CHAPTER VI. 



GENERAL DISCUSSION OF THE SPECTRA. 



EFFECT OF STRUCTURE. 



In order to learn what effect a group of atoms in a molecule has upon 

 infra-red absorption spectra, the most logical procedure is to study 

 isomeric compounds, in order to determine fully that the phenomenon 

 is intramolecular, and after that to attempt to locate the particular 

 group of atoms suspected of causing the disturbance. As already 

 mentioned, in many cases the spectra of isomers is very similar until 

 we extend our observations far into the infra-red. The total number 

 studied is so large and varied, while the change in the spectra of a pair 

 of isomers is so marked, that there can be no question that this is due 

 to structure rather than impurities. In aniline, CgHgNHa, and its isomer, 

 picoline, C6H4N(CH3), the effect of structure is very marked. The 

 benzene band at 3.25 jjl, found in aniline, is entirely obliterated by the 

 one at 3.35 fi in picoline, while in the spectrum of picoline, only one 

 band, at 10 /x, is in common with that of aniline. In the sulphocyanates 

 R — iSCN, and the mustard oils, R — NCS, the effect of structure is still 

 more pronounced. As previously mentioned, the small band of the 

 sulphocyanates at 4.68 fi is completely outclassed by the 4.78 fi band in 

 the mustard oils. As the band occurring from 3 /^ to 3.4 /u, is a char- 

 acteristic of carbohydrates, so is this band a characteristic of the mus- 

 tard oils. Of all compounds studied, the mustard oils are unique in 

 having an enormous absorption band in the region of shorter wave- 

 lengths, this side of 5 fi. In CSg the first strong band occurs at about 

 6.7 fi, in methyl iodide at 11.35 /*» ^"^ in carbon tetrachloride at 13 /a. 



In allyl mustard oil, C3H5NCS, using the large spectrometer, this 

 band was found to be complex, being opaque from 4.5 /* to 4.9 /x with 

 the maximum located at about 4.8 /m. Phenyl mustard oil, CeHjNCS, is 

 still more interesting, since it contains the 3.25 /n band, as well as several 

 others belonging to benzene, and has, in addition, this strong band of 

 the mustard oils, located at 4.8/*, just as though the CH and the CS 

 ion were vibrating side by side but independent of each other. 



Other isomers, like pinene and limonene, CjoHj^, have a great simi- 

 larity until we arrive at 10 /a, while the caproic acids are identical to 6 /* 

 and begin to show dissimilarity at 8/x. Probably the most evident 



