Ab initio studies of solvent effect on conformational equilibria and vibrational spectra of dipropionamide

Journal of Molecular Structure (Theochem) 589–590 (2002) 171–181 www.elsevier.com/locate/theochem

Ab initio studies of solvent effect on conformational equilibria and vibrational spectra of dipropionamide G. Nandini, D.N. Sathyanarayana* Department of Inorganic and Physical chemistry, Indian Institute of Science, Bangalore-560012, India Received 8 February 2002; accepted 5 April 2002

Abstract Systematic ab initio molecular orbital studies of the conformational equilibria and vibrational spectra of dipropionamide using the basis sets 6-31g(d) and 6-31þþG(d,p) have been carried out. The vibrational spectra of dipropionamide have been satisfactorily interpreted taking into account the agreement between the calculated frequencies, infrared and Raman band intensities and the shifts in the spectra of deuterated molecules with those observed. The previous assignments of most of the vibrational bands are well confirmed, a few bands need reassignment, however. The solvent effects were investigated by selfconsistent reaction field theory using dipole and self-consistent isodensity polarized continuum model methods. The introduction of a dielectric medium has only a marginal effect on the conformational equilibria and vibrational spectra. However, the calculated changes in geometry and vibrational spectra on going from the gas phase to the solution phase are in accord with the increasing weight of the dipolar resonance structure in polar solvents. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Dipropionamide; Conformation; Ab initio studies; Vibrational spectra; Solvent effect

1. Introduction Imides are analogous to dipeptides in possessing the – CONHCO – grouping and the imide group is an important structural element of many biological systems. They provide key determinant to the protein conformation. Imides can be found in purine bases, uracil and its derivatives, hydantoins, etc. Some imides are widely used in technology and pharmacology [1]. Imides are important in molecular recognition through hydrogen bonding in natural * Corresponding author. Tel.: þ91-80-309-2827; fax: þ 91-80360-1552. E-mail address: [email protected] (D.N. Sathyanarayana), [email protected] (D.N. Sathyanarayana).

systems. Consequently, it is desirable to understand the energetic and structural details of such interactions. Diformimide, diacetamide and dipropionamide form the three lower members of acyclic aliphatic imides. Imides exhibit rotational isomerism although barriers to configurational changes in these molecules have received much less attention. As the bulkiness of the alkyl groups increase, they show tendency towards cyclization due to steric hindrance. They exhibit intraand intermolecular hydrogen bonding particularly in the solid state. The imide skeleton is essentially planar because of delocalization. The aliphatic imides RCONXCOR (diacetylamines) may exist in three stable conformations, which differ in the location of the CO groups in relation to the NX bond. The three

0166-1280/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 1 2 8 0 ( 0 2 ) 0 0 2 6 1 - 0

172

G. Nandini, D.N. Sathyanarayana / Journal of Molecular Structure (Theochem) 589–590 (2002) 171–181

Fig. 1. Conformers of dipropionamide (all the conformers mentioned in the text are not shown).

