of Aromacity (HOMA) methods were used to verify the aromaticity of the pyrylium

cation and the studied complexes. Complexation results in small alterations in the equilibrium geometry of the monomers. Energetic analysis

allowed us to verify the stability order of the studied complexes and the intensity of the hydrogen bonds taking place between the monomers.

Small alterations in the electronic structure of the monomers occur, indicating that the interaction between pyrylium and water is weak and little

contributes to increasing the cation resonance.

q 2005 Elsevier B.V. All rights reserved.

Keywords: Pyrylium cation; Solvation; Hydrogen bonds; Resonance; Aromaticity

1. Introduction

Pyrylium cations are widely used in chemical synthesis and

present several potential applications [1]. Systematic studies

considering a large series of pyrylium salts and their reactions

can be found in the literature [2,3]. Pyrylium salts have been

employed as versatile synthetic intermediates in the synthesis

of many organic compounds such as aryl furan derivatives,

pyridines, nitrobenzenes, phosphabenzenes, thiopyryliums,

2,4-pentadienones, and complex heterocyclic frameworks

[4–6]. Despite the large number of pyrylium compounds

known, few organometallic pyrylium complexes have been

prepared and investigated [4,7–9].

Pyrylium compounds are widely used as sensitizers in

photographic materials and photosemiconductors, laser dyes,

Q-switchers, mode lockers in lasers, laser disc media, and

optical recording material [2,4,7,9–13]. Also, they are useful as

organic luminophores, secondary nonaqueous-electrolyle

0166-1280/$ - see front matter q 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.theochem.2005.11.020

* Corresponding author. Tel.: C55 16 602 3765; fax: C55 16 633 8151.

E-mail address: [email protected] (S.E. Galembeck).

battery [9,10], corrosion-inhibiting additives in paints [4,7],

fluorescent probes for ion detection, improved dyes for solar

collectors [11,14,15], polymerization initiators [1,16], and

homogeneous photocatalyst to achieve the abatement of

several phenolic pollutants [17–19]. In addition, pyrylium

cation and their derivatives have presented important role in

photobleaching process and photodynamic therapy [20–24].

Furthermore, pyrylium cation is part (ring C) of the

anthocyanins which are a group of natural cationic pigments

belonging to a class of low-weight phenolic compounds known

as flavonoids. These compounds are ubiquitous in the plant

kingdom and exhibit various biological roles including

participation in reproductive processes, visual indication of

flowers and fruit [25–28], and protecting the plant against stress

factors [29]. Besides their antioxidant character, various

therapeutic effects have been proposed for anthocyanins and

other flavonoids [30–35].

Despite their importance and potential applications, there are

few computational studies on pyrylium cation. In this work, we

studied the interaction of the pyrylium cation with one, two or

three water molecules. Calculations were carried out using DFT

and 6-31CG(d,p) basis set. Modifications in the equilibrium

geometry of the monomers constituting the complexes, as well as

changes in vibrational frequencies, band intensities and atomic

Journal of Molecular Structure: THEOCHEM 760 (2006) 59–73

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R.L.T. Parreira, S.E. Galembeck / Journal of Molecular Structure: THEOCHEM 760 (2006) 59–7360

charges were considered. Hydrogen bonds and electronic

alterations undergone by the monomers were analyzed by various

theories, such as Natural Bond Orbitals (NBO), Natural

Resonance Theory (NRT), Natural Steric Analysis (NSA), and

Atoms in Molecules (AIM). Energetic analysis was carried out

employing the energy decomposition method proposed by

Xantheas. The influence of the hydrogen bonds on the aromaticity

of the pyrylium cation was investigated by the Nucleus

Independent Chemical Shifts (NICS) and Harmonic Oscillator

Model of Aromacity (HOMA) methods.

2. Computational methods

Geometry optimization and vibrational frequency calcu-

lations were carried out using the GAUSSIAN 98 program [36],

using the B3LYP three-parameters hybrid functional [37–39]

and the 6-31CG(d,p) basis set [40]. Gu, Kar and Scheiner used

the same basis set in their study of the CH/O interactions

taking place between proton donors of the FnH3KnCH type and

proton acceptors, such as water (H2O), methanol (CH3OH) and

methanal (H2CO) [41]. The nature of the stationary point was

determined on the basis of the eigenvalues obtained from the

harmonic vibration analysis. Energies were corrected using the

Zero Point Energy (ZPE). Generalized Atomic Polar Tensor

(GAPT) charges were obtained from the polar atomic tensors,

which are related to the intensities of bands in the infrared

spectrum [42]. Energetic analysis was carried out by the

method of Xantheas for many-body interactions [43–45]. The

counterpoise (CP) method was employed to take the effects of

the Basis Set Superposition Error (BSSE) into account [46].

Wavefunction analysis was done by using the NBO method

[47,48], including Natural Population Analysis (NPA) [49],

NSA [50], and NRT [51–54]. These calculations were carried

out using the NBO 5.0 program [55] interfaced with the

GAUSSIAN 98 package. To help these analyses, the Molekel 4.1

visualization program was employed [56].

Topological analysis of the electron density was done using

AIM theory [57–59], which has been employed in the

characterization of hydrogen bonds in a variety of molecular

complexes [60,61]. Calculations were carried out with the

software suite PROAIM [62].

The influence of complexation on the aromaticity of the

pyrylium cation was determined using magnetic and geometric

criteria, by means of NICS [63], HOMA [64,65], and the

Harmonic Oscillator Stabilization Energy (HOSE) [64,66].

3. Results and discussion

3.1. Geometries

The equilibrium geometries of the monomers and the

complexes are presented in Fig. 1, together with the numbering

of the atoms. The complexation sites are represented by letters,

according to the scheme shown for complexes (II)–(V) and

(VII)–(IX).

All the considered complexes presented hydrogen bonds of

type H2O/H–C in the molecular plane. Complex with

hydrogen bond HO–H/O(1) in the molecular plane was not

found by using B3LYP/6-31CG(d,p) level of theory. The lack

of hydrogen bond between O(1) and water is due to the fact that

one of the main resonance structures of pyrylium exhibits a the

positive charge on O(1). The complex (Fig. 2) that has the

water molecule displayed above the cation ring is about 2 kcal/

mol less stable than (II) (the most stable complex with only one

water molecule). In addition, an interaction between O(1) and/

or C(2) and the oxygen of the water molecule was observed.

The above mentioned arguments were considered enough to

keep this complex out of the analysis.

Because the pyrylium cation exhibits weak interactions with

water molecules, the equilibrium geometries of the complexes

differ little from those of the monomers. The most affected

bond lengths are those of the C–H bonds whose hydrogen

atom interacts directly with the water oxygen. The length of

these C–H bonds increases in the complexes with only one

water molecule ((II)–(IV)). In the case of complex (II), a slight

enlargement of the C(2)–C(3) bond length and a small

reduction of the C(3)–C(4) bond length were observed.

As for complexes (V)–(X), which contain water between two

C–H groups, positions B, B 0, D, and D 0, there are no significant

alterations in the bond lengths or bond angles of the cation. In the

case of the six structures exhibiting two water molecules, only

(IX) and (X) do not have water in positions A or A 0. In (V),

lengthening of the O(1)–C(2) and C(2)–H(7) bonds occurs

while, in complex (VI), the most significant enlargements were

observed to the O(1)–C(2), C(2)–H(7), C(4)–H(9), and C(4)–

C(5) bond lengths. In this latter complex, a small shortening of

the C(5)–C(6) bond can also be observed. The C(2)–H(7) and

O(1)–C(2) bonds were elongated in (VII). A similar increase is

observed in the O(1)–C(2), O(1)–C(6), C(2)–H(7), and C(6)–

H(11) bond lengths of complex (VIII). Finally, alterations in the

equilibrium geometries of (IX) and (X) are very small, being the

shortening of the C(5)–C(6) and C(6)–O(1) bonds of complex

(IX) the most expressive change.

Considering the complexes with three water molecules

((XI) and (XII)), the O(1)–C(2), C(6)–O(1), C(2)–H(7), and

C(6)–H(11) bond lengths increase upon complexation.

Lengthening of the C(4)–H(9) bond also occurs in (XI).

The O–H bonds of the water molecules are approximately

0.002 A larger in the complexes than in the monomer.

Variations in the bond angles of pyrylium and water in the

complexes are lower than one degree. The directional character

(linearity) of the hydrogen bonds may be used to distinguish

between them and other electrostatic interactions [41]. This

linearity can be observed in complexes (II)–(VIII), (XI), and

(XII), where the water molecule interacts with only one

pyrylium C–H group, being the angles of the C–H/O bond

higher than 1708. The distances of the C–H/O hydrogen bond

in the complexes containing water in positions B, B 0, D, and D 0

lie between 2.2 and 2.8 A, being longer than the distance

observed in the remaining complexation sites (w2.0 A) and in

the water dimer (w1.95 A) [67]. The addition of a second

water molecule to complexes (II) and (IV), leading to

complexes (V)–(VIII), results in increased H/O hydrogen

bond distances. A similar situation occurs upon addition of

Fig. 1. Equilibrium geometries of the monomers and the complexes.

