Chirped-pulse Microwave Spectroscopy Research Articles

High-Resolution Spectroscopy and Structure of Heavy Carbon Subchalcogenides: Tricarbon Selenide, C3Se.

Linear tricarbon selenide, C3Se, has been studied spectroscopically for the first time using a combination of high-resolution infrared and microwave techniques. Probing laser ablation products from carbon-selenium targets in a free jet expansion with He, initial spectroscopic detection was accomplished in the infrared at a wavelength of 5 μm in search of the ν1 vibrational fundamental. Along with the band of the most abundant isotopic species C380Se found centered at about 2039 cm-1, the corresponding bands of the C382Se, C378Se, C376Se, and C377Se isotopologues were also detected. Pure rotational spectra of the five C3Se isotopologues in the 8-18 GHz frequency range were observed in a supersonic jet expansion using chirped-pulse microwave spectroscopy of the discharge products of selenophene, c-C4H4Se. Spectroscopic analyses were guided by results from high-level quantum-chemical calculations carried out at the coupled-cluster level of theory using large correlation-consistent basis sets. Using the experimentally derived ground-state rotational constants of five isotopologues and calculated zero-point vibrational corrections, an accurate semiexperimental equilibrium carbon-selenium bond length is derived.

Product-specific reaction kinetics in continuous uniform supersonic flows probed by chirped-pulse microwave spectroscopy.

Experimental studies of the products of elementary gas-phase chemical reactions occurring at low temperatures (<50K) are very scarce, but of importance for fundamental studies of reaction dynamics, comparisons with high-level quantum dynamical calculations, and, in particular, for providing data for the modeling of cold astrophysical environments, such as dense interstellar clouds, the atmospheres of the outer planets, and cometary comae. This study describes the construction and testing of a new apparatus designed to measure product branching fractions of elementary bimolecular gas-phase reactions at low temperatures. It combines chirped-pulse Fourier transform millimeter wave spectroscopy with continuous uniform supersonic flows and high repetition rate laser photolysis. After a comprehensive description of the apparatus, the experimental procedures and data processing protocols used for signal recovery, the capabilities of the instrument are explored by the study of the photodissociation of acrylonitrile and the detection of two of its photoproducts, HC3N and HCN. A description is then given of a study of the reactions of the CN radical with C2H2 at 30K, detecting the HC3N product, and with C2H6 at 10K, detecting the HCN product. A calibration of these two products is finally attempted using the photodissociation of acrylonitrile as a reference process. The limitations and possible improvements in the instrument are discussed in conclusion.

Hydrogen Bonding in the Dimer and Monohydrate of 2-Adamantanol: A Test Case for Dispersion-Corrected Density Functional Methods.

Weakly-bound intermolecular clusters constitute reductionist physical models for non-covalent interactions. Here we report the observation of the monomer, the dimer and the monohydrate of 2-adamantanol, a secondary alcohol with a bulky ten-carbon aliphatic skeleton. The molecular species were generated in a supersonic jet expansion and characterized using broadband chirped-pulse microwave spectroscopy in the 2–8 GHz frequency region. Two different gauche-gauche O-H···O hydrogen-bonded isomers were observed for the dimer of 2-adamantanol, while a single isomer was observed for the monomer and the monohydrate. The experimental rotational parameters were compared with molecular orbital calculations using density functional theory (B3LYP-D3(BJ), B2PLYP-D3(BJ), CAM-B3LYP-D3(BJ), ωB97XD), additionally providing energetic and electron density characterization. The shallow potential energy surface makes the dimer an interesting case study to benchmark dispersion-corrected computational methods and conformational search procedures.

Detecting combustion intermediates via broadband chirped-pulse microwave spectroscopy

Developing new experimental techniques that allow for species identification and quantification in the high-temperature environment of reacting flows is a continuing challenge in combustion research. Here, we combine broadband chirped-pulse microwave (rotational) spectroscopy with an atmospheric-pressure jet-stirred reactor as a novel method to identify key reactive intermediates in low-temperature and ozone-assisted oxidation processes. In these experiments, the gas sample, after being withdrawn from reactive dimethyl ether/O2/Ar, dimethoxy methane/O2/Ar, and ethylene/O2/O3/Ar mixtures, expands via a supersonic expansion into the high vacuum of a microwave spectrometer, where the rotationally cold ensemble of polar molecules is excited with short MW radiation frequency ramps (chirps). The response of the molecular ensemble is detected in the time domain and after a Fourier transformation, the spectral composition of the transient emission is obtained in the frequency domain. The observed rotational frequencies are uniquely correlated to molecular structures and allow for an unambiguous identification of the sampled species. Detection and identification of intermediates such as formaldehyde, methyl formate, formic acid, formic acid anhydride, and the primary ethylene ozonide via literature-known rotational frequencies are evidence for the superb identification capabilities of broadband chirped-pulse microwave spectroscopy. Strong-field coherence breaking is employed to identify and assign transitions due to a specific component. The observation of van der Waals complexes provides an opportunity to detect combustion intermediates and products that are impossible to detect by rotational spectroscopy as isolated molecules.

