ORIGINAL RESEARCH

Molecular structure, conformational stability, energeticand intramolecular hydrogen bonding in ground, and electronicexcited state of 3-mercapto propeneselenal

Maryam Shokhmkar • Heidar Raissi •

Fariba Mollania

Received: 26 July 2013 / Accepted: 27 November 2013

� Springer Science+Business Media New York 2014

Abstract In the present work, a conformational analysis of

3-mercapto propeneselenal is performed using several

computational methods, including DFT (B3LYP), MP2, and

G2MP2. At the DFT and G2MP2 levels the most stable

conformers of title compound are characterized by an

extended backbone structure, minimizing the steric repul-

sions between the sulfur and selenium lone pairs. Two

conformers exhibit hydrogen bonding. This feature,

although not being the dominant factor in energetic terms,

appears to be of foremost importance to define the geometry

of the molecule. The influence of the solvent on the stability

order of conformers and the strength of intramolecular

hydrogen bonding was considered using the PCM, SCI–

PCM, and IEF–PCM methods. The results of analysis by

quantum theory of ‘‘Atoms in Molecules’’ and natural bond

orbital method fairly support the DFT results. The calculated

HOMO and LUMO energies showed that charge transfer

occurs within the molecule. Further verification of the

obtained transition state structures was implemented via

intrinsic reaction coordinate analysis. Calculations of the 1H

NMR chemical shift at GIAO/B3LYP/6–311??G** levels

of theory are also presented. The excited-state properties of

intramolecular hydrogen bonding in hydrogen-bonded sys-

tems have been investigated theoretically using the time-

dependent density functional theory method.

Keywords 3-Mercapto propeneselenal � Molecular

structure � Ab-initio and DFT calculation � Intramolecular

hydrogen bond � TD-DFT

Introduction

Selenium-containing compounds have been well recog-

nized, not only because of their remarkable reactivities and

chemical properties, but also because of their diverse

pharmaceutical applications. Despite the high toxicity of

many selenium compounds, organic derivatives of sele-

nium have been synthesized as anticancer [1, 2], and for

other medicinal applications [3], as well as biologically

active substances exhibiting antiviral [4], antibacterial [5],

antihypertensive [6], and fungicidal properties [7]. As a

result, selenium-containing compounds are of increasing

interest because of their chemical properties and biological

activities. Many chemical processes in selenium-containing

compounds are modulated by the existence or the forma-

tion of intramolecular hydrogen bonds (HB) [8–10]. The

3-mercapto propeneselenal (MCPS) is an interesting sele-

nium-containing molecule, which involved in Se–H���S and

S–H���Se intramolecular HBs. The hydrogen bonding and

proton transfer play a crucial role in chemical and bio-

logical processes, especially in enzymatic reactions [11].

Both intra- and inter-molecular hydrogen bonding have a

significant effect on chemical behavior, especially on the

excited state properties [12–14]. For instance, the photo-

physics and photochemistry of chromophores in hydrogen

bonding surroundings can be remarkably tuned by the

hydrogen bonding in electronic excited states [15–21]. In

the recent years, quantum mechanical calculations of

hydrogen bonding have attracted theoretical chemists,

physicists, and biologists [22–29]. The importance of

hydrogen-bonded systems has long been known, although

the understanding of its nature is not yet complete.

The aims of this paper were (i) to determine the order of

stability of the various MCPS conformations, (ii) to predict

the most stable structure in the gas phase and in solution,

M. Shokhmkar (&) � H. Raissi � F. Mollania

Computational Chemistry Lab, Department of Chemistry,

University of Birjand, Birjand, Iran

e-mail: maryamshokhmkar@gmail.com

123

Struct Chem

DOI 10.1007/s11224-013-0381-3

and (iii) to evaluate the intramolecular HB strength in

MCPS conformers in the ground and the first excited states.

The natural bond orbital analyses (NBO) were applied as a

powerful approach for evaluation of the hydrogen bond

strength in the chelated conformers of MCPS. Finally, the

obtained results were compared with the AIM topological

parameters analysis.

Quantum chemical calculation

All computations were performed using the Gaussian 03

suite of programs [30]. The geometry optimization was

carried out at B3LYP [31] and MP2 [32] methods with

6-311??G** basis set. To have more reliable energetic,

their total energies were computed at the G2MP2 level,

which yields energies of an effective QCISD(T)/6-311G**

quality. Furthermore, harmonic vibration frequencies were

calculated at B3LYP/6-311??G** and MP2/6-311??G**

levels of theory in order to confirm the nature of stationary

points found and to account for the zero-point vibrational

energy (ZPVE) correction. Time-dependent density func-

tional theory (TD-DFT) was employed for excited state

computations [33, 34]. The reaction path has been followed

using Fukui’s theory of the intrinsic reaction coordinate

(IRC) method [35]. The topological analyses have been

performed with the AIM 2000 program [36] using the

B3LYP/6-311??G** wave functions as input. Natural

bond orbital (NBO) analysis [37] at B3LYP/6-311??G**

level was carried out to understand the orbital interactions

and charge delocalization during the course of the reaction.

The contour plot for visualization of the NBO result is

constructed on NBOView (Version 1.1) [38] software

package using the standard keywords implemented therein.

The effect of solute–solvent interactions was initially taken

into account by means of PCM [39], IEF–PCM [40], and

SCI–PCM [41] methods. The harmonic oscillator model of

aromaticity (HOMA) [42] is calculated as follows:

HOMA ¼ 1� 1

n

Xn

j�1

ai ðRopt � RjÞ2

where n stands for number of all bonds, ai is a normali-

zation factor preserving HOMA = 0 for hypothetical

Kekule structure and HOMA = 1 for fully aromatic. The

system with all bonds equal to the optimal value Ropt,

assumed to be realized for full aromatic systems; Rj

denotes bond lengths taken into calculation. In the present

work, the Ropt and a parameter for HOMA index were

calculated in gas phase at the same level of theory (for CC,

CS, and CSe bonds: Ropt, CC = 1.396 A, Ropt,

CS = 1.689 A, Ropt, CSe = 1.830 A, aCC = 88.270,

aCS = 74.590, and aCSe = 72.535). After optimization,

1H chemical shift was calculated with GIAO method [43],

using corresponding TMS shielding calculated at the same

theoretical levels as the reference. The molecular orbital

(MO) calculations such as HOMO–LUMO are also per-

formed on the conformer of MCPS with the same level of

DFT theory. Molecular electrostatic potentials (MEPs) of

MPS-1 and SPT-1 have been obtained on the 0.001 au

electron density isosurfaces. This surface has been shown

to resemble the van der Waals surface [44].