G. Nandini, D.N. Sathyanarayana / Journal of Molecular Structure (Theochem) 589–590 (2002) 171–181

possible conformations for the lower members of symmetrical imides are shown in Fig. 1. Among them, only diacetamide has generated much attention concerning its molecular geometry and vibrational spectroscopy [2], while dipropionamide has received scanty attention. The infrared and Raman spectra of dipropionamide have been studied for the trans –trans conformer by Kuroda et al. [3]. They have made the assignments by referring to infrared dichroism, isotopic shifts in deuterated molecules and classical normal coordinate analysis using the Urey –Bradley force field. The proton NMR studies by Noe and Raban [4] have shown that the cis– trans conformer predominates in solution. It was supported from a study of the NH stretching frequencies at different temperatures in non-polar solvents such as carbon tetrachloride and benzene [5]. The infrared spectra, dipole moment data and X-ray diffraction studies have been used to assign the trans– trans configuration to dipropionamide in the solid state [3,6]. However, the details of X-ray structure data are not available. Since the intrinsic features of the empirical force field used in normal coordinate analysis lie in their uncertainty, particularly with respect to the interaction force constants, it was felt desirable to carry out the ab initio molecular orbital studies at the HF/6-31g(d) and HF/6-31þ þ G(d,p) levels to determine the molecular geometry and the force field, and then examine the ground state vibrations of dipropionamide and its N-, C- and C,N-deuterated molecules, (CH3CH2CO)2ND, (CH3CD2CO)2NH and (CH3CD2CO)2ND. Detailed investigation of the spectra of isotopic molecules is invaluable in correlating the observed vibrational frequencies with the theoretical normal modes of polyatomic molecules. In continuation of our recent ab initio studies of propionamide [7], in this paper we present a systematic study of the optimized molecular geometry and vibrational spectra of dipropionamide. The infrared and Raman spectra have been assigned taking into consideration the agreement between the calculated and observed band intensities and the trend between the calculated (unscaled) and observed frequencies. The assignments are further supported by the isotope shifts in deuterated molecules. The solvent effects on the conformational equilibria and vibrational spectra are also examined by the ab initio

173

studies using self-consistent reaction field theory for solvents with dielectric constant 1 ¼ 2:2 and 78, corresponding to carbon tetrachloride and water, respectively, employing dipole and self-consistent isodensity polarizable continuum model (SCIPCM) methods. So far no ab initio studies have been reported on dipropionamide.

2. Methods 2.1. Computational details The ab initio calculations at the Hartree –Fock level using the basis sets 6-31g(d) and 6-31þ þ G(d,p) have been performed for dipropionamide using the GAUSSIAN 94 program [8] in the gaseous phase and in solvent media. The fully optimized geometry of dipropionamide was obtained by the analytical gradient methods. The Hartree – Fock cartesian force constants, vibrational frequencies and their intensities were obtained for the optimized geometry. The GMAT program of Schachtschneider [9] was employed to obtain the B and G matrices in internal coordinates for the optimized geometry. Some atoms in Fig. 1 are labeled to define the bond lengths and bond angles, and to specify the internal coordinates used in the calculation of vibrational spectra. The force constants in cartesian coordinates were transformed to those in local and symmetry coordinates through appropriate transformations. The secular equation lGF 2 Ell ¼ 0 was solved to obtain the vibrational frequencies and their potential energy distributions for dipropionamide and its deuterated molecules. The effect of solute – solvent interaction was taken into account via the self-consistent reaction field (SCRF) method [10]. This method is based on Onsager’s reaction field theory of electrostatic solvation [11] and SCIPCM [12]. In the reaction field model, the solvent is considered as a uniform dielectric characterized by a dielectric constant 1. The solute is assumed to occupy a spherical cavity of radius a0 in the medium. The permanent dipole of the solute will induce a dipole (reaction field) in the surrounding medium, which will interact with the molecular dipole leading to stabilization. In the SCRF formalism, the solute – solvent

174

G. Nandini, D.N. Sathyanarayana / Journal of Molecular Structure (Theochem) 589–590 (2002) 171–181

Table 1 Total energies (in hartrees) of the conformers of dipropionamide Conformera

Gaseous state

Solvent medium

1 ¼ 1:0

1 ¼ 2:2 (Dipole)

1 ¼ 78:0 (Dipole)

1 ¼ 78:0 (SCIPCM)

trans–trans staggered–staggered eclipsed–eclipsed gauche–gauche

2437.85045 2437.84101 2437.85023

2437.85340 2437.84387 2437.854744

2437.85758 2437.84869 2437.85851

2437.87209 2437.86215 2437.87064

cis– trans staggered–staggered eclipsed–eclipsed gauche–gauche

2437.86058 2437.85150 2437.86022

2437.861181 2437.85123 2437.86079

2437.86212 2437.85297 2437.86165

2437.87321 2437.86377 2437.87277

cis– cis staggered–staggered

2437.85034

a

Energies for other conformations are not shown.

