Theoretical study on structures and stability of HC2P isomers

 The structures and isomerization pathways of various HC2P isomers in both singlet and triplet states are investigated at the B3LYP/6-311G(d,p), QCISD/6-311G(d,p) (for isomers only) and single-point CCSD(T)/6-311G(d,p)//B3LYP/6-311G(d,p) levels. At the CCSD(T)/6-311G(d,p)//B3LYP/6-311G(d,p) level, the lowest-lying isomer is a linear HCCP structure 31 in the 3∑− state. The second low-lying isomer has a CPC ring with exocyclic CH bonding 15 in a singlet state at 10.5 kcal/mol. The following third and fourth low-lying isomers are a singlet bent HCCP structure 11 at 20.9 kcal/mol and a bent singlet HPCC structure 13 at 35.8 kcal/mol, respectively. Investigation of the HC2P potential-energy surface indicates that in addition to the experimentally known isomer 31, the other isomers 11, 13 and 15 also have considerable kinetic stability and may thus be observable. However, the singlet and triplet bent isomers HCPC 12 and 32 as well as the triplet bent isomer HPCC 33 are not only high-lying but are also kinetically unstable, in sharp contrast to the situation of the analogous HCNC and HNCC species that are both kinetically stable and that have been observed experimentally. Furthermore, the reactivity of various HC2P isomers towards oxygen atoms is briefly discussed. The results presented here may be useful for future identification of the completely unknown yet kinetically stable HC2P isomers 11, 13 and 15 either in the laboratory or in interstellar space.