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Frequency assignment of the infrared and Raman spectra of β-methyl methylcrotonate and deuterated analogs
Spectrochimica
Acta
Vol. 33A. pp. 745 to 754. Pergamon
Press 1977. Prmted
in Great Britain
Frequency assignment of the infrared and Raman spectra of /l-methyl methylcrotonate and deuterated analogs J. M. M. DROOCand W. M. A. SHIT Analytical Chemistry Laboratory, State University of Utrecht, Croesestraat 77A, Utrecht, The Netherlands (Received 18 May 1976; recked
26 November 1976)
Abstract-The i.r. and Raman spectra of b-methyl methylcrotonate and three deuterated derivatives have been investigated. Frequency assignments have been made based on band contours in the i.r. vapour spectra, depolarization ratios of the Raman lines, comparison with similar compounds and normal coordinate calculations.
INTRODUCTION
In the course of i.r. studies in our laboratory on cis- and trans-/?,j3 dialkylsubstituted @unsaturated methylesters [l] we became interested in the frequency assignment of B-methyl methylcrotonate. Although already in 1943 KOHLRAUSCH reported Raman data of this compound [2], up till now, to our best knowledge, no assignment of the vibrational frequencies has been published. Several papers on related c&unsaturated carbonyl compounds, however, have been published in the last ten years [3-l 11. The geometry of b-methyl methylcrotonate is not known yet. Acrolein, which may be regarded as the backbone of all c&unsaturated carbonyl compounds, has been established to exist in the planar s-tram conformation, both by microwave spectroscopy [12] and electron diffraction [13, 141. Electron diffraction data for methylacrylate [ 151 and simple aliphatic esters [16] indicate that the methoxy group is twisted somewhat out of the plane of the molecule. Our Raman depolarization measurements, however, seem to indi-
cate an almost planar structure for p-methyl methylcrotonate. Hence we have assumed the compound to be predominantly in the planar s-tram form (see Fig. l), leading to C, symmetry. All 48 normal vibrations are i.r. and Raman active; 30 belong to the A’ species (in plane modes), the remaining 18 modes to the A” species (out of plane modes) [ 171. The Raman lines belonging to the A’ species are polarized whereas the A” lines are depolarized. Making use of geometry data for methylformate [ 163 and acrolein [ 141 and taking tetrahedral angles and CH distances of 1.094 A for the methyl groups, we assumed the equilibrium structure as given in Table 1. The vibrational spectra have been studied, viz.
II
I
of four
isotopic
species
JY
In
Table 1. Assumed geometry for j&methyl methylcrotonate Int. coord. r4s rsh p.57 P68 ra9
rs10 rloll = k Fig. 1. Equilibrium structure and internal coordinates /?-methyl methylcrotonate.
of 745
f-lo15
Remaining 109.5 deg.
Equil. value
Int. coord.
Equil. value
1.460 A 1.360 1.219 1.470 1.086 1.345 1.505 113.0 deg
a57 %8 a78 a69 a610 a9L0 aall = %5
124.0 deg 116.0 120.0 118.8 119.8 121.4 123.4 113.2
distances:
a1115
1.094 A;
remaining
angles:
746
J. M. M. DR~~G and W. M. A. SMIT Table 2. Symmetry coordinates for b-methyl methylcrotonate
Symmetry coordinate
Description
Species A’ S, Sz S3 S4 s5
= = = = =
6-*(2r, 4 - r24 - rw) 3-*(r14 + rz4 + r3J rb5 rs6
S6
=
r66
S7
=
rB9
S8
=
r810
S9
=
2-*(rloll
r67
+
r1015)
Slo = 2-*(r toI1 - rlol5) S,, = (12)- (2r 1112 - ri113 s12
=
6-*(r
112
&3
=
(12)-f(2rll12
+
rll13 -
-
+
rll14
rll14
r1113
-
S,, = 6-*(a,,
+
aI3
+
a23
s18 =
(12)-*(2a1314
S19 = (12)-*(a +
-
1213
a1617
+ +
-
a16L8
+
+
-
-
-
a1214
a1718
+ -
-
+
-
-
r1517 -
r1518)
rls16) +
r1518)
r151S)
a531
2c(1718
a1O16
+
r1517
a1O12
-
r1517
r1s17
2r1516
a52
a1314
-
+
r1516
a51
a1213 a1214
2rt5,6
r1516
r1114
S,, = 6-*( r1112 + r1113 + r1114 S,, = 6-*(2a,, - aI2 - al3) S,, = 6-*(2a 51 - a52 - ad
+
+
-
-
al618
alOl3 a1O17
-
-
a1617)
a1014 alOld
%O
=
(12)-*(2a1012
-
a1Ol3
-
a1O14
+
2a1016
-
0L1017 -
alOld
S2,
=
V2)-*(2a1314
-
a1213
-
a1214
-
2a1718
+
a1618
%617)
S22 = (12)-*(a 1213 + alz14 +
a1314
a1718 fj23
=
(~z,-:~;;~,““‘”
s24
=
a46
a1O13
S,, = 66*(2a 58 s26
=
2-*(a78
S2, = 6-*(2a s2!3
=
2-*(a69
S30
=
2-*(k
a57
610
-
-
+
a1012 alo
a1O14
-
+
2a1016
alo alo17 +
+ alOl7
+ alo
-
a69
+
a1018)
a761
-
a9lO)
a9lO)
S,, = 6-*(2a ,115 - asll - as14 I -
CH, rock in phase CH, as bend out of phase CH3 s bend out of phase
aloLd
a57)
-
CH,t as str CH,t s str Ct-0 str O-C str C=O str C-C str C-H str C=C str C-CH, str in phase C-CH, str out of phase CH, as str in phase CH, s str in phase CH, as str out of phase CH, s str out of phase CH;t as bend CH,t s bend CH;1. rock CH3 as bend in phase CH, s bend in phase
h15)
CH, rock out of phase C-M bend W-C bend C==O rock C-C==C bend C-H rock C-C-C bend C-C-C rock
Species A”
s38
=
ita
-
%213
+
al617
-
a1618)
S39
=
t(alo13
-
alo
-
aloL7
+
alolA
s40
=
YS
s41
=
Ys
CH,t as str CH, as str out of phase CH3 as str in phase CH3t as bend CH3t rock CH, as bend out of phase CH, rock out of phase CH, as bend in phase CH, rock in phase C=O wag C-H wag
s42
=
710
C+CH3)2
s43
=
145
S31
=
2-*@24
s32
=
&Ill3
S,, = gr
1113
-
S34 = 2-*(a,,
r34) r1114
-
r1518
+
r1517)
r1114
+
r1518
-
r1517)
-
ala)
S35
=
2-*@53
s36
=
ital
-
-
a1213
a521 -
al617
+
a1618)
S37
=
!&0l3
-
a1O14
+
a1O17
-
a1618)
s44
=
?56
s45
=
568
s46
=
?810
s47
=
2-*ho11
-
~~~~~~
S48
=
2-*(51011
+
T1015)
wak?
