J . Phys. Chem. 1993, 97, 10674-10677
10674
Molecular Structure and Conformation of Gas-Phase Methyl Dichloroacetate Oleg A. Litvinov, Michail B. Zuev, and Victor A. Naumov' A . E . Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center of Russian Academy of Sciences, 420083 Kazan, Tatarstan, Russia
Hans Vidar Volden Department of Chemistry, University of Oslo, N-0315 Oslo, Norway
Kolbjerrn Hagen' Department of Chemistry, University of Trondheim, A VH,N- 7055 Trondheim, Norway Received: May 18, 1993; In Final Form: July 29, 1993"
Gas-phase electron diffraction data obtained a t 22-23 OC, together with a b initio calculations, have been used to determine the structure and conformational composition of methyl dichloroacetate (HC12C-C(=O)-o-CH3). Two conformers have been identified, 53 (13)% of a syn form with a H-C-C=O torsion angle of $1 = 0' and 47 (1 3)% of a gauche form with $1 = 152 (5)O. For both conformers the O=C-O-CH3 torsional angle was found to be $2 = Oo. Results obtained for the bond distances (rg)and valence angles (La) for the syn conformer are r(C-H) = 1.108 (13) A, r(C=O) = 1.213 (4) A, r(C-C) = 1.528 (7) A, r(0C-0) = 1.328 (6) A, r(H3C-0) = 1.437 (6) A, r(C-Cl) = 1.767 (3) A, LO-C=O = 125.4 (6)O, LC-C=O = 122.0(11)0,LC-0-C= 1 17.7(16)0,LO-C-H= 103.8 (30)0,~C-C-H= 107.8O (assumed),K-C-Cl = 109.9 (6)", and Kl-C--Cl= 11 1.7 (4)O. Error limits are given as 2 a (a includes estimates of uncertainties in voltage/height measurements and correlation in the experimental data). The results are compared with those from related molecules and with results from a b initio calculations.
Introduction We have earlier tried to determine the rotational isomerism in molecules with a C(sp2)-C(sp3) bond by studying the conformational composition of acid chlorides, aldehydes and ketones. The same type of carbonxarbon bond is also found in esters, and in the present paper we report the results of an investigation of the structure and conformation of methyl dichloroacetate, HCI2C-C(OCH3)=O (Figure 1) using gasphase electron diffraction (ED) and ab initio molecular orbital calculations. While several compounds with the general formula HzXC-C(Y)=O have been studied (see ref 1 and references therein), few compounds with formula HX2C-C(Y)=O have so far been investigated. For dichloroacetyl chloride, HClZC-C(Cl)=O, two conformers with approximately equal energy were observed in an ED investigation,z a syn form where C-H is eclipsing C-0 and a gauche form with a H-C-C=O torsional angle of 138 ( 5 ) O . Spectroscopic studies394 of methyl dichloroacetate (MCDA) have indicated that a conformational mixture exists both in liquid phase and in solutions. This was also supported by molecular mechanics (MM) calculations4 where conformers with C-H eclipsing C = O (C,symmetry, H-C-C=O torsional angle = Oo) and with C-C1 eclipsing C=O (Cl symmetry, 4 c 1 loo) were found. From the spectroscopic measurements in liquid phase the more polar gauche form with C1 symmetry was found to be about 1 kJ/mol lower in energy than the syn form with C,symmetry. The M M calculations indicated an energy difference of 0.46 kJ/mol, again with the gauche form lower in energy.
Experimental Section and Data Reduction A sample of MDCA was supplied by Dr. L. Antokhina (bp 41 OC/8 mmHg). Electron diffraction patterns were recorded with Balzers Eldigraph KDG-2 at the University of 0 s l 0 ~on * ~Kodak Abstract published in Advance ACS Absrracfs, September 15, 1993.
