Molecular Beam Fourier Transform Microwave Research Articles

Internal rotation in halogenated toluenes: Rotational spectrum of 2,5-difluorotoluene

The microwave spectrum of 2,5-difluorotoluene was investigated in a pulsed Molecular Beam Fourier Transform Microwave (MB-FTMW) spectrometer in the frequency region 4.5–24GHz. Rotational lines show large internal rotation splittings indicating that the potential barrier hindering internal rotation of the methyl group is comparatively low. The torsional splittings could be unambiguously identified and the potential barrier hindering internal rotation of the methyl top was determined. Rotational constants for the parent species, together with quartic centrifugal distortion terms were determined for a total of eight isotopologues, comprising the parent and all monosubstituted 13C species. Supporting ab initio (MP2) and density functional (B3LYP) calculations are provided. The experimental potential barrier V3=2580(12)J/mol compares satisfactorily with the ab initio prediction of 2568Jmol−1.

Laboratory microwave, millimeter wave and far-infrared spectra of dimethyl sulfide

Context. Dimethyl sulfide, CH3 SCH3 (DMS), is a nonrigid, sulfur-containing molecule whose astronomical detection is considered to be possible in the interstellar medium. Very accurate spectroscopic constants were obtained by a laboratory analysis of rotational microwave and millimeter wave spectra, as well as rotation-torsional far-infrared (FIR) spectra, which can be used to predict transition frequencies for a detection in interstellar sources.Aims. This work aims at the experimental study and theoretical analysis of the ground torsional state and ground torsional band ν 15 of DMS in a large spectral range for astrophysical use.Methods. The microwave spectrum was measured in the frequency range 2−40 GHz using two Molecular Beam Fourier Transform MicroWave (MB-FTMW) spectrometers in Aachen, Germany. The millimeter spectrum was recorded in the 50−110 GHz range. The FIR spectrum was measured for the first time at high resolution using the FT spectrometer and the newly built cryogenic cell at the French synchrotron SOLEIL.Results. DMS has two equivalent methyl internal rotors with a barrier height of about 730 cm-1 . We performed a fit, using the XIAM and BELGI-Cs -2Tops codes, that contained the new measurements and previous transitions reported in the literature for the ground torsional state ν t = 0 (including the four torsional species AA, AE, EA and EE) and for the ground torsional band ν 15 = 1 ← 0 (including only the AA species). In the microwave region, we analyzed 584 transitions with J ≤ 30 of the ground torsional state ν t = 0 and 18 transitions with J ≤ 5 of the first excited torsional state ν t = 1. In the FIR range, 578 transitions belonging to the torsional band ν 15 = 1 ← 0 with J ≤ 27 were assigned. Totally, 1180 transitions were included in a global fit with 21 accurately determined parameters. These parameters can be used to produce a reliable line-list for an astrophysical detection of DMS.

Unusual internal rotation coupling in the microwave spectrum of pinacolone

The molecular beam Fourier-transform microwave spectrum of pinacolone (methyl tert-butyl ketone) has been measured in several regions between 2 and 40GHz. Fits of the assigned spectrum using several computer programs based on different models for treating torsion–rotation interaction lead to the conclusion that no existing program correctly captures the internal dynamics of this molecule. Quantum chemical calculations at the MP2/6-311++G(d,p) level of theory indicate that this molecule does not have a plane of symmetry at equilibrium, and that internal rotation of the light methyl group induces a large oscillatory motion of the heavy tert-butyl group from one side of the Cs configuration to the other. This effect has been modeled for J=0 levels by a relatively simple two-top torsional Hamiltonian, where the magnitudes of the strong coupling terms between the tops are determined directly from the ab initio two-dimensional potential surface. A plot of the resultant 0A, 0E, 1E, 1A torsional levels on the same scale as a one-dimensional potential curve along the zig-zag path connecting the six (unequally spaced) minima bears a striking resemblance to the 1:2:1 splitting pattern of the A, E, E, B levels of an internal rotation problem with a sixfold barrier. It seems likely that rotational transitions within the 1E and 1A torsional levels are the cause of the roughly 50% of the spectrum that remains unassigned after all predicted transitions within the 0A and 0E torsional levels are removed. However, a much more complete measurement campaign and some new torsion–rotation theory will be needed to verify this hypothesis.

