We studied the photoionization of 2-pyridone and its tautomer, 2-hydroxypyridine by means of VUV synchrotron radiation coupled to a velocity map imaging electron/ion coincidence spectrometer. The photoionization efficiency (PIE) spectrum is composed of steps. The state energies of the [2-pyridone](+) cation in the X[combining tilde] ground and A excited electronic states, as well as of the [2-hydroxypyridine](+) cation in the electronic ground state, are determined. The slow photoelectron spectra (SPES) are dominated by the 0(0)(0) transitions to the corresponding electronic states together with several weaker bands corresponding to the population of the pure or combination vibrational bands of the cations. These vibrationally-resolved spectra compare very well with state-of-the-art calculations. Close to the ionization thresholds, the photoionization of these molecules is found to be mainly dominated by a direct process whereas the indirect route (autoionization) may contribute at higher energies.

Protonated and anionic artemisinin in the gas phase have respectively been studied by infrared multi-photon dissociation (IRMPD) spectroscopy and by anion photoelectron spectroscopy. Comparison of the measured IRMPD spectrum with calculated spectra of various conformations showed that the two lowest-energy protonated structures, both corresponding to protonation at the CO14 carbonyl site, were observed experimentally. The calculations also indicated that the peroxide bridge in artemisinin is only slightly modified by protonation. Additionally, stable, intact (parent) artemisinin radical anions have been obtained for the first time in the gas phase and the photoelectron spectrum supports the computational finding that the excess electron is mainly localized on the σ∗ orbital of the peroxide bond. The vertical detachment energy and adiabatic electron affinity, calculated at the MP2/6-31+G∗ level, are in good agreement with the experimental data and the O–O distance is calculated to be stretched by more than 50% in the anion.

The gas-phase structures of protonated peptides are studied by means of resonant infrared multiphoton dissociation spectroscopy (R-IRMPD) performed with a free electron laser. The peptide structures and protonation sites are obtained through comparison between experimental IR spectra and their prediction from quantum chemistry calculations. Two different analyses are conducted. It is first supposed that only well-defined conformations, sufficiently populated according to a Boltzmann distribution, contribute to the observed spectra. On the contrary, DFT-based Car-Parrinello molecular dynamics simulations show that at 300 K protonated peptides no longer possess well-defined structures, but rather dynamically explore the set of conformations considered in the first conventional approach.

Finite temperature Car-Parrinello molecular dynamics simulations are performed for the protonated dialanine peptide in vacuo, in relation to infrared multiphoton dissociation experiments. The simulations emphasize the flexibility of the different torsional angles at room temperature and the dynamical exchange between different conformers which were previously identified as stable at 0 K. A proton transfer occurring spontaneously at the N-terminal side is also observed and characterized. The theoretical infrared absorption spectrum is computed from the dipole time correlation function, and, in contrast to traditional static electronic structure calculations, it accounts directly for anharmonic and finite temperature effects. The comparison to the experimental infrared multiphoton dissociation spectrum turns out very good in terms of both band positions and band shapes. It does help the identification of a predominant conformer and the attribution of the different bands. The synergy shown between the experimental and theoretical approaches opens the door to the study of the vibrational properties of complex and floppy biomolecules in the gas phase at finite temperature.

The electronic excited state dynamics of protonated tryptamine ions generated by an electrospray source have been studied by means of photoinduced dissociation technique on the femtosecond time scale. The result is that the initially excited state decays very quickly within 250 fs. The photoinduced dissociation channels observed can be sorted in two groups of fragments coming from two competing primary processes on the singlet electronic surface. The first one corresponds to a hydrogen-atom loss channel that creates a tryptamine radical cation. The radical cation subsequently fragments to smaller ions. The second process is internal conversion due to the H-atom recombination on the electronic ground state. Time-dependent density functional theory calculations show that an excited pisigma* state dissociative along the protonated amino N-H stretch crosses both the locally excited pipi* state and the electronic ground state S(0) and thus triggers the photofragmentation reactions. The two processes have equivalent quantum yields, approximately equal to 50% of the fragments coming from the H-atom loss reaction. The two primary reaction paths can clearly be distinguished by their femtosecond pump/probe dynamics recorded on the different fragmentation channels.

