Halogen vibrational frequencies

Much of tills chapter concerns ET reactions in solution. However, gas phase ET processes are well known too. See figure C3.2.1. The Tiarjioon mechanism by which halogens oxidize alkali metals is fundamentally an electron transfer reaction [2]. One might guess, from tliis simple reaction, some of tlie stmctural parameters tliat control ET rates relative electron affinities of reactants, reactant separation distance, bond lengtli changes upon oxidation/reduction, vibrational frequencies, etc. 

Table 1.4 Vibrational frequencies in MXjj species (cm ) (M = Ru, Os X = halogen)...

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. 

Vanadates For the isostructural strontium and barium ortho-vanadates the V-—0 vibrational frequencies decrease with increasing effective nuclear charge [51]. Similar orthovanadates with the apatite structure have for a given halogen ion decreasing frequencies of the VO4 ion in the order Ca>Sr >Ba ((62) and Table 4]. However, it is impossible to see any trend towards the tiivalent lanthanide cations (59). The IR bands are so wide in this case that small changes cannot be measured. 

Kaupert, Heydtmann and Thiel"2 calculated the vibrational spectrum of monohalo-genated 1 at the HF level using the 6-31 G(d) basis set and effective core potentials with DZ + P basis sets for Cl, Br and I. Reduction from Z)3h to Cs symmetry leads to considerable coupling between modes (exceptions C—H stretching and CH2-deformation modes) of 1. Vibrational frequencies that are influenced by the halogen substituent are shifted to lower values with increasing mass of the halogen. 

Electron depopulation of the donor and concomitant population of the acceptor in the complex results in a lowering of the vibrational frequencies in the IR spectra of the donor and acceptor moieties. Additionally, complex formation can decrease the symmetry of the donor/ acceptor dyad and can lead to increased IR intensity or the appearance of new bands. For example, in halogen/alkylbenzene complexes, the stretching frequencies of the halogens are lowered, as seen in the shift of chlorine band from 557 cm-1 in free chlorine to 530 cm-1 in the benzene complex, to 527 cm-1 in the toluene complex, and to 524 cm-1 in the p-xylene complex. Increases in the intensity of some of the arene bands are also clearly seen [23b]. 

These early studies of the effect of various anionic ligands on v(M—H) alerted us to the possibility that v(M—Cl) would provide a much more generally applicable method of finding relative trans influences we could determine the stretching frequencies of metal-halogen vibrations. L. A. Duncanson, followed by D. M. Adams, proceeded to develop a... 

Vibrational Frequencies and Force Constants for the Triatomic Halogen Cations... 

Ab initio calculations by David [92] on the insertion of an isolated magnesium atom into the carbon halogen bond of fluoromethane and chloromethane included calculations of transition state energies, geometries, and vibrational frequencies. Both systems exhibited similar geometries for the transition state with C, symmetry (Scheme 34). 

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