Results and discussion

Relative stabilities

Theoretically, MCPS has 20 possible conformers. In view

of functional groups, these conformers can be classified

into three tautomeric classes: selenal (MPS), thial (SPT)

and selonoxo-thial (ST), which have 8, 8, and 4 rotamers,

respectively (see Fig. 1). The values of relative electronic

energies relative to the global minimum (at the DFT, MP2,

and G2MP2 levels) are given in Table 1. The results

of Table 1 showed that MPS-7 conformer is only

0.8 kJ mol-1 more stable than MPS-1 conformer, at the

MP2/6-311??G** level. For considering the higher order

correlation corrections, calculations were performed at the

G2MP2 level. Our theoretical calculations confirmed that

with considering higher order correlation corrections

energy gap between these two conformers becomes closer.

As shown in Table 1, the energy gap between structure

SPT-1 and structure SPT-7, which at the MP2/6-31??G**

level is already 3.8 kJ mol-1, increases slightly when

obtained at the G2MP2 level of theory. With only a few

exceptions it is worth noticing that the stability order of the

conformations given by DFT closely resembles that

obtained by MP2. Moreover, the global minimum on the

DFT potential energy surface is the second lowest energy

conformation (DE ca. 2.7 kJ mol-1) at the MP2 level. The

differences between DFT and MP2 in predicting the most

significant conformations have been reported by some

authors [45].

Our theoretical calculations on MCPS show that MPS

conformers are more stable than the other conformers.

These conformers can be divided in two forms, hydrogen-

bonded and non-hydrogen-bonded systems. In non-hydro-

gen-bonded systems, MPS conformers are about 3.01 and

46.23 kJ mol-1 more stable than SPT and ST conformers,

respectively. Furthermore the SPT conformers are about

43.22 kJ mol-1 more stable than the ST conformers. In

hydrogen-bonded systems, the MPS conformers are more

stable than the others too; that is, the extra stability of TES

conformers is due to the existence of strong C–S, C=Se,

and C=C bonds. It was found out that the ZPVE correction

Struct Chem

123

could not considerably change the energy orders being an

insensitive parameter. Selected geometrical parameters of

MCPS conformers are given in Table 2. This Table

includes also the results corresponding to the transition

state between structures MPS-1 and SPT-1 (TS 1/1). The

results predict that all the conformations of the MCPS are

fully planar except ST conformers. The absence of imag-

inary frequency for all the different conformers of MCPS

proves that each of these forms is stable (except for ST-1)

and has a particular local minimum on the potential energy

surface (PES).

Regarding the DFT calculations, the comparison of the

relative energies of the different MPS and SPT conformers

shows that MPS-7 and SPT-5 are more stable than all the

other conformers. This stability is mainly due to the ori-

entation of lone pairs of S and Se atoms.

The results of theoretical calculations on the stability

orders of MCPS conformer (Table 1) illustrate that in spite

of the presence of Se���H–S and S���H–Se HBs in MPS-1

and SPT-1 conformers, they are less stable than MPS-7 and

SPT-5 conformers, respectively. This means that this

interaction is not the dominant factor in the energy ordering

of the different conformers of MCPS. Geometrical

parameters present in Table 2 show that in MPS-1 and

SPT-1 conformers both C=C and C=X (X = Se or S) bond

lengths are increased, whereas C–C and C–X (X = Se

or S) bond lengths are decreased with respect to the other

MPS and SPT conformers, respectively. These behaviors

are caused by hydrogen bond formation, which in fact

increase the p–electron resonance in the chelated ring.

Analysis of the geometrical parameters provides evidence

that bond angles in the MPS-1 chelate ring are closer to the

Fig. 1 Possible conformers of

MCPS

Struct Chem

123

standard sp2 hybridization values in comparison to SPT-1

conformer. Due to the presence of a relatively strong

Se���H–S hydrogen bond and by considering the relative

energies, MPS-1 conformer is more stable than SPT-1

conformer.

For the ST conformers only three rotamers are minima

of the PES. It is worth noticing that ST-1 converges to ST-3

after full optimization. This is owing to electronic repul-

sion between the S and Se lone pair electrons and absence

of the p-conjugation between two double bonds (as a

consequence of the CH2 group). It is necessary to mention

that the ST conformers cannot form hydrogen bond. The

absence of p-delocalization and the proton in the ring of ST

conformers causes the hydrogen bond not to be formed.

Besides, the highest rotational constants (A) come from

the SPT-3 and MPS-7 conformers, and the highest rota-

tional constant (B) comes from the SPT-5 conformer in

Table 2. In SPT-5 conformer, dipole moment is higher than

those in SPT-3 and MPS-7 conformers.

Intramolecular HB and selenal–selenol tautomerism

As mentioned above, the conformational arrangements of

MPS-1 and SPT-1 enable the formation of intramolecular

H-bond between the SH group and the selenium (SH���Se)

and the SeH���S bond, respectively. It seems that the

energetic gap between MPS-1 and SPT-1 species is a direct

consequence of the stronger intramolecular HB of the

former. Therefore, we shall try to analyze the strength of

intramolecular HB and the parameters which showing the

formation of this interaction for these two forms. It is well

known that the geometrical parameters of the HB reflect

the strength of the bond.

