Supersonic Jet Expansion Research Articles

Conformational Landscape, Chirality Recognition and Chiral Analyses: Rotational Spectroscopy of Tetrahydro-2-Furoic Acid⋅⋅⋅Propylene Oxide Conformers.

A chiral adduct formed between a chiral carboxylic acid, tetrahydro-2-furoic acid (THFA), and a chiral ester, propylene oxide (PO), was investigated using rotational spectroscopy and DFT calculations. Isolated THFA exists dominantly as three different conformers: I, II, and III in a jet, with I and II taking on the trans-COOH configuration and III having the cis-COOH configuration. We utilized CREST, a conformational ensemble space exploration tool, to identify the possible conformations of the binary adduct, THFA⋅⋅⋅PO. Subsequent DFT geometry optimizations predicted about two hundred homochiral and heterochiral binary structures with 28 low energy structures within an energy window of 15 kJ mol-1 . A rich broadband rotational spectrum was obtained with a mixture of trace amounts of THFA+PO in neon in a supersonic jet expansion. Six THFA⋅⋅⋅PO conformers were identified experimentally. Kinetically favored binary products which contain trans-COOH I dominate among the observed conformers, while thermodynamically more stable adducts were also detected. Detailed analyses of the structures of the observed conformers show interesting chirality-controlled structural preferences. Such non-covalently bound chiral contact pairs are the foundation of chiral-tag rotational spectroscopy, an exciting new analytical application of rotational spectroscopy for determination of enantiomeric excess. Enantiomeric excess analyses were performed and the results are discussed.

Molecular Rotation Spectrum of Tetracyclic Quinolizidines: Observation of trans-Matrine and the Elusive cis-Matrine.

We characterized the bis-quinolizidine tetracyclic alkaloid (5S, 6S, 7R, 11R)-matrine in a supersonic jet expansion, using chirped-pulsed broadband microwave spectroscopy. Previous crystal diffraction analyses suggested 16 diastereoisomers associated with matrine's four carbon stereocenters but were inconclusive whether the lactamic nitrogen atom would additionally produce separated trans-/cis- diastereoisomers or if both species may interconvert through low potential barriers. Our experiment simultaneously detected trans- and cis-matrine through their rotational spectrum, confirming the possibility of conformational rearrangement in matrine alkaloids. The two matrine conformers mainly differ in the envelope or half-chair lactamic ring, as evidenced by the experimental rotational and nuclear quadrupole coupling parameters. Molecular orbital calculations with ab initio (MP2) and density functional methods (B3LYP-D3(BJ) and MN15) were tested against the experiment, additionally offering an estimation of the cis-/trans- barrier of 24.9-26.9 kJ mol-1. The experiment illustrates the structural potential of chirped-pulsed broadband microwave spectroscopy for high-resolution rotational studies of biomolecules in the range of 20-40 atoms.

Hydrogen bonding interactions in the 1,1,1,3,3,3-hexafluoro-2-propanol∙∙∙1,4-dioxane complex: Rotational spectroscopy and density functional theory calculations

The rotational spectrum of the binary complex formed between 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and 1,4-dioxane was investigated using a chirped pulse Fourier transform microwave spectrometer. HFIP is known to be an exceptional solvent that catalyzes the epoxidation of olefins by hydrogen peroxide. The addition of 1,4-dioxane can severely reduce HFIP’s ability to boost the epoxidation rate, possibly through its intermolecular interactions with HFIP. It is therefore of considerable interest to examine the non-covalent interactions between HFIP and 1,4-dioxane in detail. Theoretical conformational searches were carried out for the binary HFIP···1,4-dioxane complex and 17 minimum energy structures were identified. Seven of them are within an energy window of 7 kJ mol−1, while the three lowest energy ones are within 1.4 kJ mol−1. Experimentally, only the rotational spectrum of the most stable conformer was detected and assigned. To understand the non-observation of the other low energy conformers in the supersonic jet expansion, subsequent analyses were performed to estimate the conformational conversion barriers. The detected conformer contains a trans HFIP subunit which is hydrogen-bonded to an O atom of 1,4-dioxane and is further stabilized by weak F···H attractive interactions. The intermolecular interactions in HFIP···1,4-dioxane were analyzed and visualized using QTAIM, NCI and SAPT approaches and the interaction energies compared to the HFIP dimer and related complexes of HFIP and 1,4-dioxane with water.

Gas-phase hydration of nopinone: the interplay between theoretical methods and experiments unveils the conformational landscape.

