The Absorption Spectra of Some Aromatic Compounds

THE ABSORPTION SPECTRA OF SOME AROMATIC COMPOUNDS. Part L--Hydrocarbons. BY P. K. SESHAN. (From the lndia~z Association for the Cultivation of Scietzce, Calcutta.) Received December 29, 1935, (Communicated by Prof. K. S. Krishnan, msc.) 1. Introduction. TxE absorption spectra of benzene and some of its simple derivatives have been studied in great detail by I-Ienri x and his collaborators, and they yield much useffd information regarding the optical and other constants of the molecules. For molecules containing more than one benzene ring, the analysis of the spectrum is naturally more difficult. Some of the molecular constants, however, are easily obtained, as for example, the electronic frequencies of the molecule, and its vibration frequencies in the normal and in the excited states. It would be of interest to follow the progressive variation of these constants and of the general characteristics of the absorption bands, with the addition of extra benzene rings, both of the condensed and uncondensed types, in different positions, and to interpret these variations in relation to the structure of these molecules. The present paper concerns itself with such a study. Several aromatic hydrocarbons are studied for their absorption spectra in the vapour state, and among them are (1) naphthalene, anthracene, and naphtha- ten4, with linear condensed benzene rings; (9~) phenauthrene, chrysene, and dibenzanthracene, with side condensed rings; (3) pyrene and perylene, with close-packed rings ; (6) acenaphthene, fluorene and fhtoranthene, with incom- plete rings; (5) diphenyl, terphenyl, diphenylmethane, dibenzyl, triphenyl- methane and triphenylbenzene, with uncondensed benzene rings ; and (8) durene and hexamethylbenzene. The results are discussed in relation to the structure of the molecules and their characteristic electronic and vibrational frequencies. Since the absorption spectra of the molecules are known to be consider- ably inftuencedbythe state of aggregation of the molecules, we have also studied the absorption spectra of these substances in the state of solution in different i Henri, four. Pltysiqzee, 1922, 3, 181; 0rndorff, GibbS, McNulty and Shapiro, Iour. Aln. Chem. Soc., 1928, 50, 831. 148 The AbsorplioTz Spectra o/Some ,4 romaHc Compoutzcls--I 149 solvents, and, occasionally in the molten and in the solid states as well. The positions of the absorption bands of a substance in the different physical states are compared. 2. Experimental The absorption measurements for the vapours extended in general, from 7000 _~ to 2300 ~, and were made with a ttilger quartz F,2 spectrograph. Where the absorption bands were very sharp, Hilger E~ spectrograph, with a much higher dispersion, was used. Iron arc was used as the standard for the wave-length measurements. The source of continuous spectrum used for the absorption measurements in the ultra-violet was a specially designed water-cooled hydrogen discharge tube described in an earlier publication by the author3 giving an intense spectrum upto about 2100 X. For the visible and the near ultra-violet regions a Zeiss tungsten linear filament lamp, fitted with a quartz window, was found to be a more suitable source. The substances were in general obtained from standard firms, 3 and were further purified by erystaIlisation or sublimation. The absorption spectra of the solutions were studied in the usual manner with a graduated Baly absorption tube fitted with quartz windows. For the study of the absorption in the vapour state, the substance was introduced in an absorption tube of pyrex glass, 75 eros. long and 2 cms. in diameter, fitted with quartz windows at the ends. It was placed in a suit- ably devised air oven with a double jacket, where it could be heated and maintained at any desired temperature up to 300 ~ C. (&lt the substances studied here melt below 250 ~ C.) The temperature of the absorption tube was raised in stages of 10 ~ C., and the absorption spectrum was photographed at each stage. From these absorption spectra, corresponding to gradually increasing vapour pressures, photographed in succession the intensity wave- tength curves can be easily traced on an arbitrary scale, just as in the absorption photographs for the solution by the Baly method in which the thickness of the column of solution is gradually increased. The positions of the maxima and the minima of absorption are located in this manner more precisely than from single photographs of the absorption. The absorption and fluorescence spectra of single crystals of many of these hydrocarbons have been studied by us in detail, particularly in relation to the influence of the direction of vibration of the incident light with reference to the crystal axes on the positions and the intensities of the bands. A 2 7our. Sci. Instruments, 1935, 12, 230. a The author's thanks are due to Prof. L. F. Fieser of the Harvard University and to Prof. A. Dadieu of the Vienna University for the supply of naphthacene and perylene. A5 F' 150 P.K. Seshan report of this work is being published separately, to which the reader may be referred for a description of the experimental arrangement used for these absorption measurements, and for the details of the results. We may men- tion here that though the intensities of the various absorption bands show a remarkable dependence on the direction of the light-vector in the crystal, the positions of the bands are practically independent of the direction of the vector. The data for the positions of the absorption bands in the solid state quoted in the next section are all taken from this paper. 