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

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.