Raman and infrared spectra, conformational stability, barriers to internal rotation and ab initio calculations for 3-methyl-2-butenoyl chloride

Journal

of

MOLECULAR STRUCTURE

Journal of Molecular Structure 376 (1996) 261-275

Raman and infrared spectra, conformational stability, barriers to internal rotation and ab initio calculations for 3-methyl-2-butenoyl chloride’ James R. Duriga3*, Gamil A. Guirgis”, Department hDepartment

of

Chemistry,

qf Chemistry

Yanping

Jinb32

aLrniversity of Missouri-Kansas City, Kunsm City, MO 64110-2499, USA und Biochemistry, Univrrsif_v ofSouth Carolina, Columbia, SC 29208, USA

Received 21 August 1995; accepted 25 September 1995

The Raman (3200&10 cm-‘) spectra of liquid and solid and mid-infrared (3400&400 cm-‘) spectra of gaseous and solid 3-methyl-2-butenoyl chloride (3,3-dimethylacryloyl chloride), (CH3)2CC(H)CClO, have been recorded. Additionally, the far-infrared (350-65 cm-‘) spectrum of the gas has been recorded. These spectral data have been interpreted on the basis that the syn conformer (the C=C double bond is oriented cis to the C=O bond) is the only stable conformer present in all three phases. This conclusion is supported by ah initio calculations where a second conformer is predicted to be nearly 5 kcal mol-’ higher in energy. A complete vibrational assignment of the normal modes is provided. The structural parameters, force constants, and vibrational frequencies for the syn conformer have been determined from ab initio calculations employing the RHF/3-2lG* and RHF/6-3lG* basis sets, as well as with electron correlation at the MP2/6-3lG* level. The theoretical results are compared to the experimental values, as well as with the corresponding quantities obtained for some similar molecules where appropriate.

1. Introduction Recently, there has been substantial interest in the conformational properties of molecules with conjugated systems [l-9], such as molecules with the general formula XYC=C(X’)CO(Y’)? where X and Y are H or CH3, X’ is H, F, Cl, or CH, and Y’ is F, Cl or Br. These molecules are similar to

* Corresponding

author.

I Dedicated to Professor James E. Boggs, an excellent scientist and good friend on the occasion of his 75th birthday. ’ Taken in part from the thesis of Yanping Jin which will be submitted to the Department of Chemistry and Biochemistry in partial fulfillment of the Ph.D. degree.

1,3-butadiene and the reported data indicate that they exist in the anti or syn forms with the anti form frequently being the more stable conformer. Additionally, several studies have been reported [lo-131 questioning the reliability of ab initio calculations to predict the most stable conformer of a molecule with a CC10 group. In a recent study, we [9] examined this phenomenon and concluded that the agreement between experimental and calculated data is greatly dependent on the basis set used. As an illustration, we [9] calculated the conformational stability of but-2-enoyl chloride and found that the energy difference between the syn and anti conformers becomes closer on going from the RHF/3-21G* (750 cm-‘) to MP2/6-31G*

0022-2860/96/$15.00 80 1996 Elscvicr Science B.V. All rights reserved SSDI 0022-2860(95)09124-6

262

J.R. Durig et d.iJourtd

of Molecular Structure 376 (1996) 261-275

(208 cm-i) calculations but the syn conformer is still predicted to be the more stable rotamer, whereas the anti form is found to be the more stable rotamer experimentally. As a continuation we have re-examined the of these studies, vibrational assignment and conformational stability of 3-methyl-2-butenoyl chloride (3,3dimethylacryloyl chloride), (CH&C=C(H)CClO. with an emphasis on the theoretical calculations. In a previous study, Gupta et al. [14] reported a vibrational study of 3-methyl-2-butenoyl chloride utilizing the infrared and Raman spectra of the liquid and the infrared spectra of solutions in CC4 and CSI. These authors concluded that the molecule in the liquid phase exists as two conformers, i.e. the syn (s-cis) and anti (s-trans) forms. This is in contrast with the conclusions from an earlier electron diffraction study [ 151 where the investigators interpreted the data on the basis of a single conformer, the syn form. Therefore, we have recorded the infrared spectra of the gas and solid and the Raman spectra of the liquid and solid. The Raman spectrum of the gas could not be recorded because of the low vapor pressure of this compound. Additionally, we carried out ab initio calculations utilizing the RHF/3-2 1G* and RHF/6-3 lG* basis sets along with electron correlation at the MP2/6-31G* level to obtain structural parameters and, with the smaller RHF/3-21G* basis set, force constants and vibrational frequencies. The results of this spectroscopic and theoretical study are reported herein.

2. Experimental The sample of 3-methyl-2-butenoyl chloride was purchased from Aldrich Chemical Co. (Milwaukee, WI) with a stated purity of 95%. Further purification was carried out with a low-temperature, low-pressure fractionation column. The purified sample was stored at dry ice temperature under vacuum in a greaseless sample tube. All sample manipulations were carried out under vacuum. The Raman spectra (Fig. 1) were recorded on a Cary model 82 spectrophotometer equipped with a Spectra-Physics model 171 argon ion laser operating on the 5145 A line. The laser power at the

A

3000

1500 WAVENUMBER

1000

500

(cm-‘)

Fig. I. Raman spectra of 3-methyl-2-hutcnoyl liquid; (B) amorphous solid; (C) annealed solid.

chloride: (A)

sample was 0.2 W. The spectrum of the liquid (Fig. 1) was recorded from the sample sealed in a Pyrex capillary. The spectrum of the solid was recorded by condensing the sample on a blackened brass block cooled by boiling liquid nitrogen and contained in a cell with a quartz window. The variable temperature study of the liquid was carried out on a Cryogenics Technology cryostat equipped with quartz windows and a Lake Shore Cryotronits DTC-500 high precision temperature controller. The accuracy is expected to be at least f 2cm-i for sharp, resolvable bands. The mid-infrared spectra from 3200 to 400 cm ’ (Fig. 2) of the gas, amorphous solid and annealed solid were recorded with a Digilab FTS-14C

