Structural features of monohydrated 2-(4-fluorophenyl)ethylamine: a combined spectroscopic and computational study.

A jet-cooled singly hydrated 2-(4-fluorophenyl)ethylamine (4-FPEA-HO) cluster has been studied by ionization-loss stimulated Raman spectroscopy of the 4-FPEA photofragment and density functional calculations of the parent. Comparison of the measured spectrum of the photofragment to computed scaled harmonic Raman spectra of different conformers of the 4-FPEA-HO cluster, at the M06-2X/6-311++G(d,p) level of theory, allowed determination of the calculated spectrum that best fits the experimental one. The correlation between them was further supported by the stability of the cluster, as revealed from the calculated energies of the fully optimized geometries of the possible different clusters in the ground electronic state.

The conformational landscape of 2-(4-fluoro-phenyl)-ethylamine: consequences of fluorine substitution at the para position.

Fluorination is considered as a possible means for alteration of conformational landscapes in molecules. The effect of fluorine substitution was studied here by measuring the vibronic and vibrational spectra of gas phase 2-(4-fluoro-phenyl)-ethylamine (4-FPEA) by resonant two-photon ionization (R2PI) and by ionization-loss stimulated Raman spectroscopy (ILSRS). The measurement of survey ILSR spectra of 4-FPEA in the amino group region allowed to associate the bands in the R2PI spectrum to origin and vibronic transitions of the ground (S) to the excited (S) electronic states of three different conformers.

Structural motifs of 2-(2-fluoro-phenyl)-ethylamine conformers.

Vibronic and vibrational spectra of 2-(2-fluoro-phenyl)-ethylamine (2-FPEA) conformers were measured in a molecular beam by resonant two-photon ionization (R2PI), ultraviolet-ultraviolet hole burning (UV-UV HB) spectroscopy, and ionization-loss stimulated Raman spectroscopy (ILSRS). The measured ILSR spectral signatures in the survey spectra of the amino group region and in the broad spectral range revealed the presence of five different conformers, which were confirmed by the HB spectra. The determination of the structures of the conformers of 2-FPEA was assisted by quantum chemical calculations of the torsional potential energy surface and of the scaled harmonic Raman spectra.

Vibrational and vibronic spectra of tryptamine conformers.

Conformation-specific ionization-detected stimulated Raman spectra, including both Raman loss and Raman gain lines, along with visible-visible-ultraviolet hole-burning spectra of tryptamine (TRA) conformers have been measured simultaneously, with the aim of obtaining new data for identifying them. The slightly different orientations of the ethylamine side chain relative to the indole lead to unique spectral signatures, pointing to the presence of seven TRA conformers in the molecular beam. Comparison of ionization-loss stimulated Raman spectra to computationally scaled harmonic Raman spectra, especially in the alkyl C-H and amine N-H stretch regions together with the retrieved information on the stabilities of the TRA conformers assisted their characterization and structural identification.

Revealing the hot bands in the regions of the N-H and C-H stretch fundamentals of pyrrole.

Photoacoustic Raman spectra of gaseous pyrrole in the 3504-3535 and 3068-3152 cm(-1) energetic windows were measured, to obtain new information about the hot bands in the vicinity of the N-H(ν1) and C-H(ν2) stretch fundamentals, respectively. The observed vibrational patterns are characterized by sharp Q-branches, where the strong bands reflect the fundamentals and the weaker ones, as established from their temperature dependence, are hot bands. From the simulation of the observed spectra, the band origins and nondiagonal anharmonicities were determined.

Simultaneous Ionization-Detected Stimulated Raman and Visible-Visible-Ultraviolet Hole-Burning Spectra of Two Tryptamine Conformers.

A key first step toward probing structures and interactions of individual conformers of isolated flexible molecules is uncovering their characteristic spectral signatures. Here, conformation-specific ionization-detected stimulated Raman (IDSR) and visible-visible-ultraviolet hole-burning spectra were measured simultaneously to determine the unique signatures of the two most stable conformers of tryptamine in the gas phase. These signatures together with the comparison of the IDSR spectra to the computationally predicted Raman spectra assisted in their characterization and structural identification.

Carbohydrate hydration: heavy water complexes of α and β anomers of glucose, galactose, fucose and xylose.

The singly and doubly hydrated complexes of the α and β anomers of a systematically varied set of monosaccharides, O-phenyl D-gluco-, D-galacto-, L-fuco- and D-xylopyranoside, have been generated in a cold molecular beam and probed through infrared-ultraviolet double resonance ion-dip (IRID) spectroscopy coupled with quantum mechanical calculations. A new 'twist' has been introduced by isotopic substitution, replacing H(2)O by D(2)O to separate the carbohydrate (OH) and hydrate (OD) vibrational signatures and also to relieve spectral congestion. The new spectroscopic and computational results have exposed subtle aspects of the intermolecular interactions which influence the finer details of their preferred structures, including the competing controls exerted by co-operative hydrogen bonding, bi-furcated and OH-π hydrogen bonding, stereoelectronic changes associated with the anomeric effect, and dispersion interactions.

Isotopic hydration of cellobiose: vibrational spectroscopy and dynamical simulations.

The conformation and structural dynamics of cellobiose, one of the fundamental building blocks in nature, its C4' epimer, lactose, and their microhydrated complexes, isolated in the gas phase, have been explored through a combination of experiment and theory. Their structures at low temperature have been determined through double resonance, IR-UV vibrational spectroscopy conducted under molecular beam conditions, substituting D(2)O for H(2)O to separate isotopically, the carbohydrate (OH) bands from the hydration (OD) bands. Car-Parrinello (CP2K) simulations, employing dispersion corrected density functional potentials and conducted "on-the-fly" from ∼20 to ∼300 K, have been used to explore the consequences of raising the temperature.

Ionization-loss stimulated Raman spectroscopy for conformational probing of flexible molecules.

The approach of studying structural and dynamical properties of flexible molecules is of substantial interest, as it allows decoding the shapes and intrinsic properties of isolated molecular constituents, which have an influence on the selectivity and functionality in biological processes. Combining quantum computation methods with double resonance or infrared hole burning techniques, mainly covering hydride stretch vibrations, recently led to great progress in understanding the structure of a variety of biological building blocks. Measurements of spectra in the lower frequency range, with relatively compact and convenient laser sources, still pose major challenges.

Raman spectral signatures as conformational probes of gas phase flexible molecules.

A novel application of ionization-loss stimulated Raman spectroscopy (ILSRS) for monitoring the spectral features of four conformers of a gas phase flexible molecule is reported. The Raman spectral signatures of four conformers of 2-phenylethylamine are well matched by the results of density functional theory calculations, showing bands uniquely identifying the structures. The measurement of spectral signatures by ILSRS in an extended spectral range, with a conventional laser source, is instrumental in facilitating the unraveling of intra- and intermolecular interactions that are significant in biological structure and activity.

A new method for determining absorption cross sections out of initially excited vibrational states.

A first experimental demonstration, combining the methods of vibrationally mediated photodissociation (VMP) and ionization-loss stimulated Raman spectroscopy (ILSRS) for measuring cross sections for dissociation of vibrationally excited levels is reported. The action spectrum obtained in the VMP of methylamine exhibits enhancement of the H photofragment yield as a result of initial vibrational excitation and the ILSRS monitors the fraction of molecules being excited. The partial cross sections for H production out of the sampled vibrational states and the extent of mode selectivity were thus determined.