Chirped-pulse Fourier Transform Microwave Spectrometer Research Articles

Improvements on the FANTASIO set-up and new spectroscopic results concerning the Ar-H2O van der Waals complex in the 2OH overtone region

We present a series of improvements of the FANTASIO experimental set-up that couples a supersonic expansion to a cavity ring-down spectrometer originally reported by M. Herman, et al., Mol. Phys. 105, 815 (2007). In particular, the implementation of a new pulsed slit valve of 80 mm in length is detailed. The design is based on a powerful piezo stack actuator. Schlieren imaging is used to verify the homogeneity of the molecular beam and the quality of the pulsed slit performance. The ability to form weakly bound van der Waals complexes is assessed using the cavity ring-down spectrometer and a new chirped-pulse Fourier transform microwave spectrometer with the 12–18 GHz operating range. The water hexamer and heptamer were observed using this microwave spectrometer. Following enhancements and an extended laser coverage of the CRDS in the 2OH region, we observed and analyzed seven sub-bands of the Ar-H216O van der Waals complex, along with one sub-band of the much less abundant Ar-H218O isotopologue.

A global rho-axis method for fitting asymmetric tops with one methyl internal rotor and two 14N nuclei: Application of BELGI-2N to the microwave spectra of four methylimidazole isomers.

A number of internal rotation codes can deal with the combination of one or two internal rotors with one 14N quadrupole nucleus, but once it comes to two 14N nuclei, no such code is available even for the case of one internal rotor. We present here the extension of our internal rotor program called BELGI-2N using the rho-axis method global approach to deal with compounds containing one methyl rotor and two weakly coupling 14N nuclei. To test our new code, we applied it to the microwave data recorded for N-methylimidazole, 2-methylimidazole, 4-methylimidazole, and 5-methylimidazole using a chirped-pulse Fourier transform microwave spectrometer in the 7.0-18.5GHz frequency range. Compared to the previously published study, BELGI-2N was able to (i) significantly increase the number of assigned and fitted lines, (ii) fit the complete datasets considering both the internal rotation and the 14N nuclear quadrupole coupling effects simultaneously, and (iii) achieve standard deviations within the measurement accuracy for all methylimidazole isomers.

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.

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.

The gas-phase structure determination of α-pinene oxide: An endo-cyclic epoxide of atmospheric interest.

The gas-phase rotational spectra of α-pinene oxide have been recorded using a chirped-pulse Fourier transform microwave spectrometer in the 6-18GHz frequency range. The parent species and all heavy atom isotopologues (13C and 18O) have been observed in their natural abundance. The experimental rotational constants of all isotopic species observed have been determined and used to obtain the substitution (rs) and the effective (r0) structures of the most stable conformer of α-pinene oxide. Calculations using the density functional theories B3LYP, M06-2X, and MN15-L and the abinitio method MP2 level of theory were carried out to check their performance against experimental results. The structure of the heavy atom's skeleton of α-pinene oxide has been compared to that of α-pinene and has shown that epoxidation does not overly affect the structure of the bicycle, validating its robustness. Furthermore, the structural features have been compared to those of other bicyclic molecules, such as nopinone and β-pinene.

Steroid Hormone Androsterone Observed in the Gas Phase by Rotational Spectroscopy.

We report the investigation of the steroid hormone androsterone in the gas phase. Androsterone is a male sex steroid hormone, being the first steroid hormone from this category isolated and discovered 90 years ago. Despite the chemical compositions of steroids being well-known since long ago, studying their structures in the gas phase is still a challenging task, and to date, just a handful of detailed experimental structures for steroids have been reported. The rotational spectrum of androsterone was recorded in the 2-8 GHz frequency region with a broadband chirped-pulse Fourier transform microwave spectrometer coupled with supersonic expansion. From the weak rotational spectrum, one conformer has been detected in the isolated and cold conditions of the molecular jet. The combination of the experimental results with quantum chemical calculations allowed us to unambiguously identify the conformation of androsterone in the gas phase.

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.

