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Conformational behaviour and vibrational spectra of methyl propionate
Volume
26. number
CIIEMICAL
2
CONFORMATIONAL
PIiYSICS
LETTERS
1.5
BEHAVIOUR AND VIBRATIONAL OF METHYL PROPIONATE
M&y 1974
SPECTRA
Rose-Marie MORAVIE and Jacques CORSET Laboraroire
de ciritnie Physique
du C.N.R.S.
et Loboratoire
Received 10 January Revised manuscript received
F.N.F.P. I’.E.I.. 9432G-Tlriais,
1974 I March
France
1974
Through the analysis of the infrared spectra of methyl propionate recorded at several temperatures, two conformers were distinguished which were due to rotation about the C-C bond between the o carbon and carbonyl group. An equilibrium state is discussed, with an energy difference of 1.1 i 0.3 kcal mole-‘, between aguuche and more stabk cis isomer, where the CHs group eclipses the carbonyl group.
1. Introduction
Our studies of the solvation properties of methyl polyacrylatcs and polymethacrylates has led us to examine in detail the vibrational spectra of model molecules: methyl acetate, propionate, isobutyrate, and pivalate. Infrared spectra had previously been investigated only for methyl forrnate and acetate between 4000 and 200 cm-l [I, 3-I. ii& intend to discuss here the structure of methyl propionate and more particularly the existence of conformers in the liquid state,
as can be inferred from the infrared spectrum of this compound in various physical states. The skeleton of the methyl propionate is given below:
[5] as well as electron diffraction measurements [6] are in agreement with these results; they seem to indicate, however, for formates and acetates, that the planes OCIO and C, OC, form an angle of 30”. Nevertheless, the microwave studies made on the lighter molecules, methyl formate [7], ethyl for-mate [S], and methyl acetate [9], show that the ester group is plane and confirm the frans structure of the methoxy group. These studies also report the values of the rotational barrier around the C2-C1 and O-C, bonds which are in agreement with the infrared torsion frequency of the methoxy group [lo, 1 l] _Recent ab initio calculations on methyl formate also confirm the stability of the rrans conformation [ 12]Several studies have also been undertaken with the use of vibrational spectroscopy, in the Y (C=O) vibration region, where the observed band splittings have been assigned to either Fermi re&mances [ i3, 141 or to
conformers
[13,15,16]. .The structure of the ester group has been established principally from the data on formates and acetates. Through ultrasonic relaxation measurements [3,4], a relatively high barrier for rotation around the C,-0 bond has been found: it amounts to 10 kcal mole-l for methyl propionate; the conformer in which the acyl group is trans to the alkoxy group is more stable by 4.9 k&l mole-l than the’other. Dipole moment :
due to rotation around the Cl-O bond Similar interpretationshave been made
from studies of bands in other regions [15, 171. The v (C=O) vibration and some bands of lower frequency have also been used to detect conformations due to
rotation around the Cz-Cf bond [B-20]. In order to establish such an equilibrium; it is necessary to assign the spectrum of these molecules in various physical states as compIetely .as possible. A preliminary study by Dirlikov et al. [2 1] showed that some bands
15 hlay 1974
CHEMICAL PHYSICS LETTERS
Volume 26, number 2
of the liquid disappear in the solid. They related this phenomenon to the presence in the liquid of confarmers due to rotation around the C2-Cl bond.
examined with a variable-temperature laboratory.
cell built in
our
3. Results and discussion 2. Experimental A detailed analysis of solvent effects and of the influence of the physical state on the infrared and Raman spectra of methyl acetate, propionate, isobutyrate and pivalate shows that (a) the observed band splittings in the v (C=O) region are due to Fermi resonances [23]: and (b) for the propionate and the eisobutyrate, but not for the acetate and the pivalate [2S, 261, some bands present in the liquid are absent in the
Our sample of methyl-iz3-propionate was supplied by Fluka, quality ‘ipurum”, and carbon disulphide was obtained from Merck. They were both dried with molecular sieves 4 A. Methyl-d3-propionate was obtained by synthesis [223. Infrared spectra were measured with a Beckman IR 12 spectrometer of films and of the pure liquid or solid sandwiched between cesium iodide windows separated by a spacer_ Crystalline
solid. These latter bands are assigned to fundamentals (table 1) on the basis of both H/D substitution on the
films were obtained by freezing the liquid to liquid nitrogen temperature and annealing. Solutions were
methoxy group and Raman intensities
[24]. We at-
Table 1 Vibrational modes related to conformation CH~CHZCOOCD~
CH3CH2COOCH3
liq.
nys. 443 448
440
(466) a) 581
Infrared
Raman
Infrared
liq. 443
Cl-p.
