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We describe the results of quantum chemical calculations (DFT and MP2) on the intermolecular interactions involving ammonia and halofluoromethanes. The equilibrium C−X···N geometries are linear and the X···N distances are shorter than the sum of the van der Waals radii. The binding energies of CF3X···NH3 increase from 2 to 6 kcal/mol on following the sequence X = Cl, Br, I. Also, progressive introduction of F atoms in methyliodides raises the interaction energy from 2 kcal/mol for CH3I to 6 kcal/mol for CF3I. Therefore, halogen bonding involving perfluorinated alkylhalides and appropriate donors can be comparable in strength to strong hydrogen bonding. This agrees with recent experimental observations, that also the former can drive the construction of supramolecular edifices overcoming the low affinity between perfluorocarbons and hydrocarbons. Calculation of the atomic charges by the Atoms in Molecules method indicates that the charge-transfer contribution to the interaction energy is much less important for the present systems than for dihalogen−ammonia complexes.

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29 Si and 27 Al NMR chemical shifts have been evaluated from the NMR shielding tensors obtained using the SOS-DFPT technique. The 29 Si NMR chemical shifts have been calculated for the nine crystallographically distinct Si sites of the zeolite-β lattice. The calculations were carried out on cluster models Si(OSiH 3 ) 4 and R 3 SiOSiR 3 (R ) OSiH 3 ) representing one (1T) and two (2T) sites in the zeolite lattice, respectively. Using the 1T models, the nine signals of the experimental spectrum have been assigned with a relative error of less than 1 ppm, the absolute error being estimated at about 5 ppm. The use of a "fragmentaveraging" technique based on the results obtained with the 2T models leads to calculated absolute NMR shifts with an accuracy of (1 ppm. The 27 Al NMR shifts of AlH(OSiH 3 ) 4 models have also been calculated, after relaxation of the local geometry. It was found that the relative positions of the 27 Al chemical shifts are in agreement with the experimental spectrum.