interaction is treated as a perturbation of the hamiltonian of the isolated molecule. The reaction field is updated iteratively until self-consistency is achieved for the intramolecular electric field [10]. The solvation energy calculated by the SCRF method corresponds to the electrostatic contribution to the free energy of solvation [10]. For the basis set 6-31þ þ G(d,p), the cavity a0 was ˚ for the cis – trans calculated as 4.18 and 4.32 A and trans – trans of dipropionamide, respectively. The SCIPCM is a refinement of the isodensity polarizable continuum method which allows geometry optimization and calculation of vibrational frequency to be made for the solute molecule in a solvent [12]. The isodensity surface was given for the solvents studied a default value as a0 ¼ 0:0004: The geometry optimization was carried out by fixing the cavity in the presence of the reaction field in two different solvents of 1 ¼ 2:2 and 78.0.

3. Results and discussion The results of the ab initio calculations on the molecular conformation of dipropionamide are discussed first. A brief discussion of the assignment of the vibrational frequencies and solvent effect is then presented.

3.1. Molecular conformation Planar (see Fig. 1) and other non-planar conformations are possible for dipropionamide. The full geometry optimization was carried out for all the conformers shown in Fig. 1 at the Hartree – Fock level using the basis sets 6-31g(d) and 6-31þ þ G(d,p). The basis sets with polarization functions were used to minimize the effect of intramolecular hydrogen bonding. The total energy obtained for each of the conformations is given in Table 1 only for the latter basis set. The orientation of the methyl groups plays a key role in the stability of imide conformation. For all the three planar conformers shown in Fig. 1, staggered – staggered orientation of the two methyl groups with respect to their respective CyO groups is favoured. From the total energy, in both the basis sets, it was found that the cis –trans dipropionamide with staggered –staggered methyl orientations represents the global minima. It is more stable than the corresponding trans– trans conformer by 27 kJ/mole. The ab initio calculations refer to the molecule in the gas phase. The corresponding eclipsed orientation of the two methyl groups is the least stable. The total energy for the other orientations of the methyl groups namely staggered – gauche, staggered – eclipsed, gauche – gauche and gauche –eclipsed orientations lies in the local minima. The calculated barrier to methyl

G. Nandini, D.N. Sathyanarayana / Journal of Molecular Structure (Theochem) 589–590 (2002) 171–181

175

Table 2 Calculated geometrical parameters of dipropionamide Parametersa

trans–trans 1 ¼ 1:0

C –N C0 –N CyO C0 yO C –C C0 –C C –C(me) C –C(me) N –H C–He C–Hg C–Hi C–Hj C–Hl C–Hm N–CyO N–C0 yO N –C– C N–C–C0 C –C–C C–C–C0 C –N– C H –N –C He –C –C Hg – C–C Hh – C–C Hj – C–C Hl – C–C Hm – C–C N –C– C–C N–C0 –C– C OyC–N –C OyC0 – N– C a

cis–trans 1 ¼ 78

1 ¼ 1:0

Dipole

SCIPCM

1.393 1.393 1.187 1.187 1.520 1.520 1.523 1.523 0.995 1.088 1.088 1.085 1.083 1.085 1.083 124.01 124.01 112.59 112.59 112.94 112.94

1.391 1.391 1.190 1.190 1.519 1.519 1.523 1.523 0.996 1.088 1.084 1.084 1.088 1.084 1.084 123.9 123.9 112.49 112.49 113.23 113.23

115.85 108.08 109.89 111.02 111.0

115.64 108.09 109.76 111.04 108.09

180.0 180.0 0.0 0.0

180.0 180.0 0.0 0.0

1.390 1.390 1.190 1.190 1.519 1.519 1.523 1.523 0.995 1.088 1.085 1.083 1.088 1.085 1.083 123.9 123.9 112.50 112.50 113.32 113.32 128.73 117.71 107.90 109.78 111.11 107.90 109.76 111.04 180.0 180.0 180.0 0.0

1.380 1.396 1.195 1.196 1.518 1.523 1.523 1.524 0.995 1.088 1.084 1.085 1.083 1.085 1.083 123.49 117.23 113.25 119.29 112.10 112.19 128.58 117.4 107.89 109.80 111.10 111.42 109.78 111.11 0.0 180.0 180.0 0.0

