Ab initio HF-SCF studies of the equilibrium structures and vibrational spectra of the Be (NO< sub> 3)< sub> 2, Mg (NO< sub> 3)

ELSEVIER

THEO CHEM

Journal of Molecular Structure (Theochem) 314 (1994) 247-260

Ab initio HF-SCF studies of the equilibrium structures and vibrational spectra of the Be(NO&, Mg(NO& and Ca(NO&

molecules

V. Rossi, C. Sadun, L. Bencivenni, R. Caminiti*

Dipartimento di Chirnica, Universitb degli Studi di Roma, P. le A. Moro 5, 00185 Roma. Italy

Received 3 February 1994; accepted 20 February 1994

Abstract

HF-SCF calculations have been performed for the Mg(NO&, Ca(NO& and Be(NO,), molecules to determine their stable equilibrium structures and vibrational spectra. Bidentate and monodentate structures have been taken into account and the calculations suggest that non-planar Dzd is the most stable equilibrium structure for all the species considered in the work. The infrared spectra of the DZd and D2h structures of Mg(NO& have been simulated on the basis of the HF/6-31G* frequency and intensity calculations, and the “0 isotope shifts have been predicted in the stretching region of the NO; modes. The conclusions of the calculations have been used to assign the FT-IR spectra of molecular Cu(NO&. The paper includes calculations for the [BeNOj]+H20, [MgNOJ+H20, [CaN03]+H20, [A1N03]*+H20, [BeN02]+H20, [MgN02]+H20, [CaN02]+H20, and [A1N02]*+H20 hydrated species.

1. Introduction

Our research group has been quite active in the

structural [ 1,2] and spectroscopic [3-51 investigation of inorganic molecules having appreciable ionic character due to the interaction of a metal

cation with an inorganic anionic group. More recently, this topic has been developed by means of HF-SCF and MP2 calculations [6-91. So far, much attention has been devoted to the associa-

tion of group la metal cations M+ with NO, and PO;. It has been definitively established that the [M+][NO;] and [Mf] [PO;] ion couples prefer bidentate coordination structures in the gas phase. A valuable contribution to the study of the

*Corresponding author.

coordination models of these ion couples is provided by the theoretical investigation on the interaction of [L?] and [Na+] with [NOT] and

[PO,] [7,10]. In fact, ab initio calculations have a great capability for describing the geometrical, energetic and spectroscopic features of these mol-

ecules. It has to be borne in mind that it is always desirable for a better understanding of the chemical problem to link theory with spectroscopy because unequivocal conclusions often cannot be reached

from experiment alone. Computational chemistry, therefore, plays an important role in clarifying experimental results and predicting helpful infor- mation for further investigations by means of more sophisticated experimental techniques. This is, for instance, the case with modern FT-IR spectroscopy coupled with low-temperature matrix isolation

0166-1280/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0166-1280(94)03741-3

248 V. Rossi et al./J. Mol. Strut. (Theochem) 314 (1994) 247-260

spectroscopy which is particularly suitable for the investigation of the vibrational pattern of ion couples and their ‘80-isotope spectra.

In this paper we present, as a development of our former ab initio studies, novel results of a theoretical investigation on the interaction of the metal cations Be*+, Mg*+ and Ca2+ with NOi. A further aim of this investigation is the assignment of the FT-IR matrix isolation spectrum of the stable molecular species Cu(NO&; the theoretical predictions obtained for the probe system Mg(N03)* have been employed for this purpose.

2. Computational methods

The calculations were accomplished by the IBM version of the GAUSSIAN 90 package [ 1 l] on an IBM- 3090 computer of the University of Rome. The calculations were performed at the HF-SCF level of theory employing the split-valence basis sets 6-31G and 6-31G* for studying the Mg(NO& and Be(N03)2 systems. The calculations for the Ca(N03)2 molecule are reported at the HF/ STO-3G* and HF-SCF level with the 4-31G* basis set for the N and 0 atoms, and the 14~9~ contracted to 8s4p basis set for Ca [12]. The 6-31G, 4-31G*, 6-31G* and STO-3G* basis sets are stored in the GAUSSIAN 90 program. The vibrational frequencies were computed analytically at each optimized HF-SCF structure. Single-point energy calculations have been carried out on the optimized lowest energy structures of Mg(N03)2 and Ca(N03)2 at the HF-SCF level employing the DZP basis [12] for the nitrate group and the 6-31G* basis for the Be and Mg atoms, and the 14~9~ contracted to 8s4p basis for the Ca atom [12].

3. Experimental methods

The FT-IR spectra were measured both in a solid argon matrix at approx. 10 K and in very dilute CC& solution at approx. 300K in the frequency range 2000-400 cm-’ by using a Bruker IFS-1 13V interferometer maintained under mechanical pump vacuum. A variable optical path liquid cell equipped with CsI windows was used for recording the Fl-IR

solution spectra. Measured frequencies are accurate within *l cm-’ and the highest resolution of the FT-IR spectra is 1 cm-’ . The FT-IR matrix spectra were measured after depositing molecular CU(NO~)~ and argon (Matheson, 99.998%) onto the optical surface of a reflecting copper block frozen up to approx. 10K by means of a He closed-cycle cryo- stat (Displex CSA). A commercial sample of Cu(N03)* (Aldrich, 99.99+%) was heated under a suitable high vacuum device inside a platinum evaporator at approx. 385 K.