Ct-0 torsion O-C torsion C-C torsion C=C torsion C+CH,), torsion (E2 type) C---jCH,), torsion (A2 type)
(internal coordinates as indicated in Fig. 1; t refers to methoxy group).
Henceforth these compounds will be referred to as I, II, III and IV. For all four compounds the principal moments of inertia (IA, I, and 13 have been calculated. From these parameters the PR separations have
been calculated according to the method described by SETH-PAUL and DJJKSTRA[18,19]. The compounds are asymmetric rotors of the prolate type. Type A bands should have a normal PQR structure, type l3 bands a doublet structure. Type C
Frequency assignment of the infrared and raman spectra bands should exhibit a PQR structure with a strong Q branch and weak P and R shoulders. Since the A and B axes he in the symmetry plane, all A” modes should have C-type bands. In plane modes may have complex A/B hybrid-type bands. EXPERIMENTAL The compounds were obtained from the Laboratory of Organic Chemistry, University of Utrecht by courtesy of
Dr. L. W. van Broekhoven. The purity of the compounds was checked with GLC. No isotopic impurities could be detected with i.r. or Raman methods. The Raman spectra were recorded on a Spectra-Physics Model 700 spectrophotometer equiped with an argonion laser. Infrared spectra in the region 400&X30 cn- ’ were recorded on a Perkin-Elmer 421 grating spectrophotometer, with a spectral slit width varying between 2 and 1 cm- ’ over the spectral range. In the region SO&100 cm- 1 the i.r. spectra were run on a Perkin-Elmer 180 spectrophotometer with a resolution of 2-l cn-‘.
NORMAL COORDINATE
CALCULATIONS
Normal coordinate calculations have been carried out, based on approximate force constants, to serve as an aid in the search for the assignment. No attempt has been made to refine the initial force constants in order to reproduce all the observed fundamental frequencies. There seems to be not much sense in producing such a set of force constant values, when no definite choice of fundamental frequencies from the observed absorption bands can be made independently from the normal coordinate calculations. Moreover, even if the fundamental frequencies of all four compounds can be established, a unique assignment of the fundamental frequencies to the normal * Detailed experimental data (i.r. gas. i.r. liquid, Raman liquid) are available on request.
741
modes will be almost always impossible for such large molecules. The serious lack of data to determine uniquely the complete force field prohibits the determination of the actual form of the normal modes. This seriously hampers the assignment of fundamental modes to fundamental frequencies, especially if a considerable number of symmetry coordinates are strongly mixed over a number of normal modes, which have to be assigned to a number of neighbouring fundamental frequencies (e.g. the skeletal modes of the compounds under consideration). Nevertheless we expected the approximate calculations to be of some help in the assignment procedure. From the internal valence coordinates indicated in Fig. 1 the set of symmetry coordinates listed in Table 2 has been constructed. The force field has been obtained by transferring the appropriate force constants from reported force fields for acrolein [5], methylformate [20], methylacetate [21] and isobutene [22]. The torsions have been excluded from the secular equation. Nearly all calculated frequencies above 14OOcm-’ are in reasonable agreement with the observed ones. As was expected, the results for the lower frequencies are much poorer. Considerable changes in the forms and frequencies of many of the calculated normal modes appear on varying the force constant values within the adopted ranges. Nevertheless, viewing the resulting frequencies for all four compounds, useful trends are observed in a number of cases.
DISCUSSION
The i.r. and Raman spectra of compound I are shown in Figs. 2-5. The observed* fundamental fre-
r
P. .60
.40 20
Frequency,
cm-’
Fig. 2. Pure liquid i.r. spectrum of P-methyl methylcrotonate
(400&400 cm-‘).
J. M. M. DROOG an d W. M. A.
148
quencies have been summarized in Tables 3 and 4. Each fundamental frequency is followed by an approximate description indicating which symmetry coordinates are predominantly involved in the corresponding normal mode. Because of the uncertainty in the force constant values the extent to which a particular symmetry coordinate is involved in a normal mode can not be determined precisely. The approximate description roughly indicates the prob-
Fig. 3. Pure liquid
far-ix.
ability that a symmetry coordinate is substantially involved in the normal mode under consideration (no asterisk: high confidence level, one asterisk: medium confidence level, two asterisks: lower confidence level). The remaining observed frequencies have been collected in Table 5. They are, as far as possible, followed by binary combinations of fundamental frequencies which may be responsible for these absorption bands.
300 Frequency,
400
spectrum
SMIT
of B-methyl
200 cm-’
methylcrotonate
(5oCrlOO cm-‘).
00
60
3600
Fig. 4. Pure liquid
Raman
3200 Frequency,
spectrum
2800 cm-’
of p-methyl methylcrotonate lower line: 1.
2400
(400@2OOOcm~‘).
Upper
line: //
Frequency
assignment
of the infrared
and raman
spectra
749
80
60
I
I
db y i
1200
1600
Frequency,
Fig. 5. Pure
liquid
Raman
spectrum
800
The frequency data from the four isotopic species allow a rather definite assignment. The adopted assignment for the methoxy group is in good agreement with the one of Susr and SCHERER [20] for methylformate, while the assignment for the geminal methyl groups corresponds very well with the one of PATHAK and FLETCHER[22] for isobutene. However, there are some remarkable differences with the assignment for methyl trans-crotonate proposed by GEDRGE et al. [ll]. The consistent frequency data from the four isotopic species reported here, the close agreement with assignments for related compounds [20,22] and the good fit with the calculated CH/CD stretching frequencies indicate some misassignments to be present in the reported assignment for methyl trans-crotonate. In view of the spectral data the assignment of the 3053 cm-’ band to S, is obvious. C==O and C==C stretching modes The frequency selection for the C==0 and C==C stretching modes is straightforward. However, the calculations reveal that several symmetry coordinates are involved in these modes. It appears from the potential energy distributions resulting from the several force fields used that the contribution of the
20
cm-’
of b-methyl methylcrotonate lower line: 1.
CH/CD stretching modes
400
J
40
(2OOC1OOcm~‘).
Upper
line:
//.
“own” symmetry coordinate (viz. S5 and S,) varies from 4@50% (C==0) or 20-35x (C-c) for all four compounds. CHICD bending and rocking modes The assignment of the nine methyl bending modes and the six methyl rocking modes is not as obvious as the one of the stretching modes. The normal coordinate calculations show that many of the symmetry coordinates concerned are more or less coupled with other bending or rocking coordinates as well as with skeletal bending and stretching coordinates. The final choice given in Tables 3 and 4, shows a rather consistent picture. There is not much uncertainty about the assignment of the bending and rocking modes of the methoxy group of I and III, which is in good agreement with the one for methylformate [20]. The assignment for the deuterated species II and IV is more difficult, especially for the methyl rocking mode. Moreover, some inconsistencies arise by comparison with methylformate-d, and -d4 [20]. SUSI and SCHERER rather surprisingly assign the observed 1368 cm- ’ band of methylformate-d, to the asymmetric CD3 bending mode, while the 1371 cn-’ mode of methylformate-d, is assigned to the C-H in plane rocking mode. The assignment of the C-H in plane rocking mode proposed here for I-IV, viz. 1355, 1357,
J. M. M. DROOCand W. M. A. SMIT
750
Table
3. Assignment
of fundamental
Compound Fund.
