7655
J . Phys. Chem. 1991,95, 7655-7658
cm-l (OD) and -3230 cm-l (OH). The latter frequencies indicate that the second-sphere water molecules of these trivalent cations form hydrogen bonds to outer water that are comparable in strength to those formed by first-sphere water molecules of divalent cations, e.g., Ni2+ and Mg2+.9J1 The lowest possible number of water molecules in the second shell, giving physically meaningful difference spectra, is 8.4. This is most certainly an underestimate, and it has been shown that
the number 13 f 1, obtained from a solution X-ray diffraction study on chromium(II1) and rhodium(II1) perchlorate solutions? is consistent with the present data.
Acknowledgment. Financial support from the Swedish Natural science Research Council, the Marianne and Marcus Wallenberg Foundation, and “OKs Miljbtiftelse”, is gratefully acknowledged. Registry No. AI3+,22537-23-1; Cr3+,16065-83-1; Rh”, 16065-89-7.
Electron Dlffraction Study of the Molecular Structure and Centormation of Gaseous Chloroacetone Quang Shen Department of Chemistry, Colgate University, 13 Oak Drive, Hamilton, New York 13346
and Kolbjplrn Hagen* Department of Chemistry, University of Trondheim, A VH, N7055 Trondheim, Norway (Received: March 28, 1991)
The structure and conformation of chloroacetone (CH2CI-COCH3)has been determined from gas-phase electron diffraction data at 322 K. The majority of the molecules have a gauche conformation with an O = C - C - C I torsion angle of 4‘ = 138 (7)’. Only a small potential hump of Vo = 0.2 (2) kcal/mol separates the two equivalent gauche forms. Small amounts of a syn conformer (& = O’), where C-CI is eclipsing C 4 , may also be present; our least-squares refinement gave a mole fraction of the syn form of as= 0.05 (8). Results obtained for the bond distances ( r ) and valence an les (La) are (r(C-H)) = 1.109 (7)A, r(C=O) = 1.216 (3) A, r(CH,CI-CO) = 1.537(18) A, r(CH,-Cd) = 1.507 (16) r(C-CI) = 1.787 (3) A, LCH2CI-C=O = 121.5 (16)’, LC-C-C = 119.5 (9)O, LC-C-CI = 113.7(9)’, LC2-C)-H = 1 10.Oo (assumed), LC2-Cl-H = 107 (3)O, LH-Cl-H = 109.0’ (assumed). The results are compared with those from related molecules and with results from ab initio calculations.
1,
Introduction We have been interested in the rotational isomerism about a C(sp2)-C(sp3) bond for some time and have found that the conformational composition of molecules with the general formula X C H 2 - C Y = 0 depends strongly on the substituents X and Y. In chloroacetaldehyde’ (X = CI, Y = H) the form with C-X and C - 0 close to anti to each other is by far more stable than the syn form, which is the second conformer (see Figure 1). For chloroacetyl chloride2 (X = Y = CI), propionyl chloride) ( X = CH,, Y = Cl), and chloroacetyl f l ~ o r i d e(X ~ , ~= CI, Y = F) the syn conformer is more stable than the gauche conformer (the use of syn, anti, and gauche may be different from those used in the cited references; here it refers to C-X and M). In fluoroacetyl fluoride6 (X = Y = F) and fluoroacetyl chloride’J (X = F, Y = C1) the syn is more stable than the anti form. Replacing the aldehyde proton with a halogen atom therefore always seems to make the syn conformer the low-energy form. If we replace the aldehyde proton with a methyl group (Y = CH)), a substituent of about the same size as chlorine, but with S.;Schei, H.; Hagen, K. J . Mol. Srrucr. 1983, 102, 45. (2)Stein,,=, 0,;Shen,. Q.;H ~ K. J~. ~ strUcr. ~ , 64, 217. (3) Dyngeseth, S.;Schei, S.H.; Hagen, K. J . Mol. Struct. 1904,116,257. (1) Dyngeseth,
(4) Durig, J. R.; Zhao, W.; Lewis, D.; Little, T. S.J . Chem. Phys. 1988, 89, 1285. (5) van Eijck, B. P.; Stolwijke, V. M. J . Mol. Specrrosc. 1985, 111, 164. (6) Saegebarth, E.; Wilson, E. B. J . Chem. Phys. 1967, 46, 3088. (7) Szalanski, L. B.; Ford, R. G. J . Mol. Spectrosc. 1974, 53, 428. (8) Durig, J. R.; Phan, H. V.; Hardin, J. A.; Little, T. S.J . Chem. Phys. 1989, 90. 6840.
