Solid state intramolecular hydrogen bonds

Solid state characterization studies of the previously mentioned polymorphic systems [26-34] all utilize IR as a means to differentiate the various crystal modifications. In some cases, the observation of variations in IR absorption intensities has led to conclusions regarding intramolecular hydrogen bonding [26]. For other systems, fairly complete IR spectral band assignment has allowed for determination of structure for the polymorphic system. In one study [29], DSC-IR was used to identify the polymorphs and determine simultaneously the correlation between thermal events and structural changes. 

As was the case for dinitrobenzene, the meta and para nitroaniline isomers have essentially the same gaseous enthalpy of formation. In the gaseous phase, it is surprising to find that despite the more attractive quinonoid resonance structures92 for the para isomer (58) than for the meta (59) the met a and para nitroaniline have essentially the same gas-phase enthalpy of formation. In the solid and liquid states the intermolecular stabilization lowers the enthalpy of formation of the para isomer relative to the meta. Interestingly, the gas-phase intramolecularly hydrogen-bonded ortho isomer is of comparable stability to its isomers. In contrast, it is considerably less stable than its isomers in the solid state because it can form fewer intermolecular hydrogen bonds. All isomers of nitroaniline are more stable than calculated by additivity. 

In the solid state, 3-benzoylpyridine-2-carboxylic acid exists as the open-chain form 12, stabilized by an intramolecular hydrogen bond. In dioxane solution, the ring-chain equilibrium was observed. The IR spectrum reveals that the hydrochloride of acid 12 exists as a mixture of the protonated cyclic and open-chain forms (86MII). 

In contrast to solutions, because of the proximity of molecules in the solid state, the existence of M- -H intramolecular hydrogen bonding or its absence is controlled by competition between intermolecular classical bonds O-H- O and intramolecular interactions O-H- -M. It has been found that M- H bonding is more effective when the proton-accepting strength of the metal atom increases. 

Prior to this work, solid-state structures of TADDOL derivatives indicated the existence of an intramolecular hydrogen bond, implicating a BBA-type catalytic mechanism. For a review, see Seebach,... 

FIGURE 22. Intramolecular hydrogen bonding observed for A-saccharinperpropanoic acid 51 in the solid state... 

Simulation of spectra for (R)-acacpnH2 and its trifluoro derivative indicates that the syn conformation, predominant in solution, is that having the pn methyl group anti to the N atom, as found in the solid state.101 A similar conformer distribution is expected also for acacenH2 in view of the similarity of absorption spectra. In conclusion, it appears that these compounds exist in solution essentially in a unique conformation, maintained by strong intramolecular hydrogen bonds. 

---