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 carbenes studied in this work. Singlet carbenes form relatively strong hydrogen bonds with binding energies in the range 3-9 kcal/mol; triplet carbenes form weaker hydrogen bonds with binding energies in the range 1-4 kcal/mol. NBO analysis demonstrates that the singlet carbene hydrogen bonds are stabilized in typical fashion, through donation of electron density from the lone pair orbital on carbon into the O-H antibonding orbital. This stabilizing interaction also is present in triplet carbene hydrogen bonds; however, a back-donation from the O-H bonding orbital into the carbon lone pair orbitals also is observed, which leads to reduced charge transfer in the triplet carbene hydrogen-bonded complexes. With the exception of methylene, hydrogen bond formation is strong enough to reverse the ordering of the singlet and triplet states for the carbenes possessing triplet ground states.