Vibrational spectra of furan,pyrrole, and thiophene from a density functional theory anharmonic force field

Quartic force fields for furan, pyrrole, and thiophene have been generated using density functional theory (DFT). These were used to evaluate vibrational levels by second-order perturbation theory (PT) and also by the variational method. The results for the fundamental frequencies are in very good agreement with observation. The accuracy of overtones and combination transitions is also good, for those cases where assignments can be made. Second-order PT combines speed and quality whereas the variational approach is inherently more reliable, but converges rather slowly, requiring significant computational effort.