Single molecule infrared spectroscopy in the gas phase.

Spectroscopy is a key analytical tool that provides valuable insight into molecular structure and is widely used to identify chemical samples. Tagging spectroscopy is a form of action spectroscopy in which the absorption of a single photon by a molecular ion is detected via the loss of a weakly attached, inert 'tag' particle (for example, He, Ne, N). The absorption spectrum is derived from the tag loss rate as a function of incident radiation frequency.

Low-Temperature Gas-Phase Kinetics of Ethanol-Methanol Heterodimer Formation.

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.

Assignment-free chirality detection in unknown samples via microwave three-wave mixing.

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.

High sensitivity microwave spectroscopy in a cryogenic buffer gas cell.

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 the use of cryogenically cooled low-noise 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).

Automated, context-free assignment of asymmetric rotor microwave spectra.

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.

Rotational Characterization of the Elusive gauche-Isoprene.

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.

Enantiomeric Analysis of Chiral Isotopomers via Microwave Three-Wave Mixing.

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-D alcohol.