4. Results and discussion

4.1. The M(NO,), molecules

The theoretical study was accomplished for the Be, Mg and Ca nitrates, M(N03)*, owing to the similarity of these molecules with CU(NO~)~ which is not at present suitable for a reliable ab initio investigation. Most of the computational effort was addressed to Mg(N03)2 to achieve a general picture for the structural models and vibrational spectra expected for the M(N03)2 molecules. We have therefore optimized at the HF-6-3 1 G and HF-6-31G* levels the structures shown in Fig. 1; these structures are the bidentate models la and lb and the monodentate models lc, Id and le. Among these models, the structures la, lc and le were considered for the CU(NO~)~ molecule in earlier electron diffraction studies [13,14], with model la being preferred, while the non-planar structure lb was suggested from a later electron diffraction investigation [ 151 linked with molecular orbital calculations.

The main results obtained from our computa- tions indicate that bidentate binding is preferred over the other coordinations and, moreover, the non-planar bidentate model of DZd symmetry shown in Fig. l(b) is the lowest energy configura- tion of the Mg(N03)*, Ca(N03)* and Be(N03)2 molecules. The planar monodentate model lc (D2h symmetry), the non-planar monodentate model Id (D2,, symmetry) and the staggered model le (DJd symmetry) are unstable stationary points and do not correspond to any minimum of the total SCF energy. Thus, according to the HF-SCF

V. Rossi et al./J. Mol. Struct. (Theochem) 314 (1994) 247-260 249

a

d -

e

Fig. 1. Mg(N03)2 molecule. Bidentate structures: (a) I&h symmetry and (b) DZd symmetry; monodentate structures: (c) Dzh

symmetry structure and (d) DZd symmetry; (e) staggered D3d

symmetry.

single-point energy calculations carried out on the optimized HF-SCF geometries, the lowest energy coordination models are la and lb which are approx. 9 kJ mol-’ more stable than the monodentate models lc and Id, while the Dsd symmetry model le is the highest energy structure. In addition, the HF/6-3 lG*

vibrational frequencies, calculated in the harmonic approximation, indicate that the DM symmetry bidentate structure lb is the absolute minimum and that the Dz~ symmetry bidentate structure la is prob- ably the transition state connecting two equivalent D2d symmetry bidentate structures. Regarding the remaining high-energy structures, the results of the HF/6-3 1 G* harmonic frequencies indicate that all the monodentate models are high-order saddle points. The subsequent investigation on Ca and Be nitrates has been restricted to the bidentate coordination models of DZd and DZh symmetry and also in this case we have reached the same conclusions as for Mg(NO&. In fact, the DZd bidentate structure is the lowest energy minimum and the DZh bidentate structure is the transition state, being a first-order saddle point with one calculated imaginary A, mode. The HF-SCF single-point energy difference separating the transition state from the minimum is estimated to be approx. 4kJ mall’ for Ca(NO& and approx. 53 kJ mall’ for Be(NO&. These data, although they do not take into account electron correla- tion, indicate that such a barrier decreases remark- ably on going from Be to Mg and Ca nitrate and that tunnelling from two equivalent DZd minima through the DZh first-order saddle point is not hindered for the heavier metal nitrates. There is obviously consistency between the SCF barrier height and the value of the imaginary A, mode frequency of the Dzh bidentate structures, that is 131 icm-’ for Be(NO&, 61 icm-’ for Mg(NO& and 54 i cm-’ for Ca(NO&. A summary of the calculations is given in Table 1.

Let us now examine the most interesting structural differences between the group 2a metal nitrates and the group la nitrates of Li and Na, previously studied at the HF/6-31G* and MP2/6- 3 1 G* levels [ 10,161. It is known that the interaction of the metal cation with NO, induces a perturbation of the anion and causes the lowering of the symmetry from Djh, that of free NOj, to the local C,, symmetry of the bidentate [M2’][NOf] moiety and it causes a general rearrangement of the geometry of the anion. The HF-SCF level predicts a difference of about 0.1 A between the ring and terminal bonds of coordinated NO; for the Be, Mg and Ca nitrates and also a large devia-

250 V. Rossi et al./J. Mol. Srruct. (Theochem) 314 (1994) 247-260

Table 1.

Equilibrium geometriesa and unscaled vibrational frequencies

of the LIZ,, and & symmetry bidentate structures of Be(NO&

Mg(NO& and Ca(NO&; bond lengths in A, angles in degrees,

and frequencies in cm’

DSd Geometries

Be(NO& Mg(NO& Ca(NO&

NO 1.155 1.163 1.171

NO’. 1.268 1.264 1.252

MeO’ 1.648 2.007 2.336

O’NO’ 109.1 113.5 114.8

ONO’ 125.4 123.3 122.6

HF-SCF vibrational frequencies Description

At 2010 1947 1900 NO str. sym.

