Fourier Transform Microwave Spectrometer Research Articles

Microsolvation of ethyl carbamate conformers: effect of carrier gas on the formation of complexes.

Microsolvated complexes of ethyl carbamate (urethane) with up to three water molecules formed in a supersonic expansion have been characterized by high-resolution microwave spectroscopy. Both chirped-pulse and cavity Fourier transform microwave spectrometers covering the 2-13 GHz frequency range have been used. The structures of the complexes have been characterized and show water molecules closing sequential cycles through hydrogen bonding with the amide group. As is the case in the monomer, the ethyl carbamate-water complexes exhibit a conformational equilibrium between two conformers close in energy. The interconversion barrier between both forms has been studied by analyzing the spectra obtained using different carrier gas in the expansion. Complexation of ethyl carbamate with water molecules does not appear to significantly alter the potential energy function for the interconversion between the two conformations of ethyl carbamate.

The smell of coffee: the carbon atom microwave structure of coffee furanone validated by quantum chemistry

The rotational spectra of coffee furanone (2-methyltetrahydrofuran-3-one) have been measured in the frequency range from 2.0 to 26.5 GHz using a molecular jet Fourier transform microwave spectrometer. Quantum chemical calculations used for the conformational analysis yielded two stable twist conformers, which were described using the Cremer–Pople notation for five-membered rings. The experimental spectrum of the more stable conformer with the methyl group in equatorial position was assigned and fitted using a rigid rotor model with centrifugal distortion corrections. The spectra of all five 13C-isotopologues could be observed in natural abundance. The carbon atom structure was experimentally deduced using Kraitchman’s equations and compared with the structure calculated by quantum chemistry.

14N Nuclear quadrupole coupling and methyl internal rotation in the microwave spectrum of 2-methylpyrrole

Using two molecular jet Fourier transform microwave spectrometers, the rotational spectrum of 2-methylpyrrole was recorded in the frequency range from 2 to 40 GHz. From the torsional splittings due to the internal rotation of the methyl group a barrier height of 279.7183(26) cm−1 was deduced. Because of the 14N nucleus, all lines show a quadrupole hyperfine structure. The microwave spectra were analysed using the XIAM and BELGI-Cs-hyperfine codes. The XIAM code enabled us to reproduce the whole data set with a root-mean-square deviation of 5.6 kHz while the BELGI-Cs-hyperfine code could provide a better root-mean-square almost by a factor of 2 compared to that of XIAM. The experimental results were complemented by quantum chemical calculations. The values of the methyl torsional barrier and the 14N nuclear quadrupole coupling constants are discussed and compared with other methyl substituted pyrroles as well as other aromatic five-membered rings.

Low torsional barrier challenges in the microwave spectrum of 2,4-dimethylanisole.

Low barriers to internal rotations are especially challenging for both the experimental and theoretical determinations because they result in large tunneling splittings which are hard to assign and in potential functions that can be difficult to model. In the present work, the internal rotations of two methyl groups of 2,4-dimethylanisole were analyzed and modeled using a newly developed computer code, called ntop, adapted for fitting the high-resolution torsion-rotation spectra of molecules with two or more methyl rotors. The spectrum was measured using a pulsed molecular jet Fourier transform microwave spectrometer operating in the frequency range of 2.0-26.5 GHz, revealing internal rotation tunneling quintets with splittings of up to several gigahertz. The V3 potential barriers are 441.139(23) cm-1 and 47.649(30) cm-1 for the o- and p-methyl groups, respectively. Quantum chemical calculations predicted only one conformer with the methoxy group in the anti position related to the neighboring o-methyl group. While the results from geometry optimizations were reliable, ab initio calculations at the MP2 level did not reproduce the low torsional barriers, calling for further experiments on related systems and additional theoretical models.

State-to-State Rate Coefficients for NH3-NH3 Collisions from Pump-Probe Chirped Pulse Experiments.

The kinetics of rotational inelastic NH3–NH3 collisions are recorded using pump–probe experiments, carried out with a K-band waveguide chirped pulse Fourier transform microwave spectrometer, in which the population of one inversion doublet is altered by the pump pulse. Due to self-collisions, the resulting deviation from equilibrium propagates to other states and, thus, can be interrogated by probe pulses as a function of the pump–probe delay time. A clear hierarchy of the state-to-state collision processes is found and subsequently translated into propensity rules. State-to-state rate coefficients are estimated, first via an analysis of the kinetics, and then more robustly and accurately derived from the pressure-dependent measurements using a global fitting procedure.

Strong-field coherence breaking as a tool for identifying methyl rotor states in microwave spectra: 2-hexanone.

