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Multiple minor isotopologues of ArHCl have been observed in natural abundance using two CP-FTMW spectrometers. These include the first known microwave observation of 38 Ar isotopologues in natural abundance. The parameters derived from fits of the spectrum have been utilized in calculating multiple molecular parameters for the system which are shown to be the best determined to date. From these new spectrum, it is shown that there is a mass-dependency of the derived effective Ar Cl bond distance and the large amplitude averaging angle of HCl is relatively invariant to mass substitution at 41.5°.

As the first step of a two-part chiral tagging experiment, the spectrum and subsequent isotopologue analysis on the heavy atoms of (1S)-(-)-Verbenone is presented. The spectrum has been recorded up to 69 GHz on three spectrometers, one CP-FTMW spectrometer from the University of Virginia functional from 2 to 8 GHz, a CP-FTMW spectrometer operational in the 6–18 GHz range located at the Missouri University of Science and Technology, and a Stark-modulated spectrometer operational from 48 to 72 GHz. 1250 transitions have been assigned to the parent and isotopologues for the predominantly b -type spectrum. Rotational constants and quartic centrifugal distortion constants have been determined for the parent species while for the 11 isotopologues only rotational constants have been determined. A Kraitchman analysis has been performed and the resulting coordinates are reported. The experimental heavy-atom structure has been compared to previously studied bicyclic terpenes and the computational structure and is found to be in excellent agreement with both, showing reliability of the theoretical approaches needed for the future chiral tagging work.

Reported here are CP-FTMW experiments on N-ethylformamide (NEF), previously studied by pure rotational spectroscopy in the gas phase by Hirota and co-workers (2005). Two conformers of the monomer (trans sc and cis ac), the trans-trans sc dimer, and several NEF-water complexes have been detected. (Only trans sc was observed in the earlier experiments.) Of these species, the trans-trans sc dimer is the most interesting since its three-dimensional structure bears a striking resemblance to β-turns in folded peptides. The well-known anti-parallel β-sheet structures would be expected to have near-zero dipole moments and are therefore discriminated against by the CP-FTMW method.

Journal of molecular spectroscopy 378, 111452 (2021). doi:10.1016/j.jms.2021.111452, We report our analysis of the pure rotational spectra of low-lying vibrationally excited states and heavy-atom rare isotopologues of imidazole. To facilitate searches for imidazole in the interstellar medium, we previously described the analysis of the rotational spectrum of the imidazole main isotopologue across the 2–295 GHz range, and we extend this analysis here. Structure optimisation and anharmonic frequency calculations were performed to aid the spectral analysis. Three vibrationally excited states of imidazole were assigned in our room-temperature spectra, with energies up to approximately 670 cm$^{−1}$ above the vibronic ground state. The vibrational states could act as temperature probes in warmer star-forming regions. The $^{13}$C and $^{15}$N heavy-atom isotopologues were assigned, which allowed for the structure of imidazole in the gas phase to be determined. Structural comparisons are drawn between the related heterocyclic molecules hydantoin and imidazolidine. An experimental gas-phase structure of the former is also determined in this work. The potential detection of the isotopologues could help deduce formation pathways towards imidazole and other nitrogen-containing cyclic compounds in interstellar space., Published by Academic Press, Orlando, Fla.

Broadband chirped-pulse Fourier transform microwave (CP-FTMW) spectrometers have increased the sensitivity for molecular rotational spectroscopy. The measurement dynamic range is often large enough that isotopologues of the molecular species with the highest transition strength will also be detectable. In order to analyze the complex spectra from these broadband measurements, an automated spectral assignment program called AUTOFIT has been developed. The algorithm of AUTOFIT is described and its performance is illustrated by the analysis of the CP-FTMW spectrum of 1-hexanal obtained over the spectral range 6–40 GHz. The rotational spectra of a total of 12 conformers of 1-hexanal have been assigned using AUTOFIT to automatically identify the spectrum of conformers predicted by ab initio calculations. In addition, the rotational spectra of the 13C and 18O isotopologues for the two lowest energy conformers and the 13C isotopologues for the third and fourth lowest energy conformers are assigned. The effect of the quality of the input theoretical estimates of the rotational spectroscopy parameters on AUTOFIT performance is discussed. It is shown that the use of B3LYP-D3/aug-cc-pVTZ optimized structures can improve AUTOFIT search speeds by a factor of 10–60× compared to B3LYP structures.

