Gas Phase Spectroscopic Studies Research Articles

The evolution towards cyclic structures in the aggregation of aromatic alcohols: the dimer, trimer and tetramer of 2-phenylethanol.

Gas-phase spectroscopic studies of alcohol clusters offer accurate information on the influence of non-covalent interactions on molecular recognition, and are of paramount importance to model supramolecular and biological chemical processes. Here, we examine the role of the aliphatic side chain in the self-aggregation of aromatic alcohols, using a multi-methodological gas-phase approach which combines microwave spectroscopy and mass-resolved electronic and vibrational laser spectroscopy. Spectroscopic and electronic structure computations were carried out for the dimer, trimer and tetramer of 2-phenylethanol, extending previous investigations on smaller aromatic alcohols. While the conformational flexibility of the ethyl group anticipates a variety of torsional isomers, the intra- and inter-molecular interactions restrict molecular conformations and favour particularly stable isomers. The conformational landscape of the clusters is very shallow and multiple competing isomers were rotationally and/or vibrationally detected, including three dimer species, two trimers and two tetramers. Cluster growth is associated with a tendency to form cyclic hydrogen bond structures.

Mechanism and Scope of Phosphinidene Transfer from Dibenzo-7-phosphanorbornadiene Compounds.

Dibenzo-7-phosphanorbornadiene compounds, RPA (A = C14H10 or anthracene), are investigated as phosphinidene sources upon thermally induced (70-90 °C) anthracene elimination. Analysis of substituent effects reveals that π-donating dialkylamide groups are paramount to successful phosphinidene transfer; poorer π-donors give reduced or no transfer. Substituent steric bulk is also implicated in successful transfer. Molecular beam mass spectrometry (MBMS) studies of each derivative reveal dialkylamide derivatives to be promising precursors for further gas-phase spectroscopic studies of phosphinidenes; in particular, we present evidence of direct detection of the dimethylamide derivative, [Me2N═P]. Kinetic investigations of iPr2NPA thermolysis in 1,3-cyclohexadiene and/or benzene-d6 are consistent with a model of unimolecular fragmentation to yield free phosphinidene [iPr2N═P] as a transient reactive intermediate. This conclusion is probed by density functional theory (DFT) calculations, which favored a mechanistic model featuring free singlet aminophosphinidenes. The breadth of phosphinidene acceptors is expanded to unsaturated substrates beyond 1,3-dienes to include olefins and alkynes; this provides a new synthetic route to valuable amino-substituted phosphiranes and phosphirenes, respectively. Stereoselective phosphinidene transfer to olefins is consistent with singlet phosphinidene reactivity by analogy with the Skell hypothesis for singlet carbene addition to olefins.

Effective Strategy for Conformer-Selective Detection of Short-Lived Excited State Species: Application to the IR Spectroscopy of the N1H Keto Tautomer of Guanine.

The ultrafast deactivation processes in the excited state of biomolecules, such as the most stable tautomers of guanine, forbid any state-of-the-art gas phase spectroscopic studies on these species with nanosecond lasers. This drawback can be overcome by grafting a chromophore having a long-lived excited state to the molecule of interest, allowing thus a mass-selective detection by nanosecond R2PI and therefore double resonance IR/UV conformer-selective spectroscopic studies. The principle is presently demonstrated on the keto form of a modified 9-methylguanine, for which the IR/UV double resonance spectrum in the C═O stretch region, reported for the first time, provides evidence for extensive vibrational couplings within the guanine moiety. Such a successful strategy opens up a route to mass-selective IR/UV spectroscopic investigations on molecules exhibiting natural chromophores having ultrashort-lived excited states, such as DNA bases, their complexes as well as peptides containing short-lived aromatic residues.

The Clusters-in-a-Liquid Approach for Solvation: New Insights from the Conformer Specific Gas Phase Spectroscopy and Vibrational Optical Activity Spectroscopy.

