Fourier Transform Microwave Spectra Research Articles

Anions in Space

ASTROCHEMISTRY Although more than 100 neutral molecules and 14 molecular cations have been identified in space, polyatomic anions have eluded detection. Most detection efforts have focused on small anions for which well-resolved spectra have been measured in the laboratory. McCarthy et al. report rotational spectra for the comparatively large linear triynyl anion C6H−. In the millimeter band, absorption spectra were acquired from samples generated by dc discharge of acetylene at low pressures. In the centimeter band, Fourier-transform microwave spectra were obtained using a molecular-beam source. Spectral shifts for the deuterated isotopomer helped to confirm the assignment. These spectra proved to be an excellent match to a harmonic series observed more than a decade ago in the infrared carbon star IRC +10216 in Leo, as well as to features observed in the Taurus molecular cloud TMC-1. The large size of this anion likely helps it to retain its excess electron despite high fluxes of ionizing ultraviolet radiation in space. — PDS Astrophys. J. 652 , L141 (2006).

Tunneling motions of argon on chlorofluoromethane

The rotational supersonic jet Fourier transform microwave spectra of the 35Cl and 37Cl species of the molecular complex chlorofluoromethane-argon show that, in its equilibrium conformation, the argon atom is located out of the ClCF plane, interacting with the F and Cl atoms. All rotational transitions are split into several quadrupole components, each of them further split into two lines, due to the tunneling motion of the Ar atom between two equivalent positions, below and above the ClCF plane. The feasible low energy pathway between the structurally degenerate conformations is described, in a first approximation, by a circular motion around the C-Cl bond, with barriers estimated to be about 61 and 100 cm(-1).

Internal motions in a complex consisting of a rare gas atom and a C2v molecule: Theoretical formulations and their applications to Fourier transform microwave spectra of Ne-dimethyl ether and Ar-dimethyl ether

The internal motion of the rare gas atom, i.e., the relative motion of the two constituents, in a complex shown in the title was discussed by paying special attention to its effect on the rotational motion of the complex in order to extract as much precise information on this motion as possible from the observed rotational spectra. We have set up two theoretical formulations. One is based on a coordinate axis system attached to the C2v molecule, but its origin is floating with the motion of the rare gas atom, while keeping the orientation parallel to the original C2v molecule-fixed coordinate system. The second approach starts with counting the number of equivalent potential minima, which are well separated from the others by high potential barriers, and then collects all permutation-inversion operations, which transform the system from one minimum to another, to set up a group appropriate for the complex. By using the symmetry properties thus derived, a phenomenological Hamiltonian is set up to fit the observed spectra. The two formulations result in alike rotational energy matrices, and we have applied them to analyze the internal motions in the two complexes of present concern: neon-dimethyl ether (Ne-DME) and argon-dimethyl ether (Ar-DME). Some of the transitions observed by the present study exhibited additional splittings, which were interpreted as due to an internal rotation of the methyl groups in DME and were analyzed by the second formulation. For Ar-DME the splittings appeared only in high-K transitions, yielding the V3 potential barrier to be 778(1) cm(-1), whereas those observed for Ne-DME were ascribed to the effects of the CH3 internal rotation on the inversion splitting.

Fourier transform microwave spectroscopy of N, N-dimethylacetamide

The jet-cooled Fourier-transform microwave spectrum of N, N-dimethylacetamide was recorded in the region of 12–24 GHz, and was analyzed to determine rotational constants and nuclear quadrupole coupling constants. Coriolis-like coupling parameters characterizing interaction between internal rotation of methyl groups and the overall rotation were also determined from internal-rotation tunneling splittings of the rotational transitions. The Coriolis-like coupling parameters permitted determination of the barrier heights to internal rotation of the three methyl groups, which were found to be 677, 237, and 183 cm −1 for the C-methyl top, the trans- N-methyl top and the cis- N-methyl top, respectively.