Two methods were used to compute intramolecular HB

energy. In method 1, the difference in energy between

closed and open configurations was calculated, i.e., EHB

based on Shuster method [46]. In method 2, the HB ener-

gies EHB* could be estimated from the properties of bond

critical points [47]. The simple relationship between HB

energy and the potential energy density V(rcp) at the crit-

ical point corresponding to Se���H and S���H contacts was

assigned to be EHB* = 1/2 V(rcp). The comparison between

S–H���Se (in MPS-1) and S���H–Se (in SPT-1) hydrogen

bonds (see Table 3) shows that, not only that the acidity of

SH bond is greater than of the SeH but also basicity of Se

atom is greater than the S atom, this means that the

hydrogen bond in Se-H���S system (SPT-1) is weaker than

the S–H���Se system (MPS-1). Furthermore, the values of

Se���S distance in MPS-1 and SPT-1 conformers are 3.438

and 3.473 A, respectively, this leads MPS-1 to be more

stable than SPT-1. It is well known that formation of HB

caused the S–H and Se–H stretching modes shift to lower

frequencies and with strengthen of HB this shifting become

larger. Inspection of Table 2 reveals clearly that the S–H

stretching frequency for the MPS-1 conformer appears red

shifted by ca. 638.36 cm-1 with respect to that in MPS-3,

Table 1 Relative energies (kJ mol-1) of all possible conformations in gas phase and water solution

B3LYP MP2 G2MP2 PCM IEFPCM SCIPCM

MPS-1 5.21 (5.37) 0 (0) 10.69 20.24 (21.89) 20.23 (21.89) 5.06

MPS-2 8.33 (9.73) 6.09 (8.76) 15.24 14.70 (15.84) 14.70 (15.84) 11.47

MPS-3 10.53 (10.40) 11.43 (13.47) 21.09 14.74 (14.26) 14.74 (14.26) 13.00

MPS-4 11.38 (10.81) 12.69 (13.49) 21.67 16.11 (15.47) 16.11 (15.47) 13.86

MPS-5 9.96 (9.92) 9.26 (10.35) 19.48 15.03 (15.33) 15.03 (15.33) 12.43

MPS-6 9.32 (9.00) 8.05 (9.23) 0 12.49 (12.11) 12.48 (12.11) 10.46

MPS-7 0 (0) 0.78 (2.79) 11.45 0 (0) 0 (0) 0

MPS-8 2.69 (2.01) 3.75 (4.70) 13.21 1.52 (1.07) 1.52 (1.07) 2.49

SPT-1 6.49 (14.39) 17.17 (14.60) 18.61 30.60 (29.75) 30.60 (29.75) 16.12

SPT-2 40.86 (37.85) 18.62 (17.79) 20.09 29.12 (28.46) 29.11 (28.45) 17.65

SPT-3 4.02 (1.52) 9.77 (9.83) 13.74 14.65 (14.46) 14.64 (14.46) 5.10

SPT-4 14.03 (10.96) 19.99 (19.31) 22.67 29.02 (28.12) 29.02 (28.12) 17.83

SPT-5 1.34 (0.80) 6.35 (7.40) 9.41 20.85 (22.25) 20.85 (22.25) 8.07

SPT-6 12.17 (9.00) 15.83 (14.87) 17.10 25.99 (25.11) 25.99 (25.11) 15.22

SPT-7 14.31 (10.80) 21.01 (19.69) 23.24 29.17 (27.94) 29.16 (27.95) 18.47

SPT-8 5.31 (1.97) 11.80 (10.85) 15.04 14.87 (14.39) 14.87 (14.39) 5.74

ST-2 50.62 (54.61) 29.21 (37.52) 45.69 69.41 (75.26) 69.41 (75.26) 55.14

ST-3 49.34 (53.32) 28.96 (37.24) 45.98 69.48 (75.33) 69.47 (75.33) 55.22

ST-4 48.38 (52.98) 27.49 (36.40) 47.81 67.58 (73.92) 67.58 (73.90) 54.00

The values in parentheses refer to relative energies with considering ZPVE correction

Struct Chem

123

while the red-shifting of the Se–H stretch of SPT-1 with

respect to SPT-5 is 369.53 cm-1. The larger value corre-

sponds to MPS-1, which is the system that presents the

stronger HB. Molecular electrostatic potentials (MEPs) of

MPS-1 and SPT-1 conformers (see Fig. 2) have been

obtained on the 0.001 au electron density isosurfaces. This

surface has been shown to resemble the van der Waals

surface [44]. Furthermore, results of the aromaticity cal-

culations by HOMA index [42] are listed in Table 3.

Comparison of local aromaticity systems in chelated con-

formers elucidates that MPS-1 has more delocalized

p-electrons than SPT-1. The 1H chemical shifts for MCPS

conformers calculated by the GIAO method at the B3LYP

level are collected in Tables 2 and 4 confirming stronger

HB causes the 1H chemical shift of H to move downfield

further.

Another method for evaluating the energy of

intramolecular hydrogen bridges utilizing the rotation

Table 2 Geometrical parameters obtained at B3LYP/6-311??G** level of theory

CS CC CC CSe X–H A B C d m (X–

H)

l

MPS-

1

1.632 (1.712)

[1.717]

1.384 (1.376)

[1.370]

1.407 (1.427)

[1.424]

1.820 (1.795)

[1.783]

1.411 (1.360)

[1.359]

4.831 1.451 1.116 16.626 1,917.5 2.59

MPS-

2

1.738 (1.731)

[1.733]

1.364 (1.369)

[1.364]

1.421 (1.431)

[1.426]

1.797 (1.787)

[1.777]

1.361 (1.344)

[1.349]

4.516 1.574 1.167 4.368 2,555.8 3.82

MPS-

3

1.736 (1.731)

[1.732]

1.355 (1.358)

[1.354]

1.436 (1.450)

[1.445]

1.793 (1.778)

[1.768]

1.351 (1.338)

[1.343]

11.033 0.770 0.720 4.252 2,655.0 3.85

MPS-

4

1.743 (1.736)

[1.739]

1.354 (1.358)

[1.353]

1.436 (1.449)

[1.444]

1.793 (1.779)

