Infrared Spectroscopy and Gas Electron Diffraction

Infrared spectroscopy has the advantage that it can be applied to the same compound in all three states of matter. However, there still appears to be insufficient data from gas-phase studies of hydrogen-bonded complexes to permit a correlation between X-H stretching frequencies and bond lengths, so as to improve on the long-standing empirical Badger s Rule [158]. 

Similarly, the down-field chemical shift of the proton resonance in H-NMR spectroscopy, which is a diagnostic tool for hydrogen bonding in solution, might be related to crystal structural data through the recent development of solid-state NMR spectroscopy. 

An interesting example where infrared O-H frequencies were used to correlate structures is for choline chloride dihydrate, which is postulated to have a semi-clathrate hydrate structure by analogy with the known crystal structure of tetraethyl ammonium fluoride pentahydrate [162]. 

The connection between energies and geometry must be empirical or indirect through theoretical hypotheses and calculations. This connection is further complicated by the other intermolecular interactions in crystals. The hydrogen-bond lengths observed in crystals, which provide the vast majority of the data available, are modulated by the crystal field effects and any one observation is therefore not necessarily representative of the potential energy minima of the isolated hydrogen-bonded adducts. 

Some values for the enthalpy of hydrogen-bond formation are given in Tkble 2.5. The sequence of energies is in qualitative agreement with the bond lengths observed in crystals, OH 0=C OwH- -Ow NH- 0=C N(H)H N, but no generally accepted quantitative relationships could be proposed. 

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