Fundamental vibrational frequencies and spectroscopic constants for the methylperoxyl radical, CH3O2, and related isotopologues 13CH3OO, CH3 18O18O, and CD3OO

Abstract

Accurate spectroscopic and geometric constants for CH3O2, and its isotopologues 13CH3OO, CH3 18O18O and CD3OO, are predicted. Employing coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)], we obtain optimized equilibrium geometries using Dunning's cc-pVTZ basis set. A Taylor expansion of the potential energy surface, including all third-order and semidiagonal fourth-order terms in a basis of normal coordinates, yields anharmonic vibrational frequencies and vibrationally-averaged properties including the effects of anharmonicity. We detail the strong influence of Fermi resonances on the problematic ν6 vibrational mode of CD3OO, arriving at a value of 993 cm−1; two previous experimental measurements of this mode appear to have been incorrectly assigned. Our computed energies for the low intensity ν11 transition are in excellent agreement with experimental measurements performed for CH3 18O18O and CD3OO, inspiring confidence that our results will serve as a guide for experimental measurement of this yet-unobserved quantity for the CH3OO and 13CH3OO isotopologues. Given the reliability of our force field, and considering the results of other experiments, we make a number of reassignments to previously recorded spectra, which eliminate large disagreements between experimental observations. The vibrational averaging of the rotational constants and geometries are also discussed for each isotopologue.