Species A’ 3054 3035 2988 2958 2958 2924 2852 1744 1670 1460 1452 1452 1443 1385 1355 1355 1234 1193 1155 1080 1022 1015 920 735 735 515 385 352 325 171 Species A” 3025 2988 2958 1460 1452 1452 1155 1080 1080 855 463 385 243 130 (97Y n.0.’ n.0. n.0. t Gas “See b Not c Not
Compound
description”
s7 S, S13 s2
S 11 S 12 S s::s,, sg** S& s,,*, s24*, S15T
S16.
s,,,
s,,*,
s,*
s,7 s23**
S S::? S,,, S
S28**
s::. ss*,s,o* S
s:: se*,s2,** s,7* s,,** ST”* _”
s*,** ’
s,,,
s22*
s3*,
s*j*
S25.
s9,
s ,n**
s3**
s,*
, s1**
s;,**sib** s,,,
d,,
s2.5.
L%*
s,,,
s,*,
s24,
s2%5*.
s25**
s27,
SW*,
s6*
S,, S,, Ss2 S,,, S,,, S,, S,,, Ss,. S,, So, S,,, S,,, S,, s,, S45 S 46 S 47 S 48
S,, S,, S,, S,,
S,, S,,
for compounds
I
Approx
freq.
frequenciest
s5**
s,,;*
se**,
s26**
Fund.
freq.
Species A’ 3054 2984 2950 2925 2870 2260 2135 1741 1667 1453 1453 1385 1357 1357 1240 1164 1102 1079 1060 1020 971 902 874 725 700 515 382 342 312 165 Species A” 2984 2950 2260 1453 1453 1079 1079 1060 874 855 463 382 232 119 (93Y n.o.c n.0. no.
values first choice, i.r. liquid second choice. text for meaning of the asterisks. observed; value derived from combination bands. observed.
Approx.
I and II II description”
ST S’13
SII S12 S14 S,
**
S 33
S32 S31 S36
’
S
s::, S36 S 2. s35 s35,h S &2 s42,&o S44 s43 S45
4:. S 46
S47 S48
See Table
5.
Frequency
Table
assignment
4. Assignment
of fundamental
Compound Fund.
Species A 3053 3035 2957 2203 2203 2114 2072 1740 1653 1460 1443 1365 1365 1238 1190 1095 1045 1045 1030 1030 918 870 780 735 715 489 370 322 301 170 Species A” 3020 2237 2237 1460
s13
S11 S12
s::s,, s20** s*,s20.s,,. s5* S
s,,.SL6,SL7 s15,s1.53 s,o** S,% s*s*. SlO
S28,s*o,s**.se**
S IR
s22.s21
s2,. .%a**, s*4**, S221s9*
S,, S,, S,, S,,,
S,,
S,,, S,,
1020 1020 928 780 765 401 370 226 127 (72) n.o.d n.0.
s,, s,, S,,, S,, S,,, S,, S,, SdO s,,, s,, Sd4 SM S&5 S 46 S47
t Gas ‘See b Not c Not d Not
szo**
s,9*, s29*>s24**
1162
LO.
s2,**,
s,,*, s,5**
S48
spectra
for compounds Compound
description”
Sl S, S2
S& &*,
frequenciest
and raman
III
Approx.
freq.
of the infrared
sj**
Fund.
Approx.
freq.
751
III and IV IV description”
Species A’ 3055 2270 2208 2208 2135 2120 2086 1737 1655 1369 1369 1245 1175 1100 1100 1043 1043 1043 1018 977 870 789 735 690 665 489 365 308 297 157 Species A” 2250 2235 2235 (106O)b 1018 1018 929 870 779 764 404 365 226 118 (66) n.o.* n.0. n.0.
values first choice, i.r. liquid second choice. text for meaning of the asterisks. observed; calculated value. observed; value derived from combination bands. observed.
S 31
S32 S S::, S,, S 36 S St:. S4L s35.
s34
S41>
s39
S 37 S S::? S 44 S 43
s39
S45 S46
S47 &I
See Table
5.
Table
5. Combination
bands
and overtones
Compound I Obs. frequencies” G
L
R
3420 2740
1655 1278
480
2150 2070 2005 1650 1280 971
1295
1648 1275 958 878 825 810 480
826 814 483 Compound 3420
2210 2086
2740 2725
3040 2740 2200 2080
1298 828 775 475
475
2990 2920 2870 2850
1625 1280
2138 2033 1685 1612 1281
442
835 600 442 Compound 3420 2987 2925 2065
1295
2025 1555 1405 1347 1300 900 880 462 431
“G, L, R denote
(i.r. liquid values)
2 x 1720 2 x 1380; 1720 + 1022; 1660+ 1079; 1380 + 1351 1660 + 1074 1232 + 918; 2 x 1079 1720 + 353; 1232 + 850; 1151 + 918 1151 + 850; 1079 + 918;2 x 1007 918 + 738 1151 + 130 850 + 130 515 + 351 (R); 1228 - 351 (R) ‘> ‘)
;86 + (97)
II
2740 2196 2079 2038 1935 1860 1790 1545 1298 1210 823
Compound 3420 2985 2908 2880 2842
Assignment
2 x 1718 1659 + 1380 2 x 1380; 1718 + 1020; 1659 + 1079; 1380 + 1354 1238 + 964; 1162 + 1020; 2 x 1092 2 x 1060; 1380 + 695; 1354 + 725 1718 + 312; 1659 + 382; 1162 + 874 1238 + 695; 1060 + 874 1162 + 695 1092 + 695 1238 + 312; 1162 + 382; 848 + 695 1060 + 232;964 + 312 848 + 382: 1092 + 119
382 + (93); 312 + 165
III 2985 2910 2876 2846 2265 2038 1615 1285 890 847 446
2 x 1720 1720 + 1239 1720 + 1190 1645 + 1239 1645 + 1190 2113 + 170 1645 + 489; 1365 + 780 1720 + 323; 1239 + 780; 2 x 1029 1365 + 302; 1239 + 442;928 + 765;916 1435 + 170; 1239 + 371 1160 + 127; 780 + 127; 715 + 170; 2 x 442 928 - (75)? 371 + 228; 2 x 302 371 + (75)
+ 780
IV 2987 2923 2068 2030
1300
438
2 x 1717 1717 + 1245? 1717 + 1245’? 1717 + 1175? 1644 +431; 1368 + 690; 1175 + 870; 1093 + 973: 2 x 1041 1717 + 308; 1245 + 789; 1175 + 870; 2 x 1015 1245 + 308; 870 + 690 1093 + 308;973 + 431 1041 + 308; 973 + 365 (1060) + 226; 973 + 308 779 + 118 764 + 118 404 + (66); 308 + 157 365 + (66)
gas, i.r. liquid
and Raman 752
liquid
values
respectively.
