Proximity, photochemical reactivity, and intermolecular 1H-13C cross polarization in benzophenone-d10 and cyclohexane in the zeolite NaX

Article abstract of DOI:10.1021/ja9623368

The photochemistry, photophysics, and solid-state ¹³C CPMAS NMR of benzophenone and cyclohexane in the zeolite NaX have been analyzed to investigate their proximity, relative mobility, and intermolecular reactivity. Photochemical irradiation of benzophenone in pure cyclohexane yields benzopinacol and benzhydrol as the predominant products. In contrast, irradiation in the solid state leads to 1-cyclohexyl-1,1-diphenylmethanol as the only product by collapse of the radical pair formed after hydrogen abstraction from cyclohexane by excited benzophenone. Phosphorescence analysis at 77 K suggests a highly polar environment, but lack of emission at 300 K in the presence of cyclohexane is assigned to triplet decay via an efficient hydrogen abstraction reaction. Spectral analysis by ¹³C CPMAS NMR reveals that benzophenone and cyclohexane are adsorbed in an approximate 1:2 ratio. Changes observed in spinning and static samples before and after photolysis are interpreted in terms of molecular motions that are capable of affecting the spectral line width. That benzophenone and cyclohexane share the zeolite supercages in a close packed arrangement was shown by a relatively efficient ¹H-¹³C intermolecular cross polarization from cyclohexane to benzophenone-d₁₀. Comparison of the CPMAS intensities measured with deuterated and nondeuterated benzophenone samples under identical conditions suggest that intermolecular C-D···H-R distances between carbon atoms of deuterated benzophenone and hydrogens of cyclohexane have an average value of ca. 2.2 A?.

Full text of DOI:10.1021/ja9623368

184
J. Am. Chem. Soc. 1997, 119, 184-188
Proximity, Photochemical Reactivity, and Intermolecular
1H-13C Cross Polarization in Benzophenone-d10 and
Cyclohexane in the Zeolite NaX
Deniz Cizmeciyan, Laura B. Sonnichsen, and Miguel A. Garcia-Garibay*
Contribution from the Department of Chemistry and Biochemistry, UniVersity of California,
Los Angeles, California 90024
ReceiVed July 8, 1996X
Abstract: The photochemistry, photophysics, and solid-state 13C CPMAS NMR of benzophenone and cyclohexane
in the zeolite NaX have been analyzed to investigate their proximity, relative mobility, and intermolecular reactivity.
Photochemical irradiation of benzophenone in pure cyclohexane yields benzopinacol and benzhydrol as the predominant
products. In contrast, irradiation in the solid state leads to 1-cyclohexyl-1,1-diphenylmethanol as the only product
by collapse of the radical pair formed after hydrogen abstraction from cyclohexane by excited benzophenone.
Phosphorescence analysis at 77 K suggests a highly polar environment, but lack of emission at 300 K in the presence
of cyclohexane is assigned to triplet decay via an efficient hydrogen abstraction reaction. Spectral analysis by 13C
CPMAS NMR reveals that benzophenone and cyclohexane are adsorbed in an approximate 1:2 ratio. Changes observed
in spinning and static samples before and after photolysis are interpreted in terms of molecular motions that are
capable of affecting the spectral line width. That benzophenone and cyclohexane share the zeolite supercages in a
close packed arrangement was shown by a relatively efficient 1H-13C intermolecular cross polarization from
cyclohexane to benzophenone-d10. Comparison of the CPMAS intensities measured with deuterated and nondeuterated
benzophenone samples under identical conditions suggest that intermolecular C-D‚‚‚H-R distances between carbon
atoms of deuterated benzophenone and hydrogens of cyclohexane have an average value of ca. 2.2 Å.
Zeolites are microcrystalline aluminosilicates with periodic
structures of interconnecting channels and cages.1-3 Their
ability to include medium size organic chromophores makes
them attractive for the study of photochemical processes in
nanoscopic containers.4-9 The zeolites most often used for
photochemical studies are those with medium size cavities that
allow for intracrystalline complexation and diffusion such as
the synthetic faujasites or zeolites X and Y.1 The internal
structure of zeolites X and Y consists of 4-fold interconnected
supercages with 15 Å diameter interconnected with each other
by pores of 7.4 Å in a network that has the topology of the
diamond structure (Figure 1).
