A magnetic switch for spin-catalyzed interconversion of nuclear spin isomers

Article abstract of DOI:10.1021/ja910282p

"Figure Presented" The interconversion of ortho-hydrogen (oH ₂) and para-hydrogen (pH₂), the two nuclear spin isomers of dihydrogen, requires a paramagnetic spin catalyst such as a nitroxide. We report the design and demonstration of

Full text of DOI:10.1021/ja910282p

Published on Web 02/22/2010
A Magnetic Switch for Spin-Catalyzed Interconversion of Nuclear Spin
Isomers
†
†
†
†
†
Yongjun Li, Xuegong Lei, Steffen Jockusch, Judy Y.-C. Chen, Michael Frunzi,
†
‡
§
§
§
Jeremiah A. Johnson, Ronald G. Lawler, Yasujiro Murata, Michihisa Murata, Koichi Komatsu,
,
†
and Nicholas J. Turro*
Department of Chemistry, Columbia UniVersity, New York, New York 10027, Department of Chemistry, Brown
UniVersity, ProVidence, Rhode Island 02912, and Institute for Chemical Research, Kyoto UniVersity,
Kyoto 611-0011, Japan
Received December 4, 2009; E-mail: njt3@columbia.edu
Dihydrogen, H
nuclear singlet state with antiparallel (vV) nuclear spins, termed para-
hydrogen (pH ), and a nuclear triplet state with parallel (vv) nuclear
spins, termed ortho-hydrogen (oH
According to the Pauli Principle, singlet pH
even rotational states (J ) 0, 2, 4, ...) and triplet oH
states (J ) 1, 3, 5, ...). Since J ) 0 is the lowest rotational level of
, pH is the ground rotational state and oH a rotationally excited
state. The energy gap between the J ) 0 and J ) 1 rotational states
2
, exists as two allotropes or nuclear spin isomers: a
Chart 1. Structures of H
Blue Balls Indicate Incarcerated H
Sample Is a Mixture of H @Host and HD@Host)
2
@C60, HD@C60, H
2
@1, H
2
@2, and H
2
@3
#
(
2
or HD; Indicates That the
2
2
1
2
).
2
is required to occupy
odd rotational
2
H
2
2
2
is 120 cm- (0.343 kcal mol ). The equilibrium mixture at any
given temperature is termed eH . At room temperature (RT), triplet
oH dominates at equilibrium (eH ) 75% oH /25% pH ) because
1
-1
2
precursor to a paramagnetic nitroxide, a “magnetic switch” for forward
conversion (eH @C60 f *pH @C60) at 77 K and back conversion
*pH @C60 f eH @C60) at RT would be available. We describe the
design and demonstration of such a magnetic switch based on the
2
2
2
2
2
2
of its higher statistical weight and the small energy gap between
the two rotational levels.
(
2
2
The two allotropes of hydrogen, oH
incarcerated into C60 to form the endofullerene guest@host complexes
2 2
and pH , may be quantitatively
6
#
#
2–4
2 2
reversible nitroxide/hydroxylamine system H @2 and H @3 (Chart 1).
#
5
We investigated the spin conversion of H @1, a diamagnetic
2
oH
following procedure for producing samples of pH
in which the incarcerated H spin isomers are not in spin equilibrium
at RT: (1) adsorbing a sample of eH @C60 on the external surface of
2 2
@C60 and pH @C60, respectively. We have developed the
#
derivative of H
2
@C60, in order to determine the effect of substitution
2 2
-enriched H @C60
on the forward and back conversions of the incarcerated spin isomers
2
1
#
of H
2 2
. Figure 1a shows the initial and final H NMR spectra of H @1
2
that was put through the enrichment procedure described above. The
black curve corresponds to the initial sample of eH
NaY zeolite at RT; (2) cooling the sample to 77 K; (3) immersing the
sample in liquid oxygen at 77 K, thereby converting the sample to a
2
@1. The signal at
1
-
4.43 ppm corresponds to the H signal from oH @1, and the other
2
2 2 2
50% oH /50% pH mixture (which is eH at 77 K); (4) removing the
1
three signals correspond to the H signals from HD@1 (the triplet
results from the coupling of H with H). The red curve corresponds
to the H NMR signals after conversion. Clearly, the proportion of
2
spin catalyst (O ) by applying a vacuum; (5) rapidly bringing the
1
2
7
sample to RT before the sample can back-convert to equilibrium at
RT; and (6) extracting the sample into a solvent at RT. The sample at
1
oH
are consistent with a 50%/50% mixture of oH
sponding to *pH @1. At RT, the back conversion rate of *pH
eH @1 without a spin catalyst is similar to that of *pH
eH @C (see Table 1), showing that the substitution by itself does
2
@1 relative to HD@1 decreased as expected: the signal intensities
@1/pH @1, corre-
@1 to
RT is enriched in pH
equilibrium 25% pH ) and is termed *pH
At RT, the back conversion of *pH @C60 to eH
in the absence of an added spin catalyst. The conversion of *pH
2 2
(nonequilibrium 50% pH rather than the
2
2
2
2
@C60.