1178 1175 1207 NO’ str. sym.

930 875 853 NO; def. sym.

306 254 229 MeO’ str. sym.

B2 1996 1933 1889 NO str. asym.

1178 1171 1204 NO’ str. asym.

1035 881 851 NO; def. asym.

908 598 440 MeO’ str. asym.

BI 135 51 35 Torsion

E 1499 1475 1530 NO’ str. asym.

930 942 976 NO; def. asym.

770 788 805 NO’, def. asym.

560 360 295 MeO’ str. asym.

280 187 134 o.p. def.

92 64 43 o.p. def.

Dth Geometries

Be(NO3)2 Mg(NO& Ca(NOA

NO 1.157 1.164 1.171

NO’ 1.265 1.263 1.251

MeO’ 1.685 2.013 2.339

O’NO’ 108.8 113.3 114.7

ONO’ 125.6 123.4 122.6s


As 2004 1945 1899 NO str. sym.

1196 1178 1208 NO’ str. sym.

931 877 853 NO; def. sym.

318 255 229 MeO’ str. sym.

B3, 1469 1460 1524 NO’ str. sym

750 784 803 NO; def. sym.

208 283 264 MeO’ str. sym.

Bk 933 942 975 NO; def. sym.

183 149 119 def. sym.

Table 1. (Contd).


1991 1931 1887

1188 1173 1205

934 881 851

910 591 436

1549 1498 1539

771 789 805

454 393 312

180 86 54

942 944 977

504 225 149

88 59 39

131 i 61i 34i

NO str. asym.

NO’ str. asym.

NO: def. asym.

MeO’ str. asym.

NO’ str. asym.

NO: def. asym.

MeO’ str. asym.

Def. asym.

NO; def. asym.

def. asym.

def. asym.

a 0’ indicates the oxygen atom of the cyclic frame of the mol-

ecule.

tion of the ring angle of the anion, decreasing from 120” by about 5” in the Mg and Ca nitrates and 11” in Be(N03)2. Therefore the effect of ion-pairing on NOj is more marked in Be(NO& than in Ca(N03)2 and Mg(NOs)*. The calculations also suggest that the consequences of ion-pairing exerted by Mg and Ca atoms are nearly of the same extent as those determined for the LiNOs and NaNOs molecules [ 10,161, while the interaction of the Be atom induces a huge modification of the geometry of the anion. The extent of such effects is consistent with the M . . . N bond length, which increases passing from the Be to the Ca atom according to the ionic polarizability following the order Ca2+ > Mg*+ > Be*’ [ 171.

The conclusions reported for the Be, Mg and Ca nitrates might be useful for reinvestigating the IR spectrum of CU(NO~)~, a typical high-temperature molecule having noticeable stability in the gas phase. This molecule vaporizes as a monomer without appreciable decomposition [ 181 and therefore is suitable for matrix-isolation studies. The FT-IR spectrum of gaseous CU(NO~)~ condensed onto an argon matrix at approx. 10 K and that of a highly diluted solution of Cu(NO& in CC14 closely mirror each other. In particular, the matrix spectrum shows sharp bands at 16155cm-‘, 1204.7cm-‘, 975.6cm-’ and 785.4cm-’ and these IR-active vibrations of coordinated NO; compare well with those reported in an earlier work on

V. Rossi et a/./J. Mol. Struct. (Theochem) 314 (1994) 247-260

3 1000 6 E

‘i S5 3) c I

600 a, t;

251

3000 50 -1

cm

Fig. 2. HF-SCF infrared spectra of Mg(N03)2 calculated for the bidentate DZh and bidentate D,, symmetry structures.

Cu(NO& [19]. It is interesting to apply the theoretical predictions to the FT-IR spectrum of CU(NO~)~ in the light of the available IR measure- ments and of the electron diffraction studies [13-151. First of all, it is necessary to scale the HF/ 6-31G* frequencies for improving the agreement with the observed ones and this procedure is particularly easy in this case because the scaling factors for the SCF force constants of the nitrate anion vibrations are known from previous work [7,10].

The fact that the FT-IR spectrum shows three bands in the stretching region of the NO, anion is a typical feature of both the Dzh and DZd structures because the IR-active stretching vibrations of coordinated NOT are the two Bt, and the BZu modes, in the case of DZh symmetry, and the B2 + E modes, in the case of DZd symmetry. However, the calculated relative intensity IR patterns are different for the two symmetries (see Fig. 2). The IR pattern of the DZh symmetry is 10:6: 1 for the B,, (1617cmP1), B2,, (1258cm-‘) and Bt, (965cm-‘) bands, and that of the Dzd symmetry is 10 : 3 : 2 for the B2 (1621cm-‘), E (1229cm-‘) and B2 (976cm-‘) bands. It is therefore clear that the FT-IR spectrum of Cu(NO& fits the IR features of Dzh symmetry better than those of DZd symmetry, since the experi- mentally measured relative intensity pattern of the 1616, 1205 and 976 cm-’ bands is 10 : 5 : 4. Our conclusions would not support the most recent electron