Vibrational optical activity spectroscopies, namely vibrational circular dichroism (VCD) and Raman optical activity (ROA), have been emerged in the past decade as powerful spectroscopic tools for stereochemical information of a wide range of chiral compounds in solution directly. More recently, their applications in unveiling solvent effects, especially those associated with water solvent, have been explored. In this review article, we first select a few examples to demonstrate the unique sensitivity of VCD spectral signatures to both bulk solvent effects and explicit hydrogen-bonding interactions in solution. Second, we discuss the induced solvent chirality, or chiral transfer, VCD spectral features observed in the water bending band region in detail. From these chirality transfer spectral data, the related conformer specific gas phase spectroscopic studies of small chiral hydration clusters, and the associated matrix isolation VCD experiments of hydrogen-bonded complexes in cold rare gas matrices, a general picture of solvation in aqueous solution emerges. In such an aqueous solution, some small chiral hydration clusters, rather than the chiral solutes themselves, are the dominant species and are the ones that contribute mainly to the experimentally observed VCD features. We then review a series of VCD studies of amino acids and their derivatives in aqueous solution under different pHs to emphasize the importance of the inclusion of the bulk solvent effects. These experimental data and the associated theoretical analyses are the foundation for the proposed “clusters-in-a-liquid” approach to account for solvent effects effectively. We present several approaches to identify and build such representative chiral hydration clusters. Recent studies which applied molecular dynamics simulations and the subsequent snapshot averaging approach to generate the ROA, VCD, electronic CD, and optical rotatory dispersion spectra are also reviewed. Challenges associated with the molecular dynamics snapshot approach are discussed and the successes of the seemingly random “ad hoc explicit solvation” reported before are also explained. To further test and improve the “clusters-in-a-liquid” model in practice, future work in terms of conformer specific gas phase spectroscopy of sequential solvation of a chiral solute, matrix isolation VCD measurements of small chiral hydration clusters, and more sophisticated models for the bulk solvent effects would be highly valuable.

High-resolution synchrotron far infrared spectroscopy of thionyl chloride: Analysis of the [formula omitted] and [formula omitted] fundamental bands

Thionyl chloride (SOCl2) is a volatile inorganic compounds used extensively in industry. Its monitoring in gas phase is critical both for environmental and defense concerns. Previous high-resolution gas phase spectroscopic studies were focused on the microwave region (below 40GHz) and no rotationally-resolved study of the IR spectrum has been reported to date. We present in this article a rovibrational analysis of the two lowest frequency infrared active bending modes ν3 and ν6 of SOCl2. By means of synchrotron based Fourier-Transform far-infrared spectroscopy on the AILES beamline of the SOLEIL facility, the spectra of the symmetric ν3 (346cm−1) and asymmetric ν6 (283cm−1) fundamental bands have been rotationally resolved and analyzed.

Novel techniques in VUV high-resolution spectroscopy

Novel VUV sources and techniques for VUV spectroscopy are reviewed. Laser-based VUV sources have been developed via non-linear upconversion of laser pulses in the nanosecond (ns), the picosecond (ps), and femtosecond (fs) domain, and are applied in high-resolution gas phase spectroscopic studies. While the ns and ps pulsed laser sources, at Fourier-transform limited bandwidths, are used in wavelength scanning spectroscopy, the fs laser source is used in a two-pulse time delayed mode. In addition a Fourier-transform spectrometer for high resolution gas-phase spectroscopic studies in the VUV is described, exhibiting the multiplex advantage to measure many resonances simultaneously.

Evidence of the formation and conversion of unstable thionyl isocyanate: Gas-phase spectroscopic studies

Thionyl diisocyanate which is unstable at ambient temperature is generated from a heterogeneous reaction of gaseous thionyl dichloride with silver cyanate and studied for the first time in the gas phase at 298 K. N-isocyanatoformyl sulfinylimide is also observed with photoelectron spectroscopy in the gas phase for the first time. The conversion of thionyl diisocyanate to N-isocyanatoformyl sulfinylimide via 1,3 shift at different temperatures is studied by using photoelectron spectroscopy and mass spectroscopy with the help of theoretical computations. On the basis of combined observations and quantum chemical calculations, possible 1,3-isocyanato shifts and dissociation processes for thionyl isocyanate and its cation have been discussed in detail.

