LOW-TEMPERATURE C-13 NMR INVESTIGATIONS ON CARBON-MONOXIDE HYDROGEN-BONDED TO BRONSTED ACID SITES IN H-Y ZEOLITES

C-13 NMR investigations were carried out at 4.5 K in order to analyze the lineshape of the solid-state spectrum of (CO)-C-13 adsorbed on Bro nsted acid sites in H-Y zeolites. The spectrum was found to be dominat ed by the chemical shift anisotropy and by the heteronuclear magnetic dipolar interaction between the C-13 nucleus of the (CO)-C-13 molecule and the proton of the Bronsted acid site where the molecule is adsorb ed. Therefore, an analysis of the lineshape of the low-temperature C-1 3 NMR spectrum allows a precise determination of the hydrogen bond dis tance r(H-C) between these nuclei. A value of r(H-C)=0.21 +/- 0.01 nm has been obtained by this method which is in excellent agreement with recent quantum chemical calculations.