diffraction study [ 151 according to which the NO, groups of the molecule do not lie in the same plane and form an average dihedral angle of 84.5” [15]. Further support of the planarity of the molecules comes from single-point energy calculations at the ROHF-SCF level employing the 14s9p6d basis, contracted to 8s4p3d, for the Cu atom and 4-31G* basis sets for N and 0 atoms, carried out for the planar (D,, symmetry, 2B3, electronic ground state) and the non-planar (D2,j symmetry, 2B2 electronic ground state) structures of CU(NO~)~ based on the electron diffraction geometry data. This calculation in fact predicts that the planar structure is 137 kJmol_’ more stable than the non-planar one.

From the theoretical point of view, it is important to consider the expected IR spectra of DZh and

D2d '*O isotopically enriched divalent metal nitrate. It has to be remarked that SCF IgO isotopic frequency shifts have been employed success- fully for this purpose to explain the spectroscopic features of random “0 labelling of group la metal nitrates [7,10], metaphosphates [lo], chlorates [20] and D2d-symmetry Li2S04 and C2,-symmetry LiC104 molecules [9]. The “0 isotope pattern of the nitrate group vibrations of a D2t, and D2d symmetry M(N03)2 molecule shows some characteris- tic differences in the region of the NO; stretching vibrations. The isotopic bands of the DZh structure

252

Table 2

V. Rossi et al./J. Mol. Struct. (Theochem) 314 (1994) 247-260

HF-SCF scaled frequencies (cm-‘) of the DZh and Dz~ models of the “0 isotopomers of Mg(N03)z

O.uygen atom numbering Ol-N(02)(03) Mg (04)(05)N-06

Isotopomers

1 016-N(016)(016) Mg (016)(016)N-016

D1 A, (1628) Bjg (1226) A, (965)

B,, (1617) BZu (1258) B,, (965)

2 016-N(016)(016) Mg (016)(018)N-016

CS A’ (1628) A’ (1616) A” (1254)

A” (1221) A’ (968) A’ (948)

3 016-N(016)(016) Mg (016)(016)N-018

C,, Al (1624) A, (1591) B2 (1258)

B2 (1226) A, (967) AI (955)

4 016-N(018)(016) Mg (016)(018)N-016

CS A’ (1628) A’ (1616) A’ (1249)

A’ (1218) A’ (950) A’ (946)

5 016-N(018)(016) Mg (016)(016)N-018

CS A’ (1623) A’ (1591) A’ (1254)

A’ (1221) A’ (955) A’ (948)

6 016-N(016)(018) Mg (016)(018)N-016

C,” A, (1628) A, (1616) Bz (1249)

Bz (1218) A, (950) AI (946)

7 016-N(016)(016) Mg (016)(018)N-018

C, A’ (1624) A’ (1590) A” (1254)

A” (1221) A’ (968) A’ (935)

8 016-N(016)(016) Mg (018)(018)N-016

CZ” A, (1628) B2 (1252) A, (968)

A, (1616) B2 (1215) A, (929)

9 018-N(016)(016) Mg (016)(016)N-018

&I, BI, (1586) B2u (1257) BI, (953) A, (1598) Bjg (1226) A, (956)

10 016-N(016)(016) Mg (018)(018)N-018

C,, A, (1624) Bz (1251) A, (968) A, (1589) B2 (1213) Al (916)

11 018-N(016)(016) Mg (018)(018)N-016

C,, A, (1622) B2 (1251) A, (955) A, (1591) B2 (1214) Al (929)

12 016-N(018)(016) Mg (018)(018)N-016

CS A’ (1627) A’ (1615) A’ (1245)

A’ (1218) A’ (948) A’ (929)

13 016-N(016)(018) Mg (018)(016)N-018

CS A’ (1623) A’ (1590) A’ (1248)

Dzd A, (1623) B2 (1611) E (1229)

A, (979) B2 (976)

G A’ (1623) A’ (1611) A’ (1229)

A” (1221) A’ (978) A’ (958)

C,V A, (1618) Bz (1229) A, (978)

B2 (1586) A, (1228) B2 (964)

C, A(1623) A(1611) B (1221)

B (1221) A (960) A (957)

CS A’ (1618) A’ (1586) A’ (1228)

A” (1221) A’ (965) A’ (958)

CS ,4’ (1618) A’ (1585) A’ (1229)

A” (1219) A’ (978) A’ (944)

C,, A, (1623) B2 (1229) A, (978)

A, (1611) B2 (1211) A, (939)

Dzd B2 (1580) E (1228) B2 (963)

A, (1592) A, (965)

C,, A, (1618) B2 (1229) A, (978)

A, (1584) Bz (1209) A, (925)

Cl” A, (1618) B2 (1228) A, (964)

A, (1589) Bz (1211) Al (939)

C, A’ (1622) A’ (1611) A’ (1221)

A” (1211) A’ (958) A’ (939)

c, (1618) (1585) (1221) A’ (1217) A’ (948) A’ (935) (1219) (958) (944)

