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Issue 12, 2010
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Ionization of cytosine monomer and dimer studied by VUV photoionization and electronic structure calculations

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Abstract

We report a combined theoretical and experimental study of ionization of cytosine monomers and dimers. Gas-phase molecules are generated by thermal vaporization of cytosine followed by expansion of the vapor in a continuous supersonic jet seeded in Ar. The resulting species are investigated by single photon ionization with tunable vacuum-ultraviolet (VUV) synchrotron radiation and mass analyzed using reflectron mass spectrometry. Energy onsets for the measured photoionization efficiency (PIE) spectra are 8.60 ± 0.05 eV and 7.6 ± 0.1 eV for the monomer and the dimer, respectively, and provide an estimate for the adiabatic ionization energies (AIE). The first AIE and the ten lowest vertical ionization energies (VIEs) for selected isomers of cytosine dimer computed using equation-of-motion coupled-cluster (EOM-IP-CCSD) method are reported. The comparison of the computed VIEs with the derivative of the PIE spectra suggests that multiple isomers of the cytosine dimer are present in the molecular beam. The calculations reveal that the large red shift (0.7 eV) of the first IE of the lowest-energy cytosine dimer is due to strong inter-fragment electrostatic interactions, i.e., the hole localized on one of the fragments is stabilized by the dipole moment of the other. A sharp rise in the protonated cytosine ion (CH+) signal at 9.20 ± 0.05 eV is ascribed to the formation of protonated cytosine by dissociation of the ionized dimers. The dominant role of this channel is supported by the computed energy thresholds for the CH+ appearance and the barrierless or nearly barrierless ionization-induced proton transfer observed for five isomers of the dimer.

Graphical abstract: Ionization of cytosine monomer and dimer studied by VUV photoionization and electronic structure calculations

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Publication details

The article was received on 14 Oct 2009, accepted on 11 Jan 2010 and first published on 09 Feb 2010


Article type: Paper
DOI: 10.1039/B921498D
Citation: Phys. Chem. Chem. Phys., 2010,12, 2860-2872
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    Ionization of cytosine monomer and dimer studied by VUV photoionization and electronic structure calculations

    O. Kostko, K. Bravaya, A. Krylov and M. Ahmed, Phys. Chem. Chem. Phys., 2010, 12, 2860
    DOI: 10.1039/B921498D

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