R.L.T. Parreira, S.E. Galembeck / Journal of Molecular Structure: THEOCHEM 760 (2006) 59–73 61

the third water molecule to complexes (V) and (VI), which

results in complexes (XI) and (XII). This can be attributed to a

negative cooperativity effect in these hydrogen bonds.

3.2. Vibrational frequencies

All studied complexes were minima in the potential energy

surface as indicated by the positive vibrational frequencies.

The stretching frequencies of the C–H groups (Table 1) are

little influenced by the presence of water molecules in

positions B, B 0, D, and D 0, showing that the hydrogen

bonds in these positions must be weak. In the complexes (II),

(V), (VI), (VII), (VIII), (XI), and (XII) a diminution in the

C(2)–H(7)/C(6)–H(11) asymmetric stretching frequencies was

observed. There is a similar decrease in the C(2)–H(7)/C(6)–

H(11) symmetric stretching frequencies of complexes (VIII),

Fig. 2. Complex pyrylium–water without hydrogen bond: the water molecule is

displayed above the cation ring.


(XI), and (XII). The C(4)–H(9) stretching frequencies are also

lower in complexes (IV), (VI), and (XI). Finally, a decreased

C(3)–H(8)/C(5)–H(10) symmetric stretching frequency is

observed for complex (III). There is a feature of hydrogen

bonds that is related to the vibrational spectrum. The

frequency associated with the proton donor group is typically

red-shifted [41,68]. In the studied complexes, this shift occurs

due to a slight increase in the C–H bond lengths, when

compared to the isolated monomers. The ability of the C–H

group to form hydrogen bonds has been a subject of much

controversy [69]. The characterization of the CH/O

interaction (coulombic interaction between atoms of opposite

charges) as a hydrogen bond has been questioned after the

observation that the C–H stretching frequency may be slightly

blue-shifted, contrary to the typical red-shift of proton donors

in hydrogen bonds [69].

The hydrogen bonds also influence the intensities of the

C–H group bands. The intensity of the C(2)–H(7)/C(6)–H(11)

symmetric streching band increases in complexes (VII),

(VIII), (XI), and (XII), while the asymmetric stretching band

intensifies in complexes (II), (V), (VI), (VII), (VIII), (X),

(XI), and (XII). An increase of the intensity of the C(3)–

H(8)/C(5)–H(10) symmetric stretching band was observed for

Table 1

Vibrational frequencies (cmK1) of the C–H bond stretchings

C(2)–H(7)/C(6)–H(11)

Symmetric Asymmetric

(I) 3262 (8.1) 3258 (26.5)

(II) 3261 (11.6) 3135 (363.3)

(III) 3263 (7.6) 3259 (20.1)

(IV) 3262 (6.0) 3259 (19.6)

(V) 3261 (7.6) 3152 (316.7)

(VI) 3261 (7.8) 3152 (140.9)

(VII) 3259 (54.7) 3154 (321.4)

(VIII) 3159 (206.7) 3151 (413.8)

(IX) 3262 (5.2) 3251 (57.8)

(X) 3258 (6.2) 3254 (130.8)

(XI) 3171 (147.8) 3163 (360.1)

(XII) 3170 (199.2) 3163 (351.8)

Intensities (km/mol) are indicated in brackets.

complexes (III), (V), and (IX), while their corresponding

asymmetric stretching band are more intense in the case of

complexes (VIII), (IX), and (XI). As for the complexes

exhibiting water in E ((IV), (VI), and (XI)), the C(4)–H(9)

stretching band is also intensified. Such increase is less

pronounced in (V) and (XII). The increase in the intensity of

a band is due to the charge flux term [70], which is strongly

influenced by complexation. In the isolated molecule, the

hydrogen charge is always positive, while the charge flux is

slightly negative. Upon complexation, the latter term

becomes positive, and therefore, both the charge and charge

flux have the same algebraic sign. As the intensity of the

bond stretching is proportional to the sum of the squares of

these terms, the intensity significantly increases upon

hydrogen bond formation [70].

3.3. Atomic charges

Atomic charges were obtained by the NPA, GAPT and MK

methods (Table S1). The C(2) and C(6) atoms of the pyrylium

cation exhibit positive charges. The NPA charges differ from

the MK and GAPT ones, mainly when C(4) is concerned.

While the former method shows there is a slightly negative

charge on C(4), MK and GAPT give evidence of a

considerably positive charge on it. The atomic charges on

the O(12), O(15), and O(18) atoms of the water molecules

become more negative upon complexation, whilst the positive

charge on the hydrogen atoms of the water molecules

increases. The hydrogen atoms of the C–H groups acting as

proton donors in the hydrogen bonds present higher positive

charges when the interaction occurs in positions A, A 0, C, and

E. These charges do not display this same behavior in the

case of the complexes containing water in positions B, B 0, D,

and D 0.

3.4. Energetic analysis

The energies of the complexes were compared so that

information about the most stable complexes could be obtained

(Table 2). The relative energies (DE) and the relative energies

C(3)–H(8)/C(5)–H(10) C(4)–H(9)

Symmetric Asymmetric

3240 (19.9) 3239 (8.4) 3221 (1.2)

3242 (12.6) 3240 (12.1) 3220 (1.1)

3146 (286.8) 3239 (10.6) 3221 (3.4)

3238 (17.0) 3238 (5.1) 3137 (242.5)

3244 (32.8) 3239 (9.6) 3219 (17.4)

3240 (10.8) 3238 (9.9) 3149 (385.6)

3246 (9.1) 3242 (12.2) 3219 (0.6)

3244 (3.4) 3242 (16.7) 3219 (1.1)

3241 (52.6) 3239 (24.5) 3223 (8.6)

3246 (18.5) 3244 (3.4) 3219 (0.3)

3241 (3.6) 3240 (12.7) 3159 (225.6)

3246 (24.8) 3241 (8.5) 3221 (15.1)

Table 2

Relative energies, DE (kcal/mol), and relative energies corrected by ZPE, D(ECZPE) (kcal/mol)

Energies in relation to (II) Energies in relation to (VIII) Energies in

relation to (XI)

(III) (IV) (V) (VI) (VII) (IX) (X) (XII)

DE

0.90 0.82 0.79 0.78 0.10 0.78 0.21 0.01

D(ECZPE)

0.89 0.81 0.93 0.76 0.26 1.07 0.49 0.12


corrected by ZPE (D(ECZPE)) presented similar results for

complexes (II)–(IV). The stability order of DE and D(ECZPE)

was also the same for complexes (VII), (VIII), and (X). In

addition, the values of DE were practicality equal for (V), (VI),

and (IX). However, this tendency was not observed to D(ECZPE).

Concerning complexes (II)–(IV), the most stable is (II),

which forms a complex with water in position A, followed by

(IV) and (III), which contain water in positions E and C,

respectively. Complex (VIII), with water in positions A and A 0,

is the most stable among those containing two water molecules

((V)–(X)). This shows preferable complexation in the C–H

group adjacent to O(1). According to D(ECZPE), the

decreasing stability order for these complexes is (VIII)O(VII)O(X)O(VI)O(V)O(IX). The relative energies corrected

by ZPE (D(ECZPE)) show that the complexes bearing at least

one water molecule in positions B and/or B 0 are less

destabilized. The inverse is observed for the complexes

displaying water in positions D and/or D 0. Considering the

complexes with three water molecules, (XI) is slightly more

stable than (XII).

Results obtained by the energy decomposition method

proposed by Xantheas [43,44] are given in Table 3. The greatest

contributions to relaxation energies are due to distortions

undergone by pyrylium upon complex formation (ERpyrylium).

ERpyrylium are higher in complexes (V), (VII), (IX), (X), and

(XII); i.e. in the complexes where water interacts forming cycles

(positions B, B 0, D, and D 0). For water clusters, Xantheas found

that the relaxation energies of the water molecules are higher in

the cases where the monomers interact forming cycles [43]. The

relaxation energies of the water molecules in the complexes are

null or extremely low (0.01 kcal/mol). At the MP4/aug-cc-

pVTZ model, Xantheas determined that the total relaxation

energy for the water dimer is 0.02 kcal/mol [43]. The relaxation

energy of the pyrylium cation becomes higher upon addition of a

second water molecule.

Values obtained for the two-body interaction terms (D2E)

and binding energies (BE2) are approximately twice as high as

those calculated by Xantheas for the water dimer (K4.69 and

K4.67 kcal/mol) [43]. Considering dimers (II)–(IV), the

highest D2E is obtained for complex (II) (with water in

position A). D2E for the complexes containing water in

positions C (complex (III)) and E (complex (IV)) are close, the

latter value being 0.08 kcal/mol higher than the former.

