Two-dimensional chirped-pulse Fourier transform microwave spectroscopy

Two-dimensional (2D) correlation techniques are developed for chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy. The broadband nature of the spectrometer coupled with fast digital electronics permits the generation of arbitrary pulse sequences and simultaneous detection of the 8-18 GHz region of the microwave spectrum. This significantly increases the number of rotation transitions that can be simultaneously probed, as well as the bandwidth in both frequency dimensions. We theoretically and experimentally evaluate coherence transfer of three- and four-level systems to relate the method with previous studies. We then extend the principles of single-quantum and autocorrelation to incorporate broadband excitation and detection. Global connectivity of the rotational energy level structure is demonstrated through the transfer of multiple coherences in a single 2D experiment. Additionally, open-system effects are observed from irradiating many-level systems. Quadrature detection in the indirectly measured frequency dimension and phase cycling are also adapted for 2D CP-FTMW spectroscopy.     

Chirped-pulse Fourier transform microwave spectroscopy of perfluoroiodoethane

The pure rotational spectrum of perfluoroiodoethane between 8.0 and 11.9 GHz has been measured on a search accelerated, correct intensity Fourier transform microwave (SACI-FTMW) spectrometer. The spectra is dense with 247 measured transitions in the given region. Only the anti conformer was observed for which rotational constants are reported. Nuclear electric quadrupole coupling constants due to the iodine-127 were determined and are reported. Also, two dipole forbidden/quadrupole allowed ΔJ=2 transitions were observed in the spectra. The observation of these transitions has been rationalized on the basis of near degeneracies between energy levels connected by χ_ab.     

Fourier transform microwave spectroscopy and Fourier transform microwave-millimeter wave double resonance spectroscopy of the ClOO radical

Pure rotational spectra of the ClOO radical for the (35)Cl and (37)Cl isotopomers have been observed using Fourier transform microwave and Fourier transform microwave-millimeter wave double resonance spectroscopy. The rotational, centrifugal, spin-rotation coupling, and hyperfine coupling constants have been determined by least-squares fits of the observed transition frequencies. The molecular constants indicate that the electronic ground state is 2A". The r(0) structure is determined to be r(0)(ClO)=2.075 A, r(0)(OO)=1.227 A, and theta;(0)(ClOO)=116.4 degrees . Several highly accurate ab initio calculations have also been performed. Some of them turned out to be inaccurate because it is necessary to take into account both static and dynamic electronic correlations. Only multireference (single and double) configuration interaction calculations with large basis sets reproduce the present experimental results. The anharmonic force constants obtained by the ab initio calculations are used to determine the r(e) structure, r(e)(ClO)=2.084(1) A, r(e)(OO)=1.206(2) A, and theta;(e)(ClOO)=115.4(1) degrees . Unique features of the ClOO radical have become clear by the present experiment and the ab initio calculations.     

Fourier transform microwave and millimeter wave spectroscopy of quinazoline, quinoxaline, and phthalazine

The pure rotational spectra of the bicyclic aromatic nitrogen heterocycle molecules, quinazoline, quinoxaline, and phthalazine, have been recorded and assigned in the region 13-87 GHz. An analysis, guided by ab initio molecular orbital predictions, of frequency-scanned Stark modulated, jet-cooled millimeter wave absorption spectra (48-87 GHz) yielded a preliminary set of rotational and centrifugal distortion constants. Subsequent spectral analysis at higher resolution was carried out with Fourier transform microwave (FT-MW) spectroscopy (13-18 GHz) of a supersonic rotationally cold molecular beam. The high spectral resolution of the FT-MW instrument provided an improved set of rotational and centrifugal distortion constants together with nitrogen quadrupole coupling constants for all three species. Density functional theory calculations at the B3LYP∕6-311+G?? level of theory closely predict rotational constants and are useful in predicting quadrupole coupling constants and dipole moments for such species.     

The trans-isomer of formanilide studied by microwave Fourier transform spectroscopy

The rotational spectra of the trans-isomer of formanilide was recorded by microwave Fourier transform spectroscopy. The rotational and centrifugal distorsion constants as well as the quadrupole coupling constants have been accurately determined. It is shown that the stable conformation corresponds to a planar structure. The energy barrier between this conformation and the less stable one (the amino group lies in a plane perpendicular to the phenyl ring) has been evaluated by ab initio calculations.     