The rotational spectrum of 2-hexanone was recorded over the 8-18 GHz region using a chirped pulse Fourier transform microwave spectrometer. Strong field coherence breaking (SFCB) was utilized to selectively modulate the intensities of rotational transitions belonging to the two lowest energy conformers of 2-hexanone, aiding the assignment. In addition, the SFCB method was applied for the first time to selectively identify rotational transitions built off the two lowest energy hindered methyl rotor states of each conformer, 0a1 and 1e. Since these two states have rotational energy levels with different nuclear spin symmetries, their intensities could be selectively modulated by the resonant monochromatic pulses used in the SFCB method. The difference spectra, final fit, and structural parameters are discussed for the three assigned conformers of 2-hexanone.

Supersonic jet microwave rotational spectrum of 2,3-difluorophenol

The microwave rotational spectrum of the aromatic 2,3-difluorophenol (2,3-DFP) has been recorded and analyzed in the frequency range of 5–25 GHz using a pulsed-jet Fourier transform microwave spectrometer. Rotational transitions were measured for the parent, the 18O, and all six 13C singly substituted isotopologues in natural abundance as well as the deuterium enriched 2HO-species. In addition to the rotational and quartic centrifugal distortion parameters, the quadrupole coupling constants of the deuterium species were determined. The rotational constants for the parent species are obtained as A = 2313.37516(14) MHz, B = 1797.935693(62) MHz, C = 1011.695171(36) MHz. For the deuterated species, the 2H quadrupole coupling constants are χaa = −0.11693(80) MHz and (χbb−χcc) = 0.4112(14) MHz. The partial rs substitution structure as well as a semi-experimental r0 structure was determined from the rotational constants of the observed isotopologues. Both rs and r0 structures agree well with each other and with MP2/6-311++g(2d, 2p) and B3LYP/6-311++g(2d, 2p) predicted equilibrium structures with the biggest deviation in heavy atom bond lengths being 0.008 Å.

A General Treatment to Study Molecular Complexes Stabilized by Hydrogen‐, Halogen‐, and Carbon‐Bond Networks: Experiment and Theory of (CH2F2)n⋅⋅⋅(H2O)m

Rotational spectra of several difluoromethane-water adducts have been observed using two broadband chirped-pulse Fourier-transform microwave (CP-FTMW) spectrometers. The experimental structures of (CH2 F2 )⋅⋅⋅(H2 O)2 , (CH2 F2 )2 ⋅⋅⋅(H2 O), (CH2 F2 )⋅⋅⋅(H2 O)3 , and (CH2 F2 )2 ⋅⋅⋅(H2 O)2 were unambiguously identified with the aid of 18 isotopic substituted species. A subtle competition between hydrogen, halogen, and carbon bonds is observed and a detailed analysis was performed on the complex network of non-covalent interactions which stabilize each cluster. The study shows that the combination of stabilizing contact networks is able to reinforce the interaction strength through a cooperative effect, which can lead to large stable oligomers.

Conformational Effect on the Large Amplitude Motions of 3,4-Dimethylanisole Explored by Microwave Spectroscopy.

The microwave spectrum of 3,4-dimethylanisole, a molecule containing three methyl groups allowing for internal rotation, was recorded using a pulsed molecular jet Fourier transform microwave spectrometer operating in the frequency range from 2.0 to 26.5 GHz. Quantum chemical calculations yielded two conformers with an anti and a synconfiguration of the methoxy group, both of which were assigned in the experimental spectrum. Torsional splittings due to the internal rotations of two methyl groups attached to the aromatic ring were resolved and analyzed. The rotational-torsional transitions could be reproduced to measurement accuracy, yielding well-determined rotational and internal rotation parameters. The torsional barriers of the methyl groups at the meta and para position were deduced to be 430.00(37) and 467.90(17) cm-1, respectively, for the syn-conformer. The respective values for the anti-conformer are 499.64(26) and 533.54(22) cm-1. A labeling scheme for the G18 group written as the semidirect product ( C3 I ⊗ C3 I) (× C s was introduced.

Rotational Spectrum of Eugenol As Analyzed with Double Resonance and Grid-Based Autofit.

The molecule eugenol was extracted from cloves via steam-distillation, and its rotational spectrum from 3-18 GHz was acquired with a chirped-pulsed Fourier transform microwave spectrometer. This spectrum was analyzed via two separate methods in parallel, one employing several microwave-microwave double resonance measurements and the other using a newly written version of the Autofit program. Both methods yielded rotational constants in excellent agreement with predictions from ab initio calculations. The relative merits of the different analysis methods are discussed.

