Molecular Beam Expansion Research Articles

IR photodissociation spectra of SixH4x-4+ (x = 4–8): Evidence for Si[sbnd]H[sbnd]Si proton bridges

Although silane-type cations of the form SixHy+ are thought to play a significant role in plasma chemistry and astrochemistry, their structural and energetic properties are largely unexplored in the size range x ≥ 3. Herein, infrared photodissociation (IRPD) spectra of mass-selected SixHy+ ions (specifically SixH4x-4+ with x = 4–8, Si4H12+-Si8H28+) are recorded in the SiH stretch range and analyzed by dispersion-corrected density functional calculations at the B3LYP-D3/aug-cc-pVDZ level. The SixH4x-4+ ions are produced in a SiH4/H2/He plasma molecular beam expansion. The IRPD process leads to the loss of SiH4 ligands, which corresponds to the lowest-energy fragment channel. Spectral analysis of the IRPD spectra reveals that all SixH4x-4+ ions have at least one SiHSi bridge. The characteristic fingerprint of these three-center two-electron (3c-2e) bonds is the highly IR active antisymmetric stretch fundamental of the SiHSi bridge (σ SiHSi) occurring in the 1600–2100 cm−1 range, whose frequency strongly depends on the structural and energetic details of the SiHSi bridge. Although the investigated SixH4x-4+ ions can formally be described by the formula Si2H4+(SiH4)x-2, the cluster growth is more complex. The appearance of the σSiHSi bands confirms that all considered SixH4x-4+ ions with x ≤ 5 are formed by polymerization reactions. Larger clusters (x ≥ 6) show evidence for the additional presence of weakly-bonded SiH4 ligands attached to smaller chemically-bonded core ions. Correlations of the properties of the SiHSi bridges (bond distances, bond angles, binding energies, SiH stretch frequencies), which vary between strong symmetric 3c-2e chemical bonds and weak hydrogen bonds, are discussed.

Characterization of the isolated [Co3Ni(EtOH)]+ cluster by IR spectroscopy and spin-dynamics calculations

We experimentally and theoretically study the geometry, as well as the electronic and vibrational properties, of the heterotetranuclear magnetic cluster ${[{\mathrm{Co}}_{3}\mathrm{Ni}(\mathrm{EtOH})]}^{+}$, which is prepared in the gas phase with molecular beam expansion. We characterize the cluster and identify possible isomers through the comparison of experimentally observed infrared spectra with state-of-the-art quantum chemistry calculations, more specifically by focusing on the OH stretching frequency. Furthermore, we suggest ultrafast, laser-induced, local spin-flip scenarios on every Co atom, and report a cooperative effect, in which the spin density is localized on one Co atom, gets transiently transferred to another, and then bounces back pointing in the opposite direction. Finally, we predict a tolerance of the suggested scenarios with respect to the laser detuning of about 20 meV, which lies within an experimentally applicable range. Our joint investigation is an additional step toward the implementation of laser-controlled nanospintronic devices.

An imaging photoelectron-photoion coincidence investigation of homochiral 2R,3R-butanediol clusters.

We report an experimental investigation of homochiral cluster formation in seeded molecular beam expansions of (2R,3R)-butanediol. Synchrotron radiation vacuum ultraviolet photoionization measurements have been performed using a double imaging electron-ion spectrometer in various configurations and modes of operation. These include measurements of the cluster ion mass spectra, wavelength scanned ion yields, and threshold electron spectra. Protonated cluster ions ranging up to n = 7 have been observed and size-selected photoelectron spectra and photoelectron circular dichroism (PECD) have been recorded by velocity map imaging, recorded in coincidence with ions, at a number of fixed photon energies. Translation temperatures of the cluster ions have been further examined by ion imaging measurements. As well as the sequence of protonated clusters with integral numbers of butanediol monomer units, a second series with half-integral monomer masses is observed and deduced to result from a facile cleavage of a butanediol monomer moiety within the nascent cluster. This second sequence of half-integral masses displays quite distinct behaviours. PECD measurements are used to show that the half-integral mass cluster ions do not share a common parentage with whole integer masses. Using an analogy developed with simple theoretical calculations of butanediol dimer structures, it is inferred that the dissociative branching into integral and half-integral ion mass sequences is controlled by the presence of different butanediol monomer conformations within the hydrogen bonded clusters.