442 450
liq. 430
580
579 611
610 654 678
665
581
655 674
806
808
808 814
965
963
962
crys.
430
430 437 I
6 (CCO) 11
57s 584
565
514 583 1
-y(C=O) 1
6 (CCO) 2
596
640 659 768 808
938
633 771
806 810
639 659
1
-
p(CHz) 2
803 813 1
PKHz)
1
v(wo)
2
v(oco)
1
848
848
957 966
liq.
430 436
597
(744) 809
crys.
(474) (587)
671
Raman
940
938
938 }
1092 109s
1093
1093 1096 1
-1350 1364
1362
965 1075
-
1087
1092
Y(C2C3) 1687
1094
1093
1362
1363
1330 1362
(1100) 1330
-
1357
1360
a) (
.1362 1370 I
v(c&)
2 1
w (CH2)
2
w(CH2)
1
:
1 indicate a shoulder. -
.I -211
CHEhlICAL
Volume 26, number 2
PHYSICS
tribute this disappearance of bands to a conformational equilibrium between isomers 1 and 2 in the liquid state, due to rotation around the Cr-Cz bond with only the more stable isomer existing in the crystal. The isomerism due to rotation about the Cl-O bond, which is generaily prevented by the high energy difference between isomers, may occur in the presence of a bulky alkoxy group, as was recently demonstrated by NMR [35] and suggested by infrared [ 15, 161 studies on hindered formates. In fig. 1 are presented the infrared spectra of methyl propionate-kg between 400 and 700 cm-i (1) in carbon disulphide solution at different temperatures (2), in nujol solution at room and liquid nitrogen temperature, and (3) in crystalline form at liquid nitrogen temperature. In this region, we expect absorptions due to the skeletal bending vibrations 6 (C=O), y(C=O) and 6 (CCO)*, bands due to the other vibrations T(OCH~), r(CH3), +y(COC), 6 (CCC) and 6(COC)* being located between 130 and 350 cm-l, as detected in the Raman spectrum of the crystal. The bands of the liquid near 600 cm-l and the shoulder near 466 cm-r exist in the infrared and Raman spectra of both tz, and d, compounds (table 1). We assign these bands, respectively, to the y(C=O) and 6 (CCO) vibrations of the less stable isomer 2, because they disappear in the crystalline solid. The spectra of the carbon disulphide and nujol solutions are similar to those of the liquid. In the vitreous nujol matrix at liquid nitrogen temperature, isomer 1 exists almost alone with a small amount of isomer 2; although narrower, the bands of isomer 1 at 447 and 657 cm.-’ are not split as in the crystal. In the pure liquid and the carbon disulphide solution (fig. 1) when the temperature is lowered, the bands of isomer 2 decrease in intensity with regard to those of isomer 1I In fig. 2 are presented log Dr /D2 versus 1IRT for different band pairs, assigned in table 1. This is presented in order to measure the energy difference
between
the two conformers
* Group frequency notations are used here for-commodity; itrang coupling is expected @t&en skeletal bending vii& tions and between v&ence s&etching vibrations v(CC) id ‘u(C0) [37;38].
212
-.
15 hlay 1974
IT
h p.+.* _-’
---_,
_/ -----____
______-----
[28] 1
It can be noticed, within experimental error, that the slopes of all the straight lines related to a band pair are very close,.with A&values slightly lower for the
..-
LETTERS
:
0 400
I
I
600
500
cm-*
Fig. 1. Infrared spectra of methyl propionate in different physiml states: (I) CS2 soh&on. 50% V/V, cell thickness 117 p; tempeiatire: (a) 29”, @) 35”,.(c) 68°C. (II) Nujol snlutioti~ 0.62.M;cell thickness 700 P; temperature: 29°C (- - -) and -190% (--I. (III) Pure Iiquid (-----) 29”C, (- - -) -70°C; (z) crystaiIized solid Y190”C.
