ELSEVIER 20 September 1996 Chemical Physics Letters 260 (1996) 7-14 CHEMICAL PHYSICS LETTERS Multireference configuration interaction and second-order perturbation theory calculations for the 13A ', 11K', and 11A electronic states of vinylnitrene and vinylphosphinidene Vudhichai Parasuk 1, Christopher J. Cramer Department of Chemistry and Supercomputer Institute, University of Minnesota, 207 Pleasant St. SE, Minneapolis, MN 55455, USA Received 28 June 1996 Abstract MRCI and CASP'I2 calculations from a (4, 4) MCSCF active space predict both vinylnitrene and vinylphosphinidene to have 3~, ground states. For vinylnitrene, the lowest open-shell singlet (1K') and closed-shell singlet 0A') states lie 15 and 40 kcal/mol higher in energy, respectively. The corresponding relative singlet energies in vinylphosphinidene are 17 and 23 kcal/mol, respectively. The reduced separation between the singlet states in H2C=CHP compared to H2C=CHN is attributed to decreased conjugation between the double bond and the pnictogen atom in the phosphinidene. I. Introduction Nitrenes and phosphinidenes are interesting un- charged, monosubstituted pnictogen compounds• Certain nitrenes in particular can be useful as pho- toaffinity labeling reagents [1]. These compounds formally have four non-bonding pnictogen electrons that can be distributed into three atom-centered or- bitals; thus, nitrenes and phosphinidenes are typically ground state triplets and have singlet states that are moderately, higher in energy. Computational studies have predicted the singlet-triplet gap for a number of different nitrenes and phosphinidenes [2-10]. Spectroscopic data for ~Permanent address: Department of Chemistry, Faculty of Science, Chulalongkorn University, Phyathai Rd., Patumwan, Bangkok 10330, Thailand. some of these compounds are also available [11-17]. For the parent systems, NH and PH, the ground states are known to be triplets with splittings of 36 and 22 kcal/mol, respectively, separating them from their doubly-degenerate first excited singlet states (~A) [12,13]. The smaller gap in HP is attributed to reduced Coulomb repulsion between the unpaired electrons in the singlet state when they occupy the more diffuse 3n-level orbitals compared to 2n-level orbitals in HN. Substitution of H with a non-cylindrically sym- metric group, e.g., vinyl, phenyl, etc., lowers the molecular symmetry and causes the degenerate sin- glet states to split into one open-shell singlet (some- times referred to as a singlet biradical) and a closed- shell singlet. The substitution-induced splitting be- tween the two singlet states has been calculated to be as large as 20 kcal/mol for phenylnitrene [3,4,9] and 0009-2614/96/$12.00 Copyright © 1996 Published by Elsevier Science B.V. All rights reserved PH S0009-261 4(96)00865-2