Fermi Doublet Research Articles

The (ν1, 2ν2) Vibron–Phonon Infrared Absorption Band of Solid CO2

The infrared absorption spectrum of thick samples of solid CO2 (~4 mm path length) has been investigated in the frequency range 1100–1600 cm−1 at temperatures from 88 to 160 K. The (ν1, 2ν2) Fermi doublet is inactive because of the crystal symmetry but absorption is observed with maxima at the frequencies ν1 ± ai, 2ν2 ± ai(i = 1, 2, 3, 4). At 88 K the values of ai are 60, 75, 95, and 108 cm−1; at lower temperatures the maxima are sharper and the ai greater. These four frequencies correspond to maxima in the phonon frequency distribution. The agreement with existing theoretical calculations is not good; there is better agreement with the zone boundary frequencies determined for two symmetry directions by inelastic neutron scattering experiments. The present results are consistent with previously observed combinations with the allowed ν2 and ν3 internal modes.

High-resolution infrared spectra of CH 3I

The i.r. spectra of the perpendicular bands v 5 + v 6 and v 3 + 2 v 6 of CH 3I have been measured with a resolution of about 0.03 cm −1. These bands form a Fermi doublet and the Fermi resonance has been analysed to give a value for the cubic normal-coordinate force constant ¦ k 356¦= 20 cm −1. A second Fermi doublet of CH 3I, v 2 + v 5 and v 2 + v 3 + v 6, has been measured with a resolution about 0.09 cm −1 and a partial analysis of the rotational fine structure has been made. Several other perturbations have been detected in these bands besides the Fermi resonance.

An Infrared Spectrum of CO2 Dimers in the "Locked" Configuration

The two components of the (ν1, 2ν2) Fermi doublet of gaseous CO2 in an absorption path length of 56 m at 192 °K show a complex structure. When the allowed C16O18O absorption and the pressure-induced CO2 absorption are removed by computational procedures, the residual spectrum consists of two similar symmetric patterns of five maxima. These are interpreted in terms of the rotation and vibration of (CO2)2 dimers held by quadrupole–quadrupole interaction in the locked T position at an intermolecular distance of 4.1 Å.

The Fermi doublet, v1 and 2v2, of ClCN at high resolution

Abstract The Fermi doublet, v 1 and 2 v 2 of ClCN, at about thirteen microns in the gas-phase infrared spectrum, has been measured with a resolution of better than 0.03 cm −1 . The sample was prepared from natural chlorine and contained 35 Cl 12 C 14 N (~75%) and 37 Cl 12 C 14 N (~25%). We have determined the vibrational energies and rotational constants for a number of states of these molecules and have estimated the Fermi-resonance operators for the 10 0 0, 02 0 0 and 11 1 0, 03 1 0 diads of 35 ClCN and the 10 0 0, 02 0 0 diad of 37 ClCN.

Infrared Spectrum of Cyanogen Azide

The infrared spectrum of cyanogen azide (N3CN) has been recorded in the gas phase between 3000 and 70 cm−1. The nine vibrational fundamentals have been located. Assignments of the seven in-plane and two out-of-plane fundamentals have been made on the basis of observed and calculated vibration–rotation band contours. Two strong Fermi doublets have been observed in the triple bond stretching region of the spectrum. Valence force constants have been calculated for the in-plane vibrations of cyanogen azide, and the distribution of potential energy among the force constants obtained, in order to give qualitative descriptions of the normal modes. Thermodynamic quantities of cyanogen azide have been calculated for several temperatures and compared with corresponding quantities for methyl azide.

Study of Fermi resonance by the method of solvent variation

A method of determining the Fermi resonance parameters and of confirming the nature of the phenomenon in complex molecules is presented. It is based on the determination of the ratio of the intensities R and the frequency separation Δ of the two components of a Fermi doublet in a wide series of solvents. A plot of R vs. Δ gives an experimental curve which can generally be fitted by a theoretical curve using as adjustable parameters the Fermi coupling coefficient W and the ratio of intensities of the unperturbed transitions R 0, assumed to be constant in all solvents. The method is tested in molecules in which Fermi resonance is well established. The Fermi Resonance parameters W and Δ 0 are determined in p-benzoquinone, cyclopentanone, benzoyl chloride and ethylene carbonate.

Infrared absorption intensities of hexafluorobenzene. Assignment of the 317 cm −1 band

The infrared intensifies of the two low-frequency fundamentals of hexafluorobenzene have been measured in the vapor phase, and the value of the intensity of the Fermi doublet at 1000 cm −1 has been checked. The intensity of the 317 cm −1 band is very low, contrary to the estimate made by STEELE and WHIFFEN. Although both the low-frequency bands show the PQR structure of a parallel vibration, estimation of the PQR spacing expected for this molecule confirms the assignment of the 317 cm −1 band to the e 1 u fundamental, ν 20, made by STEELE and WHIFFEN. The CF bond moments are presented, together with a discussion of the nature of the bond moment hypothesis and an estimate of the vibronic effects to be expected in this molecule.

Fermi resonance in condensed CF 4 and CCl 4

The infrared absorption spectra of CF 4 and CCl 4 have been observed in both the pure form and in rare gas matrices at low temperatures. The region studied corresponded to the ν 3, 2ν 4, and ν 3, ν 1+ν 4 Fermi doublets of CF 4 and CCl 4 respectively. The interaction of the vibrations in Fermi resonance in these molecules changes considerably from that found in the gaseous phase. The spectral features observed are discussed and related to S trizhevskii's theory.

Absolute Infrared Intensities of the CS2 Fundamentals in CCl4 Solution and of the 768–797 cm—1 Fermi Doublet of CCl4 in Cyclohexane Solution

The absolute infrared intensities of two nonpolar solute molecules were determined in nonpolar solvents by extrapolating the apparent intensities to zero concentration. For CS2 the intensities were found to be 5.8×107 cm mole—1 in CCl4 in 5.2×107 cm mole—1 in C2Cl4 for the v3 (1523 cm—1) mode, and 9.5×105 cm mole—1 in CCl4 for the v2 (395 cm—1) mode. For CCl4 the intensity of the 768–797 cm—1 Fermi doublet was found to be 5.4×107 cm mole—1 in cyclohexane. The results are discussed in relation to the Polo-Wilson, Person, and Buckingham expressions for the gas to solution intensity ratio. Modifications of the dielectric expressions were investigated.