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 ³Σ^- 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.