Bond Information from Vibrational Spectra

DERIVATION OF BOND INFORMATION FROM VIBRATIONAL SPECTRA 

Certainly, it is possible to obtain other characteristic fragment frequencies in a systematic way although an enormous amount of synthetic work is involved to get suitable isotopomers in each case. In addition, the measured fragment frequencies will always be contaminated by some residual coupling. Therefore, one can predict that it is hardly possible to solve, just by experimental means, the problem of determining fragment-specific frequencies. 

One can use stretching frequency/bond length relationships to predict geometrical features of molecules from measured infrared spectra. This should be first checked for a case where verification of the prediction is possible due to an available geometry obtained from an ab initio calculation. 

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For the finer differences in bond energy of various bonds between similar atoms, information can be obtained from vibration frequencies in the infrared and Raman spectrum and from small variations in the bond length, which are connected with each other and with the bond energy through semi-empirical relations (Badger s rule etc.). 

These are vibrational spectra and lead to fundamental frequencies of vibration and the strength of ion-solvent interactions. When a salt is dissolved in water it will affect the frequencies of the water absorption and in favourable cases it will give rise to new peaks due to ion-solvent interactions. Alteration in the vibrational spectrum of the water due to the presence of the ion gives information regarding the effect of the ion on the water structure. The really useful information, however, comes from a study of the new lines due to the actual bonding of the ion to the solvent molecules. The frequencies and intensities of the vibrational lines give a measure of the strength of the bond between ion and solvent, and the peak areas can in favourable cases lead to hydration numbers. 

As expected from Fig. 1.2, electronic spectra are very complicated because they are accompanied by vibrational as well as rotational fine structure. The rotational fine structure in the electronic spectrum can be observed if a molecule is simple and the spectrum is measured in the gaseous state under high resolution. The vibrational fine structure of the electronic spectrum is easier to observe than the rotational fine structure, and can provide structural and bonding information about molecules in electronic excited states. 

The amount of information contained in a measured vibrational spectrum is exploited to some, but not full extent. For example, vibrational spectra are never used to characterize all bonds of the molecule and to describe its electronic structure and charge distribution in detail. Of course, aspects of such investigations can be found off and on in the literature, however, both quantum chemists and spectroscopists fail to use vibrational spectra on a routine basis as a source of information on bond properties, bond-bond interactions, bond delocalization or other electronic features. Therefore, it is correct to say that the information contained in the vibrational spectra of a molecule is not fully utilized. This has to do with the fact that the analysis of vibrational spectra is always carried out in a way that is far from chemical thinking. The basic instrument in this respect is the normal mode analysis (NMA), which describes the displacements of the atomic nuclei during a molecular vibration in terms of delocalized normal modes [1-6]. 

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