Electronic Spectra of 1,4-Cyclohexadiene and 1,3-Cyclohexadiene:  A Combined Experimental and Theoretical Investigation

Abstract

The electronically excited states of the two isomers 1,3-and 1,4-cyclohexadiene are examined by means of multiconfigurational second-order perturbation (CASPT2) theory with extended ANO-type basis sets. The calculations comprise the lower valence excited states plus all singlet 3s, 3p, and 3d members of the Rydberg series converging to the first ionization limit for the 1,3 isomer and the two first limits for 1,4. Companion optical and resonantly enhanced multiphoton ionization spectroscopic measurements were made on the 1,3 isomer. The computed vertical excitation energies were found to be in agreement with the available experimental energies for both systems. New assignments are proposed on the basis of combined experimental and theoretical efforts.

Ultrafast Dynamics of 1,3-Cyclohexadiene in Highly Excited States

“…The intensity ratio of 1 : 0.8 for the two most dominant peaks matches the ratio of the oscillator strengths for the p x (2.24 eV) and p y (2.29 eV) (Since the binding energies of the p-states shift in time, the binding energies given here correspond to the peak centers at the time of maximum electronic state population.) transitions as previously determined in a CASPT2 calculation by Merchán et al [3]. The transition to the p z state is symmetry forbidden in the one-photon excitation, and therefore much weaker [3].…”

“…transitions as previously determined in a CASPT2 calculation by Merchán et al [3]. The transition to the p z state is symmetry forbidden in the one-photon excitation, and therefore much weaker [3]. The orientation of the 3p states is such that the p z state lies along the C 2 symmetry axis, where the p y -and p x -states are perpendicularly within and out of the plane of the molecule, respectively.…”

“…Upon excitation to the 1B valence state, the reaction path carries the molecule within about 140 fs along the ring-opening coordinate through the 1B/2A and 2A/1A conical intersections back to the ground state [3][4][5][6][7]. But even while these curve-crossing transitions are now well understood, numerous questions remain.…”

“…The contour plots map the electron binding energy E B (obtained by subtracting the measured photoelectron energy E e-from the energy of the ionizing probe photon) as a function of the pump-probe time delay. Comparison with absorption spectra shows that excitation at the wavelength of the Dω 4 photon should yield a 1A 1 → 3p transition, identifying the intense band around 2.3 eV in the (4 + 2)-spectrum as the 3p Rydberg state [3]. While the admixture of valence states to 3p is not yet known, it has been suggested that 3s mixes with the doubly excited 2A state that plays an important role in the ring-opening mechanisms [4].…”

Transient X-Ray Fragmentation: Probing a Prototypical Photoinduced Ring Opening

“…This process has been extensively studied, in part because of its role in vitamin D biosynthesis. Time-resolved pump-probe techniques have been employed to understand this process [20][21][22][23][24][25]. These studies have found that the conical intersection-mediated ring-opening step in isomerization is completed in less than 200 fs [22], after which the molecule undergoes conformational changes on a range of timescales (see inset in Fig.…”