Water Molecules and Clusters in Zeolites

While these computational studies [34] were awaiting publication, a neutron diffraction study on H-SAPO-34 provided evidence for a protonated water molecule [38]. Whereas comments in the more popular press [39] stressed the apparent disagreement with previous calculations ( much of the confusion about how zeolites work stems from quantum calculations ) and used the entertaining title Quantum mechanics proved wrong , a comment to the original paper in the same issue 

This perfectly fits our picture that the proton transfer depends on the proton affinity of the adsorbed molecule the proton affinity of the water trimer (853 kj mokh Table 22.1) is about that of ammonia (858 kj mol i). 

The distances between the leff (Oj,) and the central water molecules (O,, ) are given in the left bottom part of Fig. 22.5. There are several attempts by the bw proton to jump to the central water molecule, but they are not successful. The average Ojj-Ow distance along the trajectory is 256 nm, while the energy minimum distance is 247 pm. This increase in the intermolecular distance is accompanied by a shortening of the bond from 111 pm (minimum structure) to 108 pm at 

even at higher temperature, there is no indication for a proton transfer to the central water molecule. In turn, trajectories started at the H20(H30+)H20 local minimum structure never reached the global minimum structure. This confirms the H30+(H20)2 structure for the protonated water trimer in H-SSZ-13 and points to the important role of the environment in the structure of protonated water clusters. 

We also conclude that proton transfer between the water trimer and the zeolitic Bronsted site occurs on the picosecond time scale. 

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