I02 INFRA-RED ABSORPTION SPECTRA. 



example of the influence of structure is in the aliphatic or chain-linked 

 groups of atoms, like octane, and the carbocyclic or ring compounds, 

 like benzene. If we consider simply the number of atoms in the 

 molecule, then the benzene series CnHsn.e. can be classed with the chain 

 series, CnH2n-2, CnHgn, CnHon+a- Hence, reasoning from the fact that 

 in the three groups of chain compounds studied all the conspicuous 

 bands occur in common, one would expect at least a few of these bands 

 to occur in the benzene (CnHsn-e) series. But no such coincidence 

 occurs, and only after the substitution of CHg-groups for H atoms in 

 benzene do we find bands, e. g., 3.43 /t, in common with those of the 

 chain compounds. If, then, we had no knowledge of organic chemistry, 

 the evidence presented here would be sufficient to conclude that we are 

 dealing with two distinct classes of compounds. 



Thymol and carvacrol, methyl ether and ethyl alcohol, eucalyptol 

 and terpineol, and the three isomeric xylenes are additional examples 

 showing very clearly the marked influence of the arrangement of the 

 chemical atom in the molecule upon the resulting absorption spectrum. 

 In the xylenes the bands occur in groups, as noticed in discussing their 

 curves (Chapter V), while the location of the maximum of each group 

 seems to occur in the order para, meta, ortho, with increase in wave^ 

 length. In thymol and carvacrol the change in the spectra begins to 

 manifest itself at 5 /* and 6 /x, while from 9 /x to 14 /x the spectrum is 

 entirely rearranged. In methyl ether and ethyl alcohol we have the 

 most marked change in the location of the absorption bands, which is 

 noticeable throughout the whole spectrum. 



As a whole, the present investigation substantiates the conclusions 

 of Julius (loc. cit.) of the influence of structure upon absorption spectra. 



EFFECT OF MOLECULAR WEIGHT. 



In the present work the results agree with that of Kriiss (loc. cit.) 

 in so far as it seems permissible to assume that the occurrence of a 

 certain conspicuous absorption band in a diflferent place is a real shift. 

 The benzene derivatives are the most noticeable example. In benzene, 

 CgHg, the maximum occurs at 3.25 /u,, and is shifted to 3.3 yu, in toluene, 

 CgHBCHa, to 3.38 fi in the xylenes, C^^{C}1^)^, and to 3.4 /a in mesity- 

 lene, €8113(0113)3. In other words, by substituting three CH3-groups 

 for an H atom we have shifted the maximum from 3.25 jx to 3.4 fx. Of 

 all the compounds studied, excepting the gases, this is the only example 

 where such a supposed shifting occurs. For a shift toward the shorter 

 wave-lengths certain derivatives of benzene containing nitrogen are the 

 most conspicuous, just as found by Kruss. In aniline, CgHbNHo, we 

 find the benzene band almost obliterated and the minimum shifted to 



