Normal modes analysis of Ã-state acetylene based on directly observed fundamental vibrations

Abstract

Recent experimental results permit a detailed normal modes analysis of Ã-state acetylene (C2H2) and its isotopomers (C2HD and C2D2). Using only experimentally determined frequencies and measured or estimated anharmonicities, we determine harmonic frequencies for the 11 directly observed and unambiguously assigned vibrational fundamentals. The normal modes calculation varies force constants to fit the 11 harmonic frequencies and yields a complete set of harmonic frequencies, force constants, and Coriolis coefficients for the three isotopomers. A complete set of fundamental frequencies calculated from the set of harmonic frequencies allows a comparison to and, in some cases, suggests a reassessment of frequencies for tentatively assigned fundamental vibrations.

Full dimensional Franck-Condon factors for the acetylene $\tilde{\mathbf {A}}$Ã 1 A u—$\mathbf {\tilde{X}}$X̃ $\mathbf {^1\Sigma _g^+}$Σg+1 transition. I. Method for calculating polyatomic linear—bent vibrational intensity factors and evaluation of calculated intensities for the gerade vibrational modes in acetylene

“…Parameters for theX-state equilibrium geometry and harmonic force field used in the evaluation of the coordinate transformation were taken from Halonen et al ("Model II" of the paper). 65 In order to reproduce the results of Watson, we first used theÃ-state geometry and harmonic force field from Tobiason et al 66 Using these force fields, we obtained elements for the gerade block of D (a-s) and δ (a-s) that agree with those obtained by Watson to within a phase factor of ±1. 46 The phase factor Antisymmetric stretch b u 3032.4 ν 6 Cis bend b u 801.…”

“…Table IX and Figure 4 provide a comparison between experimental absorption data and calculated relative intensities using both the Tobiason et al 66 force field and the Jiang et al 64 force field, and using the three types of coordinate transformation discussed in Sec. III.…”

“…Recent spectral analysis published since the Tobiason et al force field 66 has uncovered new parameters relevant to the force field of theÃ-state of acetylene. Most notably, 64 We have used the updated force field from "Fit Method I" in Ref.…”

“…The quantity el |μ c | el was estimated by taking the value of the ab initio electronic transition dipole 66 force field with the axis-switching transformation in Table VII (purple dash-dot curve); and the Jiang et al 64 force field with the zero-order transformation of Table VI (black dashed curve), the axis-switching transformation of Table VII ( moment (plotted in Figure 1) at the geometry q 0 (defined in Eq. (78)) that maximizes the exponential factor in the vibrational overlap integral.…”

Full dimensional Franck-Condon factors for the acetylene $\tilde{\mathbf {A}}$Ã 1 A u—$\mathbf {\tilde{X}}$X̃ $\mathbf {^1\Sigma _g^+}$Σg+1 transition. II. Vibrational overlap factors for levels involving excitation in ungerade modes

“…47 provide better agreement with experimental intensities than the parameters of Ref. 48. In the present paper, we provide a brief comparison in Sec.…”