Testing the Scalability of the HS-AUTOFIT Tool in a High-Performance Computing Environment

In the last years, the development of broadband chirped-pulse Fourier transform microwave spectrometers has revolutionized the field of rotational spectroscopy. Currently, it is possible to experimentally obtain a large quantity of spectra that would be difficult to analyze manually due to two main reasons. First, recent instruments allow obtaining a considerable amount of data in very short times, and second, it is possible to analyze complex mixtures of molecules that all contribute to the density of the spectra. AUTOFIT is a spectral assignment software application that was developed in 2013 to support and facilitate the analysis. Notwithstanding the benefits AUTOFIT brings in terms of automation of the analysis of the accumulated data, it still does not guarantee a good performance in terms of execution time because it leverages the computing power of a single computing machine. To cater to this requirement, we developed a parallel version of AUTOFIT, called HS-AUTOFIT, capable of running on high-performance computing (HPC) clusters to shorten the time to explore and analyze spectral big data. In this paper, we report some tests conducted on a real HPC cluster aimed at providing a quantitative assessment of HS-AUTOFIT’s scaling capabilities in a multi-node computing context. The collected results demonstrate the benefits of the proposed approach in terms of a significant reduction in computing time.

Copper-Catalyzed Transfer Hydrodeuteration of Aryl Alkenes with Quantitative Isotopomer Purity Analysis by Molecular Rotational Resonance Spectroscopy.

A copper-catalyzed alkene transfer hydrodeuteration reaction that selectively incorporates one hydrogen and one deuterium atom across an aryl alkene is described. The transfer hydrodeuteration protocol is selective across a variety of internal and terminal alkenes and is also demonstrated on an alkene-containing complex natural product analog. Beyond using 1H, 2H, and 13C NMR analysis to measure reaction selectivity, six transfer hydrodeuteration products were analyzed by molecular rotational resonance (MRR) spectroscopy. The application of MRR spectroscopy to the analysis of isotopic impurities in deuteration chemistry is further explored through a measurement methodology that is compatible with high-throughput sample analysis. In the first step, the MRR spectroscopy signatures of all isotopic variants accessible in the reaction chemistry are analyzed using a broadband chirped-pulse Fourier transform microwave spectrometer. With the signatures in hand, measurement scripts are created to quantitatively analyze the sample composition using a commercial cavity enhanced MRR spectrometer. The sample consumption is below 10 mg with analysis times on the order of 10 min using this instrument-both representing order-of-magnitude reduction compared to broadband MRR spectroscopy. To date, these measurements represent the most precise spectroscopic determination of selectivity in a transfer hydrodeuteration reaction and confirm that product regioselectivity ratios of >140:1 are achievable under this mild protocol.

Pure Rotational Spectrum of Benzophenone Detected by Broadband Microwave Spectrometer in the 2–8 GHz Range

The investigation on microwave spectrum of benzophenone was conducted with a recently constructed broadband chirped-pulse Fourier transform microwave spectrometer with a heating nozzle in the 2–8 GHz range. In this work, 138 b-type pure rotational transitions were assigned to bridge the measuring gap in the microwave region. The rotational constants for benzophenone were accurately determined by a combined microwave data fitting with frequency coverage between 2–14 GHz and have the following values: A = 1692.8892190(119) MHz, B = 412.6446602(43) MHz and C = 353.8745644(43) MHz.

Molecular Parameters of Tert-Butyl Chloride and Its Isotopologues Determined from High-Resolution Rotational Spectroscopy

A broadband chirped-pulse Fourier transform microwave spectrometer was used to detect the rotational spectra of the products of a chemical reaction in the gas phase from 1-18 GHz under the supersonic expansion condition. In natural abundance, pure rotational energy level transitions of tert-butyl chloride and its isotopologues (13C, 37Cl) were observed and assigned. The rotational spectral parameters (rotational constant, quadrupole coupling constant and centrifugal distortion constant) of these isotopologues were determined. The experimental results are in great agreement with the calculated values of quantum chemistry and the spectral parameters in the literature. The accuracy and the capability for chemical detection of our homemade rotational spectrometer were verified by this experiment.

