A new infrared spectroscopy technique for structural studies of mass-selected neutral polar complexes without chromophore

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 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

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

Electrons in Diffuse Orbitals

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

Multipole-bound anions of organic molecules

Multipole-bound anions are ions which possess weakly-bound excess electrons in diffuse orbitals. They can be either produced by electron attachment in the high-pressure region of supersonic expansions, leading to intense anion intensities allowing for photoelectron spectroscopy or by electron transfer from laser-excited atoms to cold molecular species. This review emphasizes applications of multipole-bound electron binding to the determination of structures and electron affinities of weakly-bound organic complexes. Studies involving charge-exchange between polar molecules and laser-excited surfaces, zwitterions and electron binding to molecules deposited on helium droplets are briefly considered

Evaluation of MP2, DFT, and DFT-D Methods for the Prediction of Infrared Spectra of Peptides

International audienc

Ultrafast excited state dynamics in protonated GWG and GYG tripeptides

International audienc

Long-range electron binding to quadrupolar molecules

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. For a molecule with a null dipole the first non-vanishing field left is the quadrupolar field. Theoretical results for quadrupole-bound anions have been rather conflicting and the few experimental reports can have alternative explanations. Following a systematic search, we report experimental evidence that such 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

Urea and methylurea dipole-bound anions

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 large-amplitude motions which possess similar characteristic frequencies

Ligand-protected gold nanoclusters probed by IRMPD spectroscopy and quantum chemistry calculations

This paper reports an attempt to structurally characterize isolated ligand-protected gold nanoclusters by means of gas-phase InfraRed Multiple Photon Dissociation (IRMPD) spectroscopy compared to quantum chemistry Density Functional Theory (DFT) calculations. The mass-selected kilodalton nanocluster complexes consist of ten or eleven gold atoms that are bound to glutathione or phosphine ligands and are produced by ElectroSpray Ionization (ESI) in the form of multiply charged anions or cations. This study allows us to build some methodology benchmarks for species that are large for IRMPD experiments and that are used for biochemistry applications. These gas-phase results on isolated ions are compared to condensed phase data from Fourier-Transform InfraRed (FTIR) spectroscopy and to theoretical IR spectra that are calculated with two different functional/basis sets, namely B3LYP/6-31G* and M06L/LanL2DZ, at the scaled static harmonic level. Although theoretical calculations are able to reproduce well the experimental IR spectra, the size of such species and the presence of many possible interactions between ligands make difficult a precise assignment among the many possible molecular arrangements