Because of their relatively large pores sizes, intracrystalline
diffusion in zeolites X and Y is relatively efficient.9-11 When
size and shape requirements are satisfied, there may be several
molecules per site allowing for the occurrence of bimolecular
reactions.6,12-21 It is of interest in these reactions to know
whether encounters between prospective reactants may occur
Figure 1. Representation of the structure of Zeolite NaX with a
supercage as a vertex of a diamond structure where bonds represent
interconnecting channels.
within the lifetime of the excited state or whether reaction
requires a preorganized arrangement of the two reactants.9 In
this paper, we have studied benzophenone and cyclohexane in
the zeolite NaX in search for a correlation between bimolecular
reactivity and intermolecular nuclear 1H-13C cross polarization.
Several photochemical, photophysical and solid-state 13C
CPMAS NMR measurements were carried out in order to
investigate their proximity, molecular motion, and relative
diffusion.
X Abstract published in AdVance ACS Abstracts, December 1, 1996.
(1) Breck, D. W. Zeolite Molecular SieVes: Structure, Chemistry and
Use; John Wiley and Sons: New York, 1974.
(2) Dyer, A. An Introduction to Zeolite Molecular SieVes; John Wiley
and Sons: New York, 1988.
(3) Rabo, J. A. Zeolite Chemistry and Catalysis; American Chemical
Society: Washington, D.C. 1975.
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1993, 26, 530-536.
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7-9.
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Chem. 1989, 93, 1309-1313.
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7710.
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Tetrahedron Lett. 1990, 31, 47-50.
(20) Lei, X.; Turro, N. J. J. Photochem. Photobiol. A 1992, 69, 53-56.
(21) Ramamurthy, V.; Sanderson, D. R.; Eaton, D. F. J. Am. Chem. Soc.
1993, 115, 10438-10439.
(6) Thomas, J. K. Chem. ReV. 1993, 93, 301-320.
(7) Yoon, K. B. Chem. ReV. 1993, 93, 321-339.
(8) Abrams, L.; Corbin, D. R.; Turro, N. J. In Characterization of Porous
Solids; Unger, K. U., Ed.; Elsevier: Amsterdam, 1988; pp 519-529.
(9) Ramamurthy, V.; Garcia-Garibay, M. A. In ComprehensiVe Supramo-
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pp 693-719.
(10) Garcia-Garibay, M.; Zhang, Z.; Turro, N. J. J. Am. Chem. Soc. 1991,
113, 6212-6218.
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(13) Karger, J.; Pfeifer, H. Zeolites 1987, 7, 90-106.
S0002-7863(96)02336-0 CCC: $14.00 © 1997 American Chemical Society

Benzophenone-d10 and Cyclohexane in the Zeolite NaX
J. Am. Chem. Soc., Vol. 119, No. 1, 1997 185
Scheme 1
Scheme 2
The abstraction of a hydrogen from cyclohexane by triplet
benzophenone is a well known reaction which requires close
proximity and adequate CdO‚‚‚H-R alignment that in fluid
solution is accomplished by random encounters. In contrast,
reaching adsorption saturation, the excess solvent was decanted, and
the samples were allowed to dry under argon.
1
intermolecular H-13C cross polarization depends on nuclear
Photolyses. Photolyses were carried out under argon with a 400
medium pressure Ace-Hanovia mercury arc lamp, and the products were
recovered by dissolving the zeolite in 1 N HCl solution followed by
organic extraction. Solution irradiations were carried out in argon-
purged cyclohexane with ca. 0.029 M benzophenone. Products were
analyzed by GLC and GLC-MS and identified by comparison of their
retention times and fragmentation patterns with those of authentic
samples. Benzhydrol and benzopinacol were commercially available.
1-Cyclohexyl-1,1-diphenylmethanol was prepared by Grignard reaction
of benzophenone with cyclohexylmagnesium bromide under argon
atmosphere. Benzopinacolone was prepared by refluxing benzopinacol
in benzene using p-toluenesulfonic acid (TsOH) as catalyst.
NMR Spectroscopy. Solid-state NMR spectra were obtained using
a Bruker MSL-300 instrument. 13C (74.8 MHz) spectra were obtained
using CPMAS, and the suppression of the spinning side bands was
implemented by the TOSS23 sequence. Hexamethylbenzene (132 ppm)
was used as the chemical shift reference, and the magic angle was
adjusted with KBr. The 90° pulse width used was 4.9 µs. Adequate
cross polarization was achieved using 2.1 ms as a contact time in the
case of protonated samples and 5 ms contact time in the case of
benzophenone-d10. A recycle delay of 2 s was employed for all zeolite
samples. In order to compare intensities, the same number of scans
were recorded for similar samples. The spectra of crystalline 1-cyclo-
hexyl-1,1-diphenylmethanol were acquired with a 20 s recycle delay.
