Cristóbal Pérez, Matt T. Muckle, Daniel P. Zaleski, Nathan A. Seifert, Berhane Temelso, George C. Shields, Zbigniew Kisiel, Brooks H. Pate

Theory predicts the water hexamer to be the smallest water cluster with a three-dimensional hydrogen-bonding network as its minimum energy structure. There are several possible low-energy isomers, and calculations with different methods and basis sets assign them different relative stabilities. Previous experimental work has provided evidence for the cage, book, and cyclic isomers, but no experiment has identified multiple coexisting structures. Here, we report that broadband rotational spectroscopy in a pulsed supersonic expansion unambiguously identifies all three isomers; we determined their oxygen framework structures by means of oxygen-18–substituted water. Relative isomer populations at different expansion conditions establish that the cage isomer is the minimum energy structure. Rotational spectra consistent with predicted heptamer and nonamer structures have also been identified.

Steven T. Shipman, Justin L. Neill, Richard D. Suenram, Matt T. Muckle, and Brooks H. Pate

The rotational spectrum of ethyl 3-methyl-3-phenylglycidate (strawberry aldehyde) has been obtained with chirped-pulse Fourier transform microwave spectroscopy. The sample is a mixture of diastereomers, cis and trans, with different relative stereochemistry around the central epoxide. The spectra of five conformers of this molecule (two of cis and three of trans) have been assigned, and carbon backbone structures for the two most populated conformers (one of cis and one of trans) were determined from all 12 carbon-13 isotopomers in natural abundance using the Kraitchman relations. Comparisons of experimentally determined structural data to ab initio calculations show that the B3LYP density functional fails to account adequately for a long-range dispersive interaction between the phenyl ring and the terminal ethyl group in cis-strawberry aldehyde. However, calculations performed using both MP2 and the M05-2X density functional are able to capture the effects of this interaction on the molecular geometry.

Amanda L. Steber, Justin L. Neill, Daniel P. Zaleski, Brooks H. Pate, Alberto Lesarri, Ryan G. Bird, Vanesa Vaquero-Vara and David W. Pratt

A chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer has been optimized for biomolecular spectroscopy. The sensitivity of this technique makes it possible to perform heavy atom structure determination using the natural abundance of the isotopes. The performance of the spectrometer is illustrated by obtaining the structure of the phenol dimer, a model system that is a challenge for theoretical methods. For application to larger biomolecule systems, it is expected that rotational spectroscopy alone will face challenges in making structural determinations. The scope of problems that can be addressed by rotational spectroscopy can be expanded through double-resonance spectroscopy approaches that provide a “second dimension” of structural information. A general method to implement laser-microwave double resonance spectroscopy is described. We also discuss the potential for developing low-cost microwave detectors for biomolecular spectroscopy that achieve savings by reducing the measurement bandwidth. This approach is particularly promising for developing low-frequency CP-FTMW spectrometers that are well-suited for large molecule rotational spectroscopy.

Alberto Lesarri, Steven T. Shipman , Justin L. Neill, Gordon G. Brown, Richard D. Suenram, Lu Kang, Walther Caminati, and Brooks H. Pate

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 ¹³C 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.

Amanda L. Steber, Brent J. Harris, Justin L. Neill, Brooks H. Pate

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.

Daniel P. Zaleski, Justin L. Neill, Matt T. Muckle, Nathan A. Seifert, P. Brandon Carroll, Susanna L. Widicus Weaver, Brooks H. Pate

The design and performance of a new chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer operating from 25-40 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 (T = 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 carbon-13 and oxygen-18 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.

Justin L. Neill, Steven T. Shipman, Leonardo Alvarez-Valtierra, Alberto Lesarri, Zbigniew Kisiel, Brooks H. Pate

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 carbon-13 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.

Gordon G. Brown, Brian C. Dian, Kevin O. Douglass, Scott M. Geyer, Steven T. Shipman, and Brooks H. Pate

In this spectrometer, a chirped pulse with 12 GHz of bandwidth is directly generated by using a 20 GS/s AWG and upconverted in a single step with an ultrabroadband mixer. The amplified molecular emission is directly detected by using a 50 GS/s digital oscilloscope with 18 GHz bandwidth. In both designs, fast Fourier transform of the FID produces the frequency domain rotational spectrum in the 7–18 GHz range. The performance of the CP-FTMW spectrometer is compared to a Balle–Flygare–type cavity-FTMW spectrometer. The CP-FTMW spectrometer produces an equal sensitivity spectrum with a factor of 40 reduction in measurement time and a reduction in sample consumption by a factor of 20. The CP-FTMW spectrometer also displays good intensity accuracy for both sample number density and rotational transition moment. Strategies to reduce the CP-FTMW measurement time by another factor of 90 while simultaneously reducing the sample consumption by a factor of 30 are demonstrated.

