REMPI spectroscopy of cytosine

Abstract

We report resonant two-photon ionization spectra of laser desorbed, jet cooled, cytosine, 1-methyl cytosine, 5-methyl cytosine, and dimers of these. Unlike other pyrimidine bases, cytosine exhibits vibronic spectra with sharp features in two spectral regions, separated by about 5000 cm À1 . We interpret these as being due to two tautomeric forms, one keto and one enol. The dimers absorb at wavelengths that are intermediate between those of the two monomer forms. By UV-UV hole burning we determined the numbers of isomers contributing to each spectrum and by delayed two color ionization we determined triplet lifetimes. We observed hydrogen transfer between bases both in collisions between monomers and after photo-excitation in clusters. Ó

Photophysics of cytosine tautomers: new insights into the nonradiative decay mechanisms from MS-CASPT2 potential energy calculations and excited-state molecular dynamics simulations

“…The notation of MS(l)-CASPT2(m,n) is also used, in which case the SA(l)-CASSCF(m,n) wavefunction is used as the reference and the l states are mixed after the perturbation. After the geometry is determined, the potential energies are recalculated by the MS-CASPT2 method with a larger active space for the reference CASSCF wavefunctions in order to provide improved energetics for a given geometry: MS(4)-CASPT2(12,9) for keto and imino, and MS(4)-CASPT2 (10,8) for enol cytosine.…”

“…22,30 It also agrees quite well with the experimental adiabatic excitation energy of 3.95 eV estimated from the resonance-enhanced multiphoton ionization (REMPI) spectrum. 10,60 It has been proposed previously that the minimum energy path (MEP) approach is appropriate for the determination of the deactivaiton pathway since it provides information about the accessibility of the conical intersection. 29,[61][62][63] There is the possibility that it leads to the seam of CIs before reaching the ( 1 ππ*) min structure, thus providing the most relevant photochemical deactivation pathway.…”

“…In this MECI search, the S 0 , 1 n O π*, and 1 ππ* states are included with equal weights in the average, and the (10,8) active space is comprised of 7 π orbitals and lone pair orbital localized on the O7 atom. The optimized structure of ( 1 n O π*/gs)′ CI (see Figure S11) exhibits pyramidalizations of the C6 and N1 atoms as seen in ( 1 n O π*/gs) CI .…”

“…[4][5][6][7][8] Cytosine in the gas phase is of special interest among the nucleic acid bases in various environments, because several tautomeric forms are likely to coexist. Experimental [9][10][11][12][13][14][15][16][17] and theoretical [11][12][13]18 studies have revealed the coexistence of at least three lowest-energy tautomers of isolated cytosine: keto (amino-keto), imino (imino-keto), and enol (amino-enol) forms. Figure 1 shows the molecular structure of these tautomers.…”

Gas-Phase IR Spectroscopy of Nucleobases

“…While the energy difference between the enol and keto forms is very small, some of their other properties, such as their vertical excitation energies, are very different. Nir et al reported REMPI and hole burning spectra and concluded that the keto and enol tautomers have band origins that differ by a remarkable half electron volt at 314 nm and 278 nm, respectively (42,43). The REMPI spectra of U and T exhibit a very broad structure with an onset in the frequency range of the origin of the enol cytosine first reported by Brady et al (4).. No spectroscopic detail could be extracted from these broad spectra and tautomeric information was limited for a long time to bulk measurements.…”