Borazine and benzene homo- and heterodimers

The homodimers of benzene and borazine as well as a heterodimer consisting of one benzene (bz) and one borazine (bor) molecule are investigated using MP2, SCS-MP2, and CCSD(T) theories in conjunction with basis sets of up to quadruple-zeta quality. Dimer geometries were completely optimized using the resolution of the identity approximation of MP2 with a QZVPP basis set and characterized by computation of harmonic vibrational frequencies using triple-zeta basis sets. While significant higher order correlation effects beyond MP2 are important for the benzene dimer, these are very small for the borazine dimer and intermediate for the heterodimer. The spin-component scaling (SCS) correction of MP2 produces binding energies for the borazine dimer that are too low but yields very good agreement with CCSD(T) for the heterodimer. The decrease in the intermolecular distance in the sandwich (S) configurations from bz(2) via bz-bor to bor(2) is accompanied by an increased binding energy and a change from second-order stationary points to a minimum for bor(2). The T isomer is less stable than the S configuration for bor(2), but it is preferred over the S and a parallel-displaced (PD) arrangement in the heterodimer. The following order of stability is obtained for the minima at the extrapolated CCSD(T) level: T(bz-bor) > S(bor(2)) > PD(bz-bor) > PD(bor(2)) > T(bor(2)) > PD(bz(2)). The most stable isomer at all levels of theory, T(bz-bor), features a NH...pi interaction.