3043
ELECTRON DIFFRACTION OF CYCLOPROPYL METHYLKETONE
Electron Diffraction Study of the Structure and Conformational Behavior of Cyclopropyl Methyl Ketone and Cyclopropanecarboxylic Acid Chloride'"
by L. S. Bartell,lbJ. P. Guillory, and Andrea T. Parks Institute for Atomic Research and Department of Chentistru, Iowa State University, Ames, Iowa (Received March 26, 1966)
~~
~
~~
~
The -COCH, and -COC1 derivatives of cyclopropane were found to exist in conformations similar to those previously reported for C3Hs-CHO. cis and trans rotational isomers were observed rather than the trans and gauche isomers characteristic of noncyclic carbonyl derivatives. The dominance of the twofold over the threefold contribution to the barrier potential seems most easily interpreted in terms of a 7-electron conjugative effect. Steric interactions in C3H6COCHS and CaH5COC1 made the trans isomers relatively less stable than in C3H5CH0. Bond lengths, bond angles, and amplitudes of molecular vibration were also determined for the molecules.
Introduction I n a recent electron diffraction investigation of cyclopropylcarboxaldehyde it was discovered that the molecule exhibits a novel conformational behavior. Unlike unstrained alkyl carboxaldehydes in which the rotation of the CHO group is governed by a (nominally) threethe cyclopropyl fold barrier to internal derivative is characterized by a twofoId barrier. The theory of restricted rotation about single bonds is, as yet, unsettled but the simplest model which accounts for the cistrans isomerization observed in the cyclic aldehyde is Walsh's a-electron modeL6 Cyclopropylcarboxaldehyde, viewed in this light, is an analog of butadiene or acrolein. It seemed desirable to study further examples of molecules with the formula of C3H5COX to determine whether the behavior of the aldehyde was representative. The present paper reports the results of an investigation of the methyl ketone (X = CH3) and the acid chloride (X = Cl).
Experimental Procedure Samples of cyclopropyl methyl ketone and cyclopropanecarboxylic acid chloride obtained from the Aldrich Chemical Co. were further purified by fractional distillation. The level of impurities detected by gas phase chromatography was less than 1.5%. Electron diffraction photographs were taken at room temperature with the diffraction unit at Iowa State
University employing an ra sector and an accelerating voltage of 40 kv. Specimen pressures of 30 and 14 torr were used for the ketone and acid chloride, respectively. The corresponding exposure times for a beam of 0.6 pa. were 5.5 and 2.7 sec. for the 21-cm. camera distances and 14.0 and 9.5 sec. for the ll-cm. camera distances. Diffraction patterns were recorded on 4 X 5 in. Kodak process plates. Intensities of the patterns were determined by measuring the absorbancies of four selected plates for each camera distance. Measurements were made with a considerablymodified Sinclair Smith microphotometer equipped to give a digital output.
Structure Analyses The data were processed and analyzed using an IBM 7074 digital computer and a method fully described elsewhere.' Leveled experimental intensity and background curves for the molecules are shown in Figures (1) (a) Contribution No. 1671. Work was performed in the Ames Laboratory of the U. S. Atomic Energy Commission; (b) to whom correspondence concerning reprints should be addressed: Department of Chemistry, University of Michigan, Ann Arbor, Mich. (2) L. S. Bartell and J. P. GuiUory, J. Chem. Phys., 43, 647 (1965). (3) J. P. Guillory and L. S. Bartell, ibid., 43, 654 (1965). (4) R. W. Kilb, C. C. Lin, and E. B. Wilson, Jr., ibid., 2 6 , 1695
(1957). (5) C. C. Lin and J. D. Swalen, Rev. Mod. Phys., 31, 841 (1959). (6) A. D. Walsh, Nature, 159, 167, 712 (1947); Trans. Faraday SOC., 45, 179 (1949).