Frequency assignment of the infrared and raman spectra 1365 and 1369cm-’ respectively is more consistent and strongly supported by our calculations and literature data on related compounds [2328]. Also the out-of-plane C-H wagging mode seems to be well established at 855 cm- ‘. No problems arise in the assignment of the A” bending and rocking modes of the geminal methyl groups. The agreement with the assignment for isobutene is very satisfactory. However, the assignment of the c&responding A’ modes is far more difficult due to strong couplings between several symmetry coordinates. Hence the results of PATHAK and FLETCHER [22] for the corresponding isobutene modes are less comparable with those for I. Nevertheless, the agreement for the compounds I and II is reasonable. More differences arise for the deuterated species III and IV, due to strong mixing of many symmetry coordinates. Our calculations suggest little mutual mixing of the bending symmetry coordinates of the geminal methyl groups (&s, Slg, SZ1, S,,) of compounds I and II. This is in accordance with results on other compounds containing geminal methyl groups [29-311. It may be noted that the assignments for the C-H bending modes as proposed here are in good accordance with the preliminary conclusions of VAN DER MAAS, LUTZ and VAN BROEKHOWN[~~. Skeletal
stretching
and bending
modes
Because of the considerable uncertainties in the force constants connected with the skeletal stretching and bending modes, no reliable assignment for all these modes can be given. The calculated frequencies and the way of mixing of the symmetry coordinates involved strongly depend on the particular choice of the force constant values and some of the geometry parameters. However, as indicated by the asterisks in Tables 3 and 4, the contribution of some symmetry coordinates to some of the skeletal modes seems to be rather well established. The contribution of S4 to the 1234, 1240, 1238 and 12451~~’ modes respectively is in accordance with the preliminary conclusions of VAN DER MAAS, LUTZ and VAN BROEKHOVEN and is confirmed by the results of MATZKE, CHAC~N and ANDRADE[21] for methylformate and methylacetate and by the results of SUSI and SCHERER[20] for
methylformate. Also the contribution of &., and SZ6 to the 325 resp. 312 cm-’ modes of I and II corresponds with Ref. 20 and 21. The contribution of S, to the 1015 resp. 97 1 cm-’ modes of I and II is well comparable with the results for methylacetate given by MATZKE et al. [21]. In good agreement with the results of PATHAK and FLETCHER[22] for isobutene, the C-+CH,), wagging coordinate S,, seems to be
153
involved in the 463 and 385cm-’ bands of compounds I and II, and in the 37&365cm-’ band of compounds III and IV. The reliability of the calculated forms for the skeletal modes with A” symmetry is very satisfactory due to little mixing of the symmetry coordinates. Torsional
modes
As can be seen from Tables 3 and 4 the methoxy torsions S,, and S,, have been assigned. The selected value for S,, corresponds well with the assignment of methylformate as given by Suer and SCHERER[20]. The frequency for Sd4 is about 100 cm- ’ lower as the corresponding value selected by SUSI and SCHERER. However, in view of the depolarization is no obvious other choice possible. C-C torsion is tentatively assigned
ratios there The skeletal to the not
observed values 97, 93, 72 and 66 for I-IV. These values have been derived from the adopted assignment of the combination bands (see Table 5). No band has been observed that can be assigned to the C===C torsion S,,. The geminal methyl torsions S,, are and S,, expected in the range of
210-160 cm-’ [32-341. The observed fundamental in this region, viz. 171, 165, 170 and 157 cm-‘, is primarily assigned to the lowest in plane bending mode. Weak torsional bands in the neighbourhood of the 170cm-’ band are not easily detectable. Hence no assignment for these torsions can be given. Combination
bands and overtones
The remaining absorption bands have been collected in Table 5. They are, as far as possible, interpreted in terms of overtones and binary combinations. In many cases more than one possibility can be given. Since no information is available on Fermi resonances, any definite choices can hardly be made. Also the possibility that the selection of fundamental frequencies is not fully correct can not be excluded, especially in the region of the skeletal modes. Acknowledgement-The authors express their gratitude to Dr. J. H. VANDER MAASfor recording the spectra on the P.E. 180 spectrometer and to Dr. L. W. VAN BROEKHOVEN for preparing the compounds.
REFERENCES [l] J. H. VANDERMAA~,E. TH. G. LUTZ and L. W. VANBROEKHOVEN, Spectrochim. Acta, 32A, 505 (1976). [2] K. W. F. KOHLRAUSCH, Ramanspektren, Akad. Verlag, Leipzig, p. 302-3 19 (1943). [3] J. C. D. BRANDand D. G. WILLLAMSON, Disc. Farad. Sot. 35, 184 (1963). [4] R. K. HARRI$ Spectrochim. Acta 2OA, 1129 (1964).
154
J. M. M. DR~~G and W. M. A. !&IT
[S) T. FUKUYAMA,K. KUCHITSUand Y. MORINO, Bull. Chem. Sot. Jap. 41, 3021 (1968). [6] H. H. JENSEN,F. HAGEN and S. J. CYVIN, .I. Mol. Structure 4, 51 (1969). [7] A. R. KYAZIM~VA, S. I. SLJBB~TIN and V. I. TYULIN, Opt. Spektrosk. 27, 237 (1969). [8] W. R. FEAIRHELLER, JR. and J. E. KATON, J. Mol. Structure 1, 239 (1968). [9] A. J. B~WLES,W. 0. GEORGEand D. B. C~NLIFFEJONES,J. Chem. Sot. (B) 1070 (1970). [lo] A. J. BOWLES,W. 0. .~EORGE And W. F. MADDAM~, .I. Chem. Sot. (B), 8 10 (1969). [ll] W. 0. GEORGE, D. V. HASSID, W. C. HARRIS and W. F. MADDAMS,J.C.S. Perkin 11 4, 392 (1975). [12] E. A. CHERNIAKand C. C. COSTAIN,J. Chem. Phys. 45, 104 (1966). [13] K. KUCHITSU,T. FUKUYAMAand Y. MORINO,J. Mol. Structure 1, 463 (1968). [14] M. TRAEI-~EBERG, Acta Chem. Stand. 24, 373 (1970). [lS] T. UKAJI, Bull. Chem. Sot. Jap. 32, 1275 (1959). [16] J. M. O’GORMAN, W. SHAND and V. SCHOMAKER, J. Am. Chem. Sot. 72, 4222 (1950). [17] F. A. COTTON,Chemical Applications of Group Theory, John Wiley, New York (1967). [18] W. A. SETH-PAUL and G. DIJKSTRA, Spectrochim. Acta 23A, 2861 (1967). [19] W. A. SETH-PAUL,J. Mol. Structure 3, 403 (1969). [20] H. SUSI and J. R. SCHERER,Spectrochim. Acta 25A, 1243 (1969).