HIGH-RESOLUTION FOURIER-TRANSFORM MICROWAVE SPECTROSCOPY OF METHYL- AND DIMETHYLNAPTHALENES

High-resolution pure rotational spectra of four alkylnaphthalenes were measured in the range of 6–15 GHz using a molecular-beam Fourier-transform microwave spectrometer. Both a- and b-type transitions were observed for 1-methylnaphthalene (1-MN), 1,2-dimethylnaphthalene (1,2-DMN), and 1,3-dimethylnaphthalene (1,3-DMN); only a-type transitions were observed for 2-methylnaphthalene (2-MN). Geometry optimization and vibrational analysis calculations at the B3LYP/6-311++G(d,p) level of theory aided in the assignments of the spectra and the characterization of the structures. Differences between the experimental and predicted rotational constants are small, and they can be attributed in part to low-lying out-of-plane vibrations, which distort the alkylnaphthalenes out of their equilibrium geometries. Splittings of rotational lines due to methyl internal rotation were observed in the spectra of 2-MN, 1,2-DMN, and 1,3-DMN, and allowed for the determination of the barriers to methyl internal rotation, which are compared to values from density functional theory calculations. All four species are moderately polar, so they are candidate species for detection by radio astronomy, by targeting the transition frequencies reported here.

Acetyl Methyl Torsion in N-Ethylacetamide: A Challenge for Microwave Spectroscopy and Quantum Chemistry.

The gas-phase structures and parameters describing acetyl methyl torsion of N-ethylacetamide are determined with high accuracy, using a combination of molecular beam Fourier-transform microwave spectroscopy and quantum chemical calculations. Conformational studies at the MP2 level of theory yield four minima on the energy surface. The most energetically favorable conformer, which possesses C1 symmetry, is assigned. Due to the torsional barrier of 73.4782(1) cm(-1) of the acetyl methyl group, fine splitting up to 4.9 GHz is found in the spectrum. The conformational structure is not only confirmed by the rotational constants, but also by the orientation of the internal rotor. The (14) N quadrupole hyperfine splittings are analyzed and the deduced coupling constants are compared with the calculated values.

The effects of methyl internal rotation and (14)N quadrupole coupling in the microwave spectra of two conformers of N,N-diethylacetamide.

The gas phase structures and internal dynamics of N,N-diethylacetamide were determined with very high accuracy using a combination of molecular beam Fourier-transform microwave spectroscopy and quantum chemical calculations at high levels. Conformational studies yielded five stable conformers with C1 symmetry. The two most energetically favorable conformers, conformer I and II, could be found in the experimental spectrum. For both conformers, quadrupole hyperfine splittings of the (14)N nucleus and torsional fine splittings due to the internal rotation of the acetyl methyl group occurred in the same order of magnitude and were fully assigned. The rotational constants, centrifugal distortion constants as well as the quadrupole coupling constants of the (14)N nucleus were determined and fitted to experimental accuracy. The V3 potentials were found to be 517.04(13) cm(-1) and 619.48(91) cm(-1) for conformer I and II, respectively, and compared to the V3 potentials found in other acetamides. Highly accurate CCSD(T) and DMC calculations were carried out for calculating the barriers to internal rotation in comparison with the experimentally deduced V3 values.

Conformational transformations of sulfur-containing rings: 2-methyltetrahydrothiophene gas-phase structures.

Stable conformations of five-member rings with the prototype cyclopentane are well-known to exist as twist or envelope structures and are of general interest in chemistry. Here, we report on the conformational analysis of the sulfur-containing ring 2-methyltetrahydrothiophene studied by a combination of molecular beam Fourier transform microwave (MB-FTMW) spectroscopy and quantum chemistry. Two twist conformers were observed, whereby highly accurate molecular parameters could be determined. In addition, the (34) S-isotopologue of the most stable conformer was assigned in natural abundances. Geometry optimizations were performed at different levels of theory and the calculated rotational constants were compared with experimental values. Two transition states optimized at the MP2/6-311++G(d,p) level using the Berny algorithm could illustrate the intramolecular conversion between both conformers.