[1.769]

1.354 (1.335)

[1.339]

11.232 0.770 0.720 3.818 2,679.3 3.28

MPS-

5

1.745 (1.738)

[1.741]

1.361 (1.366)

[1.360]

1.423 (1.435)

[1.431]

1.789 (1.779)

[1.769]

1.348 (1.335)

[1.340]

13.404 0.719 0.683 5.124 2,674.3 3.01

MPS-

6

1.756 (1.749)

[1.741]

1.358 (1.363)

[1.360]

1.426 (1.437)

[1.431]

1.787 (1.778)

[1.769]

1.347 (1.334)

[1.340]

12.471 0.747 0.705 3.612 2,684.4 4.56

MPS-

7

1.742 (1.736)

[1.738]

1.356 (1.361)

[1.356]

1.423 (1.437)

[1.432]

1.789 (1.777)

[1.768]

1.351 (1.338)

[1.342]

26.206 0.614 0.600 3.886 2,653.9 3.53

MPS-

8

1.750 (1.742)

[1.744]

1.356

(1.361){1.355]

1.426 (1.437)

[1.432]

1.788 (1.778)

[1.768]

1.347 (1.335)

[1.339]

25.808 0.617 0.603 3.435 2,673.6 4.66

SPT-

1

1.865 (1.864)

[1.855]

1.368 (1.371)

[1.365]

1.424 (1.439)

[1.435]

1.664 (1.645)

[1.646]

2.085 (2.179)

[2.259]

4.654 1.434 1.096 15.035 1,875.2 2.44

SPT-

2

1.894 (1.890)

[1.880]

1.355 (1.361)

[1.356]

1.434 (1.444)

[1.439]

1.642 (1.632)

[1.633]

1.471 (1.459)

[1.481]

11.436 0.757 0.710 2.9397 2,369.6 3.29

SPT-

3

1.888 (1.885)

[1.875]

1.351 (1.357)

[1.352]

1.437 (1.447)

[1.441]

1.642 (1.630)

[1.632]

1.473 (1.461)

[1.484]

26.647 0.613 0.600 3.842 2,372.1 3.69

SPT-

4

1.881 (1.879)

[1.869]

1.350 (1.355)

[1.350]

1.447 (1.461)

[1.453]

1.644 (1.630)

[1.633]

1.474 (1.462)

[1.484]

12.867 0.730 0.691 4.044 2,349.3 3.70

SPT-

5

1.879 (1.875)

[1.866]

1.360 (1.366)

[1.361]

1.427 (1.437)

[1.431]

1.652 (1.640)

[1.642]

1.495 (1.475)

[1.500]

4.536 1.684 1.228 4.676 2,244.8 3.24

SPT-

6

1.902 (1.899)

[1.891]

1.353 (1.358)

[1.354]

1.435 (1.446)

[1.441]

1.640 (1.630)

[1.632]

1.470 (1.458)

[1.482]

10.230 0.808 0.749 3.589 2,382.2 4.26

SPT-

7

1.887 (1.885)

[1.876]

1.349 (1.354)

[1.350]

1.447 (1.460)

[1.453]

1.644 (1.630)

[1.633]

1.471 (1.459)

[1.481]

13.339 0.730 0.692 3.692 2,367.6 3.24

SPT-

8

1.894 (1.890)

[1.882]

1.350 (1.357)

[1.352]

1.436 (1.447)

[1.441]

1.641 (1.630)

[1.632]

1.471 (1.459)

[1.482]

25.218 0.621 0.606 3.505 2,373.8 4.43

ST-2 1.622 (1.617)

[1.618]

1.497 (1.506)

[1.500]

1.501 (1.501)

[1.498]

1.767 (1.764)

[1.754]

– 7.119 0.936 0.847 – – 2.66

ST-3 1.623 (1.619)

[1.620]

1.496 (1.498)

[1.494]

1.504 (1.510)

[1.506]

1.763 (1.761)

[1.751]

– 8.366 0.855 0.796 – – 2.59

ST-4 1.619 (1.618)

[1.619]

1.509 (1.510)

[1.506]

1.502 (1.505)

[1.501]

1.763 (1.763)

[1.753]

– 10.398 0.738 0.711 – – 2.30

TS1/

1

1.679 1.38 1.408 1.849 – – – – – – –

Values in parentheses refer to calculation at MP2/6-311??G** and values in brackets refer to calculation at G2MP2. Bond lengths in [A], and

angles in (�), rotational constants (GHz), dipole moment (Debye), chemical shift of proton (d, ppm) and frequencies (cm-1) are included for all

conformers (X = Se or S atoms)

Struct Chem

123

barriers of the donor and/or of the acceptor groups is

proposed. The rotational barriers (EBR) around Se–H and

S–H bonds in MPS-1 and SPT-1, respectively, were

investigated by DFT calculations using 6-311??G** basis

set in gas phase. The results of our theoretical calculations

showed that the values of rotational barriers in SPT-1 and

MPS-1 conformers are about 70.35 and 85.38 kJ mol-1,

respectively, which are in agreement with the EHB* values.

The results corresponding to the transition state between

MPS-1 and SPT-1 structures (TS1/1) are listed in Table 2.

One can see that the geometry of TS is very close to the

geometry of S–H���Se tautomeric form (Table 2). For the

process MPS-1 ? TS, the geometrical parameters change

for the transfer (H from S to Se) to take place easily. For

atom H, firstly, the angle CSH is compressed from 94.789

to 90.624 while the angle HSeC increased from 87.39 to

90.594. Then the bond length of CS is shortened by

0.040 A from 1.718 to 1.679 A. The elongation of CSe

(from 1.849 to 1.865) and SeH (from 1.524 to 1.645 A, an

evident change) also takes place. These geometrical

parameter changes cause a decrease of length of HS from

1.776 to 1.395 A. In general, the reaction path could be

defined as the curve on the potential energy surface con-

necting the reactants and products through the transition

state (see Fig. 3). The energy barrier for proton transfer is

12.68 kJ mol-1. The ‘‘transient’’ structure (TS) represents

the structure of the highest energy along the minimum-

energy path reaction coordinate (Fig. 3).