0022-3654/93/2097- 10674$04.00/0
Figure 1. Diagram of the syn conformer of methyl dichloroacetate with atom numbering.
electron image plates with a nozzle-tip temperature of 294-295 K. The nozzle-to-plate distances were 498.31 and 248.43 mm for the long and the short camera distance experiments. The electron wavelength was calibrated against diffraction patterns of benzene (A = 0.058 72 A). Sirdiffraction photographs from each of the two camera distances were used in the analysis. Optical densities were measured using a single-beam microdensitometer at the University of Oslo, and the data were reduced in the usual way.l-9 The range of data were 2.00 Is/A-l I14.75 and 6.00 Is1A-l I29.50; thedata interval was L v = 0.25 A-I. Acalculated backgroundlo was subtracted from the data for each plate to yield experimental molecular intensity curves in the form SI&). The average experimental intensity curves are shown in Figure 2. Figure 3 shows the final experimental radial distribution (RD) curve calculated in the usual way from the modified molecular intensity curve I'(s) = sl,(s)ZoZa(Ao-'Aa-') exp(-0.002~2), where A = s2Fand Fis the absolute value of the complex electronscattering amplitudes. The scattering amplitudes and phases were taken from tables.11 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 41, 1993
Structure and Conformation of Methyl Dichloroacetate
*
A
Exp. A
\
AP-4
A
- 0,A, fi,
A
A
" \
\
A/\
f?
Theo.
\
A .a Diff.
- -
0
5
10
15
20
25
s/A-'
30
Figure 2. Intensity curves s[l,(s)] for methyl dichloroacetate. The experimental curves are averages of all plates for the two camera distances. The theoretical curve was calculated from the structural parameters shown in Table 11. The difference curves result from subtracting the relevant part of the theoretical curve from the experimental curves.
3
4
10675
I
,-,Theo.
rlA 6
Figure 4. Theoretical radial distribution curves for a mixture of 53% syn, 47% gauche conformers (A), 100% syn conformer (B), and 100%gauche conformer (C), together with the experimental curve. Only the conformationally important parts of the curves are shown.
TABLE I: Results from ab Initio Calculations for Methyl Dichloroacetate ~
qJI0
\ Diff.
0
1
2
3
4
rI.4
Figure 3. Radial distribution curves for methyl dichloroacetate. The experimental curve was calculated from the composite of the two average intensity curves with use of theoretical data for the region 0 Is / k l I 1.75 and B / A 2 = 0.0020. Difference curve is experimental minus theoretical.
Structure Analysis From the experimental R D curve and from results obtained for related molecules, trial values for the bond distances and valence angles could be obtained. Calculations of theoretical RD curves for the different possible conformers showed that no single conformer reproduced the experimental curve. However, a model with a mixture of approximately equal amounts of syn and gauche conformers gave a much better fit to the experimental data. This is clearly shown in Figure 4 where parts of the experimental RD curve is shown together with theoretical curves calculated for 100% syn conformer, 100% gauche conformer, and a mixture of 53% syn and 47% gauche conformers. In all models 0-CH3 is assumed to be syn to C=O (42 = 0'). Refinements of the structure were made by the least-squares method,l2 adjusting a theoretical sZ&) curve simultaneously to the two average experimental intensity curves, one from each of the two camera distances, using a unit weight matrix. The geometry of each conformer may be defined by six bond distances (r(C-H), r(C-=O), r ( C - 0 r(OC-O), r(H3C--O), r(C--Cl)), seven valence angles (LO-C=O, LC-C=O, LC-0-C, LO-C-H, LC-C-H, LC-C-Cl, LCl-C-Cl) and three torsion angles (I#J(H-C-C=O), 4(O=C-O-C), and C3vsymmetry was assumed for the methyl 4(C-0-C-H)). group, and in the early refinements it was assumed that only the C-C torsion angle differed in the different conformers. The vibrational properties were specified by 44 amplitude parameters for each conformer, corresponding to the different interatomic distances. The structure was defined in terms of the geometrically consistent r,-type distances. These were converted to the ra type
Eb
qJ2c
Eb
0
0.00
30 60 90 120 146 180
4.51 10.99 11.33 6.05 3.42 4.91
0 30 60 90 120 150 180
0.00 11.88 35.23 54.95 54.23 48.87 48.81