Analogous conclusions can be drawn for the two-body binding

energies (BE2). As for trimers (V)–(X), the D2E and BE2 values

that most stabilize the complexes are those corresponding to

complexation in positions A, A 0, B and B 0. In the case of

tetramers (XI) and (XII), the most stabilizing D2E and BE2

occur when water is present in positions A and A 0. However,

water in positions E and D also satisfactorily stabilize

complexes (XI) and (XII), respectively. Variations in DnE

and BEn with the successive addition of water molecules are

very small, indicating that low destabilizations take place in

most of the complexes. DnE and BEn between water molecules

are repulsive, thus destabilizing the complexes. In fact,

Xantheas obtained repulsive values for the five- and six-body

terms in calculations for the water hexamer [43].

3.5. Second-order interactions between NBOs

To better understand the main interactions involved in the

stabilization of the monomers and the complexes, second-order

interaction energies (DE(2)) between the occupied and virtual

natural bond orbitals were analyzed. Due to the great electron

delocalization in the pyrylium cation, NBO calculations were

carried out for its three main resonance structures obtained by

the NRT analysis (see Tables 4 and S2). In this analysis, the

NBOs designated p are pure ‘p’-type orbitals oriented

perpendicularly to the molecular plane. The natural bond

orbitals designated s are oriented in the molecular plane and

are hybrid orbitals formed by combination of ‘s’ and ‘p’

orbitals (Fig. 3).

Complexation in positions A, A 0, C, and E results in

approximately 2% decrease in the ‘s’ character of the snO(water)

orbital, and an increase of the same magnitude in this orbital ‘p’

character can be noted. The decrease in the ‘s’ character and

the increase in the ‘p’ character are approximately 10% in

positions B and B 0, and 1–4% in positions D and D 0. s*C–H

orbitals have slightly higher (!2%) ‘s’ character, with a

proportional decrease in the contribution of the ‘p’ character.

The pnO(water) orbital exhibits 8–9% ‘s’ contribution in the case

of the complexes with water in positions B and B 0, and

approximately 3% in the complexes with water in D. No

contribution from the ‘s’ character is observed in the pnO(water)

orbital of complex (IX), which displays water in D 0.

Pyrylium undergoes small alterations in the main DE(2)

values (Tables 4 and S2) upon complexation, indicating that its

interactions with water little influence the eletronic structure

and the resonance of the cation. Considering the first resonance

structure of the pyrylium cation, the highest DE(2) values

belong, in order of importance, to the pC(4)–C(5)/p*O(1)–C(6),

Table 3

Total (ERtotal) and monomer relaxation energies (ER), interaction energies (DnE), and binding energies (BEn) (kcal/mol) corrected by BSSE

ER D2Ea D2Ea D2Ea D2Eb D2Eb D2Eb

(I) A/A 0 B/B 0 C D/D 0 E

(II) 0.09 0.0 K10.38

(III) 0.05 0.0 K9.43

(IV) 0.04 0.0 K9.51

(V) 0.16 0.0 0.01 K10.34(A) K9.84(D) 0.38

(VI) 0.11 0.0 0.0 K10.38(A) K9.50(E) 0.21

(VII) 0.17 0.0 0.01 K10.38(A) K10.41(B 0) 0.20

(VIII) 0.11 0.0/0.0 K10.37(A) K10.37(A0) 0.23

(IX) 0.30 0.01 0.01 K10.42(B) K9.88(D0) 0.24

(X) 0.22 0.01/0.01 K10.43(B) K10.43(B 0) 0.20

(XI) 0.13 0.0/0.0 0.0 K10.35(A) K10.35(A0) K9.49(E) 0.23(AA 0) 0.22(AE) 0.22(A 0E)

(XII) 0.19 0.0/0.0 0.01 K10.32(A) K10.36(A0) K9.84(D) 0.24(AA 0) 0.38(AD) 0.19(A 0D)

D2Etotal D3Ea D3Ea D3Ea D3Eb D3Etotal D4E BE2a BE2

a BE2a BE2

b

(II) K10.38 K10.29

(III) K9.43 K9.38

(IV) K9.51 K9.46

(V) K19.81 0.48 K10.18(A) K9.68(D) 0.39

(VI) K19.67 0.54 K10.26(A) K9.39(E) 0.22

(VII) K20.59 0.55 K10.21(A) K10.23(B 0) 0.22

(VIII) K20.50 0.55 K10.25(A) K10.25(A 0) 0.24

(IX) K20.05 0.51 K10.11(B) K9.57(D 0) 0.26

(X) K20.66 0.55 K10.20(B) K10.20(B 0) 0.22

(XI) K29.52 0.54(AA 0) 0.51(AE) 0.51(A 0E) 0.0(AA 0E) 1.55 K0.02 K10.22(A) K10.22(A 0) K9.36(E) 0.24(AA 0)

(XII) K29.72 0.53(AA 0) 0.47(AD) 0.52(A 0D) 0.0(AA 0D) 1.52 K0.02 K10.13(A) K10.18(A 0) K9.65(D) 0.24(AA 0)

BE2b BE2

b BE2total BE3a BE3

a BE3a BE3

b BE3total BE4 BSSE

(II) K10.29 0.85

(III) K9.38 0.87

(IV) K9.46 0.86

(V) K19.47 K19.16 1.54

(VI) K19.43 K19.01 1.70

(VII) K20.22 K19.85 1.54

(VIII) K20.26 K19.83 1.66

(IX) K19.42 K19.23 1.48

(X) K20.18 K19.87 1.41

(XI) 0.22(AE) 0.22(A0E) K29.12 K19.80(AA 0) K18.98(AE) K18.98(A 0E) 0.68(AA 0E) K57.08 K27.86 2.50

(XII) 0.39(AD) 0.20(A0D) K29.13 K19.72(AA 0) K19.11(AD) K19.30(A 0D) 0.82(AA 0D) K57.31 K28.02 2.33

a Interaction between pyrylium and water.b Interaction between water molecules.

R.L.T.Parreira

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Table 4

Second-order stabilization energies (DE(2)) for compounds (I), (XI), and (XII)

Canonic

structure

Interactions DE(2) (kcal/mol)

(I) (XI) (XII)

OpO(1)–C(6)/p*

C(2)–C(3) 20.17 20.46 20.00

pC(2)–C(3)/p*C(4)–C(5) 22.42 23.15 23.31

pC(4)–C(5)/p*O(1)–C(6) 62.04 62.65 60.86

pC(4)–C(5)/p*C(2)–C(3) 13.30 12.78 12.79

pC(2)–C(3)/*pO(1)–C(6) 9.90 9.20 9.57

snO(12)/s*C(2)–H(7) 10.52 10.55

snO(15)/s*C(6)–H(11) 10.52 10.45

pnO(18)/s*C(3)–H(8) 0.85

snO(18)/s*C(3)–H(8) 0.49

pnO(18)/s*C(4)–H(9) 0.30

snO(18)/s*C(4)–H(9) 8.39 1.64

OpO(1)–C(2)/p*

C(5)–C(6) 20.17 20.46 20.64

pC(5)–C(6)/p*C(3)–C(4) 22.42 23.15 22.96

pC(3)–C(4)/p*O(1)–C(2) 62.05 62.65 61.91

pC(3)–C(4)/p*C(5)–C(6) 13.30 12.78 13.22

pC(5)–C(6)/*pO(1)–C(2) 9.90 9.20 9.25

snO(12)/s*C(2)–H(7) 10.52 10.55

snO(15)/s*C(6)–H(11) 10.52 10.45

pnO(18)/s*C(3)–H(8) 0.85

snO(18)/s*C(3)–H(8) 0.49

pnO(18)/s*C(4)–H(9) 0.30

snO(18)/s*C(4)–H(9) 8.39 1.64

O pC(2)–C(3)/p*nC(4) 59.54 60.22 61.44

pC(5)–C(6)/p*nC(4) 59.96 59.64 60.70

pnO(1)/p*C(2)–C(3) 30.28 30.59 29.75

pnO(1)/p*C(5)–C(6) 30.42 30.63 30.79

snO(12)/s*C(2)–H(7) 10.52 10.55

snO(15)/s*C(6)–H(11) 10.52 10.45

pnO(18)/s*C(3)–H(8) 0.85

snO(18)/s*C(3)–H(8) 0.49

pnO(18)/s*C(4)–H(9) 0.30

snO(18)/s*C(4)–H(9) 8.39 1.64

Fig. 3. NBOs: (a) pnO(1), (b) snO(water).