J.R. Durig et al.!Journal of Molecular Structure 376 (1996) 261-275

I

I

I1

I

3000

I

I

I

I

II

I

I

I

I

I

2000

2500

WAVENUMBER Fig. 2. Mid-infrared

spectra of 3-methyl-2-hutenoyl

Fourier transform interferometer equipped with a nichrome wire source, GeiKBr beamsplitter and TGS detector. The spectrum of the gas was obtained from a sample contained in a 10 cm cell equipped with CsI windows. For the solid, the spectrum was recorded by depositing the sample as a film on a KBr plate cooled with boiling liquid nitrogen and housed in a vacuum cell fitted with CsI windows. The spectrum of the annealed solid was obtained after several cycles of warming and cooling until no further changes were observed in the spectrum. The spectra of the gas and solid were recorded at effective resolutions of 0.5 and 1.0 cm-’ respectively. Interferograms obtained after 500 scans of both sample and reference were transformed by using a boxcar truncation function. The far-infrared spectrum from 350 to 65 cm-’ (Fig. 3) of the gas was recorded on a Bomem model DA3.002 Fourier transform interferometer equipped with a vacuum bench, a Globar source and a liquid-helium-cooled silicon bolometer with a wedged sapphire filter and polyethylene windows.

I

I

II

I1

1500

I

I1

I

/

263

I

1000

,I

500

(cm-l)

chloride: (A) gas: (B) amorphous solid; (C) annealed solid.

The gaseous sample was contained in a 1 m optical path cell. A 6.25 pm Mylar beamsplitter was used to record the spectrum in Fig. 3 at a resolution of 0.1 cm-‘. Typically, 512 scans were needed for both the sample and reference to give a satisfactory signal-to-noise ratio. All of the observed bands are listed in Table 1.

3. Conformational

stability

In the earlier vibrational study of 3-methyl-2butenoyl chloride, Gupta et al. [14] investigated the spectral behavior of the C=C and C=O stretching modes with the sample as a neat film in Ccl4 as a non-polar solvent and in CHCls as a polar solvent. For the C=C stretch, bands were reported at 1640 and 1610 cm-i with the higher frequency band decreasing in intensity in the CHCls solution so that the 1610 cm-’ signal was assigned to the anti (s-trans) conformer which has the larger dipole moment. In the infrared spectrum of the gas, we

264

J.R. Durig et al./Joumal

Table 1 Observed”

infrared

and Raman

frequencies

Infrared Rel. IIll.

Solid

Rel. Int.

3063

w

3050 3044

w w

2995 2986

w w

2954

w

2928

w

2878

w

1787 1774 1657

“S sh,w s w

1450 1436

m w

1386 1379 1348

m m w

1205 1088

m m

1011 967 836 Q 831 P

b w m

777

464

305 209 170

Structure

for 3,3-dimethylacryloyl

s

m

s w,bd w,bd

376 (1996)

261-275

chloride Assignment

Rama

Gas

I634 1466

(cm-‘)

of Molecular

Liquid

Rel. 1nt.

Solid

Rel. Int.

3079

VW

3078

w

CH stretch

3037 3020 2997 2987 2976 2952 2945

VW

(CH,),

antisymmetric

stretch

w

(CH,)2

antisymmetric

stretch

sh,vw VW sh,w m

(CH,), (CH,),

antisymmetric antisymmetric

stretch stretch

v,

Approximate

description

VW

2988

w

2957 2948

w w

2952

w

2910 2878 2860 1772 1754 1669 I638 1615 1469 1446 1435 1430

w w w s vs

2919 2873 2856 1786 1754 1665

sh,m YS w “VW w w VW

2916 2872 2856 1765 1146 1670

vs sh,vw VW w w VW

(CH,), symmetnc stretch Overtones or combinations

I616

VF

I604

vs

C=C stretch (CH,), antisymmetric (CHS)* antisymmetric (CH,): antisymmetric (CH,), antisymmetric

1379 1374 1348 1324 1209 1099 1071 1015 969 840 832

m s w VW m s m vs w s s

770 755 660 470 459

VW

Comhinatlon C=O stretch Combination

band band

VW

vs VW

sh,m s sh,m

s s w m m

1442

m

1378 1346

m

1442 1432 1409 1380

m In VW s

w

1346

m

1209 1095 1071 1011 965 837 834 785 169 155 661 468 460 433 428 380 306 204 174

vvw m m w w m sh w w w w m m sh m w m w w

m m

832

m

755 652

w w

457

m

429 377 304

m w In

175

vw,bd

(CH,), symmetric (CH,), symmetric CH bend CC2 antisymmetric (CH,): rock (CH& rock Cz-C3 stretch (CH,), rock CH bend

CC2 symmetric CC10 rock cc10 wag CC> rock C-Cl stretch

deformation deformation deformation deformation

deformation deformation

stretch

stretch

CC* deformation CC2 twist C, C& bend C=C twist CC10 deformation

J.K. Lhrig et nl./Journnl

of Moleruh

Structure

376 (1996)

265

261-275

Table 1 Continued Infrared Gas

Ramall

Rel

Solid

Int.

Rel. Int.

Liquid

Assignment Solid

Rel. Int.

Rel Int.

