Radiationless deactivation pathways of excited gas phase nucleobases were investigated using mass-selected femtosecond resolved pump-probe resonant ionization. By comparison between nucleobases and methylated species, in which tautomerism cannot occur, we can access intrinsic mechanisms at a time resolution never reported so far (80 fs). At this time resolution, and using appropriate substitution, real nuclear motion corresponding to active vibrational modes along deactivation coordinates can actually be probed. We provide evidence for the existence of a two-step decay mechanism, following a 267 nm excitation of the nucleobases. The time resolution achieved together with a careful zero time-delay calibration between lasers allow us to show that the first step does correspond to intrinsic dynamics rather than to a laser cross correlation. For adenine and 9-methyladenine a first decay component of about 100 fs has been measured. This first step is radically increased to 200 fs when the amino group hydrogen atoms of adenine are substituted by methyl groups. Our results could be rationalized according to the effect of the highly localized nature of the excitation combined to the presence of efficient deactivation pathway along both pyrimidine ring and amino group out-of-plane vibrational modes. These nuclear motions play a key role in the vibronic coupling between the initially excited pipi* and the dark npi* states. This seems to be the common mechanism that opens up the earlier phase of the internal conversion pathway which then, in consideration of the rather fast relaxation times observed, would probably proceed via conical intersection between the npi* relay state and high vibrational levels of the ground state.

International audienceThe present IR-UV depletion spectroscopic study extends the recent data of literature by providing evidence for the existence of a fourth form of guanine in the gas phase. The comparison of the UV and IR signatures of the four forms together with those of the 7- and 9-methylated derivatives allows us to build up a new assignment in terms of enol/keto and 7/9NH tautomerisms. From this complete picture, it turns out that the UV spectroscopy of free guanine is mainly controlled by the 7/9NH tautomerism: both 7NH tautomers observed are red-shifted compared to the 9NH ones, with the following origin transition order, from red to blue: 7NH enol (32864 ± 5 cm-1), 7NH keto (+405 cm-1), 9NH keto (+1046 cm-1), and 9NH enol (+1891 cm-1); 7-, 9- or 1-methylations are found to cause only moderate red shifts (less than 400 cm-1). The opposite trend is observed for the IR spectroscopy, which appears to be essentially controlled by the enol/keto tautomerism. This study exemplifies the need for cross-checked experimental approaches, namely the IR/UV depletion spectroscopy or the study of relevant methylated species, to reach a global and consistent assignment, even in rather simple biological systems such as purine bases

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Articles you may be interested inEffect of chemical substitutions on photo-switching properties of 3-hydroxy-picolinic acid studied by ab initio methods J. Chem. Phys. 140, 084301 (2014); 10.1063/1.4865815 On the origin of ultrafast nonradiative transitions in nitro-polycyclic aromatic hydrocarbons: Excited-state dynamics in 1-nitronaphthalene J. Chem. Phys. 131, 224518 (2009); 10.1063/1.3272536Mass-analyzed threshold ionization study of vinyl bromide cation in the first excited electronic state using vacuum-ultraviolet radiation generated by four-wave mixing in HgThe time evolution of the first excited states of ethylene, and alkyl substituted ethylenes, isomers with formula C 6 H 12 , has been studied by the femtosecond pump probe method, using mass spectrometric detection, in the region of 6 eV ͑200 nm͒. Two cyclic alkenes of the formula C 6 H 10 have also been studied. These systems exhibit a multi-exponential decay characterized by a very short time decay, ranging from 20 fs͑ethylene͒ to 100 fs ͑trans hex-2-ene͒ and a longer decay, in the picosecond range follows for most of the alkyl isomers. The short time evolution is characteristic of wave packet motion on a steep potential surface. The initial motion has been identified as the torsion about the CC double bond resulting from excitation of the valence state. The evolution of the valence excited state of excited state ethylene ͓first studied by the group of Radloff, Chem. Phys. Lett. 288, 2044 ͑1997͔͒ has been taken as a reference. The extremely rapid evolution, 20 fs, without any longer temporal component is explained by the disappearance of the wave packet from the Franck-Condon region into a conical intersection leading to the ground state surface by reference to the theoretical calculations of Ohmine ͓J. Chem. Phys. 83, 2348 ͑1985͔͒. This motion is essentially multidimensional to reach the funnel to the ground state; it combines the torsion about the CC double bond with a pyramidalization about one of the carbon atoms and/or H atom migration from one carbon to the other. Cyclic alkenes exhibit a similar behavior as ethylene with a single ultrashort decay that arises from this same mechanism. Also in the other substituted alkenes the short decay has been assigned to the wave packet motion away from the Franck-Condon region under the influence of the torsion about the double bond. The final longer decay could also be captured in the case of tetramethylethylene by a 800 nm probe as the internal conversion to the ground state via a funnel more difficult to reach. These measurements emphasize the role of conical intersections which could not be brought into evidence without time dependent methods.

