The role of accurate quantum mechanical computations in the assignment of vibrational spectra for unstable free radicals: H2CN and F2CN as test cases

The accuracy of anharmonic frequencies for semirigid free radicals obtained by a second order perturbative treatment based on CCSD(T) force fields is investigated for the prototypical H2CN and F2CN radicals. B3LYP computations show that most of the DFT errors are related to the harmonic part of the force field, so that hybrid models in which harmonic frequencies computed by coupled-cluster methods are coupled to anharmonic contributions obtained by proper density functionals perform very well. This finding paves the route toward the computation of accurate vibrational frequencies for quite large unstable open-shell species of current biological and/or technological interest.