ON ABSORPTION SPECTRA OF ORGANIC COMPOUNDS. 229 



The second effect of a solvent is when the same molecular frequency is 

 common to vapour and solution, but tlie measurements of this frequency with 

 vapour and solution do not give exactly the same values. It is this particular 

 etiect which requires discussion, because unless the phenomenon is understood 

 the relationships between the infra-red fundamental frequency and the visible 

 and ultra-violet frequencies will apparently not hold good. Without going 

 fully into the quantitative measurements it may be stated that the change in 

 the value of the molecular frequency in passing from vapour to solution depends 

 on th« nature of the solvent and on the concentration in that solvent.'^ As 

 regards the effect of concentration, the difference between values of the molecular 

 frequency as observed with vapour and solution, is greatest with concentrated 

 solutions. As the solution is diluted the value more and more nearly approaches 

 the value for tlie vapour until at very great dilution the value for the solution 

 equals that for the vapour. This change iii the molecular frequency in passing 

 from vapour to solution is not due to the fact that the quantitative relation 

 between visible or ultra-violet bands and the infra-red fundamental does not 

 hold, but to the fact that the infra-red fundamental itself varies slightly 

 in position with the nature of the solvent and the concentration in that solvent. 

 Another important fact to be recorded is that a compound in the liquid state 

 does not show exactly the same molecular frequency as it does in the state of 

 vapour. This, again, is due to a small difference in the infra-red fundamental 

 frequency in the two statee. It is obvious, therefore, that in making measure- 

 ments ot molecular frequencies the true values are those obtained with the 

 vapour. If, as frequently happens, measurements cannot be made with the 

 vapour, then very dilute solutions must be used. Abov« all, in comparing 

 together the various molecular frequencies shown by a given substance it is 

 necessary that all the measurements be made with the substance under the same 

 conditions. 



In connection with the effect of solvents on the absorption exerted by a sub- 

 stance, a brief reference may be made to the variation in the absorptive power 

 with concentration. Measurements have as yet only been, made for frequencies 

 in the ultra-violet region. At first sight it might be expected that Beer's 

 law would hold good, namely, that the molecular absorptive power would be 

 independent of the concentration. It is, however, rarely the case that Beer's 

 law holds good, and in the great majority of cases the absorptive power in- 

 creases with dilution up to a constant maximum. It has been found that if 

 K is the maximum absorptive power shown by a substance at very great dilution 

 in a given solvent, and k is the absorptive power at a definite concentration 

 k/K = l — e-av^ where V is the volume in, litres containing one gram molecule 

 of the absorbing substance and a is a constant. A more convenient form of the 

 above is log (K/K-k)=aV. 



The quantitative relationships between the various frequencies shown by a 

 molecule may now be further considered. It has already been stated that the 

 principal frequencies of all the absorption bands shown by a compound in the 

 visible and ultra-violet are always exact multiples of the principal frequency 

 of an important absorption band shown by that substance in the infra-red. 

 This is true of all the absorption bands which are shown by a substance in 

 different solvents, and which Hantzsch attempted to explain by assigning a 

 different formula for each band. Other quantitative relationships have also 

 been discovered, and these may briefly be described, because it has been 

 found possible from a knowledge of them to formulate a quantitative theory 

 which would seem capable of explaining all the observations that have been 

 made on absorption spectra. 



In the first place it may be noted that the examination of the absorption 

 exerted by a compound in the infra-red reveals the existence of many more 

 bands than the important one which has been called the infra-red fundamental, 

 and which determines the frequencies of the visible and ultra-violet hands.. 

 Further, in every case yet examined the infra-red fundamental lines were in the 

 short wave infra-red region, i.e., between the wave-lengths limits of 8/n and 3/x. 

 If the principal frequencies of all the infra-red bands are examined additional 

 interesting relationships are found. Thus the fundamental infra-red frequency 

 either is the least common multiple of certain of the long wave infra-red 