Spectroscopic Evidence for Gas-Phase Formation of Successive β-Turns in a Three-Residue Peptide Chain

We report the first gas-phase spectroscopic study of a three-residue model of a peptide chain, Ac-Phe-Gly-Gly-NH2 (Ac = acetyl), using the IR/UV double resonance technique. The existence of at least five different conformers under supersonic expansion conditions is established, most of them exhibiting rather strong intramolecular H-bonds. One of the most populated conformers, however, exhibits a different H-bonding network characterized by two weak H-bonds. Comparison of the amide A and I/II experimental data with density functional theory calculations carried out on a series of selected conformations enables us to assign this conformer to two successive beta-turns along the peptide chain, the two H-bonds being of C10 type, i.e., each of them closing a 10-atom ring in the molecule. The corresponding form is found to be more stable than the 310 helix secondary structure (not observed), presumably because of specific effects due to the glycine residues.

High-resolution spectroscopy of H 2SO 4, HDSO 4, and D 2SO 4 vapor in the region 1200–10,000 cm −1

The high-resolution (0.05 cm −1) spectra of gas-phase H 2SO 4, HDSO 4, and D 2SO 4 were measured over the frequency region 1200–10,000 cm −1 using Fourier-transform infrared spectroscopy. The increased resolution of this work compared with previous studies has lead to an improved vibrational analysis of H 2SO 4. This study has answered unresolved questions about combination bands and overlapping features from previous gas-phase spectroscopic studies of H 2SO 4 and marks the first experimental measurement of the ν 8 and ν 15 torsional vibrations in this molecule. This work leads to a brief discussion on vibrational mode mixing in sulfuric acid.

Gas phase spectroscopic studies on hydroquinone–water complex

In this paper, we examine the effect of an electron donating group in the para position, on the relative stabilization of the S1 state, in the hydroquinone–water complex. The 1 ∶ 1 complex of hydroquinone and water (HQ–W1) formed in the supersonic jet was studied using REMPI spectroscopy. Hydroquinone exists as cis and trans isomers in the gas phase and therefore the 1 ∶ 1 water complex is expected to have at least four different conformations, viz., H-bond donor and acceptor forms for each of the two isomers. However, only two HQ–W1 conformers were identified using hole-burning spectroscopy. The stable structures of all possible conformers in the ground electronic (S0) state were calculated ab initio at the Hartree–Fock (HF) and density functional theory (DFT, with a hybrid functional B3LYP) levels using the 6-31G** basis set. The experimentally observed conformers were assigned as the trans-donor HQ–W1 and cis-donor HQ–W1 conformers based on the spectroscopic results and the calculations. The optimized structures and the vibronic spectra for these isomers in the excited electronic state (S1) were calculated at the configuration interaction with singlet excitation (CIS) level using the 6-31G** basis set. A correlation was drawn between the red shift in the band origin of the 1 ∶ 1 water complexes of various phenols with the relative changes in the negative charge density on the phenolic oxygen atom upon excitation to the S1 state.

Jet-Cooled Optical Spectroscopy of FeN between 16 300 and 21 600 cm−1

We report the first gas-phase spectroscopic study of iron mononitride (FeN). FeN molecules were generated by the reaction of laser-ablated Fe atoms with NH3 under supersonic jet-cooled conditions. Laser-induced fluorescence excitation spectra were measured between 16 300 and 21 600 cm−1, and rotational analyses have been performed for about 25 vibronic bands. The Ω value of the ground state has been determined to be 52, which could be the spin–orbit component of 2Δ (… 1δ39ς2) or 4Π (… 1δ39ς14π1). The excited states in the visible absorption region show very complicated rovibronic structures due to heavy perturbations. Excited state lifetimes, 56Fe14N/56Fe15N isotope shifts, and dispersed fluorescence spectra were measured for the majority of the bands, which were classified into the five Ω = 52–XΩ = 52 band systems.

