Raman frequencies of halogen compounds

Many of the compounds in higher oxidation states are reactive, and for moisture-sensitive solids that cannot be crystallized, some of the bond lengths quoted in Table 2.1 are from EXAFS measurements [24], Raman spectroscopy is likewise well suited to studying such reactive compounds, and vibrational data for halometallates are given in Table 2.2 trends illustrated include the decrease in frequency as the oxidation state of the metal decreases, and similarly a decrease in vibrational frequency, for a given oxidation state, with increasing mass of the halogen. 

Table 2.2c lists the vibrational frequencies of triatomic halogeno compounds. The resonance Raman spectrum of the I3 ion gives a series of overtones of the Vi vibration [408,409]. The resonance Raman spectra of the l2Br and IBr2 ions and their complexes with amylose have been studied [410]. The same table also lists the vibrational frequencies of XHY-type (X,Y halogens) compounds. All these species are linear except the ClHCl ion, which was found to be bent by an inelastic neutron scattering (INS) and Raman spectral study [402]. 

Halogens form molecular compounds with organic solvents. For example, the band at 2l 3 cm of gaseous 12 is shifted to 201 cm in benzene solution, and the band at 381.5 cm of gaseous ICl is shifted to 275 cm in pyridine solution. The Raman spectra of H, Br, and ICI have been studied in many solvents. These frequencies are much lower than the corresponding gas-phase frequencies because of charge-transfer interaction with solvent molecules. 

Substiiution of one of the Y atoms of a pyramidal XYj molecule by a Z atom lowers the symmetry from Cj to C. Then the degenerate vibrations split into two bands, and all six vibrations become infrared and Raman active. The relationship between C3 and C is shown in Table II-3d. Table ll-3e lists the vibrational frequencies of pyramidal ZXY2 molecules. Simon and Paet-zold made an extensive study of the vibrational spectra of selenium compounds. The ZXYW-type molecule belongs to the C, point group, and all six vibrations are infrared and Raman active. The vibrational spectra of OSClBr and [XSnYZ] (X, Y, Z a halogen) have been reported. 

Raman spectra have also been reported on ropes of SWCNTs doped with the alkali metals K and Rb and with the halogen Br2 [30]. It is found that the doping of CNTs with alkali metals and halogens yield Raman spectra that show spectral shifts of the modes near 1580 cm" associated with charge transfer. Upshifts in the mode frequencies are observed and are associated with the donation of electrons from the CNTs to the halogens in the case of acceptors, and downshifts are observed for electron charge transfer to the CNT from the alkali metal donors. These frequency shifts of the CNT Raman-active modes can in principle be u.sed to characterise the CNT-based intercalation compound for the amount of intercalate uptake that has occurred on the CNT wall. 

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