Ab initio anharmonic force fields and spectroscopic properties for HC5N and HC5NH+, molecules of interest to astrochemistry

Large-scale coupled cluster calculations have been carried out for HC5N and HC5NH+. Accurate equilibrium geometries (ca. 0.0005 Angstrom accuracy) are predicted for both species. The equilibrium rotational constant of HC5NH+ is B-e = 1294.1 MHz, which should also be a good estimate for the ground-state value B-0. HC5NH+ has a remarkably large equilibrium dipole moment of mu(e) = 3.811 D and may thus by a suitable candidate for forthcoming investigation by microwave spectroscopy and radio astronomy. For HC5N, strong anharmonic interaction is predicted between mu(2) and 2 nu(5) as well as between nu(1), and nu(3) + nu(5). (C) 1997 Elsevier Science B.V.