566 I explain the solid state effects shown by free base porphyrins is proposed, which takes into account possible rearrangements of porphyrin molecules in the crystal. Should these calculations give positive results, general conclusions could be drawn about porphyrin tautomerism.The solid-state N M R results presented in this work do not appear to be in complete agreement with the previous X-ray analyses of the studied porphyrins. Although it is conceivable that the crystalline forms that we have employed for the CPMAS study are not the same as those used for the crystallographic analysis, it may be worthwhile to note that since hydrogen atoms are weak X-ray scatterers it is difficult to locate them by X-ray diffraction. This problem is highlighted when hydrogens are bonded to electronegative atoms like nitrogen, due to the distortion of the electron density along the N-H direction.66 However, in the case of porphine (where K = 1) it is possible to reconcile the presence of the two tautomers in the solid with the localization of the central hydrogens proposed by the X-ray analysis, if it is assumed that the tautomeric process is coupled to a rotation of (66) Taylor, R.; Kennard, 0. Acc. Chem. Res. 1984, 17, 320. the molecule as a whole in such a way that the translational symmetry of the crystal is maintained. This could be achieved by a rotation of 90° about the main molecular axis combined with the hydrogen migration process. We are currently studying this possibility using wide line spectroscopy. In any case, we have shown that 13C C P M A S N M R , when performed at different temperatures and corrected by intermolecular effects, is one of the most useful tools for studying dynanical processes in the solid state.Acknowledgment.

The hydrolysis of functional trialkoxysilanes was studied under various conditions by 'H NMR spectroscopy. The silane concentration was 1 -2% by weight in a water/acetonesystem. Under these conditions, the reaction was found to be first order. In detail, influence of pH, grade of stirring, and size and structure of functional groups of the alkyltrialkoxysilanes were investigated in order to compare the relative reaction rates of the different compounds. Einleitung

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