Ab initio molecular orbital calculations on NO+ center dot (H20)(n) cluster ions. Part I: Minimum-energy structures and possible routes to nitrous acid formation

Minimum energy geometries, harmonic vibrational frequencies, and stepwise b inding energies have been obtained for the cluster ions NO+.(H2O)(n), n = 1 -4. From systematic ab initio calculations an the lighter NO+.(H2O)(n) comp lexes (n = 1-2) at MPn, CCSD, and CCSD(T) levels of electron correlation wi th different basis sets, it was found that the MP2/6-311++G(2d,p) level of theory was reliable for the calculation of minimum-energy geometries and ha rmonic vibrational frequencies. Relative electronic energies were evaluated at the MP2/aug-cc-pVTZ//MP2/6-311++G(2d,p) level. The inclusion of zero po int energy (ZPE) corrections, as well as counterpoise corrections for basis set superposition errors (BSSE), in the calculation of binding energies wa s essential to obtain the correct energy ordering for the different isomers of a cluster ion. The nature of the stepwise hydration processes was discu ssed based on the isomeric structures obtained. A reaction route for nitrou s acid (HONO) formation when a water molecule is added to NO+.(H2O)(3) has been established.