Table 2. (Contd)


14 018-N(016)(016) Mg (016)(018)N-018

CS A’ (1598) A’ (1586) A’ (1253)

A’ (1221) A’ (955) A’ (935)

15 018%N(018)(016) Mg (016)(018)N-018

CS A’ (1597) A’ (1585) A’ (1248)

A’ (1216) A’ (937) A’ (933)

16 018-N(018)(016) Mg (018)(018)N-016

Cs A’ (1622) A’ (1590) A’ (1244)

A’ (1211) A’ (935) A’ (928)

17 018-N(018)(016) Mg (018)(016)N-016

CS A’ (1623) A’ (1590) A’ (1248)

A’ (1217) A’ (948) A’(935)

18 016-N(018)(018) Mg (018)(018)N-016

&h BI, (1615) Bzu (1258) B,, (928) A, (1627) B1, (1208) A, (931)

19 018-N(016)(016) Mg (018)(018)N-018

C,, A, (1598) Bz (1250) A, (955)

A, (1585) BZ (1213) A, (915)

20 018-N(018)(018) Mg (018)(016)N~016

CS A’ (1623) A’ (1589) A” (1244)

A” (1211) A’ (948) A’ (915)

21 018~N(016)(018) Mg (016)(018)N-018

Cl, A, (1597) B, (1248) A, (937)

A, (1585) B2 (1216) A, (933)

22 018%N(016)(018) Mg (018)(018)N-018

CS A’ (1597) A’ (1585) A’ (1244)

A’ (1210) A’ (935) A’ (915)

23 018-N(018)(018) Mg (018)(018)N-016

C,, A, (1622) B2 (1238) A, (929)

A, (1589) Bz (1207) A, (915)

24 018~N(018)@18) Mg (018)(018)N-018

Dzi, BI, (1585) B2u (1237) BI, (914) A, (1597) B,, (1206) %(917)

15N molecule”

D, h (1581) B2u (1229) BI, (964) A, (1593) B3, (1197) AZ (967)

c2 A (1592) A (1580)

B (1219) A (965)

B (1219)

A (943)

A’ (1617) A’ (1585)

A” (1211) A’ (945)

A’ (1219)

A’ (938)

Cl (1618) (1585)

(1219) (958)

(1221)

(944)

D2d B2 (1610) E (1211)

A, (1622) A, (941) B2 (938)

Cl” A, (1593) B2 (1228)

A, (1580) B, (1209) 4 (964) A, (925)

A’ (1618) A’ (1584)

A” (1209) A’ (958)

A’ (1221)

A’ (925)

A (1592) B (1219)

A (1580) B (1219)

A (946)

A (943)

A’ (1592) A’ (1580)

A” (1209) A’ (945)

A’ (1219)

A’ (925)

A, (1617) B2 (1211)

A, (1584) B2 (1209) c2, AI (939)

AI (925)

&d B2 (1580) E (1209)

A, (1592) A, (927) J32 (923)

Dzd B2 (1584) E (1210)

A, (1594) A, (965) B2 (961)

a Stretching vibrations of the anion.

are in fact due to the 2A,, 2Bi,, BZu and B3s modes, while those of the Dzd model are due to the 2Ai, 2B2 and E vibrations. Restricting the discussion to the IR-active vibrations of coordinated NOT, the main difference between the 180 pattern of the 2Bi,

and BZu stretching modes of the Dzh symmetry and that of the 2B2 and E stretching frequencies of the Dz~ symmetry arises from the breakdown of degen- eracy of the E modes occurring in the isotopomers generated by the DZd structure. The contribution of

254 V. Rossi et al./J. Mol. Struct. (Theochem) 314 (1994) 247-260

Table 3

Equilibrium geometries’ and unscaled vibrational frequencies of the [M(NO,)]‘, [M(NOJ)]+H~O, [M(NOl)]+ and [M(N02)]+HzO

species; bond lengths in A, angles in degrees and frequencies in cm-’

NO 1.130 1.147 1.160 1.118

(1.181 1.201 1.213 1.163)

NO’ 1.305 1.283 1.264 1.335

(1.347 1.323 1.3005 1.393)

MeO’ 1.529 1.934 2.265 1.753

(1.551 1.963 2.281 1.794)

O’NO’ 106.0 118.0s 113.6 107.0

(107.0 113..3 114.3 108.2)

ONO’ 127.0 123.9 123.2 126.5

(126.5 123.3 122.8 125.9)


Al 2149 2035 1950 2194

1120 1118 1174 983

1035 894 859 994

850 528 403 644

B2 1392

868

660

RI 881

318

1398 1481 1247

784 801 789

438 343 563

914 942 828

191 140 240

NO str.

NO’ str. asym.

NO’, scis.

O’Me str. sym.

NO’ str. asym.

NO; rock

O’Me str. asym.

NO; wag

Ring o.p. def.