Volume 69, Number 9 September 1966
L. S. BARTELL, J. P. GUILLORY, AND A. T. PARKS
3044
1 and 2. Indices of resolution were unity to within experimental error. Experimental radial distribution functions, f (r), were calculated using a theoretical intensity function from q = 0 to q = 14 and experimental intensity data from q = 15 to q = 120. In all comparisons of experhental and synthetic f (r) curves, the theoretical contribution to f(r)exptlis consistent with the synthetic function. Conformational analyses were made by comparing the experimental radial distribution curves with synthetic curves constructed for various distributions of rotational isomers. For both molecules the presumably continuous isomeric distribution in the angle e of internal rotation (reckoned from the cis configuration) was represented by a small set of discrete conformers. I '
22
n
I
,
'
'
I
'
LCNG CAMERA
I
R
'
I
,
'
W
I
i1
I n addition, simplifying assumptions about the ring parameters and the dependency of bond lengths and angles on e were made as described in ref. 1 to facilitate the analyses. These assumptions are believed to infiuence the reported parameters and conformational characteristics by less than the reported uncertainties. Cyclopropyl Methyl Ketone. The set of isomers chosen to represent the equilibrium distribution of the methyl ketone included s-cis (e = O"), both "rigid" and "distorted" trans (e = lSOO),two gauche-like models (0 = 1 3 3 " and =k60"),and an antigauche or twisted trans model (0 - 180" = 130"). These conformers are pictured in Figure 3. The experimental radial distribution curve for GH6COCHZ is shown in Figure 4. Bond distances, bond angles, and amplitudes of vibration determined from a least-squares analysis of this curve are reported in Table I.
H\
c-c
No
S-TRANS
20
40
€Q
DO
80
LO/
I20
dk'J
S -1RANS
X s-CIS
Figure 1. Experimental intensity and background curve8 for cyclopropyl methyl ketone. /-
ANTIGAUCHE. OR TWISTED TRANS
-
3ec
*
-I I
w
t
l 40
+
l
,
l
80
60
r
l KO
,
l
OR TWISTED C I S ~ C ) GAUCHE-LIKE
Figure 3. Molecular configurations investigated for cyclopropane deviatives. X represents a methyl group or a chlorine atom. The dashed lines in the Newman diagrams indicate the projections of double bonds to the oxygen atoms if they are regarded a~ two bent single bonds (cf. Pauling, ref. 11). Bonds are staggered in trans and gauche conformations, eclipsed in cis and antigauche.
DD
dit-9
Figure 2. Experimental intensity and background curve8 for cyclopropanecarboxylic acid chloride.
The Journal of Physical Chemistry
(7) L. 9.Bartell, L. 0. Brockway, and R. Schwendeman, J. C h m . Phys., 23, 1854 (1955); R. A. Bonham and L. S. Bartell, ibid., 31, 702 (1959); J. Am. Chem. Soc., 81,3491 (1959).
ELECTRON DIFFRACTION OF CYCLOPROPYL METHYLKETONE
3045
refer to the ring and methyl group atoms, respectively). The longest 0 CR and shortest Cn * CR distances would be expected to be approximately 3.4 d. for a gauche coonformer. The absence of a distinct peak in the 3.4-A. region of the f(r) curve indicated that few gauche isomers were present. Many different isomeric combinations were used in computing synthetic radial distribution curves. Five of the more significant compnations are shown in Figure 5 for the r = 2.6 to 5.2-A. region. A comparison of experimental and calculated curves for the s-trans model indicates that this isomer cannot be present in large concentrations. A problem arose in treating the s-trans isomer. For the sake of discussion, let us denote as ‘(rigid” the trans model constructed by adopting the bond lengths and angles determined from an analysis of the (preponderantly cis) experimental f (r). This “rigid” trans model has perfectly normal bond lengths and angles but it is found to place its ring and methyl hydrogen atoms 0.4 A. closer together than the normal 2.4-A. van der Waals diameter.