Lowest triplet (n,π*) state of 2-cyclohexen-1-one: characterization by cavity ringdown spectroscopy and quantum-chemical calculations

Jet-Cooled Phosphorescence Excitation Spectrum of the T1(n,π*) ← S0 Transition of 4H-Pyran-4-one

The 4H-pyran-4-one (4PN) molecule is a cyclic conjugated enone with spectroscopically accessible singlet and triplet (n,π*)excited states. Vibronic spectra of 4PN provide a stringent test of electronic-structure calculations, through comparison of predicted vs measured vibrational frequencies in the excited state. We report here the T1(n,π*) ← S0 phosphorescence excitation spectrum of 4PN, recorded under the cooling conditions of a supersonic free-jet expansion. The jet cooling has eliminated congestion appearing in previous room-temperature measurements of the T1 ← S0 band system and has enabled us to determine precise fundamental frequencies for seven vibrational modes of the molecule in its T1(n,π*) state. We have also analyzed the rotational contour of the 000 band, obtaining experimental values for spin-spin and spin-rotation constants of the T1(n,π*) state. We used the experimental results to test predictions from two commonly used computational methods, equation-of-motion excitation energies with dynamical correlation incorporated at the level of coupled cluster singles doubles (EOM-EE-CCSD) and time-dependent density functional theory (TDDFT). We find that each method predicts harmonic frequencies within a few percent of observed fundamentals, for in-plane vibrational modes. However, for out-of-plane modes, each method has specific liabilities that result in frequency errors on the order of 20-30%. The calculations have helped to identify a perturbation from the T2(π,π*) state that leads to unexpected features observed in the T1(n,π*) ← S0 origin band rotational contour.

Cavity Ringdown Spectrum of 2-Cyclohexen-1-one in the CO/Alkenyl CC Stretch Region of the S1(n, π*)-S0 Vibronic Band System

The 2-cyclohexen-1-one (2CHO) molecule serves as a prototype for understanding the photochemical properties of conjugated enones. We have recorded the cavity ringdown (CRD) absorption spectrum of 2CHO vapor at room temperature over the 360-380 nm range. This portion of the spectrum encompasses the S₁(n,π*) ← S₀ vibronic band system in the region of the C═C and C═O stretch fundamentals. We have assigned about 40 vibronically resolved features in the spectrum, affording fundamental frequencies for 7 different vibrational modes in the S₁(n,π*) state, including the C═C (1554 cm^-1) and OC-CH (1449 cm^-1) stretch modes. The C═O stretch character is spread over at least four different vibrational modes in the S₁(n,π*) state, with fundamentals spanning the 1340-1430 cm^-1 interval. This finding stems from a significant reduction in C═O bond order upon excitation, which leads to near-coincidence of the C═O stretch and several CH₂ wag frequencies. Such complexities make 2CHO an ideal candidate for testing excited-state computational methods. We have used the present spectroscopic results to test EOM-EE-CCSD harmonic-frequency predictions for the S₁(n,π*) state. We have also benchmarked the performance of less costly computational methods, including CIS(D) and TDDFT. For certain density functionals (e.g., B3LYP and PBE0), we find that the accuracy of TDDFT frequency predictions can approach but not meet that of EOM-EE-CCSD.