Theoretical treatment of quasibound resonances in two-color resonant four-wave mixing spectroscopy

A treatment of continuum states in the application of diagrammatic perturbation theory to calculate the signal produced in two-color resonant, four-wave mixing (TC-RFWM) spectroscopy is developed. The third-order susceptibility is significantly modified from that obtained when considering only discrete states. To illustrate the contribution of continuum states, the line profile of a quasibound resonance arising from the configuration interaction of bound and continuum states is derived. Analytic expressions for line profiles are presented for two specific experimental implementations of TC-RFWM used in gas-phase spectroscopic studies. While the TC-RFWM line profiles are found to be very distinct from the line profiles measured in linear spectroscopic techniques, the results demonstrate the important capability to characterize the TC-RFWM line profiles in terms of the same fundamental and physically significant parameters.

Ab initio geometry, quartic force field, and vibrational frequencies for P4

We have computed the equilibrium bond length and quartic force field for tetrahedral P4 using coupled-cluster methods and a variety of extended basis sets. Our calculations include the effect of correlating core, as well as valence, electrons. As with earlier P4 studies, our computed results show significant differences with current experimental estimates of both bond length and fundamental vibrational frequencies: possible reasons for this disagreement are discussed, with particular reference to the effects of the elevated temperatures used in gas-phase spectroscopic studies of P4. Our calculated endothermicity relative to two P2 molecules appears to be well converged with respect to correlation treatment and basis set, but again differs substantially from present experimental values, which we speculate are underestimates.

First observation and electronic spectroscopy of chromium mononitride: The A 4Πr←X 4Σ− transition near 745 nm

We report the first gas phase spectroscopic study of chromium mononitride. CrN molecules were generated in our laser vaporization molecular beam source and studied using laser induced fluorescence techniques. Twelve vibronic subbands of Cr14N have been rotationally analyzed in the 745–647 nm region. These bands are assigned to the A 4Πr←X 4Σ− transition. The assigned ground state electron configuration 1δ29σ1 and symmetry are identical to those of the isovalent VO, MoN, and WN molecules. The energy observed for the excited 4Πr state matches closely the first 4Π state energy derived from a recent ab initio calculation [J. F. Harrison, J. Phys. Chem. 100, 3513 (1996)]. In addition to the A-X transition, 11 other bands of Cr14N have been identified and vibrationally assigned to transitions involving two other excited states, presumably the B4Σ−, and a 2Π state. Comparisons with the isovalent VO, MoN, and WN systems have provided more insights into the bonding and electronic structure of the CrN molecule.

Electronic spectroscopy of jet-cooled iron monocarbide. The 3Δi←3Δi transition near 493 nm

We report the first gas phase spectroscopic study of iron monocarbide. FeC molecules were generated in a laser vaporization molecular beam source and detected by laser induced fluorescence. Twenty-six vibronic bands have been recorded in the 430–500 nm region. Rotational analyses have been carried out for 22 of the bands. There are three lower states involved, two with Ω″=2 and one with Ω″=3. Based on our experimental observations and the ab initio calculations on RuC [Shim et al. J. Phys. Chem. 91, 3171 (1987)] the observed bands are interpreted as 3Δi←3Δi and 1Δ←1Δ transitions. The ground state electronic configuration is assigned to 1δ39σ1. Evidence for three excited electronic states (one 3Δi state, one 1Δ state and another state with Ω′=3) and for perturbations in several of their vibrational levels is presented.

The molecular structure of the (NCCN)2and (PCCP)2van der Waals dimers

The stationary points on the potential energy surfaces of the (NCCN)2 and (PCCP)2 van der Waals homodimers have been examined at the ab initio Hartree-Fock level of theory employing a 4-31G basis set. Only a T-shaped C2v stable minimum energy structure was found for the (NCCN)2 dimer, while two energetically very similar stable minima, exhibiting near T-shaped Cs and slipped-parallel C2h spatial arrangements, were predicted for the (PCCP)2 dimer. Our predictions for the (NCCN)2 complex are in agreement with a recent gas phase spectroscopic study. No experimental gas phase study seems to have been reported for the (PCCP)2 dimer, and our results indicate that an equilibrium between a near T-shaped and a parallel form is likely to occur. The implications of including electron correlation effects and higher polarized/diffuse functions are discussed.