* I n view of the likelihood that the trans configuration would spontaneously deform its bond angles to reduce this steric stress, an alternative trans model was constructed. I n this “distorted” trans conformer, the CR-CO-CU and CR-CR-CO angles are opened to increase the H R * .HMdistance to 2.3 d. Standard deviations between the synthetic and the corresponding experimental f (r) curves were calculated for a variety of concentrations of s-cis, “rigid” trans, ‘(distorted” trans, gauche, and antigauche isomers. Graphical methods yielded a composition of 80% s-cis and 20% antigauche for the minimum standard deviation. A slightly poorer fit was obtained for the 80% s-cis, 10% “distorted” trans, and 10% antigauche combination. The standard deviation associated with an 80% s-cis and 20% “distorted” trans model was at the outer limit of acceptibility. It may be concluded, therefore, that the equilibrium composition of C3H6COCH, consists of approximately 80% s-cis conformers and 20% of forms in the general vicinity of the trans isomer. There is very weak evidence that the isomeric distribution in e about the nominally trans form is broader for the ketone than it is in the case of the corresponding aldehyde. The standard error in concentrations is appreciable, being, perhaps, f15%. Cyclopropanecarboxylic Acid Chloride. The experimental radial distribution curve for CIHSCOCl is plotted in Figure 6 . From the prominence of the 4.0-d. peak, it is evident that a majority of the molecules
-
--
--
Af(r) 0 --vm-A-n
I
Table I : Structural Parameters and Approximate Standard Errors Derived from Radial Distribution Analyses (in R. Units) Molecule
Distance
CsH6COCHa
CH CC.,
CaHbCOCl
CH CC,”
vg
&9
k“
4)
1.126 0.050’ 0.074’ O.05Ob 1.510 0.003 0.048 0.003 CO 1.225 0.020’ 0.049 0.020b C...Cav 2.556 0.020 0.048 0.030 LCCOaVd= 121.8 f 2”, LCCHaVd= 117.2 f 3” % cis-like = 80 f 15; yo trans-like = 20 f 15 1.105 0.060’ 0.080“ 0.060b 1.506 0.002 0.049 0.002 co 1.197 0.025b 0.039 0.025b cc1 1.797 0.009 0.064’ 0.009 LCCOd = 127.6 f 3”, LCCHaVd= 120.7 f 4” LCCCld = 111”f 2.5”, LOCCld = 122.4 f 2” yo cis-like = 85 f 15; % trans-like = 15 f 15
’
Amplitude of vibration as reckoned from rg. Standard errors for these poorly resolved peaks calculated using the method discussed by L. S. Bartell and B. L. Carroll, J . Chem. Phys., 42, 1135 (1965). ‘ Amplitudes corrected for failure of Born approximation. Angles correspond essentially to s-cis isomers. Not corrected for shrinkage effect.
At an early stage of the analysis it was apparent that the molecules were predominantly in the s-cis configuration. Prelimi?ary f(r)exptlcurves displayed peaks at 2.90 and 3.85 A., the positions and areas of which corresponded closely to those expected for a pure s-cis model. These peaks can be ascribed to cis 0 . * ‘CR and CM . . C R distances (where the subscripts R and M
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-
(8) L. Pauling, “The Nature of the Chemical Bond,” 3rd Ed., Cornell University Press, Ithaca, N. Y., 1960, p. 260.
volume 69,Number 9 September 1966
L. S. BARTELL, J. P. GUILLORY, AND A. T. PARKS
3046
c
-EXP.
G,H,COGY
Aft(r)
0 y"-Lwv4/L-,
I
2t\
I
I
I
80%S-CIS ~ 2 0 % ANTIGAUCHE
+
8O%S -CIS 10%S -TRANS + 10% ANTIGAUCHE
"
I
-
.v
1
I
1
3
4
5
I
rt A)
Figure 5. Experimental and calculated radial distribution curves for various assumed isomeric concentrations of cyclopropyl methyl ketone.