1 ¼ 78 Dipole

SCIPCM

1.385 1.390 1.195 1.197 1.516 1.524 1.524 1.511 0.998 1.088 1.084 1.083 1.084 1.086 1.084 122.9 117.76 113.39 119.27 113.20 112.15 130.46 117.44 107.86 109.73 111.14 108.27 109.40 111.40 0.0 180.0 180.0 0.0

1.380 1.387 1.199 1.206 1.515 1.509 1.523 1.523 0.999 1.087 1.084 1.085 1.084 1.085 1.084 123.27 117.57 113.36 113.29 119.52 112.67 130.51 116.87 107.77 109.58 111.24 109.58 109.26 111.46 180.0 180.0 0.0 180.0

Bond lengths are in angstroms and bond angles and dihedral angles in degrees; e; g; h; j; l; m denotes the atoms defined in Fig. 1

rotation is about 14 kJ/mole which is consistent with those reported for n-alkanes and alcohols [13]. Interestingly, for cis –trans and trans– trans configuration of diacetamide, the eclipsed and staggered orientation of two methyl groups represents the global minima, respectively [14]. Factors which seem to be of importance in determining the stable conformation of imides are steric effects, intermolecular hydrogen bonding, dipole –dipole interaction between the CyO groups and crystal packing forces. For isolated molecules, the

cis– trans configuration is predicted to be more stable than the trans – trans isomer because of smaller interaction between the CyO groups. In non-polar solvents and in aqueous medium too, as discussed later the cis – trans conformation is more stable than the trans – trans form. However, in the crystal, the latter conformer is more stable than the former possibly due to bifurcated hydrogen bonding between two molecules and packing forces. The optimized geometries obtained by the ab initio method for both the cis –trans and trans– trans dipropionamide for the

176

G. Nandini, D.N. Sathyanarayana / Journal of Molecular Structure (Theochem) 589–590 (2002) 171–181

Table 3 Observed and calculated vibrational frequencies and their intensities of trans–trans dipropionamide and their assignment Obsd freq. (cm21) [3] IR A1 3270vs 2980m 2900vw 2875w 1740vs 1461w 1421s 1379msh (1360) – 1075s 994m 954vw 723vs 322m 125sh B1 2985m 2920m 1459m 1257vw 1082w 810s 613w 545sh – 92 –

Calcd Freq cm21

IR (km/mole)

Ramana

PED

3877 3266 3199 3175 2050 1621 1596 1554 1518 1268 1181 1062 1031 747 348 323 128

36.3 53.7 2.1 31.1 493.8 11.1 7.5 6.3 0.04 0.02 4.1 2.0 3.8 0.3 0.0 1.1 0.6

35.9 65.7 353.3 144.7 13.4 7.0 21.9 2.7 0.3 0.3 15.8 9.1 4.0 6.4 3.8 2.4 0.07

NHs(100) CH3as(95) CH3s(93) CH2s(96) COs(81) CH3ab(82) CH2b(89) CH3sb(71) CH2w(52), CH3sb(17) CNs(26), COb(26) CH3r(31), CCs(14) CC(me)s(70) CNs(30), CNCb(20), NCCb(19) CCs(38), COb(29), CCCb(15) CCCb(50), CNs(18) COb(28), CNCb(28), NCCb(16) NCCb(43), CNCb(39)

3284 3208 1613 1400 1203 889 660 605 239 102 40

59.9 31.0 12.2 0.1 1.5 19.0 8.6 84.6 0.9 0.2 0.0

57.3 141.3 0.4 0.0 0.0 1.1 0.1 0.4 0.02 0.2 0.04

CH3as(98) CH2as(97) CH3ab(91) CH2t(76), CH3r(17) CH3r(36), CH2r(29), CH2t(20) CH3r(37), CH2r(26), PCO(21) CH2r(32), PCO(31), tCN(26) PNH(39), PCO(30) tCH3(91) tCN(38), PNH(29), tCC(18) tCC(79)