D. P. Zaleski, N. A. Seifert, A. L. Steber, M. T. Muckle, R. A. Loomis, J. F. Corby, O. Martinez Jr., K. N. Crabtree, P. R. Jewell, J. M. Hollis, F. J. Lovas, D. Vasquez, J. Nyiramahirwe, N. Sciortino, K. Johnson, M. C. McCarthy, A. J. Remijan, B. H. Pate

The detection of E-cyanomethanimine (E-HNCHCN) toward Sagittarius B2(N) is made by comparing the publicly available Green Bank Telescope (GBT) PRIMOS survey spectra to laboratory rotational spectra from a reaction product screening experiment. The experiment uses broadband molecular rotational spectroscopy to monitor the reaction products produced in an electric discharge source using a gas mixture of NH3 and CH3CN. Several transition frequency coincidences between the reaction product screening spectra and previously unassigned interstellar rotational transitions in the PRIMOS survey have been assigned to E-cyanomethanimine. A total of eight molecular rotational transitions of this molecule between 9 and 50 GHz are observed with the GBT. E-cyanomethanimine, often called the HCN dimer, is an important molecule in prebiotic chemistry because it is a chemical intermediate in proposed synthetic routes of adenine, one of the two purine nucleobases found in DNA and RNA. New analyses of the rotational spectra of both E-cyanomethanimine and Z-cyanomethanimine that incorporate previous millimeter-wave measurements are also reported.

R. A. Loomis, D. P. Zaleski, N. A. Seifert, A. L. Steber, J. L. Neill, M. T. Muckle, B. J. Harris., J. M. Hollis, P. R. Jewell, V. Lattanzi, F. J. Lovas, O. Martinez Jr., M. C. McCarthy, A. J. Remijan, J. F. Corby, B. H. Pate

We have performed reaction product screening measurements using broadband rotational spectroscopy to identify rotational transition matches between laboratory spectra and the Green Bank Telescope PRIMOS radio astronomy survey spectra in Sagittarius B2 North (Sgr B2(N)). The broadband rotational spectrum of molecules created in an electrical discharge of CH3CN and H2S contained several frequency matches to unidentified features in the PRIMOS survey that did not have molecular assignments based on standard radio astronomy spectral catalogs. Several of these transitions are assigned to the E- and Z-isomers of ethanimine. Global fits of the rotational spectra of these isomers in the range of 8-130 GHz have been performed for both isomers using previously published mm-wave spectroscopy measurements and the microwave measurements of the current study. Possible interstellar chemistry formation routes for E-ethanimine and Z-ethanimine are discussed. The detection of ethanimine is significant because of its possible role in the formation of alanine—one of the twenty amino acids in the genetic code.

Justin L. Neill, Matt T. Muckle, Daniel P. Zaleski, Amanda L. Steber, Brooks H. Pate, Valerio Lattanzi, Silvia Spezzano, Michael C. McCarthy, Anthony J. Remijan

The rotational spectrum of the higher-energy trans conformational isomer of methyl formate has been assigned for the first time using several pulsed-jet Fourier transform microwave spectrometers in the 6-60 GHz frequency range. This species has also been sought toward the Sagittarius B2(N) molecular cloud using the publicly available PRIMOS survey from the Green Bank Telescope. We detect seven absorption features in the survey that coincide with laboratory transitions of trans-methyl formate, from which we derive a column density of 3.1 (+2.6, -1.2) * 10^13 cm-2 and a rotational temperature of 7.6 +/- 1.5 K. This excitation temperature is significantly lower than that of the more stable cis conformer in the same source but is consistent with that of other complex molecular species recently detected in Sgr B2(N). The difference in the rotational temperatures of the two conformers suggests that they have different spatial distributions in this source. As the abundance of trans-methyl formate is far higher than would be expected if the cis and trans conformers are in thermodynamic equilibrium, processes that could preferentially form trans-methyl formate in this region are discussed. We also discuss measurements that could be performed to make this detection more certain. This manuscript demonstrates how publicly available broadband radio astronomical surveys of chemically rich molecular clouds can be used in conjunction with laboratory rotational spectroscopy to search for new molecules in the interstellar medium.

Justin L. Neill, Amanda L. Steber, Matt T. Muckle, Daniel P. Zaleski, Valerio Lattanzi, Silvia Spezzano, Michael C. McCarthy, Anthony J. Remijan, Douglas N. Friedel, Susanna L. Widicus Weaver, and Brooks H. Pate

Methyl formate presents a challenge for the conventional chemical mechanisms assumed to guide interstellar organic chemistry. Previous studies of potential formation pathways for methyl formate in interstellar clouds ruled out gas-phase chemistry as a major production route, and more recent chemical kinetics models indicate that it may form efficiently from radical–radical chemistry on ice surfaces. Yet, recent chemical imaging studies of methyl formate and molecules potentially related to its formation suggest that it may form through previously unexplored gas-phase chemistry. Motivated by these findings, two new gas-phase ion-molecule formation routes are proposed and characterized using electronic structure theory with conformational specificity. The proposed reactions, acid-catalyzed Fisher esterification and methyl cation transfer, both produce the less stable trans-conformational isomer of protonated methyl formate in relatively high abundance under the kinetically controlled conditions relevant to interstellar chemistry. Gas-phase neutral methyl formate can be produced from its protonated counterpart through either a dissociative electron recombination reaction or a proton transfer reaction to a molecule with larger proton affinity. Retention (or partial retention) of the conformation in these neutralization reactions would yield trans-methyl formate in an abundance that exceeds predictions under thermodynamic equilibrium at typical interstellar temperatures of ≤100 K. For this reason, this conformer may prove to be an excellent probe of gas-phase chemistry in interstellar clouds. Motivated by new theoretical predictions, the rotational spectrum of trans-methyl formate has been measured for the first time in the laboratory, and seven lines have now been detected in the interstellar medium using the publicly available PRIMOS survey from the NRAO Green Bank Telescope.