Geometries

NO 1.137 1.151 1.163 1.122

NO’ 1.291 1.277, 1.262 1.321

MeO’ 1.566 1.953 2.284 1.770

O’NO’ 106.9 112.4 113.9 107.2

ONO’ 126.5, 123.8 123.0, 126.4

M OH2 1.568 1.982 2.383 1.791

OH 0.961 0.959 0.956 0.978

HOH 110.3 106.9 104.5 107.7


Al 3916 3957 3993

2112 2014 1935

1817 1840 1875 1097 1134 1180

931 891 855

1204 621 440 442 342 282

B2 3989

1434

877

1422 1492

657 560

3681 OH str. sym.

2183 NO str. sym.

1819 H20 bending 1026 NO’ str. sym. 1008 NO; scis.

807 M. OH2 str. sym.

470 O’Me str. sym.

3718 OH str. asym.

1311 NO’ str. asym.

850 M OH2 str. asym.

V. Rossi et al./J. Mol. Strut. (Theochem) 314 (1994) 247-260 255

Table 3. (Contd)


761 786

615 415

145 51

Bl 900

435

341

109

A2 219

923

484

189

52

109 52 140 Torsion

801

330

8

946

529

136

13

787

547

76

8.51

627

253

90

NO; rock

O’Me str. asym.

M OH2 i.p. bending

NO; wag

M OH* o.p. bending

Ring o.p. def.

o.p. def.

Geometries

Pe(NOdl’ bkWW+ KIWI’ WWd12+

NO’ I .253 1.237 1.234 1 .2435

Me0 1.546 1.955 2.274 1.817

O’NO’ 107.8 112.4 113.3 110.8


Al

B2

BI

1516

1133

868

1386

692

435

Geometries

PeWdlfH20

1531 1570 1416

1024 981 1079

536 427 590

1434 1498 1336

439 385 503

396 329 519

PkWWl+H20 [C4NWl+H20 [A1(N02)]2+H20

NO str. sym.

NO* scis.

OMe str. sym.

NO str. asym.

OMe str. asym.

Ring o.p. def.

NO’ 1.247 1.237 1.234 1.250

MeO’ 1.584 1.971 2.288 1.808

O’NO’ 107.8 112.4 113.4 109.2

M OH2 1.564 1.986 2.386 1.792

OH 0.962 0.959 0.956 0.979

HOH 110.3 106.9 104.6 107.6


Al 3903 3957 3994 3667

1816 1840 1874 1818

1554 1547 1577 1472

1049 1017 976 1105

1177 624 455 827

451 351 293 471

OH str. sym.

Hz0 bending

NO str. sym.

NOa scis.

M Hz0 str. sym.

OMe str. sym.

B2 3975 4025 4063 3703 OH str. asym.

1424 1446 1501 1356 NO str. asym.

817 653 557 843 M. Hz0 str. asym.

603 422 376 522 OMe str. asym.

159 56 4 83 M.. H20 i.p. bending

J% 495 487 523 636 M. OH2 o.p. bending

404 316 314 478 Ring o.p. def.


Table 3. (Contd)


157 64 6 108 o.p. def.

A2 227 106 70 140 Torsion

a 0’ indicates the oxygen atom of the cyclic frame of the molecule. The MP2 geometries are reported in parentheses. The data calculated for free Hz0 are: HF/6-31G* (OH = 0.947& HOH = 105.5”; A, 4069, 1826; Bz 4188cm-‘); HF/4-3lG* (OH = 0.948& HOH = 105.3”; A, 4053, 1830; Bz 4169cm-‘).

the Raman-active vibrations of the DZt, and Dzd structures to the respective IR spectra is evidently lacking because the dipole derivatives &/aQ asso- ciated with the vibrations of the C,, C,, and C, isotopomers are actually zero. The number of isotopic combinations is very high in both cases (see Table 2) and it is the same for the DZh and DZd symmetry structures. However, the isotopic patterns mainly differ from each other for the Ct symmetry isotopomers of the Dti model which produce a char- acteristic pattern due to the expected band intensity ratios and frequencies. This distinctive pattern arises from the partial or complete overlap of some stretching vibrations, as can be seen from Table 2.

This also simplifies the IR spectrum because bands of distinct isotopomers are predicted to lie in the same frequency region. This evidently occurs for a series of isotopic species of both models and ultimately makes possible the distinction between the two structures.

4.2. The [MeNOj]’ and [MeN03]‘H20 species

The equilibrium geometries and vibrational spectra of [MNOs]+ and [MN0s]+H20 species (M is Be, Mg, Ca) were computed to determine the stable minimum structures and the vibrational spectra for investigating the effects of the coordination of the anion and for comparing with those of the neutral nitrates. The results of these calculations are summarized in Table 3. The models considered for the optimizations of the [MNOs]+ species are shown in Fig. 3 and the structures taken into account for the hydrated species are displayed in Fig. 4.

