Ionization of cytosine monomer and dimer studied by VUV photoionization and electronic structure calculations
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 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.
- Research Organization:
- Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, CA (US)
- Sponsoring Organization:
- Chemical Sciences Division
- DOE Contract Number:
- AC02-05CH11231
- OSTI ID:
- 981356
- Report Number(s):
- LBNL-2986E
- Journal Information:
- Physical Chemistry Chemical Physics, Journal Name: Physical Chemistry Chemical Physics Journal Issue: 12 Vol. 2010; ISSN 1463-9076
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
99 GENERAL AND MISCELLANEOUS
CYTOSINE
DIMERS
DIPOLE MOMENTS
DISSOCIATION
EFFICIENCY
ELECTRONIC STRUCTURE
ELECTROSTATICS
EVAPORATION
IONIZATION
ISOMERS
Ionization
cytosine monomer
dimer
electronic structure calculations.
MASS SPECTROSCOPY
MOLECULAR BEAMS
MONOMERS
PHOTOIONIZATION
PHOTONS
PROTONS
RED SHIFT
SPECTRA
SYNCHROTRON RADIATION