Homogeneous and Heterogeneous Noncovalent Dimers of Formaldehyde and Thioformaldehyde: Structures, Energetics, and Vibrational Frequencies

Abstract

This work provides the first characterization of five stationary points of the homogeneous thioformaldehyde dimer, (CH₂S)₂, and seven stationary points of the heterogeneous formaldehyde/thioformaldehyde dimer, CH₂O/CH₂S, with correlated ab initio electronic structure methods. Full geometry optimizations and corresponding harmonic vibrational frequencies were computed with second-order Møller–Plesset perturbation theory (MP2) and 13 different density functionals in conjunction with triple-ζ basis sets augmented with diffuse and multiple sets of polarization functions. The MP2 results indicate that the three stationary points of (CH₂S)₂ and four of CH₂O/CH₂S are minima, in contrast to two stationary points of the formaldehyde dimer, (CH2O)₂. Single-point energies were also computed using the explicitly correlated MP2-F12 and CCSD(T)-F12 methods and basis sets as large as heavy-aug-cc-pVTZ. The (CH2O)₂ and CH₂O/CH₂S MP2 and MP2-F12 binding energies deviated from the CCSD(T)-F12 binding energies by no more than 0.2 and 0.4 kcal mol–1, respectively. The (CH2O)₂ and CH₂O/CH₂S global minimum is the same at every level of theory. However, the MP2 methods overbind (CH₂S)₂ by as much as 1.1 kcal mol–1, effectively altering the energetic ordering of the thioformaldehyde dimer minima relative to the CCSD(T)-F12 energies. The CCSD(T)-F12 binding energies of the (CH₂O)₂ and CH₂O/CH₂S stationary points are quite similar, with the former ranging from around −2.4 to −4.6 kcal mol^–1 and the latter from about −1.1 to −4.4 kcal mol^–1. Corresponding (CH₂S)₂ stationary points have appreciably smaller CCSD(T)-F12 binding energies ranging from ca. −1.1 to −3.4 kcal mol^–1. The vibrational frequency shifts upon dimerization are also reported for each minimum on the MP2 potential energy surfaces.