As the first step toward understanding the influences of

the water molecules on the energy barrier, one and two water

molecules have been considered firstly based on the geom-

etries of MPS-1 and SPT-1 on three representative models. It

should be mentioned that different orientations for water

molecules were examined, which are converted and pre-

sented as models in Fig. 4. The optimized structures of the

MPS-1 and SPT-1 conformers and their H-bonded com-

plexes with one and two water molecules, calculated using

B3LYP method with 6-311??G** basis set, are shown in

Fig. 4. The first model (Fig. 4a), the hydrogen bond length

(A) between O10-atom of H2O and H7-atom in the H2O–

Table 3 The selected topological parameters (in a.u.) and the energy of

the intramolecular hydrogen bond (in kJ mol-1) and HOMA index in

MPS-1 and SPT-1 conformers in gas phase and water solution and cal-

culated electronic excitation energies (eV) and corresponding oscillator

strengths of SPT-1 and MPS-1 in the first singlet excited state S1

Gas Water

MPS-1 SPT-1 MPS-1 SPT-1

X���H 2.139 1.524 2.267 2.214

S���Se 3.438 3.473 3.512 3.535

SHSe 150.5 148.1 147.3 143.7

qX���H 0.0442 0.0431 0.0333 0.0332

r2qX���H 0.0434 0.0535 0.0494 0.054

qRCP 0.0118 0.0107 0.0102 0.0098

r2qRCP 0.0640 0.0576 0.0534 0.0506

G 0.0231 0.0241 0.0181 0.0188

V -0.0353 -0.0349 -0.0239 -0.0242

H -0.0122 -0.0108 -0.0058 -0.0053

E�HB -46.37 -45.87 -31.45 -31.69

EHB 3.12 -5.14 -5.53 -9.74

Excitation

energies (eV)

1.588 1.576

H ? L

(1.0)

H ? L

(0.879)

Oscillator

strengths

0.0001 0.0002

LP! r�X�H 42.72 39.09 25.60 24.11

O.N(LP) 1.778 1.782 1.846 1.842

O:N: r�XH

� �0.183 0.164 0.117 0.109

HOMA 0.933 0.931 0.968 0.943

X = Se or S atoms

H the highest occupied molecular orbital (HOMO), L the lowest unoc-

cupied molecular orbital (LUMO)

Fig. 2 MEP on the vdW

surface with indication of some

of the minima and maxima

values for the SPT-1 and MPS-1

conformers

Struct Chem

123

MPS-1 and H2O–SPT-1 complexes has been found to be as

follows: H2O–MPS-1 (2.255) [ H2O–SPT-1 (2.317). All

energy barriers were corrected using vibrational zero-point

energies. The energy barrier for proton transfer of MPS-

1�H2O complex respect to MPS-1 drops from 12.68 to

6.14 kJ mol-1. This reduction in the energy barrier for

proton transfer of nearly 6 kJ mol-1 has a significant impact

on the rate of rearrangement of MPS-1�H2O. The second

model (Fig. 4b), the hydrogen bond length (A) between

O-atom of H2O and H6-atom in the H2O–MPS-1 and H2O–

SPT-1 complexes is about 2.522. The energy barrier for

proton transfer reduces to 6.92 kJ mol-1 in presence of one

water molecule. Finally, for the process MPS-

1�(H2O)2 ? TS (Fig. 4c), the energy barrier for proton

transfer is 8.01 kJ mol-1. Therefore, the water molecules

play an important role in the proton transfer and reduce

considerably the energy barrier. The examination of the

structural changes from reactant to product in the first model,

shows that the C(3)S(2)H(1) bond angle undergoes com-

pression first, and then the S(2)–H(1) bond starts to lengthen.

Table 4 Geometrical parameters obtained in water solution at the PCM method

CS CC CC CSe X–H A B C d t (X–

H)

l

MPS-

1

1.718 (1.718)

[1.718]

1.376 (1.376)

[1.375]

1.411 (1.411)

[1.412]

1.821 (1.821)

[1.818]

1.383 (1.383)

[1.388]

4.655 1.409 1.081 13.808 2,207.7 4.17

MPS-

2

1.729 (1.730)

[1.733]

1.371 (1.371)

[1.368]

1.414 (1.414)

[1.416]

1.812 (1.812)

[1.806]

1.380 (1.380)

[1.356]

4.556 1.500 1.129 6.940 2,258.0 6.91

MPS-

3

1.722 (1.722)

[1.728]

1.364 (1.364)

[1.360]

1.422 (1.422)

[1.428]

1.813 (1.813)

[1.805]

1.379 (1.379)

[1.350]

11.155 0.764 0.715 5.318 2,254.8 7.13

MPS-

4

1.729 (1.729)

[1.734]

1.363 (1.363)

[1.360]

1.422 (1.422)

[1.427]

1.813 (1.813)

[1.805]

1.375 (1.375)

[1.347]

11.312 0.763 0.715 7.473 2,258.3 6.24

MPS-

5

1.730 (1.731)

[1.736]

1.370 (1.730)

[1.366]

1.412 (11.412)

[1.417]

1.806 (1.806)

[1.799]

1.373 (1.373)

[1.348]

13.535 0.714 0.678 7.310 2,328.0 5.85

MPS-

6

1.739 (1.739)

[1.744]

1.368 (1.368)

[1.364]

1.414 (1.414)

[1.418]

1.806 (1.805)

[1.798]

1.373 (1.373)

[1.347]

12.671 0.740 0.699 6.514 2,294.3 8.36

MPS-

7

1.726 (1.726)

[1.732]

1.367 (1.367)

[1.363]

1.413 (1.413)

[1.418]

1.810 (1.810)

[1.801]

1.379 (1.379)

[1.350]

26.783 0.613 0.599 6.475 2,257.8 6.96

MPS-

8

1.732 (1.732)

[1.732]

1.366 (1.366)

[1.366]

1.413 (1.413)

