Halogen switching of azacarbenes C2NH ground states at ab initio and DFT levels

Relative stabilities and singlet–triplet energy differences are calculated for 24 C₂NX azacarbenes (where X is H, F, Cl, and Br). Three skeletal arrangements are employed including azacyclopropenylidene, (imino)methylene]carbene, and cyanocarbene. Halogens appear to alternate the electronic ground states of C₂NH azacarbenes, from triplet to singlet states, at MP3/6‐311++G**, B1LYP/6‐311++G**, B3LYP/6‐311++G**, MP2/6‐311++G**, MP4(SDTQ)/6‐311++G**, QCISD(T)/6‐311++G**, CCSD(T)/6‐311++G**, CCSD(T)/cc‐pVTZ, G1, and G2 levels of theory. The aromatic characters of singlet cyclic azacyclopropenylidenes are measured using GIAO–NICS calculations. Linear correlations are found between the B3LYP/6‐311++G** calculated LUMO–HOMO energy gaps (ΔE_{HOMO ‐ LUMO}) of the singlet carbenes versus their corresponding singlet–triplet energy separations (ΔE). Electrophilic characters are found for all singlet azacarbenes in their addition reactions to alkenes with the highest electrophilicity being exhibited for X = F. © 2008 Wiley Periodicals, Inc. Heteroatom Chem 19:377–388, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20442