.
Fig- 2. Variation oflogD1/D2
~iW:
15 May 1974
CHEBIICAL PHYSICS LETTERS
Volume 26. number 2
versus (1JRT)
coefficients vary with temperature, as has been verified by the method of Hartman et al. [29] _Nevertheless we think that, because vibrational modes are close in wavenumber for the two isomers, the ratio of the absorption coefficients may be assumed temperature independent_ The energy difference thus obtained is 1.1 2 0.3 kcal mole-l. The infrared bands used by Georges et al. [ 191 to measure the energy difference between the same isomers were not correctly assigned, so it is not SWprising that we find a M value roughly ten times greater than theirs. A&a matter of fact, the bands ai 1196 and 1177 cm-’ do not disappear in the crystal and can be assigned to the v(CIO) and p(OCH3) vibrations, respectively_ Their frequencies are the same for the two isomers [24]. The 1177 cm-l band is not observed for the methyl-d3-propionate. Furthermore it can be noticed (table 2) that, whereas the rotational barrier of the methyl group (Yto the carbony1 decreases strongly from acetaldehyde to acetone to methyl acetate, the energy difference between conformers 1 and 2 due to rotation around the same C2-Cl bond is roughly the same in propionaldehyde, methylethylketone [30--341 and methylpropionate (this work). This fact is a strong indication that the energy difference is mainly due to interactions between nonbonded atoms; therefore we think that, by analogy, with similar compounds involving a carbonyl group propionaldehyde [30], methylethylketone [3 I], and propionic acid [35,36] - the more stable conformation for methylpropionate is the one where the methyl
lo3 for ~1/v2
@, 610/580,
pure methyl propionate; f, 610/580, 50% V/V cs2 solution; *, 956/970,2.5% V/V CS2 solution; 0, 1325/1356.2.5% V/v CS2 solution. pure’liquid than for the solution (band pair 610/580). Furthermore similar results can be obtained, after graphical decomposition of the bands at 6 10 and 580 cm-t, with integrated intensities. The absorption
Table 2 Comparison of methyl torsional barrier
V3 and energy difference AH between conformers by rotation around the C2 -Cl
bond
for aIdehydes, ketones and esters R = -CH3
R =
V3 (eal/mole)
ref.
-CH*-CHB Physical
ref.
H = HO(~)-Ho(l)
state.
(cal/mole) R-C-H
:: R-C-CHs
Ii 0
R-CI”kH3 II
1162+ 30
900 ilO
I391
1180
1401
1000 1030 + 130 1190+230
778 830
1411
202oi
r401
1070 c 100
307 f 50
vapor
1301 1321 1331 I341
100
liq. liq.
tiq.. vapor liq.
‘.
r311 ,I311
: IiqI
1100~300
IGl
-0.
..
.. _:
.. ..-
:
:.
I’.
._
‘. ‘. ‘_
.-
.~:
,,
.. ‘_ ..
1 ; : .._’ ,_.( 213. :.
Volume
76, number
CHEMICAL
2
group is cis to the carbonyl. liquid and in solution, should cis and gauche isomers:
PHYSICS
The equilibrium in the take place between the
OCH3
‘QH
@CH3
OCH3 1 (cis)
0 2 (gauche)
The splittings observed in the spectra of crystals of some bands in fig. 1, as well as of the bands S,(CHs) and v,(OCH,) [24] may be due to either vibrational coupling between equivalent or nonequivalent molecules or to the existence of at least two nonequivalent molecules in the unit cell of the crystal of methylpropionate which do not couple with each other. In the nujol matrix at the same temperature, the absence of splittings and the similarity of frequencies with those of the solid suggest that all the molecules in the crystal have the same conformation.
