Radiofrequency-microwave Double Resonance Research Articles

Microwave spectral studies of 2,4-difluorobenzaldehyde: O-trans conformer

The ground state microwave rotational spectra of 2,4-difluorobenzaldehyde (2,4-DFBD) vapor have been investigated in the frequency range 18.0–37.0GHz using conventional microwave as well as Radiofrequency-Microwave Double Resonance (RFMWDR) spectroscopic techniques. A least-squares analysis of 50 a-type R-branch lines provides a set of rotational and centrifugal distortion constants which corresponds to the O-trans conformer of the molecule. The rotational constants are: A=2498.977 (39)MHz, B=958.043 (2)MHz and C=692.394 (2)MHz. Detailed DFT calculations were carried out with various functional and basis sets to evaluate the spectroscopic constants, dipole moment and various structural parameters of the O-trans conformer of 2,4-DFBD for the ground state. It was found that the optimized geometry with B3LYP/6-311++g(2d,2p) basis set shows good agreement with the experimental values of the rotational constants.

Microwave spectrum of trans 3-fluorophenol in excited torsional states

The microwave rotational spectra of the trans conformer of 3-fluorophenol have been observed in excited torsional states and analyzed in the frequency range 12.0–43.0 GHz using conventional microwave and Radio-Frequency Microwave Double Resonance (RFMWDR) techniques. Analysis of the ground torsional state spectrum has been extended to higher rotational states. Least-squares analysis of three sets of rotational transitions yield rotational and centrifugal distortion constants for the ground and first two excited torsional states. A nonlinear behavior of the variation of inertial defect with the torsional quantum number was observed.

Microwave spectrum of 2,4-difluorophenol: cis conformer

The ground state microwave rotational spectra of 2,4-difluorophenol have been investigated and analyzed in the frequency region: 18.0–37.0 GHz using Radiofrequency-Microwave Double Resonance (RFMWDR) and conventional microwave spectroscopic technique. A least-squares analysis of a-type R-branch and b-type Q-branch lines yields a set of rotational and centrifugal distortion constants. The derived rotational constants correspond to the cis conformer of the molecule. A tentative r 0-structure of the cis conformer is proposed.

Microwave spectrum and quadrupole coupling constants of 13CF 3Br

Ground state microwave rotational spectra of 13CF 3Br have been investigated and analyzed in the frequency region: 20.0–37.0 GHz using radiofrequency–microwave double resonance (RFMWDR) and conventional microwave spectroscopic technique. Analysis of rotational transitions includes both isotopic species of Br, viz. 13CF 3 79Br and 13CF 3 81Br. Rotational, centrifugal distortion and quadrupole coupling constants for both isotopes are derived. Finally, an r z structure with precise C–Br, C–F bond lengths and ∠FCF bond angle using the observed rotational constants and the reported values of the parent species for both isotopes has been proposed after correction of the constants for six vibrational modes.

Microwave spectra and ab initio calculations of 1-nitropyrazole

The microwave (MW) spectrum of 1-nitropyrazole obtained at room temperature is presented, together with ab initio MP2/6-31G∗∗ calculations on its structure and harmonic vibrational frequencies. The initial assignment of the spectrum was carried out using a radio frequency—microwave double resonance (RFMWDR) technique. Both experimental and theoretical results indicate that 1-nitropyrazole is a planar molecule. The structure obtained from the MW spectrum agrees fairly well with that obtained at the MP2(FULL)/6-31G∗∗ level.

One- and two-dimensional Fourier transform spectroscopy in the microwave and radiofrequency range of trifluoromethane in the vibrational state ν 6 = 1

For trifluoromethane, CF 3H, A 1-A 2 splittings with K = | k- l| = 3 in the degenerate vibrational state ν 6 = 1 have been determined using two-dimensional radiofrequency-microwave double resonance Fourier transform spectroscopy. The observation of direct l-type resonance transitions with K = 3 has been extended to lower J values by means of microwave Fourier transform spectroscopy. The analysis yielded improved values for the rotational constant C 6, for the Δ l= 0, Δ k= ± 6 interaction constant w 6 and enabled the determination of a J dependence of the Δ l= ± 2, Δ k= ∓ 4 t-type interaction constant.