a very different electronegativity, we get acetones. Fluoroacetone (X = F, Y = CH3) hasbeen-studied by infrared and Raman spectroscopy by Durig and co-w~rkers.~They concluded that the molecules existed predominantly in the anti form in the gas phase, while in the liquid phase the syn was more stable than the anti form. The gas-phase results were consistent with an earlier microwave study reported by Saegebarth and Krisher.’O The conformational composition of chloroacetone (X = CI, Y = CH3) appeared to be different from that of fluoroacetone. Mizushima et al.” concluded, from their Raman spectroscopic study of chloroacetone (Figure 2) in the solid, liquid, and solution phases, that in the liquid phase both the syn and gauche forms were present. The torsional angle of the gauche form was estimated to be about 1 5 0 O . Bellamy and WilliamsI2 reported the presence of two conformers of chloroacetone in their infrared spectroscopic study. Tanabe and Saeki’) analyzed the infrared spectra of chloroacetone in CS2 solution and found that the gauche form (c$(CI-C-C=O) = 150’) was 0.44 (IO) kcal/mol lower in energy than the syn form. The conformational composition in the gas phase, which can be substantially different from that of the liquid and solution phases (as demonstrated in fluoroacetone),-is not well-known. In this publication we report the results obtained from our gas-phase electron diffraction study. (9) Duria. J. R.: Hardin. J. A.: Phan, H. V.: Little. T. S.Soccrrcchim. Ada 1989, 45A,-l239. (IO) Saegebarth, E.; Krishcr, L. C. J . Chem. Phys. 1970, 52. 3555. ( I !) Mizushima, S.;Schimanouchi, T.; Miyazawa, T.; Ichishima, 1.; Kuratani, K.; Nakagawa, 1.; Shido, N . J . Chem. Phys. 1953, 21, 815. (12) Bellamy, L. J.; Williams, R. L. J . Chem. Soe. 1958, 3465. (13) Tanabc, K.; Saeki, S.J . Mol. Struct. 1975. 25, 243.
0022-3654/91/2095-7655$02.50/00 1991 American Chemical Society
7656 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
Shen and Hagen
1 I
/ / \\/
-\
Figure 1. Diagrams of different conformers for molecules with general
formula CH,X-CY-O.
I . ' " ' ' / " ' " ' " " " ' " " I ' " ' ' ' ' ' ' I ~ ' "
1
0
2
r/ii
3
4
Figure 4. Radial distribution curves calculated from the curves of Figure 3. The vertical lines indicate important interatomic distances; their lengths are proportional to the weights of the distances.
Figure 2. Molecular model of the gauche conformer of chloroacetone with atomic numbering. I
, -
I
I
A
I
Theo.
I ,
2
+5
20
25
$/i-1
30
Figure 3. Average experimental intensity curves for chjoroacetoneshown together with the theoretical curve calculated from parameter values of Table 1.
Experimental Section and Data Reduction The sample of chloroacetone was obtained from Fluka Chemie AG (>95%). The sample was purified by distillation before use. Diffraction patterns were recorded with the Balzers Eldigraph KDG-2 at the University of OS IO^^,'^ on Kodak Electron Image plates with a nozzle tip temperature of 322 K. The voltage/ distance calibration was done with benzene as reference. The nozzle-to-plate distances were 497.19 and 247.25 mm for the long and the short camera experiments. Four plates from each of the two camera distances were selected for use. The optical densities were measured with a Joyce h b l e microdensitometer. The data were reduced in the usual way1*'* and a calculated backgroundI9 was subtracted from the data for each plate to yield experimental molecular intensity curves in the form SI&). The average ex(14)
,
.