The rotation rates used were between 3.00 and 3.50 kHz. All solid-
state NMR spectra were processed with line broadening of 200 Hz.
All solution spectra were obtained with a Bruker ARX 400 MHz
spectrometer in deuterated chloroform with TMS as internal reference.
dipolar interactions and requires close proximity and rigidity
and cannot occur under conditions of extensive molecular
motion where dipolar coupling disappears. Our experiment
involves conventional 13C CPMAS NMR measurements with
deuterated benzophenone and nondeuterated cyclohexane. The
CP part of the experiment involves selective 1H magnetization
followed by dipolar transfer to the less abundant 13C nuclei under
Hartman-Hahn matching conditions.22 Since observation of
the carbon spectrum relies on the availability of a polarized 1H
source, deuterated benzophenone cannot be observed unless
there is a dipolar-coupled, hydrogen-rich donor source that is
relatively rigid and at a relatively close distance. It is expected
that the CP efficiency will be best at the closest distances and
under restricted molecular motions. Finally, it is interesting
to point out that polarization transfer is expected to occur within
time scales that match the upper limit for triplet state reaction.
Thus, efficient polarization transfer from cyclohexane to deu-
terated benzophenone described below can be taken as evidence
for a relatively static bimolecular reaction arrangement during
the lifetime of the reactive triplet state.
Experimental Section
General Methods. Gas chromatography (GLC) data was obtained
on a Hewlett Packard 5890 Series II gas chromatograph, with an HP
3396 Series II integrator. An HP-1 cross-linked methyl silicone gum
column and an HP-20M Carbowax 20M column were used. Both are
25 m × 0.20 mm, with a 0.20 µm film thickness. Phosphorescence
spectra were acquired at 77 K and 300 K by front face illumination
and detection using a Spex Fluorolog Spectrofluorimeter equipped with
a pulsed lamp (10 µs width) and a double grating monochromator.
Zeolite NaX was obtained from Alfa. Benzophenone was obtained
from Fisher Scientific. All solvents were ordered from Fisher Scientific
and distilled over sodium and benzophenone under a nitrogen atmo-
sphere. 4-Methoxybenzophenone was bought from Pfaltz and Bauer,
Inc. Magnesium turnings were purchased from Malinckrodt Inc.
Benzopinacol and benzhydrol were obtained from Aldrich and Eastman
Kodak Co., respectively. Bromocyclohexane and p-toluenesulfonic acid
were ordered from Matheson, Coleman, and Bell. Benzophenone-d10
was prepared by hydrolysis of the dichlorodi(phenyl-d5)methane
prepared by AlCl3-catalyzed Friedel-Crafts reaction of benzene-d6 with
CCl4.
Preparation of Zeolite Samples. Zeolite NaX was dried at 500
°C prior to use. The adsorption of benzophenone and cyclohexane
onto zeolite NaX was achieved by vigorously stirring a suspension with
450 mg of dry zeolite in a solution containing 5 mL of cyclohexane
and 40 (45) mg of benzophenone (benzophenone-d10) under argon
atmosphere. The amount of benzophenone adsorbed was monitored
by GLC analysis of the supernatant solution after adding known
amounts of 4-methoxybenzophenone as an internal reference. After
Results and Discussion
Photochemistry in Solution and in the Solid State.
Hydrogen abstraction from cyclohexane by triplet benzophenone
in fluid media is a very well studied reaction (Scheme 2).24-26
The triplet state of benzophenone in nonpolar solvents has a
lowest energy n,π* configuration, and the hydrogen transfer
reaction in neat cyclohexane occurs with a rate of 1.2 × 106
s-1 with quantitative quantum efficiency (i.e., Φ ) 1.0).24,25 In
fluid solutions, triplet ketone and hydrogen donor encounter each
other and react by hydrogen transfer in an activated process
(i.e., kH < k-diff, Scheme 2) with Ea ) 4 Kcal/mol and log A )
9. The resulting triplet radical pair efficiently escapes the
solvent cage to form free radicals that combine to form
benzopinacol as the main isolable product along with minor
quantities of benzhydrol, 1-cyclohexyl-1,1-diphenylmethanol,
and other unidentified compounds from the cyclohexyl radical
(23) Dixon, W. T.; Shaefer, J.; Sefcik, M. D.; McKay, R. A. J. Magn.
