J . Phys. Chem. 1987, 91, 3977-3981
20
:n'
01 1
1600
be somewhat higher for the intramolecular than for the intermolecular hydrogen bonds. Infrared Spectrum in the 1600-700-cm-' Region. The relative weakness of the hydrogen bond in the studied compound is confirmed by the absence of any continuum or Evans holes in the fingerprint region. As shown in Figure 7, the bands at 1340, 1265, and 1227 cm-' disappear on deuteriation and are replaced by bands at 1175, 1164, 1100, and 1023 cm-I. These absorptions are assigned to 6NH+or 6ND+ motions coupled with other vibrational modes; the band at 935 cm-' appearing at 770 cm-' in the deuteriated analogue is likely a YNH+ mode. These bands are not very sensitive to solvent effect and are observed at 1340, 1260, and 1220 cm-' (DCNMBH) or 1170 and 1035 cm-' (DCNMBD) in CS,, and at 1267 and 1227 cm-' (DCNMBH) or 1165 and 1035 cm-] (DCNMBD) in CHC13. The submaxima observed in the vNH+ region are ascribed to a Fermi interaction with the overtones of the 1265 (2530) and 1223 (2446) cm-I levels. In the deuteriated compound, the vNC+ vibration probably interacts with a binary combination such as 1100 + 770 (1870) cm-'. Owing to the small integrated intensities of these secondary absorptions, the frequencies of the vNH+(D+) bands have not been corrected for the Fermi resonance, contrarily to the trichloro or tetrachloro-Mannich bases where a very strong vOH 726OH 2YOH interaction was d e t e ~ t e d . ~Owing . ~ to the repulsion between the fundamental and overtone levels, the real values of vNH+ can be somewhat lower and those of vND+ somewhat higher; as a consequence, the values of isotopic ratios listed in Table I1 represent the maximal values, but this does not change seriously the conclusions of the present work.
I
I
500
I
'LOO
1300
'200
1
, ilOG
1000
900
800
3977
700
Figure 7. Infrared spectra in the 1600-700-cm-' region: -, DCNMB"; - - - ,DCNMBD. 1600-1400 cm-I: S = CH2Br2,c D c N M B ~ = 0.02 M, c"MBD = 0.025 M. 1400-1000 cm-I: S = CC&,c D c N M B ~ = 0.013 M. c"MBD = 0.015 M. 1000-700 cm-I: S = CS2, tDCNMBH = 0.02 M. c x N M B D = 0.01 M. d = 0.1 cm. Hatched area denotes absorption of
the solvent. increase of the barrier height for the proton motion; when 6 varies from 5 to 20°, the height of the barrier increases from 8.5 to about 12 kcal/mo126 and as a consequence an increase of the isotopic ratio is to be expe~ted.~' It is also noteworthy that a plot of AvOH vs. ApKa obtained for numerous Mannich bases indicates that the inversion region in which the proton-transfer equilibrium takes place is shifted to somewhat higher ApK, values as compared with the intermolecular systems.' Near the inversion region, the proton potential has a single minimum or a double minimum with a low barrier. This also suggests, like the isotopic ratios, that the barrier height must
-
Acknowledgment. This work was done in the frame of the collaboration between the Universities of Leuven and Wroclaw. We thank the Catholic University of Leuven and the National Fund of Belgium for support. M.R. acknowledges the University of Leuven for a postdoctoral fellowship.
(26) Tayyari, S.F. Ph.D. Thesis, Imperial College of Technology, London, 1977. (27) Laane, J. J. Chem. Phys. 1971,55, 2514.
