Title: The triplet state of cytosine and its derivatives: Electron impact and quantum chemical study
Authors: Abouaf, R ×
Pommier, J
Dunet, H
Quan, P
Nam, PC
Nguyen, Minh Tho #
Issue Date: 2004
Series Title: Journal of Chemical Physics vol:121 issue:23 pages:11668-11674
Conference: date:Univ Paris 11, Collis Atom & Mol Lab, UMR 8625, F-91405 Orsay, France; Univ Louvain, Dept Chem, B-3001 Louvain, Belgium; HoChiMinh City Univ, Coll Nat Sci, Fac Chem, Thanh Pho Ho Chi Minh, Vietnam; Univ Danang, Fac Chem Engn, Danang, Vietnam
Abstract: The excitation of the lowest electronic states and vibrational excitation of cytosine (C) have been studied using electron energy loss spectroscopy (EELS, 0-100 eV) with angular analysis. The singlet states have been found to be in good agreement with UV-VIS absorption results on sublimed films, slightly blueshifted by about 0.1 eV. The EEL spectra recorded at residual energy below 2 eV show clear shoulders at energy losses of 3.50 and 4.25 eV (+/-0.1 eV). They are assigned to the lowest triplet electronic states of cytosine. Energies and molecular structures of the lowest-lying triplet state of C and its methylated and halogenated 5-X-C, 6-X-C, and 5-X, 6-X-C substituted derivatives (X=CH3, F, Cl, and Br) have been studied using quantum chemical calculations with both molecular orbital and density functional methods, in conjunction with the 6-311++G(d,p), 6-311++G(3df,2p), and aug-cc-pVTZ basis sets. The triplet-singlet energy gap obtained using coupled-cluster theory [CCSD(T)] and density functional theory (DFT) methods agrees well with those derived from EELS study. The first C's vertical triplet state is located at 3.6 eV, in good agreement with experiment. The weak band observed at 4.25 eV is tentatively assigned to the second C's vertical triplet excitation. For the substituted cytosines considered, the vertical triplet state is consistently centered at 3.0-3.2 eV above the corresponding singlet ground state but about 1.0 eV below the first excited singlet state. Geometrical relaxation involving out-of-plane distortions of hydrogen atoms leads to a stabilization of 0.6-1.0 eV in favor of the equilibrium triplet. The lowest-lying adiabatic triplet states are located at 2.3-3.0 eV. Halogen substitution at both C(5) and C(6) positions tends to reduce the triplet-singlet separations whereas methylation tends to enlarge it. The vibrational modes of triplet cytosine and the ionization energies of substituted derivatives were also evaluated. (C) 2004 American Institute of Physics.
ISSN: 0021-9606
Publication status: published
KU Leuven publication type: IT
Appears in Collections:Quantum Chemistry and Physical Chemistry Section
× corresponding author
# (joint) last author

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