20.3 2.9 22.7 13.0 1.5 0.0 0.2 0.1 0.6 0.0

CH3as(98) CH2as(97) CH3ab(92) CH2t(77), CH3r(16) CH3r(36), CH2r(30), CH2t(20) CH3r(39), CH2r(31), PCO(16) PCO(57), CH2r(33) tCH3(93) tCC(50), tCN(31) tCC(47), tCN(38)

77.0 14.3 2.4 5.3 4.2

CH3as(95) CH3ss(93) CH2s(96) COs(79) NHb(59), CNs(23) (continued on next page)

Raman

3270w 2984s 2882vs 2822w 1738vs 1460m 1422vs 1378sh 1178vw 1074vs 998vw 956s 340vs 322vs

2984s 2940vs 1452sh 1260w 806w 620vw 545vw – – 50 s

A2 2935vw 2900vw 1461w 1262sh 1082w 805vw 545sh – – –

806w 545vw – 65m 50vs

3284 3208 1613 1400 1205 883 591 232 59 41

B2 2980m 2920m 2875w 1681m 1515vs

2984 s 2920vs 2882w 1705vw 1508w

3266 3199 3175 1978 1677

1460m 1260w

Intensities

7.3 58.9 30 16.4 705.2

G. Nandini, D.N. Sathyanarayana / Journal of Molecular Structure (Theochem) 589–590 (2002) 171–181

177

Table 3 (continued) Obsd freq. (cm21) [3] IR

Raman

1461w 1414m 1379msh 1360vs 1181vs 1082w 1011w 846 s 613w 414m 272m

1452sh – 1378sh 1364vw 1178vw – 1018vw 846vw 620vw 414m 272m

Calcd Freq cm21

Intensities IR (km/mole)

1621 1593 1553 1510 1274 1191 1088 904 671 431 264

3.9 12.9 0.0 28.5 485.5 5.9 5.8 5.7 22.3 18.5 8.9

Ramana 1.3 1.0 0.02 0.5 0.4 2.2 4.3 0.1 1.9 1.2 0.02

PED CH3ab(85) CH2b(88) CH3sb(77) CH2w(60), CH3sb(12) CNs(55), COb(14), NHb(14) CH3r(42), CC(me)s(33) CC(me)s(34), CH3r(21), CCs(20) CCs(51), CC(me)s(17) COb(45), CCCb(18) NCCb(44), COb(31) CCCb(55), NCCb(35)

s ¼ Stretching, as ¼ asymmetric stretching, b ¼ bending, ab ¼ asymmetric bending, t ¼ twisting, r ¼ rocking, w ¼ wagging, P ¼ out of plane bend, t ¼ torsion a In a 4/mole, a ¼ Raman scattering activity.

basis set 6-31þ þ G(d,p) are reported in Table 2. However, the experimental data of molecular geometry are not available for comparison with the calculated values. The imide group – CONHCO – is planar consistent with the experiment. 3.2. Vibrational spectra Detailed experimental infrared and Raman frequencies are available only for the trans – trans conformer which exists in the solid phase. Hence, the spectra are discussed here for the trans –trans conformer. Ab initio calculations at the Hartree – Fock level using the basis sets 6-31g(d) and 6-31þ þ G(d,p) yielded all positive eigen values for the cis –trans and trans – trans dipropionamide with staggered – staggered methyl group orientations. However, with eclipsed – eclipsed methyl group orientations both conformers yielded two negative frequencies, while the other methyl group orientations yielded one negative frequency. Thus staggered – staggered methyl group orientations in both cis –trans and trans –trans conformers represent the global minima as noted above. The trans –trans conformer with staggered CH3 groups possesses C2v symmetry and the normal vibrations are classified as 17A1 þ 16B2 (in plane) and 10A2 þ 11B1 (out of plane) modes. The observed

