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Singlet and Triplet Valence Excited States of Pyrimidine
journal contribution
posted on 2003-04-05, 00:00 authored by Gad Fischer, Zheng-Li Cai, Jeffrey R. Reimers, Paul WormellThe absorption spectrum of pyrimidine vapor at 75 °C in the region of the first singlet−triplet transition,
encompassing hot bands of the first singlet−singlet transition, has been obtained and analyzed with the aid
of extensive ab initio (EOM-CCSD, CASPT2, and CIS) and density functional (B3LYP and TD-B3LYP)
vibrational analyses. The hot bands in these spectra give information about low-frequency vibrations, several
of which are vibronically active but are not particularly effective at inducing intensity. Spectra obtained at 18
°C are also reported for up to 1100 cm-1 above the singlet−singlet origin. Several singlet−singlet hot bands
have been reassigned, giving excited-state vibrational frequencies for some modes. The calculations provide
not only quantitative verification of perceived vibronic coupling and other features of the experimental
assignments but also detailed maps of the complex lowest singlet and triplet manifolds. This includes vertical
and adiabatic excitation energies, relaxation energies, excess spin densities, and normal-mode vibrational
displacement and Duschinsky rotation analyses for up to eight singlet and eight triplet excited states as well
as estimates for the structure and energy of some important interconnecting transition states and conical
intersections. As a result, revised assignments for the majority of the triplet states are suggested. In addition,
the pseudoparity selection rule, which forms the primary model for the (π*, π) spectroscopy of alternate
conjugated hydrocarbons, is found not to apply to the 3A1 manifold. The possibility of symmetry breaking in
the (π*, n) states caused by vibronic coupling to a2 vibrations is considered in detail, as is the possibility of
the excited states taking the nonplanar “boat” configuration because of vibronic coupling in b1 modes. Excited-state chemical properties such as reaction rates and hydrogen bonding are very sensitive to these effects.
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pyrimidine vaporreaction ratesinterconnecting transition statesrelaxation energiespseudoparity selection ruleabsorption spectrumvibrationalDuschinsky rotation analysesconical intersectionsb 1 modes1100 cmTriplet Valence Excited States2 vibrationstriplet manifoldsvibronicCASPTCISB 3LYP1 manifoldsinglettriplet statesab initioadiabatic excitation energies
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