exist in s-cis or cis-like forms. The isomeric set chosen for analyses included the models s-cis (6 = 0')) twistedcis, tc(6 = *15'), tc(e = *33'), tc(6 = f60"), and s-trans (6 = 180'). The s-trans form adopted represented a ('distorted" configuration in the same sense (and for the same reason) as the '(distorted" trans isomer of the methyl ketone. The CR-CO-CI, and CRCR-Co angles were increased from 110 and 118' to 114 and 121.5", respectively, in order to increase to 2.73 A. the otherwise unduly short HE- * C1 nonbonded distance. The f ( r ) curve of Figure 6 was used to determine mean bond lengths, angles, and amplitudes of vibration. Least-squares results for the skeletal parameters corresponding essentially to s-cis isomers are reported in Table I. The CR-C~-CIangle pertaining to the cis-like isomers The Journal of Physical Chemistry
cannot be established independently of the distribution in 6 of the isomers. Accidental similarities between the radial distribution for 33" tc isomers and (with somewhat shifted peaks) for the average of cis and trans isomers introduce into the diffraction analysis a decided correlation between the C-C-C1 angle and the distribution in 6. I n an attempt to resolve this problem, the optimum distribution in 6 was sought for each of three models which differed in s-cis and tc C-C-CI bond angles. Bond angles of 110, 113, and 115" were investigated since this range is representative of angles in carbonyl compounds. I n acetyl chloride, the closest model available for comparison, the angle has been reported to be 110" by electron diffraction0 and 112" by microwave spectroscopy.1° For each of the isomeric sets identified by its CCCl angle, synthetic radial distribution curves with various assumed concentrations of isomers were calculated for comparison with f (r)exptl. Examples of these curves are shown in Figure 7 (LCCCI = 110") and Figure 8 ( L CCCl = 113') illustrating the region from 2.6 to 5.2 b. which is sensitive to conformational behavior. Results of minimizing the standard deviations between observed and calculated f(r) curves with reqect to isomeric concentrations are given in Table 11. The best fit corresponds to a value of 111" for LCCCI. A rough idea of the breadth of the distribution in e is also derivable from the table. (9) Y. Morino, K. Euchitm, M. Iwasaki, K. Arakawa, and A. Takahashi, J. Chena. SOC.Japun,75, 647 (1964). (10) K. M. S h o t t , J. C h m . Phy8., 34, 851 (1961).
ELECTRON DIFFRACTION OF CYCLOPROPYL METHYL KETONE
- EXP C,H.U)CI, --- CALC.
3047
LCCCl=llOo
s-CIS
t\ L
'.
0I 4
1
33'- tc f (rl
5ox S-CIS
i
t\
S- TRANS.
\
/'
J
L
+ W%S-TRANS
42%s-cIst9%15°-tc t 3096 33O- tc t I9%S-TRAI\IS I
45% s-CIS
0
+ 45% 33%tc
+D% S-TRANS
t\
3!5%s-ust30%!5°tc +0%3S"-tct25%S-TRANS
I
0-
,
,
I
4
3
5
r(A)
Figure 8. Experimental and calculated radial distribution curves for various assumed isomeric concentrations of cyclopropanecarboxylic acid chloride; L CCCl taken to be 113".
Figure 7. Experimental and calculated radial distribution CUNH for various assumed isomeric concentrations of cyclopropanecarboxylic acid chloride; LCCCl taken to be 110"; tc represents twisted cis.
Discussion Table 11: Distribution in Angle of Internal Rotation for C3HaCOC1. Results of Minimizing, with Respect to Isomeric Concentrations, the Standard Deviation between Observed and Calculated Radial Distribution Curves concentrations for aaaumed L ccc1-
B for isomer
-Isomeric 1100'
1100'
1130b
113OC
0" 115" 133" 160" 180"
45%
35% 15% 40% 5% 5% 0.050
42% 9% 30% 0% 19% 0.060
35% 15% 10% 0% 25% 0.063
df )d
... 45%
... 10%
...
llSob
3% 47% 16% 6%
27a/o 0.098
a Analytical minimization of u(f) for set of three diecrete isoAnalytical minimization of u(f) for set of five discrete mers. Distribution giving nearly as small u(f) as the optiisomers. mum distribution. Root-mean-sauare deviation between observed and calculated radial distribution curves from 2.6 to 5.2 A.
The principal conclusion derived from this research is that the -COCHI and -COC1 derivatives of cyclopropane exhibit s-cis isomerization just as did the carboxaldehyde reported earlier. The barrier to internal rotation cannot be understood on the basis of the interpretation advanced by Pauling for R-CHO and related compounds.11 Molecules following Pauling's scheme preferentially sdqger their bent bonds at opposite ends of the central C-C bond. In the prlsint molecules, the bonds for s-cis are eclipsed. A Telectron picture in which a conjugative interaction stabilizes cis and trans isomers provides perhaps the simplest ration&,ation of the observed behavior.o It also accounts for effects found in ultraviolet spectra of dated It would seem, however, (11) L.Pauling, ref. 8, pp. 140-142.