The structure of microsolvated nopinone formed in the supersonic jet expansion is investigated in the gas phase. The rotational spectra of nopinone(H2O)n (n = 1, 2, 3) were analysed by means of Fourier transform microwave spectroscopy. In the present study, three monohydrates, two dihydrates and two trihydrates were observed and characterized. The observed structures are the lowest energy conformers predicted by quantum chemical calculations. In all the observed hydrates of nopinone, water was found to be linked to the ketone group (C[double bond, length as m-dash]O) with a strong hydrogen bond (ONOPHW) and finishing with a dispersive one (OWHNOP). The structure of nopinone was found to alter the structure of water dimer and water trimer, which make nopinone be surrounded with a chain of water molecules. A remarkable decrease in the H-bonding length was observed when the number of attached water molecules is increased. Different DFT and ab initio calculations at the equilibrium structure allowed the identification of the observed conformers. Evaluation of the B3LYP-D3 and ωB97X-D results revealed deficiencies in reproducing the structure of one observed monohydrated structure while MP2 and M06-2X reproduce all the three observed structures. A comparison with similar bicyclic ketones highlights how a small change in the bicyclic ring leads to different effects in the microsolvation of biogenic VOCs. This study presents the first step of molecular aggregation to understand the atmospheric formation of aerosols at the molecular scale.

Theoretical and experimental studies of the isotope effects for He–CO2 and Ne–CO2 complexes

In this work, we have studied the isotope effects for the He–CO2 and Ne–CO2 complexes by means of theoretical calculations and experimental measurements, which were carried out using a distributed quantum cascade laser to probe a pulsed supersonic jet expansion. Firstly, infrared spectra have been recorded for the He/Ne–12C18O2 complexes. Spectroscopic parameters including band origin ν0, rotational constants A, B, C, and centrifugal distortion constants ΔJK were obtained by fitting a Watson A-reduced Hamiltonian with 13 assigned rovibrational transitions for He–12C18O2. For Ne–12C18O2, the observed spectrum produces a set of spectroscopic parameters including the band origin, rotational constants and all the quartic centrifugal distortion constants with more than 100 rovibrational transitions (40 new transitions). Secondly, we have calculated the rovibrational energy levels, vibrational shifts, and rotational constants for the He/Ne–CO2 complexes based on potential energy surfaces (PESs) and bound state calculations for ground and vibrationally excited states. The obtained results show that the spectroscopic characteristics (vibrational shifts and rotational constants) for Ne–CO2 are analogous to those of Ar–CO2, while those for He–CO2 show some differences especially for the rotational constants. Finally, according to the available experimental data and our theoretical calculations, infrared spectra were predicted for six isotopologues with C2v symmetry of Ne–CO2 complex.

Characterization of Cosmic Grain Analogs Formed at Low Temperature from Small Hydrocarbon Precursors in the NASA Ames COSmIC Facility

Abstract Here, we present the results of the first solid-phase ex situ analysis of cosmic grain analogs produced at low temperature (<200 K) in the NASA Ames COsmic SImulation Chamber (COSmIC) from small hydrocarbon precursors, methane (CH4) and acetylene (C2H2), seeded in an argon supersonic jet expansion and submitted to a plasma discharge. The plasma-induced chemical reactions, initiated between the precursor molecules and their atomic and molecular fragments, radicals and ions, produce larger molecules and eventually solid particles that are collected in situ under controlled conditions. Scanning electron microscopy (SEM) imaging was used to provide insight on the morphology and growth structure of the grains produced in COSmIC, and to investigate how the precursors used to produce the grains affect these parameters. This SEM study has shown that under identical experimental conditions with fixed physical and chemical parameters (precursor density, temperature, energy, and reaction time), heavier precursors in the initial mixture produce larger grains and in larger quantity, most likely as a result of a more complex chemistry: most of the grains produced in the Ar/CH4 (95:5) gas mixture ranged from 15 to 385 nm in diameter with an average density of 2.1 grains μm−2, while the grains produced in the Ar/C2H2 (95:5) gas mixture ranged from 40 to 650 nm with a density of 3.5 grains μm−2. Changes in the morphology were also observed, with grains produced from acetylene (C2H2) precursors tending to be more spherical than grains produced from methane (CH4) precursors. This change in morphology could be associated with different stages of growth formation at low temperature from a more “planar” growth at first, followed by coagulation into more spherical particles. This study demonstrates that the COSmIC experimental setup can be used to investigate carbon grain formation from small gas-phase molecular precursors at low temperature (<200 K), i.e., under a temperature regime that is representative of the dust condensation zone and outer region of circumstellar envelopes.

