An ab initio and DFT comparative study of electronic effects on spin multiplicities and structures of X-C2N carbenes

Ab initio and DFT molecular orbital calculations are used to determine singlet-triplet energy splits, ΔEs-t,X, as well as relative energies, in the carbenic reactive intermediates X-C2N, at seven levels of theory: B3LYP/6-31G*, MP2/6-31G*, B3LYP/6-311++G**, MP2/6-311+G*, MP3/6-311G*, MP4(SDTQ)/6-311++G** and QCISD(T)/6-311++G**; for X=H, NH2, CN and OMe. MP3 results are more consistent to the B3LYP and/or QCISD(T) rather than the MP2 or MP4 results. Three possible structures considered for each singlet (s) and/or triplet (t) carbenes are: 3-X-2-azacyclopropenylidene (1s-X and 1t-X), [(X-imino)methylene]carbene (2s-X and 2t-X), and X-cyanocarbene (3s-X and 3t-X). Species 1t-H, 1t-NH2, 1t-OMe, 2t-NH2, 3t-NH2, 3s-OMe and 3t-OMe show imaginary frequencies, so their corresponding structures are not minima and appear as transition states on the potential energy surface of X-C2N. Cyclic triplet 1t-CN, upon optimization transforms to an acyclic carbenonitrene structure. For all X-C2N species inspected, ΔEs-t,X appears the highest for X=NH2, and the lowest for X=H. For the six H-C2N species with X=H, the order of relative energy is: 1t-H>2s-H>2t-H>3s-H>1s-H>3t-H. Likewise, for the six structures with X=CN, the order of relative energy is: 1t-CN>2s-CN>2t-CN>1s-CN>3s-CN>3t-CN. Finally, the order of relative energy for 12 structures with X=NH2 and/or OMe is: 2t-X>1t-X>2s-X>3t-X>1s-X>3s-X. The lowest energy minima, among the 12 species with X=H and/or CN, are triplet 3t-X; while the lowest energy minimum, among the six species with X=NH2 is singlet 3s-NH2. The discussion includes geometrical parameters, dipole moments, charges and bond orders of 1-3.