Determination of structural parameters for ferrocenecarboxaldehyde using Fourier transform microwave spectroscopy

Gas-phase structural parameters for ferrocenecarboxaldehyde have been determined using Fourier transform microwave spectroscopy. Rotational transitions due to a-, b-, and c-type dipole moments were measured. Eighteen rotational constants were determined by fitting the measured transitions of various isotopomers using a rigid rotor Hamiltonian with centrifugal distortion constants. Least-squares fit and Kraitchman analyses have been used to determine the gas-phase structural parameters and the atomic coordinates of the molecule using the rotational constants for various isotopomers. Structural parameters determined from the least-squares fit are the Fe-C bond lengths to the cyclopentadienyl rings, r(Fe-C)=2.047(4) A, and the distance between the carbon atoms of the cyclopentadienyl rings, r(C-C)=1.430(2) A and r(C1-C1')=1.46(1) A of ring carbon and aldehyde carbon atom. Structural parameters were also obtained using density-functional theory calculations, and these were quite helpful in resolving ambiguities in the structural fit analysis, and providing some fixed parameters for the structural analysis. The results of the least squares and the calculations indicate that the carbon atoms of the Cp groups for ferrocenecarboxaldehyde are in an eclipsed conformation in the ground vibrational state.     

Conformational study of 2-phenylethylamine by molecular-beam Fourier transform microwave spectroscopy

The conformational preferences of the simplest amine neurotransmitter 2-phenylethylamine have been investigated using molecular beam Fourier transform microwave (MB-FTMW) spectroscopy. Two new conformers have been observed together with the two previously reported by Godfrey et al. [J. Am. Chem. Soc., 1995, 117, 8204]. The (14)N nuclear quadrupole hyperfine structure has been resolved for all four conformers. Comparison of the experimental rotational and quadrupole coupling constants with those calculated theoretically provides a conclusive test for the identification of all conformers. The two most stable conformers present a gauche (folded) disposition of the alkyl-amine chain and are stabilised by a weak NH...pi interaction between the amino group and the aromatic ring. The other two conformers show an anti (extended) arrangement of the alkyl-amine chain. Tunnelling splittings have been observed in the spectrum of one of the anti conformers. The post expansion relative abundances in the supersonic jet have been also investigated and related to the conformer energies.     

Probing thymine with laser ablation molecular beam Fourier transform microwave spectroscopy

Laser ablation in combination with molecular beam Fourier transform microwave spectroscopy has been used to establish unambiguously the presence of the diketo form of thymine in the gas phase and to obtain its structure.     

Fourier transform microwave spectroscopy of the isocyanomethyl radical, CH(2)NC

The pure rotational spectrum of the isocyanomethyl radical, CH(2)NC, was measured for the first time by using a Fourier transform microwave spectrometer. The molecule was produced by a discharge of isocyanomethane, CH(3)NC, diluted in Ar or Ne. The spectral lines due to the N=1-0 and 2-1 transitions were recorded near 22 and 44 GHz, respectively. The observed spectrum showed a complicated fine and hyperfine structure because of the same order of interaction energies. Among the 39 spectral lines detected and assigned, the transitions with K(a)=1 show no hyperfine splitting due to the hydrogen nuclei, suggesting planarity for the molecule. Molecular constants such as rotational and spin-rotational parameters including centrifugal effects and hyperfine coupling constants due to both the nitrogen and the hydrogen nuclei were accurately determined. The structure and the astronomical implications of the molecule are discussed.     

Dispersive Fourier transform spectroscopy

A review is presented of recent developments in the methods of dispersive Fourier transform spectroscopy that have demonstrated the unique value of this broad band method for determining the optical constants of gases, liquids and solids.     

Fourier transform Raman spectroscopy

The basic experimental aspects of Fourier transform Raman Spectroscopy are reviewed with an emphasis on detector technology. The sensitivity is comparable to dispersive Raman Spectroscopy using visible lasers. The ease of spectral subtraction is demonstrated and examples are given showing the elimination of fluorescence.     

Bolometric Fourier transform spectroscopy

Two examples are given of the results which can be obtained by use of bolometric techniques to measure very high far-infrared reflectivities from small single crystals at low temperatures. Both a simple and a composite bolometric technique are described.     

Zeeman-modulation Fourier transform spectroscopy

Low resolution Zeeman-modulation Fourier transform spectroscopy is demonstrated. The signal is produced by applying a magnetic field to the sample which must be paramagnetic. It is a convenient way to selectively detect unstable molecules. This source modulation new technique is not, as was first expected, requiring high resolution capabilities. Indeed the detection may be possible through the overall variation of the absorption of the line, induced by the magnetic field.     

Some consequences of the Fourier transform in Fourier transform Raman spectroscopy

This paper reviews some of the differences between dispersive and Fourier transform (FT) Raman spectroscopy with the goal of highlighting some of the advantages and disadvantages of FT-Raman spectroscopy. In particular, the use of filters, Connes advantage, trading rules and the size of the multiplex and throughput advantages are discussed.     

The pure rotational spectrum of cyclobutane-d1 observed by microwave Fourier transform spectroscopy

Pure rotational transitions of both equatorial and axial conformers of cyclobutane-d₁ in their vibronic ground state have been observed between 12 and 40 GHz with a pulsed microwave Fourier transform (MWFT) spectrometer. Twenty-four transitions of the equatorial as well as 29 transitions of the axial conformer with J⩽40 have been measured. Their assignment has been confirmed by microwave-microwave double-resonance experiments. Rotational constants and the quartic centrifugal distortion constants have been determined for both conformers from the measured frequencies. The r₀ structure has been deduced from these rotational constants.     