Molecular Structure, Composition, and Energy and Entropy Differences between Conformers of Proplonyl Bromide As Determined by Gas-Phase Electron Diffraction Svenn Joar Skjerrholm and Kolbjerrn Hagen* Department of Chemistry, AVH, University of TroQdheim. N- 7000 Trondheim, Norway (Received: March 2, 1987)
The molecular structure of propionyl bromide (CH3CH2COBr)has been investigated by gas-phase electron diffraction at 293, 373, and 473 K. Two conformers were identified, a more stable form with the CH3 group syn to the carbonyl oxygen (C-C-C=O torsional angle 4 = 0') and a less stable gauche form (4 = 101 (9)'). The mole fraction of the syn form, with uncertainties estimated at 2a, was found to be 0.71 (lo), 0.58 (13), and 0.51 (16) at 293, 373, and 473 K, respectively, and correspond to AE' = Eog- E', = 5.5 ( u = 2.5) kJ mol-' and LS' = So, - So, = 6 ( g = 7) J mol-' K-I. No temperature dependence of the distances and angles was seen. The values, with estimated 2u uncertainties, of the principal distances (ra) and angles (L,) for a "best" model comprising averages of the results from the three temperatures are r(C-H) = 1.109 (9) A, r(C=O) = 1.182 ( 5 ) A, r(CI-C2) = 1.522 (9) A, r(C2-C,) = 1.524 (11) A, r(C-Br) = 1.979 (5) A, LC2-Cl=0 = 126.6 (6)O, LC2-Cl-Br = 112.5 (4)', and LC1-Cz-C3 = 111.8 (9)'.
Introduction A series of investigations in this laboratory has been concerned with the gas-phase structures of molecules containing a carbonyl group like acid halides and aldehydes. An aspect of major interest in these investigations has been the possible existence, through torsion about a carbon-carbon single bond, of more than one rotational isomer. In acid chlorides, XH2CCOC1, (X = C1, Br, CH3)1-3a mixture of two conformers has been observed, a low(1) Steinnes, 0.; Shen, Q.;Hagen, K. J . Mol. Struct. 1980, 64, 217. (2) Steinnes, 0.;Shen, Q.;Hagen, K. J . Mol. Struct. 1980, 66, 181.
0022-3654/87/2091-3977$01.50/0
energy form with X syn to the carbonyl oxygen (O=C-C-X torsion angle 4 = O'), and a higher energy gauche form with 4 = 120'. The same conformers were also found for propanal (CH3CH2COH)4,5while an electron diffraction (ED) experiment only showed the presence of a slightly nonplanar anti form for chloroacetaldehyde (C1H2CCOH).6 However, a microwave (3) Dyngeseth, S.;Schei, S.H., Hagen, K. J . Mol. Struct. 1984, 114, 257. (4) Skjerrholm, S. J.; Hagen, K. J . Mol. Strucr. 1987, 154, 155. (5) Van Nuffel, P.; Van den Enden, L.; Van Alsenoy, C.; Geise, H. J . J . Mol. Srruct. 1984, 114, 99. ( 6 ) Dyngeseth, S.; Schei, H.; Hagen, K. J . Mol. Struct. 1983, 102, 45.
0 1987 American Chemical Society
3978 The Journal of Physical Chemistry, Vol. 91, No. 15, 1987
Skjorholm and Hagen
m
_ _
Lrv---vT~, , I
5
0
Figure 1. Molecular model of the syn (6 = 0') conformer of propionyl bromide showing the atomic numbering.