[21] P. MAIZKE, 0. CHAC’ON and C. ANDRADE,J. Mot. Structure 9, 255 (1971). [22] C. M. PATHAK and W. H. FLETCHER,J. Mol. Spectrosc. 31, 32 (1969). [23] D. G. REA, Anal. Chem. 32, 1638 (1960). [24] L. J. BELLAMY,The Infrared Spectra of Complex Molecules, John Wiley, New York (1960). [25] F. C. DOLLISH,W. G. FATELEYand F. F. BENTLEY, Characteristic Raman Frequencies of Organic Corn: oounds, John Wilev. New York 11974). [26] S. SHEPPARDand D: SIMPSON, Q&t. de”. 6,21 (1952). [27] L. M. SVERDL~V,Dokl. Akad. Nauk SSSR 112, 706 (1957). [ZS] F. H. A. RUMMENS, Reck Trau. Chim. 84, 1003 (1965). [29] W. L~TTKE and S. BRUJN, Ber. Bunsenges. 71, 34 (1967). [30] J.-P. PERCHARD,M.-T. FORU. and M. L. Joam, J. Chim. Phys. 61, 632 (1964). [31] G. DELLEPIANE and J. OVEREND,Spectrochim. Acta 22A, 593 (1966). [32] W. G. FATELEYand F. A. MILLER, Spectrochim. Acta 18A, 977 (1962). [33] D. R. SMITH,B. K. MCKENNA and K. D. MOLLER, J. Chem. Phys. 45, 1904 (1966). [34] R. C. LIVINGSTON, D. M. GRANT, R. J. PUGMIRE,K. A. STRONGand R. M. BRUGGER,J. Chem. Phys. 58, 1438 (1973).
Acta
Vol. 33A. pp. 745 to 754. Pergamon
Press 1977. Prmted
in Great Britain
Frequency assignment of the infrared and Raman spectra of /l-methyl methylcrotonate and deuterated analogs J. M. M. DROOCand W. M. A. SHIT Analytical Chemistry Laboratory, State University of Utrecht, Croesestraat 77A, Utrecht, The Netherlands (Received 18 May 1976; recked
26 November 1976)
Abstract-The i.r. and Raman spectra of b-methyl methylcrotonate and three deuterated derivatives have been investigated. Frequency assignments have been made based on band contours in the i.r. vapour spectra, depolarization ratios of the Raman lines, comparison with similar compounds and normal coordinate calculations.
INTRODUCTION
In the course of i.r. studies in our laboratory on cis- and trans-/?,j3 dialkylsubstituted @unsaturated methylesters [l] we became interested in the frequency assignment of B-methyl methylcrotonate. Although already in 1943 KOHLRAUSCH reported Raman data of this compound [2], up till now, to our best knowledge, no assignment of the vibrational frequencies has been published. Several papers on related c&unsaturated carbonyl compounds, however, have been published in the last ten years [3-l 11. The geometry of b-methyl methylcrotonate is not known yet. Acrolein, which may be regarded as the backbone of all c&unsaturated carbonyl compounds, has been established to exist in the planar s-tram conformation, both by microwave spectroscopy [12] and electron diffraction [13, 141. Electron diffraction data for methylacrylate [ 151 and simple aliphatic esters [16] indicate that the methoxy group is twisted somewhat out of the plane of the molecule. Our Raman depolarization measurements, however, seem to indi-
cate an almost planar structure for p-methyl methylcrotonate. Hence we have assumed the compound to be predominantly in the planar s-tram form (see Fig. l), leading to C, symmetry. All 48 normal vibrations are i.r. and Raman active; 30 belong to the A’ species (in plane modes), the remaining 18 modes to the A” species (out of plane modes) [ 171. The Raman lines belonging to the A’ species are polarized whereas the A” lines are depolarized. Making use of geometry data for methylformate [ 163 and acrolein [ 141 and taking tetrahedral angles and CH distances of 1.094 A for the methyl groups, we assumed the equilibrium structure as given in Table 1. The vibrational spectra have been studied, viz.
II
I
of four
isotopic
species
JY
In
Table 1. Assumed geometry for j&methyl methylcrotonate Int. coord. r4s rsh p.57 P68 ra9
rs10 rloll = k Fig. 1. Equilibrium structure and internal coordinates /?-methyl methylcrotonate.
of 745
f-lo15
Remaining 109.5 deg.
Equil. value
Int. coord.