12 65 52 42 32 24 a Abbreviations branches.

v;

Approximate

m sh,m m m m in

description

Lattice modes

used: s, strong; m, moderate; w, weak; Y, very; bd, broad; sh, shoulder; P, Q, and R refer to the rotational

observed bands at 1657 and 1634 cm-’ with the latter being much more intense than the higher frequency band. In the infrared spectrum of the solid, there are three very weak shoulders at 1669, 1638 and 1589 cm ’ on the intense 1615 cm-’ band, which must be the C=C stretch. Similar bands at 1670,1636, 1604 and 1585 cm-’ arc observed in the Raman spectrum of the solid. However, none of these bands behaved as conformer bands since there was no change in relative intensities with annealing but only sharpening of the bands in both the infrared and Raman spectra.

3.50

310

270

230

Similarly, bands were reported at 1775 and 1750 cm-’ for the C=O stretch and, on the basis of the intensity variations with solvent, the 1750 cm-’ band was assigned to the anti (s-trans) conformer. These bands were observed in both the infrared and Raman spectra (Figs. 1 and 2) with about equal intensity in the spectra of the liquid and amorphous solid. With repeated annealing of the solid, the band at 1750 cm-’ became sharper, but the higher frequency band remained in the spectra with about the same intensity, which is not consistent with it arising from a second conformer.

190

WAVENUMBER Fig. 3. Far-infrared

vibrational

150

110

70

(cm-l)

spectrum of gaseous 3-methyl-2-butenoyl

chloride. Asterisk indicates HCl.

266

J.R. Durig

Table 2 Structural

parameters,

Parameter

G=Cd c3=w

G5-Cl) G-G) r(c& -C3)

0-W QL-C5) ~WIO-Cd rW11--C5) rwlzrC6) Qb-Cd ~(H,4-C6) WC,=CrG) w-G=Qd W2=C,-C5) -Cd

HC,-cl-Cd IG-C3-ChI KO4-C3-G) WI-G-W rvk+Ha) WI

constants,

and dip&

of Molecular

moments

Structure

-G-H,)

W-C5-H,d EC,-C5-HI,) W-Cs-Hd W-CS-HII) ?XW-C~-HII) WI

-C6-HI,)

WI

-C6-Hd

)IWIZ-C~-W~) w-c6-w )iWn-C6-H14) Kfh-C6-H14) W&5GH9) PWnW,Hn)

A B ;.I /f”l I,4 -!E + 365) (hartree) AE (cm-‘) a Bond lengths b Ref. [15].

376 (1996)

for 3-methyl-2-hutenoyl

cis

r(C,=C2)

rG=cl

rotational

et aLlJournal

261-275

chloride

tram

E.rxb

RHF/3-2lG*

RHF/6-31G*

MP2/6-31G*

RHF/3-21G*

RI1F/b_3l~*

~P2,6_31~*

1.330 1.461 1.193 1.512 1.511 1.810 1.068 1.081 1.086 1 086 1.075 1.086 1.086 125.0 130.5 119.9 125.8 114.3 110.8 118.7 120.2 114.8 112.0 109.7 108.8 109.7 108.8 107.6 112.1 109.1 109.5 109.1 109.5 107.4 121.0 121.4 4490 1161 933 4.353 1.780 0.000 4.703 0.172841 3886

1.335 1.413 1.173 1.508 1.505 1.793 1.072 1.082 1.086 1.086 1.076 1.087 1.087 125.4 129.9 119.1 126.6 114.3 111.6 118.5 119.6 115.0 112.3 110.1 108.6 110.1 108.6 107.0 113.3 109.2 109.1 109.2 109.1 106.8 121.1 121.8 4556 1154 931 4.196 1.703 0.000 4.528 0.753394 1745

1.352 1.466 1.206 1.501 1.498 1.810 1.084 1.092 1.096 1.096 1.086 1.096 1.096 125.2 129.9 119.0 126.1 114.9 110.9 119.2 119.3 1154 1122 110.2 108.6 110.2 108.6 106.9 112.8 109.4 109.2 109.4 109.2 106.7 121 2 121 8 4461 1151 925 4.323 1.907 0.000 4.725 0.750273 1829

1.328 1.462 1.193 1.516 1.519 1.798 1.073 1.081 1.086 1.086 1.082 1081 1.081 132.5 123.8 118.3 128.1 113.5 118.0 118.2 118.3 109.2 112.2 109.7 108.8 109.7 108.8 107.5 109.7 111.2 108.4 111.2 108.4 107.9 121.1 119.9 3591 1445 1044 4.715 1.235 0.000 4.874 0.160533

1.333 1.474 1.174 1.509 1.514 1.783 1.075 1.082 1.087 1.087 1.083 1.082 1.082 132.7 123.0 118.0 128.1 113.9 119.1 117.9 118.5 108.8 112.6 109.9 108.7 109.9 108.7 107.0 109.8 111.5 108.1 111.5 108.1 107.6 121.3 119.9 3637 1428 1038 4.518 1.295 0.000 4.700 0.745441

1.352 1.464 1.208 1.503 1.506 1.797 1.087 1.092 1.096 1.096 1.092 I .092 1.092 132 3 123.4 117.9 127.6 114.5 117.9 118.7 118.0 109.7 112.4 110.1 108.6 110.6 108.6 106.8 109.7 111.6 108.4 111.6 108.4 107.0 121.3 120.1 3561 1440 1039 4.776 1.454 0.000 4.992 0.741941

in angstrbm,

bond angles in degree, rotational

constants

in MHz. and dipole moments

in debye.