pC(2)–C(3)/p*C(4)–C(5), pO(1)–C(6)/p*

C(2)–C(3), pC(4)–C(5)/p*

C(2)–C(3), and pC(2)–C(3)/*pO(1)–C(6) interactions. DE(2) for

pC(2)–C(3)/p*C(4)–C(5) is stabilized by 1.1–1.6 kcal/mol in

complexes (II), (V), (VI), and (VIII). In the case of the pC(4)–

C(5)/p*O(1)–C(6) interaction, there is stabilization of 2.1–

4.6 kcal/mol in complexes (IV), (V), (VI), and (IX). On the

other hand, DE(2) for this same interaction is destabilized by

1.2–3.4 kcal/mol in complexes (VII), (VIII), (X), and (XII). As

for the second resonance structure, the main DE(2) are pC(3)–

C(4)/p*O(1)–C(2), pC(5)–C(6)/p*

C(3)–C(4), pO(1)–C(2)/p*C(5)–

C(6), pC(3)–C(4)/p*C(5)–C(6), and pC(5)–C(6)/

*pO(1)–C(2). Upon

complexation, DE(2) for the pC(5)–C(6)/p*C(3)–C(4) interaction

is destabilized by 1.0 kcal/mol in (III). The opposite occurs for

this same interaction in complex (VIII). Destabilization

between 2.2 and 3.4 kcal/mol can be observed for the pC(3)–

C(4)/p*O(1)–C(2) interaction in complexes (VII), (VIII), and

(X). Finally, DE(2) for the third resonance structure are the most

affected by complexation. DE(2) for the pC(2)–C(3)/p*nC(4)

interaction is stabilized by 1.7–6.0 kcal/mol in complexes (II),

(III), (V)–(X) and (XII). However, this same interaction is

destabilized by approximately 3 kcal/mol in complex (IV). In

complexes (II)–(VI) and (IX), DE(2) for the pC(5)–C(6)/p*nC(4)

interaction reflects 2.1–5.1 kcal/mol destabilization, while

stabilization of 2.4–3.5 kcal/mol is observed in complexes

(VII), (VIII), and (X). DE(2) for the pnO(1)/p*C(2)–C(3) and

pnO(1)/p*C(5)–C(6) interactions are approximately 1.0 kcal/

mol destabilized in complexes (III) and (IX).

snO(water)/s*C–H are the main interactions involved in the

formation of the hydrogen bonds. DE(2) for these interactions

are higher in the cases where water interacts with one C–H

group only (positions A, A 0, C, and E). DE(2) for such

interactions lie between 8.4 and 12.2 kcal/mol, and must help

stabilize the complexes. In the complexes with water in

positions B, B 0, D, and D 0, DE(2) values for the snO(water)/s*

C–H and pnO(water)/s*C–H interactions are low (a maximum

of 4.1 kcal/mol for snO(12)/s*C(2)–H(7) in complex (IX)). The

successive addition of water molecules leads to decreased

DE(2) for the snO(water)/s*C–H interactions in positions A, A 0,

and E.

3.6. Natural steric analysis

NSA (Table 5, Table S3) was used to study the interactions

between the occupied or partially occupied orbitals that

destabilize the monomers and the complexes. This technique

was used in a previous study of hydrogen bonds in monomers

and dimers of 2-aminoethanol [71] and to help the analysis of

hydrogen bonds between the hydroperoxyl radical and organic

acids [45]. In the case of the first resonance structure, the pO(1)–

C(6)4pC(2)–C(3), sC(2)–C(3)4snO(1), pC(2)–C(3)4pC(4)–C(5),

and sC(5)–C(6)4snO(1) interactions make the complexes 5.0–

11.0 kcal/mol less stable. Results for the second resonance

structure are analogous to those obtained for the first one, once

the only difference between these resonance structures is the

alternation of the p bonds. Therefore, destabilization of the

pO(1)–C(2)4pC(5)–C(6), sC(2)–C(3)4snO(1), pC(3)–C(4)4pC(5)–

C(6), and sC(5)–C(6)4snO(1) interactions are around 5.0–

11.0 kcal/mol. As for the last resonance structure, the sC(2)–

C(3)4snO(1), sC(5)–C(6)4snO(1), pC(2)–C(3)4pC(5)–C(6),

pC(2)–C(3)4pnO(1), and pC(5)–C(6)4pnO(1) interactions are

responsible for complex destabilization of 5.5–9.5 kcal/mol.

As shown by the NBO analysis, alterations caused by

complexation are very small. Only complex (IV) undergoes

variation higher than 1 kcal/mol in the steric exchange

interaction energy (dE(i,j)), which corresponds to an increase

of 1.63 kcal/mol in the pC(2)–C(3)4pnO(1) interaction. Hydro-

gen bonds are destabilized in the complexes mainly because of

the sC–H4snO(water) interactions. The dE(i,j) lies between 7.5

and 9.4 kcal/mol for complexes containing water in A, A 0, C

Table 5

Steric exchange interaction energies, dE(i,j), for compounds (I), (XI), and (XII)

Canonic

structure

Interactions dE(i,j) (kcal/mol)

(I) (XI) (XII)

OpO(1)–C(6)4pC(2)–C(3) 11.04 10.89 10.79

sC(2)–C(3)4snO(1) 9.00 8.64 8.53

pC(2)–C(3)4pC(4)–C(5) 5.41 5.48 5.52

sC(5)–C(6)4snO(1) 9.00 8.64 8.70

sC(2)–H(7)4snO(12) 8.15 8.12

sC(6)–H(11)4snO(15) 8.15 8.11

sC(3)–H(8)4pnO(18) 0.84

sC(3)–H(8)4snO(18) 1.34

sC(4)–H(9)4pnO(18) 0.11

sC(4)–H(9)4snO(18) 7.46 3.19

OpO(1)–C(2)4pC(5)–C(6) 11.04 10.89 10.92

sC(2)–C(3)4snO(1) 9.12 8.64 8.53

pC(3)–C(4)4pC(5)–C(6) 5.41 5.48 5.60

sC(5)–C(6)4snO(1) 9.00 8.64 8.70

sC(2)–H(7)4snO(12) 8.15 8.12

sC(6)–H(11)4snO(15) 8.15 8.11

sC(3)–H(8)4pnO(18) 0.84

sC(3)–H(8)4snO(18) 1.34

sC(4)–H(9)4pnO(18) 0.11

sC(4)–H(9)4snO(18) 7.46 3.19

O sC(2)–C(3)4snO(1) 8.97 8.69 8.53

pC(2)–C(3)4pC(5)–C(6) 9.64 9.58 9.68

pC(2)–C(3)4pnO(1) 5.55 5.59 5.55

sC(5)–C(6)4snO(1) 9.05 8.62 8.70

pC(5)–C(6)4pnO(1) 8.28 8.28 8.25

sC(2)–H(7)4snO(12) 8.15 8.12

sC(6)–H(11)4snO(15) 8.15 8.11

sC(3)–H(8)4pnO(18) 0.84

sC(3)–H(8)4snO(18) 1.34

sC(4)–H(9)4pnO(18) 0.11

sC(4)–H(9)4snO(18) 7.46 3.19


and E, between 4.3 and 5.2 kcal/mol for complexes with water

in B, B 0, and between 2.4 and 3.5 kcal/mol for complexes with

water in D or D 0.

3.7. NRT analysis

NRT analysis allowed the uncovering of the resonance

structures that most contribute to the stabilization of the studied

complexes. The weight of the three main resonance structures

are around 17–22% and complexation does not lead to large

alterations in their contributions (Table 6). A slight decrease in

the weight of the first resonance structure can be observed,

which is more pronounced in complexes (II), (V), (VI) and

(IX), and less evident in (VII) and (X). The weight of the

second resonance structure is slightly decreased in complexes

(XI) and (XII). Except for (VII) and (XI), complexation results

in increased weight of the third resonance structure, particu-

larly in (IV), (VI), (IX), and (XII), which contain one water

molecule in position E, D or D 0.

Bond orders (Table 6) show that the C(2)–C(3) and

C(5)–C(6) bonds exhibit higher double bond character. The

order of all the C–H bonds involved in hydrogen bonding

decrease as a result of the interaction between the water

oxygen with the C–H proton donor group.

3.8. Harmonic oscillator stabilization energy

On the basis of bond lengths, the HOSE method [64,66] was

used to estimate the contribution of the pyrylium resonance

structures (Fig. 4, Table 7).

Results show that resonance structure (3) is the one that

most contributes to the stabilization of the pyrylium cation,

followed by the equivalent resonance structures (4) and (5),

with the positive charge on C(2) and C(6), respectively. The

equivalent resonance structures (1) and (2), with the positive

charge on O(1), are the ones that contribute the least. It is

noteworthy that resonance structures (4) and (5) present very

small contributions (approximately 0.7%) in the NRT

analysis, differing from the contributions obtained by

HOSE. Complexation does not lead to great alterations in

the contribution of the main canonic forms. This shows that

the pyrylium resonance effect is little influenced by the

hydrogen bonds established between the monomers upon

complex formation.