spectroscopy investigation' of chloroacetaldehyde also identified a second conformer with C1 syn to the carbonyl oxygen. In propionyl fluoride* (CH3CH2COF)both the syn and the gauche conformers were again observed, while syn and anti forms were found in fluoroacetyl fluoride (FCH2COF).9 As part of our studies on the conformational effects of different substituents in molecules like these, we were interested in also studying the gas-phase structures of some acid bromides (CH2XCOBr). We have earlier published the results for bromoacetyl bromide (X = Br)Zand we report here our results for propionyl bromide (X = CH,), Figure 1. Spectroscopic investigations'Ogll have shown that propionyl bromide is a mixture of two conformers in gaseous and liquid states. These conformers were assumed to have the methyl group either syn or gauche to the carbonyl oxygen, but no accurate values are known for the torsion angles in these conformers. Values for other geometrical parameters have not been published. Experimental Section and Data Reduction A commercial sample of propionyl bromide was obtained from Merck Schuchardt. The sample purity was shown by G C to be better than 98%. Diffraction patterns were recorded with the Balzers Eldigraph KDG-2 at the University of O ~ l o ' ~on, ' Kodak ~ electron image plates with nozzle tip temperatures of 293, 373, and 473 K. The voltage/distance calibration was done with benzene as reference. The nozzle-to-plate distances were 497.92 and 248.10 mm for the long and the short camera experiments. Six plates from each of the two camera distances were used at 293 K; at 373 K four plates from the long and five plates from the short camera distances were used; and at 473 K the number of plates used at the two camera distances was three and two. The small number of plates selected at the highest temperature was due to problems with sample decomposition at this temperature. The optical densities were measured with a Joyce Loeble microdensitometer. The data were reduced in the usual way,I4-l6 and a calculated background" was subtracted from the data for each plate to yield experimental molecular intensity curves in the The average experimental intensity curves for the form sZ,(s). 293 K experiment are shown in Figure 2. Corresponding figures for the other two temperatures as well as all the intensity and (7) Ford, R. G. J . Chem. Phys. 1976, 65, 345. (8) Stiefvater, 0. L.; Bright Wilson, E. J . Chem. Phys. 1969, 50, 5385. (9) Saegebarth, E.; Bright Wilson, E. J . Chem. Phys. 1967, 46, 3088. (10) Katon, J. E.; Feairheller, Jr., W. R. J . Chem. Phys. 1966, 44, 144. (1 1) Frankiss, S . G.; Kynaston, W. Spectrochim. Acta, Part A 1975, 3ZA, 661. (12) Zeil, W.; Haase, J.; Wegmann, L.Z . Instrumentenkd. 1966, 74, 84. (1 3) Bastiansen, O., Graber, R.; Wegmann, L. Balzers High Vakuum Rep. 1969, 25, 1. (14) Hagen, K.; Hedberg, K. J . Am. Chem. SOC.1973, 95, 1003. (15) Gundersen, G.; Hedberg, K. J . Chem. Phys. 1969, 51, 2500. (16) Andersen, B.; Seip, H. M.; Strand, T. G.; Stdevik, R. Acta Chem. Scand. 1969, 23, 3224. (17) Hedberg, L. Abstracts of the Fifth Austin Symposium on Gas Phase Molecular Structure, Austin, T X , March, 1974; p 37.
15
10
,
-
-
-
- .
,
I
20
,
I
s A-'
I ,
,
i
25
Figure 2. Average experimental intensity curves for propionyl bromide at 293 K shown together with the theoretical curve calculated from the parameter values of Tables I and 11. The difference curve is experimental minus theoretical.
I
1
2
3
4 r,A
5
Figure 3. Radial distribution curves. The vertical lines indicate the most important interatomic distances; their lengths are proportional to the weights of the distances. The difference curves are experimental minus theoretical.
background data are available as supplementary material. (See paragraph at end of text regarding supplementary material.) Radial distribution (RD) curves (Figure 3) were calculated in the usual way by Fourier transformation of functions Zm'(s) = ZcZc&' Ac;' sZ,(s) exp(-Bs2) with B = 0.0025 A-z. Scattering amplitudes and phases for all calculations were obtained from tables.'* Structure Analysis The presence of more than one conformer is revealed by the temperature dependence of the outer part of the experimental RD curves in Figure 3. The area under the peak at about 4.2 8, is seen to decrease as the temperature increases while the area of the feature in the region 3.2-4.0 A increases. Theoretical R D (18) Shafer, L.; Yates, A. C.; Bonham, R. A. J . Chem. Phys. 1971, 56, 3056.
