Assignment of the perfluoropropionic acid-formic acid complex and the difficulties of including high Ka transitions.

We recently began an investigation into the perfluoropropionic acid$\cdots$formic acid complex using broadband microwave spectroscopy. This study aims to examine the possible double proton transfer between the two interacting carboxcyclic acid groups. The spectrum presented as a doubled set of lines, with spacing between transitions of $<\ 1$\ MHz. Transitions appeared to be $a$-type, R branch transitions for an asymmetric top. Assignment of all $K_a=1,0$ transitions yields decent fits to a standard rotational Hamiltonian. Treatment of the doubling as either a two state system (presumably with a double proton transfer) or as two distinct, but nearly identical conformations of the complex produce fits of similar quality. Including higher $K_a$ transitions for the $a$-type, R-branch lines greatly increases the error of these fits. A previous study involving the trifluoroacetic acid$\cdots$formic acid complex published observed similar high $K_a$ transitions, but did not include them in the published fit.\footnote{Martinache, L.Kresa, W.Wegener, M., Vonmont, U.and Bauder, A. \textit{Chem. Phys.} \textbf{148} (1990) 129-140.} We hope to shed more light on this conundrum. Similarities to other double-well potential minimum systems will be discussed

HIGH RESOLUTION MICROWAVE SPECTROSCOPY AND STRUCTURE OF THE WEAKLY BOUND Xe…OCS COMPLEX

The rotational spectrum of the weakly bound complex between xenon and carbonyl sulfide has been measured using a coaxially oriented beam resonator arrangement (COBRA) Fourier transform microwave spectrometer in Hannover. There are nine naturally occurring isotopes of xenon in addition to the many possible isotopologues of carbonyl sulfide, which allows for a detailed analysis of the structure of the van der Waals complex and subsequent comparisons to other rare gas van der Waals complexes with carbonyl sulfide that have already been reported. Of the nine isotopes of xenon, two have non-zero nuclear spins, \chem{^{131}Xe} (I=3/2) and \chem{^{129}Xe} (I=1/2). In the case of the \chem{^{131}Xe}, a hyperfine structure was observed in all transitions, revealing a non-zero field gradient at the xenon nucleus in the complex. High-level ab initio calculations were carried out to identify an accurate method for the prediction of the structure of the complex and the nuclear quadrupole coupling constants of \chem{^{131}Xe}. The van der Waals interaction energies of xenon and carbonyl sulfide will be discussed

EXPLORING THE BINDING OF METHANOL WITH FURANS

Recently, an infrared study on the complex of dimethylfuran and methanol showed only a slight preference for methanol to bind via an OH$\cdots$O hydrogen bond versus the OH$\cdots \pi$ bond.\footnote{A. Poblotzki et al, Phys. Chem. Chem. Phys. 2016, 18, 27265.} From this work, a ‘blind challenge’ was undertaken to compare the quantum chemical and experimental results of the microsolvation of a furan ring in methanol, with varying degrees of methyl substitution on furan, having a focus on methanol binding sites.\footnote{H. C. Gottschalk et al, J. Chem. Phys. 2018, 148, 014301.} The hope of the challenge is to gauge how well experiment and theory can agree on site preference when the energy difference is near $1$ kJ/mol, and ultimately to discern which theoretical models perform the best. To this end, we present the rotational spectrum and structural analysis of the dimethylfuran complex with methanol. The rotational spectrum was recorded on the Hamburg COMPACT (compact-passage acquired coherence technique) microwave spectrometer from $2-18$ GHz.\footnote{D. Schmitz et al, J. Mol. Spectrosc. 2012, 280, 77.} The observed spectrum is complicated by the internal rotation of three methyl tops, two from the dimethylfuran and one from the methanol. Each methyl top adds additional torsional angular momentum to the rotating complex, which subsequently splits the rotational energy levels yielding a complex set of torsion-rotation transitions. The resulting methyl top tunneling parameters are essential to aid in the structure determination of the complex. These results will be compared with the furan methanol results to see the effect of methylation on site preference

DO YOU EVEN BENCH, BRO?