[ 111 W.C- Fateley and F.A. Miller, Spectrochim. Acta 17 (1961) 8.57. [ 12] H. Wennerstrom. S. Forsen and B. Roos, J. Phys. Chem. 76 (1972) 2430. [ 131 31. Oki and H. Nakanishi, Bull. Chem. Sot. Japan 44 (1971) 3144. [ 14 ] Pd.Oki and H. Nakanishi, Bull. Chem. Sot. Japan 44 (1971) 3197. [ 1.5 1 II. Oki and H. Nnkanishi, Bull. Chem. Sot. Japan 43 (1970) 2558. [ 161 J-S. Byrne, P.F. Jaclcson. K.J. Bforgan and N. Unwin, J. Chem. Sot. Perkin Ii 6 (1973) 845. [ 171 W.O. George, D.V. Hassid and W.F. Maddams, J. Chem. Sot. Perkin II 7 (1972) 953. [18] W.O. George, D.V. Hassid and W.F. Maddams, J. Chem. Sot. Perkin II 12 (1972) 1798. [ 191 W.O. George. D.V. Hassid and W.F. bladdams, J. Chem. Sot. [20)
[Zl] [22] 1231 1241 1251 1261
Acknowledgement
1271
The authors thank Dr. J. Guillermet and F. Fillaux for helpful discussions and Professor M.L. Josien for
1281
her encouragements.
[29] (301 [31]
[ l] H. Susi and T. ZeII. Spectrochim. Acta 19 (1963) 1933. [2] J.K. Wiishurst. J. Mol. Struct. 1 (1957) 201. [3] J. Bailey and A.M. No+ Trans. Faraday Sot. 64 (1968)
1499. [4] J. Bailey and S. Walker, J. Mol. Struct. 6 (1970) 53. [S ] V. Shanmugasundaram and M. Meyyappan, J. Indian Chem. Sot. 49 (1972) 495. [6] J.hI. O’Gorman, W. Shand Jr. and V. Schomaker,
[7j R.F.Curl,
J.Chem. Phys. 30 (1959) 1529. [8] J.M. Riveros and E.B. W&on, J. Chem. Phys. 46 (1967) 4605.. [9] G. Williams, N-L. Owen and J. Sheridan, Trans. Faraday Sot. 61(1971) 922.
[lo] T. Miyazawa,BuIl. C’hem. Sot. Japan 34 (1961) 691.
214
[32] 1331 [34] 1351 [36] 1371
J. Am..
Chem. Sot. 72 (1950) 4222.
15 May 1974
LETTERS
[38] [39]
Perkin
A.J. Bowles,
II 8 (1972)
1029.
W.O. George
and D.D. Cunliffe-Jones.
Chem. Commun. (1970) 103. S. Dirlikor, J. Stokr and B. Schneider, Collection Czech. Chem. Commun. 36 (197 1) 3028. R. Renard and L.C. Leitch, Can. J. Chem. 34 (1956) 179. R.M. Moravie and J. Corset, to be published. R.hI. Moravie and J. Corset, J. Mol. Struct., submitted for publication. R.M. Moravie and J. Corset, to be published. W.O. George, T.E. Houston and W.C. Harris, Raman Newsletters 57 (1973) 10. T. Drakenberg and S. Forsen, J. Phys. Chem. 76 (1972) 3582. S. Mizushima, T. Shimanouchi, Ii. Kuratani and T. hliyazawa, J. Am. Chem. Sot. 74 (1952) 1378. K-0. Hartman, G.L. Carlson, R-E. Witowski and W.G. Fateley. Spectrochim. Acta 24.4 (1968) 157. S.S. Butcher and E.B. Wilson, J. Chem. Phys. 40 (1964) 1671. T. Shimanouchi, Y. Abe and hl. hlikani, Spectrochim. Acta 24A (1968) 1037. R.J. Abraham and J.A. Pople, hlol. Phys. 3 (1960) 609. G. Sbrana and V. Schettino. J. Mol. Spectry. (1970) 100. S.G. Frankiss and W. Kynaston. Spectrochim. Acta 28A (1972) 2149. B.P. van Eijck, Rec. Trav. Chim. 85 (1966) 1129. J.L. Derissen, J. Mol. Struct. 7 (1971) 81. K. Toriyama and T. Shimanouchi, Intern. Symposium on Molecular Structure and Spectroscopy, A 103-l Tokyo (1962). E-M. Popov. Zh. Strukt. Khim. 12 (1971) 61. R.W. KiIb, CC. Lin and E.B. Wilson Jr., J. Chem. Phys.