2
2
2
2
@C60 takes days
2
2
@C60 to
2
@C60
1
2
60
2
to eH @C60 (eq 1) is followed conveniently by H NMR spectroscopy
not significantly modify the rate of the back conversion.
using a sample that contains HD@C60, which does not have spin-
rotational isomers, as an internal standard.
At RT, nitroxides serve as paramagnetic spin catalysts for the back
5
Figure 1. 1H NMR analysis of the forward conversion of (a) eH
curve) to *pH @1 (red curve) and (b) eH @2 (black curve) to *pH
curve) in CDCl
2
@1 (black
@2 (red
conversion of *pH
2
@C60 to eH
2
@C60. We reasoned that if a derivative
2
2
2
2
of H @C60 could be rapidly switched from a diamagnetic nitroxide
3
.
†
‡
§
Columbia University.
Brown University.
Kyoto University.
2
The H @2 sample was cooled to 77 K for several hours according
to the usual procedure, and then the NMR spectrum was recorded as
4
042 9 J. AM. CHEM. SOC. 2010, 132, 4042–4043
10.1021/ja910282p  2010 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Relative Rates of para-ortho Conversion and Lifetimes
of H
2
, H
2
@C60, H
2
@1, and H
2
@2 at RT
conversion
relative rate
lifetime (s)
a
4
*
*
*
*
pH
2
2
2
2
f eH
2
12-18
∼1
3.6-5.4 × 10
b
5
pH
pH
pH
@C60 f eH
@1 f eH
@2 f eH
2
@bC60
∼6.5 × 10
5
∼1
∼6.5 × 10
2
@1
@2
b
>7200
<90
2
a
b
4
Data from ref 10. In deoxygenated 1,2-dichlorobenzene-d .
1
#
The intensity of the H
measure of the oH /pH
5% mixture of oH /pH
procedure, the H NMR showed that the sample was a mixture of
@3 and H @2 (Figure 2c), demonstrating that some degree of
oxidation of H @3 occurred, presumably during the treatment with
during the workup. Comparison of the H NMR signals (Figure
2d) shows that eH @3 was indeed enriched in pH while eH @2 was
not. The results can be interpreted as follows: (1) some (∼25%) of
the H @3 was oxidized to H @2 during the procedure; (2) the H @3
that survived oxidation was enriched in pH ; (3) H @2 was not
enriched in pH ; and (4) the concentration of H @2 in the sample
was insufficient to serve as a bimolecular catalyst for the back
conversion of *pH @3. These results demonstrate convincingly that
the H @3/H @2 system can act as a reversible switch that interconverts
the incarcerated oH /pH nuclear spin isomers.
These results show that the rate of nuclear spin interconversion of
encapsulated H can be markedly increased by attaching the paramagnetic
catalyst to the cage. Furthermore, comparison of the catalyzed lifetime of
signal relative to the integrated HD signal is a
ratio and corresponds to the expected 75%/
(eH at RT). After the forward conversion
3 2
Figure 2. H NMR analysis in CDCl . (a) Initial sample of H @2. (b)
#
2
#
#
After treatment of H
with liquid O at 77 K and sample workup. (d) Normalized overlap of (b)
and (c). Dotted lines indicate the H
2
@2 to produce H
2
@3. (c) After treatment of H
2
@3
2
2
2
2
2
2
2
#
2
@2 signals.
