Vibronic Analysis of the S₁−S₀ Transition of Phenylacetylene Using Photoelectron Imaging and Spectral Intensities Derived from Electronic Structure Calculations

Abstract

The vibrational structure of the S(1)-S(0) electronic band of phenylacetylene has been recorded by 1 + 1 resonance-enhanced multiphoton ionization, accompanied by slow electron velocity map imaging photoelectron spectroscopy at each resonant vibrational band. Assignments of the S(1) vibrations (up to 2000 cm(-1) above the band origin) are based upon the relative intensities of the vibronic bands calculated by complete second-order vibronic coupling, vibration-rotation (Coriolis and Birss) coupling calculations, and the vibrational structure of the S(1) resonant photoelectron spectra. Although this is an allowed electronic transition, the relative intensities of the a(1) bands are often largely determined by vibronic coupling rather than simple Franck-Condon factors, and second-order coupling is substantial. Nonsymmetric vibrations have intensities obtained through either vibronic or Coriolis coupling, and the calculations have been instrumental in discriminating between alternate possibilities in the assignments. Strong vibronic effects are expected to be present in the spectra of most monosubstituted benzenes, and the calculations presented here show that theoretical treatments based upon electronic structure calculations will generally be useful in the analysis of their spectra.

Optical Identification of the Resonance-Stabilized para-Ethynylbenzyl Radical

“…In the a-type S 0 −S 1 transition of the closely related phenylacetylene molecule, for example, inclusion of Herzberg−Teller moments up to second order is necessary to explain the prominence of several a 1 fundamentals, whereas at the Condon level the spectrum is wrongly predicted to be strongly origin-dominated. 69 In the present case, TDB3LYP calculations for displacements Q−Q 0 along several a 1 modes generally evince strong modulation of the zpolarized PEB D 0 −D 1 transition moment (Supporting Information, Figure S2), necessarily (by symmetry) owing to mixing with the 2 2 B 1 state. 4.2.2.…”

“…When a totally symmetric mode a is HT (Herzberg Teller)-active, as well as FC-active due to displacement along Q a , the intensity of every transition is affected because the term must be non-zero; however, the effect is most consequential when HT-active modes are excited explicitly. In the a -type S 0 – S 1 transition of the closely related phenylacetylene molecule, for example, inclusion of Herzberg–Teller moments up to second order is necessary to explain the prominence of several a 1 fundamentals, whereas at the Condon level the spectrum is wrongly predicted to be strongly origin-dominated . In the present case, TDB3LYP calculations for displacements Q–Q 0 along several a 1 modes generally evince strong modulation of the z -polarized PEB D 0 – D 1 transition moment (Supporting Information, Figure S2), necessarily (by symmetry) owing to mixing with the 2 2 B 1 state.…”

“…A relatively sophisticated vibronic coupling calculation may therefore be necessary for accurate vibrational intervals in D 1 , but the B3LYP harmonic D 0 frequency may be sufficiently , respectively, but a different permutation may be valid, depending on the extent of anharmonic coupling among the three D 1 levels. There is little doubt, as for the nearest analogue (6a 0 1 ) in benzyl 53 and phenylacetylene, 69 that a significant Herzberg−Teller moment causes 15 0 1 to be much stronger than FC would impute. The dominant contribution to the (linear) Herzberg−Teller moment for the fundamental transition 1 a ′ ← 0″ of a totally symmetric mode a that is HTactive and only very weakly FC-active can be expressed as…”

Coriolis interaction and the division of energy between vibrational and rotational excitation induced by photoelectron recoil

“…If a molecule with an initial angular momentum quantum number J 0 and initial rotational energy J 0 (J 0 + 1)B i undergoes a transition to a new state with the same angular momentum but with a different rotational constant B f , then there is a change in the rotational energy from J 0 (J 0 + 1)B i to J 0 (J 0 + 1)B f with a corresponding modification of the vibrational energy and the Franck-Condon factors. The effect of such Coriolis coupling has been observed in a high-resolution study of photoionization of phenylacetylene [14].…”

A rotational and vibrational investigation of phenylpropiolonitrile (C$_6$H$_5$C$_3$N)

“…If found in space, the abundance of PhC 3 N would provide a key test for models of aromatic chemistry which are poorly constrained at present. The infrared spectrum of PhC 3 N is also of interest as a point of comparison with other benzene derivatives whose vibrational spectra are often plagued by a myriad of perturbations and resonances, notably Fermi and Darling-Dennison in the C−H and C − − − C stretching regions [14,15,16], in addition to Fermi and Coriolis interactions for low frequency (∼150 cm −1 ), large amplitude modes that are prominent at room temperature [17,18,19]-even under astronomical conditions.…”