Pure Rotational Raman Spectra Research Articles

On the use of a Fabry—Perot etalon for the determination of rotational constants of simple molecules—the pure rotational Raman spectra of oxygen and nitrogen

The use of a Fabry–Perot etalon for the study of the pure rotational Raman spectra of oxygen and nitrogen is described. By using an etalon having a plate separation of 12 mm, a resolution of 0.1 cm -1 is readily achieved. The use of a Fabry-Perot etalon allows the wavelength of the individual rotational lines to be measured with high accuracy and leads to precise values for the rotational constants of oxygen and nitrogen. The rotational constants obtained are for N 2 B 0 = 1.98950 6 ±0.00002 7 cm -1 , D 0 = (5.48±0.05 5 ) x 10 -6 cm -1 ; for O 2 B 0 = 1.437682 ±0.000009 cm -1 , D 0 = (4.85 2 ± 0.01 2 ) x 10 -6 cm -1 . The values determined for oxygen are in excellent agreement with recent microwave measurements of these constants.

The pure rotational Raman spectrum of CS 2 in the ground and 01 10 vibrational states

The pure rotational Raman spectra of CS 2 in the 00 00 and 01 10 vibrational states have been observed using a low power HeNe laser (λ = 6328 Å) and a high resolution plane grating spectrograph. The Δ J = 2 transitions with J = odd in the 01 10 state are clearly resolved from the ground state transitions thus allowing the determination of some upper state rotational constants. The molecular constants determined in this work are for the 00 00 ground state, B 00 00 = 0.10912 ± 0.7 × 10 −5 cm −1, ( D J ) 00 00 = (0.83 ± 0.18) × 10 −8 cm −1 and for the 01 10 excited state B 01 10 c = 0.10935 ± 0.00002 cm −1 and ( D J ) 01 10 c = (1.5 ± 0.6) × 10 −8 cm −1.

Theoretical calculations of electronic Raman effects of the NO and O2 molecules

This paper presents a method for calculating the relative intensities and Raman shifts of the rotational structure in electronic Raman spectra of diatomic molecules. The method is exact in the sense that the wave functions used for the calculations may belong to any intermediate case of Hund's coupling schemes. Using this method, theoretical calculations of the pure rotational and electronic Raman spectrum of NO, and the pure rotational Raman spectrum of O2, are presented. Although a calculated stick spectrum for NO was previously shown by Fast et al., the details of this calculation are given here for the first time.

Rotational Raman spectrum of nitric oxide

The pure rotational Raman spectrum of nitric oxide has been recorded for the first time under high resolution. Analysis of the S-branch transitions yields values of 1.69614 cm−1 and 5.46 × 10−6 cm−1 for the rotational constants B0 and D0, respectively. A series of R-branch lines is observed and is shown to arise from transitions between levels in the 2Π3/2 substate. Some weaker R-branch lines arising from the 2Π1/2 state are also observed. A very weak feature approximately 120 cm−1 from the exciting line is interpreted as the 2Π3/2–2Π1/2 transition.

High-Resolution Raman Spectroscopy of Gases with cw-Laser Excitation*

Several techniques of exciting Raman spectra of low-pressure gases and vapors with cw laser radiation are described. Both the He–Ne laser with output at 6328 a and the Ar+ ion laser with output at 4880 and 5145 a have been used. The use of the He–Ne laser with a multiple-pass Raman tube inside the laser cavity yields Raman spectra which can be photographed in exposure times that are equivalent to those required with the classical Hg-arc and large-volume Raman tube as the source of scattered radiation. The Ar+-ion laser allows the photoelectric recording of rotational Raman spectra at atmospheric pressure with a signal/noise ratio of 10:1 when the Raman tube is located outside the laser cavity, and about 100:1 when the Raman tube is inside the laser cavity. A comparison is given for the relative efficiency when the scattering gas is located inside and outside the laser cavity. Pure rotational Raman spectra for N2, O2, CO2, and H3CC≡CH (methylacetylene) are presented. The depolarization ratios of both the Rayleigh and Raman lines of N2, O2, and CO2 have been determined and relative scattering cross section for the pure rotational Raman scattering of oxygen has been obtained.

The pure rotational Raman spectra of furan and cyclopentene

The pure rotational Raman spectra of furan and cyclopentene were photographed with a plane grating spectrograph in the thirteenth order of a 300 grooves/mm blazed grating using the line Hg 4358 Å for excitation. The spectra resemble those of a symmetric top rotor with well developed R- and S-branches. The analysis of the spectra is based on the theory of the nonrigid symmetric top rotor and yields the following molecular constants. For furan, B 0 = 1 2 (A 0 + B 0) = 0.31154 ± 0.00006 cm −1 and D J = (12.5 ± 2.5) × 10 −8, cm −1; for cyclopentene, B ̃ 0 = 1 2 (A 0 + B 0) = 0.24165 ± 0.00005 cm −1 and D J = (3.75 ± 1.25) × 10 −8, cm −1. The B ̃ 0 values are in fair agreement with those calculated from microwave data. The D J -value has not been reported previously for furan. For cyclopentene, the D J -value deduced from the Raman data shows that there are no extraordinary centrifugal distortion effects, in contrast to the conclusions derived from the analysis of the microwave rotation spectrum.