In a recent publication by Kraus and Frank,\footnote{\textit{J. Phys. Chem. A} 2018, 122, 21, 4894-4901} 89 computational methods were used to predict the gas-phase geometries of 16 dimers and trimers whose structures we determined using rotational spectroscopy of isotopically substituted species. The experimental data set, called NCDT16, sampled various non-covalent interaction of four categories: dispersion dominated interactions, hydrogen bonded dimers, CH$\cdots\pi$ \& $\pi\cdots\pi$ interactions, and a general category for trimers. For each method, a least-squares structure fit of experimental rotational constants was carried out using the respective predicted geometry as input, and the mean absolute errors were used to gauge the value of the method. The top performing methods of each category, as well as an overall category, are prescribed by the authors. This represents one of the few benchmarking studies of non-covalent interactions against experimental structures in the gas-phase. From the prescribed methods, we test the quality of the benchmarking study. With a focus specifically on CH$\cdots\pi$ interactions, we compare 7 complexes containing a halogenated methane and either acetylene or propyne (a C$\equiv$C bond). All compared complexes have structures determined by rotational spectroscopy. Our computational and fitting methods are outlined, and we discuss the strengths and weaknesses of such a study, with an emphasis on the benefits to spectroscopic research

HYDROGEN BONDING IN 4-AMINOPHENYL ETHANOL: A COMBINED IR-UV DOUBLE RESONANCE AND MICROWAVE STUDY

begin{wrapfigure}{r}{0pt} includegraphics[scale=1.0]{4ae.eps} end{wrapfigure} Both amine and hydroxyl functional groups are present in 4-aminophenyl ethanol (4-AE), and each functional group can form hydrogen bonds with carboxylic acids, such as formic acid and acetic acid. Predicting the structures of such complexes involving 4-AE is rather complex, given the many possible conformations and their similar (and method and basis-dependent) energies. In particular, the carboxyl group, -COOH, can act as both as a hydrogen bond donor or acceptor, or both at once. linebreak linebreak In this study we report the formic acid chem{-} 4-AE hydrogen bonded complex. An infrared-ultraviolet double resonance spectrometer is used to examine the shifts in IR frequencies of 4-AE from the monomer to the complex. Fourier transform microwave spectroscopy is used to determine structures of the species. Results from both experiments are compared to DFT and textit{ab initio} results. Time permitting, results of the water complex with 4-AE will also be presented

THE INFLUENCE OF FLUORINATION ON STRUCTURE OF THE TRIFLUOROACETONITRILE WATER COMPLEX

Acetonitrile, chem{CH_3CN}, and trifluoroacetonitrile, chem{CF_3CN}, are symmetric tops. In a recent study of the rotational spectrum of the acetonitrile and water complex, it was observed that the structure was also an effective symmetric topfootnote{Lovas, F.J.; Sobhanadri, J. Microwave rotational spectral study of chem{CH_3CN-H_2O} and chem{Ar-CH_3CN}. textit{J. Mol. Spetrosc.} textbf{2015}, textit{307}, 59-64.}, with the external hydrogen freely rotating about the chem{O-H} bond aligned towards the nitrogen of the cyanide of chem{CH_3CN}. Unlike the chem{CH_3CN-H_2O} complex, the chem{CH_3CN-Ar} and chem{CF_3CN-Ar} complexes were observed to be asymmetric tops. Having a series of symmetric and asymmetric top complexes of acetonitrile and trifluoracetonitrile for comparison, we report the rotational spectrum of the weakly bound complex between trifluoroacetonitrile and water. Rotational constants and quadrupole coupling constants will be presented, and the structure of chem{CF_3CN-H_2O} will be revealed. linebreak linebreak SPOILER ALERT: It's an asymmetric top

NON-COVALENT INTERACTIONS FROM THE POINT OF VIEW OF QUADRUPOLAR NUCLEUS: THE ENERGETICALLY SIMILAR STRUCTURES OF BROMOBENZENE…STYRENE OXIDE

