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Metal–ligand cluster ions are structurally characterized by means of gas-phase infrared multiple photon dissociation spectroscopy. The mass-selected complexes consist of one or two metal cations M3+ (M = Al, Fe, or Ru) and two to five anionic bidentate acetylacetonate ligands. Experimental IR spectra are compared with different density functional theory calculations, namely, PBE/TZVP, B3LYP/6-31G*, and M06/6-31+G**. Frequency analysis was also performed at different levels, namely, scaled static harmonic and unscaled static anharmonic, or with ab initio molecular dynamics simulations at the PBE/TZVP level. All methods lead to simulated spectra that fit rather well with experimental data, and the spectral red shifts of several main bands, in the 1200 cm−1–1800 cm−1 range, are sensitive to the strength of the metal–ligand interaction and to the spin state of the ion. Due to the rigidity of those complexes, first principles molecular dynamics calculations provide spectra similar to that produced by static calculations that are already able to catch the main spectral signatures using harmonic calculations at the B3LYP/6-31G* level. [ABSTRACT FROM AUTHOR]

The gas-phase conformation of the intact (parent) unprotected RGD- peptide anion has been investigated using a combination of anion photoelectron spectroscopy and quantum chemistry calculations of its low-energy stable structures. The experimentally observed RGD- species correspond to a conformation in which the guanidinium group is protonated, the C-terminus is neutral, the aspartic acid carboxyl is deprotonated, and the anion’s excess electron orbital is localized on the protonated guanidinium. This structure is reminiscent of the RGD loop, which is the peptide motif recognized by trans-membrane integrins. The parent RGD- radical anion was generated using a unique infrared desorption-photoemission-helium jet ion source, whose ability to produce radical anions of peptides may also have analytical mass spectrometric implications. [ABSTRACT FROM AUTHOR]

We report the UV and IR photofragmentation spectroscopies of protonated synephrine in a cryogenically cooled Paul trap. Single (UV or IR) and double (UV-UV and IR-UV) resonance spectroscopies have been performed and compared to quantum chemistry calculations, allowing the assignment of the lowest-energy conformer with two rotamers depending on the orientation of the phenol hydroxyl (OH) group. The IR-UV hole burning spectrum exhibits the four expected vibrational modes in the 3 μm region, i.e., the phenol OH, Cβ-OH, and two NH2+ stretches. The striking difference is that, among these modes, only the free phenol OH mode is active through IRPD. The protonated amino group acts as a proton donor in the internal hydrogen bond and displays large frequency shifts upon isomerization expected during the multiphoton absorption process, leading to the so-called IRMPD transparency. More interestingly, while the Cβ-OH is a proton acceptor group with moderate frequency shift for the different conformations, this mode is still inactive through IRPD. [ABSTRACT FROM AUTHOR]

[Display omitted] • Structures of isolated kilodalton ligand-protected gold nanoclusters are studied. • By means of gas-phase InfraRed Multiple Photon Dissociation spectroscopy compared to quantum chemistry calculations. • Nanocluster complexes consist of ten or eleven gold atoms that are bound to glutathione or phosphine ligands. • Theoretical calculations are able to reproduce well the experimental IR spectra, although the size of such species makes difficult a precise assignment among the many possible molecular arrangements. 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. [ABSTRACT FROM AUTHOR]