The pure rotational spectrum of glycolaldehyde has been recorded from 6.5–20 GHz and 25–40 GHz in two pulsed-jet chirped pulse Fourier transform microwave spectrometers. The high phase stability of the spectrometers enables deep signal integration, allowing transitions from the 13 C-substituted, 18 O-substituted, and deuterium-substituted isotopologues to be observed in natural abundance. Transitions from HCOCH 2 18 OH are reported for the first time. Additional transitions from the 13 C-substituted, deuterium-substituted, and HC 18 OCH 2 OH isotopologues, as well as previously unobserved weak lines from the main isotopologue, have been observed. Transitions from all isotopologues are used with previously reported transitions to refine the spectroscopic parameters for each isotopologue. A Kraitchman analysis was performed using the experimental rotational constants to determine the molecular structure of glycolaldehyde.

The performance of a chirped-pulse Fourier transform millimeter-wave spectrometer operating from 260 to 295 GHz is described. The spectrometer uses a high-speed arbitrary waveform generator (AWG) to create both a chirped excitation pulse and the single-frequency local oscillator (LO) used for the final down conversion detection stage. The mm-wave excitation source is an active multiplier chain (factor of 24 frequency multiplication) with power output of greater than 10 mW across the 260–295 GHz frequency range. The LO, produced by a separate active multiplier chain (factor of 12 frequency multiplication), drives a subharmonic mixer which downconverts the molecular emission to the microwave region for digitization on a 100 GS/s digital oscilloscope. All frequency sources in the experiment are locked to a 10 MHz Rb-disciplined oscillator providing direct frequency calibration for molecular transitions in the Fourier transform frequency-domain spectrum. Benchmark measurements are presented on ethyl cyanide and 1-butyne and are used to illustrate advantages and tradeoffs compared with direct absorption millimeter-wave spectroscopy.

The design and performance of a new chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer operating from 25 to 40 GHz (Ka-band) is presented. This spectrometer is well-suited for the study of complex organic molecules of astronomical interest in the size range of 6–10 atoms that have strong rotational transitions in Ka-band under pulsed jet sample conditions (Trot = 1–10 K). The spectrometer permits acquisition of the full spectral band in a single data acquisition event. Sensitivity is enhanced by using two pulsed jet sources and acquiring 10 broadband measurements for each sample injection cycle. The spectrometer performance is benchmarked by measuring the pure rotational spectrum of several isotopologues of acetaldehyde in natural abundance. The rotational spectra of the singly substituted 13C and 18O isotopologues of the two lowest energy conformers of ethyl formate have been analyzed and the resulting substitution structures for these conformers are compared to electronic structure theory calculations.

The lowest energy conformations of valeric acid (VA) and δ-valerolactam (DVL) were determined using chirped-pulsed Fourier transform microwave spectroscopy. DVL was produced by heating 5-aminovaleric acid (AVA) in a metal nozzle. A study of the reaction pathway leading to DVL identified the preferred structure of AVA and demonstrated that an n → π* interaction plays the key role in the transformation of reactant into product. An inverse kinetic isotope effect was detected for this process. Additionally, the spectra of single and double water complexes of DVL along with the 13C and 15N-substituted species (in natural abundance) were collected and analyzed.

The design of a chirped-pulse Fourier transform microwave spectrometer operating in the 2–8 GHz frequency range is presented. The linear frequency sweep is generated by an arbitrary waveform generator with a sampling rate of 20 GS/s. After amplification, the microwave pulse is broadcast into a vacuum chamber where it interacts with a supersonically expanded molecular sample. The resulting molecular free induction decay signal is amplified and digitized directly on a digital oscilloscope with a 20 GS/s sampling rate. No frequency mixing or multiplication is necessary in this spectrometer, which allows for very high pulse quality and phase stability. The performance of this spectrometer is demonstrated on the rotational spectrum of iodobenzene. All four distinct singly-substituted 13 C isotopologues have been detected in natural abundance, as well as two isotopic species of a van der Waals cluster of iodobenzene with a neon atom. Spectroscopic constants and derived structural parameters for iodobenzene and for iodobenzene–Ne are reported. In addition, the use of microwave–microwave double-resonance experiments in this spectrometer to facilitate spectral assignments is presented.

