Extreme infra-red dispersion of polar and non-polar liquids

The interaction between infra-red radiation and molecules in the liquid state reveals phenomena that are characteristic of (1) individual molecules, and (2) molecules only in the liquid state. To category (1) belong the spectral properties of molecules that are common to both the liquid and gaseous state—it is understood that there might be small shifts in the characteristic frequencies due to an interaction of the molecules in the liquid state. Most of the atomic vibration frequencies lie in the near infra-red but certain combination terms (made up of differences of near infra-red frequencies) are permitted in the spectral region considered. AIso some polyatomic molecules can have fundamental vibrational frequencies in the extreme infra-red. There is no evidence that pure rotation exists in liquids as in gases and this is understandable either by considering that shocks from neighbouring molecules are too disturbing for a free rotation to he maintained sufficiently long or, and this was experimentally verified for water, that the intermolecular fields are too powerful to permit free rotation for the quantum energies involved. This last explanation belongs to category (2) which implies a quasi-crystalline structure of liquids in which the molecules as a whole can oscillate linearly and execute oscillating rotational motions. Experimentally we can distinguish between phenomena that are characteristic of individual molecules (1) and those due to a quasi-crystalline structure, (2), by comparing the spectrum of a pure liquid with that of a solution where the intermolecular field is changed. The measurement of the absorption and reflexion of liquids not only reveals resonance phenomena; it permits the calculation of refraction and the molecular polarisation. The total molecular polarisation can be expressed as the sum of the electronic, atomic and permanent polarisation: P = P e + P a + P p = 4 π /3 N ( γ e + γ a + γ p ) = ε - 1/ε + 2 M/ d , (1) where N is the Avogadro number, γ the average polarizability, M the molecular weight, d the density and ε the static dielectric constant. This formula and those to follow neglect any interaction between molecules (except that the effective field acting is given by the Lorenz relation R = E + 4/3 P); however, we shall consider them as giving a first approximation for molecules in the liquid state.