Chirped-pulse Fourier-transform Microwave Research Articles

Broadband Rotational Spectroscopy in Uniform Supersonic Flows: Chirped Pulse/Uniform Flow for Reaction Dynamics and Low Temperature Kinetics.

ConspectusThe study of gas-phase chemical reactions at very low temperatures first became possible with the development and implementation of the CRESU (French acronym for Reaction Kinetics in Uniform Supersonic Flows) technique. CRESU relies on a uniform supersonic flow produced by expansion of a gas through a Laval (convergent-divergent) nozzle to produce a wall-less reactor at temperatures from 10 to 200 K and densities of 1016-1018 cm-3 for the study of low temperature kinetics, with particular application to astrochemistry. In recent years, we have combined uniform flows with revolutionary advances in broadband rotational spectroscopy to yield an instrument that affords near-universal detection for novel applications in photodissociation, reaction dynamics, and kinetics. This combination of uniform supersonic flows with chirped-pulse Fourier-transform microwave spectroscopy (Chirped-Pulse/Uniform Flow, CPUF) permits detection of any species with a modest dipole moment, thermalized to the uniform temperature of the gas flow, with isomer, conformer, and vibrational state specificity. In addition, the use of broadband, high-resolution, and time-dependent (microsecond time scale) micro- and mm-wave spectroscopy makes it an ideal tool for characterizing both transient and stable molecules, as well as studying their spectroscopy and dynamics.In this Account, we review recent advances made using the CPUF technique, including studies of photodissociation, radical-radical reaction dynamics, and low temperature kinetics. These studies highlight both the strength of universal and multiplexed detection and the challenges of coupling it to a high-density collisional environment. Product branching and product evolution as a function of time have been measured for astrochemically relevant systems, relying on the detailed characterization of these flow conditions via experiments and fluid dynamics simulations. In the photodissociation of isoxazole, an unusual heterocyclic molecule with a very low-energy conical intersection, we have identified 7 products in 5 reaction channels and determined the product branching, pointing to both direct and indirect pathways. We have also approached the same system from separated NO and C3H3 reactants to explore a broader range of the potential energy surface, demonstrating the power of multichannel branching measurements for complex radical-radical reactions. We determined the product branching in the C3H2 isomers in the photodissociation of the propargyl radical and identified the importance of a hydrogen atom catalyzed isomerization to the lowest energy cyclic form. This then motivated a study of direct D-H exchange reaction in radicals, in which we demonstrate that it is an important and overlooked pathway for deuterium fractionation in astrochemical environments. Recently, we have shown the measurement of low temperature kinetics inside an extended Laval nozzle, after which a shock-free secondary expansion to low temperature and density affords an ideal environment for detection by rotational spectroscopy. These results highlight the power and potential of the CPUF approach, and future prospects will also be discussed in light of these developments.

Conformational Landscapes of 2,3-, 2,4-, 2,5-, and 2,6-Difluorobenzaldehyde Unveiled by Rotational Spectroscopy.

We report the coexistence of anti-conformers and energetically unfavorable syn-conformers of 2,3-, 2,4-, 2,5-, and 2,6-difluorobenzaldehyde in the gas phase using broadband chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy. The rotational spectra of monosubstituted 13C isotopologues of the anti-conformers have also been assigned in natural abundance, which were used to derive their vibrationally averaged geometries and semi-experimental equilibrium structures. The energy differences between anti- and syn-conformations are estimated to be 10.9, 11.3, and 12.9 kJ/mol for 2,3-, 2,4-, and 2,5-difluorobenzaldehyde, respectively, at the theoretical level of DLPNO-CCSD(T)/def2-TZVP. Despite the steric repulsion caused by the close proximity between the oxygen atom of the aldehyde group and the ortho-substituted fluorine atom, our experimental results indicate the planarity of the syn-conformations. The frequencies of the large amplitude torsion between the phenyl and aldehyde groups have been estimated by experimental inertial defects, which agree with theoretical calculation results.

