Anharmonic force field and equilibrium structure of nitric acid

The quadratic, cubic and semi-diagonal quartic force field of nitric acid has been calculated at the CCSD(T) level of theory employing a basis set of triple-ζ quality. A semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. It is found that the A and B semi-experimental equilibrium rotational constants of the ¹⁸O isotopologues (for which the rotation of principal axes is large) cannot be accurately reproduced. This problem is discussed and a remedy is proposed. Finally, the semi-experimental structure is in agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of at least quadruple-ζ quality and a core correlation correction, except for the long NO single bond for which the CCSD(T) value is too short due to inadequate treatment of electron correlation. The empirical structures are also determined and their accuracy is discussed. The best equilibrium structure is: r_e(NO_syn) = 1.209(1) Å, r_e(NO_anti) = 1.194(1) Å, r_e(NO) = 1.397(1) Å, r_e(OH) = 0.968(1) Å, ∠(ONO_syn) = 115.8(1)°, ∠(ONO_anti) = 114.2(1)° and ∠(NOH) = 102.2(1)°.