The UV absorption of nucleobases: semi-classical ab initio spectra simulations

Abstract

Semi-classical simulations of the UV-photoabsorption cross sections of adenine, guanine, cytosine, thymine, and uracil in gas phase were performed at the resolution-of-identity coupled cluster to the second-order (RI-CC2) level. With the exception of cytosine, the spectra of the other four nucleobases show a two band pattern separated by a low intensity region. The spectrum of cytosine is shaped by a sequence of three bands of increasing intensity. The first band of guanine is composed by two pipi* transitions of similar intensities. The analysis of individual contributions to the spectra allows a detailed assignment of bands. It is shown that the semi-classical simulations are able to predict general features of the experimental spectra, including their absolute intensities.

A Time‐Dependent Picture of the Ultrafast Deactivation of keto‐Cytosine Including Three‐State Conical Intersections

“…Our band is blue-shifted in comparison to that obtained with EOM-CC [54] and with RI-CC2 [39] because CASSCF does not include dynamical correlation. In our spectrum we do not assign the different peaks as directly coming from the np* or pp* states, since at nonplanar geometries, the orbitals are clearly mixed.…”

“…The amplitude of each Gaussian was defined by the square of the corresponding transition dipole moment and the width is set up with a parameter that we specify later. Further details are nicely described in the recent work of Barbatti et al [39] The simulation of the electronic deactivation in the ketocytosine was done using mixed quantum-classical dynamics (MQCD) simulations. As initial conditions, the geometries of the Wigner distribution are used, starting the calculation from the bright states.…”

“…Tajti and co-workers [54] at 39 000 cm À1 and Lischka et al [39] at 37 500 cm À1 . Our band is blue-shifted in comparison to that obtained with EOM-CC [54] and with RI-CC2 [39] because CASSCF does not include dynamical correlation.…”

“…[54] More recently, Lischka and co-workers have also calculated the spectrum of cytosine (and other nucleobases) using semiclassical simulations at the RI-CC2/aug-TZVP level of theory. [39] In their work, they obtained a three-band spectra (around 37 500, 44 000 and 52 000 cm À1 ), where the first and second bands are formed mainly by a single pp* transition and the third is a broad band composed of np* plus Rydberg and pp* plus Rydberg (3s) states.…”

Ultrafast Dynamics of UV‐Excited Imidazole

“…[52] The considered basis sets were TZVPP, [50] aug-cc-pVDZ, aug-cc-pVTZ, (d)-aug-cc-pVDZ, and (d)-aug-ccpVTZ. [53] The absorption spectrum of imidazole was calculated by the method reported in reference [54]. The ground-state geometries were sampled by means of a harmonic-oscillator Wigner distribution.…”