Spatial Separation of Molecular Conformers and Clusters

Gas-phase molecular physics and physical chemistry experiments commonly use supersonic expansions through pulsed valves for the production of cold molecular beams. However, these beams often contain multiple conformers and clusters, even at low rotational temperatures. We present an experimental methodology that allows the spatial separation of these constituent parts of a molecular beam expansion. Using an electric deflector the beam is separated by its mass-to-dipole moment ratio, analogous to a bender or an electric sector mass spectrometer spatially dispersing charged molecules on the basis of their mass-to-charge ratio. This deflector exploits the Stark effect in an inhomogeneous electric field and allows the separation of individual species of polar neutral molecules and clusters. It furthermore allows the selection of the coldest part of a molecular beam, as low-energy rotational quantum states generally experience the largest deflection. Different structural isomers (conformers) of a species can be separated due to the different arrangement of functional groups, which leads to distinct dipole moments. These are exploited by the electrostatic deflector for the production of a conformationally pure sample from a molecular beam. Similarly, specific cluster stoichiometries can be selected, as the mass and dipole moment of a given cluster depends on the degree of solvation around the parent molecule. This allows experiments on specific cluster sizes and structures, enabling the systematic study of solvation of neutral molecules.

Spatial Separation of Molecular Conformers and Clusters

Gas-phase molecular physics and physical chemistry experiments commonly use supersonic expansions through pulsed valves for the production of cold molecular beams. However, these beams often contain multiple conformers and clusters, even at low rotational temperatures. We present an experimental methodology that allows the spatial separation of these constituent parts of a molecular beam expansion. Using an electric deflector the beam is separated by its mass-to-dipole moment ratio, analogous to a bender or an electric sector mass spectrometer spatially dispersing charged molecules on the basis of their mass-to-charge ratio. This deflector exploits the Stark effect in an inhomogeneous electric field and allows the separation of individual species of polar neutral molecules and clusters. It furthermore allows the selection of the coldest part of a molecular beam, as low-energy rotational quantum states generally experience the largest deflection. Different structural isomers (conformers) of a species can be separated due to the different arrangement of functional groups, which leads to distinct dipole moments. These are exploited by the electrostatic deflector for the production of a conformationally pure sample from a molecular beam. Similarly, specific cluster stoichiometries can be selected, as the mass and dipole moment of a given cluster depends on the degree of solvation around the parent molecule. This allows experiments on specific cluster sizes and structures, enabling the systematic study of solvation of neutral molecules.

Infrared Spectrum of the Si3H8+ Cation: Evidence for a Bridged Isomer with an Asymmetric Three‐Center Two‐Electron SiHSi Bond

The IR spectrum of Si3H8(+) ions produced in a supersonic plasma molecular beam expansion of SiH4, He, and Ar is inferred from photodissociation of cold Si3H8(+)-Ar complexes. Vibrational analysis of the spectrum is consistent with a Si3H8(+) structure (2(+)) obtained by a barrierless addition reaction of SiH4 to the disilene ion (H2Si=SiH2(+)) in the silane plasma. In this structure, one of the electronegative H atoms of SiH4 donates electron density into the partially filled electrophilic π orbital of the disilene cation. The resulting asymmetric Si-H-Si bridge of the 2(+) isomer with a bond energy of approximately 60 kJ mol(-1) is characteristic for a weak three-center two-electron bond, which is identified by its strongly IR active asymmetric Si-H-Si stretching fundamental at about 1765 cm(-1). The observed 2(+) isomer is calculated to be only a few kJ mol(-1) less stable than the global minimum structure of Si3H8(+) (1(+)), which is derived from vertical ionization of trisilane. Although more stable, 1(+) is not detected in the measured IR spectrum of Si3H8(+)-Ar, and its lower abundance in the supersonic plasma is rationalized by the production mechanism of Si3H8(+) in the silane plasma, in which a high barrier between 2(+) and 1(+) prevents the efficient formation of 1(+). The potential energy surface of Si3H8(+) is characterized in some detail by quantum chemical calculations. The structural, vibrational, electronic and energetic properties as well as the chemical bonding mechanism are investigated for a variety of low-energy Si3H8(+) isomers and their fragments. The weak intermolecular bonds of the Ar ligands in the Si3H8(+)-Ar isomers arise from dispersion and induction forces and induce only a minor perturbation of the bare Si3H8(+) ions. Comparison with the potential energy surface of C3H8(+) reveals the differences between the silicon and carbon species.

Can stimulated Raman pumping cause large population transfers in isolated molecules?