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). 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 the sample consumption rate.

Product Branching in the Low Temperature Reaction of CN with Propyne by Chirped-Pulse Microwave Spectroscopy in a Uniform Supersonic Flow.

A new chirped-pulse/uniform flow (CPUF) spectrometer has been developed and used to determine product branching in a multichannel reaction. With this technique, bimolecular reactions can be initiated in a cold, thermalized, high-density molecular flow and a broadband microwave spectrum acquired for all products with rotational transitions within a chosen frequency window. In this work, the CN + CH3CCH reaction was found to yield HCN via a direct H-abstraction reaction, whereas indirect addition/elimination pathways to HCCCN, CH3CCCN, and CH2CCHCN were also probed. From these observations, quantitative branching ratios were established for all products as 12(5)%, 66(4)%, 22(6)%, and 0(8)% into HCN, HCCCN, CH3CCCN, and CH2CCHCN, respectively. The values are consistent with statistical calculations based on new ab initio results at the CBS-QB3 level of theory. This work is a demonstration of CPUF as a powerful technique for quantitatively determining the branching into polyatomic products from a bimolecular reaction.

A chirped-pulse Fourier-transform microwave/pulsed uniform flow spectrometer. I. The low-temperature flow system.

We report the development of a new instrument that combines chirped-pulse microwave spectroscopy with a pulsed uniform supersonic flow. This combination promises a nearly universal detection method that can deliver isomer and conformer specific, quantitative detection and spectroscopic characterization of unstable reaction products and intermediates, product vibrational distributions, and molecular excited states. This first paper in a series of two presents a new pulsed-flow design, at the heart of which is a fast, high-throughput pulsed valve driven by a piezoelectric stack actuator. Uniform flows at temperatures as low as 20 K were readily achieved with only modest pumping requirements, as demonstrated by impact pressure measurements and pure rotational spectroscopy. The proposed technique will be suitable for application in diverse fields including fundamental studies in spectroscopy, kinetics, and reaction dynamics.

Reduced bandwidth chirped-pulse microwave spectroscopy for analysis of weakly bound dimers: Rotational spectrum and structural analysis of CH2ClF⋯FHC[dbnd]CH2

A reduced bandwidth (480MHz) chirped-pulse Fourier-transform microwave spectrometer has been used to record the rotational spectrum of the weakly bound complex between chlorofluoromethane and fluoroethylene (CH2ClF⋯FHCCH2). Spectra of both the 35Cl and 37Cl isotopologues were assigned from a 7 to 18GHz broadband scan. Derived rotational constants (35Cl: A=5602.82(28)MHz, B=918.5894(4)MHz, C=795.1888(4)MHz; 37Cl: A=5453.14(9)MHz, B=913.6372(5)MHz, C=788.4024(4)MHz) are consistent with a Cs symmetry structure in which the CH2ClF is angled toward the fluorine and hydrogen atoms at the end of the fluoroethylene subunit. The CH2ClF hydrogen atoms straddle the fluorine atom on fluoroethylene, while the chlorine atom from CH2ClF points towards the geminal hydrogen atom on fluoroethylene. Dipole moment components of μa=2.342(6)D, μb=0.022(7)D and μtot=2.342(7)D were also determined.

Interplay of Phenol and Isopropyl Isomerism in Propofol from Broadband Chirped-Pulse Microwave Spectroscopy

The conformational equilibrium of the general anesthetic propofol (2,6-diisopropylphenol) has been studied in a supersonic expansion using broadband chirped-pulse microwave spectroscopy. Three conformers originated by the combined internal rotation of the hydroxyl and the two isopropyl groups have been detected in the jet-cooled rotational spectrum. The most stable conformer exhibits tunneling splittings associated with the internal rotation of the hydroxyl group, from which we determined the torsional potential and barrier heights (905-940 cm(-1)). The carbon backbone structure was derived from the spectral assignments of all 12 (13)C monosubtituted isotopologues in natural abundance and confirmed a plane-symmetric gauche orientation of the two isopropyl groups (Gg) for this conformer. In the other two detected conformers (EG and GE) one of the isopropyl groups is eclipsed with respect to the ring plane while the other is gauche, differing in a ∼180° rotation of the hydroxyl group. Supporting ab initio calculations provided information on the potential energy surface and molecular properties of the title compound.