3.9. Harmonic oscillator model of aromaticity

The pyrylium cation aromaticity was calculated by the

HOMA index, using structural data. This index was calculated

in two ways: (i) by transforming C–O bonds into virtual C–C

[65] and (ii) by dividing ring C into two parts (one considering

C–C bonds only and the other considering C–O bonds only)

[64,65]. The factors responsible for the decreased aromaticity

are due to (i) the increase in bond length with respect to Ropt

(the EN term) and (ii) alternation in bond lengths (the GEO

term).

Compared to benzene, the HOMA index obtained for

complexes (I)–(XII) gives evidence of the significant aroma-

ticity of the pyrylium cation (Table 8) and shows that the term

which most contributes to the decreased aromaticity is related

to the alternation in the bond lengths (GEO). The contribution

of the EN term is almost half of that observed for the GEO

term.

Considering the division of ring C into two parts (Table 9), it

can be seen that the decrease in the aromaticity of the pyrylium

cation is mainly due to the long C–O bond lengths (EN term).

The length of these bonds in cations (I)–(XII) are very different

from the optimal C–O bond length (1.265 A). The opposite

occurs in the case of the C–C bond lengths, whose optimal

bond length is 1.388 A, leading to low EN term values.

The aromatic character of the pyrylium salts has already

been experimentally observed [72,73]. The complexation of

pyrylium with water molecules causes a slight decrease in the

aromaticity of the cation, which is most pronounced in

complex (XI), corroborating the conclusions that the hydrogen

bonds little alter the equilibrium geometry and electronic

structure of pyrylium and do not change its aromaticity.

3.10. Nucleus-independent chemical shifts

The NICS parameter was employed to study the influence of

the hydrogen bonds on the aromaticity of the pyrylium cation.

Table 6

Main resonance structures

Weight (%)

(I) (II) (III) (IV) (V) (VI) (VII) (VIII) (IX) (X) (XI) (XII)

Structures

O19.85 18.09 18.91 18.80 18.09 17.44 19.22 18.91 18.04 19.41 18.77 18.85

O19.74 20.48 18.96 18.63 19.74 19.58 19.51 19.15 19.82 19.32 18.50 18.38

O 17.50 19.69 19.48 21.88 19.17 21.33 16.70 19.47 20.70 18.38 17.43 20.28

Bond orders

O(1)–C(2) 1.381 1.399 1.385 1.373 1.383 1.361 1.392 1.390 1.387 1.384 1.381 1.379

C(2)–C(3) 1.473 1.453 1.473 1.480 1.466 1.495 1.461 1.464 1.466 1.467 1.469 1.473

C(3)–C(4) 1.382 1.399 1.377 1.377 1.391 1.366 1.390 1.386 1.388 1.384 1.385 1.377

C(4)–C(5) 1.382 1.372 1.384 1.377 1.367 1.390 1.389 1.386 1.366 1.384 1.385 1.385

C(5)–C(6) 1.472 1.480 1.472 1.478 1.484 1.470 1.463 1.467 1.486 1.464 1.466 1.467

C(6)–O(1) 1.383 1.371 1.373 1.376 1.374 1.388 1.385 1.385 1.367 1.386 1.386 1.385

C(2)–H(7) 0.985 0.973 0.985 0.985 0.974 0.985 0.974 0.974 0.981 0.981 0.975 0.975

C(3)–H(8) 0.983 0.983 0.972 0.983 0.981 0.983 0.983 0.983 0.982 0.982 0.983 0.981

C(4)–H(9) 0.986 0.986 0.986 0.976 0.984 0.977 0.986 0.986 0.984 0.986 0.978 0.984

C(5)–H(10) 0.983 0.983 0.983 0.983 0.983 0.983 0.982 0.983 0.980 0.982 0.983 0.983

C(6)–H(11) 0.985 0.985 0.985 0.985 0.985 0.974 0.982 0.974 0.985 0.981 0.975 0.975

R.L.T.Parreira

,S.E.Galem

beck

/JournalofMolecu

larStru

cture:

THEOCHEM

760(2006)59–73

67

O O O O O

(1) (2) (3) (4) (5)

Fig. 4. Pyrylium cation resonance structures.


This parameter was obtained at the center of the ring, NICS(0),

and 1 A above this point, NICS(1), by calculating the chemical

shieldings (Gauge-Independent Atomic Orbital (GIAO)

method) [74,75] in the nuclear magnetic resonance spectrum

(NMR). NICS(1) provides a better idea of the aromaticity in the

case of six-membered rings [76]. The benzene NICS parameter

was used as reference. Results (Table 10) show that pyrylium is

less aromatic than benzene. Furthermore, the hydrogen bonds

between pyrylium and water do not lead to any alterations in

the aromaticity of the cation. With the formation of the

hydrogen bond with water, an increase of the aromaticity of the

p-benzoquinone ring was observed by employing the NICS

method [77].

3.11. NMR analysis

The NMR chemical shifts (calculated by the GIAO method

[74,75]) obtained for the hydrogen and carbon atoms of the

pyrylium cation are presented in Table 11. The results are

close to experimental values [3,78–80], especially when the

hydrogen atoms are concerned. The deshielding effect of the

pyrylium cation is more noticeable with the C(2), C(4), and

C(6) atoms, once they exhibit a positive charge in the main

resonance structures (Fig. 4, structures (3)–(5)). Consequently,

H(7), H(9), and H(11), respectively, bonded to C(2), C(4), and

C(6), have more pronounced shifts than H(8) and H(10). As

for H(7) and H(11), there is the additional fact that they are in

ortho position with respect to the pyrylium oxygen,

accounting for their larger chemical shifts when compared

to the other hydrogen atoms of the pyrylium cation. The

complexation of this cation with water is responsible for the

increased chemical shifts observed for the carbon and

Table 7

Canonic structures contribution to compounds (I)–(XII) calculated by the HOSE m

Compound Contribution of canonic structures (Ci, (%))

(1) (2)

(I) 17.12 17.12

(II) 16.52 17.40

(III) 17.15 16.80

(IV) 16.86 16.86

(V) 16.66 16.70

(VI) 16.32 16.89

(VII) 16.95 16.82

(VIII) 16.80 16.80

(IX) 16.18 17.45

(X) 16.88 16.88

(XI) 16.54 16.54

(XII) 16.75 16.41

hydrogen atoms directly taking part in the hydrogen bonds.

Such effect is attributed to an increased positive charge on the

pyrylium hydrogens that participate in the hydrogen bonds,

particularly in positions A, A 0, C, and E, as observed in the

discussion concerning the NPA, MK, and GAPT charges, as

well as AIM (the following analysis).

3.12. AIM method

The topological analysis proposed by Bader was used to

obtain more information about the variation in electron density

during complexation. Additionally, the criteria proposed by

Popelier for the existence of a hydrogen bond were also

employed [81]. According to the latter author, the hydrogen

bond must have consistent topology [60,61,81–84]. The

electron density (rb) and its Laplacian (P2rb) at the bond

critical point (BCP) must be situated in preestablished intervals.

After formation of the hydrogen bond, a charge increase should

be noted (q(U)), as well as energetic destabilization (DE(U)),

decreased dipolar polarization (M(U)), and decreased hydrogen

atom volume (v(U)). Values for the critical point analysis (CPs,

a.u.) are presented in Tables 12 and S4–S13, while Tables 13 and

S14–S23 show the atomic properties.

3.12.1. Topology

The first condition necessary to confirm the presence of a

hydrogen bond is the correct topology of the gradient vector

field [82]. Analysis reveals the existence of a BCP between the

hydrogen atoms of the pyrylium C–H groups and acceptors of

hydrogen bonds. Complexation in positions B, B 0, D, or D 0

results in the formation of a ring between the monomers. The

critical points (RCPs) of this ring were found. The distance

odel

(3) (4) (5)

25.04 20.35 20.35

24.94 21.33 19.80

25.20 20.42 20.42

25.69 20.29 20.29

25.44 21.14 20.05

25.77 21.20 19.81

25.05 20.75 20.43

24.88 20.76 20.76

25.42 21.16 19.80

25.05 20.59 20.59

25.54 20.69 20.69

25.37 20.73 20.73

Table 8

HOMA, EN and GEO for compounds (I)–(XII)

Compound HOMA EN GEO

Benzene 0.9742 0.0257 0.0

(I) 0.7362 0.0843 0.1795

(II) 0.7286 0.0880 0.1833

(III) 0.7290 0.0868 0.1842

(IV) 0.7218 0.0899 0.1883

(V) 0.7150 0.0902 0.1948

(VI) 0.7101 0.0934 0.1965

(VII) 0.7260 0.0892 0.1847

(VIII) 0.7228 0.0918 0.1854

(IX) 0.7199 0.0877 0.1924

(X) 0.7262 0.0892 0.1846

(XI) 0.7091 0.0977 0.1932

(XII) 0.7113 0.0943 0.1943


between a BCP and an RCP can also be used as a criterion to

measure the structural stability of a hydrogen bond [61]. The

union of these two critical points represents the bond cleavage and

consequent ring opening [61]. In complex (V), the distances

between the O(15)/H(8) and O(15)/H(9) BCPs and RCP are

1.205 and 1.565 A, respectively, indicating that the latter is more

stable. The same is observed for complex (XII), in which the

distances between the O(18)/H(8) and O(18)/H(9) BCPs and

RCP are 1.232 and 1.542 A. For complex (VII), O(15)/H(10)

and O(15)/H(11) BCPs and RCP lie 0.951 and 1.782 A from

each other, respectively, showing that the O(15)/H(10) bond is

considerably less stable. In complex (IX), the O(12)/H(8)

hydrogen bond is very unstable; the distances between the

O(12)/H(8) and O(12)/H(7) BCPs and RCP are 0.417 and

2.110 A, respectively. In the same way, the O(12)/H(7) and

O(15)/H(11) hydrogen bonds are more stable than the O(12)/H(8) and O(15)/H(10) bonds in complex (X), in which the

distances between the O(12)/H(7), O(15)/H(11), O(12)/H(8), and O(15)/H(10) BCPs and RCP are 1.932, 1.927, 0.729,

and 0.737 A, respectively. The total topology of the complexes is

in agreement with the Poincare-Hopf relation [61,82], which does

not depend on gradient vector field details and can thus be used to

check topology consistency [82].