1.333 It 0.007 1.467 f 0.008 1.182 f 0.004 1.496 f 0.005 1.496 f 0.005 1.800 f 0.005 1.093 f 0.007 1.093 f 0.007 1.093 * 0.007 1.093 f 0.007 1.093 + 0.007 1.093 + 0.007 1.093 f 0.007 127.1 f 0.8 128.310.6 121.3 f 1.2 122.5 f 1.0 116.2 f 2.2 111.71to.4 120.0 + 1.0 120.7 f 7.5 112.2 + 8.3 111.4+ 1.3 111.4* 1.3 111.4f 1.3

111.4f 111.4*

1.3 1.3

111.4+

1.3

J. R. Durig er al.iJournai

of Molecular

The skeletal bending region of the spectrum is the best region for the detection of conformer peaks. The C1C2C3 deformation is expected to differ by 14 cm-’ for the two conformers, but only one band is observed at 304 cm-’ for this mode. Similarly, the CI=C2 twist is expected to be separated by 25 cm-’ for the two conformers but only one band is observed at 200 cm-’ in the Raman spectrum of the amorphous solid. Finally, the C-Cl stretch is predicted at 542 cm-’ for the anti (s-trans) conformer, but there are no bands in the 500-600 cm-’ region of either the Raman or infrared spectrum. Similarly, the CC10 rock is predicted at 609 cm-’ for the anti rotamer but, again, there are no infrared or Raman bands that can be assigned to this mode. Since both the C-Cl stretch and CC10 rock are normally observed for the second conformer of a molecule containing the CC10 group when conformers are present but are not observed for 3-methyl-2-butenoyl chloride, we have concluded that there is only one conformer present in all physical states for this molecule contrary to the conclusions from the earlier vibrational study [14]. We believe that the doublets observed for the C=C and C=O stretches are due to combination bands in Fermi resonance with the fundamentals.

4. Ab initio calculations The LCAO-MO-SCF restricted Hartree-Fock 92 calculations were performed with the GAUSSIAN program [16] using Gaussian-type basis functions. The energy minima with respect to nuclear coordinates were obtained by the simultaneous relaxation of all of the geometric parameters using the gradient method of Pulay [17]. The energy difference between the anti and syn conformers is 3886, 1745 and 1829 cm-’ from the RHF/3-2 lG*, RHF/6-3 1G* and MP2/6-3 lG* calculations respectively. These results indicate that the anti conformer cannot be the form present in the gas phase at ambient temperature. The calculated structural parameters, rotational constants, and dipole moments were determined for the syn conformer from the RHF/3-21G, RHF/6-31G*

Strucrwe 376 (1996) 261-275

2000

-_

1000

--

267

i 5 5

04 0

90

160 DIHEDRAL

270 ANGLE,

360

Cp

Fig. 4. Potential function governing internal rotation of 3methyl-2-butenoyl chloride as determined with the RHF/631G* basis set. The potential surface is calculated by allowing for optimization at the transition state and at the s-cis minimum by relaxing all of the geometric parameters.

and MP2/6-31G* calculations and are listed in Table 2. The optimized geometry for the syn conformer from the RHF/6-3 1G* calculation was used to estimate an asymmetric torsional potential surface by allowing the torsional dihedral angle to vary in 10” increments from 0” (syn) to 180” (anti). The potential function calculated in this way shows a very large energy difference for the anti conformer and gives a rough estimation that a maximum at 90” is a transition state and a minimum at 150” indicates a gauche conformer. For a more meaningful potential energy surface, the syn and anti forms and the transition and gauche structures were optimized. The results are shown in Fig. 4. The transition state was found to be at 98.2” with an energy

Cl-r

Fig. 5. Internal coordmates

for 3.methyl-2.butenoyl

chloride.

J.R. Durig et d.:Journal

268 Table 3 Symmetry

coordinates

for 3-methyl-2-butenoyl

of’Molccu/ar Structure 376

chloride Symmetry

Species

Description

A’

CzHS atrctch ICHj): antisymmetric stretch l,CH,): antisymmetric stretch (CH,): symmetric stretch (CH,), symmetric stretch C=O stretch C=C stretch (CH2): antisymmetric deformation (CH,): antisymmetric deformation (CH,), symmetrx deformation (CII,), symmetric deformation C2Hs bend CC2 antisymmetric stretch (CH,): rock c+c, stretch (CHj)2 roch CC2 symmetric stretch CC10 rock CC1 stretch CC2 deformation cc2 wag C,C2C, deformation CC10 deformation Redundancy Redundancy Redundancy Redundancy Redundancy

A”

(CH,)* antisymmetric (CII,), antisymmetric (CH,)z antisymmetric (CH,)> antisymmetric

(19961 261-275

coordinate”

stretch stretch deformation deformation

(CH,), wag (CHI)~ wag C2Hx out-of-plane wag CC10 out-of-plane wag c5cjc2 out-of-plane wag C, =Cz torsion hlethyl torsion Methyl torsion Armsymmetric torsion a Not normalixd.

difference of 2183 cm-’ higher than that of the syn conformer. The gauche minimum was found to be at 156.7‘ with an energy difference of 1712 cm-t higher than that of the syn form. The internal coordinates are shown in Fig. 5. The symmetry coordinates which were developed from

the internal coordinates are listed in Table 3. The force field in Cartesian coordinates was calculated by the GAUSSIAN 90 program [16] with the RHF/321G* basis set. The following procedure was used to transform the ab initio results in Cartesian coordinates into the form required for the iterative

J.R. Durig el ai./Journal

of Molecular

normal coordinate programs. The Cartesian coordinates obtained for the optimized structure were input into the G-matrix program written by [ 1S] together with the complete Schachtschneider set of 41 internal coordinates to calculate the B-matrix. The B-matrix was then used to convert ab initio force fields in Cartesian coordinates to the desired internal coordinates using a program developed in our laboratory. The resulting force field can be obtained from the authors. Initially, all scaling factors were kept fixed at a value of 1.0 to produce a pure ab initio calculated vibrational frequency. Subsequently, scaling factors of 0.90 for the stretching modes, 0.8 for bending modes and 1.0 for the torsional modes were input into a program to obtain the fixed scaled force fields, vibrational frequencies, and potential energy distributions (PEDs). The resulting frequencies, infrared intensities, Raman scattering activities, depolarized ratio, observed frequencies and PEDs are listed in Table 4.