From cats and blackcurrants: structure and dynamics of the sulfur-containing cassis odorant cat ketone.

Sulfur-containing odorants and flavors play an important role in flavor and food industry, especially when meaty, garlic, onion, and vegetable scents are needed. Still, many S-containing flavors also possess fruity scents and may be used in compositions of perfumes that require a fresh and fruity odor perception. They are naturally abundant in various fruits, essential oils, and food. Most of these compounds possess strong scents, and their scent composition is highly dependent on the concentration applied. At higher concentrations, they usually feature repellent off-odors, while their sweet and fruity nature is only experienced at very low concentrations. This represents a challenge for their application in perfumery, as well as in food industry. From a molecular point of view, the endless structural and scent variety of these compounds makes them fascinating, especially as their O-analogs are usually free of any malodors. Here, we report on the investigation of the gas-phase structure and dynamics of the S-containing blackcurrant odorant cat ketone (4-methyl-4-sulfanylpentan-2-one). The work was performed by combining quantum-chemical calculations and molecular-beam Fourier-transform microwave spectroscopy as complementary tools. Using this technique, we are able to determine the structures of sizeable molecules where energy differences are small and conformational distinction is not always possible by quantum-chemical calculations alone. The presented results can be used for further structure-activity correlation studies, as well as for benchmarks to improve theoretical models, especially, as there is still significant interest in characterizing the various conformers of organic molecules in terms of relative energies, structures, and dipole moments.

The Conformation of Pentanoates in the Solid and in the Gas Phase

Suitable derivatives of the four isomeric pentanoates have been structurally characterized in the solid and the gas phase. For the latter, the volatile ethyl esters of valeric, isovaleric, methylbutyric, and pivalic acid were investigated by a combination of molecular beam Fourier transform microwave (MB-FTMW) spectroscopy and theoretical calculations. Crystalline salts rather than esters were formed by reaction between the carboxylic acids and trans-1,2-diaminocyclohexane. For both gaseous and crystalline methylbutyrates, an essentially perpendicular arrangement of carboxylate and methyl group was observed; earlier structure determinations documented in the data base agree with this result. Two competing conformers of favourable energy were relevant for the corresponding isovalerates: They were associated with torsion angles around 20° and 50° between the carboxylate and the alkyl chain. Good agreements in conformation have also been achieved for our experimentally observed unbranched valerate derivatives and fully branched pivalates in solid and gas phase. Despite the apparent simplicity of the pentanoates, the identification of their lowest energy conformers represents a challenge for different methods and levels of theory.

The benzoic acid–water complex: a potential atmospheric nucleation precursor studied using microwave spectroscopy and ab initio calculations

The pure rotational, high-resolution spectrum of the benzoic acid-water complex was measured in the range of 4-14 GHz, using a cavity-based molecular beam Fourier-transform microwave spectrometer. In all, 40 a-type transitions and 2 b-type transitions were measured for benzoic acid-water, and 12 a-type transitions were measured for benzoic acid-D2O. The equilibrium geometry of benzoic acid-water was determined with ab initio calculations, at the B3LYP, M06-2X, and MP2 levels of theory, with the 6-311++G(2df,2pd) basis set. The experimental rotational spectrum is most consistent with the B3LYP-predicted geometry. Narrow splittings were observed in the b-type transitions, and possible tunnelling motions were investigated using the B3LYP/6-311++G(d,p) level of theory. Rotation of the water moiety about the lone electron pair hydrogen-bonded to benzoic acid, across a barrier of 7.0 kJ mol(-1), is the most likely cause for the splitting. Wagging of the unbound hydrogen atom of water is barrier-less, and this large amplitude motion results in the absence of c-type transitions. The interaction and spectroscopic dissociation energies calculated using B3LYP and MP2 are in good agreement, but those calculated using M06-2X indicate excess stabilization, possibly due to dispersive interactions being over-estimated. The equilibrium constant of hydration was calculated by statistical thermodynamics, using ab initio results and the experimental rotational constants. This allowed us to estimate the changes in percentage of hydrated benzoic acid with variations in the altitude, region, and season. Using monitoring data from Calgary, Alberta, and the MP2-predicted dissociation energy, a yearly average of 1% of benzoic acid is expected to be present in the form of benzoic acid-water. However, this percentage depends sensitively on the dissociation energy. For example, when using the M06-2X-predicted dissociation energy, we find it increases to 18%.