When stimulated Raman pumping (SRP) is applied to a stream of isolated molecules, such as found in a supersonic molecular beam expansion, we show that SRP can neither saturate nor power broaden a molecular transition connecting two metastable levels that is resonant with the energy difference between the pump and Stokes laser pulses. Using the optical Bloch-Feynman equations, we discuss the pumping of the hydrogen molecule from H(2) (v = 0, J = 0, M = 0) to H(2) (v = 1, J = 2, M = 0) as an illustration of how coherent population return severely reduces the SRP pumping efficiency unless the pump and Stokes laser pulses are applied with an appropriate relative delay and ratio of intensities.

IR Spectroscopy of Isolated Neutral and Protonated Adenine and 9‐Methyladenine

IR spectroscopy is employed to study isolated adenine and its derivative 9-methyladenine in both their neutral and protonated forms. The IR spectra of neutral adenine and 9-methyladenine are measured in a molecular beam expansion via IR-UV ion-dip spectroscopy in the 525 to 1750 cm(-1) region. For adenine, UV excitation selects the 9H tautomer to give a conformer-selective IR spectrum. For 9-methyladenine, only one tautomer exists because of the methyl substitution at the N(9) position. The experimental spectra agree closely with spectra computed for these tautomers at the B3LYP/6-311++G(df,pd) level of theory. These spectra complement previous tautomer-specific IR spectra in the hydrogen stretching range. The 9H-adenine spectrum obtained is compared to a previously recorded FTIR spectrum of adenine at 280 °C, which shows close agreement, although the 7H tautomer cannot be excluded from contributing. Protonated adenine and 9-methyladenine are generated by electrospray ionization and studied via IR multiple-photon dissociation (IRMPD) spectroscopy. Comparison of the experimental spectra with computed spectra allows identification of the protonation site, which suggests that the 1-9 tautomer is the dominant contributor to the spectra.

Photoionization of epichlorohydrin enantiomers and clusters studied with circularly polarized vacuum ultraviolet radiation

The photoionization of enantiomerically pure epichlorohydrin (C(3)H(5)OCl) has been studied using linearly and circularly polarized vacuum ultraviolet synchrotron radiation. The threshold photoelectron spectrum was recorded and the first three bands assigned using molecular orbital calculations for the expected conformers, although uncertain experimental conformer populations and an anticipated breakdown in Koopmans' theorem leave some ambiguity. Measurements of the photoelectron circular dichroism (PECD) were obtained across a range of photon energies for each of these bands, using electron velocity map imaging to record the angular distributions, during which a record PECD chiral asymmetry factor of 32% was observed. A comparison with calculated PECD curves clarifies the assignment achieved using ionization energies alone and further suggests a likely relative population of the conformers. Threshold photoelectron-photoion coincidence methods were used to study the ionic fragmentation of epichlorohydrin. Fragment ion appearance energies show nonstatistical behavior with clear indications that the cationic epoxide ring is unstable and lower energy decay channels proceeding via ring breaking are generally open. Extensive neutral homochiral clusters of epichlorohydrin may be formed in supersonic molecular beam expansions seeded in Ar. Electron angular distribution measurements made in coincidence with dimer and trimer ions are used to effect an examination of the PECD associated with ionization of size-selected neutral cluster species, and these results differ clearly from PECD of the neutral monomer. The shifted ionization thresholds of the n-mers (n = 2, ..., 7) are shown to follow a simple linear relationship, but under intense beam expansion conditions the monomer deviates from this relationship, and the monomer electron spectra tail to below the expected monomer adiabatic ionization potential (IP). PECD measurements made in coincidence with monomer ions obtained under different beam expansion conditions were used to identify unambiguously a contribution from dissociative photoionization of larger clusters to the monomer parent mass ion yield above and below its adiabatic IP.

Conformationally resolved structures of jet-cooled acetaminophen by UV–UV hole-burning spectroscopy

The conformational structures of jet-cooled acetaminophen were investigated in the gas phase by resonant 2-photon ionization and UV-UV hole-burning spectroscopy. In contrast to the results from a previous study, two nearly isoenergetic conformers were distinctly found in a supersonic molecular beam expansion and positively identified as the cis and trans isomers of acetaminophen by UV-UV hole-burning spectroscopy. The 0-0 bands of the cis and trans isomers were found at 33518.7 and 33485.6 cm(-1), respectively. The vibronic bands of the two isomers are close-lying and/or partially overlapping due to the small energy difference (33 cm(-1)) between the two 0-0 bands. As a consequence, the recorded resonant 2-photon ionization spectrum is highly congested in the low excitation energy region, which develops continuously into a featureless, broadened spectrum in the high energy region.