5. Vibrational assignment Although Gupta et al. [14] have previously reported a vibrational study of 3-methyl-2butenoyl chloride, their assignments were based on the results obtained from the spectrum of the liquid with comparison of the normal modes of some similar molecules. Since these investigators [ 141 did not provide a complete vibrational assignment, nor any data taken for the gas or solid, the infrared spectra of the gas and solid and the Raman spectra of the liquid and solid were investigated. The syn form of 3-methyl-Z-butenoyl chloride has C, symmetry, and the 36 vibrational modes span the irreducjble representations: 23 A’ and 13 A”. Assignment of the normal vibrational frequencies is based on the infrared band contours, infrared and Raman band intensities, group frequencies and depolarization data, and is supported by the normal coordinate calculations. The motions which preserve the symmetry plane (A’) should give rise to A-, B- or A/B-type hybrid infrared gas phase band contours and should give rise to polarized lines in the Raman spectrum. The A” modes should exhibit C-type band contours in the

Structure 376 (1996) 261-275

269

infrared spectrum of the gas and give rise to depolarized lines in the Raman spectrum. The assignment of the carbon-hydrogen stretching modes differs significantly from that given earlier [14] since the bands below 2900 cm-’ are believed to be due to overtones or combination bands of the CHj deformations. Also, the ab initio calculations predict one of the CH3 antisymmetric stretches of A’ symmetry to be about 80 cm- ’ higher in frequency than the other three antisymmetric stretches. Therefore, we have tentatively assigned the band at 3044 cm-’ as vZ, the CH3 antisymmetric stretch, and the one at 3079 cm-’ stretch, although it is possible as ~1, the C-H that these two bands are V, and an overtone in Fermi resonance with v,. The CHj symmetric stretches are assigned as being degenerate at 2919 cm-] since they are predicted to be within 6 cm-’ of each other. Our assignments also differ significantly from those provided earlier [14] in the region from 750 to 1252 cm-‘; particularly since the 1252 cm-’ band which was previously assigned as the CZ-C3 stretch (C-CC10 stretch) was not observed in our Raman spectra. We believe that the 1252 cm-’ band previously reported [14] is due to an impurity and the C2-Cj stretch is assigned to the strong infrared band at 1011 cm-’ which was previously assigned [14] to a CH3 rocking mode. The Raman band at 1346 cm-’ which had previously been assigned [14] as a CH3 deformation is assigned as the C-H in-plane bend, v12, since this frequency is too low for a CH3 deformation for a methyl group attached to a carbon atom. The two CC2 stretches were assigned earlier [14] as degenerate at 825 cm-’ but we have assigned the CC2 antisymmetric stretch at 1209 cm-’ and the symmetric mode at 770 cm-’ based on ab initio predicted frequencies and relative intensities of these bands in the infrared spectrum. The 832 cm-’ band (previously reported [14] as 825 cm-‘) is assigned to the C-H out-of-plane bend, Q,,. The band at 755 cm-’ which had previously been assigned [14] to the C-Cl stretch is now assigned to the CC10 rock with the C-Cl stretch assigned to the pronounced Raman band at 460 cm-l with an ab initio predicted frequency of 459 cm-’ Most of the skeletal bending modes have been reassigned

NO.

Speaes Vib.

Observed

Tahlc 4

stretch stretch

symmelric

(CH?), (CH? iI symmetric

1R

1419 1405 1191

I587 1571

1120 1007

1236 1076 1032 890 811 493 459 407 329 208

(CH,jz rock Cz-C, (CH,)? rock CC1 symmetricstretch CCIO rock CC1 nrerch CC2 deiormatmn CC> wag CIC2C3 deformatmn CC10 deformation

I86

300

368

412

459

752

x20

959

1210

1545 1322

C,lltknd CC, antqmmetric stretch

(CH, 11symmetnc defomutmn (CH,,I>symmctnc defomxttmn 1346

2.84 0.20 1205

14.44 066

12.79 0.70 I379

4.93 0.74

8 06 0.34 0.52 0.64

2.85

1.92 0.69

1.27 0.74

I8 I?

3.16

0.77

42.12 20.55 0.26

216.92

012

2.44 0.74

175

304

377

42Y

457

755

774

967

6.89 0.55 I011

131.99 1010

252.99

16.30 19.03 0.51 1090

Il.65

8.81

21.40

21.00 25.29 0.46 1378

37.43 30.19 0.51 1422

16S,3

12.&y 13&

4x,,. 14s>,.4lS,,

15&, 18S,,,?7&,. I IS,,, IS&,

3X&,. 27S,,.IIS,,

43S20,16&, 23&j

66&,

34s,,.33s,..14s,g

44&T, IS&:

65Slb.27S,,

SOS,,.17S,&

51.L

42S,,.19S,,

7q:

Y6S,,

93&

77&

73S,

1470

1446

(CH,!, antisymmeucdeiormation 1642

51 58 062

74S,

deiorrrratmn 165 392s

223.45 88.59 0.24 I616

1479

90&

1710

0.51 2919

I

1694

1824

antisymmetric

C=C stretch (CH,,l,

1934

stretch

C-0

91S6

2.54

9os,

244.16 26.X0 0.53 1754

3030

24.25 281.57 0.05 2919

3036

075

211

318

404

480

5X6

671

Uh5

I079

Ill5

1245

1304

1530

1573

1585

1648

1664

1842

1922

3193

3224

3285

5331

2952' 49&,47S,

3357 3290

22.81 0.62 3044' 43S>.43&

IOOS,

Ab

39.02 0.30 3079

1386

1.74

Obs = PED’

trans

mitio*

dp

chloride

ratio

Raman a~t.~

3118

2.26

3236 3lY7

inr.’