Design and performance of an E-band chirped pulse spectrometer for kinetics applications: OCS – He pressure broadening

Product channel-specific reaction kinetics at low temperature presents a significant challenge for both experiment and theory but provides essential inputs for astrochemical models of cold interstellar environments. Reaction kinetics studies using chirped pulse Fourier transform microwave (CP-FTMW) spectroscopy provide a potential solution but require the use of a buffer gas to thermalize the reactants and products; however, collisions with the buffer gas reduce the length of the detected signal, the free induction decay, through pressure broadening. The effect of this on time domain signals is largely unexplored using CP-FTMW spectrometers. A new E-band (60–90 GHz) spectrometer has been constructed using previously unavailable equipment in order to maximize its performance for detecting molecules in collisional environments. The design of this spectrometer is described in detail. The pressure broadening of OCS in He was used to test the spectrometer under collisional conditions similar to those that are routinely used to perform reaction kinetics measurements. The corresponding pressure broadening coefficients for transitions in this frequency range were determined at room temperature and the performance of the spectrometer was assessed in relation to recently reported chirped pulse in uniform flow experiments.

Conformational landscape and intricate conformational relaxation paths of 4,4,4-trifluoro-1-butanol: Rotational spectroscopy and quantum chemical calculations

Fluorine-containing aliphatic alcohols have attracted considerable interest in part because they are widely used as solvents for peptides and proteins and for some chemical reactions. In the present work, the rotational spectrum of 4,4,4-trifluoro-1-butanol was investigated using a chirped-pulse Fourier transform microwave spectrometer in combination with a pulsed molecular expansion. The long carbon chain and the OH group in 4,4,4-trifluoro-1-butanol afford substantial conformational flexibility. In total, 27 possible geometries were identified using a semiempirical tight-binding based quantum chemistry program and also by carrying out systematic manual conformational searches. These conformations were verified to be true minima at the B3LYP-D3(BJ)/def2-TZVP, B3LYP-D3(BJ)/6–311++G(2d,p) and MP2/6–311++G(2d,p) levels of theory. 14 conformers that are spectroscopically distinguishable were obtained within an energy window of 18 kJ mol−1. Rotational spectra of three conformers were assigned, including four 13C isotopologues and one 18O isotopologue of the most stable conformer in their natural abundances. One of the detected conformers exhibits an intramolecular hydrogen bond formed between a non-hydroxy hydrogen atom of the –CH2OH group and a fluorine atom of the –CF3 group, which was identified by a quantum theory of atoms in molecules (QTAIM) analysis. The intricate conformational relaxation pathways and the corresponding interconversion barriers were explored to rationalize the experimental relative abundances of the detected conformers.

The Conformational Landscape, Internal Rotation, and Structure of 1,3,5-Trisilapentane using Broadband Rotational Spectroscopy and Quantum Chemical Calculations.

The rotational spectrum of 1,3,5-trisilapentane was observed on a chirped-pulse Fourier transform microwave spectrometer and is reported. During assignment, multiple conformations of the molecule were identified in the molecular beam. Prior quantum-chemical calculations performed on the molecule show that the identified spectra correspond to the lowest three calculated energetic structures. These structures are of C2 (Conf.1), C2v (Conf.2), and C1 (Conf.3) symmetry, with relative energy ordering of Conf.1 < Conf.3 < Conf.2, which is in stark contrast to n-pentane and all known silicon-substituted n-pentane derivatives. This is found to most likely arise from the elongation of the Si-C bond and the size of the silicon atoms providing for the C2 and C1 structures relieving steric hindrance in comparison to that of the C2v. In the C2v and C1 conformers, splitting in the spectra due to internal rotation of the -SiH3 end groups of 1,3,5-trisilapentane was observed and determined. The C2v equivalent V3 values are 368.46(33) cm-1, and the C1V3 values are 347.78(21) and 360.18(88) cm-1, respectively. These barriers are compared to similar species in order to help verify their veracity and are determined to be accurate based on similar molecular silyl rotors.

A General Treatment to Study Molecular Complexes Stabilized by Hydrogen‐, Halogen‐, and Carbon‐Bond Networks: Experiment and Theory of (CH2F2)n⋅⋅⋅(H2O)m

Rotational spectra of several difluoromethane-water adducts have been observed using two broadband chirped-pulse Fourier-transform microwave (CP-FTMW) spectrometers. The experimental structures of (CH2 F2 )⋅⋅⋅(H2 O)2 , (CH2 F2 )2 ⋅⋅⋅(H2 O), (CH2 F2 )⋅⋅⋅(H2 O)3 , and (CH2 F2 )2 ⋅⋅⋅(H2 O)2 were unambiguously identified with the aid of 18 isotopic substituted species. A subtle competition between hydrogen, halogen, and carbon bonds is observed and a detailed analysis was performed on the complex network of non-covalent interactions which stabilize each cluster. The study shows that the combination of stabilizing contact networks is able to reinforce the interaction strength through a cooperative effect, which can lead to large stable oligomers.

