Pure Rotational Raman Spectra Research Articles

High resolution Raman spectroscopy of gases with laser sources. IX. The rotational spectra of cycloheptane and cyclooctane

AbstractThe pure rotational Raman spectra of cycloheptane (C7H14) and cyclooctane (C8H16) vapor were photographed using a high resolution plane grating spectrograph. Both spectra resemble those of symmetric top rotors. The spectrum of cycloheptane has well developed R‐ and S‐branches while for cyclooctane the R‐branch lines are only just observable. The molecular constants obtained for these two molecules are for cycloheptane: B =0.099 3801±0.000 0047 cm−1 and DJ =(4.08± ±0.17) × 10−8 cm−1 while for cyclooctane: B0.073 221±0.000 039 cm−1 and DJ =(0.784±0.039) × 10−8 cm−1 where the quoted errors are one standard deviation. Since both molecules exist at room temperature in several conformations these constants are average values over the various conformers.

Pure rotational Raman spectra of the chlorine species, 35Cl2 and 35Cl37Cl

The rotational constants and bond lengths of 35Cl2 and 35Cl37Cl have been redetermined from the photographically-recorded pure rotational Raman spectra. For 35Cl2 the precision of the results equals that of the best values obtained from the visible absorption spectrum; for 35Cl37Cl the precision of the present result is much greater than that of the only previous determination. Using α-values for 35Cl2 and 35Cl37Cl derived from electronic spectroscopy, Be, and hence re, values have also been Calculated. Combining the present results with the best electronic spectroscopy results gives for 35Cl2 the weighted mean bond lengths; r0= 1.991 0 ± 0.000 2 Å(1 Å= 100 pm) and re= 1.988 2 ± 0.000 2 Å. For 35Cl37Cl the present work gives r0= 1.991 7 ± 0.000 5 Å and re= 1.988 6 ± 0.000 5 Å.

Pure rotational Raman spectrum of fluorine

The pure rotational Raman spectrum of 19F2 has been recorded photographically with a reciprocal linear dispersion of 1.4 cm–1 mm–1 and analysed to yield; B0= 0.883 31 ± 0.000 04 cm–1; 106D0= 3.48 ± 0.06 cm–1; r0= 1.417 44 ± 0.000 06 A. Assuming from earlier work, B1= 0.872 ± 0.002 cm–1, the following equilibrium constants were obtained; Be= 0.889 ± 0.001 cm–1; re= 1.4131 ± 0.000 8 A.These are the most precise values yet reported for B0, D0 and r0, re of 19F2. The experimentally determined D0 value is in good agreement with the value calculated from B0 and ω0.

Pure Rotational Raman Spectra of 79Br2 and 81Br2

The pure rotational Raman spectra of the lighter halogen molecules have been studied by several authors. Andrychuk, Claassen et al., Stricker and Hochenbleicher, and Edwards et al., reported the spectra of fluorine, whereas Hochenbleicher and Schrotter and Hendra and Vear were able to record the pure rotational Raman spectrum of chlorine. Accurate values of the rotational constant B0 have been obtained for these molecules. However, because of the low value of B0 the pure rotational Raman spectrum of the heavier halogen molecules could not be resolved until now. In the course of an extensive resonance Raman spectroscopic study on natural bromine (which is a mixture of the molecules 79Br2, 79Br81Br and 81Br2) as well as on the isotopically pure molecules 79Br2 and 81Br2, we have also studied the rotational Raman spectra of the isotopically pure species 79Br2 and 81Br2. The results will be presented in this note, where it is shown that the pure rotational Raman spectra of fairly heavy linear molecules can be obtained directly with photoelectric recording.

ChemInform Abstract: RAMAN SPECTRA OF ASYMMETRIC TOP MOLECULES PART 3, INTENSITY BEATS IN THE RAMAN SPECTRA OF NEAR PROLATE TOPS

Intensity beats have been observed in the pure rotational Raman spectra of several near prolate asymmetric top molecules. Comparison of the observed spectra with computer simulated ones allows, from a knowledge of A0 and text-decoration:overlineB0, the determination of both B0 and C0 for butadiene: B0= 0.148 7 ± 0.000 2 cm–1, C0= 0.133 9 ± 0.000 2 cm–1.

