The influence of halogenating Si in zeolite type model systems (SiH3-nFn-OH-AlH3 and SiH3-nCln-OH-AlH3 with n=0, 1, 2, and 3) and silanols (SiH3-nFn-OH and SiH3-nCln-OH with n=0, 1, 2, and 3) was investigated by calculating different properties of the hydroxyl group (IR frequencies, integrated IR intensities) and comparing these with gas-phase acidities, Delta G(acid). The trends in these quantities are explained in terms of the competition between electronegativity and softness as a function of the systems considered. The results of this competition are reflected in the electronic charges on the hydrogen and oxygen in the hydroxyl group and compared to previously reported trends for halogenated silanols.