Rotational spectroscopy and structure of 1,1-dichloro-1-silacyclohex-2-ene

The ground state rotational spectrum of 1,1-dichloro-1-silacyclohex-2-ene has been recorded using a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer. Several isotopologues in their natural abundances have been observed in the free-jet expansion, and their spectra assigned, making it possible to present a partial heavy-atom substitution structure. Furthermore, the high resolution of this technique allows the complicated hyperfine splitting pattern to be largely deconvoluted. As a result, the on-diagonal nuclear quadrupole coupling constants for the two chlorine atoms have been determined for all observed isotopologues. Additionally, χbc is determined for both chlorine atoms of the parent species. The quadrupole coupling tensors for the parent species have been diagonalised, noting some assumptions have been made pertaining to the off-diagonal nuclear quadrupole coupling constants in the principal axis system, to yield reasonable values of χzz and η which are then compared.

Rotational spectra of ten new fluoroethylene/CO2 clusters, (C2H3F)x(CO2)y: Application of data-centered methods to the assignment of spectra in complex mixtures

Chirped-pulse Fourier-transform microwave (CP-FTMW) spectroscopy has been used to measure the spectra of ten previously unobserved fluoroethylene (FE)/CO2 clusters, (FE)x(CO2)y, with x from 1 to 4 and y from 0 to 4. Multiple spectra were recorded with varying concentrations of CO2 in the sample, and at least 400,000 free induction decays averaged per data set. This allowed implementation of data-centered approaches, using intensity variation, to identify subsets of transitions belonging to the same cluster species or those of similar composition, simplifying the assignment process for the complex mixture of clusters present. All spectra were fitted to Watson A-reduction Hamiltonians, including quartic distortion constants, with very few species requiring higher order distortion constants for satisfactory fits to be obtained. Computational data at MP2/6-311++G(2d,2p) and ωB97X-D/6-31+G(d,p) levels suggested FE:CO2 ratios and likely structural arrangements of each cluster based on comparisons with experimental rotational constants.

A Rotational Study of 2-tert-Butylphenol and Its 1 : 1 Argon Complex.

The chirped-pulse Fourier Transform microwave spectrum of 2-tert-butylphenol, an industrial intermediate for the production of antioxidants, has been investigated in the 2-8 GHz frequency range. The spectral analysis has allowed obtaining precise structural information on the most stable conformer and its complex with argon. The conformation of the monomer reveals that the hydroxyl group is coplanar with the ring but points in the opposite direction to the tert-butyl group, reducing steric interactions. In the tert-butyl group one methyl group is coplanar and the other two are symmetrically staggered respect to the ring. The complex shows the rare gas sitting above the aromatic ring. Interestingly, neither the monomer nor the complex exhibit large-amplitude hydroxyl torsion motions, previously observed in 2,6-disubstituted phenols such as 2,6-di-tert-butylphenol or propofol. The experimental results are supported by computational calculations, validating the molecular structure. Additionally, symmetry-adapted perturbation theory has allowed determining the van der Waals intermolecular interaction energy of the complex.

Accurate Experimental Structure of 1-Chloronaphthalene.

The structure of isolated 1-chloronaphthalene has been investigated in a supersonic expansion by high-resolution chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy in the 2-8 GHz frequency range. Accurate values of the rotational, centrifugal distortion, and nuclear quadrupole coupling constants for the only availabe conformer have been determined. The intensity of the spectrum allowed us to observe all the heavy atoms isotopologues in natural abundance, determining their rotational constants. From the extensive experimental dataset we derived accurate structures for 1-chloronaphthalene using different methodologies and compared with related compounds.

Rotational spectroscopy of methyl tert-butyl ether with a new Ka band chirped-pulse Fourier transform microwave spectrometer.

Chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy is a powerful tool for performing broadband gas-phase rotational spectroscopy, and its applications include discovery of new molecules, complex mixture analysis, and exploration of fundamental molecular physics. Here we report the development of a new Ka band (26.5-40 GHz) CP-FTMW spectrometer that is equipped with a pulsed supersonic expansion source and a heated reservoir for low-volatility samples. The spectrometer is built around a 150 W traveling wave tube amplifier and has an instantaneous bandwidth that covers the entire Ka band spectral range. To test the performance of the spectrometer, the rotational spectrum of methyl tert-butyl ether (MTBE), a former gasoline additive and environmental pollutant, has been measured for the first time in this spectral range. Over 1000 spectroscopic transitions have been measured and assigned to the vibrational ground state and a newly-identified torsionally excited state; all transitions were fit using the XIAM program to a root-mean-square deviation of 22 kHz. The spectrum displays internal rotation splitting, nominally forbidden transitions, and an intriguing axis-switching effect between the ground and torsionally excited state that is a consequence of MTBE's extreme near-prolate nature. Finally, the sensitivity of the spectrometer enabled detection of all singly-substituted 13C and 18O isotopologues in natural abundance. This set of isotopic spectra allowed for a partial r0 structure involving the heavy atoms to be derived, resolving a structural discrepancy in the literature between previous microwave and electron diffraction measurements.

Conformational adaptation and large amplitude motions of 1-phenyl-2,2,2-trifluoroethanol with two water molecules: a rotational spectroscopic and ab initio investigation.

The 1 : 2 adduct of 1-phenyl-2,2,2-trifluoroethanol (PhTFE), a chiral fluoroalcohol, with two water molecules (PhTFE⋯2H2O) was investigated via chirped pulse Fourier-transform microwave (CP-FTMW) spectroscopy and theoretical calculations. A systematic search of the PhTFE⋯2H2O conformational landscape identified 38 stable minima at the B3LYP-D3BJ/def2-TZVPPD level of theory, 27 of which are within an energy window of 10 kJ mol-1 after applying zero-point energy corrections. Rotational spectra of a single PhTFE⋯2H2O conformer along with eight deuterated and three oxygen-18 isotopologues were assigned. Interestingly, the observed PhTFE⋯2H2O conformer contains PhTFE II, the second most stable monomer conformer, and the most stable PhTFE I dihydrate is ca. 4 kJ mol-1 higher in energy. In contrast, PhTFE I⋯H2O was identified experimentally and theoretically as the most stable 1 : 1 conformer. Furthermore, the observed dihydrate structure experiences large amplitude motions connecting three theoretical minima which differ only in which water oxygen lone pairs are involved in the hydrogen-bonds, i.e., the free OH pointing directions. Additionally, the ortho and para-H2O tunnelling splittings were detected and attributed to the interchange water hydrogen atoms which interact with the aromatic part of PhTFE but not for the water interacting with PhTFE hydroxy group. Extensive theoretical modelling was carried out to gain insight into the associated large amplitude motions including tunnelling, supported by the experimental isotopic and tunnelling splitting data.

Rotational spectra of five cyano derivatives of fluorene.

The recent interstellar detection of individual polycyclic aromatic hydrocarbons (PAHs) in the dense molecular cloud TMC-1 brings interest in related species that could be present in this astronomical environment. These detections, that include pure PAHs and their cyano-derivative counterparts, were performed through the interplay between laboratory rotational spectroscopy experiments and radioastronomical observations. Here, we present the laboratory rotational spectroscopic study of the five cyano-derivatives of the PAH fluorene (C13H10). The samples for these five species were synthetized in the laboratory and then characterized in the gas phase using a chirped-pulse Fourier-transform microwave spectrometer operating between 2 and 12 GHz. The analysis of the rotational spectra allowed us to derive accurate molecular constants for the five isomers used to obtain frequency predictions that enable astronomical searches of these molecules in the interstellar medium.

Noncovalent Interactions in the Molecular Geometries of 4-Methylthiazole···H2O and 5-Methylthiazole···H2O Revealed by Microwave Spectroscopy.

