Lowest triplet (n, π*) electronic state of acrolein: Determination of structural parameters by cavity ringdown spectroscopy and quantum-chemical methods

Abstract

The cavity ringdown absorption spectrum of acrolein (propenal, CH(2)=CH-CH=O) was recorded near 412 nm, under bulk-gas conditions at room temperature and in a free-jet expansion. The measured spectral region includes the 0(0)(0) band of the T(1)(n, π*) ← S(0) system. We analyzed the 0(0)(0) rotational contour by using the STROTA computer program [R. H. Judge et al., J. Chem. Phys. 103, 5343 (1995)], which incorporates an asymmetric rotor Hamiltonian for simulating and fitting singlet-triplet spectra. We used the program to fit T(1)(n, π*) inertial constants to the room-temperature contour. The determined values (cm(-1)), with 2σ confidence intervals, are A = 1.662 ± 0.003, B = 0.1485 ± 0.0006, C = 0.1363 ± 0.0004. Linewidth analysis of the jet-cooled spectrum yielded a value of 14 ± 2 ps for the lifetime of isolated acrolein molecules in the T(1)(n, π*), v = 0 state. We discuss the observed lifetime in the context of previous computational work on acrolein photochemistry. The spectroscopically derived inertial constants for the T(1)(n, π*) state were used to benchmark a variety of computational methods. One focus was on complete active space methods, such as complete active space self-consistent field (CASSCF) and second-order perturbation theory with a CASSCF reference function (CASPT2), which are applicable to excited states. We also examined the equation-of-motion coupled-cluster and time-dependent density function theory excited-state methods, and finally unrestricted ground-state techniques, including unrestricted density functional theory and unrestricted coupled-cluster theory with single and double and perturbative triple excitations. For each of the above methods, we or others [O. S. Bokareva et al., Int. J. Quantum Chem. 108, 2719 (2008)] used a triple zeta-quality basis set to optimize the T(1)(n, π*) geometry of acrolein. We find that the multiconfigurational methods provide the best agreement with fitted inertial constants, while the economical unrestricted Perdew-Burke-Ernzerhof exchange-correlation hybrid functional (UPBE0) technique performs nearly as well.

Tropospheric Photochemistry of 2-Butenedial: Role of the Triplet States, CO and Acrolein Formation, and the Experimentally Unidentified Carbonyl Compound—Theoretical Study

“…T1 and T2 both correspond to electron excitation to the π4* orbital from the in-phase (n1) and an out-ofphase (n2) combination of n orbitals (lone pairs of the oxygen atoms), Figure 2. Both have planar geometries, but in analogy to acrolein [42][43][44], twisted geometries could also be possible. We focused on planar geometries because ISC is more favorable, and a detailed analysis of the topology of the T1 and T2 states was beyond the scope of this work.…”

Photochemical Valence Isomerization to High Energy Products—Bicyclobutanes and Oxabicyclobutanes†

“…In addition to the π to π * transitions, the presence of oxygen leads to an n to π * excitation occurring at lower energies. Furthermore, radiationless intersystem crossing from singlet to triplet is very efficient from both primary excited states leading to radical intermediates (48). Thus, the photochemistry of this chromophore yields a wealth of products not related to oxabicyclobutanes or oxetes (from the s-cis conformer) for that matter (49).…”

A comparison of methods for theoretical photochemistry: Applications, successes and challenges

“…The study shows that chains with even values of n are more stable than those where n ¼ odd, and that there is a nonlinear relationship between chain length and vertical excitation energy. Hlavacek et al 73 studied the π-conjugated acrolein molecule with CASPT2 and CCSD(T), in order to determine structural properties of the its T 1 state. The methods are used to simulate cavity ringdown (CRD) 74 spectra, finding good agreement between the experimentally obtained and simulated spectra.…”