Hydroxymethyl radical (CH₂OH) plays an important role in combustion and environmental chemistry as a reactive intermediate. Reisler's group published¹ the first rotationally resolved spectroscopy of CH₂OH with determined band origins for fundamental CH symmetric stretch state, CH asymmetric stretch state and OH stretch state, respectively. Here CH₂OH was first studied via sub-Doppler infrared spectroscopy in a slit-jet supersonic discharge expansion source. Rotationally resolved direct absorption spectra in the CH symmetric stretching mode were recorded. As a result of the low rotational temperature and sub-Doppler linewidths, the tunneling splittings due to the large amplitude of COH torsion slightly complicate the spectra. Each of the ground vibration state and the CH symmetric stretch state includes two levels. One level, with a 3:1 nuclear spin statistic ratio for K_a=0⁺/K_a=1⁺, is labeled as "+". The other tunneling level, labeled as "-", has K_a=0⁻/K_a=1⁻ states with 1:3 nuclear spin statistics. Except for the K_a=0⁺ ← 0⁺ band published before ², more bands (K_a=1⁺ ← 1⁺ , K_a=0⁻ ← 0⁻ and K_a=1⁻ ← 1⁻) were identified. The assigned transitions were fit to a Watson A-reduced symmetric top Hamiltonian to improve the accuracy of the band origin of CH symmetric state. The rotational parameters for both ground and CH symmetric stretch state were well determined.
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¹L. Feng, J. Wei and H. Reisler, J. Phys. Chem. A, Vol. 108. ²M. A. Roberts, E. N. Sharp-Williams and D. J. Nesbitt, J. Phys. Chem. A 2013, 117, 7042-7049

Halo-substituted methanes constitute a class of molecules that are important in various fields, from spectroscopy to quantum-chemical calculations. They are also gaining interest due to their potential adverse impact on the atmospheric chemistry.¹ In the series of the CH₂IX iodomethanes where X = {F, Cl, Br}, only the rotational spectra of CH₂IF ² and CH₂ICl ³ have been published. We present our investigations on the high-resolution rotational spectroscopy of the two bromine isotopologues of bromoiodomethane, CH₂I⁷⁹Br and CH₂I⁸¹Br. Due to the lack of spectroscopic information available for this compound, high-level quantum-chemical calculations were essential to guide the microwave and millimeterwave spectral assignments of both μ_a- and μ_b-type transitions. They provided rotational and centrifugal distortion constants (quartic and sextic), as well as the quadrupole-coupling tensor of the iodine (I_I = 5/2) and bromine (I_Br = 3/2) nuclei. More than 1900 lines have been analyzed, leading to an accurate determination of molecular constants for both isotopologues. The experimental structure (r₀) of the title species has been derived from the two sets of rotational constants.
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¹S.B. acknowledges support from the Laboratoire d'Excellence CaPPA (Chemical and Physical Properties of the Atmosphere) through contract ANR-10-LABX-005 of the Programme d'Investissement d'Avenir. ²C. Puzzarini, G. Cazzoli, J. C. López, J. L. Alonso, A. Baldacci, A. Baldan, S. Stopkowicz, L. Cheng and J. Gauss, J. Chem. Phy. 62, 174312 (2011). ³S. Bailleux, H. Ozeki, S. Sakai, T. Okabayashi, P. Kania and D. Duflot, J. Mol. Spectrosc. 270, 51 (2011).

We report the rotational and distortion constants of 3 conformers of 2-fluorobenzoic acid, 2 conformers of 3-fluorobenzoic acid and a single conformer of 4-fluorobenzoic acid fitted from the assignment of pure rotational transitions measured in the microwave region from 4-12 GHz. We also recorded the microwave spectrum and assigned the b-dipole transitions of a very large dimer of 3-fluorobenzoic acid which has very small rotational constants. The b-dipole transitions were split and assigned to the two states 0+ and 0- vibrational states with rotational constants: A⁺= 1157.01939( 75) MHz, B⁺ = 95.45061(199) MHz and C⁺ = 88.21514(124) and A⁻= 1157.02249( 46), B⁻ = 95.45110( 74) and C⁻88.24425( 70). This large dimer is a milestone in our groups efforts at "climbing" the ladder to very large dimers that contain dynamics information. Figure

