Microwave-microwave Double Resonance Research Articles

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 spectroscopic investigation of carbonyl sulfide solvated with helium atoms

Rotational spectra of small- to medium-sized HeN–OCS clusters with N from 2 to 8 were investigated using a pulsed molecular beam Fourier transform microwave spectrometer and a microwave-microwave double resonance spectrometer. Spectra of five to eight isotopomers were measured for each HeN–OCS cluster. Nuclear quadrupole hyperfine structures were observed for S33 and O17 containing isotopomers. The isotopic data, together with the microwave-microwave double resonance experiments, were used to establish unambiguous assignments for these clusters. The assignment of N, the number of solvating helium atoms in a particular cluster, was supported by the observed intensity change under different backing pressure and nozzle cooling conditions and in particular by the systematic infrared vibrational band shifts from the concurrent infrared study. This sequence of clusters contains some of the largest clusters that have thus far been investigated with high-resolution spectroscopic techniques. This study provides detailed and important information for the understanding of the quantum solvation process of dopant molecules in helium nanodroplets.

Single and Double Resonance Microwave Spectroscopy in SuperfluidH4eClusters

Purely rotational transitions of a molecule, such as HCCCN, embedded in large $^4$He clusters have been detected for the first time. The saturation behavior shows that, in contrast to previous expectations, the microwave line profiles are dominated by inhomogeneous broadening mechanisms. Spectral holes and peaks produced by microwave-microwave double resonance have widths comparable to those of single resonance lines, indicating that relaxation occurs among quantum states of the inhomogeneous distribution of each rotational level, at a rate approximately 10 times faster than rotationally inelastic relaxation.

The Rotational/Concerted Torsional Spectrum of the g′ Ga Conformer of Ethylene Glycol

The microwave spectrum of the g′ Ga conformer of ethylene glycol has been assigned for the first time using microwave-microwave double resonance and molecular beam Fourier transform microwave spectroscopy. Such techniques were necessary in order to select the desired lines in the crowded spectrum, which does not follow a rigid rotator pattern because of the large-amplitude motion displayed by the molecule. This motion, consisting of a concerted torsion of both OH groups, leads to a tunneling splitting of 7 GHz and significantly alters the rotational energy level pattern because it is coupled to the overall rotation of the molecule through a Coriolis coupling. This coupling is accounted for using an IAM-like theoretical model, and this allows us to satisfactorily analyze the microwave frequencies of the normal and of the O- d 2 species. Information on the equilibrium geometry of the molecule and on the tunneling path is extracted through the values obtained for the parameters involved in this Coriolis coupling. A dipole moment analysis yields values of all three dipole moments components and a determination of their relative signs is attempted.

Novel coherence effects in microwave-microwave double resonance pulse Fourier transform spectroscopy

A two pulse sequence in a microwave-microwave double resonance experiment has exhibited a new set of coherences between rotational energy levels in 35ClCN. The derivation of a set of five-level Bloch equations is presented and used to interpret the experimentally observed dependence on pump power and source detuning. Applications to spectral assignment are outlined.

Extracting vibrational information from rotational spectra—A microwave investigation on excited vibrational states of SF 4

A method of extracting harmonic information from the inertial defects of non-totally symmetric vibrational states of orthorhombic molecules has been developed. As a result it has been possible to exploit additional information for a normal coordinate analysis. This information has been obtained from microwave-microwave double resonance experiments on SF 4 in excited vibrational states.

Pure rotational transitions and electric dipole moment of allene- d4 in excited vibrational states

Pure rotational transitions ( J=1←0) in allene- d 4 have been observed for the degenerate vibrational states (ν 10 = 1) and (ν 11 = 1) with a pulsed microwave Fourier transform spectrometer. The assignments have been confirmed by microwave-microwave double resonance. Both double-resonance experiments and Stark effect measurements have been used to determine the magnitude of the vibrationally induced dipole moments. For each vibrational state, the rotational constant B has been improved.

ChemInform Abstract: Rotational Spectrum, Dipole Moment, and Ring‐Puckering Potential of Cyclobutane‐1,1‐d2

Abstract The microwave spectrum of cyclobutane-1,1-d2 has been measured over 12–40 GHz with a pulsed Fourier transform spectrometer. Rotational transitions up to J = 40 have been assigned. Some assignments have been checked by microwave-microwave double resonance with continuous pump radiation. Transitions with J > 2 showed a doubling due to the ring-puckering motion. Rotational constants and centrifugal distortion constants have been fitted with the help of a coupled state Hamiltonian for the two puckering states simultaneously. The permanent electric dipole moment has been estimated from quantitative Stark effect measurements and from microwave-microwave double-resonance experiments. A combined analysis of the rotational constants of the d2 and d1 species and of the ring-puckering transition frequencies of the d0 and d8 species provided an improved double-minimum potential for the puckering motion. The flexible model treatment included the coupling with the CH2 rocking motion. An energy difference of 95.2(37) MHz was found between the two lowest puckering states of cyclobutane-1,1-d2 from this analysis.