Rotational spectroscopy is often used to determine the gas-phase structures of weakly bond complexes. When combined with the power of quantum chemical calculations, relative stabilities of various conformations can be confirmed. Occasionally, complexes with more than one structure are observed, and experimentally the relative intensities of the rotational transitions lead to a nearly 50/50 estimation of the conformational population in the measurement. If it happens that the relative energies predicted computationally are nearly identical for two conformers, the question of which structure is favored is left unanswered. To this end, we present a rotational spectroscopic study on two conformations of a bromobenzene-styrene oxide complex that are nearly equivalent in energy, but structurally different. In the 2-8 GHz region, the observed nuclear hyperfine structure from the $^{79}$Br and $^{81}$Br nuclei (I=3/2) are used to fit the nuclear quadrupole couple tensor. For each bromine isotope, the diagonalized nuclear quardrupole tensor is used to confirm the favored structure. Results are presented along side symmetry-adapted perturbation theory (SAPT) calculations of the interaction energies. A brief comparison to similar halogenated analogs is also presented. %\begin{wrapfigure} %\includegraphics[scale=0.3]{two_complex_figure.eps} %\end{wrapfigure

THE POSITION OF DEUTERIUM IN THE HOD_N2O AS DETERMINED BY STRUCTURAL AND NUCLEAR QUADRUPOLE COUPLING CONSTANTS

A recent investigation of the chem{HOD-N_2O} complex measuring the OH + OD excited band in the near-IR was completed by Foldes textit{et al}.footnote{F\"{o}ldes, T; Lauzin, C.; Vanfleteren, T.; Herman, M.; Li`{e}vin, J.; Didriche. K. High-resolution, near-infrared CW-CRDS and ab initio investigations of chem{N_2O-HDO}.textit{Mol. Phys.} textbf{2015}, textit{113(5)},473-482.} During this study, one of us (WAK) was contacted about the position of deuterium in the chem{HOD-N_2O} complex, as his group completed the original microwave study of chem{H_2O-N_2O} and its deuterated isotopologuesfootnote{Zolandz, D.; Yaron, D.; Peterson, K.I.; Klemperer, W. Water in weak interactions: The structure of the water-nitrous oxide complex. textit{J. Chem. Phys.} textbf{1992}, textit{97},2861.} in 1992. The results of this microwave study did not give the orientation of chem{HOD} in the complex, however, we present here a supplementary study to the original microwave work using a Balle-Flygare cavity instrument, attempting to determine the orientation of chem{HOD} relative to the chem{N_2O}. In addition to a Kraitchman and a least-squares inertial structure fit of the molecule, we present the nuclear quadrupole coupling tensor of deuterium to determine the position of chem{HOD} in the complex

FOURIER TRANSFORM MICROWAVE SPECTROSCOPIC STUDIES OF DIMETHYL ETHER AND ETHYLENE FLAMES

Microwave spectroscopy has been a proven technique for the detection of short-lived molecules produced from a variety of molecular sources. With the goal of observing more reactive intermediates produced in combustion reactions, the products of a home-built flat flame burner were measured on a coaxially oriented beam resonator arrangement (COBRA) Fourier transform microwave spectrometer.footnote{J.-U. Grabow, W. Stahl, H. Dreizler, Rev. Sci. Instrum. 67, 4072, 1996} The products are coupled into a molecular beam using a fast-mixing nozzle styled after the work of Gutowsky and co-workers.footnote{T. Emilsson, T. D. Klots, R. S. Ruoff, H.S. Gutowsky, J. Chem. Phys. 93, 6971, 1990}_x000d_ _x000d_ Probing the flame at various positions, the relative abundance of products can be observed as a function of flame depth. One dimensional intensity profiles are available for formaldehyde, ketene, acetaldehyde, and dimethyl ether, where either a dimethyl ether fuel or an ethylene fuel was burned in the presence of oxygen. The current arrangement allows only for stable species produced in the flame to be observed in the molecular beam. This combination of species source and detection shows promise for future work in observing new, short-lived, combustion intermediates._x000d_ _x000d

Internal dynamics in the molecular complex of CF3CN and H2O

The rotational spectrum of trifluoroacetonitrile–water complex has been studied by pulsed-nozzle, Fourier transform microwave spectroscopy. Both a-type and b-type transitions have been observed. The rotational constants, centrifugal distortion constants, and the 14N nuclear quadrupole coupling constants have been determined. The complex is T-shaped, with the oxygen atom from the water located 3.135 Å from the carbon atom of CF3 of the trifluoroacetonitrile molecule