[1.413]

1.809 (1.809)

[1.809]

1.374 (1.374)

[1.374]

26.485 0.616 0.602 6.358 2,294.5 8.77

SPT-

1

1.671 (1.671)

[1.668]

1.421 (1.422)

[1.423]

1.370 (1.370)

[1.369]

1.866 (1.866)

[1.866]

1.499 (1.499)

[1.509]

4.681 1.391 1.072 12.670 2,052.2 4.18

SPT-

2

1.658 (1.658)

[1.652]

1.423 (1.423)

[1.427]

1.363 (1.363)

[1.360]

1.881 (1.881)

[1.885]

1.470 (1.470)

[1.470]

11.536 0.756 0.709 5.538 2,420.0 5.98

SPT-

3

1.660 (1.660)

[1.653]

1.424 (1.424)

[1.428]

1.360 (1.360)

[1.357]

1.875 (1.875)

[1.880]

1.473 (1.47)

[1.472]

27.072 0.615 0.601 4.754 2,402.4 6.78

SPT-

4

1.661 (1.661)

[1.655]

1.435 (1.435)

[1.439]

1.357 (1.357)

[1.355]

1.871 (1.871)

[1.874]

1.473 (1.473)

[1.473]

12.894 0.729 0.690 4.993 2,385.1 6.36

SPT-

5

1.664 (1.644)

[1.659]

1.422 (1.422)

[1.424]

1.365 (1.365)

[1.363]

1.876 (1.876)

[1.877]

1.485 (1.485)

[1.488]

4.546 1.620 1.194 4.673 2,319.7 5.54

SPT-

6

1.656 (1.656)

[1.650]

1.424 (1.424)

[1.428]

1.360 (1.361)

[1.358]

1.890 (1.890)

[1.893]

1.469 (1.469)

[1.469]

10.435 0.800 0.743 4.653 2,421.5 7.30

SPT-

7

1.661 (1.661)

[1.654]

1.435 (1.435)

[1.439]

1.877 (1.357)

[1.354]

1.877 (1.877)

[1.881]

1.470 (1.470)

[1.470]

13.416 0.728 0.690 5.114 2,403.7 5.78

SPT-

8

1.660 (1.660)

[1.653]

1.424 (1.424)

[1.428]

1.359 (1.359)

[1.357]

1.880 (1.880)

[1.885]

1.470 (1.470)

[1.470]

25.867 0.622 0.607 4.550 2,410.5 7.83

ST-2 1.627 (1.627)

[1.625]

1.493 (1.493)

[1.494]

1.497 (1.497)

[1.498]

1.772 (1.772)

[1.771]

– 6.786 0.961 0.865 – – 4.13

ST-3 1.629 (1.629)

[1.628]

1.493 (1.493)

[1.493]

1.499 (1.499)

[1.500]

1.769 (1.769)

[1.767]

– 7.896 0.878 0.813 – – 4.06

ST-4 1.625 (1.625)

[1.625]

1.505 (1.505)

[1.505]

1.498 (1.498)

[1.498]

1.770 (1.768)

[1.769]

– 10.517 0.735 0.709 – – 3.63

Rotational constants (GHz), chemical shift of proton (d, ppm), frequencies (cm-1), and dipole moment (Debye) are included for MCPS

conformers (values in parentheses refer to calculation at the IEF–PCM method and values in brackets refer to calculation at the SCI–PCM

method), X = Se or S atoms

Struct Chem

123

The C(3)S(2)H(1) bond angle is compressed from an equi-

librium value of 95.28�–90.66�, and 4.85 % decrease. The

S(2)–H(1) bond is then stretched from 1.392 to 2.053 A´

, an

increase of 32.20 % to reach the transition state. Also it can

be seen that, in this reaction process, the Se(6)���H(1) bond

length gradually decreases while S(2)–H(1) bond length

increases. In a word, the structural changes reflect the

process of the formation of a new bond (Se(6)���H(1)) and the

rupture of an old bond (S(2)–H(1)). This means H(1) atom

transfers from S(2) atom to Se(6) atom. For third model

(Fig. 4c), the C(3)S(2)H(1) bond angle is compressed from

an equilibrium value of 95.92�–91.58�, a 4.52 % decrease.

The S(2)–H(1) bond is stretched from 1.383 to 1.663 A´

, an

increase of 16.84 % to reach the transition state. Also it can

be seen that the Se(6)���H(1) bond length gradually decrea-

ses, from 2.249 to 1.722 A´

, an increase of 23.43 %, while

S(2)–H(1) bond length increases.

Consideration hydrogen-bonded systems in the first

excited state

The geometric structures of SPT-1 conformer and its tau-

tomer (MPS-1 conformer) in the first singlet excited state,

S1, have been optimized using TD-DFT method. One can

find that Se–H���S and S–H���Se intramolecular hydrogen

bonds can be formed in SPT-1 and its proton-transferred

tautomer (MPS-1), respectively. Geometric and energetic

data suggest that the H-bond is weakened slightly in the

p–p* state, compared to ground state (S0). Our theoretical

results showed that the distance between the S and Se atoms

Fig. 3 B3LYP/6-311??G** calculated relative energy values ver-

sus intrinsic reaction coordinates for compound MPS-1 ? SPT-1,

using mass-weighted internal coordinates

Fig. 4 Complexes of MPS-1

and SPT-1 conformers with

water molecules, interaction of

water molecule with thiol site

(a), selenal site (b), and both

sites (c)

Struct Chem

123

in the S1 is longer than corresponding value in S0. We

observe that for SPT-1 that the distance between H and S

atoms in intramolecular hydrogen bonding Se–H���S=C is

significantly lengthened from 1.524 A in the ground state to

2.868 A in the excited state. The result testifies that the

intramolecular hydrogen bond Se–H���S=C is significantly

weakened upon excitation to the S1 state. Furthermore, we

can find that the distance between atoms H and Se in

intramolecular hydrogen bonding S–H���Se=C is signifi-

cantly lengthened from 2.139 A in the ground state to

2.426 A in the excited state. Meanwhile, the bond lengths of

both groups Se=C and S–H in the excited state are slightly

increased in comparison to those in the ground state. The

result testifies that the intramolecular hydrogen bond

S–H���Se=C is significantly weakened upon excitation to the

S1 state. Moreover, it is noted that in the S1 state the

hydrogen bond length of Se–H���S=C is longer than that of

S–H���Se=C. This indicates that the hydrogen bond in the S1

state of MPS-1 is more significantly strengthened than in

SPT-1.

