The pure rotational spectra of the reaction mixture of perfluoropropionic acid, CF₃CF₂COOH, and allyl phenyl ether, C₆H₅OCH₂CH=CH₂, have been studied by a pulsed nozzle, chirped-pulse Fourier transform microwave spectrometer in the frequency range of 8-14 GHz. Transitions corresponding to multiple species, two of which being starting materials allyl phenyl ether and perfluoropropionic acid, have been observed and analyzed. Determination of the reaction products was carried out by matching observed rotational constants with ab initio quantum chemical calculations of predicted products and will be discussed. Rotational constants, centrifugal distortion constants and the assignment of allyl phenyl ether and reaction products spectra will all be discussed.

In nature and as products of chemical syntheses, chiral molecules often exist in mixtures with other chiral molecules. The analysis of these complex mixtures to identify the components, determine which enantiomers are present, and to measure the enantiomeric excesses (ee) is still one of the challenging but very important tasks of analytical chemistry. These analyses are required at every step of modern drug development, from candidate searches to production and regulation. Figure We present here a new method of identifying individual enantiomers in mixtures of chiral molecules in the gas phase.^1,2 It is based on broadband rotational spectroscopy and employs a sum or difference frequency generation three-wave mixing process that involves a closed cycle of three rotational transitions. The phase of the acquired signal bares the signature of the enantiomer (see figure), as it depends upon the combined quantity, μ_aμ_bμ_c, which is of opposite sign between members of an enantiomeric pair. Furthermore, because the signal amplitude is proportional to the ee, this technique allows for both determining which enantiomer is in excess and by how much. The high resolution of our technique allows us to perform molecule specific measurements of mixtures of chiral molecules with μ_aμ_bμ_c ≠ 0, even when the molecules are very similar (e.g. conformational isomers). We introduce the technique and present results on the analysis of mixtures of the terpenes, carvone, menthone, and carvomenthenol.
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¹D. Patterson, M. Schnell, J. M. Doyle, Nature. 497, 475-477, 2013. ²V. A. Shubert, D. Schmitz, D. Patterson, J. M. Doyle, M. Schnell, Ang. Chem. Int. Ed. 53, 1152-1155,2014.

The strucure of cyclopropane cyanosilane has been studied using chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy in the 6.5-18 GHz band. Two conformers of similar intensity were detected, one with a gauche orientation of the cyanosilane group with respect to the plane of the ring, and the other with a staggered conformation. The sensitivity of the CP-FTMW experiment was sufficient enough to assign spectra for all common singly-substituted heavy atom isotopologues (¹³C, ^29/30Si, ¹⁵N) for each conformer, resulting in a full heavy atom Kraitchman structure of the molecule in good agreement with the predicted structure. Additionally, the hyperfine effects have been analyzed for the ¹⁴N-containing parent species. Results will also be presented on the potential tunneling spectrum arising from the symmetric double well torsional potential of the gauche conformer. Some observed transitions, especially with frequencies near the upper end of the measured band, exhibit splittings that could potentially be associated with a tunneling splitting. However, the resolution is not sufficient to provide a complete quantitative analysis of this effect.

The rotational spectrum of 1-hexanal is used as a test case for developing automated assignment algorithms in molecular rotational spectroscopy, for the purpose of lowering the barrier to new users of rotational spectroscopy. There are two ways that the automated fitting algorithm, implemented in the AUTOFIT program, is used: 1) Assignment of the rotational spectrum of a molecule expected to be in the sample mixture (in this case, a conformer of 1-hexanal), using quantum chemistry estimates of the spectroscopic parameters to efficiently guide the search for the experimental spectrum. 2) Once a new spectrum is assigned, the algorithm is used to automatically assign isotopologue spectra (sensitivity permitting) to provide verification of the molecular structure. Using a combination of quantum chemical calculations and automated spectral assignment, 12 conformations of 1-hexanal have been identified using chirped-pulse Fourier transform (CP-FTMW) spectroscopy in the 6.5-18, 18-26 and 26-40 GHz bands. Of these 12 conformers, the four lowest energy conformers were intense enough to resolve each of the six ¹³C isotopologues for each conformer, and sufficient intensity was achieved to assign the ¹⁸O isotopologues of the two lowest energy conformers. The full set of assignments were made using the AUTOFIT program, and a summary of results for all 38 observed species via automated assignment will be presented. Additionally, by using all 12 conformers of 1-hexanal as a benchmark set, a discussion of dispersion-corrected density functional theory for the purpose of automated broadband spectroscopic searches will be presented, with specific results regarding rotational constant prediction. Results will also be presented on the correlation between the predicted conformational energetics predicted by multiple levels of theory and the sensitivity limits of the 1-hexanal CP-FTMW spectrum.

