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

TL;DR

This study combines experimental and computational methods to analyze the T1(n, π*) state of 2-cyclopenten-1-one, revealing vibrational frequencies and testing the accuracy of various computational models.

Contribution

The paper introduces a new computational hybrid method (CC/DFT) and compares its performance with other methods for predicting excited-state vibrational frequencies.

Findings

The REMPI spectrum of 2CP in the T1(n, π*) state reveals eight fundamental vibrational frequencies.

Out-of-plane vibrational modes show significant frequency reduction upon electronic excitation.

TDDFT methods outperform EOM-EE-CCSD in predicting excited-state fundamentals.

Abstract

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 T
1(n, π*) excited state of 2CP mediates relaxation processes that
can lead to photoproducts. In this paper, we report the T
1(n, π*) ← S
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 T
1(n, π*) ← S
0 origin band at 25,956 cm–1. Vibronic analysis of the REMPI spectrum yielded fundamental frequencies
for eight vibrational modes in the T
1(n, 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*
← n chromophore. We used the measured T
1(n, 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 T
1(n, 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 T
1(n, 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.

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