Reson. 1982, 49, 341-345.
(24) Topp, M. R. Chem. Phys. Lett. 1975, 32, 144-149.
(25) Giering, L.; Berger, M.; Steel, C. J. Am. Chem. Soc. 1974, 96, 953-
958.
(26) Inbar, S.; Linschitz, H.; Cohen, S. G. J. Am. Chem. Soc. 1981, 103,
1048-1054.
(22) Hartmann, S. R.; Hahn, E. L. Phys. ReV. 1962, 128, 2042-2053.

186 J. Am. Chem. Soc., Vol. 119, No. 1, 1997
Cizmeciyan et al.
Figure 2. Phosphorescence spectra of (solid line) benzophenone
coadsorbed with cyclohexane in Zeolite NaX at 77 K and (dotted line)
pure benzophenone in NaX under vacuum at 300 K.
(Scheme 2). The photochemical reaction in the zeolite proceeds
in a different manner.
The adsorption of benzophenone and cyclohexane in NaX
was accomplished by stirring freshly dried samples of zeolite
in cyclohexane solutions of benzophenone (ca. 0.05 M). Ad-
sorption analysis showed saturation within 24 h and loading
levels of 8.5 to 9.1 w/w corresponding to ca. 0.80-0.83 mole-
cules per supercage. It was shown that benzophenone is ther-
mally stable when complexed with NaX at ambient temperatures
as it could be recovered in essentially quantitative yields. In
contrast, no starting material or products could be retrieved from
samples that had been irradiated with λ > 300 nm for 7 h. Failed
attempts to adsorb benzopinacol and 1-cyclohexyl-1,1-diphen-
ylmethanol in NaX showed that the expected photoproducts do
not fit through the 7.4 Å pores and remain irreversibly trapped
when formed inside the zeolite. In fact, extraction of 80%
product and 20% unreacted benzophenone could only be
accomplished after destructive dissolution of the zeolite in a 1
N HCl solution. The only product obtained in the zeolite was
identified as 1-cyclohexyl-1,1-diphenylmethanol. Results ob-
tained with benzophenone-d10 were identical to those observed
with non deuterated ketone, and formation of 1-cyclohexyl-1,1-
di(phenyl-d5)methanol was confirmed.27 The formation and
exclusive coupling of cyclohexyl and diphenylketyl radicals
indicates that the radical pair cannot escape such that reaction
proceeds under a very efficient cage effect (Scheme 2).10
The effect of the zeolite on the photophysical properties of
benzophenone was investigated by phosphorescence measure-
ments (Figure 2). While zeolites stabilize ketone triplets to the
extent that phosphorescence may be observed at ambient
temperatures,28-31 no emission could be detected from ben-
zophenone in NaX in the presence of cyclohexane at 300 K.
However, spectra obtained at 77 K showed a broad emission
similar to that observed at ambient temperatures in the absence
of cyclohexane and in good agreement with spectra previously
reported in zeolite Na-ZSM-5.28 Phosphorescence decays were
not monoexponetial but reasonably good double exponential fits
were obtained with lifetimes of 6.5 and 46.6 ms for benzophe-
none and 7.2 and 45.2 ms for benzophenone-d10. These
lifetimes suggest a predominant π,π* configuration and like
other diarylketones show essentially no isotope effect.32-35
While it is known that π,π* states are not reactive toward
hydrogen abstraction,36,37 the efficient reaction at 300 K indicates
that the reactive n,π* state may be populated at ambient
temperatures. Reactive quenching in the presence of cyclo-
Figure 3. 13C NMR (74.8 MHz) CPMAS spectra of (a) pure
cyclohexane adsorbed on Zeolite NaX, (b) pure benzophenone in NaX,
(c) cyclohexane and benzophenone coadsorbed on Zeolite NaX, (d)
cyclohexane and benzophenone coadsorbed on Zeolite NaX after
photolysis (spectrum recorded with a spinning sideband suppression
sequence), and, (e) pure polycrystalline 1-cyclohexyl-1,1-diphenyl-
methanol.
hexane was also suggested by the observation of ambient
temperature phosphorescence emission in evacuated samples of
pure benzophenone in NaX which has a decay that was fit to
double exponential values of 0.13 and 2.3 ms.