The pure rotational spectra of 4-methylthiazole···H2O and 5-methylthiazole···H2O were recorded by chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy. Each complex was generated within the rotationally cold environment of a gas sample undergoing supersonic expansion in the presence of an argon buffer gas. The spectra of five isotopologues of each complex have been measured and analyzed to determine the rotational constants, A0, B0, and C0; centrifugal distortion constants, DJ, DJK, and d1; nuclear quadrupole coupling constants, χaa(N3) and [χbb(N3) - χcc(N3)]; and parameters describing the internal rotation of the CH3 group, V3 and ∠(i,b). The experimentally deduced parameters were obtained using the XIAM and the BELGI-Cs-hyperfine code. For each complex, parameters in the molecular geometry are fitted to experimentally determined moments of inertia. DFT calculations have been performed at the ωB97X-D/aug-cc-pVQZ level in support of the experiments. Each complex contains two hydrogen bonds; a comparatively strong, primary interaction between the N of thiazole and an O-H of H2O, and a weaker, secondary interaction between O and either the hydrogen atom attached to C2 (in 5-methylthiazole···H2O) or the CH3 group attached to C4 (in 4-methylthiazole···H2O). The barrier to internal rotation of the CH3 group, V3, is slightly lower for 4-methylthiazole···H2O (XIAM result is 340.05(56) cm-1) than that for the 4-methylthiazole monomer (357.6 cm-1). This is likely to be a result of internal charge redistribution within the 4-methylthiazole subunit following its coordination by H2O. At the precision of the experiments, V3 of 5-methylthiazole···H2O (XIAM result is 325.16(38) cm-1) is not significantly different from V3 of the 5-methylthiazole monomer (332.0 cm-1).

Conformational landscapes of symmetrically fluorine-substituted benzoic acids I: Microwave spectroscopic and theoretical studies on 3,5-difluorobenzoic acid

We report our combined theoretical and spectroscopic studies on 3,5-difluorobenzoic acid. Using a chirped pulse Fourier transform microwave (CP-FTMW) spectrometer, we recorded and analyzed the rotational spectrum spanning the frequency range of 6 – 12.5 GHz. Quantum chemical calculations were employed to analyze the conformational changes and landscapes of 3,5-difluorobenzoic acid. These calculations focused on studying the potential energy surfaces along the CCCO and OCOH dihedral angles at the B3LYP/6-311G level. Based on the computational results, we identified the global minimum conformer 1 as well as the local minimum conformer 2. We discussed and interpreted the geometric structures of the relevant conformations, with a particular emphasis on the interactions between the carboxylic group and the substituted fluorine atoms. Furthermore, these findings were compared to benzoic acid in internal strains. In our spectral analysis, we successfully identified conformer 1 and its seven 13C singly substituted isotopologues. We derived highly accurate rotational constants for 3,5-difluorobenzoic acid, displaying good agreement with computational results. We established the effective structure of its ground vibrational state using Kraitchman’s equations. Similar to benzoic acid, the global minimum conformation of 3,5-difluorobenzoic acid adopts a planar structure, corroborating our computational outcomes.

Conformational landscapes of symmetrically fluorine-substituted benzoic acids II: Calculations and measurements for the rotational spectrum and structure of 3,4,5-trifluorobenzoic acid

Calculations on the structure of 3,4,5-trifluorobenzoic acid were made using the Gaussian 16 program. The potential energy surfaces were scanned along CCCO and OCOH dihedral angles at the B3LYP/6-311G level to analyze its conformational landscape. Two conformations were identified and reoptimized at the B3LYP/aug-cc-pVTZ level. The result indicates that 3,4,5-trifluorobenzoic acid prefers a planar structure in its global minimum conformation. The pure rotational spectra of 3,4,5-trifluorobenzoic acid were measured in the frequency range of 6 – 12.5 GHz using a chirped pulse Fourier transform microwave (CP-FTMW) spectrometer. The spectra of the parent, seven 13C, and one deuterium singly substituted isotopologues were analyzed and fitted to measurement accuracy for a semi-rigid asymmetric top molecule. The rotational constants and centrifugal distortion constants were accurately determined. The rotational constants for the parent isotopologue are A = 1535.31408(32) MHz, B = 650.31751(16) MHz, and C = 456.98499(12) MHz. The effective structure of its ground vibrational state was determined from the spectra of the mono-substituted isotopologues. The agreement between the calculated and experimental spectroscopic constants is excellent.

