Surface-alkoxylated silica can be prepared by reacting precipitated silica with high-boiling and polymeric alcohols. The structure of the products was investigated through 13C and 29Si MAS NMR experiments. The influence of the alkoxy group structure and density on the hydrolytic stability of the alkoxylated silicas was determined through contact with boiling water. Use of polymeric or branched alcohols increased the hydrolytic stability of surface-alkoxylated silicas compared to straight-chain alcohols. The rate of surface substitution is increased by both acid and base catalysis.

Three aspects of the esterification reaction of silica surfaces with alcohols that are relevant for the synthesis of functional materials have been studied:  surface coverage, hydrolytic stability, and the introduction of functional groups. The chemisorption of alcohols on silica can be described by the Langmuir adsorption model, which allows precise control of the surface coverage. It was shown that the hydrolytic stability of the materials obtained could be significantly improved by reacting the esterified silica with hexamethyldisilazane. The use of alcohols containing mercapto groups gave silica with surface-tethered mercapto groups. These groups are available for further chemical reactions, but the hydrolytic stability of the functional material decreases in comparison to silica esterified with the corresponding nonfunctional alcohol.

The magnitude and orientation of the principal components of the carbonyl carbon chemical shift tensor of a typical aryl aldehyde, 3,4-diben~yloxybenzaldehyde-~~C,~H,, 1, have been determined from the analysis of I3C NMR spectra of static powder samples obtained at 4.7 and 9.4 T. The principal components of the carbon shielding tensor of formaldehyde, 2, have been calculated from the experimental I3C nuclear spinrotation tensor, which is available in the literature. The experimental results are compared with those of ab initio chemical shielding calculations using the GIAO (Gauge-Zncluding Atomic Orbitals) method. For 1, the most shielded component of the carbonyl carbon chemical shift tensor, 833, is perpendicular to the 2H--'3C=0 plane, whereas the least shielded component, 81 I , is within this plane approximately perpendicular to the C-0 bond. The orientation of the carbon shielding tensor of formaldehyde is analogous to that of 1; however, the precise orientation of the three principal components is dictated by the higher molecular symmetry (e.g., 8 2 2 is coincident with the C-0 bond). The deshielding associated with 811 is attributed to a large local magnetic dipole transition moment of the an* type. Analysis of I3C NMR spectra of solid 3,4-diben~yloxybenzaldehyde-'~C~~H, obtained with magic-angle spinning yields a value for the indirect 13C-'H spin-spin coupling constant, 1J(13C,2H)iSo = 26.4 f 0.5 Hz. From 2H NMR spectra of a static powder sample of 1, the 2H nuclear quadrupolar coupling constant was estimated to be 151.5 f 1 kHz. rotation data available for formaldehyde or the relationship between nuclear spin-rotation tensors and nuclear magnetic shielding tensors. Finally, we compare experimental carbonyl carbon shielding tensors of the aldehyde moiety to those calculated using the GIAO (Gauge-Including Atomic Orbitals) method. NMR Larmor frequency, yc is the I3C magnetogyric ratio, Bo

Several solid-state NMR techniques have been employed to characterize phosphorus chemical shift tensors and 31P−31P spin−spin coupling interactions for bisphosphine molybdenum complexes where only one phosphorus of the bisphosphine ligand is coordinated to the metal. The bisphosphine ligand of each complex was either tetraphenyldiphosphine, tpdp, or bis(diphenylphosphino)methane, dppm. For one compound, (OC)5Mo(η1-dppm), single crystals were examined by 31P NMR and X-ray diffraction. For the metal-bound phosphorus, the most shielded direction of the chemical shift tensor is near the Mo−P bond axis. For the non-coordinated phosphorus, the most shielded direction is oriented in the direction of the formal electron lone pair on phosphorus. For the other compounds, stationary powder samples were examined using dipolar-chemical shift and 2D spin−echo techniques. Powder samples were also examined under slow magic-angle spinning, variable-angle spinning, and rotational-resonance conditions. Analysis of these 31P NMR spectra suggested chemical shift tensor orientations analogous to those measured for the single crystal; however, in the case of powders only the relative orientations of the two chemical shift tensors with respect to the P−P axis can be determined. Our investigations indicate that spectra from powder samples should be analyzed at two or more applied magnetic fields. The 2D spin−echo experiment proved to be invaluable for obtaining the effective dipolar P,P coupling constant, R, and the relative signs of R and the indirect spin−spin coupling constant, J. Values of n J(31P,31P) measured in the solid state often differ significantly from the conformationally averaged values obtained in solution. The tpdp derivatives appear to have a reduced R, whereas for the dppm systems, R agrees very well with the value calculated from the known P,P separations.

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