FONO: A difficult case for theory and experiment

The equilibrium structures, vibrational spectra, and relative energetics of the FNO2, cis-FONO, and trans-FONO isomers have been investigated using high-levels of ab initio theory. These methods include second-order Mo/ller–Plesset perturbation theory (MP2), singles and doubles coupled-cluster (CCSD) theory, and the CCSD(T) method, which incorporates a perturbational estimate of the effects of connected triple excitations. FNO2 is determined to be 36.9±2.5 kcal/mol (0 K) more stable than cis-FONO, which is more stable than trans-FONO by 2.5±1.0 kcal/mol (0 K). The molecular structure of cis-FONO is shown to have typical F–O and central O–N single bond distances, indicating that cis-FONO has a structure analogous to HONO. The computed vibrational spectrum of cis-FONO is shown to be consistent with infrared matrix isolation experiments, including isotopic shifts. The experimentally deduced structure of FONO (based on a normal coordinate analysis), however, is shown to be incorrect because one of the vibrational bands included in the normal coordinate analysis is either a combination band or an overtone.