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: R-1e(NC) = 1.1615 Angstrom, R-2e = 1.370 5 Angstrom, R-3e = 1.2134 Angstrom, R-4e = 1.3607 Angstrom, R-5e = 1.2102 A ngstrom, R-6e = 1.3047 Angstrom and R-7e(NC) = 1.1831 Angstrom, with uncert ainties on the order of 0.0005 Angstrom. The ground-state rotational consta nt is predicted to be 580.2 +/- 0.2 MHz and the equilibrium dipole moment i s obtained to be mu(e) = 1.38 D. In agreement with matrix isolation IR spec troscopy, only three intense vibrational transitions are calculated for the most abundant isotopomer between 2000 and 2300 cm(-1) Inclusion of anharmo nicity effects is mandatory to obtain this result; an unusual anharmonicity effect between the fundamentals nu(1) and nu(2) is predicted. Isotopic sub stitution significantly changes the intensities of the two stretching vibra tional bands with highest wavenumber. (C) 2000 Elsevier Science B.V. All ri ghts reserved.