ChemInform Abstract: ROTATIONAL RAMAN SPECTRUM OF SULPHUR DIMER

The pure rotational Raman spectrum of S2 has been photographed. After allowance has been made for line blending, the rotational constants obtained were: B0= 0.294 43(± 0.000 05) cm–1 and D0= 19.0(± 0.5)× 10–8 cm–1.

The analysis of symmetric rotor Raman spectra: The pure rotational spectrum of 11BF3

The pure rotational Raman spectrum of 11BF3 has been photographed. Great care was taken in the analysis to consider all the unresolved components under each observed Raman line profile. If this is ignored, systematic errors result. The final set of molecular constants obtained was B0 = 0.34502(±3 × 10−5)cm−1, DJ = 4.38(±0.10) × 10−7cm−1, and DJK = −9.1(±1.0) × 10−7cm−1.

Raman spectra of asymmetric top molecules. Part 3.—Intensity beats in the Raman spectra of near prolate tops

Intensity beats have been observed in the pure rotational Raman spectra of several near prolate asymmetric top molecules. Comparison of the observed spectra with computer simulated ones allows, from a knowledge of A0 and text-decoration:overlineB0, the determination of both B0 and C0 for butadiene: B0= 0.148 7 ± 0.000 2 cm–1, C0= 0.133 9 ± 0.000 2 cm–1.

High resolution Raman spectroscopy of gases with laser sources. The rotational spectrum of cyanuric fluoride-C 3N 3F 3

The pure rotational Raman spectrum of cyanuric fluoride vapor was photographed using a high resolution plane grating spectrograph. The spectrum was excited with the λ = 4880 A ̊ radiation emitted by a single-mode argon-ion laser. Two sets of molecular constants were determined from the R and S branches. The preferred results are those determined from the S-branch data. These are: B 0 = 0.0655954 ± 14 × 10 −7 cm −1, D J = (2.52 ± 0.17) × 10 −9 cm −1 and H J = (−1.59 ± 0.59) × 10 −14 cm −1, where the uncertainties are one standard deviation. Possible effects of line shifts due to unresolved K structure and the presence of hot bands on the accuracy of the values of the molecular constants are discussed. The B 0 value is compared to the rotation constant computed with the structural parameters determined with the electron diffraction technique; the agreement between these two rotation constants is only fair.

Rotational Raman spectrum of sulphur dimer

The pure rotational Raman spectrum of S2 has been photographed. After allowance has been made for line blending, the rotational constants obtained were: B0= 0.294 43(± 0.000 05) cm–1 and D0= 19.0(± 0.5)× 10–8 cm–1.

Raman spectra of asymmetric top molecules. Part 2.—ΔK=± 2 transitions in the rotational spectrum of butadiene

ΔK= 2 transitions have been observed in the pure rotational Raman spectrum of butadiene. Rotational constants were determined for the ground state, ν13 torsional mode, ν24 bending mode and the 2ν13 torsional state. Evidence of a Coriolis interaction between ν13 and ν24 was found and the rotational constants derived from those two states were corrected for this specific interaction. A–text-decoration:overlineB values obtained for the four states were 1.247 82 ± 0.000 09 cm–1, 1.226 70 ± 0.000 24 cm–1, 1.2703 ± 0.0010 cm–1 and 1.167 ± 0.002 cm–1 for the ground, ν13, ν24 and 2v13 vibrational states respectively. In addition DK distortion constants of (3.61 ± 0.18)× 10–6 cm–1 and (2.6 ± 0.5)× 10–6 cm–1 were determined for the ground and ν13 states respectively.

