C. Desfrançois

ELECTRON ATTACHMENT TO ISOLATED NUCLEIC ACID BASES

Uracil, thymine, and adenine anions were produced in charge‐exchange collisions with laser‐excited Rydberg atoms. Anion creation rates for uracil and thymine exhibit Rydberg electron energy dependences which are interpreted as due to the creation of both dipole‐bound and conventional (valence) anions while only dipole‐bound anions are observed for adenine.

GROUND-STATE DIPOLE-BOUND ANIONS

Ground-state dipole-bound anions are fragile molecular species which excess electrons are almost entirely located in a very diffuse orbital outside the molecular frame. They can be created by attachment of very low energy electrons to polar molecules or small clusters which dipole moments are larger than a practical critical value of 2.5 D. They present analogies with Rydberg atoms and their geometrical structures are nearly identical to those of their neutral parents. Experimentally, dipole-binding of electrons to polar systems is a non-perturbative and reversible ionization process, in contrast with conventional valence-binding. Examples of applications such as mass-spectrometric isomer selection of clusters or determination of electron attachment properties of isolated nucleic acid bases are given.

On the binding of electrons to nitromethane: Dipole and valence bound anions

Conventional (valence) and dipole‐bound anions of the nitromethane molecule are studied using negative ion photoelectron spectroscopy, Rydberg charge exchange and field detachment techniques. Reaction rates for charge exchange between Cs(ns,nd) and Xe(nf ) Rydberg atoms with CH3NO2 exhibit a pronounced maximum at an effective quantum number of n*≊13±1 which is characteristic of the formation of dipole‐bound anions [μ(CH3NO2)=3.46 D]. However, the breadth (Δn≊5, FWHM) of the n‐dependence of the reaction rate is also interpreted to be indicative of direct attachment into a valence anion state via a ‘‘doorway’’ dipole anion state. Studies of the electric field detachment of CH3NO−2 formed through the Xe(nf ) reactions at various n values provide further evidence for the formation of both a dipole‐bound anion as well as a contribution from the valence bound anion. Analysis of the field ionization data yields a dipole electron affinity of 12±3 meV. Photodetachment of CH3NO−2 and CD3NO−2 formed via a supersonic...

Cluster size effects upon anion solvation of N-heterocyclic molecules and nucleic acid bases

Abstract Rydberg electron transfer spectroscopy is used for the experimental determination of slightly negative electron affinities of N-heterocyclic molecules, pyridine, pyridazine, pyrimidine, pyrazine and DNA bases, adenine, cytosine, thymine and uracil. These molecules are solvated inside clusters of rare gases, water, ammonia or toluene in order to stabilize their anions against autodetachment, and the anion formation process is studied as a function of the number of solvent atoms or molecules. Determination of the cluster size thresholds above which valence anions are observed provides estimations of the electron affinities which are compared with the results of other experimental or theoretical determinations.

Ultrafast deactivation mechanisms of protonated aromatic amino acids following UV excitation

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 pi sigma* state and the initially excited pi pi* state.

Resonant infrared multiphoton dissociation spectroscopy of gas-phase protonated peptides. Experiments and Car–Parrinello dynamics at 300 K

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.

Investigation of the protonation site in the dialanine peptide by infrared multiphoton dissociation spectroscopy

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.

Electron binding to valence and multipole states of molecules: Nitrobenzene, para- and meta-dinitrobenzenes

Nitrobenzene anions (NB−) in both valence and dipole bound states are examined using laser (photodetachment) photoelectron and Rydberg electron transfer (RET) spectroscopies. Photoelectron spectroscopy of the valence NB− anion yields a valence (adiabatic) electron affinity of 1.00±0.01 eV. The reaction rates for charge transfer between atoms of cesium and xenon in high Rydberg states [Cs(ns,nd) and Xe(nf )] and NB exhibit a prominent peak in their n-dependencies consistent with the formation of a dipole bound anion having an electron affinity of 28 meV. Para-dinitrobenzene (pDNB) has a zero dipole moment and a large quadrupole moment. RET studies with pDNB show a complex n-dependence. The rate of formation of pDNB− ions exhibits a broad peak at low n-values and a second very broad feature extending to large n-values. The peak at low n is tentatively attributed to charge exchange into a quadrupole bound state (EAqb=25 meV). The absence of field-detachment for these ions suggests that if these are in a quad...

Dipole binding: An experimental test for small cluster structure calculations

An experimental method for the discrimination between nearly degenerate isomers of size selected clusters of closed‐shell polar molecules is presented. It is based upon electron attachment properties of dipole fields since a minimum value (≊2.5 D) of molecular dipoles is required for electron binding. When neutral clusters are created in different configurations only those with large enough resulting dipole moments bind electrons and give birth to stable anions. These dipole‐bound anions have the geometries of their neutral parents which are here calculated within the framework of the exchange perturbation theory as developed by Claverie. Anions are created by Rydberg electron transfer to cold neutral clusters containing up to six molecules of acetonitrile, water, ammonia, or methanol. Structure and dipole moment calculations account well for the ‘‘magic’’ numbers observed in mass spectra distributions. Detailed comparisons between experimental data and calculated geometries lead to informations on the st...

Photo-induced dissociation of protonated tryptophan TrpH+: A direct dissociation channel in the excited states controls the hydrogen atom loss

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 Cα–Cβ 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 ππ*/πσ* state, the ππ* state being mainly located on the indole chromophore while the πσ* is mainly on the protonated terminal amino group. This πσ* 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.