Molecular Structure and Conformational Composition of 1,3-Dihydroxyacetone Studied by Combined Analysis of Gas-Phase Electron Diffraction Data, Rotational Constants, and Results of Theoretical Calculations. Ideal Gas Thermodynamic Properties of 1,3-D

摘要

The molecular structure of 1,3-dihydroxyacetone (DHA) has been studied by gas-phase electron diffraction (GED), combined analysis of GED and microwave (MW) data, ab initio, and density functional theory calculations. The equilibrium r_e structure of DHA was determined by a joint analysis of the GED data and rotational constants taken from the literature. The anharmonic vibrational corrections to the internuclear distances (r_e - r_a) and to the rotational constants (B_e~(i) - B_0~(i)) needed for the estimation of the r_e structure were calculated from the B3LYP/cc-pVTZ cubic force field. It was found that the experimental data are well reproduced by assuming that DHA consists of a mixture of three conformers. The most stable conformer of C_(2v) symmetry has two hydrogen bonds, whereas the next two lowest energy conformers (C_s and C_1 symmetry) have one hydrogen bond and their abundance is about 30% in total. A combined analysis of GED and MW data led to the following equilibrium structural parameters (r_e) of the most abundant conformer of DHA (the uncertainties in parentheses are 3 times the standard deviations): r(C=O) = 1.215(2) A, r(C-C) = 1.516(2) A, r(C-O) = 1.393(2) A, r(C-H) = 1.096(4) A, r(O-H) = 0.967(4) A, angle C-C=O = 119.9(2)°, angle C-C-O = 111.0(2)°, angle C-C-H = 108.2(7)°, angle C-O-H = 106.5(7)°. These structural parameters reproduce the experimental B_0~(i) values within 0.05 MHz. The experimental structural parameters are in good agreement with those obtained from theoretical calculations. Ideal gas thermodynamic functions (S°(T), C_p°(T), and H°(T) - H°(0)) of DHA were calculated on the basis of experimental and theoretical molecular parameters obtained in this work. The enthalpy of formation of DHA, -523 ± 4 kJ/mol, was calculated by the atomization procedure using the G3X method.