Infrared and electronic spectroscopy of p-C 6H 4Cl 2+–L n clusters with L = Ar, N 2, H 2O, and p-C 6H 4Cl 2

Mass-selected clusters of the para-dichlorobenzene cation (p-C 6H 4Cl 2 +, pDCB +) are characterized by vibrational and electronic photodissociation spectroscopy in a molecular beam expansion. Infrared (IR) spectra of pDCB +–L n with L = Ar ( n = 1–7) and L = H 2O ( n = 1) and (pDCB) 2 +–Ar n with n ≤ 2 are recorded in the C–H stretch range in the 2B 2g ground electronic state of pDCB +. The IR spectra with L = Ar display essentially no shift as a function of the cluster size and approximate to high accuracy the hitherto unknown IR spectra of pDCB + and its (pDCB) 2 + dimer. In addition to the two IR active C–H stretch fundamentals at 3095 and 3107 cm −1, several intense combination bands are detected and assigned in the 2800–3100 cm −1 range for pDCB +. Comparison with the spectrum of neutral pDCB demonstrates structural changes upon ionization, which are consistent with the results of quantum chemical calculations performed at the B3LYP/6-311++G** level. Analysis of the photofragmentation branching ratios for pDCB +–Ar n yields a binding energy of ∼550 cm −1 for the roughly equivalent π-bonded Ar ligands. The IR spectrum of pDCB +–H 2O displays broader transitions with only minor redshifts (∼5–15 cm −1) due to the stronger intermolecular interaction. The C–H stretch spectrum is consistent with a π-bonded charge-dipole configuration. The IR spectrum of (pDCB) 2 +, as inferred from the spectrum of the Ar-tagged species, is rather different from those of pDCB + and pDCB, indicative of a charge-resonance type interaction in this homodimer cation. The electronic spectra of the B 2B 3u ← X 2B 2g transition near 500 nm are reported for pDCB +–Ar, pDCB +–N 2, and pDCB +–H 2O. Only small shifts of the electronic origin transition are observed upon complexation, indicating little change in the intermolecular interaction energy upon electronic excitation. This conclusion is also supported by the modest excitation of unresolved intermolecular modes, suggesting that also the geometry changes in these clusters are small upon B ← X excitation.

Rotational and Vibrational Cooling in Pulsed High-Pressure Molecular Beam Expansions from 3 bar into the Supercritical Regime

Experiments employing pulsed high-pressure (< or = 90 bar) supersonic jet expansions into a laser REMPI time-of-flight mass spectrometer are presented. Due to the very short opening time of the high-pressure valve, a compact arrangement with moderate pumping speed requirements is sufficient, which can be used up to 50 Hz repetition frequency, still maintaining an operating pressure in the expansion chamber of ca. 10(-4) mbar. p-Fluorotoluene was used as a rotational and vibrational "molecular thermometer" to characterize the cooling capabilities of high-pressure pulsed argon and CO(2) expansions into vacuum, spanning a wide range of stagnation pressures into the supercritical regime. Rotational and torsional temperatures were deduced from the contour of the S(0) --> S(1)(0-0) transition by an asymmetric-top/free-internal-rotor simulation and compared with the results of a simpler rigid-rotor asymmetric-top fit. The experiments show that average rotational temperatures of ca. 1-2 K in argon and 13-15 K in CO(2) can be reached at the highest pressures studied. The population of higher |m| torsional levels of p-fluorotoluene is clearly demonstrated by the appearance of characteristic features on the blue edge of the contours, which are more pronounced in the warmer CO(2) expansions. While the rotational temperatures in argon expansions compare well with estimated gas dynamical terminal translational temperatures, there are considerable differences in the case of CO(2). Still, the degree of internal cooling reached with CO(2) is sufficiently low, so that a setup of this kind might provide good opportunities for future studies of thermally labile low-volatile molecules to cool them to cryogenic temperatures low enough to achieve a sufficient simplification of their spectra.

