Thermodynamic calculations for molecules with asymmetric internal rotors. II. Application to the 1,2‐dihaloethanes

Abstract

The thermodynamic properties of three halocarbon molecules relevant in atmospheric and public health applications are presented from ab initio calculations. Our technique makes use of a reaction path-like Hamiltonian to couple all the vibrational modes to a large-amplitude torsion for 1,2-difluoroethane, 1,2-dichloroethane, and 1,2-dibromoethane, each of which possesses a heavy asymmetric rotor. Optimized ab initio energies and Hessians were calculated at the CCSD(T) and MP2 levels of theory, respectively. In addition, to investigate the contribution of electronically excited states to thermodynamic properties, several excited singlet and triplet states for each of the halocarbons were computed at the CASSCF/MRCI level. Using the resulting potentials and projected frequencies, the couplings of all the vibrational modes to the large-amplitude torsion are calculated using the new STAR-P 2.4.0 software platform that automatically parallelizes our codes with distributed memory via a familiar MATLAB interface. Utilizing the efficient parallelization scheme of STAR-P, we obtain thermodynamic properties for each of the halocarbons, with temperatures ranging from 298.15 to 1000 K. We propose that the free energies, entropies, and heat capacities obtained from our methods be used to supplement theoretical and experimental values found in current thermodynamic tables.

Complete basis set, hybrid-DFT study, and NBO interpretations of the conformational behavior of 1,2-dihaloethanes

“…The CBS-QB3 calculated DG 1 à values for compounds 1-3 are 2.43, 5.01, and 4.31 kcal mol -1 . The B3LYP/Def2/TZVPP calculated DG 1 à values for compounds 1-3 are in agreement with the previously reported CCSD(T)/cc-Pvtz//MP2/ccPvtz results [13]. Based on the B3LYP results, the calculated DG 1 à values increase from compound 1 to compound 4.…”

“…The CBS extrapolation then computes the final energies [34]. This method is not available for iodine atom [13].…”

“…Composite complete basis set (CBS) and hybrid-DFT calculations were carried out using the CBS-4 and B3LYP/ Def2-TZVPP levels of theory with the GAUSSIAN 98 package of programs [13]. The CBS models are compound methods that extrapolate to the CBS limit by using the N-1 asymptotic convergence of MP2 pair energies calculated from pair natural orbital expansions [29][30][31][32][33][34][35].…”

“…Haloethanes are an alternative to chlorofluorocabons, CFCs, believed to be major contributors to the seasonal ozone depletion over the Antarctic continent [14][15][16][17][18]. The widespread use of these CFC replacements makes it desirable to investigate their properties [13].…”

“…The structures and rotational barriers about the C-C bonds of 1,2-difluoroethane (1) and 1,2-dichloroethane (2) have been investigated experimentally [1][2][3][4][5][6][7][8][9][10][11] and theoretically [11][12][13]. Haloethanes are an alternative to chlorofluorocabons, CFCs, believed to be major contributors to the seasonal ozone depletion over the Antarctic continent [14][15][16][17][18].…”