We describe an instrument which can be used to analyze complex chemical mixtures at high resolution and high sensitivity. Molecules are collisionally cooled with helium gas at cryogenic temperatures (∼ 4-7 K), and subsequently detected using chirped pulse microwave spectroscopy. Here we demonstrate three significant improvements to the apparatus relative to an earlier version: (1) extension of its operating range by more than a factor of two, from 12 -18 GHz to 12 -26 GHz, which allows a much wider range of species to be characterized; (2) improved detection sensitivity owing to use of cryogenically-cooled lownoise amplifiers and protection switches, and (3) a versatile method of sample input that enables analysis of solids, liquids, gases, and solutions, without the need for chemical separation (as demonstrated with a 12 -16 GHz spectrum of lemon oil). This instrument can record broadband microwave spectra at comparable sensitivity to high Q cavity spectrometers which use pulsed supersonic jets, but up to 3000 times faster with a modest increase in sample consumption rate.

Many achiral molecules can be made chiral by appropriate positioning of an isotope. Accurate detection of this type of chirality has remained elusive, and there is as yet no general method for detection of isotopically chiral species. Here, we present the first application of microwave three-wave mixing to isotopically chiral molecules, detecting enantiomeric excess in (R/S)-benzyl-α-D1 alcohol. Our method is expected to be applicable to a broad range of isotopically chiral molecules with a prochiral parent species.

We present a new algorithm, Robust Automated Assignment of Rigid Rotors (RAARR), for assigning rotational spectra of asymmetric tops. The RAARR algorithm can automatically assign experimental spectra under a broad range of conditions, including spectra comprised of multiple mixture components, in ≲100 s. The RAARR algorithm exploits constraints placed by the conservation of energy to find sets of connected lines in an unassigned spectrum. The highly constrained structure of these sets eliminates all but a handful of plausible assignments for a given set, greatly reducing the number of potential assignments that must be evaluated. We successfully apply our algorithm to automatically assign 15 experimental spectra, including 5 previously unassigned species, without prior estimation of molecular rotational constants. In 9 of the 15 cases, the RAARR algorithm successfully assigns two or more mixture components.

Isoprene (2-methyl-1,3-butadiene) is highly abundant in the atmosphere, second only to methane in hydrocarbon emissions. In contrast to the most stable trans rotamer, structural characterization of gauche-isoprene has proven challenging: it is weakly polar, present at the level of only a few percent at room temperature, and structurally complex due to both torsional and methyl tunneling motions. gauche-Isoprene has been observed by two distinct but complementary experimental approaches: chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy coupled with cryogenic buffer gas cooling, and cavity-enhanced FTMW spectroscopy with a pulsed discharge source. Thermal enhancement of the gauche population (from 1.7% to 10.3%) was observed in the cryogenic buffer gas cell when the sample was preheated from 300 to 450 K, demonstrating that high-energy rotamers can be efficiently isolated under our experimental conditions. Rotational parameters for the inversion states (0+/0–) have been determined for the first time, aided by calculations at increasing levels of theoretical sophistication. From this combined analysis, the inversion splitting ΔE and the F bc Coriolis coupling constant between the two inversion states have been derived.

We demonstrate for the first time high-precision differential microwave spectroscopy, achieving sub-Hz precision by coupling a cryogenic buffer gas cell with a tunable microwave Fabry–Perot cavity. We report statistically limited sub-Hz precision of (0.08 ± 0.72) Hz, observed between enantiopure samples of (R)-1,2-propanediol and (S)-1,2-propanediol at frequencies near 15 GHz. We confirm highly repeatable spectroscopic measurements compared to traditional pulsed-jet methods, opening up new capabilities in probing subtle molecular structural effects at the 10−10 level and providing a platform for exploring sources of systematic error in parity-violation searches. We discuss dominant systematic effects at this level and propose possible extensions of the technique for higher precision.