H-C-C=O torsion angle. HF/6-31G* relative energies in kJ/ mol, E, = -1 184.616 478 hartrees. e O=C-0-C torsion angle. required by the scattered intensity formula by using values of centrifugal distortion constants (6r), perpendicular amplitude corrections ( K ) and root-mean-square amplitudes of vibration ( I ) calculated from a harmonic force field (ri = ra - 12/r K 6r = rg- l*/r). The initial force constants were transferred from force fields developed for related m o l e ~ u l e s . ~Small J ~ changes were made in these force constants to obtain a better fit to the vibrational frequencies observed for MCDA.3 In the early refinements a H-C-C=O torsion angle of $1 = 103 (8)' was found for the gauche conformer. For this model a R factor of 0.087 was obtained. However, if the refinements were started with 41> 120°, convergence was obtained with a torsion angle of 41= 150', and for this model R = 0.077. To get more information about the conformational properties of MCDA, it was decided to do some theoretical calculations. Geometries of MCDA were fully optimized at the a b initio HF/ 6-31G* level with the use of the program GAUSSIAN 9214 for the different conformers. For the gauche conformer a torsion angle of 41 = 146' was obtained. Attempts to start the calculations a t 41 = 100' also resulted in a final gauche angle of 146', indicating that the earlier gauche model we obtained from ED was a false minimum. An even lower energy was obtained for a syn conformer (41 = 0') and the energy difference was PE = E, - E , = 3.4 kJ/mol ( E , = -1 184.616 478 hartrees). These ab initio calculations also made it possible to modify our least-squares model in such a way that calculated differences in corresponding bond distances and valence angles between the syn and the gauche conformers could be introduced as constraints in our refinements. The calculated difference between the two C-C-Cl angles in the gauche conformer was also introduced in the model. Ab initio calculations were made for several H-C-C=O torsion angles between 41 = 0" and 180' in steps of 30'. The results of these calculations are shown in Table I and Figure 5 . In most esters a O=C-0-R (R = alkyl) torsion angle of 42= 0' have been observed. This was also indicated by ab initio calculations made for different values of 42. These results
+ +
10676 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993
I
\
\
0.0 -90
0
90
180
+
270
360
ddeg
Figure 5. Ab initiocalculatedpotential energy for methyl dichloroacetate in kJ/mol, using the HF/6-31G* basis set, as a function of the H-C-C4 torsion angle, 61.
Litvinov et al.
TABLE 11: Structural Parameters for Methyl Dichloroacetate ED' ab initio parameter SYn gauche syn gauche 1.108 (13) r(C-H) 1.110 1.078 1.080 1.213 (4) 1.208 1.185 r(C=O) 1.180 1.528 (7) 1.532 1.525 r(C-C) 1.529 1.328 (6) 1.336 1.307 1.315 r(0C-0) 1.438 1.422 1.437 (6) 1.423 r(H3C-0) 1.774 1.767 (3) 1.770 1.777 r(C-Cl) 125.4 (6) 125.2 125.9 125.7 LO-c=o 122.0 (11) 126.1 121.1 LC-C=O 125.2 117.7 (16) 117.5 117.1 LC-0-c 116.9 103.8 (30) 103.9 107.8 LO-C-H 107.9 109.8 109.8 107.8 LC-C-H .1107.81. Lc-C-cl6 }109.9 (6) 107.4 110.8 LC-c-c17 110.9 LCI-C-CI 111.7 (4) 111.6 111.9 111.8 .,
:;!:: ~~
L6lb
0.0
L62c
PI
L63d
[1801 0.53 (1 3)
ae
0.0 0 180
152(5) [ll [1791 0.47 (1 3)
146 1 179
a Distances (rg)arein angstroms, angles (&) indegrees. Parenthesized values are 2u and include estimates of uncertainty in voltage/nozzle heights and of correlation in the experimental data. Values in square brackets were kept constant in the final refinement. H-C-CLO torsional angle. C-0-C=O torsional angle. H - C - 0 - C torsional angle. e Mole fraction of each conformer.
TABLE III: Selected Distances and Vibrational Amplitudes for Methyl DichloroacetatP
-90
0
90
180
270
360
+2/deg
Figure 6. Ab initiocalculatedpotentialenergy for methyl dichloroacetate in kJ/mol, using the HF/6-31GS basis set, as a function of the O=C-0-C torsion angle, 62. are shown in Table I and Figure 6. Introducing a conformer with 42 = 180° in the ED analysis made the fit to the experimental data worse and resulted in a R factor of 0.1 13. In all our later refinements only the syn and the gauche conformers, described above with 42 = Oo, were therefore included. LC-C-H could not be determined well in the least-squares refinement, and it was kept constant a t the calculated value. 43 (the H-C-0-C torsion angle) was also kept constant at the calculated value. Some of the vibrational amplitudes were refined together as groups. The amplitudes which could not be refined were kept constant at thevalues calculated in the normal coordinate analysis. In the final refinement 12 geometrical parameters, 6 amplitude parameters, and the conformational composition were refined simultaneously. The results of this refinement are given in Table I1 where also the values from the a b initio calculations for the independent parameters for the low-energy conformer are given. Selecteddistances together with calculated and refined vibrational amplitudes are given in Table 111. The intensity curve for the final model is shown in Figure 2 together with experimental and differences curves. Figure 3 shows the corresponding R D curves, and the coorelation matrix for the refined parameters is given in Table IV.
