Density functional calculations, structure, and vibrational frequencies of 2-cyclopenten-1-one in its S-0, S-1(n,π*), T-1(n,π*) and T-2(π,π*) states

Abstract

Density functional calculations have been carried out on the S-0, S-1(n,pi*), and T-1(n,pi*) states of 2-cyclopentenl-one (2CP) to complement the experimental study of the triplet state using cavity ringdown spectroscopy described in the previous paper. Structures and vibrational frequencies were calculated for each state at both the B3LYP/6-31 + G(d,p) and B3LYP/6-311 + G(d,p) levels. The structural information was used to obtain the kinetic energy part of the ring-bending Hamiltonian, for the analysis of triplet-state spectral data. The density functional calculations show the molecule in its S-0 and S-1 states to be planar, but to have a small barrier to planarity in the T-1(n,pi*) triplet state. This is in line with potential-energy fits to the experimental ring-bending levels for each state. The calculated barrier for the T-2(pi, pi*) state is 999 cm(-1). This provides further confirmation that the cavity ringdown data, from which a 43-cm(-1) barrier was determined, correspond to the T-1(n,pi*) state. The calculated vibrational frequencies are in excellent agreement with the experimental data for the S-0 state and also for the most part for the S-1 and T-1 states. Notably, the frequency calculated for the very anharmonic ring-bending vibration cannot be expected to be very accurate.