Rotational spectrum, dipole moment, and ring-puckering potential of cyclobutane-1,1- d2

The microwave spectrum of cyclobutane-1,1- d 2 has been measured over 12–40 GHz with a pulsed Fourier transform spectrometer. Rotational transitions up to J = 40 have been assigned. Some assignments have been checked by microwave-microwave double resonance with continuous pump radiation. Transitions with J > 2 showed a doubling due to the ring-puckering motion. Rotational constants and centrifugal distortion constants have been fitted with the help of a coupled state Hamiltonian for the two puckering states simultaneously. The permanent electric dipole moment has been estimated from quantitative Stark effect measurements and from microwave-microwave double-resonance experiments. A combined analysis of the rotational constants of the d 2 and d 1 species and of the ring-puckering transition frequencies of the d 0 and d 8 species provided an improved double-minimum potential for the puckering motion. The flexible model treatment included the coupling with the CH 2 rocking motion. An energy difference of 95.2(37) MHz was found between the two lowest puckering states of cyclobutane-1,1- d 2 from this analysis.

Applications of pulsed microwave-microwave double resonance: I. Relaxation studies in gases

The time-resolved response of a gas absorber exposed to two independent sources of microwave radiation, one steady and the other pulsed, is described using a formalism based on the Bloch equations for a three-level system. This predicts that a series of experiments can be made on the resonant and off-resonant recoveries of the absorptive and dispersive properties of a single sample by which the individual collisional relaxation parameters that characterise its rotational states may be separately measured. Some of these can be determined only by double-resonance experiments. The construction and operation of amicrowave double-resonance bridge spectrometer capable of carrying out these experiments is described, and results obtained for the (J=2 from 1)-(J=1 from 0) system of OCS are discussed in detail. The polarisation relaxation rate for the 2 from 1 transition is found to be 38.2(2) mu s-1 Torr-1 and its population relaxation rate, including cross-relaxation effects, is found to be 38.9(1.9) mu s-1 Torr-2. Values for the cross-relaxation parameters are obtained for the first time.

Microwave spectrum and barrier to internal rotation of 1-chloro-2-butyne

Rotational transitions of the μ a and μ b type have been identified with microwave-microwave double resonance measurements for 1-chloro-2-butyne in the ground vibrational state. In the first excited state of the methyl torsion only μ a -type transitions have been identified. The A-type transitions of the ground vibrational state can be described perfectly by the rigid rotor approximation with centrifugal corrections. Using the internal axis method the barrier to internal rotation was determined from the A, E splittings: V 3 = 10.05 ± 0.09 cm −1. A model which allowed for geometry relaxation upon internal rotation was used to fit one set of parameters to the transition frequencies of both ground state and first excited torsional state. The sixfold contribution to the barrier was found to be negligible: V 6 = −0.4 ± 0.3 cm −1.

ChemInform Abstract: Microwave Spectra and Barriers to Internal Rotation of Trifluoroacetic Acid and Trifluoroacetyl Fluoride.

Abstract The rotational transitions of trifluoroacetic acid and trifluoroacetyl fluoride were identified with radiofrequency-microwave and microwave-microwave double resonance spectroscopy. Isotopic substitution of the hydrogen atom in trifluoroacetic acid showed that the hydrogen atom is cis with respect to the CO bond. A second conformation was not found. From A,E splittings in higher vibrational levels the internal rotation barriers were calculated: for trifluoroacetic acid, V3 = 241.8 ± 0.5 cm−1 (v = 4); for trifluoroacetyl fluoride V3 = 383.6 ± 0.5 cm−1 (v = 5).

Microwave spectra and barriers to internal rotation of trifluoroacetic acid and trifluoroacetyl fluoride

The rotational transitions of trifluoroacetic acid and trifluoroacetyl fluoride were identified with radiofrequency-microwave and microwave-microwave double resonance spectroscopy. Isotopic substitution of the hydrogen atom in trifluoroacetic acid showed that the hydrogen atom is cis with respect to the CO bond. A second conformation was not found. From A, E splittings in higher vibrational levels the internal rotation barriers were calculated: for trifluoroacetic acid, V 3 = 241.8 ± 0.5 cm −1 ( v = 4); for trifluoroacetyl fluoride V 3 = 383.6 ± 0.5 cm −1 ( v = 5).

