A COUPLED-CLUSTER STUDY OF THE MOLECULAR-STRUCTURE, VIBRATIONAL-SPECTRUM, AND HEATS OF FORMATION OF XONO(2) (X=H,F,CL)

The equilibrium structures, harmonic vibrational frequencies, dipole moments, and IR intensities of nitric acid, fluorine nitrate, and chlorine nitrate have been investigated by using the singles and doubles coupled-cluster method that also includes a perturbational estimate of the effects of connected triple excitations, CCSD-(T). A standard triple-zeta double-polarized basis set was utilized. The equilibrium geometries and vibrational spectra of HONO2 and ClONO2 are shown to be in excellent agreement with the available experimental data. The ab initio vibrational spectrum of FONO2 is also shown to be in excellent agreement with experiment. Unlike the FOOF and FON molecules, but similar to the cis- and trans-FONO molecules, FONO2 is shown to possess normal bond distances. The bonding in FONO2 is shown to be more similar to that in ClONO2 than that in HONO2, although there are still significant differences, especially in the partial atomic charges as deduced from Mulliken populations. This causes FONO2 to possess almost no dipole moment, which is very different to both HONO2 and ClONO2. By using large atomic natural orbital basis sets, CCSD(T) energies are computed for four isodesmic reactions in order to determine an accurate heat of formation for FONO2. Our best estimate for Delta H degrees(f,298)(FONO2) is 3.1 +/- 2.0 kcal/mol, indicating that the F-ONO2 bond energy is 31.3 kcal/mol.