infrared and Raman frequencies are compared with the calculated frequencies obtained for the basis set 631þ þ G(d,p) in Table 3 wherein also given are the calculated potential energy distributions and band intensities. The assignments have been made taking into consideration the trend between the (unscaled) calculated and observed infrared and Raman frequencies as well as their band intensities. The vibrational assignments for the A1 modes of dipropionamide generally agree with those of Kuroda et al. [3]. The present calculations, however, do not support their assignment of a very weak band at 1257 cm21 to a coupled C– N stretching vibration corresponding to the calculated (unscaled) frequency at 1268 cm21 and similarly the 1379 cm21 band to CH2 wagging corresponding to the calculated frequency 1518 cm21. These bands expected near 1180 and 1360 cm21 are possibly not observed in the infrared spectrum due to their inherent very weak intensity consistent with their calculated infrared intensities or possibly overlap with the corresponding bands of B2 species. A weak Raman band is observed at 1178 cm21 in accord with the calculated Raman intensity. Similarly the CC stretching mode coupled with CyO bending could be assigned to an infrared band at 723 cm21 consistent with its calculated frequency. The assignments for the in-plane B2 modes of dipropionamide are in accord with those of Kuroda et al. [3].

178

G. Nandini, D.N. Sathyanarayana / Journal of Molecular Structure (Theochem) 589–590 (2002) 171–181

Table 4 Calculated and experimental [3] isotope shifts for the fundamental frequencies in cm21 of trans–trans dipropionamide Parent

N –Deut

C –Deut

C,N–Deut

Obsd

Calcd

DObsd

DCalcd

DObsd

DCalc

DObsd

DCalc

A1 3270 2980 2900 2875 1740 1461 1421 1379 1360 1178 1075 994 954 723 340 322 125

3877 3266 3199 3175 2050 1621 1596 1554 1518 1268 1181 1062 1031 747 348 323 128

855 0 0 0 0 0 0 0 2 5 4 2 21 23 0 1 –

1035 0 0 0 1 0 0 0 0 0 1 0 22 2 0 1 1

10 0 5 775 10 2 334 0 89

0 0 1 860 1 3 399 3 107 33 85 28 124 26 6 5 2

850 0 5 775 10 2 334 0 91 – 73 50 111 19 28 12 –

1035 0 1 860 2 3 206 3 107 34 88 47 125 28 6 6 3

3284 3208 1613 1400 1203 889 660 605 239 102 40

0 0 0 0 3 5 5 135 – – –

0 0 0 0 0 1 20 143 1 2 0

5 780 3 370 26 110 6 44 – – –

1 829 1 364 30 111 30 61 4 2 1

5 740 3 367 13 110 112 – – 1 –

1 829 1 365 30 114 108 4 7

3284 3208 1613 1400 1205 883 591 232 59 41

0 0 3 5 3 7 0 – – –

0 0 0 0 0 0 0 0 0 1

0 760 2 370 20 129 81 – – –

1 829 1 370 27 115 81 2 0 6

45 755 5 372 28 133 81 – – –

1 829 1 370 27 133 81 2 0 6

3266 3199 3175 1978 1677

0 0 0 16 582

0 0 0 14 633

0 15 775 11 2

1 1 756 1 2

0 0 20 1 730 856 17 16 571 630 (continued on next page)

