Effects of phase on spectra

Inelastic neutron scattering spectrum of [Ni(Ti -C3H5)2]. The bands at 68 and 87cm have not been observed in either IR or Raman spectra. Redrawn from [8] with permission of The Royal Society of Chemistry. 

Parts of IR spectra of a sample (a) as a gas, (b) as an amorphous solid, (c) as a crystalline solid, and (d) isolated in a matrix of solid argon. 

In condensed phases bands are often narrower, so that nearby vibrations are less likely to be obscured, and the peak heights are greater than in the gas phase. Frequencies are also shifted from those in the gas phase, due to interactions between a molecule and its neighbors. In liquids and solutions there is little short-range order, and at 

In crystalline solids the effects of randomness of environment and of quantized rotation are absent, and very sharp vibration bands can be observed (Rgure 8.7(c)), especially in the Raman Effect. Surface reflection effects vary a lot in the vicinity of an IR absorption band (because the refractive index of the sample varies - a phenomenon known as anomalous dispersion) and distort its apparent profile (known as the Christiansen effect). Samples must be prepared very carefidly by fine grinding, to ensme that high quality spectra can be obtained. 

Calculated phonon spectrum for phase I NH3 showing (a) the internal modes of vibration and (b) the external lattice modes of vibration. Note the calculated band intensities are not indicative of the experimental values. Adapted with permission from [10]. Copyright 2008 American Chemical Society. 

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