The optical rotatory power of quartz on either side of an infra-red absorption band

Abstract

Drude’s equation for natural optical rotations depends on the existence of “ions” of characteristic frequency, of which each type contributes a partial rotation to the total rotatory power of the medium. There is nothing in the equation to limit the range of these frequencies, but in discussing the rotatory power of quartz (the only substance for which sufficient data were then avail­able), Drude himself concluded that “the kinds of ions whose natural periods lie in the infra-red are inactive” (‘Physical Optics,’ 1907, translation, p. 413). An alternative view was put forward by R. W. Wood, who concluded that a "spurious” anomaly could be produced in colourless media by the combined influence of an infra-red and an ultra-violet absorption band (‘Physical Optics,’ 1919 edition, p. 492); and C. E. Wood and Nicholas have applied the same theory in the hope of deducing the configuration of the molecule from the sign of an infra-red term of which the existence is not yet established by experiment. Experiments on these lines have, however, already been made by Ingersol, who found that the rotatory powers of a series of typical compounds decreased progressively in the infra-red region, right up to the limit of transparency of the medium. This experimental result is in agreement with the conclusions of Kuhn (private communication, of ‘Z. Phys. Chem.,’ B, vol. 4, p. 14 (1929), who anticipates that all rotations will diminish asympto­tically to zero in the infra-red; but it is directly opposed to the theoretical deductions of the authors cited above, whose diagrams represent the rotations as increasing asymptotically to ± ∞ on approaching a characteristic frequency in the infra-red. Moreover, since the rotatory powers of linonene and pinene decrease progressively, even when passing through a region which includes three absorption bands, it is clear that the phenomenon discovered by Cotton in coloured organic compounds is not reproduced in these infra-red bands.