B1 2985 2920 1459 1257 1082 810 613 545 92 A2 2939 2900 1461 1262 1082 805 545

B2 2980 2920 2875 1681 1515

73 34 111 25 – 9

1

G. Nandini, D.N. Sathyanarayana / Journal of Molecular Structure (Theochem) 589–590 (2002) 171–181

179

Table 4 (continued) Parent

N –Deut

Obsd

Calcd

DObsd

1461 1414 1379 1360 1181 1082 1011 846 613 414 272

1621 1593 1553 1510 1274 1191 1088 904 671 431 264

2 2 0 19 124 3 4 5 5 4 1

C –Deut DCalcd 0 3 2 27 181 8 1 1 5 4 1

The assignments for the out of plane A2 and B1 modes of dipropionamide are generally consistent with those of Kuroda et al. [3]. The A2 modes are infrared inactive. We suggest that the coupled CH3 rocking of A2 species could be assigned to a weak band at 1082 cm21 instead of to an intense infrared band at 1075 cm21 since the A2 modes are infrared inactive. Further the coupled CH2 rocking mode of B1 species could be assigned to a weak infrared band at 613 cm21 instead of to a strong band at 723 cm21 consistent with its calculated infrared intensity and also with the trend noted between the calculated and observed frequencies. The present assignments for dipropionamide are consistent with those of n-propionamide [7]. The calculated infrared and Raman band intensities are generally in satisfactory qualitative agreement with the experiment. The calculated infrared intensities for the B2 modes are generally higher than those of the A1 modes and vice-versa for the Raman modes as are to be expected. 3.2.1. Deuterated molecules Detailed investigation of the spectra of isotopic molecules is invaluable in checking the consistency in the assignment of the normal modes of polyatomic molecules. The calculations have been extended to the deuterated isotopomers of dipropionamide, namely, (CH3CH2CO)2ND, (CH3CD2CO)2NH and (CH3CD2CO)2ND. The calculated and observed shifts between the parent molecule and its deuterated isotopomers are compared in Table 4. It is seen from Table 4 that the

C,N–Deut

DObsd

DCalc

DObsd

5 359 1 103 14 55 168 27 6 11 4

4 370 3 131 20 74 180 26 8 8 4

5 327 1 5 90 88 192 27 – 13 6

DCalc 4 369 3 0 60 56 184 26 14 12 5

calculated isotope shifts are in reasonably good agreement with the experimental data. The deviation between the calculated and observed frequencies in the parent molecule results in larger deviation between the observed and calculated isotopic shifts and it could also be traced to the larger anharmonicity associated with hydrogen involving vibrations. Thus all the infrared and Raman frequencies of dipropionamide have been satisfactorily assigned. 3.3. Solvent effect Electrostatic effects are usually important in the gas phase. If these molecules were placed in a solvent of high dielectric constant, the electrostatic energies would decrease and other factors may become more significant. The effect of solvents on the energies of organic compounds is often reasonably well related to the dielectric constant ð1Þ of the solvent so long as specific solvent effects such as hydrogen bonding and donor – acceptor interactions are not present. Solvent effects on conformational equilibria and vibrational spectra using self-consistent reaction field theory have been investigated for dipropionamide using the dipole and SCIPCM procedures [10,12] by the ab initio calculations using the basis set 6-31þ þ G(d,p) for two solvents of 1 ¼ 2:2 and 78.0. The total energies calculated for various conformers of dipropionamide in solvent media are given in Table 1. The results showed that the staggered – staggered cis– trans dipropionamide is more stable than the corresponding trans – trans isomer by

180

G. Nandini, D.N. Sathyanarayana / Journal of Molecular Structure (Theochem) 589–590 (2002) 171–181

Table 5 Selected ab initio vibrational frequencies in cm21 in gaseous and solvent media for trans– trans dipropionamide 1 ¼ 1:0

Dipole

SCIPCM

1 ¼ 2:2

1 ¼ 78

1 ¼ 2:2

1 ¼ 78

A1 3877 3175 2050 1518 1268 1062

3871 3178 2043 1519 1269 1063

3863 3182 2030 1520 1272 1064

3868 3180 2031 1520 1273 1064

3851 3189 1996 1523 1283 1068

B1 3208 660 605 40

3211 661 609 37

3213 665 616 36

3214 663 614 36

3223 675 625 29

B2 3175 1978 1677 1510 1274 1088 904 671 431 264

3177 1972 1679 1510 1277 1088 904 671 431 263

3181 1960 1682 1511 1283 1090 906 673 432 262

3180 1961 1683 1511 1284 1090 906 673 432 262

3188 1930 1691 1514 1303 1093 908 675 434 258

21 kJ/mole in a non-polar solvent of 1 ¼ 2:2 (carbon tetrachloride) as calculated by the dipole method and in the aqueous medium ð1 ¼ 78Þ; by 11.9 and 2.9 kJ/ mole by the dipole and SCIPCM methods, respectively, the difference is smaller than that in the gas phase. The calculated total energy by the SCIPCM method is the lowest in the solvent of 1 ¼ 78 than that in the other media. Both the dipole and SCIPCM methods predict for dipropionamide stable cis –trans structure with staggered orientation of the methyl groups in solvents of 1 ¼ 2:2 and 78 and yield all positive vibrational