Ab initio study of the stationary points on the potential energy surface for the acetylene hydrogen-bonded tetramer

Ab initio SCF calculations with an STO/4–31G basis set were done for two stationary points on the energy hypersurface for the acetylene hydrogen-bonded tetramer (C 2H 2) 4: both the “cyclic” and “T/H” configurations. Harmonic vibrational frequencies and IR and Raman intensities were calculated analytically. The relative stability is discussed and comparison with the results of a recent gas-phase spectroscopic study reveals good agreement between theory and experiment.

Structural and spectral consequences of ion pairing II. a theoretical study of CIO −3Li + and ClO −3Na +

The structure of ClO − 3M + ion pairs (M = Li, Na) has been examined with the aid of ab initio molecular orbital calculations. The calculations support recent experimental findings that ClO − 3 M ion paris have C 3v rather than C s configurations. Results from these studies also show that the metal cation prefers to coordinate at the base of the chlorate anion pyramidal structure along the C 3 axis. Calculated vibrational frequencies, intensities, isotopic frequency shifts, and isotopic substituted frequency and intensity patterns render support to this conclusion. New vibrational assignments of observed and some unassignable bands from previous matrix-isolation and molten gas phase spectroscopic studies of ClO − 3M + ion pairs are provided.

Laser induced excitation spectroscopy of copper trimer in various stages of supersonic expansion: Observation of fluorescence from dissociative levels

Gas phase spectroscopic studies of copper trimer are reported. Using laser induced excitation spectroscopy and a trimer source producing concentrations some three to six orders of magnitude in excess of those available from alternative devices, we obtain the visible spectrum of the trimer. This spectrum has been associated with a 2E″–2E′ transition of a D3h molecule in which both electronic states undergo Jahn–Teller distortion. Spectra obtained under a wide variety of experimental conditions encompass the wavelength range 6050–5180 Å corresponding to a range of effective temperatures. Data obtained in the present study are correlated with the recent studies of Morse et al. (laser vaporization, R2PI, and depletion spectroscopies) and Moskovits and co-workers (resonance Raman-matrix isolation). The observed spectroscopic features are in one-to-one correspondence with the detailed excited state vibronic calculations of Thompson et al. and in some respects may agree with the ‘‘first order’’ ground state vibronic calculations of Moskovits. The generated concentration of trimer molecules is such as to allow the characterization of strongly predissociative levels lost to the R2PI technique. The extent of the observed spectra allow the separation of level structure associated on the one hand with the excited state symmetric stretch mode and on the other with the vibronic levels resulting from the coupling of the asymmetric stretch and bending modes. Significant changes in those features which dominate observed excitation spectra as a function of effective temperature are interpreted in terms of a change in the ground state population distribution and hence an alteration in the configuration space connecting populated ground state and accessed excited state levels. These temperature dependent effects and observed hot band structure imply that the pseudorotation barrier in the ground electronic state of the trimer is considerably smaller than previously suggested.

Calculated ground state potential surface and excitation energies for the copper trimer

The results of an SCF/SDCI treatment are presented for selected portions of the ground state potential energy surface for the Cu3 molecule. For equilateral triangle geometries (D3h) the lowest state is 2E′ arising from 4sa′214se′1. The 2E′ state exhibits strong Jahn–Teller distortion, leading to 2A1 (acute angle) and 2B2 (obtuse angle) minima in C2v symmetry. Here the 2B2 minimum is a true minimum on the surface while the 2A1 minimum is a saddle point or very shallow secondary minimum connecting adjacent 2B2 minima. This strong Jahn–Teller distortion is consistent with the observed ground state vibrational levels in the gas phase spectroscopic studies of Morse, Hopkins, Langridge-Smith, and Smalley and in the matrix Raman studies of Moskovits. The 2B2 minimum is also consistent with the observed ESR spectrum of Cu3 in a matrix, which has been interpreted as an obtuse angle structure with most of the spin density on the end Cu atoms. A linear 2Σ+u state is found to be 0.26 eV higher. Two possible candidates have been found for the upper state in the spectrum of Morse et al. (i) a 3s Rydberg state of 2A1 symmetry and (ii) a 3d→4s state of 2E″ symmetry. Both of these states are consistent with the observed selection rules. The 2E″ state would be expected to be weakly Jahn–Teller distorted in agreement with the fit to the upper state levels by Morse et al. and by Thompson, Truhlar, and Mead.