Equil. value
1.460 A 1.360 1.219 1.470 1.086 1.345 1.505 113.0 deg
a57 %8 a78 a69 a610 a9L0 aall = %5
124.0 deg 116.0 120.0 118.8 119.8 121.4 123.4 113.2
distances:
a1115
1.094 A;
remaining
angles:
746
J. M. M. DR~~G and W. M. A. SMIT Table 2. Symmetry coordinates for b-methyl methylcrotonate
Symmetry coordinate
Description
Species A’ S, Sz S3 S4 s5
= = = = =
6-*(2r, 4 - r24 - rw) 3-*(r14 + rz4 + r3J rb5 rs6
S6
=
r66
S7
=
rB9
S8
=
r810
S9
=
2-*(rloll
r67
+
r1015)
Slo = 2-*(r toI1 - rlol5) S,, = (12)- (2r 1112 - ri113 s12
=
6-*(r
112
&3
=
(12)-f(2rll12
+
rll13 -
-
+
rll14
rll14
r1113
-
S,, = 6-*(a,,
+
aI3
+
a23
s18 =
(12)-*(2a1314
S19 = (12)-*(a +
-
1213
a1617
+ +
-
a16L8
+
+
-
-
-
a1214
a1718
+ -
-
+
-
-
r1517 -
r1518)
rls16) +
r1518)
r151S)
a531
2c(1718
a1O16
+
r1517
a1O12
-
r1517
r1s17
2r1516
a52
a1314
-
+
r1516
a51
a1213 a1214
2rt5,6
r1516
r1114
S,, = 6-*( r1112 + r1113 + r1114 S,, = 6-*(2a,, - aI2 - al3) S,, = 6-*(2a 51 - a52 - ad
+
+
-
-
al618
alOl3 a1O17
-
-
a1617)
a1014 alOld
%O
=
(12)-*(2a1012
-
a1Ol3
-
a1O14
+
2a1016
-
0L1017 -
alOld
S2,
=
V2)-*(2a1314
-
a1213
-
a1214
-
2a1718
+
a1618
%617)
S22 = (12)-*(a 1213 + alz14 +
a1314
a1718 fj23
=
(~z,-:~;;~,““‘”
s24
=
a46
a1O13
S,, = 66*(2a 58 s26
=
2-*(a78
S2, = 6-*(2a s2!3
=
2-*(a69
S30
=
2-*(k
a57
610
-
-
+
a1012 alo
a1O14
-
+
2a1016
alo alo17 +
+ alOl7
+ alo
-
a69
+
a1018)
a761
-
a9lO)
a9lO)
S,, = 6-*(2a ,115 - asll - as14 I -
CH, rock in phase CH, as bend out of phase CH3 s bend out of phase
aloLd
a57)
-
CH,t as str CH,t s str Ct-0 str O-C str C=O str C-C str C-H str C=C str C-CH, str in phase C-CH, str out of phase CH, as str in phase CH, s str in phase CH, as str out of phase CH, s str out of phase CH;t as bend CH,t s bend CH;1. rock CH3 as bend in phase CH, s bend in phase
h15)
CH, rock out of phase C-M bend W-C bend C==O rock C-C==C bend C-H rock C-C-C bend C-C-C rock
Species A”
s38
=
ita
-
%213
+
al617
-
a1618)
S39
=
t(alo13
-
alo
-
aloL7
+
alolA
s40
=
YS
s41
=
Ys
CH,t as str CH, as str out of phase CH3 as str in phase CH3t as bend CH3t rock CH, as bend out of phase CH, rock out of phase CH, as bend in phase CH, rock in phase C=O wag C-H wag
s42
=
710
C+CH3)2
s43
=
145
S31
=
2-*@24
s32
=
&Ill3
S,, = gr
1113
-
S34 = 2-*(a,,
r34) r1114
-
r1518
+
r1517)
r1114
+
r1518
-
r1517)
-
ala)
S35
=
2-*@53
s36
=
ital
-
-
a1213
a521 -
al617
+
a1618)
S37
=
!&0l3
-
a1O14
+
a1O17
-
a1618)
s44
=
?56
s45
=
568
s46
=
?810
s47
=
2-*ho11
-
~~~~~~
S48
=
2-*(51011
+
T1015)
wak?
Ct-0 torsion O-C torsion C-C torsion C=C torsion C+CH,), torsion (E2 type) C---jCH,), torsion (A2 type)
(internal coordinates as indicated in Fig. 1; t refers to methoxy group).
Henceforth these compounds will be referred to as I, II, III and IV. For all four compounds the principal moments of inertia (IA, I, and 13 have been calculated. From these parameters the PR separations have
been calculated according to the method described by SETH-PAUL and DJJKSTRA[18,19]. The compounds are asymmetric rotors of the prolate type. Type A bands should have a normal PQR structure, type l3 bands a doublet structure. Type C
Frequency assignment of the infrared and raman spectra bands should exhibit a PQR structure with a strong Q branch and weak P and R shoulders. Since the A and B axes he in the symmetry plane, all A” modes should have C-type bands. In plane modes may have complex A/B hybrid-type bands. EXPERIMENTAL The compounds were obtained from the Laboratory of Organic Chemistry, University of Utrecht by courtesy of
Dr. L. W. van Broekhoven. The purity of the compounds was checked with GLC. No isotopic impurities could be detected with i.r. or Raman methods. The Raman spectra were recorded on a Spectra-Physics Model 700 spectrophotometer equiped with an argonion laser. Infrared spectra in the region 400&X30 cn- ’ were recorded on a Perkin-Elmer 421 grating spectrophotometer, with a spectral slit width varying between 2 and 1 cm- ’ over the spectral range. In the region SO&100 cm- 1 the i.r. spectra were run on a Perkin-Elmer 180 spectrophotometer with a resolution of 2-l cn-‘.
NORMAL COORDINATE
CALCULATIONS
Normal coordinate calculations have been carried out, based on approximate force constants, to serve as an aid in the search for the assignment. No attempt has been made to refine the initial force constants in order to reproduce all the observed fundamental frequencies. There seems to be not much sense in producing such a set of force constant values, when no definite choice of fundamental frequencies from the observed absorption bands can be made independently from the normal coordinate calculations. Moreover, even if the fundamental frequencies of all four compounds can be established, a unique assignment of the fundamental frequencies to the normal * Detailed experimental data (i.r. gas. i.r. liquid, Raman liquid) are available on request.
741
modes will be almost always impossible for such large molecules. The serious lack of data to determine uniquely the complete force field prohibits the determination of the actual form of the normal modes. This seriously hampers the assignment of fundamental modes to fundamental frequencies, especially if a considerable number of symmetry coordinates are strongly mixed over a number of normal modes, which have to be assigned to a number of neighbouring fundamental frequencies (e.g. the skeletal modes of the compounds under consideration). Nevertheless we expected the approximate calculations to be of some help in the assignment procedure. From the internal valence coordinates indicated in Fig. 1 the set of symmetry coordinates listed in Table 2 has been constructed. The force field has been obtained by transferring the appropriate force constants from reported force fields for acrolein [5], methylformate [20], methylacetate [21] and isobutene [22]. The torsions have been excluded from the secular equation. Nearly all calculated frequencies above 14OOcm-’ are in reasonable agreement with the observed ones. As was expected, the results for the lower frequencies are much poorer. Considerable changes in the forms and frequencies of many of the calculated normal modes appear on varying the force constant values within the adopted ranges. Nevertheless, viewing the resulting frequencies for all four compounds, useful trends are observed in a number of cases.
DISCUSSION
The i.r. and Raman spectra of compound I are shown in Figs. 2-5. The observed* fundamental fre-
r
P. .60
.40 20
Frequency,
cm-’
Fig. 2. Pure liquid i.r. spectrum of P-methyl methylcrotonate
(400&400 cm-‘).
J. M. M. DROOG an d W. M. A.
148
quencies have been summarized in Tables 3 and 4. Each fundamental frequency is followed by an approximate description indicating which symmetry coordinates are predominantly involved in the corresponding normal mode. Because of the uncertainty in the force constant values the extent to which a particular symmetry coordinate is involved in a normal mode can not be determined precisely. The approximate description roughly indicates the prob-
Fig. 3. Pure liquid
far-ix.
ability that a symmetry coordinate is substantially involved in the normal mode under consideration (no asterisk: high confidence level, one asterisk: medium confidence level, two asterisks: lower confidence level). The remaining observed frequencies have been collected in Table 5. They are, as far as possible, followed by binary combinations of fundamental frequencies which may be responsible for these absorption bands.
300 Frequency,
400
spectrum
SMIT
of B-methyl
200 cm-’
methylcrotonate
(5oCrlOO cm-‘).
00
60
3600
Fig. 4. Pure liquid
Raman
3200 Frequency,
spectrum
2800 cm-’
of p-methyl methylcrotonate lower line: 1.