Electron localization is studied in formamide cluster anions. The isolated formamide molecule has a large dipole moment and its clusters can give birth to multipole-bound anions as well as valence anions. The vertical valence electron affinity of the isolated molecule is determined by electron transmission spectroscopy. The anion formation process is studied as a function of cluster size with Rydberg electron transfer spectroscopy. DFT calculations of the neutral and negatively-charged cluster structures show that the anion excess electron localizes on a single molecule. The adiabatic valence electron affinity of isolated formamide is deduced from the observation of the cluster size threshold for valence attachment.

Using Fourier transform ion cyclotron resonance mass-spectrometry (FT-ICR-MS) in combination with infrared multiphoton dissociation (IRMPD) spectroscopy, at the free electron laser (FEL) facility CLIO in Orsay (France), we obtain the IR spectra, in the 900–1900 cm−1 range, of two model protonated dipeptides: N-acetyl-alanine (AcNH-Ala) and alanyl-histidine (Ala-His). By comparison with simulated spectra, calculated at the B3LYP/6-31++G** level for many low-lying possible conformers of these two species, we are able to assign the position of the protonation site, on the acetyl oxygen for AcNH-Ala and on the side-chain imidazole nitrogen for Ala-His, and to obtain some more information on the low-lying equilibrium structures of these two species in the gas phase.

The excited-state dynamics of protonated tryptophan ions is investigated by photoinduced fragmentation in the gas phase. In contrast to the neutral molecule that decays on the nanosecond time scale, the protonated species exhibits an ultrafast decay with two time constants of about 400 fs and 15 ps. In addition, after UV excitation by a pump photon at 266 nm, specific photofragments, and in particular the NH3-loss channel, can be enhanced by the absorption of a probe photon at 800 nm. The bond-cleaving reactions can thus be controlled by a variation of the pump/probe delay.

Deactivation pathways of electronically excited states have been investigated in three protonated aromatic amino acids: tryptophan (Trp), tyrosine (Tyr) and phenylalanine (Phe). The protonated amino acids were generated by electrospray and excited with a 266 nm femtosecond laser, the subsequent decay of the excited states being monitored through fragmentation of the ions induced and/or enhanced by another femtosecond pulse at 800 nm. The excited state of TrpH(+) decays in 380 fs and gives rise to two channels: hydrogen atom dissociation or internal conversion (IC). In TyrH(+), the decay is slowed down to 22.3 ps and the fragmentation efficiency of PheH(+) is so low that the decay cannot be measured with the available laser. The variation of the excited state lifetime between TrpH(+) and TyrH(+) can be ascribed to energy differences between the dissociative pisigma* state and the initially excited pipi* state.

The paper presents IR spectroscopic data and theoretical frequency calculations, in the C–H and N–H stretch region, on jet-cooled formamide molecules and dimers. By using dipole-bound or quadrupole-bound anion formation, one is able to monitor IR absorption in the neutral beam on mass-selected species. For isolated formamide, resonant IR excitation leads to autodetachment of the excess electron in the dipole-bound anion. For the formamide dimer, the IR spectrum is characteristic from the cyclic dimer, with two neighbouring bonded N–H stretches and a complex spectrum with several lines and a broad absorption band in the C–H region absorption, as for the cyclic formic acid dimer. In this double H-bond configuration, IR absorption cannot lead to vibrational predissociation but it probably produces hot neutral dimers with large-amplitude intermolecular motions that reinforce autodetachment of the nascent weakly bound anions. Thus, the present original IR spectroscopic technique, which allows for rigorous mass-selection of the neutrals, seems to be relevant for any polar neutral species.

. Critical binding of electrons or positrons to molecular systems possessing large enough dipole moments is considered. Predictions of models are compared to quantum chemistry calculations and experimental determinations.