1829

3193

3200

3286

antisymmetnc

(CH,),

stretch

3411 3369

aotnymmctr~

Fixed scaledb

Ab mmo*

as

(cm-‘) and PEDs for cis- and rmns-3-methyl-2-hutenoyl

(CH,),

stretch

frequencies

C2H, stretch

Fundamental

and calculated

Fwxl

1R

Raman

1732

189

286

365

444

542

609

808

991

1028

1134

1193

1376

1406

3.56 0.62

0.43

524

2.63

069 1.50 061

249

2.13 0.49

12.78' 8.03 0.36

53.27 15.69 0 15

81.50

4.46 0 36

3.55 075

15.52 32.73

8.36 0.34

170.02

0.44

029

7.34 0 36 865 31.27 2026

85.23

X.08

Ii.13 17.00 0.59

3.10 17.50 049

070 1417

101x

27.12 33.29 066

502

45.07 53.68 030 1475

1488

410.81 134.74 040

005

900 13320 1819

3029

075 945 119.20 003

14.30 3786 3059

3117

3.72 81.14 0 29

ratm

dp

17.30 64.32 0 75

act!

3184

in!’

3121

scaledb

IO&

49.L 49%

54Szl,IOS,,,21S2,

68S~O.14&g

51&v. 32&s. 13&o

l3S*,,14s,+ llS,*.22S,.. 10.5,"

19s,,,17s,9,30s*,, 12&?, 10.5&

62&r, 33S,, 67&l

3lS,,.27&e

48&,,25S,>

34s,,,12S,,,2OS,~, II&

77s,,

Y2S,,

8?&,

SIS,

75S&.IIS,,

74s,

9156

55&.39&

SSS,,4OS,

BUS,

n&s2

9%

PED’

(CH,), (CHs)z Cl1 wag cc10

y8

~9

“3,

Y,

rock

rock

Antisymmetric

qa

torsion

“15

torsion

Methyl Methyl

YI

wag

C&Cl C, =C* torsion

U,?

Y,

torsion

antqmmetric

antisymmetric

antisymmetnc

antisymmetnc

wag

(CH& (CH,),

uza

(CH,),

y7

(CH,),

y,

y,

Fundamental

165 I35 64

64

698

761

165

900

1015

135

1017

464

1126

1258 1137

221

1474

1648

520

I506

1684

deformation deformation

223

3081 3071

3246 3236

stretch stretch

Fixed scaled”

Ab initio”

cis

IR

0.41

0.05

0.04

1.70

8.40

5.54

4025

11.57

11.63

4.07

25.34

2.34

20.21

mtC

Raman

0.12

0.18

0.48

0.50

5.59

2.ho

10.19

1.47

4.97

29.41

4.48

24.21

174.15

actd

17&,

505’14

56.L

93s1,

0.75

1001

511

38.S~

185

19

170

19

167

I83

248

485

725

248

666

1014

17s,,

896

II17

1120

147’)

1496

3075

3127

1253

1654

1672

Flxed scaledb

, zos,,

36&x, IO&,,

9JS13 74

14s,, 24s~~

82S,>, 13&

6hS,,

77s,,.

0.75

464 209

0.75 0.75

73&s, 71S,,.

0.75

832 660

0.75

0.75 0.75

1090 1071

0.75

0.75

74&,

1466 1436’ 77&T, 16&e

0.75

3240

3296

9lSz4

2976 9lSX

2995

0.75

Ab

0.75

PED’ initio

Ohs.’

ratio

dp

Wi”S

a

Calculated with the RHF/3-21G* basis set. ’ Scaled ab initio calculations with factors of 0.9 for stretches, 8.0 for bends, and 1.0 for torsions using the RHF/3-2lG* ’ Calculated infrared intensities in km mole-’ d Calculated Raman activities in A4 arm-‘. ’ Frequencies are taken from the infrared and/or Raman spectrum of the gas or liquid. f Calculated with the RHFj3-21G* basis set. Contributiuns less than 10% are omitted.

A”

NIJ

Spcc~esVib

Table 4 Continued

IR

Ramsn

dp

0.04

0.41

hasps set.

0.28

00s

0.16

108 001

6.73 0.14

8 53

4.75

12.96 488

38.22

3.04 0.78

11.77 8.88

19.05 21.86

8.09 12.80

0.75

0.75

075

075

075

0 75

0 75

075

0.75

0.75

0.75

075

rat10

0.75

41.66

a~t.~

11.78 101.58

2.36

~nt.~

(OS*,

46Sn

50&

169s:

83s~

33&s,

56.L

29S,,,

67&

67S,,,

79S,,

IO&,

lOS,,

17Sj2.25.S~r

43&3

hlS,,

IZ&,,

27S,,

IS&;,

70&$,. 24&O

73S,,,

+L%,. 47SB

47Sx.

50&,

50&,

PED’

272

J.R. Duriz et al.~Journal of Molecular Structure 376 (1996) 261-275

since the 230 cm-’ Raman line previously assigned [ 141 as the asymmetric torsion was not observed in our spectrum and the 304 cm:’ Raman line had been previously assigned to the two CC2 in-plane bending modes whereas we assign these vibrations at 377 and 429 cm-‘. The complete vibrational assignment is listed in Table 4 and it differs significantly from that previously reported [14]. The skeletal bending modes are somewhat broader than one might expect but we believe part of this breadth is due to the low frequencies for the three torsional modes which result in significant excited state populations of these modes at ambient temperature. In the Raman spectrum of the amorphous solid, most of the lines resulting from the skeletal modes are significantly sharper than the corresponding lines in the spectrum of the liquid (Fig. 1).