Solvent Interactions and Conformational Choice in a Core N-Glycan Segment: Gas Phase Conformation of the Central, Branching Trimannose Unit and its Singly Hydrated Complex

The intrinsic conformational preferences and structures of the branched trimannoside, alpha-phenyl 3,6-di-O-(alpha-D-mannopyranosyl)-alpha-D-mannopyranoside (which contains the same carbohydrates found in a key subunit of the core pentasaccharide in N-glycans) and its singly hydrated complex, have been investigated in the gas phase isolated at low temperature in a molecular beam expansion. Conformational assignments of their infrared ion dip spectra, based on comparisons between experiment and ONIOM (B3LYP/6-31+G(d):HF/6-31G(d)) and single-point MP2 calculations have identified their preferred structures and relative energies. The unhydrated trimannoside populates a unique structure supported by two strong, central hydrogen bonds linking the central mannose unit (CM), and its two branches (3M and 6M) closely together, through a cooperative hydrogen-bonding network: OH4(CM)-->OH6(3M)-->OH6(6M). A closely bound structure is also retained in the singly hydrated oligosaccharide, with the water molecule bridging across the 3M and 6M branches to provide additional bonding. This structure contrasts sharply with the more open, entropically favored trimannoside structure determined in aqueous solution at 298 K. In principle this structure can be accessed from the isolated trimannoside structure by a simple conformational change, a twist about the alpha(1,3) glycosidic linkage, increasing the dihedral angle psi[C1(3M)-O3(3M)-C3(CM)-C2(CM)] from approximately 74 degrees to approximately 146 degrees to enable accommodation of a water molecule at the centrally bound site occupied by the hydroxymethyl group on the 3M ring and mediation of the water-linked hydrogen-bonded network: OH4(CM) -->OH(W)-->OH6(6M). The creation of a "water pocket" motif localized at the bisecting axis of the trimannoside is strikingly similar to the structure of more complex N-glycans in water, suggesting perhaps a general role for the "bisecting" OH4 group in the central (CM) mannose unit.

Photofragment translational spectroscopy of propargyl radicals at 248nm

The photodissociation of propargyl radical, C(3)H(3), and its perdeuterated isotopolog was investigated using photofragment translational spectroscopy. Propargyl radicals were produced by 193 nm photolysis of allene entrained in a molecular beam expansion and then photodissociated at 248 nm. Photofragment time-of-flight spectra were measured at a series of laboratory angles using electron impact ionization coupled to a mass spectrometer. Data for ion masses corresponding to C(3)H(2)(+), C(3)H(+), C(3)(+), and the analogous deuterated species show that both H and H(2) loss occur. The translational energy distributions for these processes have average values E(T)=5.7 and 15.3 kcal/mol, respectively, and are consistent with dissociation on the ground state following internal conversion, with no exit barrier for H loss but a tight transition state for H(2) loss. Our translational energy distribution for H atom loss is similar to that in a previous work on propargyl in which the H atom, rather than the heavy fragment, was detected. The branching ratio for H loss/H(2) loss was determined to be 97.62.4+/-1.2, in good agreement with previous calculations.

Monosolvation of R-1-Phenyl-2,2,2-trifluoroethanol with Amines: Configurational Effects on the Excitation, Ionization, and Fragmentation of Diastereomeric Complexes

Wavelength and mass selected resonant two-photon ionization spectra of molecular clusters between R-1-phenyl-2,2,2-trifluoroethanol (FER) and methylamine (M) or the enantiomers of 2-aminobutane (AR and AS) were recorded after supersonic molecular beam expansion and analyzed with the aid of ab initio molecular orbital calculations. The experimental results agree with theoretical calculations pointing to the predominance of the two most stable conformers of monosolvated FER whose CF3 group establishes intense NH...F interactions with the selected amines so as to orient them away from the aromatic ring. This reduces the enantioselectivity of FER toward the 2-aminobutane enantiomers as compared to that exhibited by the R-1-phenylethanol (ER) analogue, where obviously NH...F interactions are absent.

High resolution electronic study of 16O 14N 16O, 16O 14N 18O and 18O 14N 18O: A rovibronic survey covering 11 800–14 380 cm −1

The 11 800–14 380 cm −1 frequency range has been scanned for rotationally resolved rovibronic transitions in the A 2 B 2– X 2 A 1 electronic band system of the symmetric ( C 2 v ) 16O 14N 16O and 18O 14N 18O isotopologues and in the corresponding electronic band system of the asymmetric ( C s ) 18O 14N 16O isotopologue. The rotational analysis—reflecting minor differences in mass—in combination with symmetry induced spectral differences allows an identification of 68 16O 14N 16O vibronic levels, 26 18O 14N 18O vibronic levels and 51 18O 14N 16O vibronic levels. The bands are recorded using near infrared fluorescence spectroscopy and a piezo valve based pulsed molecular beam expansion of premixed 18O 2 and 14N 16O in Ar. The majority of the observed bands is rotationally assigned and can be identified as transitions starting from the vibrational ground state of one of the isotopologues. Numerous hot bands have also been identified. A comparison of the overall spectroscopic features of C 2 v vs. C s symmetric species provides qualitative information on symmetry dependence of vibronic couplings.