Rotational spectroscopy of the two higher energy conformers of 2-cyanobutane

We present high-resolution rotational spectroscopy of the two higher energy conformers of 2-cyanobutane (C4H9CN). We analysed spectra starting from 12 to 27 GHz using the Cologne chirped-pulse (CP) Fourier transform microwave spectrometer, which we discuss for the first time. Spectra between 36 and 402 GHz were also recorded by means of frequency modulated (FM) absorption spectroscopy. In addition, quantum chemical calculations were performed assisting the assignments. Furthermore ground state energies of the conformers have been calculated and subsequent experimental analysis of line intensities have been performed. This study provides precise spectroscopic constants, relative energies and vibrational frequencies of the three conformers for the search of 2-cyanobutane in star-forming regions. Additionally, this molecule poses an interesting case to test our knowledge of the abundance of chiral molecules in space.

Fourier Transform Microwave Spectra of the Nitrogen Molecule-Ethylene Sulfide and Nitrogen Molecule-Dimethyl Sulfide Complexes.

We recorded the rotational spectra of N2-ethylene sulfide (ES) and N2-dimethyl sulfide (DMS) including the 15N2 and 15N14N isotopomers in the frequency range of 5-25 GHz by using a Fourier transform microwave spectrometer. The b-type transitions for the ortho and para states of 14N2-ES and 15N2-ES and c-type transitions of 14N2-DMS and 15N2-DMS were observed. The 15N14N-ES and 15N14N-DMS species were found to exist in two isomeric forms: inner (14N15N-ES and 14N15N-DMS) and outer (15N14N-ES and 15N14N-DMS). Neither the -ES nor -DMS complexes showed weak accompanying spectra, which had been observed for N2-ethylene oxide (EO). This is because the potential barriers to internal rotation of ES and DMS are higher than that of EO. The spectra were analyzed by an A-reduced asymmetric-top rotational program with less than 4 kHz standard deviation, except for the 15N14N-DMS and 14N15N-DMS complexes. Rotational, centrifugal distortion, and nuclear electric quadrupole coupling constants were determined by the spectral analysis. The V3 potential barrier to internal rotation of the two equivalent methyl groups of DMS in the ortho and para states of the 15N2-DMS complex was determined to be about 740 cm-1. We performed ab initio calculations in order to complement the information on the intracomplex motions obtained from the experimental spectra.

Microwave spectrum of the complex of 3,3,3-trifluoro-2-(trifluoromethyl)propanoic acid and formic acid

ABSTRACTWe report the first high resolution spectroscopic observation of the complex of 3,3,3-trifluoro-2-(trifluoromethyl)propanoic acid (TTPA) and formic acid. The rotational spectra were measured using a broadband chirped pulse and a narrow band cavity-based Fourier transform microwave spectrometer in the 8–10 GHz range. The conformational landscape of the TTPA-formic acid complex was explored at several levels of theory. The two most stable TTPA-formic acid conformers are of similar stability and feature the usual cyclic carboxylic double hydrogen bonded ring. Based on the broadband spectra obtained, only one stable heterodimer conformer was observed. We explain the absence of the second conformer to be a result of a double hydrogen tunnelling motion which converts the less stable heterodimer to the one observed in the jet expansion. Further CCSD(T) relative energy calculations confirm that the heterodimer conformer detected contains the most stable TTPA monomeric subunit. It is interesting to note that hydrogen bonding with formic acid offers a new path to effectively convert the less stable TTPA subunit to the most stable one in the jet expansion, while both TTPA monomeric conformers were detected in the same experiment.

Fourier-transform microwave spectroscopy of CoNO

The rotational spectra of CoNO were measured using a Fourier-transform microwave (FTMW) spectrometer. The CoNO molecules were generated in supersonic jets using the pulse-discharge reaction of Co(CO)3NO diluted with Ar. The amount of CoNO generated under the present experimental conditions was significantly more than that of CoCO, although the CO contained in a molecule of the precursor Co(CO)3NO is three times more than NO. The quadrupole parameter of the N nucleus was determined for a transition metal mononitrosyl complex for the first time and compared with those of other nitrosyl compounds.

Shape and non-bonding interactions in the formic acid-difluoromethane complex by rotational spectroscopy

The rotational spectrum of the formic acid-difluoromethane complex was measured by using supersonic-jet Fourier transform microwave spectrometer. Experimental results and ab initio calculations support a conformation formed through a relative strong OH⋯F and a bifurcated weak CH2⋯O hydrogen bonds. The distances of the non-bonding interactions were determined to be 1.960(2) Å and 2.797(3) Å for OH⋯F and CH2⋯O, respectively. The dissociation energy of the complex is estimated to be about 1346 cm−1. The bonding nature of the intermolecular interactions was revealed by non-covalent interaction analysis and electron density difference analysis.

Acetyl Methyl Torsion in Pentan-2-one As Observed by Microwave Spectroscopy.

