A system of physical chemistry - Index of

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A system of physical chemistry - Index of

APPENDIX III 199

dipoles are surrounded by a swarm of rotating electrons which no

longer occupy a position in the middle of the displaced towards one end.

figure-axis, but are

It follows that these molecules must

possess gyroscopic propeities, and cannot rotate, but experience precessional

vibrations. The infra-red bands referred to would correspond

therefore to precessional vibrations and not to rotations. [Note that

the infra-red bands produced by the " ordinary " vibration of atoms

with respect to one another (in a manner analogous to the vibration of

atoms in solids) occur in the short infra-red region, viz. 2 to 5/a. We

are at present dealing with bands in the much further infra-red.] In

general both ideas, rotation and precessional vibration, lead to the

same results; a distinction does indeed exist, viz. the position of the

absorption band on the basis of precessional vibration should be

practically independent of the temperature, whilst in the case of

rotations the position should be displaced towards the shorter wave-

length region as the temperature rises. Direct measurements upon

this point are wanting.

An indirect test of the is question possible, however. Lord Rayleigh

{Phil. Mag.., [v.], 2i, 410 (1892)) has calculated that a linear

oscillator, vibrating with frequency vq and rotating with frequency v^

must radiate or absorb at the three frequencies, vq, vq -f vi, and vq - vj.

As a rule v^ is small compared with vq. If, instead of rotation, we

substitute precessional vibration with frequency Vj, exactly the same

frequencies should be brought into play ;

be altered.

their intensities will, however,

Rayleigh's considerations have been applied by Bjerrum {Nernst

Festschrift, 1912, p. 90; Verh. d. D. phys. Ges., 16, 640, 737 (1914))

to explain the infra-red absorption spectra of gases. He points out

that an absorption band in the short wave part of the infra-red spectrum,

which is due to linear vibrations, must be split into three owing to rotation

of the molecule. In this way he has explained the broadening of

the 3-5/* line of HCl, the broadening of the infra-red lines of CO2, and

the splitting of the S-g/x band of water vapour, as due to rotational

frequencies existing in the far infra-red region. [The rotational

frequency repeats itself, as it were, close to a true vibrational frequency,

the rotational frequency being separated from the central vibrational

frequency by the small quantity ± Vj.]

The absorption bands in the short infra-red region have been

measured very accurately by Burmeister [Verh. d. D. phys. Ges., 15,

who found

589 (1913)) and by E. von Bahr [ibid., 731, 1150 (1913)),

that, in general, double bands are characteristic of this part of the

spectrum, which would correspond to the theory of Bjerrum provided

one assumes, as does E. von Bahr that, on account of insufficient dis-

persion, the middle sharp line does not appear.

The distance of the two decomposition bands must depend on

temperature on account of the variation of the rotational frequency with

the temperature— that is, assuming that rotation is the cause of the

phenomenon. On the other hand, this distance should be practically

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