,
,
,
,
,
,
3
, , , , , , , , . , , , '
rlii
4
Figure 5. Theoretical radial distribution curves for dynamic model (A), 100% syn conformer (B) with Lq5 = Oo, 10096 gauche conformer (C)with h$ = 120°, and 100% anti conformer with L& = 180° (D), together with the experimental curve. Only the conformationallyimportant parts of the curves are shown.
Diff.
40
,
a i l , W.; Haase, J.; Wegmann. L.Z. Instrumentenred. 1966,6484.
(IS) Bastiansen, 0.; Graber, R.; Wegmann. L. Balzers High Vac. Rep. 1969,25, 1. (16) Andersen, B.;Seip, H. M.; Strand, T. G. Stdevik, R. Acta Chem. Scad. 1969. 23. 3224. (17) Gundersen, G.; Hcdberg. K. J . Chem. Phys. 1969,51, 2500. (18) Hagen, K.; Hedberg. K. J . Am. Chem. Soe. 1973,95, 1003.
(19) Hcdbcrg, L. Absrracrs of Papers, 5th Austin Symposium on GasPhase Molecular Structure, Austin, TX, March 1974; p 37.
perimental intensity curves for the two camera distances are shown in Figure 3. Radial distribution (RD) curves (Figure 4) were calculated in the usual way by Fourier transformation of functions I',,,(s) = ZcZc,Ac-lAc,-'sZ,,,(s)exp(-Bs2) with B = 0.0020A-2. Scattering amplitudes and phases for all calculations were taken from tables.*O
Structure Analysis From the experimental R D curve and from the results obtained for related molecules, trial values for the bond distances and valence angles could be obtained. Calculations of theoretical RD curves for different values of the CI-C-(2-0 torsion angle $J showed that the majority of the molecules probably had a gauche or gauche/anti conformation. However, no single conformer or no mixture of two conformers (syn-gauche or syn-anti) gave a satisfactory fit to the experimental data. In Figure 5 the experimental RD curve is shown together with theoretical curves calculated for 100%syn ($J = OO), 100% gauche (4 = 1209, and 100% anti (4 = 1 8 0 O ) . The goodness of fit factor R (R = [ ~ W i ~ 2 / ~ W i ( s , I i ( o b where ) ) z ] 1Ai~ =z ,siZi(obsd) - siIi(calcd)) for these three models are 0.327,0,211, and 0.287, respectively. In several earlier investigationswe have had to use dynamic models in order to describe the C-C torsional potential function. For chloroacetaldehyde (X = CI, Y = H) we found that a double minimum potential with a hump at the anti position gave a good (20) Schafer, L.;Yates, A. C.; Bonham, R. A. J . Chem. Phys. 197L 56, 3056.
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 1651
Structure of Gaseous Chloroacetone fit to the experimental data. Such a potential may be described by the function V = Vo(1 - 2(7/70)' + ( 7 / 7 0 ) ~ ) where V, is the height of the hump at the anti position, and * T O (7 = 180 - 4) is the position of the two minima. Interatomic distances were calculated for 7 = Oo, *15', f30°, f45', *60°, and f75O and were given weights according to the potential function shown above. In addition to these conformers, the possibility of having some of a syn form also present, was tested. This had been suggested by the spectroscopic investigations. The theoretical RD curve calculated for this dynamic model is also shown in Figure 5. In order to correct for shrinkage effects in the experimental distance measurements, appropriate distance conversions are needed. These are given by r, = rg - P/r = r, 6r K - P / r , where the centrifugal distortions, 6r, the perpendicular amplitude corrections, K,and the root-mean-square amplitudes of vibration, I, can be calculated from a quadratic vibrational force field. The force field used to make these calculations was obtained from earlier published force field^.'