Fourier-transform Microwave Research Articles

Laboratory and astronomical discovery of the cyanovinyl radical H2CCCN

We report the first laboratory and interstellar detection of the α-cyano vinyl radical (H2CCCN). This species was produced in the laboratory by an electric discharge of a gas mixture of vinyl cyanide, CH2CHCN, and Ne. Its rotational spectrum was characterized using a Balle-Flygare narrowband-type Fourier-transform microwave spectrometer operating in the frequency region of 8–40 GHz. The observed spectrum shows a complex structure due to tunneling splittings between two torsional sublevels of the ground vibronic state, 0+ and 0−, derived from a large-amplitude inversion motion. In addition, the presence of two equivalent hydrogen nuclei makes it necessary to discern between ortho- and para-H2CCCN. A least-squares analysis reproduces the observed transition frequencies with a standard deviation of ca. 3 kHz. Using the laboratory predictions, this radical was detected in the cold dark cloud TMC-1 using the Yebes 40 m telescope and the QUIJOTE1 line survey. The 40, 4-30, 3 and 50, 5-40, 4 rotational transitions, composed of several hyperfine components, were observed in the 31.0–50.4 GHz range. Adopting a rotational temperature of 6 K, we derived a column density of (1.4±0.2)×1011 cm−2 and (1.1±0.2)×1011 cm−2 for ortho-H2CCCN and para-H2CCCN, respectively. The reaction of C + CH3CN emerges as the most likely route to H2CCCN in TMC-1, and possibly that of N + CH2CCH as well.

Microwave Spectrum and Novel Molecular Structure of (Z)-1-Chloro-3,3,3-trifluoropropene-Acetylene.

The gas-phase heterodimer formed between (Z)-1-chloro-3,3,3-trifluoropropene and acetylene is investigated using quantum chemistry calculations and observed via chirped-pulse Fourier transform microwave (FTMW) spectroscopy. Subsequent analysis of higher resolution spectra, including those using a sample enriched in H13C13CH, obtained with a Balle-Flygare FTMW spectrometer reveals a novel structure, as predicted by theory, for the complex, in which the acetylene functions as the gas-phase (Lewis) base and the halopropene as the acid. In the equilibrium structure, the acetylene molecule is located perpendicular to the symmetry plane of (Z)-1-chloro-3,3,3-trifluoropropene with the triple bond interacting with the two olefinic hydrogens. Mapped electrostatic potential surfaces suggest that this structure results from a reduction in the nucleophilicity of the halogen atoms as compared to previously studied acetylene halo-olefin complexes and a concomitant increase in the electrophilicity of the hydrogen atoms.

Low Barrier Methyl Internal Rotations and 14N Quadrupole Coupling in the Microwave Spectrum of 2,4-Dimethylthiazole.

The microwave spectrum of 2,4-dimethylthiazole was recorded using a pulsed molecular jet Fourier-transform microwave spectrometer operating in the frequency range from 2.0 to 26.5 GHz. Torsional splittings into quintets were observed for all rotational transitions due to internal rotations of two inequivalent methyl groups. Hyperfine structures arising from the nuclear quadrupole coupling of the 14N nucleus were fully resolved. The microwave spectra were analyzed using the modified version of the XIAM code and the BELGI-Cs-2Tops-hyperfine code. The barriers to methyl internal rotation of the 4- and 2-methyl groups were determined to be 396.707(25) cm-1 and 19.070(58) cm-1, respectively. The very low barrier hindering the 2-methyl torsion was a challenge for the spectral analysis and modeling, and separately fitting the five torsional species together with combination difference loops was the key for a successful assignment. The methyl torsional barriers were compared with those of other thiazole derivatives, showing the influence of the methyl group position on the barrier height. The experimental results were supported by quantum chemical calculations.