The Journal of Physical Chemistry, Vol. 91, No. 15. 1987 3979
Conformers of Propionyl Bromide
TABLE I: Values of Parameters Used To Define the Structure of Propionyl Bromide“ EX P.
r(C-H) r(C=O) (r(C-C)) Ar(C-C) r(C-Br) LC-C=O LC-C-Br LC-C-C LC,-C2-H6 LH,-Cz-H, LC,-C,-H
THEO. A ~
B
@IC
C
9Zd TSe
D -
% syn
Rf
3 4 r,A 5 Figure 4. Theoretical radial distribution curves for conformers with CH3 and 0 either syn (A), gauche (B), or anti (C) to each other and for a mixture of 70% syn and 30% gauche (D), together with the experimental curve. Only the conformationally important parts of the curves are shown.
curves were calculated for models with the CH3group syn, gauche, or anti to the carbonyl oxygen and for mixtures of these conformers. The outer conformationally important part of some of these curves are shown in Figure 4 together with the experimental curve from the 293 K experiment. The peak near 4.2 8, corresponds to the C3-.Br distance in the syn conformer. The area of this peak is too large in the curve calculated for 100% syn (A), and the area present a t 3.2-4.0 A in the experimental curve is not seen in curve A. Curves B and C were calculated for 100% gauche and 100% anti conformers, respectively, and the last theoretical curve of Figure 4 was calculated for a mixture of 70% syn and 30% gauche conformers. The structure refinements were carried out by the method of least squares,Ig adjusting a theoretical intensity curve to the two averaged experimental curves, one from each of the two camera distances, using a unit weight matrix. We first assumed that the geometries of the two forms differed only in the O=C-C-C torsion angle. Later we also tested the effect of allowing some of the valence angles to have different values in the two conformers, but this led to no significant improvement in the fit between the experimental and theoretical data. We also assumed all the C-H bonds to have the same length. The parameters chosen to define the model were r(C-H), r(C=O), (r(C-C)) = 0.5(r(C1-C2) r(C&)), Ar(C-C) = r(C&) - r(C1-C2),r(C-Br), LC-C=O, LC-C-Br, LC-C-C, LC,-C,-H, LH6-C2-H7, LCz-C3-H, d1 (O=C-C-C torsion angle in the gauche conformer), $2 (HC3-C2-C1 torsion angle), and T~ (the rms torsion amplitude for the syn conformer). Some of the parameters involving hydrogen atoms could not be determined very well in the refinements and they were therefore kept constant at reasonable values. Refinements showed that & had to be close to 60°, and in the final refinement it was kept constant at this value. Calculation of vibrational quantities were made with a valence force field developed from the force field for propionyl chloride3 with the necessary modification for a bromine instead of a chlorine atom being present in the molecule. Perpendicular amplitudes ( K ) and centrifugal distortion constants (ar) were used to convert the structurally consistent set of r, distances to rg and ra for use in the scattered intensity formula. The root-mean-square amplitudes of vibration ( I ) calculated from the force field were kept constant in the least-squares refinements. A dynamic model was used for the syn conformer. Five torsional pseudoconformers were introduced for this form as described
+
(19) Hedberg, K.; Iwasaki, M. Acta Crystallogr. 1964, 17, 529.
293 K 1.104 (13) 1.183 (6) 1.521 (6) 0.002 (25) 1.978 (7) 126.7 (8) 112.5 (6) 111.6 (12) [ 1101 [ 1091 [lo91 105 (15) [601 [15.3] 71 (10) 0.082
373 K 1.112 (17) 1.180 (8) 1.524 (7) 0.010 (35) 1.979 (8) 126.5 (10) 112.6 (8) 112.0 (17) Ill01 [io91 [ 1091 102 (15) [601 [17.3] 58 (13) 0.103
473 K 1.119 (22) 1.184 (10) 1.527 (8) -0.008 (46) 1.982 (9) 126.7 (11) 112.5 (10) 112.1 (24) [1101 [ 1091 [io91 98 (16) [601 [ 19.41 51 (16) 0.119
best modelb 1.109 (9) 1.182 (5) 1.523 (4) 0.003 (19) 1.979 (5) 126.6 (6) 112.5 (4) 111.8 (9) [1101 [ 1091 [io91 101 (9) 1601
Distances (fa) in angstroms, angles (L,) in degrees. Quantities in parentheses are estimated 2a. Quantities in square brackets were assumed. Weighted average of results from three temperatures. c $ l is the O=C-C-C torsion angle in the gauche conformer. d $ 2 is the C1-C2-C3-HB torsion angle, @2 = 0 when C3-Hs is eclipsed with C2C1. e ~ isS the rms torsion amplitude in the syn conformer. f R = [ ~ ~ , A , ~ / ~ w , ( s , I , ( o b swhere d ) ) ~ A, ] ~=/ ~s,I,(obsd) - s,l,(calcd).