The rotational resonance Raman spectrum of nitrogen dioxide and the determination of electronic state symmetry from resonance Raman selection rules

The pure rotational Raman spectrum of nitrogen dioxide has been observed and shown to be consistent with existing determinations of molecular parameters. Upon observation at 600 Torr pressure and 0.4 cm −1 resolution a well-defined rotational spectrum is obtained. This spectrum is overlaid with a number of fluorescence lines. The fluorescence lines are separated from the Raman spectrum by a comparison of Stokes and anti-Stokes branches of the rotational spectrum. Out of seven strong fluorescence lines seen with 5145 Å excitation, five probably are identifiable with vibration-rotation fluorescence progressions observed by Abe. The most striking feature of these observations is the potential use of the resonance Raman effect for the analysis of complicated electronic spectra. When this rotational spectrum is observed with excitation by 5309 Å or 5145 Å excitation, the Raman spectrum follows a-axis selection rules and the Q-branches are in the noise level or barely out of it. However, at 4880 Å the Δ K = 2 Q-branches become a major feature of a spectrum, indicating that an appreciable part of the absorption at this wavelength is occurring through the operation of b- or c-axis selection rules. These findings are consistent with present notions of a 2 B 2 excited state dominating absorption at longer wavelengths, while at shorter wavelengths a 2 B 1 excited state becomes important. Given a tunable laser, one could map the relative importance of these two possible selection rules for NO 2 without any theoretical analysis more sophisticated than that presented in this paper. A simplified statement of the selection rules for resonance rotational Raman spectra of asymmetric tops has been developed in the course of this investigation. No attempt has been made to refine the rotational parameters of NO 2 since all of the lines seen areunresolved multiplets. Our data should be regarded as a search spectrum preliminary to investigation on a high resolution instrument.

Study of the rotational Raman spectra of14N15N and15N2, using a Fabry–Perot etalon

A Fabry–Perot etalon has been used to study the pure rotational Raman spectra of 14N15N and 15N2. The rotational constants obtained are For 14N15N B0= 1.923604 ± 0.000020 cm–1, D0=(5.297± .038)× 10–6 cm–1. For 15N2B0= 1.857672 ± 0.000027 cm–1, D0=(5.201± .048)× 10–6 cm–1.A value is deduced for the equilibrium N2 bond length as re= 1.097632 (±0.000020)A.

Pure rotational Raman scattering by symmetric top molecules

The distribution of intensities between the branches of the pure rotational Raman spectrum of a symmetric top molecule is calculated according to the quantum mechanical method of Placzek and Teller. The total intensities are obtained by summing over the individual line intensities associated with transitions from rotational state J,K to state J′,K′. The exact distribution depends on the temperature of the system and the detailed geometry of the symmetric top molecule. In the classical (i.e., high temperature) limit, the fraction of intensity due to transitions for which Δ J=0 is found to be 0.25 for a linear molecule, to decrease to a minimum of approximately 0.167 for a prolate symmetric top, and to increase to a maximum of 0.254 for the limit of a disk. This fraction for an oblate symmetric top is in the range 0.20–0.254. Thus, measurements of the distribution of intensities of the branches of the pure rotational Raman spectrum can be used to obtain structural information for symmetric top molecules.

Quantum scattering theory of rotational relaxation and spectral line shapes in H2–He gas mixtures

A systematic study is presented of the rotational relaxation and spectral line shape properties of dilute gas mixtures of H2 in He, in an effort to determine the radial and angular dependence of the H2–He intermolecular potential. The quantum mechanical theory of relaxation in gases is reviewed, and we express the results in terms of a matrix of cross sections that determines each correlation function, and thus the relaxation properties of the system. The cross section is calculated from binary collision transition amplitudes, or S matrix elements, for H2–He scattering. A Morse-spline-fitted-van der Waals potential of the form V0(R)+V2(R)P2(cosθ) is assumed and we treat the hydrogen molecule as a rigid rotor. The Schrödinger equation is solved in the close-coupling approximation using the method of Gordon. We apply the theory to sound absorption measurements of rotational relaxation, NMR spin-lattice relaxation times, and the S and Q branches of the pure rotational Raman spectrum of H2–He mixtures. Elastic phase shift and reorientation effects are examined, along with the effects of energetically inelastic collisions. We find asymptotically closed channels to be unimportant, but the rigid rotor approximation breaks down in calculating the Raman line shifts due to the vibration-rotation interaction. This effect is accounted for by assuming different radial potentials V o(R) for scattering in two spectroscopic (vibrational) states. We find a potential that can accurately fit all the experimental data as well as the theoretical calculations of the long- and short-range behavior of the intermolecular potential.

