Looking for the PC bond in space: HPCO and HPCS as possible tracers

This paper looks at the possibility of astronomical detection of molecules of prebiotic interest containing phosphorus. Attention has been focused on the most stable species that can be formed with the four [C,H,O,P] atoms. Ab initio coupled cluster molecular orbital methods and density functional theory have been used. It is found that the best candidate for detection is singlet HPCO (corresponding to singlet HNCO), which lies 23.5 kcal/mol lower than the next isomer (HOCP) and has a non-negligible dipole moment of 0.8 D. Theoretical rotational constants and infrared (IR) spectra have been determined for HPCO and the closely related HPCS. By correcting the known inadequacies in the calculations with the average theoretical to experimental ratio from two benchmark molecules (HOCO+ and HNCO), it is possible to obtain rotational constants and IR frequencies of considerably higher accuracy. The corrected values of B = 5.5322 GHz and C = 5.4071 GHz for HPCO, and B = 3.0416 GHz and C = 3.0014 GHz for HPCS should be accurate to within a few tenths of a percent. It can also be predicted that only the 2000 cm(-1) (HPCO) and 1400 cm(-1) (HPCS) IR bands have sufficient intensity to be searched in the low-abundance conditions prevailing in space. (C) 2001 Published by Elsevier Science B.V.