Multiplicity vs. stability in C2HP carbenes and their halogenated analogues: an ab initio and DFT study

Systematic studies are carried out on the multiplicity vs. the stability for 24 phospha-carbenes, C2XP, at seven levels of theory: B3LYP/6-31G**, B3LYP/6-311++G**, MP2/6-31G*, MP3/6-31G*, MP3/6-311+G**, MP4(SDTQ)/6-311++G** and QCISD(T)/6-311++G** (for XH, F, Cl and Br). Twelve of these carbenes are singlet (s) and the rest are in their triplet (t) states. Eight of these 24 C2XP have 3-X-2-phosphacyclopropenylidene (1) structures. The other eight have [(X-phosphino)methylene]carbene (2) constitutions. The rest are with phosphinidynemethyl-X-carbene (3) forms. Calculations at B3LYP/6-311++G** level show linear correlations between the singlet-triplet energy separations and the energy gaps flanked by the lowest unoccupied and the highest occupied molecular orbitals (LUMO-HOMO) of their corresponding singlet C2XP carbenes. In the study of the structure vs. energy the order of singlet-triplet energy separations are found to follow electro-negativity (F>Cl>Br>H), for structures 1 and 2. However, in structure 3, where X is directly attached to the carbenic center, a different order is encountered: H>Cl>Br>F. The global minima are triplet states of 3 for XH, Cl and Br. This is in contrast to XF, where the global minimum appears to be the singlet state of 2.