The rotational spectrum of the CHClF 2 –H 2 O weakly bound dimer has been measured using both chirped-pulse and resonant cavity Fourier-transform microwave spectroscopy in the 5–18 GHz range. The structure of the complex has been determined by analysis of the moments of inertia of five isotopologues of the dimer. The primary interaction between the two monomers is a weak C–H⋯O contact ( R H⋯O = 2.332(3) A) with a C–Cl⋯H–O contact also present ( R Cl⋯H = 2.749(13) A). The observed structure is in reasonable agreement with ab initio calculations at the MP2/6-311++G(2d,2p) level, although these predict a Cl⋯H distance that is significantly longer than the experimental results indicate. The rotational transitions of all isotopologues containing H 2 O or D 2 O were doubled, with relative intensities of the observed transitions consistent with an internal rotation of the water molecule leading to exchange of equivalent hydrogen atoms. Fitting the upper and lower components of the transitions using an effective Hamiltonian with the ERHAM program has yielded an energy difference between the tunneling states of 16.0(4) GHz, resulting in an estimate of the barrier to internal rotation of 195(5) cm −1 (to be compared with an ab initio estimate of ∼117 cm −1 ). The binding energy of the complex is estimated to be ∼5.5(2) kJ/mol (∼460 cm −1 ) from a pseudo-diatomic approximation and assumption of a Lennard–Jones intermolecular potential.

S. Kojima et.al., J. Chem. Phys. , 20Q. Williams, J. T. Cox, W. Gordy, J. Chem. Phys. , 20G. Herrmann, J. Chem. Phys. , 22Z. Kisiel, AABS package, available at http://info.ifpan.edu.pl/ H. M. Pickett, SPFIT/SPCAT package, available at http://spec.jpl.nasa.gov.

The microwave spectrum of the sugar alcohol 1,2-propanediol (CH3CHOHCH2OH) has been measured over the frequency range 6.5–25.0 GHz with several pulsed-beam Fourier-transform microwave spectrometers. Seven conformers of 1,2-propanediol have been assigned and ab initio electronic structure calculations of the 10 lowest energy forms have been calculated. Stark effect measurements were carried out on several of the lowest energy conformers to provide accurate determinations of the dipole moment components and assist in conformer assignment.

Author Institution: Department of Chemistry, Kent State University, Kent, OH 44242; Department of Chemistry, University of Virginia, McCormick Road, Charlottesville, VA 22904

A description of Fourier transform microwave (FTMW) detected infrared (IR) spectroscopy is presented. A series of measurements demonstrates the versatility of using a narrowband high Q cavity FTMW spectrometer as a rotationally resolved detector for molecular-beam infrared spectroscopy. The IR-FTMW spectrometer performance is characterized by comparing the spectrum of the acetylenic C–H stretch fundamental of 1-butyne to that obtained using a high-resolution electric resonance optothermal spectrometer (EROS). Two different measurement schemes, corresponding to IR excitation before or after the microwave polarization pulse, are compared. The ability to measure rotationally resolved spectra in the region of the first overtone of the acetylenic C–H stretch is demonstrated through measurements on tertbutylacetylene and trimethylsilylacetylene. One major advantage of the current technique is that large frequency scan rates can be achieved. This feature is useful for molecules exhibiting fast intramolecular vibrational energy redistribution (IVR) as shown by measurements of the highly perturbed acetylenic C–H stretch fundamental region of cyclopropylacetylene. The second strength of the technique is that the high-sensitivity of FTMW spectroscopy makes it possible to obtain the infrared spectra of molecules in low abundance in the pulsed jet expansion. This capability is exploited to measure the infrared spectrum of the 13 C isotopomer of cyclopropylacetylene in natural abundance and to obtain the spectra of the trans and gauche conformational isomers of 4-fluorobut-1-yne. The technique is also well-suited to measuring the vibrational spectra of weakly bound complexes. The spectral simplification achieved by the double-resonance measurement has made it possible to identify weak perturbations in the acetylenic C–H stretch fundamental region of the H–C C–H–NH 3 complex. The acetylenic C–H stretch fundamental spectrum of propyne–ammonia has also been obtained using the technique.