~~~
0 01
~
0 02
003
00 4
1IT K - l
Figure 5. Van’t Hoff plot of conformational composition data. KW = N , / N , . Half-lengths of the bars are estimated standard deviations. Least-squares straight line.
earlier.14-20 This corresponds to assuming a Gaussian torsional potential for the syn form. The rms torsional amplitude ( T ~ describes the width of the potential. The values of T~ determined from the least-squares refinements had very large uncertainties, and in the final refinements values for T~ calculated from the determined torsional frequency” were used. The final results are given in Tables I and 11. Table I11 is the correlation matrix for the 293 K experiment.
Discussion From Tables I and I1 it can be seen that, of the refined parameters, only the conformer composition shows any systematic variation with temperature. The “best” values for the other refined parameters have therefore been calculated as weighted averages in the usual way and these values are included in Table I. In Table IV the geometrical parameters for propionyl bromide are compared with those for some related molecules. Average values are reported for molecules investigated at more than one temperature. The geometry of propionyl bromide is very similar to that observed for propionyl ~hloride.~ The only major difference is the torsion angle for the gauche conformer; in propionyl bromide (20) Hagen, K.; Hedberg, K. J . Am. Chem. SOC.1984, 106, 6150.
)
3980 The Journal of Physical Chemistry, Vol. 91, No. 15, 1987 TABLE II: Selected Distances and Amplitudes in Propionyl Bromide" 293 K rab ,,i D x 103 Tab C-H 1.104 (13) 0.079 15 1.112 (17) 1.183 (6)' 0.038 4 1.180 (8) 0.049 3 1.520 (17) 1.520 (13) 4 1.528 (20) 1.522 (15) 0.052 2 1.979 (8) 1.978 (7) 0.056 2.524 (24) 0.075 3 2.514 (17) 2.410 (14) 2 2.418 (11) 0.060 1 2.920 (13) 2.918 (11) 0.066 1 2.774 (8) 0.061 2.773 (8)
"Values
Skjorholm and Hagen
373 K
,,i 0.079 0.038 0.052 0.052 0.059 0.079 0.063 0.073 0.065
D x 103 16 5 3 5 2 3 2 1 1
rab 1.119 (22) 1.184 ( i o j 1.531 (23) 1.523 (26) 1.982 (9) 2.531 (34) 2.428 (17) 2.930 (14) 2.776 (10)
473 K iml, 0.079 0.039 0.051 0.054 0.064 0.087 0.067 0.079 0.07 1 0.137 0.087 0.149 0.180 0.133
2 1 5 10 12
0.160 0.259 0.120 0.209 0.397 0.397 0.266
11 5 17 17 36 27 16
2.812 4.305 3.087 4.582 5.055
(32) (11) (10) (14) (13)
0.113 0.073 0.136 0.160 0.122
1 1 4 7 9
Syn Form 2.821 (44) 0.122 4.315 (13) 0.079 3.082 (11) 0.141 4.599 (18) 0.170 5.072 (14) 0.128
2 1 4 8 10
2.841 (55) 4.321 (17) 3.321 (17) 4.611 (25) 5.084 (16)
3.366 3.467 3.887 2.957 2.992 3.915 4.399
(70) (126) (23) (117) (261) (171) (96)
0.130 0.206 0.110 0.180 0.323 0.322 0.218
7 3 12 12 24 18 11
Gauche Form 3.346 (74) 0.144 3.520 (125) 0.231 0.1 14 3.889 (28) 0.193 2.936 (1 11 3.071 (178) 0.358 3.990 (166) 0.358 4.451 (96) 0.241
9 4 14 14 29 22 13
3.337 3.571 3.896 2.927 3.141 4.058 4.498
are in angstroms. Parenthesized quantities are estimated 2u.
