Ionization-loss Stimulated Raman Spectroscopy Research Articles

Dispersion-Bound Isolated Dimers in the Gas Phase: Observation of the Shortest Intermolecular CH⋅⋅⋅H-C Distance via Stimulated Raman Spectroscopy.

The triphenylmethane and all‐meta tert‐butyl triphenylmethane dimers, (TPM)2 and (T tBuPM)2, respectively, were studied with ionization loss stimulated Raman spectroscopy in molecular beam experiments to resolve structure sensitive vibrations. This answers the question whether the recently reported linear head‐to‐head arrangement in (T tBuPM)2 results from crystal packing or prevails also in the gas phase, and therefore must result from extraordinarily strong London dispersion (LD) interactions. Our study clearly demonstrates that the head‐to‐head arrangement is maintained even under isolated molecular beam conditions in the absence of crystal packing effects. The central Raman‐active aliphatic C−D vibration of appropriately deuterated (T tBuPM)2 associated with an unusually short C−D⋅⋅⋅D−C distance exhibits a strong blue‐shift compared to the undisturbed case. As the LD stabilizing tert‐butyl groups are absent in (TPM)2, it displays an approximately S 6‐symmetric tail‐to‐tail arrangement.

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-H2O) 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-H2O 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 corresponding structure consists of a water molecule, which is hydrogen-bonded to the nitrogen lone pair of the folded ethylamino side chain in the most stable gauche conformer of 4-FPEA. The presence of the hydrogen bond and other bonding and non-bonding interactions was also tested by atoms in molecules and noncovalent interaction analyses. The former approach showed no critical points in electron density, while the latter revealed regional topologies of reduced density gradients, indicating the formation of this hydrogen-bond and other attractive and repulsive interactions. The monohydration of 4-FPEA provides an insight into the intra- and inter-molecular interactions that play a role in stabilizing the cluster.

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. Comparison of the measured ILSR spectra with the calculated Raman spectra allowed us to identify one gauche structure with the ethylamino side chain folded toward the fluorine atom, two gauche structures with the ethylamino side chain folded to the opposite side and two anti conformers with extended tails. The effect of fluorination on the spectra and on the stability and structures of these species is discussed.

Photofragment ionization-loss stimulated Raman spectroscopy of a hydrated neurotransmitter: 2-phenylethylamine–water

The first use of photofragment ionization-loss stimulated Raman spectroscopy, in a broad range, for structural studies of clusters is reported.

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. For this reason, the method of ionization-loss stimulated Raman spectroscopy (ILSRS) has been developed and applied for monitoring the spectral features of the 2-phenylethanol prototype. The bands observed in the Raman spectra of its two conformers uniquely identify their structures and are in accord with anharmonic results obtained by density functional theory calculations. These findings point to future opportunities for ILSRS as a powerful conformational probe and set new standards for detailed interrogation of structure and intra- and inter-molecular interactions.

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.

Intermolecular vibrations of naphthalene trimer by ionization-detected stimulated Raman spectroscopy

We present nonlinear Raman spectra of intermolecular vibrational transitions in four naphthalene trimer isotopomers. The spectra, measured at 0.03 cm−1 by mass-selective ionization-loss stimulated Raman spectroscopy, reveal distinctly shaped pendular band contours, which, upon comparison to simulated pendular contours, lead to ready assignments for almost all of the observed bands. The results show clearly that the trimer has C3h (or very nearly C3h) symmetry with the naphthalenes arranged such that their long axes are parallel to one another. Comparison of the experimental results to the results of calculations of naphthalene-trimer intermolecular spectra performed by assuming harmonic intermolecular modes and by taking the intermolecular potential energy surface to be a pair-wise additive one reveals significant qualitative agreement. This agreement strongly suggests a relatively rigid cluster with no large amplitude intermolecular motions within 100 cm−1 of the zero-point level. Finally, the results highlight the dominance of librational motions in giving rise to scattering intensity in the intermolecular portion of the cluster’s Raman spectrum.

