Coupled cluster calculations for HC7N, HC7NH+ and C7N, molecules of interest to astrochemistry

Large-scale coupled cluster calculations were carried out for linear HC7N, HC7NH+ and C7N (2Π and 2Σ states). Accurate equilibrium geometries (bond lengths accurate to better than 0·001 å) are established. The ground-state rotational constant of HC7 NH+ is predicted to be B 0= 553·8 ±0·5 MHz. Owing to its large equilibrium dipole moment of 6·35 D it may be a suitable candidate for forthcoming experimental investigation by microwave spectroscopy or radio astronomy. Various predictions are made for HC7N and HC7NH+ in order to facilitate their detection by means of infrared spectroscopy. Partially restricted open-shell coupled cluster calculations including connected triple substitutions yield the 2Π and 2Σ states of C7N to be very close in energy and substantial rovibronic interaction is expected.