Molecular Beam Rotational Spectrum of Cyclobutanone–Trifluoromethane: Nature of Weak CH⋅⋅⋅OC and CH⋅⋅⋅F Hydrogen Bonds

The molecular beam Fourier transform microwave spectrum of cyclobutanone-trifluoromethane has been assigned and measured. The carbon atom of trifluoromethane lies in the plane of the heavy atoms of cyclobutanone. The complex is stabilized by one C-H...O=C and two C-H...F-C weak hydrogen bonds. The C-H...O=C interaction, involving one carbonylic oxygen, is studied for the first time in detail with rotationally resolved spectroscopy. The two C-H...F-C weak hydrogen bonds involve two fluorine atoms of trifluoromethane and two hydrogens of the same methylenic group in the alpha position.

Fourier-transform microwave spectroscopy of N-methylaniline

The jet-cooled Fourier-transform microwave spectrum of N-methylaniline (C 6H 5-NHCH 3) was recorded in the region of 10–26 GHz, and was analyzed to determine rotational constants and nuclear quadrupole coupling constants. Furthermore, a Coriolis-like coupling parameter characterizing an interaction between an internal rotation of a methyl group and an overall rotation was also determined from A– E splittings observed in pure rotational transitions with high K a values. The Coriolis-like coupling parameter permitted the determination of the barrier to internal rotation of the methyl group which was found to be 975 cm −1.

CH···O and CH···F Links Form the Cage Structure of Dioxane−Trifluoromethane

The molecular beam Fourier transform microwave spectrum of 1,4-dioxane-trifluoromethane has been assigned and measured. The two subunits form a cage stabilized by one C-H...O and two C-H...F weak hydrogen bonds. The C-H...O link involves the axial lone pair of one of the two equivalent ring oxygens, while the two C-H...F bridges connect trifluoromethane to the two axial hydrogens in positions 3 and 5. The dissociation energy has been estimated from the D(J) centrifugal distortion parameter to be approximately 6.8 kJ/mol.

Free internal rotation in CH 3–C [tbnd]C–CF 3

The molecular beam Fourier transform microwave spectrum in the ground vibrational state of the symmetric rotor CH 3–C C–CF 3 has been detected and analyzed in the frequency interval 6–18 GHz. The molecule was generated in a pulsed dc electric discharge of trifluoropropyne and propyne seeded in neon. The observed spectrum exhibits a complex pattern of lines for each rotational transition consistent with free internal rotation of the two tops. The rotational parameters B, D J , D Jm , D JK , D JKm and H Jm have been determined from the fit of the observed lines.

Molecular beam Fourier transform microwave spectrum of the dimethylether–xenon complex: tunnelling splitting and 131Xe quadrupole coupling constants

The rotational spectra of three isotopomers of the complex dimethyl ether–Xe ( 132Xe, 129Xe, 131Xe) have been measured by molecular beam Fourier transform microwave spectroscopy. An evenly spaced doubling of ca. 0.2 MHz, due to the internal rotation of the dimethyl ether subunit in the complex, has been observed for the μ c-type transitions. The corresponding vibrational spacing, Δ 01, has been determined to be 105(1) and 106(1) kHz for the 132Xe and 129Xe species, respectively. This datum has been useful to size the tunnelling barrier of Xe from above to below the COC plane. The nuclear quadrupole coupling constants of the 131Xe ( I=3/2) species have been precisely determined, χ aa=4.57(3), χ bb=−2.93(3), and χ cc=−1.64(3) MHz, respectively. These values indicate that the nuclear contributions to the field electric gradient at the Xe nucleus are smaller than the electronic ones.

