Anchoring the Potential Energy Surfaces of Homogenous and Heterogenous Dimers of Formaldehyde and Thiofomaldehyde

Holy, Christina M. (2014) Anchoring the Potential Energy Surfaces of Homogenous and Heterogenous Dimers of Formaldehyde and Thiofomaldehyde. Undergraduate thesis, under the direction of Dr. Gregory Tschumper from Chemistry and Biochemistry, The University of Mississippi.

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Abstract

This work characterizes five stationary points of the formaldehyde dimer, (CH2O)2, two of which are minima, seven newly-identified stationary points of the formaldehyde/thioformaldehyde (mixed) dimer, CH2O/CH2S, four of which are minima, and five newly-identified stationary points of the thioformaldehyde dimer, (CH2S)2 , three of which are minima. Full geometry optimizations and corresponding harmonic vibrational frequencies were performed on CH2O and CH2S as well as each of the dimer configurations (Figures 3-5). The computations were carried out with second order Møller-Plesset perturbation theory (MP2), with the heavy-auc-cc-pVTZ (haTZ) basis set. Additionally, thirteen density functional theory methods were employed: B3LYP, B3LYP-D3, B3LYP-D3(BJ), TPSS, TPSS-D3, TPSS-D3(BJ), APF, APF-D, M06-2X, M06-2X-D3, N12SX, MN12SX, and VSXC, in conjunction with the 6-311+G(2df,2pd) basis set. Six of these functionals are dispersion corrected with either the original D3 damping function (DFT-D3) or the Becke-Johnson damping function [DFT-D3(BJ)]. Binding energies were computed via the supermolecular approach. Single-point energies were also computed for all optimized structures using explicitly correlated MP2-F12 and CCSD(T)-F12 methods with the haTZ basis set. The (CH2O)2 and CH2O/CH2S global minimum are the same at the MP2, MP2-F12, and CCSD(T) levels of theory. However, MP2 methods overbind (CH2S)2 by as much as 1.1 kcal mol-1, effectively altering the energetic ordering of the (CH2S)2 minima relative to the CCSD(T)-F12 energies.

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