Figure The broadband rotational spectrum of ibuprofen ((RS)-2-(4-isobutylphenyl)-propanoic acid), a well-known drug, will be presented. As it is used to relieve pain, reduce fever, and inhibit inflammation, the knowledge of its biological activity is very interesting. Insights to the conformational flexibility of this drug might lead to a better understanding of the class of non-steroidal anti-inflammatory drugs that ibuprofen belongs to.
The spectrum was recorded with our broadband chirped-pulse Fourier transform microwave spectrometer in the frequency range of 2.0 - 8.3 GHz. With the obtained results, we are able to identify several conformers of ibuprofen and to determine their rotational constants. Density functional theory calculations were performed and used to support the conformational assignments. Fragments of ibuprofen could be also identified in the spectrum, which can be explained by thermal decomposition during the heating process for vaporizing it. The analysis of this fragmentation process as a function of temperature might provide us with some interesting insights into its mechanism.

The rotational spectra of several different amino alcohols including D/L-allo-threoninol, 2-amino-1,3-propanediol and 1,3-diamino-2-propanol over the 6.5-18.5 GHz range have been investigated under jet-cooled conditions using chirped-pulsed Fourier transform microwave spectroscopy. Despite the small size of these molecules, a great variety of conformations have been observed in the molecular expansion. While the NH₂ group is typically thought of as a H-bond acceptor, it often acts both as acceptor and donor in forming H-bonded networks. With three adjacent H-bonding substituents (a combination of OH and NH₂ groups), many different hydrogen bonding patterns are possible, including H-bonded chains and H-bonded cycles. Since many of these structures differ primarily by the relative orientation of the H-atoms, the analysis of these rotational spectra are challenging. Only through an exhaustive conformational search and the comparison with the experimental rotational constants, nuclear quadrupolar splittings, and line strengths are we able to understand the complex nature of these interactions. The ways in which the presence and number of NH₂ groups affects the relative energies, and distorts the structures will be explored.

The weakly bound complexes between 1-fluoroethylene (FE), 1,1-difluoroethylene (DFE), and 1,1,2-trifluoroethylene (TFE) and carbon dioxide have been investigated using reduced bandwidth chirped-pulse (CP) and resonant-cavity Fourier-transform microwave (FTMW) spectroscopy. In FE…CO₂, two distinct planar isomers are observed, corresponding to the CO₂ interacting with the CHF end of the FE (side-binding) or roughly parallel to the C=C bond (top-binding). Both structures contain a C-H…O contact between one FE hydrogen atom and CO₂. In DFE…CO₂, only a top-binding configuration is possible, consistent with the observed structure. Finally, although both top- and side-binding orientations are possible for TFE…CO₂, only the side-binding conformation has been observed. The C-H…O distances in the four species vary from 2.58 Å to 2.73 Å, while the observed F…C distances are much more consistent, varying by only about 0.05 Å across the series. Ab initio calculations at the MP2/6-311++G(2d,2p) level have provided exceptionally accurate estimates of the rotational constants of these CO₂ complexes, although the energy ordering is, in several cases, inconsistent with the observed geometries.

The rotational spectrum of the CH…π bonded complex between benzene and acetylene has been measured in the 6−20 GHz range using chirped-pulse Fourier-transform microwave spectroscopy. The spectra for the normal isotopologue, three unique ¹³C substituted species, and the d₁-benzene…HCCH species have allowed determination of the dimer structure. The spectrum is that of a symmetric top, with effective C_6v symmetry, and a CH…π distance of 2.4921(1) Å. The dipole moment has been measured using the Stark effect, and is 0.438(11) D. In addition to the ground state spectrum, three additional sets of transitions corresponding to similar rotational constants have been observed, likely due to excitation of the three low energy intermolecular vibrational modes of the dimer. Analysis of these excited state transitions is in progress. Comparison of the binding energy and structure of the benzene…HCCH dimer with other H…π complexes will be presented.