Structural studies on banana oil, isoamyl acetate, by means of microwave spectroscopy and quantum chemical calculations

The rotational spectrum of isoamyl acetate, H3C–COO–(CH2)2–CH(CH3)2, has been recorded and assigned using a molecular beam Fourier transform microwave (MB-FTMW) spectrometer in the frequency range of 3–26.5 GHz. One conformer has been observed. By comparing the spectroscopic data with the quantum chemical data, it was found that the conformer observed does not have Cs symmetry. The rotational and centrifugal distortion constants were determined. The barrier to internal rotation of the acetate methyl group was found to be 93.98 cm−1. Due to the high number of the conformers, a systematic nomenclature will be presented.

Evolution of Microwave Spectroscopy at the National Bureau of Standards (NBS) and the National Institute of Standards and Technology (NIST)

This paper describes the beginning and evolution of microwave rotational spectroscopic research starting in 1954 at the National Bureau of Standards (NBS), located at that time in Washington, DC, through the present at NIST in Gaithersburg, MD. David Lide was hired in 1954 to start this research employing Stark modulated waveguide septum cells. When Donald R. Johnson joined the lab in 1968, he developed parallel plate cells coupled with rf and DC discharge methods to study free radicals and transient species. In the mid 1980s Lovas and Suenram constructed a pulsed molecular beam Fourier Transform microwave (FTMW) spectrometer to study hydrogen bonded and van der Waals dimers and trimers. This article describes the types of molecules studied and the type molecular properties derived from these measurements as well as some of the instruments developed for these studies. The two major areas of application described are atmospheric chemistry and molecular radio astronomy.

Effects of Fluorine Substitution on the Shape of Neurotransmitters: the Rotational Spectrum of 2‐(2‐Fluorophenyl)Ethanamine

The effects of fluorination on the conformational landscape of adrenergic neurotransmitters is exemplified trough the conformation analysis of 2-(2-F-phenyl)ethanamine (2FPEA) carried out by microwave spectroscopy and quantum chemical calculations. Five different conformers of the nine possible stable ones for 2FPEA are observed by molecular-beam Fourier-transform microwave spectroscopy. Their unambiguous identification is possible by comparing the experimental rotational constants and the quadrupole coupling constants with those obtained by quantum chemical calculations carried out at the MP2/6-311++G(d,p) level of theory. The relative abundances of the conformers in the jet are estimated from the relative intensities in the observed spectra. A qualitative agreement between experimental and theoretical energies was found, and the remaining deviations are explained by population transfer taking place during the adiabatic expansion. The energy landscape, which also takes the interconversion barriers between the conformers into consideration, is thus characterized completely by the strong interplay of quantum chemical methods and precise experimental data. Significant changes in energy and structure of the 2FPEA conformers are found compared to those obtained for the prototype molecule 2-phenylethanamine (PEA).

The microwave spectrum of neurotransmitter serotonin

A laser ablation device in combination with a molecular beam Fourier-transform microwave spectrometer has allowed the observation of the rotational spectrum of serotonin for the first time. Three conformers of the neurotransmitter have been detected and characterized in the 4-10 GHz frequency range. The complicated hyperfine structure arising from the presence of two (14)N nuclei has been fully resolved for all conformers and used for their identification. Nuclear quadrupole coupling constants of the nitrogen atom of the side chain have been used to determine the orientation of the amino group probing the existence of N-Hπ interactions involving the amino group and the pyrrole unit in the Gauche-Phenyl conformer (GPh) or the phenyl unit in the Gauche-Pyrrole (GPy) ones.

Conformation, structure, quadrupole coupling constants and van der Waals potential energy surface of dichloromethane–Ar

The rotational spectra of the 35Cl 2 and 35Cl 37Cl species of dichloromethane–argon have been investigated by molecular beam Fourier-transform microwave spectroscopy. Information on the configuration, structure and internal dynamics of the complex has been obtained, and the 35Cl and 37Cl quadrupole coupling constants have been determined, based on the experimental data.