The rotational spectra of two conformers of pentan-2-one (also known as methyl propyl ketone) were recorded in the frequency range from 2 to 40 GHz using two molecular jet Fourier transform microwave spectrometers. Fine splittings due to the internal rotations of the acetyl methyl group CH3CO- and the butyryl methyl group -COCH2CH2CH3 were resolved and analyzed with high accuracy. The torsional barriers of the acetyl and the butyryl methyl groups are 238.14 and 979 cm-1, respectively, for the lowest energy conformer, as well as 188.384 and 1032 cm-1, respectively, for the other one. From the results obtained for the acetyl methyl group, a general rule to predict its torsional barrier in ketones based on the molecular symmetry is proposed.

Conformational dynamics of 1-phenyl-2,2,2-trifluoroethanol by rotational spectroscopy and ab initio calculations

The rotational spectrum of 1-phenyl-2,2,2-trifluoroethanol, a chiral organic alcohol, was investigated using two chirped pulse Fourier transform microwave spectrometers. The molecule is a derivate of 2,2,2-trifluoroethanol, and the phenyl substitution plays a major role in altering the conformational landscape. Three equilibrium minima were identified at the MP2 and B3LYP-D3BJ levels of theory, of which only two exist as stable conformers after applying harmonic zero-point-energy correction. Rotational spectra of the most stable conformer and its eight 13C isotopologues were assigned and the rotational constants obtained were used to refine the ab initio structure. The second conformer was not observed experimentally, and subsequent analysis indicates that the conformational conversion barrier was too low to trap this second conformer, even in a helium jet expansion.

AC Stark Effect Observed in a Microwave-Millimeter/Submillimeter Wave Double-Resonance Experiment.

Microwave-millimeter/submillimeter wave double-resonance spectroscopy has been developed with the use of technology typically employed in chirped pulse Fourier transform microwave spectroscopy and fast-sweep direct absorption (sub)millimeter-wave spectroscopy. This technique offers the high sensitivity provided by millimeter/submillimeter fast-sweep techniques with the rapid data acquisition offered by chirped pulse Fourier transform microwave spectrometers. Rather than detecting the movement of population as is observed in a traditional double-resonance experiment, instead we detected the splitting of spectral lines arising from the AC Stark effect. This new technique will prove invaluable when assigning complicated rotational spectra of complex molecules. The experimental design is presented along with the results from the double-resonance spectra of methanol as a proof-of-concept for this technique.

Accurate rest frequencies for propargylamine in the ground and low-lying vibrational states

Context.To date, several complex organic molecules have been detected in the interstellar medium, and they have been suggested as precursors of biologically important species. Propargylamine (HC ≡C−CH2−NH2) is structurally similar to a number of other organic molecules which have already been identified by radioastronomy, making it a good candidate for astrophysical detection.Aims.This work provides accurate rest frequencies of propargylamine, from the centimeter-wave to the submillimeter-wave region, useful to facilitate the detection of this molecule in the interstellar medium.Methods.An extensive laboratory study of the rotational spectrum of propargylamine has been performed using a pulsed-jet Fourier Transform Microwave (FTMW) spectrometer (7–19 GHz frequency range) and a frequency modulation microwave spectrometer (75–560 GHz). Several hundred rotational transitions of propargylamine were recorded in the ground and three lowest excited vibrational states. The experiments were supported by high-level ab initio computations, mainly employed to characterize the vibrational state structure and to predict spectroscopic parameters unknown prior to this study.Results.The measured transition frequencies yielded accurate rotational constants and the complete sets of quartic and sextic centrifugal distortion constants for propargylamine in its vibrational ground state.14N-nuclear quadrupole coupling constants were also determined. Rotational and quartic centrifugal distortion constants were also obtained for the low-lying excited statesv13= 1 (A′),v20= 1 (A″), andv21= 1 (A″). Thea-type Coriolis resonance which couples thev13= 1 andv21= 1 levels was analyzed.Conclusions.The determined spectroscopic constants allowed for the compilation of a dataset of highly accurate rest frequencies for astrophysical purposes in the millimeter and submillimeter regions with 1σuncertainties that are smaller than 0.050 MHz, corresponding to 0.03 km s−1at 500 GHz in radial equivalent velocity.

The microwave spectrum of phenylpropiolic acid

The microwave spectrum for phenylpropiolic acid was measured in the 4.8–10 GHz frequency range using a Flygare-Balle type pulsed-beam Fourier transform microwave spectrometer. 34 a-type and 11 b-type rotational transitions were measured and assigned for the most abundant isotopologue. Based on the measured transitions, the rotational constants were determined to be A = 3859.823(33) MHz, B = 443.54379(10) MHz, and C = 398.09128(13) MHz. The centrifugal distortion constants were determined to be DJ = 0.00286(66) kHz and DJK = 0.1030(82) kHz. DFT(B3LYP) and MP2 calculations were performed with aug-cc-pVTZ basis set and the calculated rotational constants compare well with experimentally determined values.