The rotational spectra of H₂-AgCl and H₂-AuCl have been measured using a cavity FTMW spectrometer equipped with a laser ablation source. A combination of isotopic substitution, including HD and D₂ substitutions, and the spin-spin interaction of ortho hydrogen were used to determine the structures of these species. Trends in these structures and the strengths of the H₂ interaction will be discussed. Previous work with hydrogen containing complexes have shown that separate spectra are observed for the both the ortho and para hydrogen species.^1;2;3 In this work, ortho and para hydrogen are assigned together. The a-axis in the present species is coincident with internal rotation axis of hydrogen. This symmetry, along with a covalent interaction of the H₂ with the metal chlorides, allows for a straightforward global assignment of the ortho and para species. The differences in the present study from the previous works will be discussed, as well as the assignment of the combined ortho and para fits.
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¹Y. Zhenhong, K. J. Higgins, W. Klemperer, M. C. McCarthy and P. Thaddeus, J. Chem. Phys., 123(2005) 221106. ²J. M. Michaug, W. C. Topic, W. Jäger, J. Phys. Chem. A., 115(2011) 9456. ³M. Ishiguro, K Harada, K. Tanaka, Y. Sumiyoshi, Y. Endo, Chem. Phys. Lett., 554(2012) 33.

Figure The microwave spectrum of the molecule 3-bromo-1,1,1,2,2-pentafluoropropane has been measured on a newly constructed CP-FTMW spectrometer along with a FTMW spectrometer relocated from Oxford University to Missouri S&T. 3-bromo-1,1,1,2,2-pentafluoropropane has been cited as a possible safer alternative for replacing CFCs as refrigerants and this is the first of a series of studies to understand the chemistry of 3-bromo-1,1,1,2,2-pentafluoropropane with other atmospheric cleaning agents. Rotational constants, centrifugal distortion parameters, and nuclear quadrupole coupling constants will be discussed. The CP-FTMW utilized in this experiment will be discussed in great detail. The new machine has been assembled to directly create and digitize up to 18 GHz signals without the need of mixing on the broadcast or detection side of the experiment allowing for the elimination of many microwave components typically needed in both CP-FTMW and FTMW experiments.

Previous studies of argon complexes with fluoroethylenes have revealed a preference for a geometry that maximizes the contact of the argon atom with heavy atoms on the fluoroethylene.¹ We have observed a continuation of this trend when one of the fluorine atoms is replaced by chlorine. The argon-(E)-1-chloro-1,2-difluoroethylene complex provides two competing heavy atom cavities, FCCF and FCCl, and the opportunity to examine whether the number of heavy atoms or the associated increase in polarizability is determinative of structure. The 5.6 - 18.1 GHz chirped-pulse Fourier transform microwave spectrum of this species provides initial assignments and predictions for spectra obtained in a more sensitive and higher precision Balle-Flygare instrument. Transitions for both the ³⁵Cl and ³⁷Cl isotopologues are observed and analyzed to provide geometric parameters for this non-planar complex. The spectrum is consistent with the argon atom located in the FCCl cavity, and the structure agrees well with ab initio predictions. Comparisons are made with Ar-1-chloro-1-fluoroethylene, (Z)-1-chloro-2-fluoroethylene, and Ar-vinyl chloride.
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¹Z. Kisiel, P.W. Fowler, and A.C. Legon, J. Chem. Phys. 95, 2283 (1991).

In all previous examples of complexes formed between protic acids and haloethylenes, we have observed similar modes of binding regardless of the specific identity of the acid, HF, HCl, or HCCH. Although details of the structures, such as hydrogen bond length and amount of deviation from linearity, do reflect the strength of the interaction and show clear correlations with the gas-phase acidity, the complexes of a given haloethylene with any of the acids have identical structural motifs. Vinyl chloride, on the other hand, has been observed to adopt different modes of binding in its interactions with HF and HCCH. The HF complex, reported two years ago, has a geometry with HF interacting across the double bond of vinyl chloride and forming a secondary interaction with the hydrogen cis to the chlorine atom, but in the complex with acetylene, reported last year, HCCH locates at one end of the vinyl chloride with the secondary interaction occurring with the geminal hydrogen atom. This variety continues and is expanded in the vinyl chloride-HCl complex. Ab initio theory predicts a complex that has the HCl molecule interacting across the double bond, but located out of the vinyl chloride plane. The microwave spectrum of the most abundant isotopologue of this complex is consistent with theoretical predictions and additionally shows the presence of large amplitude motion connecting two equivalent structures.