Moreover, the electronic excitation energies as well as the

corresponding oscillator strengths of the hydrogen-bonded

systems for the S0 ? S1 transition are calculated using the

TD–DFT method and listed in Table 3. Herein, we can find

that the electronic excitation energy of the S1 state of SPT-1

conformer is 1.576 eV, while that of the MPS-1 conformer is

1.588 eV. The electronic excitation energy of S1 state in

SPT-1 conformer is reduced compared with that of the MPS-

1 conformer. As it is obvious from Table 3, the oscillator

strength of the MPS-1 conformer in S1 state is 0.0001, which

is close to that of the SPT-1 (0.0002) in the same state.

HOMO–LUMO analysis

Highest occupied molecular orbital (HOMO) and lowest

unoccupied molecular orbital (LUMO) are very important

parameters for chemical reaction [48]. The HOMO repre-

sents the stability to donate an electron, LUMO as an elec-

tron acceptor represents the ability to obtain an electron. The

energy-gap between HOMO and LUMO is a critical

parameter in determining molecular electrical transport

properties [49]. The energy gap between HOMO and LUMO

explains the biological activity [50] of the molecule, which is

due to the change in partial charge and to the change in total

dipole moment [51]. The plots of HOMOs and LUMOs are

shown in Fig. 5. The energy values of HOMO are computed

-6.35 and -6.11 and LUMO are -3.11 and -3.08 eV, and

the energy gap values are 3.25 and 3.03 eV in the gas phase

for SPT-1 and MPS-1 conformers, respectively. Moreover,

these orbital significantly overlap in MPS-1 and lower

energy gap explains the eventual charge transfer interactions

taking place within the molecule. For these conformers, the

HOMO and LUMO have p and p* characters, respectively.

The HOMO of MPS-1 conformer shows antibonding char-

acter at S–C and Se–C bonds. There is no electronic pro-

jection over the C–H group.

Atoms in molecules analysis

The AIM analysis was used to determine the presence of

bond critical points (BCPs) of the intramolecular bonds

XH���Y and to evaluate their energies. The most often used

criteria of the existence of hydrogen bonding interactions are

Fig. 5 HOMO and LUMO

compositions of the frontier

molecular orbital for MCPS

Struct Chem

123

the electron density q(rc) and the Laplacian of the electron

density r2q(rc) at the BCPs. These parameters for the

intramolecular XH���Y along with the lengths and angles of

the corresponding hydrogen bonds in the studied molecules

are given in Table 3. The calculated electron density prop-

erties of MPS-1 and SPT-1 conformers demonstrate that

Y���H bonding has low q and positive r2q values but the

corresponding HBCP values are negative, which means the

interaction is at least partly covalent. Comparison between

the electron density and the energy density of MPS-1 and

SPT-1 shows that these values for the MPS-1 are slightly

greater than the corresponding values of SPT-1 in the gas

phase and water solution. In SPT-1 the hydrogen atom

involved in the intramolecular HB has a quite small positive

charge, while the same hydrogen in MPS-1 species exhibits a

substantially higher positive charge. Consequently, the HB

in MPS-1 is stronger than the SPT-1. For the systems ana-

lyzed here the pseudo-ring containing Se���H–S and S���H–

Se intramolecular hydrogen bond is created and hence also

the RCP exists. The characteristics of RCPs of the systems

analyzed here are given in Table 3. It is known that the

greater electron density at RCP corresponds to the stronger

intramolecular hydrogen bonding.

There is, however, another factor which enhances the

stability of species MPS-1, associated with a typical reso-

nance-assisted hydrogen-bonding (RAHB) mechanism [52].

The values of the charge densities at the bcp’s reveal that the

existence of an intramolecular HB in MPS-1 favors a sig-

nificant delocalization of charge within the cyclic structure.

A much smaller charge delocalization takes place in the case

of SPT-1 form. This is easily understood by looking at the

evolution of the charge density on going from the selenol

SPT-1 to the thiol MPS-1 through the transition state (TS1/

1). In species SPT-1, due to the large electron affinity of the

sulfur atom and the fact that the thio group is difficult to

perturb, a quite localized structure with alternate double and

single bonds is strongly favored. When the hydrogen atom

moves closer to the sulfur, there is a significant charge

transfer from the latter to the former, to finally form S–H

bond. This charge transfer enhances the electronegativity of

the sulfur atom, which withdraws charge from the CS link-

age. This results in a polarization of the thione carbon, which

is transmitted along the C–C–C chain of bonds and favored

by the fact that Se is only somewhat electronegative and

highly polarizable. The result is a significant charge delo-

calization in the CCCSe moiety, which enhances the stability

of the MPS-1 form. This also explains why the gap between

SPT-1 and the open-chain structure SPT-7 is higher than that

found between species MPS-1 and MPS-7.

NBO analysis

The natural bond analyses (NBO) [37] were applied for the

evaluation of the hydrogen bond strength in MPS-1 and SPT-

1 conformers. Table 3 shows the NBO occupation numbers

for r�ðS�HÞ and r�ðSe�HÞ antibonds, the sulfur and selenium

lone pair electrons (nS and nSe, respectively), and their

second order perturbation stabilization energies, E(2). In the

NBO analysis of hydrogen bond system, the charge transfer

between the lone pairs of proton-acceptor and anti-bonds of

the proton-donor is the most important. The results of NBO

analysis showed that in the chelated structures of the MCPS

conformers, two lone pair electrons of sulfur (or selenium)

atoms participate as donor and r�ðS�HÞ or r�ðSe�HÞ antibonds as

acceptors. The comparison between the NBO analysis of

MPS-1 and SPT-1 conformers shows that values of second

order perturbation energy E(2) for orbital interaction

ðn2Se ! r�S�HÞ in MPS-1 is higher than the n2Se ! r�S�H

value in SPT-1. Hence, the strength of hydrogen bond in

MPS-1 is greater than the SPT-1. Figure 6 displays the two-

dimensional contour plots of the interaction between the

electron lone pair of sulfur and selenium (nS and nSe,

respectively) with antibonding Se–H or S–H orbitals in

MPS-1 and SPT-1 conformers, respectively.