We report gas-phase experimental and theoretical results on the configurations of weakly-bound neutral polar complexes without chromophores: the water dimer and the formamide-water complex. Experimental data are obtained by combining infrared (IR) absorption spectroscopy, in the 2800-3800 cm(-1) domain, with the Rydberg electron transfer (RET) technique leading to dipole-bound anion (DBA) formation. In the absence of IR excitation, RET to neutral complexes with a given total dipole moment, and thus a given molecular structure, leads to DBAs which are observed without any possible fragmentation. In the presence of the IR laser, prior to ionisation, resonant IR absorption of intramolecular vibrations of the parent neutral complexes can either induce the breaking of the weak intermolecular bonds (vibrational predissociation of the neutral) or the fast departure of the excess electron after RET (autodetachment of the DBA). Anion signal depletion, monitored at the parent mass, is then a signature of resonant IR absorption from mass- and structure-selected neutral complexes. The validity of the present experimental method and of different types of quantum chemistry calculations is discussed by comparison between calculated harmonic or anharmonic frequencies, the present experimental gas-phase IR spectra, and previous experimental data on these two test-case hydrogen-bonded complexes.

An excess electron can be bound to a molecule in a very diffuse orbital as a result of the long-range contributions of the molecular electrostatic field. Following a systematic search, we report experimental evidence that quadrupole binding occurs for the trans-succinonitrile molecule (EA=20+/-2 meV), while the gauche-succinonitrile conformer supports a dipole-bound anion state (EA=108+/-10 meV). Theoretical calculations at the DFT/B3LYP level support these interpretations and give electron affinities of 20 and 138 meV, respectively.

Protonated tryptophan ions (TrpH(+)) are generated by electrospray ionization and dissociated by irradiation with a UV laser. Different photo-fragments are observed among which a new photo-induced dissociation channel leading to the loss of a hydrogen atom that is not observed in conventional collision-induced dissociation. A tryptophan radical cation (Trp(+)) is produced in this process that subsequently leads to the m/z = 130 fragment through a C-alpha-C-beta bond cleavage, a typical fragmentation product of the Trp(+) radical cation generated either by electron impact or by photo-ionization. These results can be understood considering the excited states of protonated tryptophan: UV excitation of TrpH(+) produces a mixed pipi*/pisigma* state, the pipi* state being mainly located on the indole chromophore while the pisigma* is mainly on the protonated terminal amino group. This pisigma* state is repulsive along the N-H bond coordinate and leads either to hydrogen atom detachment producing a Trp(+) radical cation that undergoes further fragmentations or to internal conversion to the ground state of the protonated TrpH(+) ion.

Protonated dialanine cations have been isolated in a Fourier transform ion cyclotron resonance mass-spectrometer (FT-ICR-MS) and subjected to infrared multiphoton dissociation (IRMPD) at the free electron laser facility CLIO in Orsay (France). The spectral dependence of the IR induced fragmentation pattern in the mid-infrared region (800-2000 cm(-1)) is interpreted with the help of structure and vibrational spectrum calculations of the different protonated conformers. This comparison allows for the assignment of the proton on the terminal amino group, as the most favourable proton site, the neighbouring amide bond being in the trans conformation.

We report experimental (Rydberg electron transfer) and theoretical results on dipole-bound anions of urea, deuterated urea, 1,1- and 1,3-dimethylureas and tetramethylurea. For the 1,1-dimethylurea and tetramethylurea molecules, which possess only one low-lying energy conformer, the experimental excess electron energies are found to be in good agreement with neutral structure quantum chemistry calculations and semi-empirical model calculations of the corresponding dipole-bound anions. For 1,3-dimethylurea two low-lying energy conformers (trans–trans and cis–trans) should contribute to the dipole-bound anion formation behaviour but cannot fully account for it. The large-amplitude motion, associated with the anti/syn conformations of urea molecules, has been studied by performing experiments on the deuterated species and by calculating the potential energy surfaces, for both the neutral and the dipole-bound anion, along the corresponding coordinate. Even if this low-frequency mode is very likely to be involved in the anomalous anion formation behaviour, a full understanding of this process could only come from a dynamic theory, beyond the Born–Oppenheimer approximation, of both the electron and the large-amplitude nuclear motions which possess similar characteristic frequencies.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Using Rydberg Electron Transfer Spectroscopy, formation of dipole-bound anion complexes of formamide, N-methylformamide, N,N-dimethylformamide and N-methylacetamide with water has been studied. Each neutral complex can exist with several configurations and the lowest energy structures have been identified through comparison between Density Functional Theory calculations of the neutrals and measured electron binding energies of the observed weakly-bound anions.