2400
(400@2OOOcm~‘).
Upper
line: //
Frequency
assignment
of the infrared
and raman
spectra
749
80
60
I
I
db y i
1200
1600
Frequency,
Fig. 5. Pure
liquid
Raman
spectrum
800
The frequency data from the four isotopic species allow a rather definite assignment. The adopted assignment for the methoxy group is in good agreement with the one of Susr and SCHERER [20] for methylformate, while the assignment for the geminal methyl groups corresponds very well with the one of PATHAK and FLETCHER[22] for isobutene. However, there are some remarkable differences with the assignment for methyl trans-crotonate proposed by GEDRGE et al. [ll]. The consistent frequency data from the four isotopic species reported here, the close agreement with assignments for related compounds [20,22] and the good fit with the calculated CH/CD stretching frequencies indicate some misassignments to be present in the reported assignment for methyl trans-crotonate. In view of the spectral data the assignment of the 3053 cm-’ band to S, is obvious. C==O and C==C stretching modes The frequency selection for the C==0 and C==C stretching modes is straightforward. However, the calculations reveal that several symmetry coordinates are involved in these modes. It appears from the potential energy distributions resulting from the several force fields used that the contribution of the
20
cm-’
of b-methyl methylcrotonate lower line: 1.
CH/CD stretching modes
400
J
40
(2OOC1OOcm~‘).
Upper
line:
//.
“own” symmetry coordinate (viz. S5 and S,) varies from 4@50% (C==0) or 20-35x (C-c) for all four compounds. CHICD bending and rocking modes The assignment of the nine methyl bending modes and the six methyl rocking modes is not as obvious as the one of the stretching modes. The normal coordinate calculations show that many of the symmetry coordinates concerned are more or less coupled with other bending or rocking coordinates as well as with skeletal bending and stretching coordinates. The final choice given in Tables 3 and 4, shows a rather consistent picture. There is not much uncertainty about the assignment of the bending and rocking modes of the methoxy group of I and III, which is in good agreement with the one for methylformate [20]. The assignment for the deuterated species II and IV is more difficult, especially for the methyl rocking mode. Moreover, some inconsistencies arise by comparison with methylformate-d, and -d4 [20]. SUSI and SCHERER rather surprisingly assign the observed 1368 cm- ’ band of methylformate-d, to the asymmetric CD3 bending mode, while the 1371 cn-’ mode of methylformate-d, is assigned to the C-H in plane rocking mode. The assignment of the C-H in plane rocking mode proposed here for I-IV, viz. 1355, 1357,
J. M. M. DROOCand W. M. A. SMIT
750
Table
3. Assignment
of fundamental
Compound Fund.
Species A’ 3054 3035 2988 2958 2958 2924 2852 1744 1670 1460 1452 1452 1443 1385 1355 1355 1234 1193 1155 1080 1022 1015 920 735 735 515 385 352 325 171 Species A” 3025 2988 2958 1460 1452 1452 1155 1080 1080 855 463 385 243 130 (97Y n.0.’ n.0. n.0. t Gas “See b Not c Not
Compound
description”
s7 S, S13 s2
S 11 S 12 S s::s,, sg** S& s,,*, s24*, S15T
S16.
s,,,
s,,*,
s,*
s,7 s23**
S S::? S,,, S
S28**
s::. ss*,s,o* S
s:: se*,s2,** s,7* s,,** ST”* _”
s*,** ’
s,,,
s22*
s3*,
s*j*
S25.
s9,
s ,n**
s3**
s,*
, s1**
s;,**sib** s,,,
d,,
s2.5.
L%*
s,,,
s,*,
s24,
s2%5*.
s25**
s27,
SW*,
s6*
S,, S,, Ss2 S,,, S,,, S,, S,,, Ss,. S,, So, S,,, S,,, S,, s,, S45 S 46 S 47 S 48
S,, S,, S,, S,,
S,, S,,
for compounds
I
Approx
freq.
frequenciest
s5**
s,,;*
se**,
s26**
Fund.
freq.
Species A’ 3054 2984 2950 2925 2870 2260 2135 1741 1667 1453 1453 1385 1357 1357 1240 1164 1102 1079 1060 1020 971 902 874 725 700 515 382 342 312 165 Species A” 2984 2950 2260 1453 1453 1079 1079 1060 874 855 463 382 232 119 (93Y n.o.c n.0. no.
values first choice, i.r. liquid second choice. text for meaning of the asterisks. observed; value derived from combination bands. observed.
Approx.
I and II II description”
ST S’13
SII S12 S14 S,
**
S 33
S32 S31 S36
’
S
s::, S36 S 2. s35 s35,h S &2 s42,&o S44 s43 S45
4:. S 46
S47 S48
See Table
5.
Frequency
Table
assignment
4. Assignment
of fundamental
Compound Fund.
Species A 3053 3035 2957 2203 2203 2114 2072 1740 1653 1460 1443 1365 1365 1238 1190 1095 1045 1045 1030 1030 918 870 780 735 715 489 370 322 301 170 Species A” 3020 2237 2237 1460
s13
S11 S12
s::s,, s20** s*,s20.s,,. s5* S
s,,.SL6,SL7 s15,s1.53 s,o** S,% s*s*. SlO
S28,s*o,s**.se**
S IR
s22.s21
s2,. .%a**, s*4**, S221s9*
S,, S,, S,, S,,,
S,,
S,,, S,,
1020 1020 928 780 765 401 370 226 127 (72) n.o.d n.0.
s,, s,, S,,, S,, S,,, S,, S,, SdO s,,, s,, Sd4 SM S&5 S 46 S47
t Gas ‘See b Not c Not d Not
szo**
s,9*, s29*>s24**
1162
LO.
s2,**,
s,,*, s,5**
S48
spectra
for compounds Compound
description”
Sl S, S2
S& &*,
frequenciest
and raman
III
Approx.
freq.
of the infrared
sj**
Fund.
Approx.
freq.
751
III and IV IV description”
Species A’ 3055 2270 2208 2208 2135 2120 2086 1737 1655 1369 1369 1245 1175 1100 1100 1043 1043 1043 1018 977 870 789 735 690 665 489 365 308 297 157 Species A” 2250 2235 2235 (106O)b 1018 1018 929 870 779 764 404 365 226 118 (66) n.o.* n.0. n.0.
values first choice, i.r. liquid second choice. text for meaning of the asterisks. observed; calculated value. observed; value derived from combination bands. observed.
S 31
S32 S S::, S,, S 36 S St:. S4L s35.
s34
S41>
s39
S 37 S S::? S 44 S 43
s39
S45 S46
S47 &I
See Table
5.