Straightforward identification of chiral molecules in multi-component mixtures of unknown composition is extremely challenging. Current spectrometric and chromatographic methods cannot unambiguously identify components while the state of the art spectroscopic methods are limited by the difficult and time-consuming task of spectral assignment. Here, we introduce a highly sensitive generalized version of microwave three-wave mixing that uses broad-spectrum fields to detect chiral molecules in enantiomeric excess without any prior chemical knowledge of the sample. This method does not require spectral assignment as a necessary step to extract information out of a spectrum. We demonstrate our method by recording three-wave mixing spectra of multi-component samples that provide direct evidence of enantiomeric excess. Our method opens up new capabilities in ultrasensitive phase-coherent spectroscopic detection that can be applied for chiral detection in real-life mixtures, raw products of chemical reactions and difficult to assign novel exotic species.

The structures of gas-phase noncovalently bound clusters have long been studied in supersonic expansions. This method of study, while providing a wealth of information about the nature of noncovalent bonds, precludes observation of the formation of the cluster, as the clusters form just after the orifice of the pulsed valve. Here, we directly observe formation of ethanol−methanol dimers via microwave spectroscopy in a controlled cryogenic environment. Time profiles of the concentration of reagents in the cell yielded gas-phase reaction rate constants of k Me-g = (2.8 ± 1.4) × 10 −13 cm 3 molecule −1 s −1 and k Me-t = (1.6 ± 0.8) × 10 −13 cm 3 molecule −1 s −1 for the pseudo-secondorder ethanol−methanol dimerization reaction at 8 K. The relaxation cross section between the gauche and trans conformers of ethanol was also measured using the same technique. In addition, thermodynamic relaxation between conformers of ethanol over time allowed for selection of conformer stoichiometry in the ethanol−methanol dimerization reaction, but no change in the ratio of dimer conformers was observed with changing ethanol monomer stoichiometry.

We present a new algorithm, Robust Automated Assignment of Rigid Rotors (RAARR), for assigning rotational spectra of asymmetric tops. The RAARR algorithm can automatically assign experimental spectra under a broad range of conditions, including spectra comprised of multiple mixture components, in 100 seconds. The RAARR algorithm exploits constraints placed by the conservation of energy to find sets of connected lines in an unassigned spectrum. The highly constrained structure of these sets eliminates all but a handful of plausible assignments for a given set, greatly reducing the number of potential assignments that must be evaluated. We successfully apply our algorithm to automatically assign 15 experimental spectra, including 5 previously unassigned species, without prior estimation of molecular rotational constants. In 9 of the 15 cases, the RAARR algorithm successfully assigns two or more mixture components. BackgroundMicrowave spectroscopy provides our most accurate measurements of molecular structures. Typical spectra recorded by modern instruments are comprised of thousands of lines, each corresponding to a specific rotational transition of a specific species. Assigning the correct quantum numbers to lines in an observed spectrum is a prerequisite for extracting meaningful structural information about the molecule. Today, such assignments are typically performed using a combination of quantum chemical simulation, which can in most cases predict rotational constants to within a few percent, and often laborious inspection of the observed spectrum. Assignment of complex spectra remains very much an art, and veteran spectroscopists employ diverse tricks as well as deep intuition to find requisite patterns amid congested spectra [1].Robust automatic fitting of rotational spectra or rovibronic spectra has been a longstanding goal of the spectroscopy community. Attempts at fully automated algorithms include genetic algorithms [2], broad searches combined with quantum chemical calculation [3,4], assignment via nonlinear spectroscopy [5,6], and artificial neural networks [7]. Colin Western's PGOPHER program [8] includes an implementation of the automated fitting routine described in reference [3]. Of particular note are the genetic algorithms demonstrated by Meerts and Schmitt, which have successfully assigned rotationally resolved electronic spectra with no need for prior estimation of molecular constants [9,10]. These algorithms furthermore can be applied to a wide range of candidate Hamiltonians, in contrast to the rigid rotor Hamiltonian assumed in this work.Automated, context-free assignment and structure determination from high-dimensional NMR data is now an integral part of modern NMR analysis [11,12]. In many cases approaches to spectral assignment leverage the ability of modern quantum chemical calculations to predict the structure of a compound, and thus the approximate rotational constants, from the elemental composition and connectivity of the compound, vastly reducing the size of the search space. This...