Hm. Kao et al., Determining the structure of trimethylphosphine bound to the Bronsted acidsite in zeolite HY: Double-resonance NMR and ab initio studies, J PHYS CH B, 104(20), 2000, pp. 4923-4933
Solid-state NMR methods and ab initio calculations have been employed to in
vestigate the structure of the trimethylphosphine (TMP)-Bronsted acid site
complex in zeolite HY. Al-27/P-31 and Al-27/H-1 rotational echo double-reso
nance NMR experiments performed at -150 degrees C were utilized to measure
Al-P and Al-H-B distances for the acid site-TMP complex of 3.95 (+/-0.05) a
nd 2.8-3.1 Angstrom, respectively, where H-B is the Bronsted acid site prot
on. A more accurate measurement of the Al-H-B distance was not possible sin
ce models that assume the presence of isolated Al-H spin pairs are not vali
d in this case. A P-H-B distance of 1.40 (+/-0.02) Angstrom was obtained by
fitting the spinning sidebands in the H-1 magic angle spinning (MAS) NMR s
pectrum. These internuclear distances are within the range of the Al-P, Al-
H-B, and P-H-B distances obtained from ab initio calculations for the ion p
air (IP) TMPH+-zeolite complex that is formed by transferring a Bronsted ac
id proton to TMP. In contrast to the P-31 MAS NMR spectra, which indicated
that the only stable species was TMPH+, ab initio calculations on small clu
ster models predicted that the neutral complex should be more stable than t
he IP complex. However, use of a larger zeolite fragment in the calculation
s enhanced the relative stability of the IP structure.