Stratospheric-trace-gas-profile retrievals from balloon-borne limb imaging of mid-infrared emission spectra

Abstract. The Limb Imaging Fourier Transform Spectrometer Experiment (LIFE) instrument is a balloon-borne prototype of a satellite instrument designed to take vertical images of atmospheric limb emission spectra in the 700–1400 cm−1 wavenumber range from the upper-troposphere–lower-stratosphere (UTLS) altitude region of the atmosphere. The prototype builds on the success of past and existing instruments while reducing the complexity of the imaging design. This paper details the results of a demonstration flight on a stabilized stratospheric balloon gondola from Timmins, Canada, in August 2019. Retrievals of vertical trace gas profiles for the important greenhouse gases H2O, O3, CH4, and N2O, as well as HNO3, are performed using an optimal estimation approach and the SASKTRAN radiative transfer model. The retrieved profiles are compared to approximately coincident observations made by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) solar occultation and Microwave Limb Sounder (MLS) instruments. An evaluation of the LIFE measurements is performed, and areas of improvement are identified. This work increases the overall technical readiness of the approach for future balloon, aircraft, and space applications.

Rotational spectra and astronomical search of five ethynyl derivatives of propene

ABSTRACT The interstellar discovery of the five cyano derivatives of propene brings interest in analogous molecules that could be detected in similar astronomical environments. In this context, ethynyl-propene derivatives are good candidates to be observed in the Taurus Molecular Cloud (TMC-1), where other cyano and ethynyl derivatives of hydrocarbons such as ethane, allene, propene, and cyclopentadiene, among others, have been detected. We have investigated the pure rotational spectra of the five ethynyl-propene derivatives to enable their interstellar detections. These molecular species have been produced by an electric discharge and their rotational spectra were characterized using a Balle–Flygare narrow band-type Fourier-transform microwave spectrometer operating in the frequency region of 4-40 GHz. The spectral analysis allowed us to derive accurate spectroscopic parameters to obtain reliable frequency predictions for astronomical searches. We do not detect any of these species in TMC-1 but we derive upper limits for the abundances for the five isomers.

Microwave measurements and calculations for the glyoxylic acid – Formic acid hydrogen-bonded complex

The gas-phase doubly hydrogen-bonded glyoxylic acid - formic acid hydrogen-bonded complex was obtained by mixing a heated sample of glyoxylic acid with room-temperature formic acid in argon. High-level DFT and MP2 calculations with various basis sets were performed and the structures and rotational constants were determined for the lowest energy dimers of glyoxylic acid - formic acid. The microwave spectrum was measured in the 6-12 GHz frequency range using two Flygare-Balle type pulsed beam Fourier transform microwave (FTMW) spectrometers. The rotational constants were determined to have the following values: A = 5533.911(3), B = 923.3883 (5), and C = 792.1132(6) MHz using 18 transitions. B3LYP/ cc-pVQZ calculations for the lowest energy dimer yielded calculated rotational constants of A = 5551.2, B = 922.9 and C = 791.3 MHz.

Microwave spectrum and structure of a glyoxylic acid – formic acid complex☆

The microwave spectrum of a second doubly hydrogen-bonded glyoxylic acid - formic acid hydrogen-bonded complex was measured in the 6–12 GHz frequency range using two Flygare-Balle type pulsed beam Fourier transform microwave (FTMW) spectrometers. The rotational constants were determined to have the following values: A = 4113.081(4), B = 1026.9957(5), and C = 822.9584(6) MHz. The doubly hydrogen bonded structures and rotational constants were calculated for the lowest energy dimers of glyoxylic acid - formic acid using DFT and MP2 calculations with various basis sets. M11/ def2QZVPP calculations for the third lowest energy dimer yielded rotational constants of A = 4248.7, C = 1031.4, and C = 829.9 MHz, in good agreement with experimental values.

The microwave spectra and molecular structures of (Z)-1-chloro-3,3,3-trifluoropropene and its gas-phase heterodimer with the argon atom

The microwave spectra of (Z)-1-chloro-3,3,3-trifluoropropene and its gas-phase heterodimer with the argon atom in the 5.6 to 18.1 GHz frequency range are first observed and assigned using broadband, chirped pulse, Fourier transform microwave (FTMW) spectroscopy. Subsequent analysis of higher-resolution spectra obtained between 5 and 21 GHz with a narrowband, Balle-Flygare FTMW instrument provides spectroscopic constants, including nuclear quadrupole coupling constants, for five isotopologues of the propene molecule and two isotopologues of the complex with argon. Structural parameters obtained from these spectra show the existence of an intramolecular hydrogen bond between one of the fluorine atoms of the trifluoromethyl group and the hydrogen atom on the adjacent carbon atom. No evidence is seen for internal rotation of the trifluoromethyl group. The location of the argon atom in the heterodimer is consistent with the expectation that it will be positioned so to interact with the greatest number of heavy atoms, and in particular, the polarizable chlorine atom.