26 (1957) 1695. I401 Acta 19 _ _ W.G. Fateley and F.A. Miller, Spectrochim. (1963) 389_[41] R. Nelson and L. Pierce, J. Mol. Spectry. 18 (1965) 344.
26. number
CIIEMICAL
2
CONFORMATIONAL
PIiYSICS
LETTERS
1.5
BEHAVIOUR AND VIBRATIONAL OF METHYL PROPIONATE
M&y 1974
SPECTRA
Rose-Marie MORAVIE and Jacques CORSET Laboraroire
de ciritnie Physique
du C.N.R.S.
et Loboratoire
Received 10 January Revised manuscript received
F.N.F.P. I’.E.I.. 9432G-Tlriais,
1974 I March
France
1974
Through the analysis of the infrared spectra of methyl propionate recorded at several temperatures, two conformers were distinguished which were due to rotation about the C-C bond between the o carbon and carbonyl group. An equilibrium state is discussed, with an energy difference of 1.1 i 0.3 kcal mole-‘, between aguuche and more stabk cis isomer, where the CHs group eclipses the carbonyl group.
1. Introduction
Our studies of the solvation properties of methyl polyacrylatcs and polymethacrylates has led us to examine in detail the vibrational spectra of model molecules: methyl acetate, propionate, isobutyrate, and pivalate. Infrared spectra had previously been investigated only for methyl forrnate and acetate between 4000 and 200 cm-l [I, 3-I. ii& intend to discuss here the structure of methyl propionate and more particularly the existence of conformers in the liquid state,
as can be inferred from the infrared spectrum of this compound in various physical states. The skeleton of the methyl propionate is given below:
[5] as well as electron diffraction measurements [6] are in agreement with these results; they seem to indicate, however, for formates and acetates, that the planes OCIO and C, OC, form an angle of 30”. Nevertheless, the microwave studies made on the lighter molecules, methyl formate [7], ethyl for-mate [S], and methyl acetate [9], show that the ester group is plane and confirm the frans structure of the methoxy group. These studies also report the values of the rotational barrier around the C2-C1 and O-C, bonds which are in agreement with the infrared torsion frequency of the methoxy group [lo, 1 l] _Recent ab initio calculations on methyl formate also confirm the stability of the rrans conformation [ 12]Several studies have also been undertaken with the use of vibrational spectroscopy, in the Y (C=O) vibration region, where the observed band splittings have been assigned to either Fermi re&mances [ i3, 141 or to
conformers
[13,15,16]. .The structure of the ester group has been established principally from the data on formates and acetates. Through ultrasonic relaxation measurements [3,4], a relatively high barrier for rotation around the C,-0 bond has been found: it amounts to 10 kcal mole-l for methyl propionate; the conformer in which the acyl group is trans to the alkoxy group is more stable by 4.9 k&l mole-l than the’other. Dipole moment :
due to rotation around the Cl-O bond Similar interpretationshave been made
from studies of bands in other regions [15, 171. The v (C=O) vibration and some bands of lower frequency have also been used to detect conformations due to
rotation around the Cz-Cf bond [B-20]. In order to establish such an equilibrium; it is necessary to assign the spectrum of these molecules in various physical states as compIetely .as possible. A preliminary study by Dirlikov et al. [2 1] showed that some bands
15 hlay 1974
CHEMICAL PHYSICS LETTERS
Volume 26, number 2
of the liquid disappear in the solid. They related this phenomenon to the presence in the liquid of confarmers due to rotation around the C2-Cl bond.
examined with a variable-temperature laboratory.