Conformer-specific microwave spectroscopy of 3-phenylpropionitrile by strong field coherence breaking

Strong field coherence breaking (SFCB) was used with a chirped-pulse Fourier Transform microwave spectrometer to obtain conformer-specific rotational spectra of 3-phenylpropionitrile in the 8–18 GHz region. Transitions belonging to anti and gauche conformers were identified and assigned and accurate experimental rotational constants were determined to provide insight to the molecular structure. Experimental rotational transitions provided relative abundances in the supersonic expansion. A modified line picking scheme was developed in the process to modulate more transitions and improve the overall efficiency of the SFCB multiple selective excitation technique.

Rotational spectra of 4,4,4-trifluorobutyric acid and the 4,4,4-trifluorobutyric acid-formic acid complex

The pure rotational spectra of 4,4,4-trifluorobutyric acid, CF3CH2CH2COOH, and its complex with formic acid were studied by a pulsed nozzle, chirped-pulse Fourier transform microwave spectrometer in the frequency range of 7–13 GHz. The rotational constants and centrifugal distortion constants were determined for the first time. Quantum chemical calculations were carried out exploring possible conformations of 4,4,4-trifluorobutyric acid and the structure of the 4,4,4-trifluorobutyric acid-formic acid complex using B3LYP/aug-cc-pVTZ and MP2/aug-cc-pVTZ calculations. Only one conformer was observed for 4,4,4-trifluorobutyric acid and its complex with formic acid, in agreement with the calculations.

Newly assigned microwave transitions and a global analysis of the combined microwave/millimeter wave rotational spectra of 9-fluorenone and benzophenone

Microwave spectra of 9-fluorenone and benzophenone have been observed using a broadband chirped-pulse Fourier Transform Microwave (cp-FTMW) Spectrometer. An analysis of the microwave spectra allowed for the assignment of 178 b-type rotational transitions for 9-fluorenone in the 8.0–13.0GHz region, the assignment of 166 b-type transitions for benzophenone in the 8.0–14.0GHz region, and effectively quadrupled the total number of pure rotational transitions observed for these molecules. This new microwave data and the previously published millimeter wave data of Maris et al. have been analyzed together in a global fit, where the resulting rotational constants accurately reproduce the spectra over the entire 8–80GHz region for both molecules. In addition, the resulting constants have been found to be consistent with the expected planar C2v structure for 9-fluorenone and the paddle-wheel like C2 structure of benzophenone. The rotational constants of the combined global fit have allowed for a more precise determination of the inertial defects (Δ) and second moments of inertia (Pcc) for 9-fluoreneone and benzophenone. Specific focus has been paid to the second moment of benzophenone, which when used in conjunction with theory strongly suggests an ∼32.9° torsional angle out of the ab-plane for the paddle-wheel structure of the gas-phase molecule.

Microwave spectrum, structure and dipole moment of 4-fluorophenylacetylene (4FPA)

Using a chirped-pulse Fourier-transform microwave (CP-FTMW) spectrometer, a 6–18 GHz spectrum of 4-fluorophenylacetylene (4FPA) was measured and only a-type R-branch transitions were observed up to J = 9. Rotational constants and quartic centrifugal distortion constants for the normal isotopic species were determined based on Watson-S reduction: A = 5652.812(22) MHz, B = 966.92885(11) MHz, C = 825.67680(11) MHz, DJ = 0.01377(60) kHz, and DJK = 0.2468(61) kHz, with other three distortion constants fixed as DK = 0.6629 kHz, d1 = 2.386 Hz, and d2 = 0.989 Hz from ab initio results. For six kinds of carbon-13 isotopic species, 10–15 transitions were detected by a resonant cavity FTMW spectrometer in natural abundance, and rotational constants of each species were also determined by fitting transition frequencies. Gaseous molecular structures of 4FPA were derived via the least-squares fitting (r0) and substitution (rs) methods, and ab initio optimization (re). They were compared to the structures of benzene derivatives having fluorine and the acetylenic group as substituents. In addition, dipole moment component of 4FPA was also determined to be μa = μtotal = 0.8935(9) D from Stark effect measurements.