Vibrational Spectroscopy via Inelastic Neutron Scattering

Inelastic neutron scattering (INS) is a well-established technique for vibrational spectroscopy and for testing ab initio calculations. The use of vibrational INS as a technique in molecular spectroscopy is described and illustrated in this chapter. INS is particularly sensitive to the motions of hydrogen atoms relative to that of most other atoms including deuterium. The use of H/D isotopic partitioning in INS is described. When there are multiple exchangeable sites and a fixed H/D ratio, the arrangement that lowers the zero-point level is favored at low temperature. Overtone and combination transitions are allowed in INS even in the harmonic approximation. Measurement of intensity as a function of momentum transfer can distinguish the order of the transition as fundamental, first overtone, etc. and differs for harmonic and quartic oscillators. INS spectra have no symmetry selection rules and thus transitions that are otherwise forbidden can be observed. Neutrons interact with the motion of nuclei rather than to the response of the electrons to the motions of nuclei. INS intensities can be computed with good reliability using known atomic cross-sections. Modern quantum chemical methods provide accurate intensities and so spectral assignments are very reliable. Applications of these principles to several hydrocarbons and to the short strong symmetric hydrogen bond of the H₅O₂⁺ Zundel cation are presented. In this case, the coupling of motions in neighboring unit cells is very strong as revealed in spectral changes seen with isotopic dilution. For hydrocarbons where intermolecular interactions are very weak, INS can be used to establish molecular conformations in crystals.