Issue 19, 2010

Ab initio investigation of the methylation and hydration effects on the electronic spectra of uracil and thymine

Abstract

In this work we investigated the lowest-lying electronic excitations for a series of methyl-substituted uracil derivatives, i.e., uracil, 1-methyluracil, 3-methyluracil, thymine, 1-methylthymine, 1,3-dimethyluracil, 3-methylthymine, 1,3-dimethylthymine, and their microhydrated complexes by means of coupled cluster singles and approximate doubles (CC2) and density functional theory (DFT) methods. The bulk water environment was mimicked by a combination of microhydration and the conductor-like screening model (COSMO). We find that the shift of the electronic excitation energies due to methylation and hydration depend on the character of the wave function and on the position of the methyl substituent. The lowest-lying singlet and triplet n → π* states are insensitive to methylation but are strongly blue-shifted by microhydration and bulk water solvation. The largest red-shift of the first 1(π → π*) excitation occurs upon methylation at N1 followed by substitution at C5 whereas no effect is obtained for a methylation at N3. For this state, the effects of methylation and hydrogen bonding partially cancel. Upon microhydration with six water molecules, the order of the 1(n → π*) and 1(π → π*) states is reversed in the vertical spectrum. Electrostatic solute–solvent interaction in bulk water leads to a further increase of their energy separation. The n → π* states are important intermediates for the triplet formation. Shifting them energetically above the primarily excited 1(π → π*) state will considerably decrease the triplet quantum yield and thus increase the photostability of the compounds, in agreement with experimental observations.

Graphical abstract: Ab initio investigation of the methylation and hydration effects on the electronic spectra of uracil and thymine

Supplementary files

Article information

Article type
Paper
Submitted
07 Dec 2009
Accepted
02 Mar 2010
First published
27 Mar 2010

Phys. Chem. Chem. Phys., 2010,12, 4915-4923

Ab initio investigation of the methylation and hydration effects on the electronic spectra of uracil and thymine

M. Etinski and C. M. Marian, Phys. Chem. Chem. Phys., 2010, 12, 4915 DOI: 10.1039/B925677F

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