COUPLED-CLUSTER CALCULATIONS FOR HC7N, HC7NH+ AND C7N, MOLECULES OF INTEREST TO ASTROCHEMISTRY

Large-scale coupled cluster calculations were carried out for linear H C7N, HC7NH+ and C7N ((2) Pi and (2) Sigma states). Accurate equilibriu m geometries (bond lengths accurate to better than 0.001 Angstrom, are established. The ground-state rotational constant of HC7NH+ is predic ted to be B-0 = 553.8 +/- 0.5 MHz. Owing to its large equilibrium dipo le moment of 6.35 D it may be a suitable candidate for forthcoming exp erimental investigation by microwave spectroscopy or radio astronomy. Various predictions are made for HC7N and HC7NH+ in order to facilitat e their detection by means of infrared spectroscopy. Partially restric ted open-shell coupled cluster calculations including connected triple substitutions yield the (2) Pi and (2) Sigma states of C7N to be very close in energy and substantial rovibronic interaction is expected.