INVESTIGATION WITH A ROCK-SALT PRISM. 99 



Like most of the other sulphides, thiophene is quite transparent. 

 Like allylsulphide, it very abruptly changes in opacity at 6 /m, but as a 

 whole is more transparent than that substance. The small band at 

 4.65 fi is of interest, since it coincides with the CSg band found at this 

 place. With the large spectrometer the band at 6.4 /x was found double, 

 the maxima being at 6.28 fi and 6.5 /a. 



As a whole the thiophene spectrum is entirely different from the one 

 of benzene. This is rather unexpected, since the two compounds are 

 so similar in their physical properties that they are easily confounded. 

 In previous discussions of groups of compounds having similar chem- 

 ical properties we have always observed like similarities in their absorp- 

 tion spectra. This is an interesting exception, and is as strong evidence 

 in favor of the argument that the bonding of the atom in the molecule 

 determines the absorption spectrum as is the evidence, deduced from 

 the mustard oils, that a particular group of atoms causes certain absorp- 

 tion bands. 



Pyrrol. CHiCNH). (Fig. 124.) 



„_ p„ In pyrrol we have the four-membered carbon chain of 



jj I thiophene united by the bivalent imide group. It is, 



WC. XU therefore, a secondary amine. It also shows great simi- 

 N larity to the phenols. We are not surprised, then, to find 



a strong band at 2.95 fi, as in ammonia and in certain com- 

 pounds containing nitrogen or NH. The 3.22 /* band of thiophene is 

 almost obliterated, while the 4.3, 4.6, and 6.4 /* bands are shifted to the 

 longer wave-lengths. Beyond 7 /a the spectrum is entirely different. 

 The whole reminds us of the bands of the benzene nucleus, which per- 

 sist, even in some of its very complex derivatives. It also shows 

 that the S atom has little influence at 3.22 ju, and that the greater opacity 

 of pyrrol is due to the NH group. For pyrrol the film was only o.oi 

 mm. in thickness, while in thiophene it was 0.08 mm. Pyrrol decom- 

 poses very readily, and for that reason was distilled just before using. 

 The unusually sharp, narrow band at 2.95 fi, and its weaker component 

 at 3.2 fi, which occurs as a deep, narrow band in thiophene, were not 

 quite resolved with the small spectrometer, showing that a similarly 

 asymmetrical band at 7.05 ix is no doubt likewise double. 