Easy assignment of rotational spectra of slightly abundant isotopic species in natural abundance: 13C and 15N isotopic species of pyrimidine

The microwave spectra of 4- 13C and 1- 15N isotopic species of pyrimidine in natural abundance have been easily assigned by using the radiofrequency-microwave double resonance technique. The pump radiofrequency radiations were connecting several K −1 almost degenerate levels, these have been effective thanks to the μ a dipole moment component induced from the asymmetric isotopic substitution.

Radiofrequency-microwave double resonance with a dipole moment component induced by asymmetric isotopic substitution: Microwave spectrum of C 4O 2Cl 2

The microwave spectrum of C 4O 2Cl 2 (1,2-dichlorocyclobuten-3,4-dione) has been investigated for both the C 4O 2 35Cl 2 and the C 4O 2 35Cl 37Cl isotopomers. The spectrum of the latter has been studied in greater detail because a number of μ b rotational transitions were measured with the radiofrequency-microwave double-resonance technique by radiofrequency pumping of some K −1 doublets. This pumping has been made possible since a small μ a dipole moment component appears owing to the asymmetric isotopic substitution.

Radiofrequency Microwave Double Resonance Experiments in Fourier Transform Technique

Abstract Radiofrequency microwave double resonance has proved as a valuable method in microwave spectroscopy in the frequency domain. We present comparable experiments in the time domain Fourier transform spectroscopy.

Nuclear quadrupole structure of potassium hydroxide in the 32 GHz microwave region

A high temperature Stark and radiofrequency-microwave double resonance spectrometer was used to observe the nuclear quadrupole structure of the J=2←1 transition of 39KOH in a number of vibrational states. First order analysis of the hyperfine components gave nuclear quadrupole coupling constants of eQq=−7.23 (29) MHz for the (0000) state, eQq=−7.16 (33) MHz for the (1000) state, and eQq=−7.29 (37) MHz for the (0110) state.

Interpretation of the microwave spectrum of 2-methoxy ethylamine using its ab initio structures

The 7 3 ← 6 3 μ a K −1 doublet line series of 2-methoxy ethylamine, previously recorded with the radiofrequency microwave double resonance technique, was interpreted for the T and G conformations of this compound with a number of structural models including the 4-21G optimized geometries. A single model of G can reproduce the doublet series as the vibrational progression of the CO skeletal mode, starting with the υ CO = 0 groundstate at 35 457 MHz, and stepping in approximately 95 MHz intervals up to the vibrational satellite of υ CO = 9, with the υ CO = 5 doublet missing. When model calculations are performed for T with the structural parameters of G which reproduce the 7 3 ← 6 3 K −1 doublet series, except for rotating the NH 2 group by 120° to obtain T, the frequency of its groundstate doublet is found at lower frequencies than that of G. In contrast to this, when the empirically corrected 4-21G parameters of T are used in the analysis, the calculated groundstate of T coincides with the expected υ CO = 6 doublet of G. The 4-21G geometries of G and T are in good agreement with the rotational constants and the conformational energy difference derived from the microwave spectra. Thus, this analysis clearly confirms the assignment of the observed 7 3 ← 6 3 K −1 series as the progressions of the υ CO = 0 to υ CO = 4 states of G, and the υ CO = 0 to υ CO = 3 states of T.

Microwave spectral study of 3-fluorobenzonitrile

The microwave spectrum of 3-fluorobenzonitrile has been observed and analyzed in the KU (12.5–18.0 GHz) and the K (22.0–26.0 GHz) bands. Both Stark and radio frequency-microwave double resonance spectroscopy have been used in the observation and assignments. Analysis of the spectrum yields three rotational constants (all in MHz), A = 3388.59 ± 0.02, B = 1186.64 ± 0.01, C = 878.72 ± 0.01, and five quartic centrifugal distortion terms (all in MHz), d J = (1.7 ± 0.6) × 10 −4 , d JK = (−4.9 ± 1.1) × 10 −3 , d K = (−10.5 ± 2.4) × 10 −3 , d WJ = (−1.0 ± 1.0) × 10 −7 , d WK = (4.8 ± 1.1) × 10 −6 , for the molecule. An r 0 structure has been proposed which does not suggest serious distortion of the benzene ring. The inertia defect of the molecule is 0.1 amu A ̊ 2 .