6. Discussion Comparing the ab initio results, the energy differences between the syn and anti conformers are 3886, 1745 and 1829 cm-’ from the RHF/3-21G*. RHF/6-3 lG* and MP2/6-3 lG* calculations respectively. All the differences favor the syn conformer to be the more stable form. If there is a second conformer present in any appreciable amount, one would expect to observe spectral features in the fluid phases which disappear from the spectrum of the solid. No evidence could be found for any bands present in the spectra of the fluid phases but not present in either the infrared or Raman spectra of the solid. It should be noted that there are two bands at 1786 and 1750 cm-’ in the infrared spectrum of the amorphous state which are at 1784 and 1754 cm-’ after the final annealing. It should also be noted that the C=O stretching mode in both the infrared and Raman spectra shows two bands in the spectra for both the liquid and solid states. The two bands change their relative intensity during the annealing process. However, one of the bands does not disappear as would be expected if it arose from the C=O stretch of a second conformer. To further examine this band, we carried out a temperature study of the Raman spectrum of the liquid. The

relative intensities of the bands at 1752 and 1784 cm-’ at seven different temperatures ranging from 24” to -43°C were measured. There was some change in the relative intensity of the two bands but we believe it is due to the bands becoming sharper at lower temperature and there is less overlap of the lower frequency band with the higher frequency one. Because of the overlap we could not measure the areas under the two bands. The small relative intensity change would indicate a AH value of 200 cm-’ or less and if the energy difference is this small then other conformer pairs should be observed in other regions of the spectrum particularly for the skeletal modes. Therefore, we conclude that only one conformer exists in all physical states. One frequently observes multiplet bands for the carbonyl stretching mode even though the molecule does not have more than one conformer. It is believed that these additional bands are due to combination bands in Fermi resonance with the carbonyl stretching mode. Similar observations are sometimes observed for the C=C stretching mode for the same reasons. Therefore, we believe the multiplet structures for the C=O and C=C stretches for 3-methyl-2-butenoyl chloride have these origins. In order to demonstrate the utility of ab initio calculations and the presence of only one conformer, we have calculated the Raman (Fig. 6) and infrared (Fig. 7) spectra for 3-methyl-2-butenoyl chloride using the frequencies, scattering activities and intensities determined from the RHF/3-21G* 90 program [ 161 with the calculations. The GAUSSIAN option of calculating the polarizability derivatives analytically was used. The Raman scattering cross sections, a~j/an, which are proportional to the Raman intensities, can be calculated from the scattering activities and the predicted frequencies for each normal mode using the relationship [19]:

where v0 is the exciting frequency, v, is the vibrational frequency of the jth normal mode and S, is the corresponding Raman scattering activity. To obtain the polarized Raman scattering

J.R. Durig et al./Journal

of Molecular

Structure

376 (1996)

213

261-275

higher than those observed, the frequency axis of the theoretical spectrum was compressed by a factor of 0.9. The experimental spectrum of the liquid state is shown in Fig. 6A and the predicted Raman spectrum of the syn conformer is shown in Fig. 6B. The calculated spectrum is remarkably similar to the experimental spectrum and provides support for the assignment of the observed bands. Infrared intensities were calculated based on the dipole moment derivatives with respect to Cartesian coordinates. The derivatives were taken from the ab initio calculations and transformed to normal coordinates by

3000

2500

2000

1500

1000

500

0 where Qi are the ith normal coordinates, Xj are the jth Cartesian displacement coordinates, and Ljiis the transformation matrix between the Cartesian displacement coordinates and normal coordinates. The infrared intensities were then calculated by

(cm-l)

WAVENUMBER

Fig. 6. Raman spectra of 3-methyl-2-butenoyl chloride: (A) experimental Raman spectrum of the liquid; (B) calculated spectrum of the s-cis conformer.

cross-sections, the polarizabilities are incorporated into Sj by Sj[(l - pj)/(l + p,)] where p, is the depolarization ratio of the jth normal mode. The Raman scattering cross-sections and calculated frequencies are used along with the Lorentzian function to obtain the calculated spectrum. Due to the calculated frequencies being approximately 10%

I1

I1

I

I I1

3000

111

1 I1

2500

The experimental infrared spectrum of the gas is shown in Fig. 7A along with the predicted infrared spectrum of the syn conformer. There is good agreement between the two spectra which again

I I 11 1 ’ 11 11 ‘1 2000

WAVENUMBER Fig. 7. Infrared s-cis conformer.

spectra of 3-methyl-2-butenoyl

chloride:

