Open AccessPhysico-chemical studies of complex organic molecules. Part I.―Monochromatic irradiation
Author(s) -
F. P. Bowden,
Charles C. Snow
Publication year - 1934
Publication title -
proceedings of the royal society of london. series b, containing papers of a biological character
Language(s) - English
Resource type - Journals
eISSN - 2053-9185
pISSN - 0950-1193
DOI - 10.1098/rspb.1934.0038
Subject(s) - molecule , polyatomic ion , radical , chemistry , spectral line , chemical physics , raman spectroscopy , computational chemistry , physics , organic chemistry , quantum mechanics
The physical study of polyatomic molecules has proved to be a task of great difficulty. It now appears likely that, by combining observations of infra-red absorption spectra with observations of Raman emission spectra, and by making use of several subsidiary relationships, accurate information can now be given as to the resistance both to stretching and to bending of all the important links of molecules such as C2 H2 , C2 H4 , C2 H6 , CH2 O, and N2 O4 . But it is evident that a limit has nearly been reached and that molecules of greater complexity present to the physicist an almost insoluble problem. Thus, whilst we may perhaps hope for a complete analysis of the structural elements of acetone, in the not very distant future, a similar analysis of methyl ethyl ketone seems at present to be impossible.A fortiori , the complex molecules which are of so much importance in biology, are entirely beyond the scope of these methods of examination. Nevertheless much valuable information may be obtained by the application of physical methods, in view of the fact that the mutual influence of the binding electrons is limited to a very narrow range, and is usually confined (even for strongly polar radicals) to not more than about three atoms of a long chain. While, therefore, we cannot hope to provide a completemechanical description of such a model, in terms of itsvibration spectrum, we may be able to elucidate itselectronic states with some ease and certainly (Mulliken, 1933). Thus the long-established chemical practice of regarding the absorption band at about 3000 A., which is characteristic of all ketones and aldehydes, as caused by an electronic transition in the >C=O group, is justified by the new orbital theory and by more precies experiments, with the qualification that any attempt to localize the excitation in the electrons of the double bond itself is definitely not valid. For all >C=O groups in the "aldehydic" or "ketonic" environment there will be an energy of excitation of about 4 volts, corresponding to the band at 3000 A.; this energy of excitation will raise the group (and hence the molecule) to its first excited state. This first excited state will be at the same energy level through the series of molecules above, but will be much modified in different environments,e. g. , by the proximity of -OH or -NH2 in -CO . OH or -CO . NH2 or by the presence of conjugated double bonds,e. g. , -C=C-C=O.