= r,
(80) (131) (131) (38) (193) (170) (100)
D x 103 16 5
3 6 2 4 2 1 2
+ 6r + K - 12/r = re + D - 12/r = rg - 12/r,
TABLE III: Correlation Matrix (X100) for Parameters of Prorionvl Bromide at 293 K nua rl r2 r3 Ar4 r5 1 r(C-H) 0.003 1 100 -36 -5 -21 5 100 -10 37 -3 i-(C=O) 0.0014 2 100 23 -2 0.0012 3 (4C-C)) 100 38 0.0064 Ar(C-C) 4 100 r(C-Br) 0.0014 5 LC-C=O 0.27 6 LC-C-Br 0.19 7 0.39 LC-c-c 8 5.00 9 91' % syn 3.33 10 ~~
L6
10
5 -4 21 47 100
L7
L8
L9
-27 42 -6 28 -15 -70 100
14 -14 -35 -13 -22 15 -29 100
-4 1
96 9 -7
1
1 5 6 18 -12 -3 24 100
15 -3 I
-4 11 100
'Standard deviations from least squares. Distances (r,) in angstroms, angles in degrees. bC-C-C=O torsion angle for the gauche conformer.
TABLE IV. Parameter Values for Molecules with General Formula CHIYCOX X = F X = CH3 X = H Y = CH3 parameter" Y = CH, Y = CHp 1.213 (2) 1.18Id r(C=O) 1.219 (2) 1.348 [ 1.5181 1.121 (4) r(C-X) 1.529 (2) 1.550 [ 1.5181 r(C-Y) 1.505 [1.531] 1.510 (2) r(C-C) 128.4 121.3 (14) 124.8 (3) LC-C=O 111.0 (5) 116.1 (16) 1 10.0 Lc-c-x 112.0 113.5 (3) LC-C-Y 113.5 (13) 0 0 0 dlb 120 124 (3) 11 101 92< 8 21 ref 4, 5
x = c1
Y = CH3 1.185 (3) 1.797 (3) 1.525 (10) 1.521 (10) 127.0 (4) 112.2 (3) 112.5 (5) 0 121 (6) 3
X = Br Y = CH3 1.182 (5) 1.979 (5) 1.524 (11) 1.522 (9) 126.6 (6) 112.5 (4) 111.8 (9) 0 101 (9) this work
X = Br Y = Br 1.175 (13) 1.987 (20) 1.915 (20) 1.513 (20) 129.4 (17) 110.7 (15) 111.7 (18) 0 105 2
4Distances are in angstroms, angles are in degrees. Quantities in parentheses are estimated 20, those in square brackets were assumed. 'Y-CC=O torsion angle for the low-energy conformer. cY-C-C=O torsion angle for the high-energy conformer. duncertainties were not reported for propionyl fluoride.
this form has a significantly smaller torsion angle, much closer to 90°. This is probably a result of more steric repulsion due to the larger bromine atom. Also in bromoacetyl bromide2 a similar torsion angle was observed. All the acid halides show a smaller value for r(C=O) and a larger value for LC-C=O than the aldehydes or the ketones. The central C-C bond length is nearly the same in all the molecules in Table IV. The observed temperature dependence of the sample composition afford estimates of the energy and entropy differences between the two conformers by use of the formula N g / N s = 2eASIRe-AElRT
and the factor 2 is the ratio of the statistical weights of the two forms. Figure 5 shows the usual plot of the data in the form of the van't Hoff equation, where Kq = N , / N , . The best-fit (least-squares) line leads to values with estimated standard deviations AEo = Ego - Eso = 5.5 f 2.5 kJ mol-' and ASo = Sg0 - Sso= 6 f 7 J mol-' K-I. These values are very close to those found in propionyl chloride3 (6 f 2 kJ mol-' and 7 6 J mol-l K-l), AEo for propionyl bromide is also close to the values measured for AEo in propionyl fluoride using MW (5.40 0.20 kJ mol-') and Raman spectroscopy,"-22 (5.82 0.54 and 5.37 kJ
where N g ,and N , are the fractions of gauche and syn molecules
245.
