Prediction of vibrational frequencies of polyatomic molecules using density-functional theory (DFT) methods has become common because of its accuracy and therefore consistency with experimental data. However, the utility of DFT methods in predicting vibrational frequencies and normal mode descriptions of excited state intermediates has not been addressed so far. In this paper we have evaluated the performance of Hartree−Fock (HF) and various density-functional (DF) and hybrid Hartree−Fock/density-functional (HF/DF) methods in predicting the structure, vibrational frequencies, and normal mode descriptions of transient intermediates, taking p-benzoquinone (BQ) as an example. The structures, bond orders, harmonic vibrational frequencies, and isotopic shifts for BQ and its lowest triplet state, semiquinone radical, and semiquinone radical anion have been calculated using all of these methods employing 6−31G(d) and 6−31G(d,p) basis sets. Assignments of the calculated vibrational frequencies were made with the help of normal mode analysis. The calculated structural parameters and bond orders indicate that the structure of the triplet state is intermediate between those of the ground state and radical anion. The semiquinone radical shows a mixed aromatic and quinonoid structure. Geometrical changes involved in the triplet excited state and semiquinone radical anion with respect to ground state structure are explained on the basis of the calculated electronic structures. Of all the methods tested, the three parameter hybrid HF/DF methods (B3LYP, B3P86, and B3PW91) were found to give the most accurate geometries. These methods are also found to reproduce the experimental frequencies of the ground state as well as transient species within 2−4% error, whereas the isotopic shifts calculated for the deuterated species using the BP86 method are superior and show excellent agreement with experiment. Calculations using all of the methods show that both 6-31G(d) and 6-31G(d,p) basis sets yield very similar results.

Quinones and their radical ion intermediates have been much studied by vibrational spectroscopy to understand their structure-function relationships in various biological processes. In this paper, we present a comprehensive analysis of vibrational spectra in the structure-sensitive region of both the naphthoquinone (NQ) and 2-methyl-1,4-naphthoquinone (MQ, menaquinone) radical anions using time-resolved resonance Raman and ab initio studies. Specific vibrational mode assignments have been made to all the vibrational frequencies recorded in the experiment. It is observed that the carbonyl and CdC stretching frequencies show considerable coupling in NQ and MQ radical anions. Further, the asymmetric substitution present in MQ with respect to NQ shows important signatures in the radical anion spectrum. It is concluded that assignments of vibrational frequencies of asymmetrically substituted quinones must take into consideration the influence of asymmetry on structure and reactivity.

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In recent times, perfluorinated organic compounds have been investigated extensively to understand the influence of perfluorination on structure-reactivity correlations. Here, we report the time-resolved resonance Raman (TR3), ab initio Hartree-Fock (HF), and density functional theoretical (DFT) studies on the photogenerated transient states of perfluoro-p-benzoquinone (Fluoranil, FA). In particular, for the triplet excited states, radical anion and ketyl radical Raman spectra have been recorded. The observed Raman excitation profiles and the decay rate constants of triplet excited states of FA satisfactorily reproduce, respectively, the absorption spectra and decay rate constants reported previously from transient absorption studies. The structure and vibrational spectra of all these intermediates of FA have been calculated using both ab initio unrestricted Hartree-Fock (UHF) and density functional (UBP86) methods with standard 6-31G(d) basis set. The assignments for all the experimentally observed resonance Raman bands are made using the calculated frequencies and the normal coordinate analysis. Potential energy distributions (PEDs) are also presented. Perfluoro effect is found to be more pronounced in the triplet excited state than in the ground state or the radical anion, whereas the effect in the ground state seems to be higher than that in the radical anion. The lowest triplet excited state of FA has been identified as the ππ* state ( 3 B 3G ) in nonpolar solvents and the nπ* state ( 3 B 1G ) in polar solvents. The solvent polarity appears to play a major role in the nature of the lowest triplet excited states, since these two states are very close to each other.

This paper reports the TR3 spectral studies on perfluorinated organic systems with the objective to understand the influence of perfluorination on the excited states. We have recorded the TR3 spectra and Raman excitation profiles of the triplet excited states of decafluorobenzophenone and fluoranil. It is found that the influence of perfluorination is more pronounced in the triplet excited state than the ground state and thus leads to enhanced reactivity for perfluorinated compounds through larger structural distortions.