The rotational spectrum of epifluorohydrin measured by chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy is presented. A new CP-FTMW spectrometer capable of measuring the entire 7.5–18.5 GHz spectrum with a single polarizing pulse is described briefly. The CP-FTMW spectrometer takes advantage of recent advances in digital electronics by utilizing a 4.2 GS/s arbitrary waveform generator as a frequency source and a 12 GHz digital oscilloscope to digitize the down converted molecular free induction decay (FID). Signal averaging in the time domain is used to increase the signal-to-noise ratio. The rotational constants of three unique conformers of epifluorohydrin were measured, as well as the rotational constants of the three unique 13 C isotopomers and the 18 O isotopomer (in natural abundance) of the most stable conformer. The rotational constants of the two less stable conformers differ significantly from those previously reported [F.G. Fujiwara, J.L. Painter, H. Kim, J. Mol. Struct. 41 (1977) 169–175]. Ab initio calculations were performed for all three conformations and are compared to experimental values.

The rotational spectra of 3-fluoropropyne in the ground and first excited acetylenic C–H stretch vibrational state have been measured. The pure rotational spectrum of the normal species and the 13C isotopomers were measured using FTMW-cwMW double-resonance spectroscopy based on the Autler–Townes (AC Stark) effect. The lineshape properties of this measurement make it possible to determine the transition strength, ΔJ-selection rules, and the relative energy ordering of the quantum states. The frequency accuracy of this technique is tested against a previous pure rotational study of 3-fluoropropyne. The rotational spectrum of vibrationally excited state was obtained through IR-FTMW-cwMW methods. In this technique a single-longitudinal-mode pulsed infrared laser source vibrationally excites the acetylenic C–H stretch with J-selectivity. The rotational spectrum of the excited state is then obtained by FTMW and FTMW-cwMW double-resonance methods. The excited-state measurements have a signal-to-noise ratio comparable to the pure rotational spectrum. The residuals in the excited-state fit are larger than those obtained in the ground-state fit. This greater deviation from a standard asymmetric top spectrum is most likely due to weak perturbations to the acetylenic C–H spectrum.

Author Institution: National Institute of Standards and Technology, Gaithersburg, MD 20899; University of Rochester, Rochester, NY 14627; Cornell University, Ithaca, NY 14853

The pure rotational spectrum of the less stable conformer of methyl vinyl ether has been measured in an electric-resonance optothermal spectrometer. Analysis of the microwave spectrum shows that the less stable conformer is effectively planar with a low barrier between two gauche forms of the conformer. The rotational constants for the high-energy conformer, obtained from a least-squares fit to a threefold barrier internal rotation Hamiltonian are A = 39 192.490(26) MHz, B = 4527.5496(39) MHz, and C = 4181.2146(43) MHz. The threefold barrier to internal rotation is determined to be V3 = 427.516(71) cm-1. The dipole moment for the conformer has been measured using the Autler-Townes splitting of states in a strong, resonant microwave field. The dipole moment is µtot = 1.9(2) D [µa = 1.3(1) D, µb = 1.4(1) D]. The effectively planar structure of the conformer and the magnitude of the dipole moment are in good agreement with previous ab initio calculations. The experimentally determined threefold barrier to methyl rotation of the present study also agrees well with the value calculated from ab initio methods, 515 cm-1. Copyright 1999 Academic Press.

The eigenstate-resolved, high-resolution (5 MHz) infrared spectrum of the asymmetricdbond;CH2 hydride stretch of isobutene has been measured using an electric resonance optothermal spectrometer (EROS). This vibrational band near 3087 cm-1 was rotationally assigned with ground state microwave-infrared double-resonance spectroscopy. IVR rates from rotationally homogeneous IVR multiplets at values of total angular momentum, J ranging from J = 0 to J = 3 and Ka values of Ka = 0 to Ka = 2, were obtained. The average IVR lifetime for this vibrational mode is 105 ps and independent of rotational state. The experimental state density of the rotationless 000 vibrational state, approximately 150 states/cm-1, is in good agreement with the direct count result of 99 states/cm-1 using the C+3 v multiply sign in circle C-3 v (G36) molecular symmetry group. The lineshape profiles of the IVR multiplets are investigated in order to elucidate information concerning the dependence of IVR rates on the symmetry of the torsional wavefunction. We find that there is a common IVR rate among the various torsional symmetries. IVR lifetimes observed in the present study are compared to other asymmetric ethylenic hydride stretch IVR rates measured in our laboratory. Copyright 1999 Academic Press.

Obtention de la table de caractere pour le groupe de symetrie G 162 et de la table de multiplication pour le groupe