Solid State. 13C CPMAS NMR Studies. Although the use
of solid-state NMR to study molecular motion in zeolites
includes line shape analysis, relaxation,38 and pulse field gra-
dient measurements,12,13 a large amount of qualitative informa-
tion may be obtained by simple examination of CP-MAS and
static spectra. Initial measurements and assignments were
carried out with cyclohexane and benzophenone followed by
measurements with samples prepared with cyclohexane and
benzophenone-d10.
From bottom to top, Figure 3 shows the 13C CPMAS spectra
of cyclohexane (3a), benzophenone (3b), cyclohexane coad-
(29) Casal, H. L.; Scaiano, J. C. Can. J. Chem. 1985, 63, 1308-1314.
(30) Casal, H. L.; Scaiano, J. C. Can. J. Chem. 1984, 62, 628-629.
(31) Scaiano, J. C.; Casal, H. L.; Netto-Ferreira, J. C. ACS Symp. Ser.
1985, 278, 211-222.
(32) Miller, J. C.; Borkman, R. F. J. Chem. Phys. 1972, 56, 3727-3729.
(33) Borkman, R. F. Mol. Photochem. 1972, 4, 1-19.
(34) Okamoto, Y.; Teranishi, H. J. Am. Chem. Soc. 1986, 108, 6378-
6380.
(35) Garcia-Garibay, M. A.; Gamarnik, A.; Bise, R.; Jenks, W. S. J.
Am. Chem. Soc. 1995, 117, 10264-10275.
(36) Wagner, P.; Park, B.-S. In Organic Photochemistry; Padwa, A. Ed.;
Marcel Dekker: New York, 1991; pp 227-366.
(37) Wagner, P. J.; Nakahira, T. J. Am. Chem. Soc. 1973, 95, 8474-
8475.
(27) Arnold, R. T.; Liggett, R. W. J. Am. Chem. Soc. 1942, 64, 2875-
2877.
(28) Okamoto, S.; Nishiguchi, H.; Anpo, M. Chem. Lett. 1992, 1009-
1012.
(38) Lechert, H.; Basler, W. D. J. Phys. Chem. Solids 1989, 50, 497-
521.

