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.
IntroductionWe 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. 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 camer...