^^^^ The contributions to the vibrational amplitudes and the perpendicular amplitude corrections from the C,-C2 torsion were eliminated since this torsion is accounted for in the dynamic model. Refinements of the structure were made by the least-squares method,22 adjusting a theoretical SI&) curve simultaneously to the two average experimental intensity curves, one from each camera distance, using a unit weight matrix. Assuming the different conformers of chloroacetone to have the same structure apart from the torsion angle 4, the geometry of chloroacetone may be defined by r(C-H), (r(C-C)), Ar(C-C) = r(C1-C2) - r(C2-C3), r ( M ) , r(C-CI), LCHZCkC-O, LC-C-C, LC-C-CI, LC2-C,-H, LH-C,-H, LC,-C,-H. All C-H distances were assumed equal and C,,symmetry was assumed for the CH3group. In addition V, and T~ were used to describe the major conformer, and a O=C2-C,-H torsion angle 42was used to describe the torsion of the CH3 group. Also needed to define the model is the amount of syn conformer possibly present and rms vibrational amplitudes for all interatomic distances. Not all the angles involving hydrogen could be determined in the least-squares refinements, and they were given values obtained in a b initio calculations which we performed to get more information about the structure of chloroacetone. From electron diffraction alone it was also very difficult to determine the sign of Ar(C-C). The magnitude of this parameter could be determined, but equally good fit could be obtained with r(CI-C2) > r(C'-C3) or with r(CI-C2) < r(C243). Again the information from the ab initio calculations was used to establish the sign of Ar(C-C). The geometry of chloroacetone was fully optimized at the ab initio Hartree-Fcck level using G A U S S I A N - ~ ~for ' ~ both the major conformer (0-C-C-Cl torsion angle 4 = 135') and for the possible second conformer (syn, 4 = OO). The 6-31G* basis set was used and minima in the torsional potential function were found for 4 = 0' and 4 = 142'. A calculation was also carried out for 4 = 180' in order to determine the potential barrier at the anti position. The results from the final least-squares analysis of the ED data are shown in Table I, where the results from the ab initio calculations and the normal-coordinate analysis are also included. Table 11 is the correlation matrix for the model used, and intensity and R D curves for this model are shown in Figures 3 and 4, together with experimental and difference curves.
TABLE I: Structural Parameters for Cbloroacetonea oarameter r./LI* I(calcd) K 1.1@-(7) 1.216 (3) 1.537 (18) 1SO7 (1 6) 1.787 (3) 121.5 (16) 119.5 (9) 113.7 (9) [ 1 10.01 107 (3) [ 109.01 138 (7) 0.18 (17) 28 (14) 5 (8) 0.088
+ +
Discussion
The geometry of chloroacetone determined from the electron diffraction data is in general in good agreement with that obtained Crowder, G. A.; Pruettiankura, P. J . Mol. Srrucr. 1973, I S , 197. (22) GAUSSIAN86; Frisch, M. J.; Binkley, J. S.;Schlegel, H.H.; Raghavchari, K.; Melius, C. F.; Martin, R. L.; Stewart, J. J. P.;Bobrowicz, F. W.; Rohlfing, C. M.; Kahn, L. R.;Defrees. D. J.; Seeger, R.; Whiteside, R. A.; FOX,D.J.; Fleuder, E. M.; Pople, J. A. Carnegie-Mellon Quantum Chemistry Publishing Unit, Pittsburgh, PA, 1984. (21)
[0.079] 0.043 0.056 (4) 0.056 0.053 (4)
1
0.079 0.039 0.052 0.052 0.053
+ 8r
0.027 0.012 0.006 0.007 0.014
ab initio 1.082 1.190 1.524 1.508 1.788 118.1 119.2 114.0
110.0 109.0 109.0 142.0 0.25 21
Dependent Distanices 2.149 (17) 2.136 (49) 2.405 (9) 2.348 (13) 2.622 (1 3) 2.773 (IO) 3.493 (49) 3.107 (100) 2.740 (79) 2.522 (64) 2.899 (84) 3.186 (49) 2.404 (40) 3.200 (29) 3.782 (12) 2.976 (27) 3.481 (25) 2.962 (53) 2.492 (62) 4.276 (33) 3.178 (176) 3.092 (209)
[0.109] [0.107] [0.059] [0.060] [0.063] 0.079 (7) [0.105] [0.163] [0.161] [0.141] [0.129] [0.129] [0.116] 0.066 (33) 0.101 (18) [0.144] [0.105] [0.120] [O. 1341 [O.l52] [0.225] [0.198]
