Brian M. Hays

Theoretical Examination of O( 1 D) Insertion Reactions to Form Methanediol, Methoxymethanol, and Aminomethanol

A computational study of O((1)D) insertion reactions with methanol (CH3OH), dimethyl ether (CH3OCH3), and methyl amine (CH3NH2) was performed to guide laboratory investigations of the insertion product molecules methanediol (HOCH2OH), methoxymethanol (CH3OCH2OH), and aminomethanol (HOCH2NH2), respectively. The minimum energy and higher energy conformer geometries of the products were determined at the MP2/aug-cc-pVTZ level of theory, and CCSD(T)/aug-cc-pVTZ calculations were performed on the reactants, products, and transitions states to examine the insertion reaction energetics. Torsional barriers for internal motion in methanediol, methoxymethanol, and aminomethanol were also determined. It was found that O((1)D) insertion into the C-H bond was the most energetically favored reaction pathway, proceeding through a direct and barrierless insertion mechanism. The pathways of O((1)D) insertion into N-H or O-H bonds are also possible, though these reactions are less energetically favored, as they proceed through an association product intermediate before proceeding to the insertion products. Predictions are presented for the pure rotational spectra for the methanediol, methoxymethanol, and aminomethanol products based on the determined molecular parameters. These results provide an excellent starting point to guide laboratory spectral studies of the products.

Multipass millimeter/submillimeter spectrometer to probe dissociative reaction dynamics.

We present here the instrument design and first experimental results from a multipass millimeter/submillimeter spectrometer designed to probe dissociative reaction dynamics. This work focuses on benchmarking the instrument performance through detection of the CH3O and H2CO products from methanol dissociation induced by a high-voltage plasma discharge. Multiple rotational lines from CH3O and H2CO were observed when this plasma discharge was applied to a sample of methanol vapor seeded in an argon supersonic expansion. The rotational temperature of the dissociation products and their abundance with respect to methanol were determined using a Boltzmann analysis. The minimum detectable absorption coefficient for this instrument was determined to be αmin ≤ 5 × 10(-9) cm(-1). We discuss these results in the context of future applications of this instrument to the study of photodissociation branching ratios for small organic molecules that are important in complex interstellar chemistry.

Weakly Bound Clusters in Astrochemistry? Millimeter and Submillimeter Spectroscopy of trans-HO3 and Comparison to Astronomical Observations.

The emergence of chemical complexity during star and planet formation is largely guided by the chemistry of unstable molecules that are reaction intermediates in terrestrial chemistry. Our knowledge of these intermediates is limited by both the lack of laboratory studies and the difficulty in their astronomical detection. In this work, we focus on the weakly bound cluster HO3 as an example of the connection between laboratory spectroscopic study and astronomical observations. Here, we present a fast-sweep spectroscopic technique in the millimeter and submillimeter range to facilitate the laboratory search for trans-HO3 and DO3 transitions in a discharge supersonic jet and report their rotational spectra from 70 to 450 GHz. These new measurements enable full determination of the molecular constants of HO3 and DO3. We also present a preliminary search for trans-HO3 in 32 star-forming regions using this new spectroscopic information. HO3 is not detected, and column density upper limits are reported. This work provides additional benchmark information for computational studies of this intriguing radical, as well as a reliable set of molecular constants for extrapolation of the transition frequencies of HO3 for future astronomical observations.

Fast sweep direct absorption (sub)millimeter-wave spectroscopy

Direct absorption spectroscopy has been the mainstay for spectral acquisition in the millimeter and submillimeter wavelength regimes because of the sensitivity offered by standard hot electron bolometer detectors. However, this approach is limited in its utility because of the slow spectral acquisition speeds. A few rapid acquisition techniques that offer reasonable levels of sensitivity have been developed, but these rely on specialized and costly equipment. We present here a new instrument design for a (sub)millimeter spectrometer that offers both rapid spectral acquisition and highly sensitive detection while using equipment from existing chirped-pulse Fourier transform spectrometers and direct absorption spectrometers. We report on spectrometer design and performance and compare the results to standard lock-in detection techniques.

AC Stark Effect Observed in a Microwave–Millimeter/Submillimeter Wave Double-Resonance Experiment

Microwave-millimeter/submillimeter wave double-resonance spectroscopy has been developed with the use of technology typically employed in chirped pulse Fourier transform microwave spectroscopy and fast-sweep direct absorption (sub)millimeter-wave spectroscopy. This technique offers the high sensitivity provided by millimeter/submillimeter fast-sweep techniques with the rapid data acquisition offered by chirped pulse Fourier transform microwave spectrometers. Rather than detecting the movement of population as is observed in a traditional double-resonance experiment, instead we detected the splitting of spectral lines arising from the AC Stark effect. This new technique will prove invaluable when assigning complicated rotational spectra of complex molecules. The experimental design is presented along with the results from the double-resonance spectra of methanol as a proof-of-concept for this technique.