cell built in
our
3. Results and discussion 2. Experimental A detailed analysis of solvent effects and of the influence of the physical state on the infrared and Raman spectra of methyl acetate, propionate, isobutyrate and pivalate shows that (a) the observed band splittings in the v (C=O) region are due to Fermi resonances [23]: and (b) for the propionate and the eisobutyrate, but not for the acetate and the pivalate [2S, 261, some bands present in the liquid are absent in the
Our sample of methyl-iz3-propionate was supplied by Fluka, quality ‘ipurum”, and carbon disulphide was obtained from Merck. They were both dried with molecular sieves 4 A. Methyl-d3-propionate was obtained by synthesis [223. Infrared spectra were measured with a Beckman IR 12 spectrometer of films and of the pure liquid or solid sandwiched between cesium iodide windows separated by a spacer_ Crystalline
solid. These latter bands are assigned to fundamentals (table 1) on the basis of both H/D substitution on the
films were obtained by freezing the liquid to liquid nitrogen temperature and annealing. Solutions were
methoxy group and Raman intensities
[24]. We at-
Table 1 Vibrational modes related to conformation CH~CHZCOOCD~
CH3CH2COOCH3
liq.
nys. 443 448
440
(466) a) 581
Infrared
Raman
Infrared
liq. 443
Cl-p.
442 450
liq. 430
580
579 611
610 654 678
665
581
655 674
806
808
808 814
965
963
962
crys.
430
430 437 I
6 (CCO) 11
57s 584
565
514 583 1
-y(C=O) 1
6 (CCO) 2
596
640 659 768 808
938
633 771
806 810
639 659
1
-
p(CHz) 2
803 813 1
PKHz)
1
v(wo)
2
v(oco)
1
848
848
957 966
liq.
430 436
597
(744) 809
crys.
(474) (587)
671
Raman
940
938
938 }
1092 109s
1093
1093 1096 1
-1350 1364
1362
965 1075
-
1087
1092
Y(C2C3) 1687
1094
1093
1362
1363
1330 1362
(1100) 1330
-
1357
1360
a) (
.1362 1370 I
v(c&)
2 1
w (CH2)
2
w(CH2)
1
:
1 indicate a shoulder. -
.I -211
CHEhlICAL
Volume 26, number 2
PHYSICS
tribute this disappearance of bands to a conformational equilibrium between isomers 1 and 2 in the liquid state, due to rotation around the Cr-Cz bond with only the more stable isomer existing in the crystal. The isomerism due to rotation about the Cl-O bond, which is generaily prevented by the high energy difference between isomers, may occur in the presence of a bulky alkoxy group, as was recently demonstrated by NMR [35] and suggested by infrared [ 15, 161 studies on hindered formates. In fig. 1 are presented the infrared spectra of methyl propionate-kg between 400 and 700 cm-i (1) in carbon disulphide solution at different temperatures (2), in nujol solution at room and liquid nitrogen temperature, and (3) in crystalline form at liquid nitrogen temperature. In this region, we expect absorptions due to the skeletal bending vibrations 6 (C=O), y(C=O) and 6 (CCO)*, bands due to the other vibrations T(OCH~), r(CH3), +y(COC), 6 (CCC) and 6(COC)* being located between 130 and 350 cm-l, as detected in the Raman spectrum of the crystal. The bands of the liquid near 600 cm-l and the shoulder near 466 cm-r exist in the infrared and Raman spectra of both tz, and d, compounds (table 1). We assign these bands, respectively, to the y(C=O) and 6 (CCO) vibrations of the less stable isomer 2, because they disappear in the crystalline solid. The spectra of the carbon disulphide and nujol solutions are similar to those of the liquid. In the vitreous nujol matrix at liquid nitrogen temperature, isomer 1 exists almost alone with a small amount of isomer 2; although narrower, the bands of isomer 1 at 447 and 657 cm.-’ are not split as in the crystal. In the pure liquid and the carbon disulphide solution (fig. 1) when the temperature is lowered, the bands of isomer 2 decrease in intensity with regard to those of isomer 1I In fig. 2 are presented log Dr /D2 versus 1IRT for different band pairs, assigned in table 1. This is presented in order to measure the energy difference
between
the two conformers
* Group frequency notations are used here for-commodity; itrang coupling is expected @t&en skeletal bending vii& tions and between v&ence s&etching vibrations v(CC) id ‘u(C0) [37;38].
212
-.