Pinacolone-Alcohol Gas-Phase Solvation Balances as Experimental Dispersion Benchmarks.

The influence of distant London dispersion forces on the docking preference of alcohols of different size between the two lone electron pairs of the carbonyl group in pinacolone was explored by infrared spectroscopy of the OH stretching fundamental in supersonic jet expansions of 1:1 solvate complexes. Experimentally, no pronounced tendency of the alcohol to switch from the methyl to the bulkier tert-butyl side with increasing size was found. In all cases, methyl docking dominates by at least a factor of two, whereas DFT-optimized structures suggest a very close balance for the larger alcohols, once corrected by CCSD(T) relative electronic energies. Together with inconsistencies when switching from a C4 to a C5 alcohol, this points at deficiencies of the investigated B3LYP and in particular TPSS functionals even after dispersion correction, which cannot be blamed on zero point energy effects. The search for density functionals which describe the harmonic frequency shift, the structural change and the energy difference between the docking isomers of larger alcohols to unsymmetric ketones in a satisfactory way is open.

Understanding (coupled) large amplitude motions: the interplay of microwave spectroscopy, spectral modeling, and quantum chemistry

Abstract A large variety of molecules contain large amplitude motions (LAMs), inter alia internal rotation and inversion tunneling, resulting in tunneling splittings in their rotational spectrum. We will present the modern strategy to study LAMs using a combination of molecular jet Fourier transform microwave spectroscopy, spectral modeling, and quantum chemical calculations to characterize such systems by the analysis of their rotational spectra. This interplay is particularly successful in decoding complex spectra revealing LAMs and providing reference data for fundamental physics, astrochemistry, atmospheric/environmental chemistry and analytics, or fundamental researches in physical chemistry. Addressing experimental key aspects, a brief presentation on the two most popular types of state-of-the-art Fourier transform microwave spectrometer technology, i.e., pulsed supersonic jet expansion–based spectrometers employing narrow-band pulse or broad-band chirp excitation, will be given first. Secondly, the use of quantum chemistry as a supporting tool for rotational spectroscopy will be discussed with emphasis on conformational analysis. Several computer codes for fitting rotational spectra exhibiting fine structure arising from LAMs are discussed with their advantages and drawbacks. Furthermore, a number of examples will provide an overview on the wealth of information that can be drawn from the rotational spectra, leading to new insights into the molecular structure and dynamics. The focus will be on the interpretation of potential barriers and how LAMs can act as sensors within molecules to help us understand the molecular behavior in the laboratory and nature.

High-resolution spectroscopy of the [formula omitted] and [formula omitted] bands in the [formula omitted] transition of [formula omitted

We present a high-resolution spectroscopic study of the A~1B2-X~1A1 transition of SiC2 by laser-induced fluorescence. The gas-phase SiC2 molecules are produced in a discharge plasma and cooled in a supersonic jet expansion. By employing a home-made single longitudinal mode optical parametric oscillator as the narrow bandwidth laser source, laser excitation spectra have been recorded with a transition frequency accuracy of ~0.004 cm-1 for the 000,201,301 and 311 bands. Rotational analyses of the high-resolution data have yielded accurate spectroscopic constants, including the centrifugal distortion constants, for the upper vibronic states studied here.

Probing the Endo/Exo Isomeric Variants of a Binary Complex of Cyclopropylamine with p-Fluorophenol by LIF Spectroscopy: A Comparative Study with Ammonia Complex.

Conformational isomers of an O-H···N hydrogen-bonded binary complex between para-fluorophenol (pFP) and a nonrigid primary amine base, cyclopropylamine (CPA), have been probed by means of laser-induced fluorescence (LIF) spectroscopy in a supersonic jet expansion. Two closely spaced electronic origin bands have been identified in the measured LIF excitation spectrum, and their assignments have been corroborated by making comparisons with the spectra of the parent pFP-NH3 complex recorded under the same expansion condition. The observation is consistent with the presence of endo and exo isomeric variants of the complex predicted by electronic structure theory methods, and the endo isomer is stabilized by ∼2 kcal/mol additionally owing to the formation of a C-H···O and a C-H···π type of weak hydrogen bonds between the two moieties. In the fluorescence excitation (FE) spectrum, the low-frequency bands for different intermolecular modes gain substantial intensity, and this spectral feature is in contrast to that of the pFP-NH3 complex. The Franck-Condon intensity of the bands has been simulated invoking Duschinsky rotation scheme, taking into consideration the ground- and excited-state geometries.

