Mr Purvis, The absorption spectra of mesitylene, etc. 89 



Tlie absorption spectra of mesitylene and trichloromesitylene. 

 By J. E. Purvis, M.A., St John's College. 



[Read 8 February 1909.] 



The absorption spectrum of mesitylene in the ultra violet region 

 has been described by Hartley and Huntington (Phil. Trans. 

 vol. CLXX. (1879), pp. 257 — 274). They noted a band between 

 A, 27 5 — A- 245 in strong solutions. By diluting this solution in 

 the ratio of 1 : 2000, the strong band is resolved into three small 

 sharp bands, which remain visible in a solution of 1 : 8000. 



Drossbach {Ber. Chem. Ges. vol. xxxv. (1902), pp. 1486—1489, 

 noted that general absorption occurred at A, 336, but this occurs 

 only in a very thick layer. He did not appear to have noticed 

 any bands. 



Coblentz (Astrophy. Jour. vol. xx (1904), pp. 207—223), has 

 observed the presence of a considerable number of bands and lines 

 in the ultra red. 



The author has compared the absorption spectrum of trichloro- 

 mesitylene with that of mesitylene, when equivalent quantities of 

 substances were in alcoholic solutions. He noticed that the bands 

 and the general absorption were shifted in the trichloromesitylene 

 more towards the red end of the spectrum when compared with 

 their positions in the mesitylene solution. For example, through 

 a thickness of 40 mm. of solution and iVyiOOO concentration the 

 strong band in mesitylene lay between \ 269 — \ 254, whilst that of 

 the trichloromesitylene was between \ 287 — A, 263. Also, a most 

 important difference was the greatly increased persistence of the 

 absorption curve of the trichloromesitylene as compared with that 

 of the mesitylene. The loading of the ring by the chlorine atoms 

 influenced the vibration of the molecule so that the absorption 

 band was shifted towards the red end of the spectrum and agrees 

 with the effect previously observed by Hartley in the case of 

 substituted compounds, and, at the same time, the persistence of 

 the band was greatly increased. 



It has been shown by Baker and Baly {Jour. Ghem. Soc. 

 vols. xci. and xcii. p. 1122), that, in bhe case of pyridine, the intro- 

 duction of chlorine atoms into the nucleus not only shifted the 

 bands towards the red, but also greatly increased their persistence, 



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