Theoretical thermochemistry and spectroscopy of weakly bound molecules
The weakly bound association products of atmospherically relevant radical species (O₂, OH, NO₂, HO₂ and NO) have been studied theoretically using quantum-chemical methods. The thermodynamic stabilities, which are crucial to determining the probability of formation in Earth's atmosphere, were calculated for the hydrotrioxy radical (HOOO) and peroxynitrous acid (HOONO, an isomer of nitric acid) relative to the radical dissociation products. In the case of HOONO, the experimentally determined values were confirmed. For HOOO, the predicted stability was significantly lower than the experimentally determined value; a conclusion that was supported by later experimental work and indicates that HOOO will not form in significant quantities in Earth's atmosphere. The fundamental and multi-quantum vibrational transitions were also predicted for both the HOONO and HOOO systems. The theoretical work on the HOONO system aided the assignment of experimental spectra and was used to correct equilibrium rotational constants. The HOOO system presented a challenge for the methods used here and work to apply other approaches in describing the vibrational modes is ongoing. Second-order vibrational perturbation theory, combined with a correlated quantum-chemical method and a moderately sized basis set, provides a method for accurately predicting fundamental and low-order multi-quantum transition energies and intensities for many systems (HOOO being an exception). Here coupled cluster theory, at a level which treats one- and two-electron correlation with a correction for three-electron correlation, and atomic natural orbitals basis sets were used in the vibrational calculations. To predict the dissociation energies of weakly bound species with the precision required (due to the small energy differences involved), high-order correlation contributions (a full treatment of three-electron correlation and a correction for four-electron correlation) are included, as is extrapolation to the basis set limit. Other contributions, such as that for the zero-point energy, were also considered. For the HOOO system, one-dimensional potential curves along the dissociation and torsional coordinates were constructed with standard single-reference and equation-of-motion coupled-cluster methods. The latter is better able to describe the nature of a system in the bond-breaking region and the complex electronic structure of a species formed from two radical fragments, one doubly degenerate in the ground state: X²[Pi] OH and X³[Sigma] O₂. A possible barrier to dissociation and the torsional potential for HOOO were investigated.