Microwave spectral study of 2-fluorophenol: cis conformer

The microwave spectra of 2-fluorophenol and its deuterated species have been observed and analyzed in the frequency ranges 12.5–18.0 GHz (KU band) and 21.5–26.0 GHz (K band) in the ground vibrational state at room temperature. For the normal species, the radio frequency-microwave double resonance spectrum has been recorded in the frequency range 30.0–38.0 GHz. Three rotational and five quartic centrifugal distortion constants for the normal species, A ̃ = 3337.86 ± 0.02 , B ̃ = 2231.92 ± 0.01 , C ̃ = 1337.52 ± 0.01 , d J = (3.5 ± 2.9) × 10 −4, d JK = (−4.9 ± 1.5) × 10 −3, d K = (−3.2 ± 1.0) × 10 −3, d WJ = (−2.0 ± 1.0) × 10 −7, d WK = (2.6 ± 0.8) × 10 −6 (in MHz), and three rotational constants for the deuterated species, A ̃ = 3324.70 ± 0.03 , B ̃ = 2177.95 ± 0.03 , C ̃ = 1315.96 ± 0.03 (in MHz), have been obtained. Consideration of the r s coordinate of the hydroxyl hydrogen atom leads to the assignment of the spectra to the cis conformer of the molecule. An r 0 structure for the cis conformer has been proposed. The nonbonded OH ⋯ F distance is lower by about 0.3 Å than the sum of the van der Waals radii.

Rotational spectrum of trifluoroidomethane

The rotational spectra for the ground vibrational states of 12CF3I and 13CF3I have been re-examined with greater precision and at frequencies up to 204 GHz or J″= 66 and up to 115 GHz for J″= 37, respectively. The rotational spectra of the excited vibrational states ν3= 1 and ν6= 1 have been measured and assigned, with the help of radiofrequency–microwave double resonance, for the 12CF3I species up to J″= 40 and 45, respectively; i.e. frequencies of 125 and 140 GHz. The states are distinguished from each other by the clear effects of the angular momentum of ν6= 1 state, for which |l|= 1. Anomalous values of the centrifugal distortion terms are examined and shown to be due to a Coriolis resonance between these two states which are only ca. 20 cm–1 apart. Full details of the rotational parameters are given in tables 1 and 2.

Measurement of hyperfine splitting in the OH radical by a radio-frequency microwave double resonance method

The RFMDR technique is used to investigate the hydroxyl radical spectrum. The experiment allows to analyse the magnetic hyperfine structure for J = 7/2 and 9/2 in the 2Π 3/2 state and gives more complete and accurate results than in previous works.

Ein Beitrag zur Radiofrequenz-Mikrowellen-Doppelresonanz-Methode / A Contribution to the Radiofrequency Microwave Double Resonance Method

Abstract Experiments are reported which extend the useful Radiofrequency Microwave Double Resonance Method to higher Radiofrequencies. A special absorption cell was constructed for this purpose.

Radio frequency-microwave double resonance as a tool in the analysis of microwave spectra

The practical application of radio frequency-microwave double resonance as a spectroscopic technique in the analysis of microwave spectra has been investigated. Application of a high-power amplitude-modulated radio-frequency field to a gaseous sample in a waveguide cell provides a means of detecting microwave transitions, analogous to the situation in microwave-microwave double resonance. The sensitivity of the technique compares favorably with that of Stark modulation. Near-symmetric top molecules with allowed K-type doublet transitions are especially amenable to the technique. Accidental near degeneracies of dipole-connected rotational levels in asymmetric top molecules are also useful. A description of a radio frequency-microwave double resonance spectrometer is given together with a discussion of the special problems associated with radio frequency circuits. Applications of the technique are discussed with examples.