15 hlay 1974
IT
h p.+.* _-’
---_,
_/ -----____
______-----
[28] 1
It can be noticed, within experimental error, that the slopes of all the straight lines related to a band pair are very close,.with A&values slightly lower for the
..-
LETTERS
:
0 400
I
I
600
500
cm-*
Fig. 1. Infrared spectra of methyl propionate in different physiml states: (I) CS2 soh&on. 50% V/V, cell thickness 117 p; tempeiatire: (a) 29”, @) 35”,.(c) 68°C. (II) Nujol snlutioti~ 0.62.M;cell thickness 700 P; temperature: 29°C (- - -) and -190% (--I. (III) Pure Iiquid (-----) 29”C, (- - -) -70°C; (z) crystaiIized solid Y190”C.
.
Fig- 2. Variation oflogD1/D2
~iW:
15 May 1974
CHEBIICAL PHYSICS LETTERS
Volume 26. number 2
versus (1JRT)
coefficients vary with temperature, as has been verified by the method of Hartman et al. [29] _Nevertheless we think that, because vibrational modes are close in wavenumber for the two isomers, the ratio of the absorption coefficients may be assumed temperature independent_ The energy difference thus obtained is 1.1 2 0.3 kcal mole-l. The infrared bands used by Georges et al. [ 191 to measure the energy difference between the same isomers were not correctly assigned, so it is not SWprising that we find a M value roughly ten times greater than theirs. A&a matter of fact, the bands ai 1196 and 1177 cm-’ do not disappear in the crystal and can be assigned to the v(CIO) and p(OCH3) vibrations, respectively_ Their frequencies are the same for the two isomers [24]. The 1177 cm-l band is not observed for the methyl-d3-propionate. Furthermore it can be noticed (table 2) that, whereas the rotational barrier of the methyl group (Yto the carbony1 decreases strongly from acetaldehyde to acetone to methyl acetate, the energy difference between conformers 1 and 2 due to rotation around the same C2-Cl bond is roughly the same in propionaldehyde, methylethylketone [30--341 and methylpropionate (this work). This fact is a strong indication that the energy difference is mainly due to interactions between nonbonded atoms; therefore we think that, by analogy, with similar compounds involving a carbonyl group propionaldehyde [30], methylethylketone [3 I], and propionic acid [35,36] - the more stable conformation for methylpropionate is the one where the methyl
lo3 for ~1/v2
@, 610/580,
pure methyl propionate; f, 610/580, 50% V/V cs2 solution; *, 956/970,2.5% V/V CS2 solution; 0, 1325/1356.2.5% V/v CS2 solution. pure’liquid than for the solution (band pair 610/580). Furthermore similar results can be obtained, after graphical decomposition of the bands at 6 10 and 580 cm-t, with integrated intensities. The absorption
Table 2 Comparison of methyl torsional barrier
V3 and energy difference AH between conformers by rotation around the C2 -Cl
bond
for aIdehydes, ketones and esters R = -CH3
R =
V3 (eal/mole)
ref.
-CH*-CHB Physical
ref.
H = HO(~)-Ho(l)
state.
(cal/mole) R-C-H
:: R-C-CHs
Ii 0
R-CI”kH3 II
1162+ 30
900 ilO
I391
1180
1401
1000 1030 + 130 1190+230
778 830
1411
202oi
r401
1070 c 100
307 f 50
vapor
1301 1321 1331 I341
100
liq. liq.
tiq.. vapor liq.
‘.
r311 ,I311
: IiqI
1100~300
IGl
-0.
..
.. _:
.. ..-
:
:.
I’.
._
‘. ‘. ‘_
.-
.~:
,,
.. ‘_ ..
1 ; : .._’ ,_.( 213. :.
Volume
76, number
CHEMICAL
2
group is cis to the carbonyl. liquid and in solution, should cis and gauche isomers:
PHYSICS
The equilibrium in the take place between the
OCH3
‘QH
@CH3
OCH3 1 (cis)
0 2 (gauche)
The splittings observed in the spectra of crystals of some bands in fig. 1, as well as of the bands S,(CHs) and v,(OCH,) [24] may be due to either vibrational coupling between equivalent or nonequivalent molecules or to the existence of at least two nonequivalent molecules in the unit cell of the crystal of methylpropionate which do not couple with each other. In the nujol matrix at the same temperature, the absence of splittings and the similarity of frequencies with those of the solid suggest that all the molecules in the crystal have the same conformation.
