Effects of Solvation and Hydrogen Bond Formation on Singlet and Triplet Alkyl or Aryl Carbenes

Formation of hydrogen-bonded complexes involving singlet and triplet alkyl or aryl carbenes and the impacts of solvation and hydrogen bonding upon the carbene singlet–triplet gaps have been investigated using computational methods. Single-point CCSD(T)-F12 and MRCI+Q methodologies have been employed with aug-cc-pVDZ and aug-cc-pVTZ basis sets to determine accurate singlet–triplet gaps of carbenes and hydrogen-bonded complexes involving carbenes, with geometries and vibrational frequencies obtained at the B3LYP-D3/aug-cc-pVTZ level. Using the PCM continuum solvent method and density functional theory (B3LYP/aug-cc-pVTZ), the singlet–triplet gaps of the carbenes are found to exhibit significant solvent effects; due its higher polarity, the singlet carbene is stabilized to a greater degree than the corresponding triplet carbene, impacting the singlet–triplet gap by as much as 4.4 kcal/mol. In addition, water and methanol, acting as hydrogen bond donors, form hydrogen bonds with all the singlet and triplet ca...