Microwave spectrum, structure, and dipole moment of 2-fluorophenylacetylene

The rotational spectra of the parent and eight 13C isotopomers of 2-fluorophenylacetylene were measured using chirped-pulsed Fourier-transform microwave spectroscopy in the frequency range 6–18 GHz. The rotational constants for the parent isotopomer were determined to be A0 = 2949.41825(17) MHz, B0 = 1495.78081(13) MHz, and C0 = 992.23008(9) MHz. The total dipole moment was determined to be 1.720(2) Debye from Stark measurements. The molecular geometry parameters were obtained experimentally using Kraitchman substitution and semi-experimental equilibrium structure. These structural parameters and dipole moments were compared with the data calculated at the B3LYP/6-311+G(2d,p) level and analyzed using the natural bond orbital method.

Microwave measurements, calculations, and analysis for the gas phase ammonia-formic acid dimer

The gas-phase doubly hydrogen-bonded formic acid-ammonia complex was obtained by heating a sample of ammonium formate in neon. High-level DFT and MP2 calculations with various basis sets were performed and barriers for proton tunneling and internal rotation were determined. The microwave spectrum was measured in the 7–16 GHz frequency range using a Flygare-Balle type pulsed beam Fourier transform microwave (FTMW) spectrometer. Double resonance transitions were observed near 20 GHz. Rotational transitions were measured and fitted for two vibrational states to obtain the rotational constants and quadrupole coupling constants. The rotational constants were determined to have the following values: A = 12017.0(2), B = 4337.331(2), and C = 3227.279(2) for the 0+ state and A = 12017.0, B = 4302.02(1), and C = 3238.161(1) for the 0− state and have been fit to 16 transitions and 9 transitions respectively. Each state was fit to a rigid model showing quadrupole splitting and compared with computations.

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.

Fourier-transform microwave spectroscopy of the s-trans-3-propenalyl (CH2CHĊO) and 3-propenolyl (ĊH2CHCO) radicals.

Pure rotational transitions of two conformers of the CH2CHCO radical have been observed by Fourier-transform microwave spectroscopy, where one conformer is called the s-trans-3-propenalyl radical and the other the 3-propenolyl radical. The observed two conformers have different electronic states. The former, the s-trans-3-propenalyl radical, has the 2A' electronic state and can be written as CH2CHĊO, where the unpaired electron resides mainly on the terminal CO carbon. On the other hand, the latter, 3-propenolyl radical has the 2A'' electronic state and can be written as ĊH2CHCO. We were able to observe pure rotational transitions of the two conformers. Since both of the species have an unpaired electron, there exist spin-rotation interactions due to the unpaired electron and the magnetic hyperfine interactions due to the three coupling protons. The observed very complicated spectra, caused by these interactions, were assigned, leading to detailed molecular constants including the fine and hyperfine coupling constants for both of the species. The determined molecular constants support the electronic structures of the two conformers. There exists a controversy as to which of the two conformers is the lowest energy one. Our present observation led to the conclusion that s-trans-3-propenalyl is the lowest energy conformer.

Microwave spectra of dinitrotoluene isomers: a new step towards the detection of explosive vapors.

