Resonance-Enhanced Multiphoton Ionization Spectrum and Computational Study of 2-Cyclopenten-1-one in Its <i>T</i>₁(<i>n</i>, π*) State

The 2-cyclopenten-1-one molecule (2CP) is a cyclic conjugated enone that participates in a variety of photochemical reactions. Prior computational work indicates that the <i>T</i>₁(<i>n</i>, π*) excited state of 2CP mediates relaxation processes that can lead to photoproducts. In this paper, we report the <i>T</i>₁(<i>n</i>, π*) ← <i>S</i>₀ vibronically resolved spectrum of 2CP, recorded in a supersonic free-jet expansion using resonance enhanced multiphoton ionization (REMPI) detection. The REMPI spectrum covers the region extending to +900 cm^-1 with respect to the <i>T</i>₁(<i>n</i>, π*) ← <i>S</i>₀ origin band at 25,956 cm^-1. Vibronic analysis of the REMPI spectrum yielded fundamental frequencies for eight vibrational modes in the <i>T</i>₁(<i>n</i>, π*) state, including four modes that could not be observed in the jet-cooled phosphorescence excitation spectrum we reported previously. We observe that the out-of-plane, but not in-plane modes, undergo dramatic frequency reduction upon electronic excitation. This distinction sharpened our understanding of the π* ← <i>n</i> chromophore. We used the measured <i>T</i>₁(<i>n</i>, π*) fundamental frequencies to test a computational method, termed coupled-cluster/density functional theory (CC/DFT) hybrid, that was developed by Puzzarini and Barone for predicting spectroscopic properties of medium-sized molecules (up to about 10 heavy atoms). In our implementation of CC/DFT, we employed the unrestricted coupled cluster singles and doubles with perturbative triples (CCSD(T)) ab initio technique to calculate harmonic frequencies of the <i>T</i>₁(<i>n</i>, π*) state of 2CP. We used second-order vibrational perturbation theory (VPT2) to obtain anharmonic corrections, in conjunction with anharmonic force constants computed using unrestricted DFT. The calculation predicts <i>T</i>₁(<i>n</i>, π*) fundamental frequencies that deviate by only 8 cm^-1, on average, from those measured in the REMPI spectrum. We used the CC/DFT results as a reference to evaluate the performance of more economical hybrid methods for predicting excited-state fundamentals. These methods incorporate equation-of-motion excitation energies coupled cluster singles and doubles (EOM-EE-CCSD) or time-dependent density functional theory (TDDFT) to calculate harmonic frequencies. Notably, the economical TDDFT approaches outperform the EOM-EE-CCSD ab initio technique in this application.