NC5NC: results of coupled cluster calculations and an unusual anharmonicity effect

On the basis of CCSD(T) calculations, an accurate equilibrium geometry has been established for linear NC5NC: R1e(NC)=1.1615 Å, R2e=1.3705 Å, R3e=1.2134 Å, R4e=1.3607 Å, R5e=1.2102 Å, R6e=1.3047 Å and R7e(NC)=1.1831 Å, with uncertainties on the order of 0.0005 Å. The ground-state rotational constant is predicted to be 580.2±0.2 MHz and the equilibrium dipole moment is obtained to be μe=1.38 D. In agreement with matrix isolation IR spectroscopy, only three intense vibrational transitions are calculated for the most abundant isotopomer between 2000 and 2300 cm−1. Inclusion of anharmonicity effects is mandatory to obtain this result; an unusual anharmonicity effect between the fundamentals ν1 and ν2 is predicted. Isotopic substitution significantly changes the intensities of the two stretching vibrational bands with highest wavenumber.