Mg+NO and Mg+ON: potentially important ionospheric species

In order to determine the lowest energy isomer of the Mg+ - NO complex, sel f-consistent field (SCF), configuration interaction including all single an d double excitations (CISD), coupled cluster singles and doubles (CCSD), an d CCSD with perturbatively evaluated triples [CCSD(T)] ab initio electronic structure methods were employed. Equilibrium geometries, relative energies , dipole moments, harmonic vibrational frequencies, and associated infrared (IR) intensities for the lowest triplet and closed-shell singlet structure s were determined. At the CCSD(T) level with the largest basis set, triple- zeta plus double polarization augmented with one set of higher angular mome ntum and one set of diffuse functions (TZ2PF + diff), the global minimum wa s predicted to be closed shell,(1)A' Mg+ NO with equilibrium geometry r(e)( Mg - N) = 2.378 Angstrom, r(e)(N - O) = 1.147 Angstrom, and theta(e) = 122. 6 degrees. At this same level of theory, (1)A' Mg+ NO was predicted to lie approximately 14 kcal mol(-1) below the Mg+ + NO dissociation asymptote. At levels of theory below CCSD(T), the (3)Pi state of Mg+ NO is erroneously p redicted to be the ground state. The dipole moment with respect to the cent er of mass is predicted to be 4.9 debyes for the (1)A' ground state of MgNO. (Int J Mass Spectrom 185/186/187 (1999) 961-975) (C) 1999 Elsevier Scie nce B.V.