Rotational spectrum of 1,1,1-trifluoro-2-butanone using chirped-pulse Fourier transform microwave spectroscopy

The pure rotational spectra of 1,1,1-trifluoro-2-butanone and its four (13)C isotopologues have been studied using the new chirped-pulsed Fourier transform microwave spectrometer at the University of Manitoba in combination with a conventional Balle-Flygare-type instrument. Quantum chemical calculations, at the MP2/6-311++G(d,p) level, were carried out to obtain information about the structure, relative stability, and difference in populations of the three lowest energy conformers corresponding to dihedral angles of 0°, 82.8°, and 119.2° along the carbon backbone. The observed spectra are that of conformer I (dihedral angle 0°), and, based on analysis of the observed splitting, the V(3) barrier to internal rotation of the methyl group has been determined to be 9.380(5) kJ mol(-1). The spectroscopic constants of the five isotopologues were used to precisely derive the r(s) and partial r(0) geometries of this conformer based on an assumed planar carbon backbone (as supported by the spectra and ab initio calculations).     

Detection of the triplet HC4N radical by Fourier transform microwave spectroscopy

The triplet HC₄N radical in a linear carbon-chain form has been detected by Fourier transform microwave spectroscopy for the first time. Rotational transitions with fine and hyperfine structures, about 130 lines in total, were observed in a pulsed-discharge-nozzle supersonic expansion of the HC₃N sample diluted in Ar buffer gas. The spectrum was assigned to the linear HC₄N radical by various combinations of discharge gases, by the rotational, fine and hyperfine structures, and most decisively, by comparison of the determined molecular constants with those of the HCCN radical.     

Hyperfine resolved spectrum of the bromomethyl radical, CH2Br, by Fourier transform microwave spectroscopy

Pure rotational spectra of the bromomethyl radical, CH(2)Br, were measured by using a Fourier transform microwave (FT-MW) spectrometer in order to fully resolve hyperfine structures arising from both the bromine and hydrogen nuclei. We detected a total of 124 lines for the (79)Br and (81)Br isotopomers, including K(a)=0 (ortho species) and K(a)=1 (para species). No hyperfine splitting due to the hydrogen nuclei was observed for the para species, directly confirming the planarity of the radical. We conducted a global analysis of our present FT-MW results and previous measurements in the millimeter-wave region and obtained an exhaustive list of molecular constants. The sign of the Fermi constant of the bromine nucleus was unambiguously determined to be positive, which is opposite to that found in previous work in the millimeter-wave region and in electron spin resonance experiment on this radical. The present study permitted a systematic comparison to be made of the hyperfine coupling constants of both the halogen and hydrogen nuclei for CH(2)X-type compounds, where X=F, Cl, and Br.     

Fourier transform microwave spectrum of CO-dimethyl ether

Two sets of 32 rotational transitions were observed for the carbon monoxide-dimethyl ether (CO-DME) complex and two sets of 30 transitions for both (13)CO-DME and C(18)O-DME, in the frequency region from 3.5 to 25.2 GHz, with J ranging from 1<--0 up to 7<--6, by using a Fourier transform microwave spectrometer. The splittings between the two sets of the same transition varied from 2 to 15 MHz, and the two components were assigned to the two lowest states of the internal rotation of CO with respect to DME governed by a twofold potential. A preliminary analysis carried out separately for the two sets of the observed transition frequencies by using an ordinary asymmetric-rotor Hamiltonian indicated that the heavy-atom skeleton of the complex was essentially planar, as evidenced by the "pseudoinertial defects," i.e., the inertial defects, which involve the contributions of the out-of-plane hydrogens of the two methyl groups, I(cc)-I(aa)-I(bb) of -5.764(23) and -5.753(16) uA(2) for the symmetric and antisymmetric states, respectively. All of the observed transition frequencies were subsequently analyzed simultaneously, by using a phenomenological Hamiltonian which was described in a previous paper on Ar-DME and Ne-DME [Morita et al., J. Chem. Phys. 124, 094301 (2006)]. The rotational constants thus derived were analyzed to give the distance between the centers of gravity of the two component molecules, DME and CO, to be 3.682 A and the angle between the CO and the a-inertial axes to be 75.7 degrees ; the C end of the CO being closer to the DME. Most a-type transitions were observed as closely spaced triplets, which were ascribed to the internal rotation of the two methyl tops of DME. The V(3) potential barrier was obtained to be 772(2) cm(-1) from the first-order Coriolis coupling term between the internal rotation and overall rotation, which is about 82% of V(3) for the DME monomer, whereas the second-order contribution of the coupling to the B rotational constant led to V(3) of 705(3) cm(-1). By assuming a Lennard-Jones-type potential, the dissociation energy was estimated to be E(B)=1.6 kJ mol(-1), to be compared with 1.0 and 2.5 kJ mol(-1) for Ne-DME and Ar-DME, respectively.