An electronic structure theory investigation of the physical chemistry of the intermolecular complexes of cyclopropenylidene with hydrogen halides

The proton accepting and donating abilities of cyclopropenylidene (c‐C₃H₂) on its complexation with hydrogen halides H? X (X = F, Cl, Br) are analyzed using density‐functional theory with three functionals (PBE0, B3LYP, and B3LYP‐D) and benchmarked against second‐order Møller–Plesset (MP2) theory. Standard signatures including, inter alia, dipole moment enhancement, charge transfer from the carbenic lone pair to the antibonding σ*(H? X) orbital, and H? X bond elongation are examined to ascertain the presence of hydrogen bonding in these complexes. The latter property is found to be accompanied with a pronounced red shift in the bond stretching frequency and with a substantial increase in the infrared intensity of the band on complex formation. The MP2/aug‐cc‐pVTZ c‐C₃H₂···H? F complex potential energy surface turns out to be an asymmetric deep single well, while asymmetric double wells are found for the c‐C₃H₂···H? Cl and c‐C₃H₂···H? Br complexes, with an energy barrier of 4.1 kcal mol^?1 for proton transfer along the hydrogen bond in the latter complex. Hydrogen‐bond energy decomposition, with the reduced variational space self‐consistent field approach, indicates that there are large polarization and charge‐transfer interactions between the interacting partners in c‐C₃H₂···H? Br compared to the other two complexes. The C···H bonds are found to be predominantly ionic with partial covalent character, unveiled by the quantum theory of atoms in molecules. The present results reveal that the c‐C₃H₂ carbene divalent carbon can act as a proton acceptor and is responsible for the formation of hydrogen bonds in the complexes investigated. © 2012 Wiley Periodicals, Inc.