The pure rotational Raman spectra of 1,4-cyclohexadiene and 1,3,5-cycloheptatriene

The pure rotational Raman spectra of 1,4-cyclohexadiene and 1,3,5-cycloheptatriene were photographed with a plane grating spectrograph in the thirteenth order of a 300 groove/mm blazed grating. The spectra resemble those of symmetric top rotors with well-developed R- and S-branches. The molecular constants were found to be B ̃ 0 = 1 2 (A 0 + B 0) = 0.1671 1 ± 0.00006 cm −1 and D J = (2.1 ± 0.4) × 10 −8 cm −1 for 1,4-cyclohexadiene and B ̃ 0 = 1 2 (A 0 + B 0) = 0.1226 4 ± 0.00005 cm −1 and D J = (1.4 ± 0.5) × 10 −8 cm −1 for 1,3,5-cycloheptatriene. The results of this study lend support to the molecular structures that have been derived on the basis of infrared and Raman vibrational spectra.

The pure rotational Raman spectrum of chloroform

The pure rotational Raman spectrum of CHCl 3 vapor has been photographed under high dispersion in the thirteenth order of a 300 groove/mm plane grating. The spectrum is that of a symmetric top rotator with an average line spacing of 0.44 cm −1 for the S-branch. Transitions were observed in the S-branch from J = 35 to J = 84. Using the value B 0 = 0.110141 cm −1 determined by microwave spectroscopy for CHCl 3 35, the Raman data yield for the centrifugal distortion constant D J the value D J = (6.2 ± 2) × 10 −8, cm −1. This result is in fair agreement with the value calculated from force constants but differs considerably from the results of the most recent microwave investigations.

HIGH-RESOLUTION RAMAN SPECTROSCOPY OF GASES: XVII. THE PURE ROTATIONAL RAMAN SPECTRUM OF IODOACETYLENE

A study of the pure rotational Raman spectrum of iodoacetylene has yielded a value of 0.10622 cm−1 for the ground-state rotational constant. From this value, and from assumed C≡C and C—H bond lengths of 1.203 Å and 1.055 Å respectively, the C—I bond length is calculated to be 1.988 Å.

The pure rotational Raman spectrum of cyclopentane-d 0 and cyclopentane-d 10

The pure rotational Raman spectra of C 5H 10 and C 5D 10 were photographed under high dispersion in the 13th order of a 300 grooves/mm plane grating using the line Hg 4358 Å for excitation. The spectra are quite weak and diffuse and resemble that of a symmetric top rotator. The line spacing in the S branches is nearly constant with a value of about 0.864 cm −1 (C 5H 10) and 0.644 cm −1 (C 5D 10). Transitions with J up to 67 in the ordinary and up to 76 in the deuterated compound were observed in the S-branch. A number of R-branch lines were also observed. The analysis of the spectra is based upon the symmetric rotor theory and yields the following ground-state constants: C 5H 10 : B 0 = 0.2160 5 ± 0.0010 cm −1, D = (5.0 ± 1.4) × 10 −8 cm −1; C 5D 10 : B 0 = 0.1610 2 ± 0.00012 cm −1, D j = (4.0 ± 2.2) × 10 −8 cm −1; If a planar model is assumed for the carbon ring ( D 5h point group) the length of the CC bond is computed to be r CC = 1.537 0 ± 0.0024 A ̊ . The cyclopentane molecule is not, however, a strict symmetric top as is known from the careful analysis of the thermodynamic functions and the vibrational spectrum. The present study does not admit any choice between the permissible C 2, C S , and C 1 symmetries.

The Raman spectrum of gaseous oxygen

The pure rotational and rotation-vibrational Raman spectrum of oxygen gas at 1 atmosphere pressure has been photographed under high dispersion in the first order of a 15,000 lines/inch, 21-foot concave grating in Wadsworth mounting. The molecular constants are δ ν0=δG 1 2 =1556.26, B e=1.4457, α=0.0158, D 0=D ν=5.6 × 10 −6 cm −1 These results are in good agreement with those obtained from the study of the electronic spectrum.

