A b i n i t i o characterization of the S1–S2 conical intersection in pyrazine and calculation of spectra

Abstract

Potential-energy functions of the three lowest singlet states of pyrazine have been computed on the self-consistent field, second-order Mo/ller–Plesset (SCF/MP2) level as a function of ab initio determined ground-state normal coordinates. The electronic-structure calculations confirm the existence of a conical intersection of the S1(nπ*) and S2(ππ*) surfaces near the minimum of the latter surface. A vibronic-coupling model involving four spectroscopically active vibrational modes is constructed on the basis of the ab initio data. Absorption, resonance-Raman, fluorescence, and femtosecond time-resolved pump–probe spectra are computed for this model using previously developed methods. The results are compared with experimental data where available.

The visible spectrum of zirconium dioxide, ZrO2

“…This is probably the top of the barrier of a double-well mode whose potential is distorted by the vibronic coupling with the higher neighboring B 1 A 1 state. 23,24 The CASSCF/LanL2DZ equilibrium R and θ of 1.884 Å and 96.6 • differ significantly from the TD-DFT/LanL2DZ values of 1.843 Å and 101.8 • . There is no direct experimental measurement of R and θ for theà 1 B 2 , but the REMPI and LIF excitation spectra could be best modeled using 1.828 Å and 99.0 • (vide infra), which are near the TD-DFT values.…”

“…Various combinations of the anharmonic terms were attempted. In the end, a fit of the excitation spectra was obtained by varying only χ 12 , χ 23 , and χ 33 in addition to ω 1 , ω 2 , and ω 3 for theà 1 B 2 state. The standard deviation of the fit (σ = 9.64 cm −1 ) is commensurate with estimated combined measurement uncertainty of the LIF and MS-REMPI spectra.…”

Dynamics of radiationless transitions in large molecular systems: A Franck–Condon-based method accounting for displacements and rotations of all the normal coordinates

“…Indeed, that problem has attracted considerable interest over the past years and therefore experimental and theoretical data are abundant in the literature. [37][38][39][40][41][42][43][44][45][46][47] The absorption spectrum of pyrazine exhibits a well structured band for the S 0 → S 1 (π, π * ) transition and a broad, less structured band for the S 0 → S 2 (n, π * ) one. 37,38 The most significant feature of the S 1 absorption band is the presence of an anharmonic progression of the b 1g mode (ν 10a ); since the molecular symmetry (D 2h ) is retained upon transition to S 1 , 43,46,48 odd transitions of that progression are not allowed in the Franck-Condon approximation (strong frequency variation could be responsible for even transitions), so that other mechanisms must be invoked.…”

Efficient on-the-fly ab initio semiclassical method for computing time-resolved nonadiabatic electronic spectra with surface hopping or Ehrenfest dynamics

“…The pyrazine molecule exhibits a conical intersection between the first (S 1 ) and second (S 2 ) bright excited states, which influences the dynamics after excitation and the resulting vibronic spectra. Because the pyrazine molecule provides an almost ideal benchmark, its vibronic spectra have been extensively studied both experimentally [37][38][39][40][41][42][43][44][45][46] and theoretically, 6,25,26,31,[47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62] giving us an opportunity not only to assess the accuracy of the MSDR in relation to experiment but also to compare it with exact quantum calculations on simplified models developed by others. In addition, due to efficiency of the MSDR, we were able to go beyond simplified model systems and to compute the absorption spectrum using the ab initio electronic structure computed on the fly in all 24 dimensions.…”