Fundamental vibrational frequencies and spectroscopic constants for the methylperoxyl radical, CH₃O₂, and related isotopologues ¹³CH₃OO, CH₃¹⁸O¹⁸O, 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.

Keywords:

• anharmonic
• vibrational frequencies
• methylperoxyl radical
• Fermi resonances
• isotopologues

Acknowledgements

This work was supported by the Department of Energy, Office of Basic Sciences (Grant No. DEFG02-97-ER14748). J.A. thanks D.S. Hollman for helpful discussions.