HIGH RESOLUTION RAMAN SPECTROSCOPY OF GASES: XII. ROTATIONAL SPECTRA OF C2H4 AND C2D4 AND THE STRUCTURE OF THE ETHYLENE MOLECULE

The pure rotational Raman spectra of C2H4 and C2D4 were photographed in the second order of a 21-ft grating. The resolution achieved was such that several lines due to single transitions were observed (and identified) in the spectra of both molecules. An analysis of these lines based on the non-rigid asymmetric top yielded the following rotational constants for the ground states:[Formula: see text]The structural parameters obtained from these constants are r0(C=C) = 1.339 ± 0.002 Å, r0(C—H) = 1.086 ± 0.003 Å, and [Formula: see text]. These values are in agreement with the recent preliminary results of the electron diffraction data. There is also agreement between the above values and those obtained from a recent study of the rotation–vibration spectra of C2H4 and C2D4, although the agreement between the ground state rotational constants is not as satisfactory.

HIGH RESOLUTION RAMAN SPECTROSCOPY OF GASES: VIII. ROTATIONAL SPECTRA OF ACETYLENE, DIACETYLENE, DIACETYLENE-d2, AND DIMETHYLACETYLENE

The pure rotational Raman spectra of acetylene (C2H2), diacetylene (C4H2 and C4D2), and dimethylacetylene (C4H6) have been photographed with a 21-ft. grating spectrograph. The rotational constants were found to be B0(C2H2) = 1.1769 cm−1, B0(C4H2) = 0.14689 cm−1, B0(C4D2) = 0.12767 cm−1, and B0(C4H6) = 0.1122 cm−1. An analysis shows that r0(C—H) in diacetylene is shorter than in acetylene and cyanoacetylene; in dimethylacetylene at least one of the carbon–carbon bonds is longer than in the closely related molecule methylacetylene.

HIGH RESOLUTION RAMAN SPECTROSCOPY OF GASES: VII. ROTATIONAL SPECTRA OF s-TRIAZINE AND s-TRIAZINE-d3

The pure rotational Raman spectra of s-triazine (C3H3N3) and s-triazine-d3 (C3D3N3) vapor were photographed in the second order of a 21-ft. grating spectrograph. The rotational constants were found to be B0(C3D3N3) = 0.21460 ± 0.00008 cm−1 and B0(C3D3N3) = 0.19358 ± 0.00015 cm−1. These values, together with the assumption that the molecule is planar and has a CH bond length of 1.084 Å, give the following values for the structural parameters: r0(CN) = 1.338 ± 0.001 Å, [Formula: see text]NCN = 127°, and [Formula: see text]CNC = 113°.

HIGH RESOLUTION RAMAN SPECTROSCOPY OF GASES: VI. ROTATIONAL SPECTRUM OF SYMMETRIC BENZENE-d3

The pure rotational Raman spectrum of C6H3D3 vapor at a pressure of 15 cm. Hg was photographed in the second order of a 21 ft. grating. The value of the rotational constant was found to be B0 = 0.17165 ± 0.0001 cm−1. This result confirms the earlier spectroscopic values of the internuclear distances in the benzene molecule.

HIGH RESOLUTION RAMAN SPECTROSCOPY OF GASES: V. ROTATIONAL SPECTRA OF ALLENE, ALLENE-d4, AND ALLENE-1,1-d2

The pure rotational Raman spectra of allene, allene-d4 and allene-1,1-d2 were photographed in the second order of a 21 ft. grating. Two plates of each spectrum were analyzed yielding the following values for the rotational constants:[Formula: see text]These constants lead to the value r0(C=C) = 1.3088 ± 0.001 Å. Also if a value of r0(C—H) = 1.07 ± 0.01 Å is assumed, then [Formula: see text]. It is noted that the length of the carbon–carbon double bond in allene is significantly shorter than that in ethylene.

HIGH RESOLUTION RAMAN SPECTROSCOPY OF GASES: II. ROTATIONAL SPECTRA OF C6H6 AND C6D6, AND INTERNUCLEAR DISTANCES IN THE BENZENE MOLECULE

The pure rotational Raman spectra of benzene and benzene–d6 at a pressure of [Formula: see text] atm. were photographed in the second order of a 21 ft. grating. Both spectra were resolved and analyzed, yielding for the rotational constants the values B0(C6H6) = 0.18960 ± 0.00005 cm.−1, B0(C6D6) = 0.15681 ± 0.00008 cm.−1and, therefore, for the moments of inertia about an axis perpendicular to the figure axis[Formula: see text]If it is assumed that the benzene molecule has the planar hexagonal structure, the moments of inertia just given yield for the internuclear distances in benzene the values[Formula: see text]

The Infrared and Raman Spectra of Formaldehyde-d1 Vapor

The infrared spectrum of HDCO vapor has been investigated in the region 2.5μ—25μ and all six fundamentals have been observed. Four of the fundamentals have also been observed in the Raman spectrum of the vapor. The perpendicular component of an hybrid band at 3.5μ and the pure rotational Raman spectrum were resolved and analyzed. The rotational constant (A—B̄) for the symmetric top approximation was found to be 5.47±0.03 cm-1. This value, combined with the rotational constants of H2CO obtained by Dieke and Kistiakowsky, yields approximate molecular dimensions for the ground state of formaldehyde.