The spectroscopic characterization of explosive taggants used for TNT detection is a research topic of growing interest. We present a gas-phase rotational spectroscopic study of weakly volatile dinitrotoluene (DNT) isomers. The pure rotational spectra of 2,4-DNT and 2,6-DNT were recorded in the microwave range (2-20 GHz) using a Fabry-Perot Fourier-transform microwave (FP-FTMW) spectrometer coupled to a pulsed supersonic jet. Rotational transitions are split by hyperfine quadrupole coupling at the two 14N nuclei leading to up to 9 hyperfine components. The spectral analysis was supported by quantum chemical calculations carried out at the B98/cc-pVTZ and MP2/cc-pVTZ levels of theory. Based on 2D potential energy surfaces at the B98/cc-pVTZ level of theory, the methyl group internal rotation barriers were calculated to be V3 = 515 cm-1 and 698 cm-1 for 2,4- and 2,6-DNT, respectively. Although no splitting due to internal rotation was observed for 2,6-DNT, several splittings were observed for 2,4-DNT. The microwave spectra of both species were fitted using a semi-rigid Hamiltonian accounting for the quadrupole coupling hyperfine structure. Based on the internal axis method (IAM), an additional analysis was performed to retrieve an accurate value of the rotationless A-E tunneling splitting which could be extracted from the rotational dependence of the tunneling splitting. This yielded in the case of 2,4-DNT to an experimental value of 525 cm-1 for the barrier height V3 which agrees well with the DFT value. The coupled internal rotations of -CH3 and -NO2 are investigated in terms of 2-D surfaces, as already done in the case of 2-nitrotoluene [A. Roucou et al., Chem. Phys. Chem., 2020, 21, 2523-2538].

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.

Laboratory rotational spectroscopy and astronomical search of ethynyl substituted naphthalene

ABSTRACT The recent interstellar detection of cyanonaphthalenes brings interest in related aromatic molecular species that could be present in similar astronomical environments. In this context, ethynyl derivatives of naphthalene are promising candidates to be observed in the Taurus Molecular Cloud (TMC-1), where cyanonaphthalenes together with cyano- and ethynyl-derivatives of cyclopentadiene and benzene have been detected. To enable the interstellar searches for ethynyl derivatives of naphthalene, their pure rotational spectra need to be investigated in the laboratory. We have observed for the first time the rotational spectra of 1- and 2-ethynylnaphthalene species using a broad-band Fourier-transform microwave spectrometer operating in the 2–8 GHz frequency region. Accurate spectroscopic parameters are derived from the analysis of the experimental spectra, allowing for reliable predictions for astronomical searches. Our searches in TMC-1 for both isomers provide upper limits for the abundances of these species.

Characterizing the Interactions of Dimethyl Sulfoxide with Water: A Rotational Spectroscopy Study.

The interaction of dimethyl sulfoxide with water has been investigated by Fourier-transform microwave spectroscopy of the 1:1 complex and its isotopologues, complemented with quantum chemical calculations. The rotational spectra of 34S and 13C isotopologues in natural abundance and the H218O and deuterated water enriched isotopologues have been measured, allowing a partial structure determination and establishing the position of water in the complex. In the most stable conformation water was found to be the donor of a primary OH···OS bond to the oxygen atom of dimethyl sulfoxide and acceptor of two weak CH···OH bonds with the methyl hydrogen atoms of dimethyl sulfoxide. From the structural determination confirmed by quantum chemical calculations, the water molecule lies in the symmetry plane of dimethyl sulfoxide, and the complex has an overall Cs symmetry. The experimental findings are supported by atoms in molecules and symmetry-adapted perturbation theories, which allowed for determining the hydrogen bond and intermolecular interaction energies, respectively.

Conformational flexibility and hydrogen bonding in 5-aminopentanol

Rotational spectra of two conformers of 5-aminopentanol have been recorded using a Fourier-transform microwave spectrometer. For conformer C-1, eighty-two hyperfine components from the twenty-four a-, b-, and c-type transitions measured were fit to the quadrupole coupling constants, χaa = −2.954(2) MHz, χbb = 2.386(3) MHz. For conformer C-2, the fit of the seventy-six hyperfine components from the twenty-four a- and b-type transitions measured yielded χaa = −3.636(1) MHz and χbb = 2.087(2) MHz. Rotational and centrifugal distortion constants determined from fits of the resulting unsplit line centers to the Watson A-reduction Hamiltonian are A = 3322.169(1) MHz, B = 1958.7382(9) MHz, C = 1402.5957(8) MHz, ΔJ = 0.60(2) kHz, ΔJK = −0.21(8) kHz, ΔK = 0.99(3) kHz, δJ = 0.203(7) kHz, and δK = 1.1(1) kHz for conformer C-1 and A = 3249.2215(6) MHz, B = 2027.9327(3) MHz, C = 1432.5846(3) MHz, ΔJ = 0.545(6) kHz, ΔJK = 0.25(2) kHz, ΔK = 0.39(5) kHz, δJ = 0.18(4) kHz, and δK = 0.96(3) kHz for conformer C-2. The two experimental conformations are consistent with the two lowest energy ab initio MP2/6-311++G(d,p) structures. Both conformations of 5-aminopentanol are stabilized by an intramolecular hydrogen bond from the alcohol proton to amino nitrogen. The flexibility introduced by the five carbons in the alkyl group separating the amino and alcohol functional groups resulted in the first appearance of multiple low energy conformers being detected relative to the four, three, and two carbons in 4-aminobutanol, 3-aminopropanol, and 2-aminoethanol respectively in which only one experimental conformation was observed.

