J. P. Schermann

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...

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...

Electron transfer collisions between small water clusters and laser‐excited Rydberg atoms

The Letters to the Editor section is divided into four categories entitled Communications, Notes, Comments, and Errata. Communications are limited to three and one halfjournal pages, and Notes, Comments, and Errata are limited to one and three-fourths journal pages as described in the Announcement in the I July 1991 issue.

Infrared Signature of DNA G-Quadruplexes in the Gas Phase

DNA oligonucleotide ions forming G-quadruplex structures were studied in the gas phase using IRMPD spectroscopy. Data interpretation on these large biomolecule ions was made using carefully chosen control experiments. The major finding is a fingerprint of hydrogen bonding in the gas phase in the guanine C6O6 stretching mode that allows probing of the conservation of G-quartets in the gas phase. The experiments demonstrate the conservation of G-quadruplex hydrogen bonds in the human telomeric sequence d(TTAGGG)4.

Experimental and ab initio theoretical studies of electron binding to formamide, N-methylformamide, and N,N-dimethylformamide

The influence of methylation upon adiabatic electron affinities of formamide (F), N- methylformamide (NMF), and N,N-dimethylformamide (DMF) is experimentally investigated by means of Rydberg electron transfer spectroscopy and calculated with the use of high-level ab initio methods. In the anions of these systems the excess electrons are captured in diffuse dipole-bound states. The methylation of formamide results in a slight increase of the dipole moment and in an increased molecular size. The two factors have opposite effects on the electron affinity. Both experimental data and theoretical results are in agreement, showing that the molecular size effect dominates and that the electron affinity noticeably decreases with the methylation.

Experimental observation of the transition between gas-phase and aqueous solution structures for acetylcholine, nicotine and muscarine ions

Structural information on acetylcholine and its two agonists, nicotine, and muscarine has been obtained from the interpretation of infrared spectra recorded in the gas-phase or in low pH aqueous solutions. Simulated IR spectra have been obtained using explicit water molecules or a polarization continuum model. The conformational space of the very flexible acetylcholine ions is modified by the presence of the solvent. Distances between its pharmacophoric groups cover a lower range in hydrated species than in isolated species. A clear signature of the shift of protonation site in nicotine ions is provided by the striking change of their infrared spectrum induced by hydration. On the contrary, structures of muscarine ions are only slightly influenced by the presence of water.

Neutral and negatively-charged formamide, N-methylformamide and dimethylformamide clusters

Abstract The structure and low energy electron attachment properties of small clusters of formamide, N-methylformamide and dimethylformamide have been studied. The experimental technique used is Rydberg electron transfer (RET) between laser-excited atoms and the molecular systems. The richness of hydrogen bonding possibilities of these three highly polar molecules leads to very different structures and resultant dipoles or quadrupoles which we determine by means of an empirical model. A corresponding wide variety of cluster anions is observed and interpreted, ranging from dipole-bound or quadrupole-bound anions in their ground state or first excited state to valence-type anions.

Experimental and theoretical ab initio study of the influence of N-methylation on the dipole-bound electron affinities of thymine and uracil

The influence of N-methylation on the dipole-bound electron affinities of pyrimidine nucleic acid bases, uracil and thymine, has been investigated theoretically using ab initio quantum mechanical calculations, and experimentally using Rydberg electron transfer spectroscopy. Both experiment and theory are consistent in showing that replacement of hydrogen atoms by methyl groups reduces electron affinities corresponding to formation of dipole-bound anions of these systems. Also, the distortion of the anion geometries with respect to the geometries of the neutral parents are reduced with the methylation.