Nonlinear Raman spectroscopy of intermolecular vibrations in benzene-(water) n clusters

We report species-selective spectra of intermolecular vibrational transitions in benzene-(water) n clusters ( n = 1–5), as measured by mass-selective, ionization-loss stimulated Raman spectroscopy. The spectra exhibit prominent Raman activity in the range 35–65 cm −1. In addition, Raman bands at less than 10 cm −1 are found for the n = 1 and n = 3 species, and rotational Raman features are observed for all of the clusters. It is argued that much of the Raman activity is due to intermolecular vibrations in which water moieties move collectively across the plane of the benzene.

The size dependence of ground-state collective vibrational modes in molecular clusters. Benzene dimer through pentamer

We report the measurement of species-selective Raman spectra of collective vibrational modes in the ground states of benzene dimer, trimer, tetramer, and pentamer by mass-selective, ionization-loss stimulated Raman spectroscopy at 0.2 cm −1 resolution. The spectra are markedly size-dependent and become complex rapidly with increasing size. The polarization characteristics of the spectra and the deuterium-isotope effects on them suggest that almost all of the observed bands correspond to intermolecular vibrations that have some librational character. The results are discussed in terms of the properties of the clusters and are compared to spectra of intermolecular vibrations in condensed-phase samples of benzene.

Raman/vibronic double-resonance spectroscopy of benzene-doped argon clusters

Results of mass-selective, ionization-loss stimulated Raman spectroscopy on benzene-(Ar) n , n = 1–22, clusters are reported. The results prove the existence of at least two different gross cluster types in the size range from n = 16–20. The evidence suggests that these cluster types are distinguished from one another by the number of Ar atoms bound in the plane of the benzene moiety.

Size evolution of solvent vibrational structure in neutral solute-(solvent)n clusters: benzene-(nitrogen)n, n = 1-32

Size-selective Raman spectra of the N{sub 2} moieties in benzene-(N{sub 2}){sub n} clusters (n = 1-32) have been measured by mass-selective ionization-loss stimulated Raman spectroscopy. The evolution of the spectra is interpreted in terms of the structure of the solvent N{sub 2} species about the benzene solute. 27 refs., 1 fig.

Ionization-loss stimulated Raman spectroscopy of jet-cooled hydrogen-bonded complexes containing phenols

Stimulated Raman spectra of a variety of hydrogen-bonded, one-to-one complexes of phenol and substituted phenols are presented and discussed. The Raman spectra were recorded with ionization-loss stimulated Raman spectroscopy on supersonic molecular beam samples. Complexation-induced vibrational frequency shifts and vibrational line widths are reported for both the proton-donating moieties (the phenols) and the proton-accepting moieties in the complexes. The vibrational frequency shifts are discussed in terms of the strength of the hydrogen bond and the usefulness of different vibrational modes as indicators of hydrogen bonding. Particular attention is paid to the phenolic CO and OH stretch fundamentals. The line-width measurements suggest that vibrational modes closely associated with the hydrogen bond (i.e., the phenolic CO and OH stretches) relax more efficiently than other modes in the complexes. Even so, the lifetimes of the excited OH stretches are at least a few picoseconds for several of the complexes. Evidence for mode selective dynamics in the OH stretch region of the substituted phenol-water complexes is also presented and discussed. 58 refs., 12 figs., 6 tabs.

Applications of ionization-detected stimulated Raman spectroscopy in molecular-beam studies

We consider two ionization-detected stimulated Raman spectroscopic techniques that are suitable for studying the ground-state spectroscopy of species in molecular beams: ionization-gain stimulated Raman spectroscopy (IGSRS) and ionization-loss stimulated Raman spectroscopy (ILSRS). The merits of IGSRS and ILSRS in such studies are discussed in relation to each other and in relation to stimulated-emission spectroscopy. In addition, some advantages of the Fourier-transform implementations of IGSRS and ILSRS are considered. Experimental results on numerous species are presented that show that IGSRS and ILSRS have ample sensitivity and species specificity for the study of weakly bound complexes.