3. Results. The experimental results are collected together in this section. The different substances are considered separately, and the special characteristics of the bands are described under each. In the tables are entered the wave- lengths of the absorption maxima, and the corresponding wave numbers in era. -1 The difference between the wave-numbers of successive bands are entered in the next column. The intensities of the bands are indicated by arbitrary numbers. The absorption data for the substance in sdution in alcohol, and for the substance in the solid state, are also given for comparison with the vapour data. On comparing the absorption spectra for the solid, liquid and the solution with that of vapour, we find that the positions of the bands are widely different; even for the solutions they are diiterene for different solvents. A closer comparison of the spectra in the different physical states, however, shows that except for a bodily shift of the absorption spectra towards the longer-wave-length side as we go from the vapour to the solution, and further in the same direction as we go from solution to molten liquid and s~ill further when we pass to the solid, and a progressive change in the diffuseness of the bands, the relative positions of different components of the bands system are more or !ass the same for the different physical states. There is thus an approxi- mate one to one correspondence between the prominent absorption bands in the different states ; the corresponding bands in the different states are also easy to identify. In the following tables the data for the corresponding bands in the different physical states are entered in the same row, so as to facilitate comparison. The actual absorption spectrograms are also reproduced, from which the general nature of the absorption bands and their other characteristics wilI be clear. Among the substances studied here naphthalene vapour has been already studied in great detail by Henri and Laszlo.* Some preliminary. 4 Henri and Laszlo, Proc. Roy. Sot., A, 1926, 105, 35S; Laszlo, Zeits. Phys. Chem., t925, 118, 369. The Absorption Spectra of Some AromaLic Cov@ou~zds--[ 151 absorption measurements are available for the anthracene vapour, s and for solid anthracene and phenanthrene at low temperatures. 6 In the state of solution most of these substances have previously been studied, by Baly, 7 Clar, s Dadieu 9 and others. The solution data given in the tables are those obtained by us, and they agree well with those given by these earlier in- vestigators. Alcohol was the usual solvent in our measurements. In general the absorption spectrum consists of a large number of prominent bands more or less equally spaced ; the spacing interval is about 1300 to ]400 cm. -1 These bands are accompanied by feeble satellites usually one each, on the shorter-wave-length side, separated from the main band by about 400 cm.-1 They do not generally exhibit any rotational fine structure; naphthalene and aeenaphthene are striking exceptions. Accompanying this banded absorption there is also a strong background of continuous absorption, which consists of two distinct regions separated by a region of transmission. The first absorption region, usually in the near ultra-violet, begins more or less abruptly, and reaches a feeble maximum. Beyond it is a region of relative transparency, and further beyond begins the second region of absorption, which is much stronger than the first. For some of the substances there is also a second region of feeble transmission in the ultra-violet. The bands in the first region stand ottt from the continuous background much more prominently than those in the second region ; indeed in the second region two or three bands only can be observed, near the long-wave-length end of the region. We now proceed to the detailed report of the results. \/\// See Figs. 1 (a) and (b), Plate V. Many of the bands in the long-wave- length region exhibit fine-structure and they are indicated by an asterisk; a double asterisk indicates that the fine-structure is very prominent. There is a region of transmission from %00 .~ to 9~250 ~, beyond which there is again absorption; the bands in the 1utter region are broad and diffuse.

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