Fig. 2. Resonance structures of the amide group.

frequencies. However, in the case of analogous trans – trans conformation, while the dipole method yielded all positive frequencies, the SCIPCM method produced one or two negative frequencies for different orientations of the methyl groups. This suggests that the global minima of trans – trans conformer in aqueous medium is possibly not planar. For the cis – trans conformer too the other orientations of the methyl groups yielded one or two negative frequencies in solvent media as in the case of the gaseous phase. Table 5 lists selected vibrational frequencies for trans – trans dipropionamide in two different solvent media as obtained by the dipole and SCIPCM methods. The calculated infrared and Raman band intensities too show some variations in the solvents. Since they are difficult to interpret, the values have not been tabulated. The shifts obtained on going from a solvent of low 1 to higher 1 is generally more pronounced in the SCIPCM method than those obtained by the dipole method but the same trend is maintained. The calculated equilibrium geometry only in aqueous medium by these two methods are shown in Table 2. Interestingly, the CyO bond length increases and C – N bond length decreases on going from the gas phase to the solvent phase particularly as calculated by the SCIPCM method. The calculated changes in the C – N and CyO bond lengths though negligibly small are in accord with the increasing weight of the dipolar resonance structure shown in Fig. 2(b) for the amide group in solvents of high dielectric constant. Consequently, the CyO stretching frequencies decrease by about 50 cm21 and the C – N stretching frequencies because of their coupled nature register a smaller rise (15 – 25 cm21) as calculated by the SCIPCM method. The 1274 cm21 (calcd) band which is predominately due to asymmetric CN stretching (B2 mode) shows increase by nearly 30 cm21 in a solvent of high dielectric constant. The out-of-plane NH and CyO bending modes too show increase by 10 – 15 cm21 in a solvent of 1 ¼ 78: Interestingly the NH stretching frequency decreases by 26 cm 21 because of increased positive character on nitrogen in the dipolar structure in solvents of high dielectric constant.

G. Nandini, D.N. Sathyanarayana / Journal of Molecular Structure (Theochem) 589–590 (2002) 171–181

Acknowledgments The authors gratefully acknowledge the financial support from the CSIR, New Delhi under grant No. 01(1475)/971 EMR II.

References [1] O.H. Wheeler, O.R. Osado, in: J. Zabicky (Ed.), The Chemistry of Amides, Interscience, London, 1970, p. 335. [2] F. Ramondo, S. Numziante Cesaro, L. Bencivenni, J. Mol. Struct. 291 (1993) 219 and references therein. [3] Y. Kuroda, K. Machida, T. Uno, Spectrochim. Acta 30A (1974) 47. [4] E.A. Noe, M. Raban, J. Am. Chem. Soc. 97 (1975) 5811. [5] J. Jadzyn, B. Zywcki, J. Mol. Struct. 145 (1986) 195. [6] K.L. Gallaher, S.H. Bauer, J. Chem. Soc. Farad. Trans. 2 (1975) 1423.

181

[7] G. Nandini, D.N. Sathyanarayana, J. Mol. Struct. (Theochem) 586 (2002) 125. [8] M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T.A. Keith, G.A. Peterson, J.A. Montegomeryj, K. Raghavachari, M.A. AlLaham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. Head-Gordon, C. Gonzalez, J.A. Pople, Gaussian, Inc., Pittsburgh, PA, 1995. [9] J.H. Schachtschneider, Tech Report. No. 57, Shell Development Co., Emeryville, CA, 1965. [10] M.W. Wong, K.B. Wiberg, M.J. Frisch, J. Am. Chem. Soc. 114 (1992) 1645. [11] L. Onsager, J. Am. Chem. Soc. 58 (1936) 1486. [12] V. Barone, M. Cossi, J. Tomasi, J. Comput. Chem. 19 (1998) 404. [13] K.B. Wiberg, in: P.V.R. Schleyer (Ed.), Encyclopedia of Computational Chemistry, vol. 4, 1995, p. 2518. [14] G. Nandini, D.N. Sathyanarayana, unpublished results.