The vibration spectrum of the ammonia molecule

Abstract

1. 1. The infra-red absorption spectrum of ammonia in carbon tetrachloride and aqueous solutions has been studied with a registering quartz spectrograph. The visible spectrum in aqueous solution has also been examined with a Hilger El spectrograph. 2. 2. The absorption bands obtained are quite analogous to those usually observed with ammonia gas. The positions of certain of the bands depend upon the solvent while others do not. 3. 3. Of particular interest is a series of bands, 2.916, 1.51, 1.035, 0.795 0.652 and 0.556 μ, whose positions are apparently independent of the presence or nature of the solvent. The frequencies of these bands obey the equation: v n = 97 ± 3400 n − 70 n 2. They are believed to arise from anharmonic vibrations within the NH bond for the following two reasons: 3.1. a. The energy of the oscillator, S.i equivalent volts, calculated from the above equation agrees approximately with the energy of a single NH linkage, 3.99 equivalent volts, computed chemically. 3.2. b. This series of bands persists, with systematic minor shifts, in the spectra of primary and secondary organic derivatives of ammonia. 4. 4. The appearance of the constant term, 97 cm −1, in the equation is unusual. It is interpreted as arising from a change in potential energy of the molecule, before or while absorption by vibration takes place, arising from a slight inter-nuclear rearrangement. 5. 5. The whole infra-red and visible vibration spectrum of the NH 3 molecule can be correlated on the basis of three fundamental bands, 10.55, 6.132 and 2.916 μ. 6. 6. The second, as well as the last, of these bands is believed to be associated with the NH bond, inasmuch as it persists in the spectra of many of the organic derivatives of ammonia. It possibly has non-harmonic overtones of a positive deviation type.