The Six Isomers of the Cyclohexanol Dimer: A Delicate Test for Dispersion Models

AbstractThe cyclohexanol homodimer acts as a delicate test model of the role of dispersion forces in intermolecular association. Whereas phenol produces a single dimer, the suppression of π interactions and the larger conformational flexibility in cyclohexanol results in multiple isomerism, with six competing dimers of the free molecule being observed in a supersonic jet expansion. Rotational spectroscopy reveals accurate structural data, specifically the formation of homo‐ and heterochiral diastereoisomers and the presence of both equatorial and axial forms in the dimers. Four dispersion‐corrected density‐functional molecular orbital calculations were tested against the experiment, with B3LYP‐D3(BJ) offering good structural reproducibility with an Alrich's triple‐ζ basis set. However, the prediction of the dimer energetics is largely model‐dependent, thus offering a testbed for the validation of dispersion‐corrected computational models.

The Six Isomers of the Cyclohexanol Dimer: A Delicate Test for Dispersion Models.

The cyclohexanol homodimer acts as a delicate test model of the role of dispersion forces in intermolecular association. Whereas phenol produces a single dimer, the suppression of π interactions and the larger conformational flexibility in cyclohexanol results in multiple isomerism, with six competing dimers of the free molecule being observed in a supersonic jet expansion. Rotational spectroscopy reveals accurate structural data, specifically the formation of homo- and heterochiral diastereoisomers and the presence of both equatorial and axial forms in the dimers. Four dispersion-corrected density-functional molecular orbital calculations were tested against the experiment, with B3LYP-D3(BJ) offering good structural reproducibility with an Alrich's triple-ζ basis set. However, the prediction of the dimer energetics is largely model-dependent, thus offering a testbed for the validation of dispersion-corrected computational models.

Nuclear spin symmetry conservation studied by cavity ring-down spectroscopy of ammonia in a seeded supersonic jet from a pulsed slit nozzle

We report high-resolution cw laser spectra of the symmetric stretching fundamental in ammonia NH measured with a cavity ring-down setup for mid-IR transitions (around ). In a supersonic jet expansion ( seeded in Ar), we have obtained rotational temperatures below . For the gas expansion, a pulsed slit nozzle with length and 70 µm width has been used. The measured relative line intensities demonstrate that the A and E nuclear spin symmetry isomers do not interconvert in the supersonic jet expansion under these conditions (nuclear spin symmetry conservation). The lateral velocity of the gas expansion has been probed by a Doppler profile measurement, showing a preference for an expansion with an angular distribution following Lambert's cosine law perpendicular to the slit direction and an approximate molecular velocity of about . The experimental setup is described in detail, also in view of high-resolution spectroscopy of chiral molecules as candidates to measure the parity violating energy difference between enantiomers with a barrier to interconversion around .

The first microsolvation step for furans: New experiments and benchmarking strategies

The site-specific first microsolvation step of furan and some of its derivatives with methanol is explored to benchmark the ability of quantum-chemical methods to describe the structure, energetics, and vibrational spectrum at low temperature. Infrared and microwave spectra in supersonic jet expansions are used to quantify the docking preference and some relevant quantum states of the model complexes. Microwave spectroscopy strictly rules out in-plane docking of methanol as opposed to the top coordination of the aromatic ring. Contrasting comparison strategies, which emphasize either the experimental or the theoretical input, are explored. Within the harmonic approximation, only a few composite computational approaches are able to achieve a satisfactory performance. Deuteration experiments suggest that the harmonic treatment itself is largely justified for the zero-point energy, likely and by design due to the systematic cancellation of important anharmonic contributions between the docking variants. Therefore, discrepancies between experiment and theory for the isomer abundance are tentatively assigned to electronic structure deficiencies, but uncertainties remain on the nuclear dynamics side. Attempts to include anharmonic contributions indicate that for systems of this size, a uniform treatment of anharmonicity with systematically improved performance is not yet in sight.

Infrared spectra of (H2)1,2-C6D6 and Rg1,2-C6D6 complexes, Rg = He, Ne, Ar

Spectra of dimers and trimers containing C6D6 and He, Ne, Ar, or H2 are observed in the region of the C6D6 ν12 fundamental using a pulsed supersonic jet expansion and a tunable optical parametric oscillator laser source. These are the first reported infrared spectra of hydrogen – benzene complexes. Two bands coupled by anharmonic resonance are studied for each complex: the ν12 fundamental itself (≈2289 cm−1) and the ν2 + ν13 combination band (≈2276 cm−1). Vibrational shifts for the trimers are found to be approximately twice those of the corresponding dimer. Effective intermolecular separations for (H2)1,2-C6D6 are about 3.46 Å, similar to those determined previously from electronic spectra of (H2)1,2-C6H6 and also similar to (Ne)1,2-C6D6.