Table 9

HOMA, EN and GEO for compounds (I)–(XII)

Compound HOMA C–C contribution

HOMA EN G

(I) 0.7282 0.9562 0.0002 0

(II) 0.7206 0.9609 0.0003 0

(III) 0.7210 0.9562 0.0002 0

(IV) 0.7137 0.9452 0.0006 0

(V) 0.7068 0.9571 0.0001 0

(VI) 0.7019 0.9495 0.0004 0

(VII) 0.7179 0.9625 0.0002 0

(VIII) 0.7146 0.9686 0.0002 0

(IX) 0.7117 0.9533 0.0001 0

(X) 0.7180 0.9626 0.0002 0

(XI) 0.7008 0.9591 0.0006 0

(XII) 0.7031 0.9626 0.0002 0

3.12.2. Electron density of the bond critical point (rb)

This property is related to bond order and, consequently, to

bond strength [61,85]. According to the criteria proposed by

Popelier, the electron density value (rb) of the bond critical

point (BCP) must fall between 0.002 and 0.04 a.u. for a

hydrogen bond to be formed [81]. In all the studied complexes,

rb values are within the established limit. The higher rb values

show that the hydrogen bonds in positions A, A 0, C, and E are

more favored than those observed in positions B, B 0, D, and D 0.

Complexation does not cause significant alterations in the

electron density of the bond critical points of the pyrylium ring,

as observed by means of the NBO and NRT analyses, which

show that complexation little influences the pyrylium

electronic structure.

Ellipticity indicates preferential charge accumulation in a

given plane, besides providing information about structural

stability [61]. An increase in ellipticity may reflect an increase

in the structural instability or in the p character of the bond. In

this way, higher stability of the hydrogen bonds in positions A,

A 0, C, and E can be verified once again. The ellipticity also

reveals increased double bond character in O(1)–C(2) and

C(5)–C(6). However, a direct relationship between the

complexation site and variation in this property cannot be

established.

3.12.3. Laplacian of the electron density of the bond

critical point (P2rb)

All the hydrogen bonds in the complexes present positive

P2rb values and are within the interval [0.014–0.139 a.u.]

proposed in the literature [61,82].

3.12.4. Charges (q(U))

The hydrogen atoms involved in the hydrogen bonds

exhibit increased net charges after complexation, obeying the

criteria proposed by Popelier [81]. As for the other atoms,

the charges obtained by the AIM method are negative for the

pyrylium and water oxygen atoms only. Moreover, in the case

of the carbon atoms, the positive charge is concentrated on

C(2) and C(6), being close to zero in the remaining atoms.

Therefore, the AIM charges exhibit distinct behavior from

that observed with the NPA, MK, and GAPT methods. After

C–O contribution

EO HOMA EN GEO

.0435 0.2723 0.7277 0.0

.0388 0.2398 0.7602 0.0

.0435 0.2505 0.7493 0.0001

.0542 0.2507 0.7493 0.0

.0427 0.2060 0.7933 0.0006

.0500 0.2065 0.7933 0.0001

.0372 0.2287 0.7712 0.0001

.0312 0.2066 0.7933 0.0

.0466 0.2287 0.7712 0.0002

.0371 0.2288 0.7712 0.0

.0403 0.1841 0.8158 0.0

.0371 0.1840 0.8158 0.0002

Table 11

Chemical shifts (d, ppm)

C(2) C(3) C(4) C(5) C(6)

Exp.a 169.32 127.74 161.21 127.74 169.32

(I) 163.7 124.0 155.6 124.0 163.7

(II) 170.9 124.0 154.0 122.8 162.7

(III) 164.3 127.7 156.0 123.4 162.2

(IV) 162.5 124.2 161.7 124.2 162.4

(V) 169.7 126.5 158.1 122.3 161.5

(VI) 169.4 124.2 159.6 123.2 161.6

(VII) 169.1 122.8 152.9 124.8 167.8

(VIII) 169.6 122.8 152.5 122.8 169.6

(IX) 168.0 125.1 158.4 125.6 161.4

(X) 167.6 124.7 153.3 124.7 167.6

(XI) 168.1 123.2 157.9 123.2 167.7

(XII) 168.3 125.5 156.8 122.3 167.7

Reference: TMS (CZ192.7 ppm and HZ31.6 ppm).a Experimental data [3,78–80].

Table 12

Critical points analysis (CPs) for compounds (I) and (XI)

BCP (I)

rb P2rb 2

O(1)–C(2) 0.2940 0.0943 0.0345

C(2)–C(3) 0.3284 K0.9612 0.2405

C(3)–C(4) 0.3113 K0.8608 0.1457

C(4)–C(5) 0.3113 K0.8610 0.1457

C(5)–C(6) 0.3284 K0.9612 0.2405

C(6)–O(1) 0.2940 0.0943 0.0345

C(2)–H(7) 0.2985 K1.2304 0.0353

C(3)–H(8) 0.2897 K1.1154 0.0111

C(4)–H(9) 0.2924 K1.1439 0.0053

C(5)–H(10) 0.2897 K1.1154 0.0111

C(6)–H(11) 0.2986 K1.2304 0.0353

O(12)–H(13)

O(12)–H(14)

O(12)–H(7)

O(15)–H(16)

O(15)–H(17)

O(15)–H(11)

O(18)–H(19)

O(18)–H(20)

O(18)–H(9)

Ring 0.0222 0.1754

O–H (water) 0.3644 K2.0695 0.0273

O–H (water) 0.3644 K2.0695 0.0273

Table 10

Values for the NICS (0) and (1) parameters

Compound NICS (0) NICS (1)

Benzene K8.15 K10.21

(I) K5.79 K8.92

(II) K5.65 K8.88

(III) K5.73 K8.94

(IV) K5.68 K8.89

(V) K5.58 K8.88

(VI) K5.53 K8.84

(VII) K5.56 K8.87

(VIII) K5.51 K8.84

(IX) K5.61 K8.89

(X) K5.57 K8.87

(XI) K5.39 K8.77

(XII) K5.42 K8.79


complexation, there is an increase in the O(1) negative charge

and a decrease in the positive charge of the carbon atoms.

3.12.5. Energetic destabilization (DE(U))

DE(U) is defined as the difference between the energies of

the same atom in the complex and in the monomer. The

hydrogen atoms involved in the hydrogen bonds exhibit

increased atomic energies (DE(U)O0) after complexation,

specially in the complexes containing water in positions A, A 0,

C, and E. The successive addition of water molecules causes a

decrease in the energetic destabilization of the hydrogen atoms.

Complexation destabilizes O(1), but the carbon atoms of the

pyrylium cation are stabilized, except for C(6) in complex (III),

which hardly undergoes any alteration in energy.

