92 INFRA-RED ABSORPTION SPECTRA. 



As a whole, the terpenes are more opaque to infra-red radiation than 

 benzene and the sulphides. The absorption spectra are characteristic 

 of the group, just as was found in the fatty acids. The curves show 

 numerous well-defined absorption bands in all of the compounds inves- 

 tigated. The contrast between them and those of the simple com- 

 pounds, like CoHgOH and CH3I, is worthy of notice. All curves show 

 a sudden increased general absorption from 7/x to 12 fi. This is espe- 

 cially noticeable in eucalyptol (CioHigO). 



The number and especially the position of the absorption bands are 

 very striking when compared with similar groups of other compounds 

 in Table VI.^ 



The ever-recurring 5.85 [x band in other compounds is formed at its 

 usual place in Venice turpentine and resin, while it is shifted beyond 

 6.05 /x in limonene and pinene. In eucalyptol that region appears 

 complex. 



The large absorption band at 3.43 fi is no doubt complex in the ter- 

 penes, while the benzene bands have disappeared. In pinene and limo- 

 nene the maximum occurs at 3.43 [x, with a slight depression at 3.78 /x, 

 being most marked in limonene. On the other hand, in Venice turpen- 

 tine the maximum occurs at 3.78 fx, with a slight depression at 3.45 /x. 

 The terpene compounds were separated with great care in the chemical 

 laboratory of the university, and were considered very pure. The 

 slight depression of the curves at 1.6 /x to 1.7 /u, is probably due to an 

 absorption band found by Puccianti and Donath at 1,7 /«.. 



Limonene. doHw. (Fig. iii.) 



"T 



Limonene (CioHig) belongs to the simpler " mono- 



' _„ cyclic terpenes," and, with pinene (CiqHiq) , is an excel- 



j^ I lent example for studying the constitution of the mole- 

 ^Q=l~cu ^ cule, especially since they have the same number of 

 atoms. In this work d-limonene (CioH^g ; boil, pt., 176° 

 to 177° ; rot. -]-ioo°.7) was used, and 102 points were found on the curve. 

 A number of points are of special interest in examining these curves. 

 First, it will be noticed that the thickness of the 1-pinene is about double 

 that of the limonene. Yet the general absorption is about the same up 

 to 3 ju,, beyond which the difference is very marked. Again, if the 

 curve did not extend beyond g fx, the limit of previous investigations, 

 one would feel that here is an example in which the arrangement of the 

 atoms in the molecule exerts little influence. The transmission minima 

 coincide out to 8.9 fx, where the first new one occurs in limonene. The 

 deep bands of limonene at 11.3/i. and 12.6 /* find their counterpart in 

 d-pinene at 1 1.4 /x and 12.7 fx, both of which are complex, while the slight 

 depression at 1 1.8 /x. in the latter does not occur in the former. 



^The tables will be found at pages 136 et seq. 