The rotational spectrum of cyclohexyl formate, chemically prepared within a supersonic expansion

The pulsed supersonic jet expansion microwave spectra of the parent and of the CD=O monodeuterated species of cyclohexylformate have been measured in the 6.5–18 GHz range. The samples have been obtained by direct esterification of HCOOH (or DCOOH) with cyclohexyl alcohol during a supersonic expansion in a microwave cavity. Most of the observed transitions are split into two component lines, due to the tunnelling motion connecting two equivalent gauche forms. The tunnelling barrier has been determined to be B2 = 267 (10) cm−1. Structural information has been obtained from the 6 available rotational constants.

Reinvestigate the C2Π-X2Π(0,0) band of AgO

The C2Π-X2Π(0,0) band of AgO has been reinvestigated by laser induced fluorescence spectroscopy with a spectral resolution of ∼0.02 cm−1. The AgO molecules are produced by discharging a gas mixture of O2/Ar with silver needle electrodes in a supersonic jet expansion. By employing a home-made narrowband single longitude mode optical parametric oscillator (SLM-OPO) as the laser source, high-resolution spectra of the C2Π-X2Π(0,0) band have been recorded for both 107Ag16O and 109Ag16O isotopologues. The spectroscopic constants of the C2Π state are consequently determined, with the 109Ag16O one being reported for the first time. The nature of the spin-orbit coupling effect in the C2Π state is proposed to be due to state mixing with the nearby repulsive 4Σ− and 4Π states.

The Ethylene-Carbon Dioxide Complex and the Double Rotor Model.

The infrared spectrum of the weakly bound C2H4-CO2 complex is investigated in the region of the ν3 fundamental band of CO2 (≈2350 cm-1), using a tunable OPO laser source to probe a pulsed supersonic slit jet expansion. The spacing of the various K-subbands in this perpendicular (ΔK = ±1) spectrum is very irregular, and the pattern of irregularity is quite different from that observed previously in another C2H4-CO2 band by Bemish et al. [ J. Chem. Phys. 1995 , 103 , 7788 ]. But by allowing for the different symmetry of the ν3 (CO2) upper vibrational state, both results can be strikingly well explained using the "double internal rotor" model as described by Bemish et al.

Sulfur hydrogen bonding and internal dynamics in the monohydrates of thenyl mercaptan and thenyl alcohol.

The monohydrates of thenyl alcohol and thenyl mercaptan have been probed in a supersonic jet expansion using chirped-pulse and Fabry-Perot Fourier-transform microwave spectroscopy. The rotational spectra revealed a single isomer for each of the dimers. The thenyl alcohol hydrate is stabilized by an O-HOw hydrogen bond between the alcohol and water, with water acting as a proton acceptor and additionally engaging in an Ow-Hπ interaction with the thenyl ring. Conversely, water behaves as a proton donor in the thenyl mercaptan hydrate, linking to the thiol group though an Ow-HS hydrogen bond and secondary Ow-Hπ interactions with the ring. In both dimers water retains internal mobility, as tunneling doublings in the spectrum confirm an internal rotation motion of water inside the cluster. The experimental results have been complemented with density-functional-theory molecular orbital calculations, binding energy decomposition and a topological analysis of the electronic density, providing a comparative description of the effects of hydrogen bonding of water to the alcohol and thiol groups in the dimers, relevant to understand hydrogen bonding to sulfur centers.

The reduced cohesion of homoconfigurational 1,2-diols.

By a combination of linear FTIR and Raman jet spectroscopy, racemic trans-1,2-cyclohexanediol is shown to form an energetically unrivalled S4-symmetric heterochiral dimer in close analogy to 1,2-ethanediol. Analogous experiments with enantiopure trans-1,2-cyclohexanediol reveal the spectral signature of at least three unsymmetric homochiral dimers. A comparison to signal-enhanced spectra of 1,2-ethanediol and to calculations uncovers at least three transiently homochiral dimer contributions as well. In few of these dimer structures, the intramolecular OHO contact present in monomeric 1,2-diols survives, despite the kinetic control in supersonic jet expansions. This provides further insights into the dimerisation mechanism of conformationally semi-flexible molecules in supersonic jets. Racemisation upon dimerisation is shown to be largely quenched under jet cooling conditions, whereas it should be strongly energy-driven at higher temperatures. The pronounced energetic preference for heterochiral aggregation of vicinal diols is also discussed in the context of chirality-induced spin selectivity.