Simulation of the Resonance Raman Spectrum for Uracil

Abstract

The resonance Raman spectrum of uracil is simulated using the Herzberg-Teller short-time dynamics formalism. The ground-state geometry is optimized at the levels of PBE0/aug-cc-pVTZ and B3LYP/aug-cc-pVTZ, respectively. The gradient of the bright excited state is computed using time-dependent density functional theory and spin-flip time-dependent density functional theory. The excited-state calculations are carried out in both the gas phase and implicit water using the conductor-like polarizable continuum model. The ground-state equilibrium structure is found to impact the resulting resonance Raman spectrum significantly. The simulated resonance Raman spectrum using the long-range corrected functionals, that is, CAMB3LYP and LC-BLYP, and based on the PBE0/aug-cc-pVTZ optimized ground-state structure shows better agreement with the experimental spectrum than using standard hybrid functionals, that is, PBE0 and B3LYP. The solvation effect leads to a change in the energetic order of the n → π* and π → π* excited states, and it improves the agreement with the experimental spectrum, especially with regard to the relative intensities of the peaks with frequencies greater than 1600 cm(-1).

Multiconfigurational Effects in Theoretical Resonance Raman Spectra

“…By contrast, density functional theory (DFT)/time‐dependent DFT (TDDFT) calculations yielded a large overestimation of the peak intensity of the highest lying C=O stretching vibrational mode. However, in contradiction to most other (gas‐phase) calculations, which determine the first bright transition to occur from the electronic ground state (S 0 ) to the second excited spin‐singlet state S 2 , HF/CIS predicted the bright transition to be of S 0 →S 1 character . In view of the latter, the rather unexpected accuracy of HF/CIS may be a result of a fortuitous cancellation of errors originating in particular from an unbalanced treatment of non‐dynamical and dynamical electron correlation effects rather than from basis set incompleteness (see, for example, Ref.…”

“…Figure 2 in Ref. ). It compares reasonably well with the experimental RR spectrum shown in the uppermost panel of Figure , with the notable exception of the normal mode #26 (C2=O8 stretching mode) at 1752 cm −1 , the intensity of which is largely overestimated.…”

“…Note that Table 1 in Ref. apparently contains a misprint for the vibrational frequency ω of normal mode 24 and the scaling factor for frequencies higher than 1000 cm −1 , which should read ω =1622 cm −1 and 0.9568, respectively. The correct values were reported in Table 5 of that paper and subsequently entered their calculation of the RR spectra.…”

“…Concerning the following analysis and discussion of the RR spectra in Section 4.4, we therefore refer the reader to Section 3.1 and Table 1 of Ref. where a list of all normal modes and vibrational frequencies that are necessary to analyze the experimental RR spectrum of uracil is available.…”

“…These assumptions greatly simplify the calculation of RR intensities within the transform theory approach as was discussed in detail in a landmark paper by Neugebauer and Hess . Moreover, we do not take into account solvation effects, which do not appear to play a major role for a qualitatively correct description of the characteristic peaks in the RR spectrum of uracil …”