Table
5. Combination
bands
and overtones
Compound I Obs. frequencies” G
L
R
3420 2740
1655 1278
480
2150 2070 2005 1650 1280 971
1295
1648 1275 958 878 825 810 480
826 814 483 Compound 3420
2210 2086
2740 2725
3040 2740 2200 2080
1298 828 775 475
475
2990 2920 2870 2850
1625 1280
2138 2033 1685 1612 1281
442
835 600 442 Compound 3420 2987 2925 2065
1295
2025 1555 1405 1347 1300 900 880 462 431
“G, L, R denote
(i.r. liquid values)
2 x 1720 2 x 1380; 1720 + 1022; 1660+ 1079; 1380 + 1351 1660 + 1074 1232 + 918; 2 x 1079 1720 + 353; 1232 + 850; 1151 + 918 1151 + 850; 1079 + 918;2 x 1007 918 + 738 1151 + 130 850 + 130 515 + 351 (R); 1228 - 351 (R) ‘> ‘)
;86 + (97)
II
2740 2196 2079 2038 1935 1860 1790 1545 1298 1210 823
Compound 3420 2985 2908 2880 2842
Assignment
2 x 1718 1659 + 1380 2 x 1380; 1718 + 1020; 1659 + 1079; 1380 + 1354 1238 + 964; 1162 + 1020; 2 x 1092 2 x 1060; 1380 + 695; 1354 + 725 1718 + 312; 1659 + 382; 1162 + 874 1238 + 695; 1060 + 874 1162 + 695 1092 + 695 1238 + 312; 1162 + 382; 848 + 695 1060 + 232;964 + 312 848 + 382: 1092 + 119
382 + (93); 312 + 165
III 2985 2910 2876 2846 2265 2038 1615 1285 890 847 446
2 x 1720 1720 + 1239 1720 + 1190 1645 + 1239 1645 + 1190 2113 + 170 1645 + 489; 1365 + 780 1720 + 323; 1239 + 780; 2 x 1029 1365 + 302; 1239 + 442;928 + 765;916 1435 + 170; 1239 + 371 1160 + 127; 780 + 127; 715 + 170; 2 x 442 928 - (75)? 371 + 228; 2 x 302 371 + (75)
+ 780
IV 2987 2923 2068 2030
1300
438
2 x 1717 1717 + 1245? 1717 + 1245’? 1717 + 1175? 1644 +431; 1368 + 690; 1175 + 870; 1093 + 973: 2 x 1041 1717 + 308; 1245 + 789; 1175 + 870; 2 x 1015 1245 + 308; 870 + 690 1093 + 308;973 + 431 1041 + 308; 973 + 365 (1060) + 226; 973 + 308 779 + 118 764 + 118 404 + (66); 308 + 157 365 + (66)
gas, i.r. liquid
and Raman 752
liquid
values
respectively.
Frequency assignment of the infrared and raman spectra 1365 and 1369cm-’ respectively is more consistent and strongly supported by our calculations and literature data on related compounds [2328]. Also the out-of-plane C-H wagging mode seems to be well established at 855 cm- ‘. No problems arise in the assignment of the A” bending and rocking modes of the geminal methyl groups. The agreement with the assignment for isobutene is very satisfactory. However, the assignment of the c&responding A’ modes is far more difficult due to strong couplings between several symmetry coordinates. Hence the results of PATHAK and FLETCHER [22] for the corresponding isobutene modes are less comparable with those for I. Nevertheless, the agreement for the compounds I and II is reasonable. More differences arise for the deuterated species III and IV, due to strong mixing of many symmetry coordinates. Our calculations suggest little mutual mixing of the bending symmetry coordinates of the geminal methyl groups (&s, Slg, SZ1, S,,) of compounds I and II. This is in accordance with results on other compounds containing geminal methyl groups [29-311. It may be noted that the assignments for the C-H bending modes as proposed here are in good accordance with the preliminary conclusions of VAN DER MAAS, LUTZ and VAN BROEKHOWN[~~. Skeletal
stretching
and bending
modes
Because of the considerable uncertainties in the force constants connected with the skeletal stretching and bending modes, no reliable assignment for all these modes can be given. The calculated frequencies and the way of mixing of the symmetry coordinates involved strongly depend on the particular choice of the force constant values and some of the geometry parameters. However, as indicated by the asterisks in Tables 3 and 4, the contribution of some symmetry coordinates to some of the skeletal modes seems to be rather well established. The contribution of S4 to the 1234, 1240, 1238 and 12451~~’ modes respectively is in accordance with the preliminary conclusions of VAN DER MAAS, LUTZ and VAN BROEKHOVEN and is confirmed by the results of MATZKE, CHAC~N and ANDRADE[21] for methylformate and methylacetate and by the results of SUSI and SCHERER[20] for
methylformate. Also the contribution of &., and SZ6 to the 325 resp. 312 cm-’ modes of I and II corresponds with Ref. 20 and 21. The contribution of S, to the 1015 resp. 97 1 cm-’ modes of I and II is well comparable with the results for methylacetate given by MATZKE et al. [21]. In good agreement with the results of PATHAK and FLETCHER[22] for isobutene, the C-+CH,), wagging coordinate S,, seems to be
153
involved in the 463 and 385cm-’ bands of compounds I and II, and in the 37&365cm-’ band of compounds III and IV. The reliability of the calculated forms for the skeletal modes with A” symmetry is very satisfactory due to little mixing of the symmetry coordinates. Torsional
modes
As can be seen from Tables 3 and 4 the methoxy torsions S,, and S,, have been assigned. The selected value for S,, corresponds well with the assignment of methylformate as given by Suer and SCHERER[20]. The frequency for Sd4 is about 100 cm- ’ lower as the corresponding value selected by SUSI and SCHERER. However, in view of the depolarization is no obvious other choice possible. C-C torsion is tentatively assigned
ratios there The skeletal to the not
observed values 97, 93, 72 and 66 for I-IV. These values have been derived from the adopted assignment of the combination bands (see Table 5). No band has been observed that can be assigned to the C===C torsion S,,. The geminal methyl torsions S,, are and S,, expected in the range of
210-160 cm-’ [32-341. The observed fundamental in this region, viz. 171, 165, 170 and 157 cm-‘, is primarily assigned to the lowest in plane bending mode. Weak torsional bands in the neighbourhood of the 170cm-’ band are not easily detectable. Hence no assignment for these torsions can be given. Combination
bands and overtones
The remaining absorption bands have been collected in Table 5. They are, as far as possible, interpreted in terms of overtones and binary combinations. In many cases more than one possibility can be given. Since no information is available on Fermi resonances, any definite choices can hardly be made. Also the possibility that the selection of fundamental frequencies is not fully correct can not be excluded, especially in the region of the skeletal modes. Acknowledgement-The authors express their gratitude to Dr. J. H. VANDER MAASfor recording the spectra on the P.E. 180 spectrometer and to Dr. L. W. VAN BROEKHOVEN for preparing the compounds.
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154
J. M. M. DR~~G and W. M. A. !&IT
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