0.109 0.107 0.059 0.060 0.063 0.075 0.105 0.163 0.161 0.141 0.129 0.129 0.116 0.106 0.082 0.144 0.105 0.120 0.134 0.152 0.225 0.198
0.021 0.021 0.014 0.013 0.004 0.003 0.016 0.012 0.019 0.019 0.026 0.029 0.030 0.001 0.001 0.005 0.005 0.006 0.007 0.008 0.008 0.006
ODistances ( r ) are in angstroms, angles (L) are in degrees. Parenthesized values are 20 and include estimates of uncertainty in voltage/nozzle heights, and of correlation in the experimental data. Quantities in square brackets were kept constant in the final refinement. Torsion dependent distances are reported for a CI-C-C=O torsion angle of 135". *Quantities in braces were refined as a group. = 180 T~ is the minimum in the torsional potential for the gauche conformer. 4 = Oo when C C I is eclipsing C = O . V, is the barrier height in kcal/mol when C = O and C-CI is anti to each other. 'H-C3-Cz=0 torsion angle. 'For definition of R,see text. from the ab initio calculations. The C=O distance and the C,-C2=0 angle are calculated somewhat smaller than the experimental results, and all other parameters are in very good agreement with those obtained from ED. Because of the small amount of syn conformer present at 322 K, we cannot get an accurate value for the energy difference between conformers. If we assume the two forms to have the same entropy apart from the multiplicity (this may not be a very good assumption because of the small value of Vo),5% of syn conformer correspond to an energy difference AEo = 1.5 kcal/mol. The value obtained from the ab initio calculations is AEo = 1.04 kcal/mol. Both these values are significantly larger than the value (0.44 f 0.10 kcallmol) determined by Tanabe and Saeki13 for chloroacetone in CS2 solution. The ED value for the potential barrier a t the anti position (0.18 (17) kcal/mol) is in good agreement with the a b initio value (0.25 kcal/mol). In Table 111 our results for the structure of chloroacetone are compared with those for some related molecules with the general formula CH2X-CY=O. The carbon-carbon single bond has almost the same value in these four molecules, while r ( C 4 ) is significantly longer when Y = H or CH3, than when Y = CI. This shortening of r(C=O) for acid halides has also been observed in other investigations. It is usually followed by an opening of the C-C=O angle, as is also seen in Table 111. The valence angle
-
1658 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
Shen and Hagen
TABLE 11: Correlation Matrix (X100) for Parameters of Cbloncetone"
parameter r(C-H) r(C=O) (r(C-C)) &(C-C) r(C-CI) LCI-C,=O LC-C-CI LC-C-C LC2-Cl-H
VO
'60 L62
I(C=O) I(C-CI) I(C,.CI) I(C3.CI) I(0.CI)
OLS
0.0025 0.00 IO 0.0009 0.01 16 0.0008 0.55 0.31 0.32 2.22 0.06 2.54 4.96 0.0013 0.0013 0.0022 0.01 15 0.0063
r2
rl
-3
100
100
r3
r4
r5
& '
-10 -24
-41
-6
-1
-1
35
9 18
24 12 -38 -89 -15 100
100
100
100
Ll
L8
4
'0
LII
L12
I13
I14
[I5
I16
Ill
32 8 -27 -78 -29 67 100
-1
-7 -19 15 23 21 -19 -45 -82
-4 9 -5 -6 3 18 3 -16 9
5 -20
15 30 -21 -46 -18 48 46 55 -67 16 -42
22 33 -44 -72 -13 76 60 16 -36
-5 38 -34 -42 -10 55 38 22 -29 7 -23 41 59
-17 21 8
14 3 -21 -26 -10 24 -10 -7 -4 -18 27 26 20
-4 -6 -3 6 -1 -6 -16 -20 16
34 4 2 -7 -8 27 100
100
100
10
17 -1
-31 -21 -4 12 -85 100
100
IO -25 52 100
100
10
4 -5 12 40 -20 5 -13 5 8 18 100
IO -18 100
1
29 24 -4 -3 -6 18 100
'Standard deviations from least-squares refinements. Distances ( r ) and amplitudes ( I ) are in angstroms, angles (L) is in degrees, and potential (V) is in kcal/mol. TABLE 111: Geometries of Molecuks with Formulr CH2X-CY=O"
chloroacetaldehyde (C = c1, Y = H) r(C=O) r(C-C) r(C-X) r(C-Y) LC-C=O LC-c-Y LC-c-x L61b L62b