A fitting program for structural determination of molecular clusters from rotational spectroscopy

The characterization of the structures of molecular clusters, which serve as building blocks for bulk substances, provides crucial insight into the interactions between constituent units. Chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy, combined with state-of-the-art quantum chemical calculations, is a powerful tool for characterizing the structures of molecular clusters, as the rotational spectra are directly related to the mass distribution of a molecule or cluster. However, determining the structures of large or complex clusters from experimental rotational spectra remains challenging due to their structural flexibility. Ab initio and density functional theory calculations for searching their stable structures could be significantly time-consuming and method-dependent. To address these challenges, we have developed an approach that relies on the experimental rotational constants to search for potential molecular structures without quantum chemical optimization. Our approach involves creating an initial set of conformers through either a semi-empirical sampling program or the quasi-Monte Carlo method. After-ward, the trust region reflective algorithm is utilized for structure fitting. This procedure enables us to quickly generate potential conformers and gain access to precise structural information. We apply our fitting program to water hexamer and benzaldehyde-water clusters, and the resulting topological structures align extremely well with the experimental results.

Supersonic jet chirped pulse microwave spectroscopy of ring-like methanol : water pentamers.

The potential energy surfaces of pure methanol and mixed methanol-water pentamers have been explored using chirped pulse Fourier-transform microwave spectroscopy aided by ab initio calculations. Rotational constants, anharmonic corrections, dipole moments, and relative energies were calculated for different conformers. Predicted rotational transitions were then fit to experimental spectra from 10-18 GHz and the assignments were confirmed using double resonance experiments where feasible. The results show all 23 of the lowest energy conformers are bound in a planar ring of hydrogen bonding that display a steady decrease in the RO-O distance along this ring as methanol content is increased. Interspersed methanol and water conformers have comparable relative abundances to those with micro-aggregation, but structures with micro-aggregated methanol and water have a higher rigid rotor fitting error. The computational methods' high degree of accuracy when compared to our experimental results suggests the strong donor-acceptor hydrogen bonding in these clusters leads to well-defined minima on the intermolecular potential energy surface.

Chirped pulse Fourier-transform microwave spectroscopy of alcohol and water tetramers.

In an effort to build towards quantitative models of alcohol:water microaggregation in liquid mixtures, the present works characterizes the energy landscape and structures of pure ethanol and mixed ethanol:water tetramers using Chirped Pulse Fourier-transform Microwave spectroscopy. Many conformers of each type of tetramer are available, and those with sufficiently strong dipole moments are experimentally examined. This analysis considers, but does not explicitly fit, the splitting of rotational states due to internal rotation of the methyl groups present, as well as utilizes isotopic substitution experiments to verify the conformer variations observed. Implications of the listed results include a suggestion of the stability of micro-aggregated structures as opposed to homogeneously mixed clusters, informing future work on characterization of larger clusters and any potential modeling of the hydrogen bond network at play.

Uniform supersonic flow sampling for detection by chirped-pulse rotational spectroscopy.

Chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy is a powerful near-universal detection method finding application in many areas. We have previously coupled it with supersonic flows (CPUF) to obtain product branching in reaction and photodissociation. Because chirped-pulse microwave detection requires monitoring the free induction decay on the timescale of microseconds, it cannot be employed with good sensitivity at the high densities achieved in some uniform supersonic flows. For application to low-temperature kinetics studies, a truly uniform flow is required to obtain reliable rate measurements and enjoy all the advantages that CP-FTMW has to offer. To this end, we present a new setup that combines sampling of uniform supersonic flows using an airfoil-shaped sampling device with chirped-pulse mmW detection. Density and temperature variations in the airfoil-sampled uniform flow were revealed using time-dependent rotational spectroscopy of pyridine and vinyl cyanide photoproducts, highlighting the use of UV photodissociation as a sensitive diagnostic tool for uniform flows. The performance of the new airfoil-equipped CPUF rotational spectrometer was validated using kinetics measurements of the CN + C2H6 reaction at 50K with detection of the HCN product. Issues relating to product detection by rotational spectroscopy and airfoil sampling are discussed. We show that airfoil sampling enables direct measurements of low temperature reaction kinetics on a microsecond timescale, while rotational spectroscopic detection enables highly specific simultaneous detection of reactants and products.

The molecular structure and curious motions in 1,1-difluorosilacyclopent-3-ene and silacyclopent-3-ene as determined by microwave spectroscopy and quantum chemical calculations.

