Physical Nature of Differential Spin-State Stabilization of Carbenes by Hydrogen and Halogen Bonding: A Domain-Based Pair Natural Orbital Coupled Cluster Study

The variation in the singlet-triplet energy gap of diphenylcarbene (DPC) upon interaction with hydrogen (water and methanol) or halogen bond (XCF₃, X = Cl, Br, I) donors to form van der Waals (vdW) complexes is investigated in relation to the electrostatic and dispersion components of such intermolecular interactions. The domain-based local pair natural orbital coupled cluster method, DLPNO-CCSD(T), is used for calculating accurate single-triplet energy gaps and interaction energies for both spin states. The local energy decomposition scheme is used to provide an accurate quantification to the various interaction energy components at the DLPNO-CCSD(T) level. It is shown that the formation of vdW adducts stabilizes the singlet state of DPC, and in the case of water, methanol, and ICF₃, it reverses the ground state from triplet to singlet. Electrostatic interactions are significant in both spin states, but preferentially stabilize the singlet state. For methanol and ClCF₃, London dispersion forces have the opposite effect, stabilizing preferentially the triplet state. The quantification of the energetic components of the interactions through the local energy decomposition analysis correlates well with experimental findings and provides the basis for more elaborate treatments of microsolvation in carbenes.