The stable minimum structures of both the [MeNOs]+ and [MeNO$H20 species (see Figs. 3(a) and 4(a)) have planar and bidentate shapes,

the same as the Li and Na nitrates [7,10]. The HF/6-31G* calculations, restricted to the monodentate model of [MgNOs]‘, suggest that monodentate binding (see Fig. 3(b)) is approx. 164 kJ mol-’ , unstable with respect to the bidentate interaction. The monodentate model is, furthermore, a second- order saddle point with imaginary B2 and B, frequencies. The CsV symmetry model of [MeNO$ (see Fig. 3(c)) lies very high in energy at the SCF level and therefore any further discussion of it is useless.

It is possible to examine in detail the effect of ion- pairing in the structures and vibrational spectra of the bidentate [MeNOs]+ and [A1N0s12+ complexes. As it concerns the structural effects, such an inter-

a

0

+

Mg 0

0

b

C

Fig. 3. Structures of [MN03]+: (a) bidentate Cl, symmetry, (b) monodentate C,, symmetry and (c) C,, symmetry.


H

3 0 H

H

Ib 0

H

a

H

<

0

H

b

C

Fig. 4. Structures of [MN03]+H20: (a) bidentate planar C,, symmetry and(b) bidentate non-planar Cz, symmetry; (c) bidentate planar C,, symmetry, structure of [MNO:]H20.

action causes a large rearrangement of the geometry of the anion, as can be seen in the large difference between the lengths of the ring and terminal NOT bonds of NOT, indicated by AZ, and in the magni- tude of the deviation of the ring angle of the anion from 120” (the free anion bond angle), indicated by Aq5. These variations have the same sign and value at both the HF/6-31G* and MP2/6-31G* levels and therefore the discussion is based on the results of the highest computational level. Regarding the

bond distances, Al shows the highest values in [AlNOs]*+ (+0.23OA) and [BeNOs]+ (+O. 166 A), and decreases progressively to +O. 122 A in [MgNOs]+ and +O.O88A in [CaN03]+. In the same way, A4 follows the same trend, i.e. -12” and - 13” for [AlNOs]*+ and [BeNOs]+ respectively, and -7” and -6” for [MgNO$ and [CaNOs]+. Evidently, these effects are particularly enhanced with respect to those of LiNOs (Al = +0.070 A and A# = -4”) and of the neutral


M(NO& species. The vibrational effects expected upon ion-pairing are quite strong in the vibrational pattern of NO; owing to the splitting of the E’ stretching and bending modes and the IR activity of the A’, stretching mode, occurring when the Dst, symmetry of the anion lowers to CzV. These results, and the comparison with the vibrational features of bidentate LiNOs where NO, is weakly perturbed by Lif, are very helpful for this purpose. We have in fact observed that the splitting of the E’ stretching of NOJ increases as the perturbation of the metal cation M+ increases, being 360cmP’ for Li+, 469cmP’ for Ca*+, 637cmP’ for Mg*+, 757 cm-’ for Be*+ and 947 cm-’ for A13+. The IR intensity of the totally symmetric stretching mode of coordinated NOT, i.e. the Raman-active A’, vibration of free NOT, gains intensity appreciably as the distortion of the anion becomes larger. It is quite interesting to see that the relative intensity pattern of the three stretching vibrations of the C2, symmetry anion undergoes significant changes, depending on the extent of the perturbation induced by the cation. In fact, the relative intensity pattern of the AI/B2/AI stretching modes is 10/9/l for LiNOs, 10/9/2 for [CaNO$ and 10/S/ 3 for [MgN03]+ and presents intensity inversion between the A, and B2 modes for the remaining species, i.e. g/10/2 for [BeNOs]+ and 8/10/2 for [AlN0312+. As it concerns the bending vibrations of coordinated NO,, related to the E’ and A: modes of free NO: calculated at 797cm-’ and 984cmP’ respectively, these vibrations correlate the B2 (NO2 rocking) and Ai (NO2 scissoring), and the B1 (NO2 wagging) modes in the C2,-symmetry molecules. The bending frequencies follow the order B, > Ai > B2 in the case of bidentate LiN03 and the relative intensity pattern of the Bi and A, vibrations is 10/6/O, the intensity of the B2 component being negligible. The perturbation in the [CaN03]+ and [MgN03]’ species is manifested in the reinforcement of the intensity of the Ai component and the calculated pattern of the Bi and Ai vibrations is in fact 10/4 and 10/6 in the case of [CaNOs]+ and [MgNOs]+. The effects are predicted to be extremely large for the [BeN03]+ and [A1N0s12+ species where the perturbation is quite strong. Here, the three com- ponents follow the frequency order A, > Bi > B2 and the intensity distribution of the triplet is

thoroughly altered with respect to that of [MgN03]+ and [CaNOs]*+ and, moreover, the intensity of the B2 component is still weak but not completely negligible. Thus, the predicted intensity pattern is 10/2/l for [BeN03]+ and 10/2/ 0 for [A1N0312+. It has to be observed that the coupling between the totally symmetric bending mode of the anion and the stretching mode of the cation is stronger for the [BeN03]+ and [AlNOs]*+ species than for the others, and this determines the increase of the Al bending vibration of the anion. The extent of the splitting of the E’ bending mode is also related to the distortion of the anion. The separation of the Ai and B2 com- ponents of this bending vibration is, however, smaller than that occurring for the stretching modes, just 1 10cmP1 for [MgN03]+ and 58 cm-’ for [CaN03]+, where the perturbation due to Mg*+ and Ca2+ IS small. However, such a splitting reaches its highest extent when the distortion of the anion is large: 167cmP’ in [BeN03]+ and 205 cm-’ in [AlNOs]*+.