The molecules 1,1-difluorosilacyclopent-3-ene (3SiCPF2) and silacyclopent-3-ene (3SiCP) have been synthesized and studied using chirped pulse, Fourier transform microwave (CP-FTMW) spectroscopy. For 3SiCP this is the first ever microwave study of the molecule and, for 3SiCPF2, the spectra reported in this work have been combined with that of previous work in a global fit. The spectra of each contain splitting which has been fit using a Hamiltonian consisting of semirigid and Coriolis coupling parameters. A refit of the original 3SiCPF2 work was also carried out. All fits and approaches are reported. Analyses of the spectra provide evidence that the molecule is planar which is in agreement with the high-level calculations, but the source of the splitting in the spectra has not been determined.

High throughput chirped pulse Fourier-transform microwave spectroscopy of ethanol and water clusters.

Here we discuss the design and performance of a novel high-throughput instrument for Chirped Pulse Fourier-transform Microwave (CP-FTMW) spectroscopy, and demonstrate its efficacy through the identification of the lowest energy conformers of the ethanol trimer and mixed water : ethanol trimers. Rotational constants for these trimers were calculated from observed lines in the spectra from 10 to 14 GHz, and compared to the results of anharmonic ab initio computations. As predicted, all trimers share a cyclic donor-acceptor hydrogen bonding structure, with the ethanol monomer favoring the gauche conformation in the lowest energy structures. The increased speed of data collection and resulting sensitivity opens a new avenue into rotational studies of higher order clusters.

Rotational spectra and conformational analysis of 2-bromobutane

The rotational spectra of 2-bromobutane were obtained by using a chirped-pulse Fourier-transform microwave (CP-FTMW) spectrometer in the frequency range 8–18 GHz. Hyperfine structures of both bromine isotopologues (I = 3/2) for the three conformers (G+, A, and G−) have been investigated to determine the rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling constants of bromine (Br). The conformational compositions for G+, A, and G− in the supersonic jet, estimated by relative intensity measurements, are 55%, 28%, 17%, respectively, for the 79Br species, and 52%, 33%, 15%, respectively, for the 81Br species. However, the small zero-point energy differences of isotopomers can be negligible and can be considered as an average value instead of individual values, which is 54%, 30%, 16% for G+, A, and G− species. The nuclear quadrupole coupling tensors (χxx, χyy, and χzz) of Br and the quadrupolar angles were obtained, and the χzz value of 2-bromobutane is compared with those of other alkyl bromides. The spectroscopic constants, rs coordinates of Br and the dipole moments of the most stable G+ conformer are predicted well from the ab initio calculations at the MP2/6–311++G(2d,2p) level.

Higher-Energy Hexafluoroisopropanol···Water Isomer and Its Large Amplitude Motions: Rotational Spectra and DFT Calculations.

Rotational spectra of the 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP)···water complex were measured using a chirped pulse Fourier-transform microwave spectrometer. The spectral analyses, aided by density functional theory calculations, reveal two HFIP···water isomers: one previously reported, trans HFIP (HFIPt)···water (Phys. Chem. Chem. Phys. 2015, 119, 5650-5657), and a new isomer, gauche HFIP (HFIPg)···water. To confirm the identity of the new isomer, rotational spectra of seven of its deuterated species were also measured and analyzed. Both the experimental and theoretical pieces of evidence indicate that the intermolecular interaction with water preferentially stabilizes the HFIPg monomer configuration over the global minimum configuration, HFIPt. The relative energy difference between these monomeric forms is 4.1 kJ mol-1 and decreases to 2.5 kJ mol-1 in the respective monohydrates at the B3LYP-D3(BJ)/def2-QZVP level of theory. Both rigid and relaxed potential energy surface scans were carried out to gain insights into the large-amplitude water motions in HFIPg···water. The nonobservation of a water tunneling splitting in HFIPt···water has been explained to be a result of a barrier-less (after zero-point-energy correction) pathway for the water motion, whereas in HFIPg···water, a relatively large water tunneling barrier was identified as the cause of barely resolved water tunneling splittings. Noncovalent interaction and quantum theory of atoms and molecule analyses were used to evaluate the changes in HFIPg···water when going from the minimum to the transition state in terms of attractive interactions such as the OH···H and OH···F contacts. The effect of fluorination is discussed by comparing the vastly different binding topologies of isopropanol···water and HFIP···water.