Benzophenone-d10 and Cyclohexane in the Zeolite NaX
J. Am. Chem. Soc., Vol. 119, No. 1, 1997 187
sorbed with benzophenone (3c), a spectrum of the latter sample
after photolysis for 6.5 h (3d), and the spectrum of crystalline
1-cyclohexyl-1,1-diphenylmethanol (3e). The spectra in Figure
3a-d were recorded with a 90° pulse of 4.9 µs, a contact time
of 2.1 ms, and a recycle delay of 2.0 s. The spectrum in Figure
3e was acquired with a 20 s recycle delay. The spectrum of
pure cyclohexane in NaX displays a symmetric peak at 20 ppm
and relatively sharp line width of 230 Hz (Figure 3a). Pure
benzophenone in NaX gives a broad signal (983 Hz) centered
at 122 ppm that is assigned to the aromatic carbons. Coad-
sorption of benzophenone and cyclohexane broadens the cy-
clohexane signal relative to that of pure cyclohexane from 230
to 469 Hz and the signal of the benzophenone ring carbons at
135 ppm relative to that of pure benzophenone from 983 to
1516 Hz. The integrated areas of the aromatic and aliphatic
signals in Figure 3c show an approximate cyclohexane to
benzophenone ratio of the order of 2:1 suggesting a fairly
packed, if disordered, structure. Several variations in contact
times and recycle delays failed to produce the signal of the
carbonyl group, which was expected in the range of 160-180
ppm. This is a reminder that integration on solid samples must
only be taken as a rough approximation. CPMAS analysis of
the photolyzed sample (Figure 3d) gave a spectrum consistent
with that expected for 1-cyclohexyl-1,1-diphenylmethanol (Fig-
ure 3e) which cannot be independently adsorbed in the zeolite,
presumably because of its large kinetic diameter. The spectrum
of the photoproduct within the zeolite consists of aromatic peaks
at 120 and 141 ppm, a carbinol carbon at 75 ppm, and a broad
aliphatic signal assigned to the cyclohexyl group and to
unreacted cyclohexane. This is consistent with the spectrum
obtained from pure crystalline samples of the same compound
shown in Figure 3e. We interpret the systematic broadening
of the cyclohexyl signal in the spectra of Figure 3a,c,d in terms
of changes in molecular motion. Since all spectra are obtained
under high power 1H dipolar decoupling, the 2-fold increase in
line width in going from 3a to 3c is a strong indication of
spectral heterogeneity resulting from restricted molecular motion
which limits spectral averaging in a highly heterogeneous
environment. This interpretation is supported by the further
30% increase in the line width of the cyclohexyl peak, up to
616 Hz, after the sample was photolyzed to form the product.
Appearance of spinning side bands in the aromatic region of
the spectrum of 1-cyclohexyl-1,1-diphenylmethanol (which were
removed with TOSS23 in Figure 3) also suggests a decrease in
motion as radicals from benzophenone and cyclohexane become
bound to form the photoproduct. Accordingly, static sample
measurements shown in Figure 4 reveal a considerable increase
in the chemical shift anisotropy of the aromatic region of the
photoproduct as expected for a molecule unable to undergo rapid
molecular tumbling. The static line widths of the aromatic peaks
change from 3.63 to 8.15 kHz, and those of cyclohexyl groups
change from 1.21 to 2.40 kHz before and after photolysis. These
line widths are close to those of the static polycrystalline sample
of the photoproduct.
Figure 4. 13C NMR (74.8 MHz) CP spectrum of (a) stationary samples
of cyclohexane and benzophenone adsorbed on Zeolite NaX, (b)
stationary samples of cyclohexane and benzophenone adsorbed on
Zeolite NaX after photolysis, and (c) stationary sample of polycrystalline
1-cyclohexyl-1,1-diphenylmethanol.
reasonable analogy may be drawn between the nuclear CP
experiment and the dipolar interactions that are familiar to
photochemists in the Forster type energy transfer mechanism.40
Both depend on the distance and orientation between the donor
and the acceptor as well as on their spectral overlap. The main
difference with optical experiments is that spectral overlap in
the magnetic CP experiment can be artificially optimized within
a spin-locking RF-field where the Hartman-Hahn condition has
1
been satisfied.22 A dismal spectral overlap with H and 13C
resonance frequencies at 300 and 75 MHz, respectively, can be
converted into a perfect frequency match at fields of ca. 40
kHz of the spin-locking RF pulse. Under these conditions,
isoenergetic 1H-13C coupled transitions transfer magnetization
1
from abundant H to rare 13C, thus providing the well-known
sensitivity advantage.41
The efficiency of cross polarization depends on geometric
1
factors and on the strength of the H-13C dipolar coupling. It
1
is inversely proportional to the H-1H homonuclear dipolar
1
coupling.39 Experimentally, 13C signals grow during H-13C
contact times (TCT) of a few milliseconds and then they decay
(usually in slightly longer times) by virtue of their spin-lattice
relaxation in the rotating frame (T1F). For samples with
randomly oriented molecules where many orientations and
distances are possible, the strength of the dipolar interactions
and the rate of cross polarization (TCT-1) are maximized when
molecules are relatively static. It has been documented that
1
cross polarization can occur if the average H-13C distances
in different molecules are of the order of 10 Å.42,43 Experiments
1
demonstrating H-29Si cross polarization across silicate inter-
The signals of benzophenone in Figures 3b and 4 result
primarily from intramolecular magnetization transfer. It is
expected that hydrogens directly attached to benzophenone
transfer magnetic polarization to benzophenone carbons much
more efficiently that those of cyclohexane. In contrast, the
aromatic signals in spectra obtained with benzophenone-d10
coadsorbed with cyclohexane must originate from intermolecular
polarization transfer. Our expectations from intramolecular and
intermolecular 1H-13C polarization (CP) transfer are based on
analysis of the cross polarization experiment as originally
presented by Schaefer and Stejskal.39 For our purposes, a
faces have also been carried out to distances up to 3-5 Å.44,45
Thus, it may be expected that intracavity intermolecular cross
(39) Schaefer, J.; Stejskal, E. O. Topics Carbon-13 NMR Spectrosc. 1979,
3, 283-324.
(40) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cum-
mings Publishing Co.: Menlo Park, 1978.
(41) Pines, A.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys. 1973, 59,
569-590.
(42) Parmer, J. F.; Dickinson, L. C.; Chien, C. W.; Porter, R. S.
Macromolecules 1987, 20, 2308-2310.