Analysis and fit of the Fourier-transform microwave spectrum of the two-top molecule N-methylacetamide

The jet-cooled Fourier-transform microwave spectrum of N-methylacetamide (CH 3NHC(O)CH 3), a molecule containing two methyl tops with relatively low barriers to internal rotation, has been recorded and fit to nearly experimental uncertainty. Measurements were carried out between 10 and 26 GHz, with the nitrogen quadrupole splittings resolved for many transitions. The permutation-inversion group for this molecule is G 18 (not isomorphic to any point group), with irreducible representations A 1, A 2, E 1, E 2, E 3, and E 4. One of these symmetry species and the usual three asymmetric rotor quantum numbers J K a K c were assigned to each torsion–rotation level involved in the observed transitions. F values were assigned to hyperfine components, where F= J+ I N . Transitions involving levels of A 1 and A 2 species could be fit to an asymmetric rotor Hamiltonian. The other transitions were first fit separately for each symmetry species using a Pickett-like effective rotational Hamiltonian. Constants from these fits show a number of additive properties which can be correlated with sums and differences of effects involving the two tops. A final global fit to 48 molecular parameters for 839 hyperfine components of 216 torsion–rotation transitions involving 152 torsion–rotation levels was carried out using a newly written two-top computer program, giving a root-mean-square deviation of observed-minus-calculated residuals of 4 kHz. This program was written in the principal axis system of the molecule and uses a free-rotor basis set for each top, a symmetric-top basis set for the rotational functions, and a single-step diagonalization procedure. Such an approach requires quite long computation times, but it is much less prone to subtle programming errors (a consideration felt to be important since checking the new program against precise fits of low-barrier two-top molecules in the literature was not possible). The two internal rotation angles in this molecule correspond to the Ramachandran angles ψ and φ often defined to describe polypeptide folding. Barriers to internal rotation about these two angles were found to be 73 and 79 cm −1, respectively. Top–top coupling in both the kinetic and potential energy part of the Hamiltonian is relatively small in this molecule.

Pure rotational spectrum and model calculations of indole–water

The molecular beam Fourier transform microwave spectra of two isotopomers of the 1:1 complex between indole and water have been measured. The water molecule has been reliably located in the complex from these experimental data. The complex has a Cs symmetry with an N–H⋯O hydrogen bond and the plane of the H2O molecule perpendicular to the indole plane. The two-dimensional potential energy surface of the internal rotation and inversion of water in the complex, evaluated with B3LYP/6-31G** or MP2/6-31G** quantum chemical calculations, suggests the tunneling motion of water to take place with the contribute of both motions. The experimental evidence combined with flexible model calculations, indicate, however, that the tunneling motion is mainly an internal rotation of water around its C2 symmetry axis.

High resolution spectroscopic studies of 1-(1-naphthyl)ethylamine in S 0 and S 1: exploring the dependence of circular dichroism on conformational structure

Rotationally resolved spectra of gas-phase samples of 1-(1-naphthyl)-ethylamine (NEA) and amine deuterated forms have been obtained in the microwave and ultraviolet regions, with the isotopomers initially prepared in their zero-point vibrational levels by cooling in pulsed jet and molecular beam supersonic expansions. A single parameter set that includes inertial parameters and 14N nuclear quadrupole constants has accounted for nearly all transitions observed in the Fourier-transform microwave spectrum at 2 K and indicates the presence of only one geometrical isomer in the jet-cooled expansion. The rotational constants, dipole moment orientation, and the amine hydrogen atom positions have been used to identify the conformation of the attached chiral group from among nine possible isomeric forms. The S 1 rotational constants and electronic transition moment orientation have been obtained from high resolution molecular beam data of the band origin at 31771.56(2) cm −1. Excited state “gas phase” predictions from ab initio theory at the CIS/6-31G(d,p) and CIS/6-311 + G(d) levels are compared with the observed S 1 results and with circular dichroism (CD) data obtained for solution phase samples of ( S)-NEA in cyclohexane. Distinguishing spectral features are found in the calculated CD spectra of the three lowest energy conformers arising from rotation about the C–CH(NH 2)CH 3 bond, all of which are thermally populated at room temperature. While the predictions at both levels are in fair agreement with the observed gas phase results, CIS/6-31G(d,p) theory is found to be inadequate to model the condensed phase CD spectrum. In constrast, the calculated spectrum of the equilibrium mixture of conformers at the CIS/6-311 + G(d) level is in good qualitative agreement with the observed CD results and indicates the importance of diffuse functions for the accurate prediction of chiroptical properties of NEA. The most noteworthy exception is the predicted rotatory strength of the S 1 origin, which is overestimated by more than 10-fold. Possible reasons for this discrepancy with experiment include a reversal of S 1(L b) and S 2(L a) states from the absence of electron correlation and/or the neglect of room-temperature solvent effects. These results provide for rigorous benchmark tests of theoretical models and further elucidate the importance of the conformational structure for determinations of absolute stereochemistry from CD spectra.

