Optimization, Standardization, and Testing of a New NMR Method for the Determination of Zeolite Host−Organic Guest Crystal Structures

An optimized and automated protocol for determining the location of guest sorbate molecules in
highly siliceous zeolites from 29Si INADEQUATE and 1H/29Si cross polarization (CP) magic-angle spinning
(MAS) NMR experiments is described. With the peaks in the 29Si MAS NMR spectrum assigned to the
unique Si sites in the zeolite framework by a 2D 29Si INADEQUATE experiment, the location of the sorbate
molecule is found by systematically searching for sorbate locations for which the measured rates of 1H/29Si cross polarization of the different Si sites correlate linearly with 1H/29Si second moments calculated
from H−Si distances. Due to the 1H/29Si cross polarization being in the “slow CP regime” for many zeolite−sorbate complexes, it is proposed that the CP rate constants are best measured by 1H/29Si cross polarization
drain experiments, if possible, to avoid complications that may arise from fast 1H and 29Si T1ρ relaxations.
An algorithm for determining the sorbate molecule location is described in detail. A number of ways to
effectively summarize and display the large number of solutions which typically result from a prediction of
the structure from the CP MAS NMR data are presented, including estimates of the errors involved in the
structure determinations. As a working example throughout this paper, the structure of the low loaded
p-dichlorobenzene/ZSM-5 complex is determined under different conditions from solid-state 1H/29Si CP
MAS NMR data, and the solutions are shown to be in excellent agreement with the known single-crystal
X-ray diffraction structure. This structure determination approach is shown to be quite insensitive to the
use of relative rate constants rather than absolute values, to the detailed structure of the zeolite framework,
and relatively insensitive to temperature and motions.