AB-INITIO CALCULATIONS OF STRETCHING VIBRATIONAL TRANSITIONS FOR THE LINEAR-MOLECULES HCN, HNC, HCCF AND HC3N UP TO HIGH OVERTONES

Accurate electric dipole moment functions have been calculated for the stretching vibrational coordinates of the linear molecules HCN, HNC, HCCF and HC3N by means of coupled cluster theory with single and doubl e excitation operators plus a quasi-perturbative treatment of connecte d triples (CCSD(T)). Combining these with anharmonic stretching vibrat ional wavefunctions absolute IR intensities for stretching vibrational transitions up to high overtones are obtained. For HCN, excellent agr eement with experiment is observed up to 7 nu(1) + nu(3) at 23047 cm(- 1). HCCF and DCCF show unusual behavior, with the nu(2) band with orig in at 2239.2 cm(-1) being strongest in HCCF and the nu(1) band with or igin at 2645.1 cm(-1) being the most intense in DCCF. The Fermi resona nce system 2 nu(3)/nu(2) Of DCCF is analysed in detail. The calculated IR intensities of the stretching fundamentals of HC3N and DC3N, which are difficult to obtain with high accuracy, are in very good agreemen t with the existing experimental data.