H(7) H(8) H(9) H(10) H(11)

9.59 8.40 9.20 8.40 9.59

9.5 8.5 9.3 8.5 9.5

12.1 8.5 9.2 8.4 9.5

9.5 11.1 9.3 8.4 9.4

9.4 8.6 11.9 8.6 9.4

11.9 9.3 10.3 8.3 9.4

11.9 8.5 11.6 8.4 9.3

11.9 8.4 9.1 9.0 10.7

11.9 8.4 9.1 8.4 11.9

10.9 8.9 10.2 9.4 9.3

10.7 8.9 9.1 8.9 10.7

11.7 8.4 11.5 8.4 11.7

11.7 9.2 10.1 8.4 11.7

(XI)

rb P2rb 2

0.2912 0.0902 0.0754

0.3286 K0.9596 0.2381

0.3111 K0.8583 0.1394

0.3111 K0.8583 0.1394

0.3286 K0.9596 0.2381

0.2912 0.0902 0.0754

0.2966 K1.2682 0.0323

0.2892 K1.099 0.0126

0.2921 K1.1814 0.0038

0.2892 K1.099 0.0126

0.2966 K1.2682 0.0323

0.3600 K2.0877 0.0256

0.3600 K2.0877 0.0256

0.0216 0.0649 0.0960

0.3600 K2.0877 0.0256

0.3600 K2.0877 0.0256

0.0216 0.0649 0.0960

0.3603 K2.0850 0.0257

0.3603 K2.0850 0.0257

0.0194 0.0583 0.1010

0.0223 0.1751

Table 13

Atomic properties (a.u.) for atoms in compounds (I) and (XI)

Atom (I) (XI)

q(U) M(U) v(U) KE(U) q(U) M(U) v(U) KE(U)

O(1) K1.0929 0.284 97.71 76.0572 K1.1003 0.268 98.31 76.0426

C(2) 0.5928 0.801 69.83 37.7061 0.5440 0.829 71.99 37.7382

C(3) 0.0799 0.210 80.83 38.0520 0.0562 0.186 81.77 38.0609

C(4) 0.0640 0.115 78.70 38.0497 0.0307 0.192 80.53 38.0708

C(5) 0.0800 0.210 80.83 38.0520 0.0562 0.186 81.80 38.0609

C(6) 0.5928 0.801 69.83 37.7061 0.5441 0.829 71.98 37.7382

H(7) 0.1609 0.108 39.90 0.5649 0.2227 0.083 31.97 0.5386

H(8) 0.1205 0.108 42.27 0.5772 0.0992 0.113 43.65 0.5865

H(9) 0.1202 0.108 42.28 0.5800 0.1780 0.081 34.81 0.5571

H(10) 0.1205 0.108 42.27 0.5772 0.0992 0.113 43.64 0.5865

H(11) 0.1609 0.108 39.90 0.5649 0.2227 0.083 31.96 0.5386

O(12) K1.2020 0.246 147.91 75.7470

H(13) 0.6094 0.159 20.70 0.3465

H(14) 0.6094 0.159 20.69 0.3465

O(15) K1.2020 0.245 147.91 75.7470

H(16) 0.6094 0.159 20.70 0.3465

H(17) 0.6094 0.159 20.70 0.3465

O(18) K1.2006 0.248 148.57 75.7439

H(19) 0.6071 0.160 20.90 0.3478

H(20) 0.6071 0.160 20.90 0.3478

O(water) K1.1658 0.202 152.96 75.7079

H(water) 0.5829 0.171 22.12 0.3631

H(water) 0.5829 0.171 22.12 0.3631


3.12.6. Dipolar polarization (M(U))

The atomic integration of a position vector times the

electron density gives rise to the first momentum, M(U) [61].

The formation of the hydrogen bond leads to a loss of

nonbonded density of the hydrogen atom, and consequently, a

decrease in its dipolar polarization is observed [81,82].

Results show decreased dipolar polarization of the hydrogen

atoms after complexation, especially in the case of the

complexes with water in positions A, A 0, C, and E, indicating

that the hydrogen bonds taking place in these positions are

stronger.

3.12.7. Atomic volume (v(U))

The formation of the hydrogen bond must lead to a

reduction in the volume of the hydrogen atom [61,81,82].

The volume of the hydrogen atoms participating in the

hydrogen bonds undergoes a reduction of 7.5–9.0 a.u. for

complexation in positions A, A 0, C, and E. In positions B, B 0,

D, and D 0, such reduction is less than 5 a.u. Alterations in the

volume of the other atoms are not significant.

4. Conclusions

The complexation of pyrylium with 1–3 water molecules

does not lead to significant alterations in the equilibrium

geometries of the monomers, which can be attributed to the

weak hydrogen bonds formed between the oxygen of the

water molecule and the C–H group of the pyrylium cation.

The frequency associated with the stretching of the proton

donor group (C–H) exhibits the typical red-shift.

The behavior of the atomic charges depends on the method

employed for their determination (NPA, MK, GAPT, and AIM).

According to main resonance structures of the pyrylium cation

(Fig. 4), a slightly negative charge and a highly positive charges

should be exhibited by O(1), C(2), and C(4) atoms, respectively.

In this way, the GAPT and MK charges provide better chemical

insights than NPA and AIM charges.

Energetic analysis shows preferential complexation in

positions A and A 0. This same analysis shows that the

complexes containing at least one water molecule in positions

B and/or B 0 are less destabilized. The inverse is observed for

the complexes with water in positions D and/or D 0. The energy

analysis proposed by Xantheas reveals that the greater

contributions to the total relaxation energies (ERtotal) are due

to distortions undergone by pyrylium upon complex formation

(ERpyrylium). In comparison with the complexes containing two

water molecules, the addition of a third water molecule leads to

small alterations in DnE and BEn values.

The Natural Bond Orbitals undergo changes in their ‘s’ and

‘p’ character contributions upon complex formation. However,

the slight alterations in the second order interaction energies

(DE(2)) indicate that the electronic structure of the cation is

little influenced by the hydrogen bonds.

The aromaticity of the pyrylium cation was studied by

various methods. Different results concerning the contribution

of the resonance structures were obtained by using the HOSE

and NRT analysis. These two methods, together with the

HOMA and NICS indexes, reveal that the hydrogen bonds

between pyrylium and water do not exert any effect on the

aromaticity of the cation.


The hydrogen bonds were characterized by the AIM

method, and the results corroborate the fact that complexation

does not lead to significant alterations in the electron density of

the bonds critical points in the pyrylium ring.

Acknowledgements

The authors thank Professors Frank Weinhold, Tadeuz M.

Krygowski, Alexandru T. Balaban, Claudio F. Tormena and for

the referee for their valuable suggestions. SEG thanks CNPq for

the research scholarship (grant no. 301957/88-6). RLTP thanks

FAPESP for the PhD scholarship (grant No. 01/06101-6). The

authors also thank Laboratorio de Computacao Cientıfica

Avancada da Universidade de Sao Paulo for the generous

allocation of computational resources, and Ali Faiez Taha for

technical support.

Supplementary Material

Supplementary data associated with this article can be

found, in the online version, at 10.1016/j.theochem.2005.11.

020

References

[1] C. Fiorini, S. Delysse, J.-M. Nunzi, R. Karpiez, V. Gulbinas, M. Veber,

Synth. Met. 115 (2000) 133.

[2] M.A. Miranda, H. Garcıa, Chem. Rev. 94 (1994) 1063.

[3] A.T. Balaban, A. Dinculescu, G.N. Dorofeenko, G.W. Fischer,

A.V. Koblik, V.V. Mezheritskii, et al., Syntheses reactions and physical

properties, in: A.R. Katritzky (Ed.), Pyrylium salts Advances in

Heterocyclic Chemistry, 2, Academic Press, New York, 1982.

[4] M.J. Shaw, S.J. Afridi, S.L. Light, J.N. Mertz, S.E. Ripperda,

Organometallics 23 (2004) 2778.

[5] A.M. Bello, L.P. Kotra, Tetrahedron Lett. 44 (2003) 9271.

[6] J.D. Tovar, T.M. Swager, J. Org. Chem. 64 (1999) 6499.

[7] M.J. Shaw, J. Mertz, Organometallics 21 (2002) 3434.

[8] G.L. Ning, X.C. Li, W.T. Gong, M. Munakata, M. Maekawa, Inorg.

Chim. Acta 358 (2005) 2355.

[9] G.L. Ning, X.C. Li, M. Munakata, W.T. Gong, M. Maekawa,

T. Kamikawa, J. Org. Chem. 69 (2004) 1432.

[10] W.-T. Gong, G.-L. Ning, X.-C. Li, L. Wang, Y. Lin, J. Org. Chem. 70

(2005) 5768.

[11] R.M. Abd El-Aal, A.I.M. Koraiem, Z.H. Khalil, A.M.M. El-Kodey, Dyes

Pigment 66 (2005) 201.

[12] A.M. Bonch-Bruevich, E.N. Kaliteevskaya, T.K. Razumova,

A.D. Roshal’, A.N. Tarnovski, Opt. Spectrosc. 89 (2000) 216.

[13] D. Li, J. Zhang, M. Anpo, Opt. Mater. 27 (2005) 671.

[14] R.F. Khairutdinov, J.K. Hurst, Nature 402 (1999) 509.

[15] R.F. Khairutdinov, J.K. Hurst, J. Am. Chem. Soc. 123 (2001) 7352.

[16] J.A. Mikroyannidis, Macromolecules 35 (2002) 9289.

[17] A.M. Amat, A. Arques, S.H. Bossmann, A.M. Braun, M.A. Miranda,

R.F. Vercher, Catal. Today 101 (2005) 383.

[18] M.A. Miranda, F. Galindo, A.M. Amat, A. Arques, Appl. Catal., B 30

(2001) 437.

