Theoretical study of the electronic structure of XCCP molecules (X = H, F, Cl, Br, I): carbene vs. phosphinidene

Abstract

The molecular and electronic structures of XCCP molecules, with X = H, F, Cl, Br and I, were investigated using both unrestricted and restricted CCSD(T), CASSCF/CASPT2 and B3LYP methods, with basis sets up to 6-311++G(3df,2p) and cc-pVTZ. Our results indicate that these molecules possess two distinct types of electronic structure, namely phosphinidene and carbene. The triplet phosphinidene is clearly favoured over the singlet carbene. In the ethynyl-phosphinidene (X–CC–P) framework, both triplet and open-shell singlet states feature a linear geometry (3Σ−); the corresponding singlet–triplet energy gaps vary from 70 to 80 kJ mol−1. Except for the closed-shell singlet of HCCP which is also linear, the phosphaethynyl-halocarbenes (X–C–CP) are characterized by a bent form (1A′) with rather small barriers to linearity. The standard heats of formation (ΔfH° at 298.15 K) of the triplet phosphinidenes have been evaluated as follows (values in kJ mol−1): HCC–P, 421; FCC–P, 292; ClCC–P, 420; BrCC–P, 465; and ICC–P, 569.