Alignment of ethylene molecules in supersonic seeded expansions probed by infrared polarized laser absorption and by molecular beam scattering

Relaxation and alignment of the rotational angular momentum of ethylene molecules are here shown to be induced naturally by collisions occurring in supersonic seeded expansions, which lead to generation of continuous molecular beams. Dependences of the achieved alignment degree on the rotational level, on the final molecular speed and on the divergence angle with respect to the molecular beam expansion direction have been investigated using two separate complementary experimental techniques, IR spectroscopy and molecular beam scattering. Results obtained for this planar hydrocarbon are of interest toward the goal of planning collisional alignment experiments on molecules with different mass and geometry.

Modelling neurotransmitter functions: a laser spectroscopic study of (1S,2S)-N-methyl pseudoephedrine and its complexes with achiral and chiral molecules

Wavelength and mass resolved resonance-enhanced two photon ionization (R2PI) excitation spectra of (1S,2S)-N-methyl pseudoephedrine (MPE) and its complexes with several achiral and chiral solvent molecules, including water (W), methyl (R)-lactate (L(R)), methyl (S)-lactate (L(S)), (R)-2-butanol (B(R)), and (S)-2-butanol (B(S)), have been recorded after a supersonic molecular beam expansion and examined in the light of ab initio calculations. The spectral patterns of the selected complexes have been interpreted in terms of the specific hydrogen-bond interactions operating in the diastereomeric complexes, whose nature in turn depends on the structure and the configuration of the solvent molecule. The obtained results confirm the view that a representative neurotransmitter molecule, like MPE, "communicates" with the enantiomers of a chiral substrate through different, specific interactions. These findings can be regarded as a further contribution to modelling neurotransmitter functions in biological systems.

Using Volatile Solvents for Ion Formation in Liquid Molecular Beam Expansion Mass Spectrometry

Cocrystallization between analyte and matrix is required by matrix-assisted laser desorption/ionization and can represent a significant limitation of the technique. A molecular beam expansion, mass spectrometric method has been developed to explore the possibility of using pure solvents as matrix to avoid cocrystallization. Two kinds of solvent, liquid CS2 and liquid or supercritical CO2, have been studied with 266-nm UV laser irradiation. We successfully ionized a number of compounds, including caffeine, guanine, cholesterol, and mixed fullerenes. Under some conditions, the mass spectra reflect parent radical cations formed by photoionization. Under other conditions, protonated, sodiated (and with CS2 even sulfated) ions are seen reflecting a nonunimolecular ionization process. When UV-transparent CO2 is used as a solvent, only analyte molecules with a UV chromophore are detected. However, with UV-absorbing CS2, we demonstrate ionization of molecules lacking a UV chromophore. This work provides strong evidence that one can form solvent clusters containing analyte, that laser photoionization of the solvent precedes ionization of the analyte, and that solvent evaporation along with the indirect ionization leads to reduced parent ion fragmentation. The exploration of this now demonstrated concept with other solvents would appear fruitful for future work.

Excitation, Ionization, and Fragmentation of Chiral Molecules in Asymmetric Microenvironments: A Mass-Resolved R2PI Spectroscopic Study

One- and two-color, mass selected R2PI spectra of the S(1) <-- S(0) transitions in the bare (R)-(+)-1-phenyl-1-propanol and its complexes with bidentate solvent molecules, like the (R)-(-)- and (S)-(+)-3-hydroxytetrahydrofuran enantiomers, have been recorded after a supersonic molecular beam expansion. The one-color R2PI excitation spectra of the diastereomeric complexes are characterized by three main peaks, one red-shifted and the other two blue-shifted relative to the band origin of the most stable anti conformer of the bare chromophore. The opposite direction of these spectral shifts is ascribed to the occurrence of three different hydrogen bonded isomeric structures for each individual complex, while their different magnitude depends on the configuration of the bidentate solvent molecule as well as its specific hydrogen bond interaction center, whether the ethereal oxygen atom or the hydroxyl group. The same factors play a major role in determining the magnitude of the phenomenological activation barriers for the loss of an ethyl radical from the ionized diastereomeric complexes.