(43) Gobbi, G. C.; Silvestri, R.; Russel, T. P.; Lyerla, J. R.; Fleming,
W. W.; Nishi, T. J. Polym. Sci. Part C: Polym. Lett. 1987, 25, 61-65.

188 J. Am. Chem. Soc., Vol. 119, No. 1, 1997
Cizmeciyan et al.
creased by about 40% as a result of spin-lattice relaxation in
the rotating frame (T1F). That the shape of the spectra of benzo-
phenone-d10-cyclohexane samples are essentially identical to
those with nondeuterated benzophenone suggests that all carbons
are on average equally accessible to cyclohexane hydrogens.
Spectra obtained after photolysis showed similar trends.
Based on analysis by Klein Dowel et al.,44-46 one may
propose that cross polarization to benzophenone carbons in
deuterated [I(D)] and nondeuterated [I(H)] samples is related to
the inverse sixth power of the average C-H and C-D‚‚‚H-R
distances responsible for intra- and intermolecular polarization
transfer. These efficiencies may be roughly estimated from the
intensity of signals obtained under identical conditions. Thus,
1/6
using the relations [d(C-D‚‚‚H-R)/d(C-H)] ) [I(H)/I(H)-I(D)
]
∼
[I(H)/I(D)] ,
1/6 46 the intensities obtained with 2.1 ms contact times
Figure 5. 13C NMR (74.8 MHz) CPMAS spectrum of (a) cyclohexane
and benzophenone adsorbed on Zeolite NaX obtained with a contact
time of 2.1 ms and 9.5K scans, (b) cyclohexane and benzophenone-d10
adsorbed on Zeolite NaX obtained with a contact time of 5.0 ms and
9.5K scans, (c) cyclohexane and benzophenone adsorbed on Zeolite
NaX after photolysis, obtained with a contact time of 2.1 ms and 85K
scans, and (d) cyclohexane and benzophenone-d10 adsorbed on Zeolite
NaX after photolysis, obtained with a contact time of 5.0 ms and 85K
scans.
(i.e., I(H)/I(D) ∼ 8), and a C-H bond distance of 1.54 Å, one
can estimate an average carbon-hydrogen C-D‚‚‚H-R distance
of the order of 2.2 Å. It is interesting to note that this is the
value expected for a C-D‚‚‚H distance where benzophenone
and cyclohexane are within their van der Waals distances,47
which is a reasonable result given the loading of benzophenone
and cyclohexane present in the zeolite. Thus, the proximity
and rigidity suggested by the photochemical results is strongly
supported by the 13C CPMAS measurements. This study shows
that the 13C CPMAS NMR can be used as a qualitative tool for
probing the proximity and mobility of distinct molecules within
the confined environment of a zeolite and may be used to
complement studies involving intermolecular reactivity. The
line shape similarities between intra- and intermolecular cross
polarization reflect the nearly spherical symmetry of the
supercages in the zeolite NaX. It may be expected that
polarization transfer in zeolites with less spherical sites may
result in different transfer efficiencies to different nuclei if less
isotropic contact between donor and acceptors are enforced by
the structure. Studies in progress in our group are currently
addressing this and other aspects of zeolite complexation and
photochemistry.
polarization may be possible given the fact that intermolecular
distances between donor and acceptor should be much less than
15 Å, which is the diameter of the supercages in zeolite NaX.
The optimized CPMAS spectra of the benzophenone-d10-
cyclohexane samples before and after photolysis are shown in
Figure 5 (b and d) along with the optimized spectra of the
nondeuterated ketone (spectra 5a and 5c). The first thing to
notice is that the relatively strong signal for the benzophenone-
d10 carbons clearly demonstrates the occurrence of intermo-
lecular cross-polarization. As expected, the most important
difference between deuterated and nondeuterated samples comes
from their cross polarization efficiencies. The optimum signal
intensity for the intramolecularly cross-polarized aromatic
carbons of benzophenone, and its photoproduct were obtained
with a contact time of 2.1 ms (Figure 4). In contrast, identical
spectral acquisition with benzophenone-d10 and its photoproduct
under these conditions failed to produce the aromatic signals
along with the strong cyclohexyl peak. When the contact times
were increased, up to 5 ms, the appearance of aromatic signals
was observed at the same time that the cyclohexyl peak de-
Acknowledgment. We gratefully acknowledge the National
Science Foundation, the Petroleum Research Fund, and the
University of California for their generous support of this
research.
JA9623368
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