MPS-1 SPT-1

Fig. 6 NBO contour plots

illustrating the interaction

between the electron lone pair

of sulfur and selenium atoms

with an antibonding Se–H and

S–H orbitals in SPT-1 and MPS-

1 conformers, respectively

Struct Chem

123

Water solution

Different methods exist for quantum mechanical calcula-

tions of solvent effects, depending on models for the cav-

ity. For example, the polarizable continuum model (PCM)

model uses a sphere of radius 1.2 times the van der Waal’s

radius around each atom of the molecule, and puts charges

on the surface resulting from the intersecting spheres to

simulate the external field of the solvent. The SCI–PCM

model carries out the calculation in a self-consistent fash-

ion, using the electron density of the solute itself (in any

one iteration) to determine both the shape of the cavity and

the external field potential. An integral equation formula-

tion (IEF–PCM) has been used to obtain good results for

aqueous ionic solutions, including solvation energies for

neutral molecules as well as ions. In this work, the Ropt and

a parameters for HOMA index were calculated (in water

solution) at the B3LYP/6-311??G** level of theory (for

CC, CS, and CSe bonds: Ropt, CC = 1.396 A, Ropt,

CS = 1.689 A, Ropt, CSe = 1.832 A, aCC = 87.54,

aCS = 74.57, and aCSe = 74.41). The geometries of the

studied systems do not change appreciably when solvent

effects are taken into account using exclusively a contin-

uum model (see Tables 2, 4). However, the relative sta-

bilities of the MCPS conformers change significantly when

the solvent effect was applied. The agreement between

PCM and IEF–PCM optimized values is fairly good. The

most stable conformer in solution is predicted to be MPS-7

conformer (see Table 1). It is noteworthy to mention that

the hydrogen bond strength in MPS-1 and SPT-1 con-

formers in solution is weaker than the gas phase (see

Table 3). The hydrogen bond energy value EHB* for the

Se���H–S bridge in MPS-1 reduces to -31.45 kJ mol-1 in

water solution (in gas phase is -46.37) whereas EHB* for

S���H–Se bridge in SPT-1 conformer in water solution is

-31.69 kJ mol-1, which increases to -45.87 in gas phase.

It is noteworthy to mention that the S���H and Se���H dis-

tances show the hydrogen bond strength in SPT-1 and

MPS-1 conformers in water solution are weaker than the

gas phase which is in agree with the calculated hydrogen

bond strength. Our theoretical calculations confirmed that

MCPS conformers in water solution are more stable than

the gas phase. Furthermore, we can say that in going from

the gas phase to the solvent phase, the dipole moment value

increases (Tables 2, 4). As shown in Table 1, solute–sol-

vent interactions affect significantly the relative stabilities

of the cyclic hydrogen-bonded SPT-1 and MPS-1species.

Furthermore, the open chain MPS-7 and MPS-8 conform-

ers become sizably stabilized. The enhanced stability of

MPS-7 and MPS-8 forms with respect to MPS-1 has a

double origin; on one hand, the open-chain species have a

larger dipole moment (3.5 and 4.6 D, respectively, at the

B3LYP/6-311??G** level) than the cyclic one (2.5 D),

and on the other hand they interact in a more efficient way

with the solvent.

Ionization potential

The ionization potential and chemical hardness of the

molecule were calculated using Koopman’s theorem [53]

and are given by

g ¼ ðIP� EAÞ2

where IP & -E(HOMO), EA & -E(LUMO); IP is

ionization potential (eV), EA is electron affinity (eV).

g ¼ ELUMO � EHOMOð Þ2

The ionization potential calculated for MPS-1 (1.52 eV)

has lower potential than that of SPT-1 (1.62 eV). Consid-

ering the chemical hardness, large HOMO–LUMO gap

means a hard molecule and small HOMO–LUMO gap

means a soft molecule. One can also relate the stability of

the molecule to hardness, which means that the molecule

with least HOMO–LUMO gap is more reactive. The

HOMO–LUMO band gap of the MPS-1 conformer displays

the lowest energy (by *3.03 eV), and it can therefore be

considered softer than the SPT-1 conformer.

Conclusions

Theoretical calculations are applied to conformational

study of MCPS and harmonic vibrational frequencies also

calculated to confirm the nature of the stationary points

found and to discuss the ZPVE correction. The NBO and

AIM analyses were used to discuss the origin of confor-

mational preference and hydrogen bond strength. Further-

more, the electronic excited state properties of hydrogen-

bonded MCPS conformers were investigated by TD-DFT

method. Theoretical calculations show that in general the

MPS conformers of MCPS are about 3–30 kJ mol-1 more

stable than the corresponding SPT analogs. Our theoretical

calculation results revealed that the HB strength increases

from SPT-1 to MPS-1 (SPT \ MPS). The use of contin-

uum models indicates that both open-chain conformers

(MPS-7 and MPS-8) are significantly stabilized by solute–

solvent interactions, and they should predominate in

aqueous solution. The comparison between the stability

order and hydrogen-bond energies leads us to suggest that

the hydrogen bond is not the conquering factor in deter-

mining the preferred conformation. We concluded that the

hydrogen bond strength in MPS-1 and SPT-1 conformers

(Se���H–S and S���H–Se) in water solution is weaker than in

the gas phase. Our results also showed that ZPVE

Struct Chem

123

correction does not have any significant effect on stability

order of MCPS conformers.

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