Valdemir Ludwig

Theoretical absorption and emission spectra of 1H- and 2H-benzotriazole

Excitation energies, dipole moments, and transition properties for the lowest-lying 1(π, π*) and 1(n, π*) electronic states of two benzotriazole tautomers have been studied at the complete active space (CAS) SCF and multiconfigurational second-order perturbation (CASPT2) levels. With these results, the UV absorption spectra of 1H- and 2H-benzotriazole were first described individually, and then they were considered simultaneously in order to better understand the experimental, observed spectrum of benzotriazole. Aspects of the fluorescence of the compound have been also analyzed. The spectrum of benzotriazole is finally related to those of other azo-compounds.

DNA nucleobase properties and photoreactivity: Modeling environmental effects*

The accurate ab initio quantum chemical (QM) method multiconfigurational second-order perturbation (CASSPT2)/complete active space self-consistent field (CASSCF) has been used in conjunction with molecular mechanics (MM) procedures to compute molecular properties and photoinduced reactivity of DNA/RNA nucleobases (NABs) in isolation and within a realistic environment, in which the double helix strand, the aqueous media, and the external counterions are included. It is illustrated that the use of an MM model is helpful both to account for short- and long-range effects of the system surrounding the QM molecular core and to provide the proper structural constraints that allow more accurate QM geometry determinations.

Theoretical studies of hydrogen bonding in water -cyanides and in the base pair Gu - Cy

Abstract Density-functional (DFT) and many-body-perturbation theories (MBPT/CC) are used to study the hydrogen bonding in the water–cyanide complexes H–CN⋯H2O, H3C–CN⋯H2O and (CH3)3C–CN⋯H2O. Structures, binding energies and changes in vibrational frequencies are analyzed. The calculated CN stretching frequency is found to shift to the blue upon complexation in H–CN⋯H2O and H3C–CN⋯H2O. To investigate electron correlation effects on the binding energies of these complexes, single-point calculations are performed at the MBPT/CC (MP2, MP3, MP4, CCSD and CCSD(T)) levels using the optimized MP2 geometries. Binding energies are also obtained at different levels of DFT (B3LYP and PW91) and compared with the MBPT/CC results. All calculations include corrections for basis set superposition error (BSSE) and zero-point vibrational energies. Additionally, the triple hydrogen-bonded guanine–cytosine (Gu–Cy) base pair is analyzed. The binding energy of the Watson–Crick model for Gu–Cy is calculated using the Hartree–Fock calculations and DFT (B3LYP and BP86) methods. The results for the hydrogen bonding distances and binding energies are in good agreement with experimental and recent theoretical values. The calculated dipole moment of the Gu–Cy complex is compared with the direct vector sum of the isolated bases. After taking into account the BSSE effects we find that the electron polarization due to the hydrogen binding leads to an increase of ∼20% of the calculated dipole moment of the complex.