Molecular beam Fourier transform microwave spectra of (chloromethyl)cyclopropane and (chloromethyl)oxirane

Pulsed molecular beam Fourier transform microwave spectroscopy has been used in order to remove disagreements in previous assignments of the rotational spectra of (chloromethyl)cyclopropane and (chloromethyl)oxirane. For the latter, two rotameric forms have been detected in a supersonic jet. It has been possible to determine, in addition to the rotational and quartic centrifugal distortion constants, the complete chlorine quadrupole coupling tensors including all of their off-diagonal elements for the 35Cl and 37Cl isotopomers of all gauche conformers. From these data, modified plausible structures could be deduced. These findings are supported by the results of quantum chemical calculations.

Weak, improper, C-O...H-C hydrogen bonds in the dimethyl ether dimer.

The ground-state rotational spectrum of the dimethyl ether dimer, (DME)(2), has been studied by molecular beam Fourier transform microwave and free jet millimeter wave absorption spectroscopies. The molecular beam Fourier transform microwave spectra of the (DME-d(6))(2), (DME-(13)C)(2), (DME-d(6))...(DME), (DME-(13)C)...(DME), and (DME)...(DME-(13)C) isotopomers have also been assigned. The rotational parameters have been interpreted in terms of a C(s) geometry with the two monomers bound by three weak C-H...O hydrogen bonds, each with an average interaction energy of about 1.9 kJ/mol. The experimental data combined with high-level ab initio calculations show this kind of interaction to be improper, blue-shifted hydrogen bonding, with an average shortening of the C-H bonds involved in the hydrogen bonding of 0.0014 A. The length of the C-H...O hydrogen bonds, r(O...H), is in the range 2.52-2.59 A.

Fourier Transform Microwave Spectrum and ab Initio Study of Dimethyl Methylphosphonate

The rotational spectrum of dimethyl methylphosphonate (DMMP) has been studied using a pulsed molecular beam Fourier transform microwave spectrometer. The spectrum is complicated by the internal rotation motions of the three methyl tops in the molecule as well as an interconversion motion of the two methoxy groups. Here, we present the microwave spectrum, the ab initio calculations, and the assignment of the rigid-rotor A-symmetry state of the molecule. The rotational constants for this state are A=2828.753(2) MHz, B=1972.360(3) MHz, and C=1614.267(2) MHz. In the following paper, a group theoretical analysis is developed for DMMP. The observed conformation of the molecule has no point-group symmetry and all three electric-dipole selection rules are active, with c-type transitions being the most intense. Ab initio calculations were carried out at both the Hartree–Fock 6-31G* and MP2/6-311G* levels of theory. These calculations indicate that two low-energy conformations are possible separated by energies of less than 170 cm−1. Furthermore, the calculated lowest energy conformer is in agreement with the one observed experimentally. The relative energies of the two low-energy conformers rise from 34 cm−1 at the HF level to 170 cm−1 at the MP2 level.