[ 111 W.C- Fateley and F.A. Miller, Spectrochim. Acta 17 (1961) 8.57. [ 12] H. Wennerstrom. S. Forsen and B. Roos, J. Phys. Chem. 76 (1972) 2430. [ 131 31. Oki and H. Nakanishi, Bull. Chem. Sot. Japan 44 (1971) 3144. [ 14 ] Pd.Oki and H. Nakanishi, Bull. Chem. Sot. Japan 44 (1971) 3197. [ 1.5 1 II. Oki and H. Nnkanishi, Bull. Chem. Sot. Japan 43 (1970) 2558. [ 161 J-S. Byrne, P.F. Jaclcson. K.J. Bforgan and N. Unwin, J. Chem. Sot. Perkin Ii 6 (1973) 845. [ 171 W.O. George, D.V. Hassid and W.F. Maddams, J. Chem. Sot. Perkin II 7 (1972) 953. [18] W.O. George, D.V. Hassid and W.F. Maddams, J. Chem. Sot. Perkin II 12 (1972) 1798. [ 191 W.O. George. D.V. Hassid and W.F. bladdams, J. Chem. Sot. [20)
[Zl] [22] 1231 1241 1251 1261
Acknowledgement
1271
The authors thank Dr. J. Guillermet and F. Fillaux for helpful discussions and Professor M.L. Josien for
1281
her encouragements.
[29] (301 [31]
[ l] H. Susi and T. ZeII. Spectrochim. Acta 19 (1963) 1933. [2] J.K. Wiishurst. J. Mol. Struct. 1 (1957) 201. [3] J. Bailey and A.M. No+ Trans. Faraday Sot. 64 (1968)
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214
[32] 1331 [34] 1351 [36] 1371
J. Am..
Chem. Sot. 72 (1950) 4222.
15 May 1974
LETTERS
[38] [39]
Perkin
A.J. Bowles,
II 8 (1972)
1029.
W.O. George
and D.D. Cunliffe-Jones.
Chem. Commun. (1970) 103. S. Dirlikor, J. Stokr and B. Schneider, Collection Czech. Chem. Commun. 36 (197 1) 3028. R. Renard and L.C. Leitch, Can. J. Chem. 34 (1956) 179. R.M. Moravie and J. Corset, to be published. R.hI. Moravie and J. Corset, J. Mol. Struct., submitted for publication. R.M. Moravie and J. Corset, to be published. W.O. George, T.E. Houston and W.C. Harris, Raman Newsletters 57 (1973) 10. T. Drakenberg and S. Forsen, J. Phys. Chem. 76 (1972) 3582. S. Mizushima, T. Shimanouchi, Ii. Kuratani and T. hliyazawa, J. Am. Chem. Sot. 74 (1952) 1378. K-0. Hartman, G.L. Carlson, R-E. Witowski and W.G. Fateley. Spectrochim. Acta 24.4 (1968) 157. S.S. Butcher and E.B. Wilson, J. Chem. Phys. 40 (1964) 1671. T. Shimanouchi, Y. Abe and hl. hlikani, Spectrochim. Acta 24A (1968) 1037. R.J. Abraham and J.A. Pople, hlol. Phys. 3 (1960) 609. G. Sbrana and V. Schettino. J. Mol. Spectry. (1970) 100. S.G. Frankiss and W. Kynaston. Spectrochim. Acta 28A (1972) 2149. B.P. van Eijck, Rec. Trav. Chim. 85 (1966) 1129. J.L. Derissen, J. Mol. Struct. 7 (1971) 81. K. Toriyama and T. Shimanouchi, Intern. Symposium on Molecular Structure and Spectroscopy, A 103-l Tokyo (1962). E-M. Popov. Zh. Strukt. Khim. 12 (1971) 61. R.W. KiIb, CC. Lin and E.B. Wilson Jr., J. Chem. Phys.
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