Microwave spectra of partially deuterated acetic acid: Internal rotation, potential energy function, and methyl group geometry

The microwave spectra of CH 2DCOOH and CHD 2COOH have been studied by means of microwave-microwave double resonance. For the asy rotamers torsional splittings (5898 and 530 MHz, respectively) and effective rotational constants were determined in the ground state. Effective barrier parameters were provisionally estimated and used to predict excited-state spectra. Here significant interaction between sy and asy rotamers occurred, and a Hamiltonian based on an extension of the IAM method to the case of an asymmetric internal rotor was used to account for the spectra. A few direct sy-asy transitions were observed as well as spectra originating from the second excited torsional state. Effective potential energy coefficients, V 1 through V 6, were determined accurately; apart from V 3 and V 6, which are comparable to values in CH 3COOH and CD 3COOH, large V 2 terms occur (28.5 cm −1 in CH 2DCOOH and −25.4 cm −1 in CHD 2COOH). These terms provide localization in the ground state wave functions, and can be rationalized as arising from the zero-point energies of the other normal vibrations. Also determined were Fourier components of the rotational constants, which were in fair agreement with results from model calculations when geometry relaxation was included. After correction of the ground state inertial moments for effects of the torsion a consistent set of inertial moments was obtained for the various isotopic species, and a complete substitution structure could be determined. The HCH angles in the methyl group were found to differ by 2.7°.

Microwave spectrum of torsionally excited acetic acid

The microwave spectrum corresponding to the first excited state of the methyl torsion in acetic acid has been identified by means of microwave-microwave double resonance. Although the A- E splittings are extremely large, a reasonable fit has been obtained for the v = 0 and v = 1 states simultaneously by using a Hamiltonian which allows for geometry relaxation upon internal rotation. Barrier parameters are V 3 = 169.90 ± 0.06 cm −1 and V 6 = −6.74 ± 0.02 cm −1. An interpretation of the parameters describing nonrigidity is given in terms of a model with two relaxing bond angles, which is qualitatively supported by ab initio calculations.

Rotational spectrum, hyperfine spectrum and structure of lithium isocyanide

The rotational spectrum of LiNC has been measured for the first time. We succeeded in producing a supersome molecular beam (= 1% LiNC in Ar). The molecular-beam electric-resonance technique has been employed to obtain high-resolution microwave spectra. Two rotational transitions (J = 1 → 0 and J = 2 → 1) of 7LiNC in the ground vibrational state were observed. The hyperfine structure was resolved and identified with the help of microwave-microwave double resonance. The rotational constants B0 and D0 and the hyperfine coupling constants eQq(Li), eQq(N), and c(Li) could be deduced. The J = 1 → 0 rotational transition of 6LiNC was observed in natural abundance. From this we conclude that LiNC has a linear isocyanide structure. The results for the effective structural parameters are: rLiN = 1.760 Å, and rNC = 1.168 Å. We did not observe transitions in excited vibrational states or from LiCN. The agreement between the experimental results and recent ab initio calculations is good.

Microwave–microwave double resonance using a Fabry–Perot cavity spectrometer

The design and operation of a relatively simple, yet versatile spectrometer for microwave–microwave double resonance (MMDR) studies is described. The system consists of a high-Q, semiconfocal Fabry–Perot cavity resonator which is crossed at right angles by a second free-space microwave beam. We excite the cavity with a low-power unmodulated signal source, while the free-space irradiation source is square-wave frequency modulated and is of moderately high power (several hundred milliwatts). Both microwave sources are scannable; the signal source is locked to the cavity while the pumping klystron is free running. The signal (cavity) frequency has been operated in the 15–40-GHz range, and pump frequencies can be selected and easily implemented above or below the signal microwave frequency.

The construction of a microcomputer controlled microwave-microwave double resonance spectrometer incorporating two crossed fabry-perot resonators

The construction of a semi-automatic MW-MW double resonance spectrometer is described, emphasizing the circuitry for the mirror movement, the role of the coupling irises for the pump- and signal resonator respectively, and the realization of high sensitivity.

Modern Aspects of Microwave Spectroscopy

G W Chantry (ed) 1979 London: Academic Press xiv + 513 pp price £38.80 This collection of articles provides an introduction to some of the newer techniques available in microwave spectroscopy, aimed mainly at readers who already have a grounding in the classical techniques of the subject, but wish to be brought up to date on modern developments. After an introductory chapter summarising conventional spectrometer techniques, there follow chapters on microwave-microwave and infrared-microwave double resonance, which stress the extra information and sensitivity to be gained over conventional methods, though at the expense of added experimental complexity.

Internal rotation of molecules with two inequivalent methyl groups: The microwave spectrum of methyl acetate

The microwave spectrum of methyl acetate has been investigated in the frequency range from 8 to 40 GHz. Internal rotation of the two methyl groups splits each overall rotational transition into a quintet. Microwave-microwave double resonance has been used extensively in order to assign these quintets. The dynamical constants for overall and internal rotations have been adjusted from the measured transition frequencies. Accurate transition frequencies have been calculated by diagonalizing large sections of the energy matrices directly. The potential barrier to internal rotation of the acetate CH 3 group has been determined to be 99.56(8) cm −1 whereas that of the methoxy CH 3 group has been determined as 425(31) cm −1.