Threshold photoelectron spectra (TPES) of the isotopomers of the methyl radical (CH(3), CH(2)D, CHD(2), and CD(3)) have been recorded in the 9.5-10.5 eV VUV photon energy range using third generation synchrotron radiation to investigate the vibrational spectroscopy of the corresponding cations at a 7-11 meV resolution. A threshold photoelectron-photoion coincidence (TPEPICO) spectrometer based on velocity map imaging and Wiley-McLaren time-of-flight has been used to simultaneously record the TPES of several radical species produced in a Ar-seeded beam by dc flash-pyrolysis of nitromethane (CH(x)D(y)NO(2), x + y = 3). Vibrational bands belonging to the symmetric stretching and out-of-plane bending modes have been observed and P, Q, and R branches have been identified in the analysis of the rotational profiles. Vibrational configuration interaction (VCI), in conjunction with near-equilibrium potential energy surfaces calculated by the explicitly correlated coupled cluster method CCSD(T*)-F12a, is used to calculate vibrational frequencies for the four radical isotopomers and the corresponding cations. Agreement with data from high-resolution IR spectroscopy is very good and a large number of predictions is made. In particular, the calculated wavenumbers for the out-of-plane bending vibrations, nu(2)(CH(3)(+)) = 1404 cm(-1), nu(4)(CH(2)D(+)) = 1308 cm(-1), nu(4)(CHD(2)(+)) = 1205 cm(-1), and nu(2)(CD(3)(+)) = 1090 cm(-1), should be accurate to ca. 2 cm(-1). Additionally, computed Franck-Condon factors are used to estimate the importance of autoionization relative to direct ionization. The chosen models globally account for the observed transitions, but in contrast to PES spectroscopy, evidence for rotational and vibrational autoionization is found. It is shown that state-selected methyl cations can be produced by TPEPICO spectroscopy for ion-molecule reaction studies, which are very important for the understanding of the planetary ionosphere chemistry.

To cite this version:C. Canuel, Mohamed Elhanine, M. Mons, F. Piuzzi, B. Tardivel, et al.. Time-resolved photoelectron and photoion fragmentation spectroscopy study of 9-methyladenine and its hydrates: a contribution to the understanding of the ultrafast radiationless decay of excited DNA bases.. Physical Chemistry Chemical Physics, Royal Society of Chemistry, 2006, 8, pp.3978-3987. <10.1039/b606437j>. AbstractThe excited state dynamics of the purine base 9-methyladenine (9-MeAde) has been investigated by time-and energy-resolved photoelectron imaging spectroscopy and massselected ion spectroscopy both in vacuum and water-cluster environment. The specific probe processes used, namely a careful monitoring of time-resolved photoelectron energy distributions and of photoion fragmentation, together with the excellent temporal resolution achieved enable us to derive additional information on the nature of the excited (ππ*, nπ*, πσ*, triplet) states involved in the electronic relaxation of adenine. The two-step pathway we propose to account for the double exponential decay observed agrees well with the recent theoretical calculations. The near UV photophysics of 9MeAde is dominated by the direct excitation of the ππ* ( 1 L b ) state (lifetime of 100 fs), followed by internal conversion, proceeding to the nπ* state (lifetime in the ps range) via conical intersection. No evidence for the involvement of a πσ* or a triplet state was found. 9MeAde-water clusters have been studied, focussing onto the fragmentation of these species after the probe process. A careful analysis of the fragments allowed us to provide evidence for a double exponential decay profile for the hydrates. The very weak second component observed, however, leads to conclude to a very different photophysics as compared to the isolated base, assigned to a competition between i) a direct one-step decay of the initially excited state (ππ* L a and/or L b , stabilised by hydration) to the ground state and ii) a modified two-step decay scheme, qualitatively comparable to that occurring in the isolated molecule.

The solvation of group II metals has not been studied in great details as compared to alkali metals and we show that due to the closed shell nature of these metals, the clusters possess unique features. Besides clusters allow the observation of singly ionised solvated ions that do not exist in solution and the characterisation of their properties is important since many of these singly ionised systems are reactive. The ionisation thresholds of magnesium atom solvated by ammonia have been investigated by tuneable laser ionisation of clusters prepared in a simple pickup source. The ionisation potential IP of clusters Mg(NH3)n have been measured for n =1 up to 35. The solvation of the magnesium ion is more efficient than for the closed shell neutral atoms resulting in a steep decrease of the ionisation energy of clusters with ammonia of increasing sizes (1.3 eV for n=1, 2.4 eV for n=2 and 3.2 eV for n=3 etc...). The effect becomes smaller large clusters (n¿20)for higher order clusters but the potential continues to decrease and the asymptote does not appear to be reached at n=35 , corresponding to increased electrostatic stabilisation of the ion cluster with these sizes. Quantum chemical calculations have been performed which reveal the unique features of the solvation of the neutral closed shell metal, ns2 and the metal ion ns1 by ammonia molecules. The formation of neutral clusters are dominated by singly coordinated cyclic sub-units containing 3 ammonia, while the ion is best stabilised in triply coordinated magnesium. The experimental ionisation energies are in excellent agreement with calculated values for singly coordinated, neutral cluster ionised to locally adiabatic states, doubly coordinated with ammonia.

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