A Coupled-Cluster Study of XON (X=H, F, Cl), and the XON (left and right arrow) XNO Transition States

The XON molecules (X=H, F, and Cl) have been studied using the singles and doubles coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations, CCSD(T), in conjunction with a double polarized triple-zeta basis set. The equilibrium geometries, dipole moments, harmonic vibrational frequencies and infrared intensities have been predicted. The X-O bond distance is shown to be abnormally long for X=F and Cl, and this is attributed to the degree of ionic bonding and the stability of NO(+). Based on Mulliken population analyses, it is shown that there is a significant degree of X(-), ON(+) ionic bonding character for FON and ClON, whereas for HON the ionic character is reduced and best described as H(+), NO(-). The stability of the XON molecule relative to the XNO isomer is shown to increase in the order HON