The interconversion between two equivalent configurations of a system via exchange of protons is an important process with implications in areas ranging from chemical dynamics to molecular biology. In this work, we present microwave spectroscopic results and ab initio calculations on the doubly hydrogen bonded complex HNO₃-HCOOH. Spectra of seven isotopologues of the system confirm a doubly hydrogen bonded geometry, analogous to that of the well studied carboxylic dimers. Unlike most carboxylic acid dimers, however, the two hydrogen bond lengths (and their associated heavy atom distances) are substantially different, causing the double proton transfer process to be accompanied by significant heavy atom motion. Splittings in the a-type rotational spectrum are observed but disappear for HNO₃-HCOOD, indicating a tunneling motion of protons across the hydrogen bonds. Ab initio calculations of the binding energy and intermolecular potential surface are compared with those on the more thoroughly studied carboxylic dimers and the role of heavy atom motion is discussed. Spectroscopic constants provide accurate structural information and quantify the degree to which the electronic structure of the HNO₃ is altered upon complexation.

Organic rings are common building blocks in biologically active molecules. In contrast to six-membered rings, cycloalkanes with seven or more ring atoms have a less rigid structure which allows for conformational diversity and complex internal dynamics. The broadband in-phase/quadrature-phase-modulation passage-acquired-coherence technique (IMPACT) FT-microwave spectroscopy is the ideal experimental method to gain insight into the conformational freedom and structure of these cyclic motifs.
The combination of high spectral resolution ( < 10 kHz) and frequency accuracy with a 1 GHz wideband acquisition range per experimental pulse (over the entire 2 to 26.5 GHz range) makes the IMPACT spectrometer a time-saving tool when it comes to the investigation of fine or hyperfine effects and/or wide tunelling splittings. In this communication we will present the rotational spectra of several organic seven-membered rings, reporting on their conformational preferences and molecular structures.

I have demonstrated that triple resonance performed on a three-rotational-level system of a chiral molecule of C₁ symmetry exhibits signals opposite in phase for different enantiomers, thereby making enantiomer differentiation possible by microwave spectroscopy¹. This prediction was realized by Patterson et al. on 1,2-propanediol and 1,3-butanediol². We thus now add a powerful method: microwave spectroscopy to the study of chiral molecules, for which hitherto only the measurement of optical rotation has been employed. Although microwave spectroscopy is applied to molecules in the gaseous phase, it is unprecedentedly superior to the traditional method: polarimeter in resolution, accuracy, sensitivity, and so on, and I anticipate a new fascinating research area to be opened in the field of molecular chirality. More versatile and efficient systems should be invented and developed for microwave spectroscopy, in order to cope well with new applications expected for this method For C₂ and C_n (n ≥ 3)chiral molecules, the three-rotational-level systems treated above for C₁ molecules are no more available within one vibronic state. It should, however, be pointed out that, if we take into account an excited vibronic state in addition to the ground state, for example, we may encounter many three-level systems. Namely, either one rotational transition in the ground state is combined with two vibronic transitions, or such a rotational transition in an excited state may be connected through two vibronic transitions to a rotational level in the ground state manifold. The racemization obviously plays a crucial role in the study of molecular chirality. However, like many other terms employed in chemistry, this important process has been “defined” only in a vague way, in other words, it includes many kinds of processes, which are not well classified on a molecular basis. I shall mention an attempt to obviate these shortcomings in the definition of racemization and also to clarify the implicit assumptions made in Hund’s paradox³.
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¹E. Hirota, 3rd Molecular Science Symposium, Nagoya, September 2009, E. Hirota, Proc. Jpn. Acad. Ser. B, 88, 120 (2012). ²D. Patterson, M. Schnell and J. M. Doyle, Nature 497, 475 (2013), D. Patterson and J. M. Doyle, Phys. Rev. Lett. 111, 023008 (2013). ³F. Hund, Z. Phys. 43, 805 (1927).

Chirality induction in a model system, i.e. the 2,2,2-trifluoroethanol (TFE)- -propylene oxide (PO) adduct, was investigated in detail using chirped pulse and cavity based Fourier transform microwave spectroscopy, complemented with high level ab initio calculations. Hydrogen-bonding interaction of TFE with the permanently chiral PO molecule results in eight binary TFE- -PO diastereomers. Rotational spectra of four of them were observed experimentally and unambiguously assigned and identified. Unlike the TFE dimer where an extreme case of chirality synchronization was previously reported, diastereomers due to both the g+ and g- forms of TFE were observed, indicating that the tunneling between the two isoenergetic gauche forms of TFE was quenched. Comparison to the previous studies reveals that perfluorination increases the hydrogen-bonding energy by about 70% over its ethanol counterpart. TFE- -PO serves as a prototype system for chirality induction which leads to chirality amplification rather than a system with chirality synchronization.