[19] M.A. Miranda, M.L. Marın, A.M. Amat, A. Arques, S. Seguı, Appl.

Catal., B 35 (2002) 167.

[20] I. Polyzos, G. Tsigaridas, M. Fakis, V. Giannetas, P. Persephonis,

J. Mikroyannidis, Chem. Phys. Lett. 369 (2003) 264.

[21] M. Fakis, J. Polyzos, G. Tsigaridas, J. Parthenios, A. Fragos,

V. Giannetas, et al., Chem. Phys. Lett. 323 (2000) 111.

[22] Y.-f. Zhou, S.-y. Feng, Chem. Phys. Chem. (2002) 969.

[23] I. Polyzos, G. Tsigaridas, M. Fakis, V. Giannetas, P. Persephonis, Opt.

Lett. 30 (2005) 2654.

[24] D.M. Teegarden, W.G. Herkstroeter, W.C. McColgin, J. Imaging Sci.

Technol. 37 (1993) 149.

[25] R.E. Koes, F. Quattrochio, J.N.M. Mol, BioEssays 16 (1994) 123.

[26] J.M.D. Markovic, N.A. Petranovic, J.M. Baranac, J. Agric. Food. Chem.

48 (2000) 5530.

[27] J.B. Harbone, R.J. Grayer, in: J.B. Harbone (Ed.), The Flavonoids:

Advances in Research since 1980, Chapman & Hall, London, 1988.

[28] R. Brouillard, in: J.B. Harbone (Ed.), The Flavonoids: Advances in

Research since 1980, Chapman & Hall, London, 1988.

[29] D. Strack, V. Wray, in: J.B. Harbone (Ed.), The Flavonoids: Advances in

Research since 1986, Chapman & Hall, London, 1994.

[30] D. Heber, S. Bowerman, J. Nutr. 121 (2001) 3078S.

[31] A. Hagiwara, K. Miyashita, T. Nakanishi, M. Sano, S. Tamano,

T. Kadota, et al., Cancer Lett. 171 (2001) 17.

[32] S. Meiers, M. Kemeny, U. Weyand, R. Gastpar, E. von Angerer,

D. Marko, J. Agric. Food. Chem. 49 (2001) 958.

[33] C.-Q. Hu, K. Chen, Q. Shi, R.E. Kilkuskie, Y.-C. Cheng, K.-H. Lee,

J. Nat. Prod. 57 (1994) 42.

[34] S. Christie, A.F. Walker, G.T. Lewith, Phytother. Res. 15 (2001) 467.

[35] C. Gomez-Cordoves, B. Bartolome, W. Vieira, V.M. Virador, J. Agric.

Food. Chem. 49 (2001) 1620.

[36] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb,

J.R Cheeseman, et al., GAUSSIAN 98, Gaussian Inc., Pittsburgh PA, 1998.

[37] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.

[38] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.

[39] P.J. Stephens, F.J. Devlin, C.F. Chabalowski, M.J. Frish, Phys. Chem. 98

(1994) 11623.

[40] J.B. Foresman, E. Frisch, Exploring Chemistry with Electronic Structure

Methods, second ed., Gaussian, Inc, Pittsburgh, 1996.

[41] Y. Gu, T. Kar, S. Scheiner, J. Am. Chem. Soc. 121 (1999) 9411.

[42] J. Cioslowski, J. Am. Chem. Soc. 111 (1989) 8333.

[43] S.S. Xantheas, J. Chem. Phys. 100 (1994) 7523.

[44] S.S. Xantheas, J. Chem. Phys. 104 (1996) 8821.

[45] R.L.T. Parreira, S.E. Galembeck, J. Am. Chem. Soc. 125 (2003) 15614.

[46] S.F. Boys, F. Bernardi, Mol. Phys. 19 (1970) 553.

[47] J.E. Carpenter, F. Weinhold, J. Mol. Struct. 169 (1988) 41.

[48] A.E. Reed, L.A. Curtiss, F. Weinhold, Chem. Rev. 88 (1988) 899.

[49] A.E. Reed, R.B. Weinstock, F. Weinhold, J. Chem. Phys. 83 (1985) 735.

[50] J.K. Badenhoop, F. Weinhold, J. Chem Phys. 107 (1997) 5406.

[51] E.D. Glendening, PhD Thesis, in: E.D. Glendening (Ed.), PhD Thesis,

University of Wisconsin, Madison, WI, 1991.

[52] E.D. Glendening, F. Weinhold, J. Comput. Chem. 19 (1998) 593.

[53] E.D. Glendening, F. Weinhold, J. Comput. Chem. 19 (1998) 610.

[54] E.D. Glendening, J.K. Badenhoop, F. Weinhold, J. Comput. Chem. 19

(1998) 628.

[55] E.D. Glendening, J.K. Badenhoop, A.E. Reed, J.E. Carpenter,

J.A. Bohmann, C.M. Morales, et al., Theoretical Chemistry Institute,

University of Winsconsin, Madison, 2001. NBO 5.0.

[56] P. Flukiger, H.P. Luthi, S. Portmann, J. Weber, et al., MOLEKEL 4.1,

Swiss Center for Scientific Computing, Manno, Switzerland, 2001,

p. 2000.

[57] R.F.W. Bader, Atoms in Molecules A Quantum Theory, Oxford

University Press, Oxford, UK, 1990.

[58] R.F.W. Bader, Chem. Rev. 91 (1991) 893.

[59] R.F.W. Bader, Phys. Chem. A 102 (1998) 7314.

[60] P. Hobza, J. Sponer, E. Cubero, M. Orozco, F.J. Luque, J. Phys. Chem. B

104 (2000) 6286.

[61] P.L.A. Popelier, J. Phys. Chem. A 102 (1998) 1873.

[62] F. Biegler-Konig, R.F.W. Bader, T.H.J. Tang, J. Comput. Chem. 3 (1982)

317.

[63] P.V.R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao, N.J.R. van Eikema,

J. Am. Chem. Soc. 118 (1996) 6317.

[64] T.M. Krygowski, M.K. Cyranski, Chem. Rev. 101 (2001) 1385.

[65] T.M. Krygowski, M.K. Cyranski, Tetrahedron 52 (1996) 10255.

[66] C.W. Bird, Tetrahedron 53 (1997) 13111.

[67] S. Aloisio, J.S. Francisco, J. Am. Chem. Soc. 122 (2000) 9196.

http://dx.doi.org/doi:10.1016/j.theochem.2005.11.020

http://dx.doi.org/doi:10.1016/j.theochem.2005.11.020


[68] S. Scheiner, T. Kar, J. Phys. Chem. A 106 (2002) 1784.

[69] S. Scheiner, Y. Gu, T. Kar, J. Mol. Struct. (THEOCHEM) 500 (2000) 441.

[70] K.C. Lopes, F.S. Pereira, R.C.M.U. de Araujo, M.N. Ramos, J. Mol.

Struct. 565-566 (2001) 417.

[71] I. Vorobyov, M.C. Yappert, D.B. DuPre, J. Phys. Chem. A 106 (2002) 668.

[72] M. Gdaniec, I. Turowska-Tyrk, T.M. Krygowski, J. Chem. Soc., Perkin

Trans. 2 (1989) 613.

[73] I. Turowska-Tyrk, T.M. Krygowski, P. Milart, J. Mol. Struct. 263

(1991) 235.

[74] R. Ditchfield, Mol. Phys. 27 (1974) 789.

[75] K. Wolinski, J.F. Hinton, P. Pulay, J. Am. Chem. Soc. 112 (1990) 8251.

[76] J.M. Schulman, R.L. Disch, H. Jiao, P.V.R. Schleyer, J. Phys. Chem. A

102 (1998) 8051.

[77] T.K. Manojkumar, H.S. Choi, P. Tarakeshwar, K.S. Kim, J. Chem. Phys.

118 (2003) 8681.

[78] A.T. Balaban, V. Wray, Org. Magn. Reson. 9 (1977) 16.

[79] D. Farcasiu, S. Sharma, J. Org. Chem. 56 (1991) 126.

[80] I. Degani, F. Taddei, C. Vincenzi, Boll. Sci. Fac. Bologna 25 (1967) 61.

[81] P.L.A. Popelier, Atoms in Molecules: An Introduction, Pearson Education

Limited, Edinburgh Gate, Harlow, England, 2000.

[82] U. Koch, P.L.A. Popelier, J. Phys. Chem. 99 (1995) 9747.

[83] I. Alkorta, I. Rozas, J. Elguero, Ber. Bunsen-Ges. Phys. Chem 102 (1998)

429.

[84] A. Hocquet, Phys. Chem. Chem. Phys. 3 (2001) 3192.

[85] K.B. Wiberg, R.F.W. Bader, C.D.H. Lau, J. Am. Chem. Soc. 109

(1987) 1001.

Documents

Computational study of pyrylium cation–water complexes: hydrogen bonds, resonance effects, and aromaticity