Ab initio calculations of molecular properties of low-lying electronic states of 2-cyclopenten-1-one--link with biological activity.

Geometries, vibrational frequencies, vertical and adiabatic excitation energies, dipole moments and dipole polarizabilities of the ground and the three lowest electronic excited states, S₁(n, π*), T₁(n, π*), and T₂(π, π*) of the 2-cyclopenten-1-one molecule (2CP) were calculated at the CCSD and CCSD(T) levels of approximation. Our results indicate that two triplets T₁(n, π*) and T₂(π, π*) are lying very close each to other, while the singlet S₁(n, π*) is well above them. There are dramatic changes in dipole moments for (n, π*) excited states in respect to the ground state. On the other hand the T₂(π, π*) state has a similar dipole moment as the ground state. These changes can be interpreted within the MO picture using electrostatic potential maps and changes in model IR spectra. Our CCSD(T) dipole moment data for the ground state and almost isoenergetic triplets T₁(n, π*) and T₂(π, π*) are 1.469 a.u., 0.551 a.u., and 1.124 a.u., respectively. Dipole polarizabilities of investigated excited states are much less affected by electron excitations than dipole moments. These are the first dipole moment and polarizability data of 2CP in the literature. The changes of molecular properties upon excitation to S₁(n, π*) and T₁(n, π*) correlate with the experimental data on the biological activity of 2CP related to the α, β-unsaturated carbonyl group.