Conformational Behavior of Norephedrine, Ephedrine, and Pseudoephedrine

A conclusive identification of the different conformers of the neurotransmitters ephedrine, norephedrine, and pseudoephedrine has been carried out in the gas-phase by molecular beam Fourier transform microwave (MB-FTMW) spectroscopy. Three conformers of norephedrine, three conformers of ephedrine, and four conformers of pseudoephedrine have been unequivocally assigned through the analysis of their rotational spectra and the comparison of the experimental rotational and (14)N quadrupole coupling constants with those calculated ab initio. The conformational preferences have been rationalized in terms of the various intramolecular forces at play. The main stabilizing interaction is an O-H...N hydrogen bond established in the side chain of the neurotransmitters which adopt an extended disposition in their most stable form.

Tailoring the Key in a Molecular Lock-and-Key Model System: The Propylene Oxide···2-Fluoroethanol Complex

The conformational isomerism of the propylene oxide (PO)...2-fluoroethanol (FE) complex has been investigated using molecular beam Fourier-transform microwave spectroscopy complemented with high level ab initio calculations. Rotational transitions of three different binary conformers have been observed experimentally. On the basis of the agreement of the experimental and calculated rotational constants, they could be identified as the three most stable structures, anti G-g+, anti G+g-, and syn G+g-. All the observed structures exhibit a primary O-H...O hydrogen bond, an intramolecular O-H...F hydrogen bond and two secondary intermolecular C-H...F contacts. The two anti conformers, with FE and the PO methyl group on the opposite sides of the oxirane ring, show higher abundances than the syn conformer. In all three observed conformers, FE remains approximately in its favorable compact gauche conformation. The monofluorination of the molecular lock-and-key model system PO...ethanol increases not only the number of possible binary conformers, but also the discrimination energy among them. The superior discrimination ability of FE as compared to ethanol classifies it as a tailored key to the PO lock.

Rotational spectrum, structure and internal dynamics of the pyrrole·ammonia complex

The rotational spectrum of the weakly hydrogen-bound pyrrole·ammonia complex has been observed in the frequency range from approx. 3 to 13 GHz using molecular beam Fourier transform microwave (MB-FTMW) spectroscopy. The assignment of the spectra of two isotopologues (pyrrole· 15NH 3 and pyrrole· 14NH 3) was facilitated by ab initio predictions of the structure of the complex. Tentative assignments from broadband scans were finally confirmed by application of a double resonance technique. A total of 14 and 12 a-type rotational lines for each isotopologue, respectively, were observed, exhibiting hyperfine structures due to 14N nuclear quadrupole coupling. From the spectra, rotational and centrifugal distortion constants as well as 14N nuclear quadrupole coupling constants were derived. The results are discussed in this paper in view of structural parameters and dynamical properties of the complex.

Inversion, internal rotation, and nitrogen nuclear quadrupole coupling of p-toluidine as obtained from microwave spectroscopy and ab initio calculations

A highly accurate combined experimental and theoretical investigation has been conducted on p-toluidine (4-methylaniline) in its vibrational and electronic ground state. Rotational transitions have been recorded using pulsed molecular beam Fourier transform microwave (MB-FTMW) spectroscopy in the frequency range 7–20 GHz. The rotational spectrum of this molecule is complicated by the inversion of the amino group, the internal rotation of the methyl group, and the 14N nuclear quadrupole hyperfine structure. Second-order Møller–Plesset perturbation theory and coupled-cluster singles-and-doubles with a perturbative correction for connected triples have been used in conjunction with high-level basis sets for the quantum chemical investigation. The torsional fine and quadrupole hyperfine structures of the spectra could be interpreted by means of these theoretical data.

Rotational spectrum and structure of 1,2,3,4-tetrafluorobenzene

The rotational spectra of 1,2,3,4-tetrafluorobenzene and its three monosubstituted 13C-isotopic species were recorded in the frequency range from 2.8 to 14 GHz with a molecular beam Fourier-transform microwave (MB-FTMW) spectrometer. The centrifugal distortion analysis of the spectra yielded rotational and fourth-order centrifugal distortion constants. From the principal moments of inertia, partial r s and r m (1) structural parameters were derived. The experimental structures are compared with the results from ab initio calculations.