Last year, at the 68th International Symposium of Molecular Spectroscopy, the rotational spectrum of a complex formed between C₂H₂ and AgCCH was presented. The geometry was found to be T-shaped with the silver atom coordinated to the center of the CC bond in acetylene. Evidence for a new complex formed between AgCCH and C₆H₆ is now presented in the form of deep-averaged broadband rotational spectra. The spectra are observed only when both C₂H₂ and C₆H₆ are present in the gas sample. The relative intensities of the observed spectra are consistent with the naturally-occurring abundance ratio of the isotopes of silver. The shift on substitution of ¹⁰⁷Ag for ¹⁰⁹Ag implies a silver atom positioned close to the center of mass. The isotopic shifts observed when C₂D₂ is used as a precursor instead of C₂H₂ are also consistent with assignment to a complex formed between C₆H₆ and AgCCH/D. The geometry of the complex is yet to be precisely established.

A chirped-pulse Fourier transform microwave spectrometer has been used to measure the microwave spectrum of CF₃I …PH₃ between 6.5 and 18.5 GHz. The complex was stabilised by supersonic expansion of a gas sample containing small percentages of CF₃I and PH₃ in a balance of 6 bar of argon. The observed spectrum is consistent with a C_3v prolate symmetric top structure which displays evidence for internal rotation of the PH₃ subunit about the principal axis. Over two hundred hyperfine transitions across eleven J"← J′ transitions have been assigned to the internal rotor A-state allowing the rotational (B₀) and centrifugal distortion (D_J and D_JK) constants as well as the nuclear electric quadrupole coupling constant of iodine (χ_aa(I)) to be assigned for this state. For the E-state, the additional distortion terms D_Jm and D_JKm have been determined. The length of the halogen bond between the iodine and phosphorus atoms and the force constant of this bond have also been determined.

A chirped-pulse Fourier transform microwave spectrometer has been used to measure the microwave spectra of both C₂H₂…AuI and C₂H₄…AuI. These complexes are generated via laser ablation at 532 nm of a gold surface in the presence of CF₃I and either C₂H₂ or C₂H₄ and argon and are stabilized by a supersonic expansion. Rotational (A₀, B₀, C₀) and centrifugal distortion constants (∆_J, ∆_JK and δ_J) of each molecule have been determined as well the nuclear electric quadrupole coupling constants of gold and iodine atoms (χ_aa(Au), χ_bb−χ_cc(Au), χ_aa(I) and χ_bb−χ_cc(I)). The spectrum of each molecule is consistent with a C_2v structure in which the metal atom interacts with the π-orbital of the ethene or ethyne molecule. Isotopic substitutions of atoms within the C₂H₂ or C₂H₄ subunits are in progress and in conjunction with high level ab initio calculations will allow for accurate determination of the geometry of each molecule.

The water hexamer is the first cluster size where three dimensional structures become the most stable. For cluster sizes up to the water decamer, the hexamer is unique because there are three low-lying isomers with distinct oxygen framework geometries that can be observed in pulsed molecular beams: the prism, cage, and book. Each structure has a different number of hydrogen bonds (prism: 9, cage: 8, book: 7). The prediction of the lowest energy form by different water potentials has become a benchmark in computational studies of water clusters. The experimental determination that the cage is the lowest energy form was presented by our group in 2012 using the population changes that occur when different carrier gases are used in the molecular beam expansion. Recently, it has been proposed by Babin and Paesani that the relative energy ordering of these isomers might be useful for testing the ability of theory to include zero-point energy effects.¹ Their calculations suggested that the prism might become the lowest energy isomer in the fully deuterated water hexamer. At the simplest level, this can explained by the fact that the prism has the most hydrogen bonds and would, therefore, experience the most energy lowering upon isotopic substitution. Broadband rotational spectroscopy in a pulsed supersonic expansion has been used to study the cage, prism and book isomers of deuterated water hexamer. These data, in conjunction with new computational advances, quantify the changes in the oxygen framework structure respect to the normal water hexamers. Moreover, by using different gases in the expansion we have established that the prism isomer becomes the minimum energy structure as suggested by Babin and Paesani.
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¹V. Babin and F. Paesani, “The curious case of the water hexamer: Cage vs. Prism”, Chem. Phys. Lett. 580, 1-8 (2013).