Molecular structure and conformation of gas-phase methyl dichloroacetate

Abstract

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...

Molecular structure and conformational composition of methyl chloroacetate: An electron-diffraction and ab initio molecular orbital investigation

“…r(C@O) is calculated shorter than the experimental value, but this has been observed in many of these molecules. The OACH 3 bond distance is found to be surprisingly long, both compared with calculated values and with experimental values observed in related molecules [12,13]. We have no good explanation for this.…”

Theoretical Investigation on the Mechanism and Kinetics of Atmospheric Reaction of Methyldichloroacetate with Hydroxyl Radical

“…As shown in Figures 1 and S1, the methyldichloroacetate (MDCA) is having two conformers, cis and trans, that are formed due to the internal rotation around the C1−C2 bond, i.e., dihedral angle, (H1−C1−C2−O1) of MDCA. The energy calculated for optimized geometry of cis and trans conformers show that the cis conformer is the most stable conformer by 3 kcal/mol, which is in agreement with the earlier studies of Olega et al 30 Hence, the reaction of MDCA with OH radical is studied with cis conformer.…”

Conformational Analysis with Both Experimental and Computational Data for Both Gaseous and Crystalline Phases: Unexpected Interactions in N-Methyldichloroacetamide

“…Rotational isomerism in several α-halo-substituted acetic acids and their derivatives has been intensively studied by ab initio, spectroscopic, and diffraction methods. − With regard to 2,2-dichloroacetic acid, its methyl ester, and acyl chloride, several studies indicate clearly the presence of two low-energy conformers derived from rotational isomerism of the dichloromethyl group. They are syn conformers in which the HCCO torsion angle is ∼0°, and gauche conformers in which this torsion angle is ∼140° (with eclipsing of Cl and O atoms).…”

“…Rotational isomerism in several R-halo-substituted acetic acids and their derivatives has been intensively studied by ab initio, spectroscopic, and diffraction methods. [1][2][3][4][5][6] With regard to 2,2dichloroacetic acid, 7 its methyl ester, 8 and acyl chloride, 9 several studies indicate clearly the presence of two low-energy conformers derived from rotational isomerism of the dichloromethyl group. They are syn conformers in which the HCCO torsion angle is ∼0°, and gauche conformers in which this torsion angle is ∼140°(with eclipsing of Cl and O atoms).…”