Volume 69,Number 9 September 1066
3048
that a conjugative interaction is not needed to account for the length of the central C-C bond,13 as discussed in ref. 2. Although the conformational behavior of the methyl ketone and acid chloride resembles that encountered in the aldehyde, there are pronounced differences. For the aldehyde, nearly equal concentrations of s-cis and s-trans isomers were observed. By contrast, quite low concentrations of trans isomers were found for the methyl ketone and acid chloride derivatives which have bulkier groups to be accommodated. The reason for the difference is undoubtedly steric. As discussed in the foregoing, the methyl and chloride groups have insufficient clearances unless several bond angles are strained open. The clearances in the trans-methyl ketone resemble tthose in gauche n-a1kanes,14 and the conformational free energy difference reported for the alkanes is quite sufficient to account for the present observations. For cyclopropylcarboxaldehyde, a barrier to rotation in excess of 2.5 kcal./mole was found. A barrier of this magnitude implies a root-mean-square amplitude of oscillation of the cis form of 20" or less from the 0 = 0" configuration. The methyl ketone data are consistent with such an amplitude. In the case of the acid chloride, however, a rather insidious pitfall was encountered which nearly led to a (probably) false conclusion. Since the example provides a good illustration of the danger in relying upon automated programs of analysis, it is perhaps worth discussing briefly. In initial analyses of the acid chloride distribution in e, avalueof 110"for L CCClwas adopted, corresponding to the angle reported in an electron diffraction study of CH3COCL9 The resulting distribution, listed in Table 11, appears to be materially broader and flatter than the corresponding distributions in the aldehyde and ketone. As discussed in the preceding section, this seems to be an artifact of a correlation between assumed values for the C-C-C1 angle and the distribution. At first glance the broadening toward gauche-like forms appears like a physically plausible result stemming
The Journal of Physical Chem&ry
L. S. BARTELL, J. P. GUILLORY, AND A. T. PARKS
from the H R * * * cis C ~nonbonded distance. The HR *..C1 distance associated with a value of 110" for L CCCl is abnormally short and would lead to warping of the molecule out of cis planarity. When the L CCCl is opened, however, the derived distribution approaches that of the aldehyde-and, simultaneously, the "physical" basis for a broad, flat distribution vanishes because the implied H R * .C1 distance increases. It seems more pleasing, subjectively, to accept the latter analysis with the larger G-C-C1 angle and narrower distribution in e similar to that for the aldehyde and ketone, though the diffraction data do not firmly discriminate between the alternatives. The deceptive aspect is the fortuitous self-consistency of the physical and analytical implications of both alternatives. Finally, it is interesting to note that Hoffmann has made calculations, based on his extended Huckel MO scheme, of the potential energy for internal rotation of the methyl ketone.15 He correctly predicted, before the electron diffraction results were made known, that the barrier function has minima in the vicinity of cis and trans rather than trans and gauche, and that the cis isomer is more stable than the trans. The predicted energy difference between cis and trans, which was uncorrected for molecular deformation to relieve steric stresses, was roughly within a factor of 2 of that crudely determined by electron diffraction. The above agreement, together with Hoff mann's earlier qualitative success in the case of cyclopropyl carboxaldehyde,2 is quite pleasing. It suggests that the Huckel model captures, in a simple manner, a significant portion of the molecular physics involved.
-
Acknowledgment. We are pleased to acknowledge the assistance of Miss Margo Dunlap in many phases of the calculations. (12) E. M.Kosower, PTOC.Chem. SOC.,25 (1982); N. H.Gromwell and G. V. Hudson, J . Am. C h m . SOC.,75, 872 (1953). (13) It can be calculated that this bond is about 1.49 A. in length if the ring bonds in the derivatives are assumed to have the same length as in cyclopropane itself. (14) L. 5. Bartell and D. A. Kohl, J . Chem. Phys., 39, 3097 (1963). (15) R. Hoffmann, private communication, 1964.