Discussion
In Table V some of the parameter values obtained for MCDA are compared with those reported for methyl acetate and dichloroacetyl chloride. Also listed are the calculated a b initio values for all three compounds using the 6-3 lG* basis set. Most of the corresponding experimental values in these three molecules
distance r(C-H) r(C=O) r(C-C) r(C2-03) r(C4-03) r(C-C1) r(C1..Od r(03-0~) r(C1-04 r(CrC4) r( CrCI) r( ClvCl7) r(HyC1) r(C14) r( C4-0~) r(O&l), r(O3-C1), r(C&I), r(OPC16)g
's
1.108 (13) 1.213 (4) 1.528 (7) 1.328 (6) 1.437 (6) 1.767 (3) 2.397 (1 5) 2.257 (8) 2.375 (13) 2.356 (17) 2.698 (11) 2.922 (7) 2.356 (12) 3.710 (10) 2.703 (35) 3.615 (14) 3.044 (24) 4.342 (24) 3.418 (49) 4 0 ~ ~ 1 ~ ) 3.31 ~ 1 (61) r(cI"Cl6)g 4.554 (52) r(Os..C1dg 3.009 (39) r(Os..Ch)g 3.791 (32) r(C~C17)~ 4.974 (26) a
(ref
0.054 (4)
0.085 (7) 0.073 (4)
0.238 (56) 0.161 (31)
0.141 (68)
lalc
0.077 0.037 0.050 0.045 0.048 0.05 1 0.060 0.056 0.066 0.0066 0.068 0.083 0.102 0.071 0.100 0.173 0.230 0.137 0.207 0.241 0.136 0.152 0.123 0.098
Distances and amplitudes are in angstroms. Parenthesized values
are 2u and include estimatesof uncertainty in voltage/nozzle heights and of correlation in the experimental data.
are very similar. The carbonyl bond is significantly shorter in the acid chloride than in the esters. This effect is also reproduced in the a b initio calculations, and similar shortenings are observed when acid chlorides are compared with aldehydes or ketones.' Comparing the results for the two esters, it is seen that r(OC-0) is significantly shorter and r(C-C) significantly longer in the chloro-substituted compound. Again the same trend is also reproduced in the theoretical calculations. Except for thecarbonoxygen bonds, where the calculated values are systematically too short when using the 6-31G* basis set, the agreement between ED and a b initio values are surprisingly good. If we assume the two conformers of MCDA to have approximately the same entropy, the experimental conformational composition corresponds to an energy difference of AE = E87 E, = 2.0 f 1.3 kJ/mol. The value obtained from a b initio is
The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 10677
Structure and Conformation of Methyl Dichloroacetate
TABLE I V Correlation Matrix (X100) for Parameters of Methyl Dichloroacetate ~~
parameter r(C-H) r(C4) r(C-C) r(C2-03) r(C4-03) r(C-CI) LO-C=O LC-C4 LC-0-C LO-C-H LC-C-CI LC1-C-C1 LQ 1 I(C-Cl) I(C2..CI) I(ClS..C17)
I(OyCl), I(03..CI), I(CyC17), a a
ULS
0.0044 0.0013 0.0023 0.0019 0.0019 0.0007 0.21 0.37 0.58 1.07 0.46 0.13 1.89 0.0010 0.0020 0.0012 0.0196 0.0108 0.0239 0.0460
rl 100
r2 r3 25 -11 100 10 100
r4
n
27 53 34 100
10 27 33 54 100
r6
2 -9 8 7 5 100
LE 9 13 -22 34 2 10 -38 59 -17 41 -14 2 100 -43 100 ~7
L9
LIO
-10 -18 -18 22 -58 17 -50 21 -45 3 -21 32 35 2 -11 -5 100 -2 100
LII
LIZ
L13
114
18 -6 -23 5 -3 -6 -35 12 -25 -37 100
-13 -23 -18 -23 -20 -41 18 -3 35 -21 35 100
-7