The final comment about the calculated frequencies of these charged species regards the B, out-of- plane ring deformation mode which has a large displacement of the cation with respect to the plane of the NO,. This low frequency is calculated at 182 cm-’ for the LiNOs molecule, and it is predicted to fall in the same range for the Mg (191 cm-‘) and Ca (140cmPr) species. However, this bending frequency would occur higher for the Al (240 cm-‘) and Be (318 cm-‘) complexes, in agreement with the strong interaction of the Be2+ and A13+ ions with NOT.

Let us now consider the hydrated species [MeN03]+H20 and [MeN02]+H20, where the H20 molecule interacts with the metal. For the [MeN03]+H20 complexes, two bidentate C2V- symmetry structures were considered. These are the models shown in Fig. 4(a) where Hz0 is placed in the plane of [MeN03]+; an alternative model is that where H20 is placed in the plane perpen- dicular to [MeN03]+. The minimum energy con- figuration of [MeN0s]+H20 corresponds to the C2” symmetry planar structure of Fig. 4(a), as also evidenced by frequency calculations. The C2, symmetry non-planar structure (see Fig. 4(b)) is, however, a first-order saddle point, due to the


100

0

. Wql’

q I~,l+Hzo

4000 50 cm -1

Fig. 5. HF-SCF infrared spectra of [BeN03]+ and [BeN03]+H20 calculated for the lowest energy minima structures

presence of one A2 imaginary frequency. At the SCF level, the two structures are separated by small energy differences ranging from 1 kJ mall’ for [CaN03]+H20 to 2 kJmol_’ for [MgN03]+H20, 4 kJmol_’ for [A1N03]2fH20, and 10 kJmol_’ for the [BeN0,]+H20 complex. The optimized geometries and vibrational frequencies of the hydrated complexes are summarized in Table 3. The typical structural feature obtained from the calculations for all the hydrated species is that the MO’ and M. . . OHz bond distances assume very close values, indicating in this way that the interactions of the metal cation with the anion and with Hz0 are comparable. The hydration is mainly responsible for frequency shifts of the totally symmetric stretching mode MO’ and the effects are particularly strong in the [BeNO$ and [A1N0312+ complexes (see Table 3). The frequency of this vibration strongly decreases from 850 to 442 cm-i on passing from [BeNOX]+ to [BeNO,]+H,O; this is the largest calculated red shift, the others being less appreciable. In fact, the corresponding red shifts of the other hydrated species are - 186cm-’ for [MgN03]+H20, - 174 cm’ for [AlN03]*+H20 and -121 cm-’ for [CaN0$H20. In contrast, the red shifts expected for the B2-type MeO’ stretching vibrations are small, ranging from -16cn-’ to -45 cm-‘. The calculations are useful for estimating

the M . . OH2 stretching vibrations occurring in these complexes. The values of this totally symmetric mode for [BeN0$H20, [MgN0s]+H20, [CaN03]‘H20 and [A1N03]2+H20 compare favourably with those of the simplest hydration models M”+ . . OH2 (X = 2, 3) which, as expected, have nearly identical MX+ . . . OH2 bond distances [21]. In fact, the calculated Ml+ . I . OH, A, stretching frequencies of [BeN03]+H20, [MgNO$H20, [CaN03]+H20 and [A1N03]2+H20 are 1204, 621, 440 and 807cm-‘, respectively, whereas the values of this mode are 1057cm-’ for the [Be2+]H20 complex, 562cm-’ for [Mg2+]H20, 399cmP’ for [CaZf]H20 and 790cm-’ for [Al+]H20. Fig. 5 shows the calculated IR spectra of [BeNOs]+ and [BeN03]+H20, revealing the most evident vibrational changes occurring upon hydration.

Finally, to complete the study of the interaction of divalent cations with NO;, it is worthwhile considering the ion pairs formed by M2+ and NOT, where M is Be, Mg, Ca, and that formed by A13+ and NO;. Also in this case, the formation of the hydrated complexes [MN02]+H20 (see Fig. 4(c)) exerts appreciable effects on the vibrational spectra of [BeN02]+ and [AlN02J2+, while effects are negligible for the other species. There are obviously close analogies between the vibrational and structural features of the hydrated [MeN02]+H20 and


[MN03]+H20 species and these may be readily observed by inspection of Table 3.

Acknowledgements

This work has been supported from the CASPUR project through a CPU time grant. The authors thank Dr. Fabio Ramondo for valuable discussions.

References

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121

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[41

[51

161

[71

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Documents

Ab initio HF-SCF studies of the equilibrium structures and vibrational spectra of the Be (NO< sub> 3)< sub> 2, Mg (NO< sub> 3)