Fourier transform microwave spectra of the very weakly bound He–CO2 dimer

Spectra of pure rotational transitions of the very weakly bound He–CO2 van der Waals complex were investigated using a pulsed molecular beam Fourier Transform microwave spectrometer. The complex exhibits very large amplitude internal vibrational motions and the rotational transitions were, as a consequence, difficult to locate and to assign. Eight isotopomers, namely He–CO2, He–13CO2, He–OC18O, He–O13C18O, He–18OC18O, He–18O13C18O, He–OC17O, and He–O13C17O, were studied in order to establish the assignments. The hyperfine structures due to the quadrupolar 17O nucleus were resolved and analyzed in the two 17O containing isotopomers. The observed nuclear quadrupole hyperfine structures not only provide further confirmation for the assignment but also give new information about the angular anisotropy of the He–CO2 interaction potential.

Nuclear Spin–Rotation Coupling Constants and Absolute Nuclear Shielding Parameters of ClPO2

High-resolution Fourier transform microwave spectra of the unstable species ClPO2 have been measured, and the P and Cl spin–rotation coupling constants have been determined from the resolved hyperfine splittings. These constants have been used to derive the absolute nuclear shielding parameters for the Cl and P nuclei of ClPO2. Ab initio values for the spin–rotation coupling constants have also been calculated theoretically; the results are compared with the experimental determinations.

Fourier Transform Microwave Spectroscopic Study of Gas-Phase ClPO2: Molecular Geometry and Electronic Structure Parameter Determinations

This paper reports the first spectroscopic study of free gas-phase ClPO2. Experimental geometric parameters have been determined for the first time and insight into the electronic structure of the molecule has been gained, both from the analysis of the measured Fourier transform microwave spectra. The sample was generated inside a pulsed nozzle by passing an electric discharge through PCl3 and O2 in He and was subsequently stabilized in a supersonic jet expansion. The determined ClPO2 parameters have been compared with those of related species.

The Hyperfine Structures of ScCl and ScF

The Fourier transform microwave spectra of gaseous ScF and ScCl have been measured in their 1Σ+ ground electronic states. Transitions have been observed for ScF (J = 1–0) and for both Sc35Cl and Sc37Cl (J = 1–0 and J = 2–1) in the ground vibrational state, and for Sc35Cl (J = 1–0 and J = 2–1) in the first excited vibrational state. The nuclear quadrupole coupling constants of Sc and of both isotopes of Cl, the spin–rotation constants for all the nuclei and the nuclear spin–nuclear spin constants for both molecules, have been determined. From these hyperfine constants, the electronic structures of ScF and ScCl have been investigated and comparisons have been made to similar molecules. ScCl has been found to be highly ionic. For Sc35Cl, the equilibrium bond length has been reevaluated and the vibration frequency and dissociation energy have been estimated. A simple approach to account for the Sc quadrupole coupling constants has been shown to be flawed.

Fourier Transform Microwave Spectrum, Geometry, and Hyperfine Coupling Constants of Phosphenous Fluoride, OPF

The microwave spectrum of phosphenous fluoride, OPF, has been measured using a pulsed-jet cavity Fourier transform microwave spectrometer. With the exception of a mass spectroscopic detection of the molecule, chemically prepared for use in a matrix IR study, this is the first observation of free gas-phase OPF. The samples were prepared from mixtures of PF3 and O2 in Ne carrier gas, using an electric discharge. Rotational transitions of two isotopomers (16O31P19F and 18O31P19F) have been measured in the 4−26 GHz frequency range. The determined rotational constants have been used to calculate r0, rz, and approximate re molecular geometries. In contrast to the nitrogen analogue, ONF, OPF has been found to show no irregularities in its geometry. Small hyperfine splittings due to the spin-1/2 31P and 19F nuclei have been analyzed in terms of nuclear spin−rotation interactions. Because the determined coupling constants were of similar magnitudes and could not be unambiguously assigned, the related nuclear shiel...