*
*
*
(21) Abe, M.; Kuchitsu, K. J.; Shimanouchi, T. J. Mol. Srruct. 1969, 4,
The Journal of Physical Chemistry, Vol. 91, No. 15, 1987 3981
Conformers of Propionyl Bromide
b r ~ m i d e , ~and ' fumaryl chloride28 all have substantial amounts of a second syn or gauche conformer present in gas phase. All of these molecules are conjugated and this may have something to do with the difference in conformational behavior. The torsional potential for propionyl bromide, assumed to have the form V(4) =
-180
-120 gauche
-60
0 SY"
60
120
gauche
180
Figure 6. Potential for torsion around the C,-C2 single bond.
mol-'). Our value for AEO is in good agreement with the value (4.34 kJ mol-') reported from variabletemperature IR and Raman spectroscopy." The three propionyl halides therefore seem to have very similar energy differences between gauche and syn conAlso for propanal the same two conformers were observed for". ( A E O = 6.3 f 2.9 kJ mol-'). It is a little surprising that AEo for the aldehyde is so close to the values found in the acid halides. In p r ~ p e n a l , ~ ~ and fumaraldehyde2s the major parts of the molecules have the double bonds anti to each other, while propenoyl propenoyl chloride,20oxalyl ~hloride,'~ oxalyl (22) Guirgis, G. A.; Barton, Jr., B. A.; Durig, J. R. J . Chem. Phys. 1983, 79, 5918. (23) Blom, C. E.; Grassi, G.; Bauder, A. J . Am. Chem. SOC.1984, 106, 7427. (24) Durig, J. R.; Tong, C. C.; Li, Y. S. J. Chem. Phys. 1972,57,4425. (25) Paulen, G.; Traetteberg, M. Acta Chem. Scund., Sect. A 1975, A28, 1155. (26) Durig, J. R.; Church, J. S.; Compton, D. A. C. J. Chem. Phys. 1979, 71, 1175.
1/2CV,(1- cos i4)
may be determined through three terms from the torsional frequency observed from spectroscopy," the value of the torsional angle at which V ( 4 ) has a minimum, and the value of V(4) at this angle (V(4) = AEo = 5.5 f 2.5 kJ mol-'), as described earlier.14s20 The results, with standard deviations estimated from the standard deviations in AEO and &, are Vl = 17.4 f 4.8, V2 = -7.1 f 4.9, and V3 = 8.9 f 1.8, all in kJ mol-'. Determination of the same potential, through four terms, has been attempted from spectroscopy," but here it was assumed that the torsion angle for the gauche conformer was 120' and this wrong assumption will strongly affect the values of the potential constants. A plot of our result for the torsional potential is shown in Figure 6. It shows a relatively low barrier between the syn and the gauche position and a much higher barrier at the anti position. This is not surprising since the methyl group and the large bromine atom are eclipsed in the anti position.
Acknowledgment. We are grateful to Ragnhild Seip for help with the electron diffraction experiment and to Ms. Snefrid Gundersen for technical assistance. Financial support from the Norwegian Research Council for Science and the Humanities is acknowledged. Supplementary Material Available: Tables of total scattered intensities, s4Zt(S),and calculated backgrounds for each plate (Tables V and VI), and figures of average experimental, theoretical, and difference intensity curves for the 373 and 473 K experiments (Figures 7 and 8) (12 pages). Ordering information is given on any current masthead page. (27) Hagen, K.; Hedberg, K. J . Am. Chem. SOC.1973, 95, 4796. (28) Hagen, K. J . Mol. Struct. 1985, 128, 139.