High resolution Raman spectroscopy of gases with laser sources. VII: The rotational spectrum of cyclohexane‐C6H12 and C6D12

AbstractThe pure rotational Raman spectra of cyclohexane‐d0 and cyclohexane‐d12 have been photographed under high resolution using a single mode argon laser as the exciting source. The spectra consist of reasonably sharp lines which form well developed R‐ and S‐branches. The molecular constants found from the analysis of the spectra are, for C6H12: B0 = 0.143429 ± 2 × 10−6 cm−1, DJ = (1.56 ± 0.03) × 10−8 cm−1 and, for C6D12: B0 = 0.109469 ± 1 × 10−6 cm−1 and DJ = (0.61 ± ± 0.01) × 10−8 cm−1. These results and possible molecular structures are discussed in terms of the results of recent electron diffraction studies of cyclohexane.

A Raman instrument for rotation‐vibrational spectra using laser excitation

AbstractA description is given of an instrument designed for the registration of rotation‐vibrational Raman spectra. The gas sample cell has a volume of 20 ml and forms one end of the cavity of an argon‐ion laser. The laser beam is focused to a diameter of 50 μ inside the cell. The scattered light passes a double premonochromator and is registered photographically in a grating spectrograph.The resolution obtained is about 0.3 cm−1, determined by the pressure broadening in the sample. The detectability for weak lines is high, as demonstrated by the observation of previously unobserved vibrational bands.The instrument may also be used for pure rotational Raman spectra, primarily for weak scatterers. A pure rotational spectrum of CO2 is obtained in an exposure of ½ min.

High resolution Raman spectroscopy of gases with laser sources. VI. The rotaitional spectra of 1,3,5‐trifluorobenzene and hexafluorobenzene

AbstractThe pure rotational Raman spectra of 1,3,5‐trifluorobenzene and hexafluorobenzene have been photographed under high resolution using a single mode argon laser as the exciting source. The observed spectra consist of sharp, highly symmetric lines which form well developed R‐ and S‐branches. The analysis of the spectra was performed on the basis of the theory of the non‐rigid, symmetric top rotor. The following ground state constants were determined. For 1,3,5‐trifluorobenzene, Bo = 0.0586517 ± (5 × 10−7) cm−1 and Dj = (1.89 ± 0.04) × 10−9 cm−1; for hexafluorobenzene, Bo = 0.0343165 ± 9 × 10−7 cm−1 and Dj = (0.70 ± 0.04) × 10−9 cm−1. The CF bond lengths for these molecules were determined by assuming the values of the carbon ring parameters of benzene. The CF bond lengths (r0‐values) of 1,3,5‐trifluorobenzene and hexafluorobenzene are then, respectively, 1.304 Å and 1.321 Å. These values are in good agreement with the CF bond lengths determined by means of the electron diffraction technique.

The pure rotational laser Raman spectrum of chlorine

The pure rotational laser Raman spectrum of chlorine, in natural and isotopically enriched 35Cl 2 forms were photographed at high resolution. The spectral data was analysed to give rotational constants for the two major isotopic species, 35Cl- 35Cl and 35Cl- 37Cl. Values obtained were 35Cl- 35Cl, B 0″ = 0.2433 ± 0.0006 cm −1; 35Cl- 37Cl, B 0″ = 0.2365 ± 0.0006 cm −1. Various features of the spectra are discussed with reference to a synthetic spectrum.

Raman and Rayleigh spectroscopy and molecular motions

Extensive measurements of vibrational and, in a few cases, pure rotational Raman spectra of HCl, DCl, HBr and DBr dissolved in SF6, C2F6, CCl4 and SO2 have been carried out at room temperature. Band profiles of both isotropic and anisotropic scattering have been studied and discussed in the light of existing theories on molecular motions in liquids. From isotropic scattering band-fitting it is concluded that, in the case of the three non-polar solvents, broadening due to vibrational fluctuations is small and that the band shape comes essentially from vibration-rotation coupling. Thus information about reorientational motions can be obtained directly from anisotropic scattering. For SO2 solutions this statement is no longer valid because of the important contribution of vibrational fluctuations to the profile of the isotropic scattering. In this case the rotational correlation function can be determined, as for pure liquid hydrochloric and hydrobromic acids, by elimination of the vibrational correlation fu...