Spectroscopic Characterization of 3-Aminoisoxazole, a Prebiotic Precursor of Ribonucleotides.

The processes and reactions that led to the formation of the first biomolecules on Earth play a key role in the highly debated theme of the origin of life. Whether the first chemical building blocks were generated on Earth (endogenous synthesis) or brought from space (exogenous delivery) is still unanswered. The detection of complex organic molecules in the interstellar medium provides valuable support to the latter hypothesis. To gather more insight, here we provide the astronomers with accurate rotational frequencies to guide the interstellar search of 3-aminoisoxazole, which has been recently envisaged as a key reactive species in the scenario of the so-called RNA-world hypothesis. Relying on an accurate computational characterization, we were able to register and analyze the rotational spectrum of 3-aminoisoxazole in the 6–24 GHz and 80–320 GHz frequency ranges for the first time, exploiting a Fourier-transform microwave spectrometer and a frequency-modulated millimeter/sub-millimeter spectrometer, respectively. Due to the inversion motion of the −NH2 group, two states arise, and both of them were characterized, with more than 1300 lines being assigned. Although the fit statistics were affected by an evident Coriolis interaction, we were able to produce accurate line catalogs for astronomical observations of 3-aminoisoxazole.

Methyl Internal Rotation in Fruit Esters: Chain-Length Effect Observed in the Microwave Spectrum of Methyl Hexanoate.

The gas-phase structures of the fruit ester methyl hexanoate, CH3-O-(C=O)-C5H11, have been determined using a combination of molecular jet Fourier-transform microwave spectroscopy and quantum chemistry. The microwave spectrum was measured in the frequency range of 3 to 23 GHz. Two conformers were assigned, one with Cs symmetry and the other with C1 symmetry where the γ-carbon atom of the hexyl chain is in a gauche orientation in relation to the carbonyl bond. Splittings of all rotational lines into doublets were observed due to internal rotation of the methoxy methyl group CH3-O, from which torsional barriers of 417 cm−1 and 415 cm−1, respectively, could be deduced. Rotational constants obtained from geometry optimizations at various levels of theory were compared to the experimental values, confirming the soft degree of freedom of the (C=O)-C bond observed for the C1 conformer of shorter methyl alkynoates like methyl butyrate and methyl valerate. Comparison of the barriers to methyl internal rotation of methyl hexanoate to those of other CH3-O-(C=O)-R molecules leads to the conclusion that though the barrier height is relatively constant at about 420 cm−1, it decreases in molecules with longer R.

The microwave spectrum of the difluorocyanomethyl radical, ĊF2CN

The pure rotational spectrum of the open shell difluorocyanomethyl radical, ĊF2CN, has been measured using two Balle-Flygare-type cavity Fourier-Transform-Microwave (FTMW) spectrometers both equipped with pulsed discharge nozzles. A total of 156 transitions (from N = 1 – 0 to 6 – 5, and Ka = 0, 1, 2, 3) in the electronic ground state were observed between 6.5 GHz and 38.4 GHz with a typical linewidth of ∼5 kHz full-width-half-maximum. A Hamiltonian that included semi-rigid rotor, spin-rotation, and nuclear hyperfine parameters was fit to the observed data set and these parameters have been interpreted and compared to similar radicals. Excellent agreement between experimental and uB3LYP/aug-cc-pVQZ calculated rotational constants, the experimental inertial defect, −0.6858(2) uÅ2, and the failure of a coupling scheme in which the fluorine nuclei are treated as identical and related by a C2v symmetry axis combine to indicate a nonplanar structure for the ĊF2CN radical.