H-1-NMR CHEMICAL-SHIFT AND INTRINSIC ACIDITY OF HYDROXYL-GROUPS - AB-INITIO CALCULATIONS ON CATALYTICALLY ACTIVE-SITES AND GAS-PHASE MOLECULES

For the molecules H(n)XOH (X = H, B, C, N, O, F, Al, Si, P, S, Cl); C2H5OH, CF3CH2OH, and CF3OH; B(OH)3, Al(OH)3, Si(OH)4, and OP(OH)3; H3AlOPH2OH and H3POAlH2OH; HO(H)Al(OH)3, H3SiO(H)Al-(OH)3, H3SiO(H)AlH3, and H3SiO(H)BH3; and (AlH2OH)2 and ((HO)2AlOH)2, some of which are models of surface hydroxyl groups, ab initio SCF calculations are performed of both the H-1 NMR chemical shifts and the deprotonation energies. As the latter are a measure of acidity the results obtained can be used to check whether the postulated property-reactivity relation between the chemical shift of the hydroxyl proton and its acidity exists. We find that this is not the case for the general set of systems studied. The reason is that the lone pairs on oxygen and the X-O bond make non-constant and non-negligible contributions to the chemical shift. However, for the limited set of surface hydroxyls which are bonded to B, Al, Si, or P completely coordinated by oxygen atoms, such a relation can be justified.