(A) experimental

1500

loo0

11 1 ’ L 500

b?I-‘) infrared

spectrum

of the gas: (B) calculated

spectrum

of the

274

J.R. Durig et ul./Joumal u/hloleculur Structure 376 (1996) 261-275

supports the vibrational assignment provided in the present study. It should also be noted that even a very small basis set can provide excellent infrared and Raman spectral predictions. Comparing the optimized structural parameters obtained from the ab initio calculations with those of but-2-enoyl chloride which has a very similar structure, it is shown that the C-H bond distances are very similar for each molecule. The differences are within &O.OOl A. The Ci=C? distance for 3methyl-2-butenoyl chloride is longer than that for but-2-enoyl chloride by 0.010 A with each basis set. The same is true for the C5-C, or C-C1 distances which are 0.010 A larger. Also, the C-Cl distance is 0.007 A longer than that for but-2-enoyl chloride with all three basis sets. The C2-Cs and C=O distances are in good agreement Obetween the two molecules, i.e. within ho.004 A and f 0.001 A respectively. The angles of the two molecules show considerably greater variation. Comparing the structural parameters obtained at the RHF limit with the two basis sets, the differences in bond angles are less than 1”. However, the heavy atom bond distances for the same structural parameters differ significantly with the two basis sets, whereas the C-H bonds are calculated to be essentially the same. The structural parameters obtained from the MP2/6-31G* calculation are significantly different. The C-H bond distances are usually ~0.010 A longer than those calculated at the RHF level. In general, the structural parameters obtained from the MP2 calculation are in better agreement with the results obtained from the electron diffraction technique [I 51 than those obtained at the RHF level. It is especially true for the C-H bond distances but the double bond distances are too long. The vibrational assignment for this molecule is mostly based on the ab initio calculations and PED predictions. For the most part, they are consistent with the well documented group frequencies for the various parts of the molecule. Of particular interest is the C-Cl stretch which has been assigned to the band at 457 cm-’ based on the ab initio calculations. However, based on the group frequencies and the infrared band intensity, it would normally be assigned to the band at 755 cm-‘. However, instead of assigning the CC10 rock in the 400

cm--’ region, we assign this mode to the band at 755 cm-’ based on its calculated infrared intensity and the PED value with the C-Cl stretch assigned to the lower frequency band. It should be noted that many of the vibrational modes are extensively mixed. The band at 755 cm-’ has a 34% contribution from the CC10 rock, a 33% contribution from the CC2 symmetric stretch and a 14% contribution from the C-Cl stretch. The band at 457 cm-’ has a contribution of 66% C-Cl stretch, 12% CC* deformation and 13% CC2 wag, whereas the band at 429 cm-’ has 43% CC2 deformation, 16% CC10 rock and 23% CC10 deformation. The band at 377 cm-’ has a 38% contribution from the CC2 wag, 27% CC10 rock, and 11% C-C3 stretch. Therefore, the mixing is extensive and the simplified descriptions for the heavy atom vibrations is more for “bookkeeping” than to provide descriptions of the molecular motions. Neither methyl torsional mode was observed but they are predicted to be very weak in both the infrared and Raman spectra. Similarly, the asymmetric torsion is also predicted to be very weak in the infrared and Raman spectra. It has tentatively been assigned at 74 cm -’ in the infrared spectrum of the vapor, but polyethylene has a band near this frequency so it is possible that the 74 cm-’ band is due to the windows on the gas cell which are not completely compensated by the background. The remaining fundamentals have been rather confidently assigned and, in general, the ab initio predictions are very satisfactory. The results clearly show the importance of carrying out ab initio calculations when investigating conformational stabilities of these small molecules.

References [l] J.R. Durig, J.S. Church and D.A.C. Compton. J. Chem. Phys.. 71 (1979) 1175. [2] J.R. Durig, PA Brletic and J.S. Church, J. Chem. Phys., 76 (1982) 1723. [3] B.C. Laskowski. R.L. Jaffe and A. Komornicki, J. Chem Phys., 82 (1985) 5089. [4] J.R. Durig. R.J. Berry and P Groncr, J. Chem. Phys., 87 (1987) 6303. [5] J.R. Durig, A.Y. Wang, T.S. Little, P.A. Brletic and J.R. Bucenell, J. Chem. Phys., 91 (1989) 7361.

J.R. Durig et al.~Journal qf Molecular Structure 376 (1996) [6] J.R. Durig, P.A. B&tic, Y.S. Li, A Y. Wang and T.S Little, J. Mol. Struct., 223 (1990) 291. [7] J.R. Durig, A.Y. Wang, T.S. Little and P.A. B&tic. I. Chem. Phys., 93 (1990) 905. [8] J.R. Durig, A.Y. Wang, T.S. Little and P.A. B&tic, I. Phys. Chem., 95 (1991) 3569. [9] J.R. Durig. C.V. Groner, T.G. Costner and A.Y. Wang, J. Raman Spectrosc., 24 (1993) 335. [IO] H. Mack and H. Oberhammer, I. Mol. Struct . 200 (1989) 277. [Ill V. Jonas and G. Franking, Chem. Phys Lett., 177 (1991) 175. [12] M.T. Nguyen, M.R. Hajnal and L.C. Vanqwckenborne, J. Mol. Struct., 231 (199 I) 185. [13] f1. Mack and H. Oberhammer, J. Mol. Struct., 258 (1992) 197

261-275

275

[I41 R.K. Gupta, R. Prasad and H.L. Bhatnagar, Spectrochim. Acta, Part A, 45 (1989) 595. [15] T. Nordtomme and K. Hagen, J. Mol. Struct., 128 (1985) 127. [I61 M.J. Frisch, M. Head-Gordon, G.W. Trucks, J.B. Foresman, H.B. Schlegel, K. Raghavachari, M.A. Robb, J.S. Binkley, C. Gonzalez, D.J. DeFrees, D.J. Fox, R.A. Whiteside, R. Seeger, C.F. M&us, I. Baker, R.L. Martin, L.R. Kahn, J.J.P Stewart, S. Topiol and J.A Pople, OAUSSIAN90. Gaussian Inc., Pittsburgh, PA, 1990. [17] P.P. P&y, Mol. Phys., 17 (1969) 197. [IX] J.H Schactschneider, in Vibrational Analysis of Polyatomic Molecules, Parts V and VI, Tech. Report Nos. 231 and 57. Shell Development Co., Houston, TX, 1964 and 1965. [I91 G.W. Chantry, m A. Anderson (Ed.), The Raman Effect, Vol. 1, Marcel Dckker, New York, 1991.