alp ,2c
temp, K ref
1.206 (3) 1.521 (5j 1.782 (4) 1.093 ( 1 2) 123.3 (6) 1 12.4 (38) 110.4 (3) 0 160 0.06 (7) 0.94 (7) 315 1
chloroacetone (X = CI, Y = CH,) 1.215 (3) 1.535 (18) 1.785 (3) 1.504 (16) 121.5 (16) 119.5 (9) 1 1 3.7 (9) 0 138 (7) 0.05 (8) 0.95 (8) 322 this work
chloroacetvl chloride (X = CI, Y = CI)
(X = CH,, Y = CI)
1.182 (4) 1.521 (9) 1.772 (16) 1.782 (18) 126.9 (9) 110.0 (7) 112.9 (17) 0 116 (8) 0.77 (7) 0.23 (7) 29 1 2
1.187 ( 5 ) 1.525 (16) 1.523 (16) 1.795 (5) 127.0 (7) 112.1 (4) 112.7 (7) 0 118 (11) 0.77 (IO) 0.23 (IO) 293 3
Distances ( r a ) are in angstroms, angles (f,) are in degrees. *&$,and L $ Jare ~ O=C-C-CI C-CI is eclipsing C=O. is the mole fraction for the two observed conformers. C-C-Y is significantly larger for Y = CH3 compared with Y = H or CI; the rest of the bond distances and valence angles are very similar in these molecules. There are, however, major differences in the conformational properties of these four molecules. With Y = H or CH3, the major conformer has C-X and C 4 pointing away from each other in a gauche or gauche/anti conformation. Only a very low barrier exists at the anti position. With Y = CI,on the other hand, the major form has C-X and C=O syn to each other. A minor gauche conformer, with a O=C-C-X torsion angle close to 120°, is also observed. When Y = H or CH3, only a very small amount of a syn form is present in the gas phase. These results may be understood if we assume that the torsional potential is determined by the steric repulsions and the electrostatic interactions between the different parts of the molecules. In chloroacetaldehyde and chloroacetone (Y = H, CH3) there is one large bond moment on each of the central carbon atoms (C-CI and C=O). The molecules will have lowest energy when these two dipoles point as far away from each other as possible. The steric repulsion is increasing, however, as X and Y get closer to each other, and since H is smaller than CH3,it is to be expected that the major form in chloroacetaldehyde has a torsion angle closer to anti, and a lower barrier at the anti position, compared with chloroacetone. In chloroacetyl chloride and propionyl chloride, there are two bond moments on C,, pointing away from each other
Dropionyl chloride
torsion angles for the two conformers.
L$J= 0'
when
(C-CI and C 4 ) . In these molecules the dipole-dipole effects are therefore not much different in the syn and the anti conformers. Since the steric repulsions between X and 0 probably are smaller than the repulsions between X and CI, it is therefore not surprising that the major conformer in chloroacetyl chloride and propionyl chloride is the one with C=O and C-X syn to each other. In chloroacetaldehyde and chloroacetone, the electrostatic interactions are therefore the most important factor in determining the conformational properties, while steric interactions are most important in chloroacetyl chloride and propionyl chloride. These results are also in agreement with those found for fluoroacet~ne.~-'~ Here the dipoledipole repulsion is large enough and the steric repulsion (between F and CH3) small enough to make the anti conformer the major form in gas phase.
Acknowledgment. We are grateful to Snefrid Gundersen and Hans Vidar Volden, The University of Oslo, for help in obtaining the electron diffraction data. We thank Kenneth W. Hedberg, Oregon State University, for help with the ab initio calculations, and the OSU Computer Center for computer time for these calculations. Financial support from The Norwegian Research Council for Science and the Humanities (NAVF) and a travel grant from NATO is gratefully acknowledged. Registry No. CH2CI-COCH3,78-95-5.
