Hydrogen crystalline environment

Since the experimental EPR specttum shows unambiguously that in the solid state the three H(N) atoms are equivalent (probably due to tunneling) [138], only average values of hcc s on H(N) atoms are considered. Note that the introduction of environmental effects does not modify the results obtained for the isolated radical, which are already in fair agreement with experiment. Thus, the magnetic properties of glycine radical in this zwitterionic form are scarcely affected by the crystalline environment, and the EPR spectra for this system are well reproduced by computations for the isolated radical. In particular, the hcc of the a-hydrogen atom has a value close to that of typical aliphatic jt-radicals. 

A study of the photochemical reactivity of salts of the amino ketone (44) with enantiomerically pure carboxylates has been reported. The irradiations involved the crystalline materials using A, > 290 nm and the reactions are fairly selective which is proposed to be the result of hindered motion within the crystalline environment. Some of the many results, using (S)-(—)-malic acid, R-(+)-malic acid and (2R,3R)-(+)-tartaric acid, are shown in Scheme 1. The principal reaction in all of the examples is a Norrish Type II hydrogen abstraction and the formation of a 1,4-biradical. This leads mainly to the cis-cyclobutanol (45) by bond formation or the keto alkene (46) by fission within the biradical. A very minor path for the malate example is cyclization to the trn 5-cyclobutanol (47). A detailed examination of the photochemical behaviour of a series of large ring diketones (48) has been carried out. Irradiation in both the solid phase and solution were compared. Norrish Type II reactivity dominates and affords two cyclobutanols (49), (50) and a ring-opened product (51) via the conventional 1,4-biradical. Only the diketone (48a) is unreactive... 

The base plan unit-cell projection (Figure 2) shows the positions of the water molecules in the unit-cell as deduced from an analysis of the equatorial x-ray diffracted intensities for a fiber diagram recorded at about 50X relative humidity. The water molecules cluster in distinct areas and form a helical column whose symmetry matches that of the xylan chains. In fact, the water of hydration may dictate the symmetry of the xylan chains. The energetically (theoretical) most stable conformation of the xylan chain involves twofold symmetry, whereas in the hydrated crystalline environment, as deduced from x-ray diffraction, the xylan chains possess three-fold symmetry. The water molecules stabilize the three-fold structure by the formation of hydrogen bonds. This structure is an example of columnar hydration which allows a symmetric... 

Some recent calculations on the (OH)3-Al-OH-Si-(OH)3 prototype molecule representing an acidic site of a zeolites interacting with an NH3 molecule clearly demonstrated that isolated (in vacuo) clusters are in appropriate to describe correctly the ionic complex, which is formed in sodalite cages [224]. It seems that the effect of the crystalline environment plays a crucial role in the stabilization of the proton-transferred form (0H)3A10Si(0H)3. .. NH. Similar qualitative conclusions were drawn from earlier studies on the hydrogen bonded subunits in the hydroxonium perchlorate crystal [208]. 

Many pharmaceuticals are prepared as hydrochloride salts for reasons of solubility, crystallinity and/or stability. Schurko and coworkers describe the potential of Cl SSNMR spectroscopy as a tool for pharmaceutical hydrochloride polymorph fingerprinting and recognition [12,34]. By examining a large series of samples, it is shown that the chlorine EFG and CS tensors are generally distinct for each pharmaceutical compound, and for each polymorph. The authors also correlate the quadrupolar coupling parameters to the local hydrogen bond environment of the chloride ions and report several trends, some of which corroborate those reported previously [31,35]. 

While crystal structures of rubredoxins have been known since 1970 (for a full review on rubredoxins in the crystalline state, see Ref. (15)), only recently have both crystal and solution structures of Dx been reported (16, 17) (Fig. 3). The protein can be described as a 2-fold symmetric dimer, firmly hydrogen-bonded and folded as an incomplete /3-barrel with the two iron centers placed on opposite poles of the molecule, 16 A apart. Superimposition of Dx and Rd structures reveal that while some structural features are shared between these two proteins, significant differences in the metal environment and water structure exist. They can account for the spectroscopic differences described earlier. 

---