1
quickly as possible after the sample was warmed to RT. Within the
fastest workup possible (90 s), no spin enrichment was observed [in
Figure 1b, the final H signals (red and black curves) are indistinguish-
able]. We hypothesize that the paramagnetic nitroxide substituent of
2
H @2 is such an effective spin catalyst that at RT it causes a rapid
back conversion of *pH
workup of the sample.
To test the hypothesis that spin enrichment to *pH
is rapidly reversed to eH
a diamagnetic “masked” nitroxide, H
H
2
2
2
9
1
1
O
2
2
2
2
2
@2 produced at 77 K to eH
2
@2 during the
2
2
2
2
2
2
@2 occurs but
@2 during workup, we synthesized (eq 2)
@3 (Chart 1). In addition, we
@2 would be regenerated by oxidation of H
2
2
8
2
#
2
2
#
#
expected that H
2
2
@3,
2
2
creating a reversible magnetic switch for interconverting incarcerated
nuclear spin isomers.
2
2
2
5
H
2
@2 (< 90 s) with the uncatalyzed lifetime of ∼7.5 days (6.5 × 10 s)
for H @1 (Table 1) demonstrates that the rate of back conversion may
2
be varied by ∼4 orders of magnitude by turning the catalyst on and off.
We are attempting to couple the ability to control the rate of
conversion with the generation of nuclear polarization of the oH
2
produced by the interconversion. These results have potential for
magnetic resonance imaging applications employing fullerenes con-
taining H
2
as an imaging agent and for the application of the nitroxide
@2 for dynamic nuclear polarization.
The broadened NMR (Figure 2a) and ESR (Figure S1a in the
Supporting Information) spectra of the initial sample are consistent
and H components of H
2
2
with those expected for paramagnetic eH
with phenylhydrazine resulted in the conversion of paramagnetic
eH @2 to diamagnetic eH @3. As expected, the resulting NMR signals
were sharpened (Figure 2b) and the ESR signal disappeared (Figure
2
@2. Treatment of the sample
Acknowledgment. The authors thank the National Science
Foundation for its generous support through Grant CHE 07-17518.
2
2
Note Added after ASAP Publication. The structures of the
S1b), consistent with essentially quantitative conversion of eH
2
@2 into
fullerene derivatives were corrected on February 25, 2010.
8
eH
and ESR spectra of eH
of eH @3 back to eH @2 in >95% yield.
The hypothesis concerning the lack of a net conversion when eH
is cycled to 77 K and back to RT (Figure 1b) could be tested next,
since eH @3 is a “caged” diamagnetic analogue of eH @2. The
strategy to test the hypothesis was to convert eH @3 to *pH @3 at
7 K (analogous to the result for eH @1 in Figure 1a), return the
sample to RT, “uncage” the nitroxide to produce the paramagnetic
pH @2, and determine the extent of conversion.
The specific execution of the strategy for testing the hypothesis was
as follows: First, treatment of eH @3 under the forward conversion
conditions (NaY/77 K/liquid O ) was expected to cause conversion
of eH @3 to a sample of enriched *pH @3 that would be stable to
back conversion at room temperature. Treatment of *pH @3 with
Cu(OAc) at room temperature would generate enriched *pH @2. If
the hypothesis of rapid interconversion of the nuclear spin isomers of
pH @2 is correct, then at RT, that portion of the sample that was
oxidized to *pH @2 would be rapidly converted to eH @2.
The results of applying this strategy are shown in Figure 2. The H
NMR spectrum of the initial sample of eH @3 is shown in Figure 2b.
2
@3. Treatment of the eH
2
@3 with Cu(OAc)
2
produced the NMR
Supporting Information Available: Synthesis and experimental
details, including EPR analysis. This material is available free of charge
via the Internet at http://pubs.acs.org.
2
@2, demonstrating the reversible conversion
2
2
2
@2
References
2
2
(
1) Farkas, A. Orthohydrogen, Parahydrogen and HeaVy Hydrogen; Cambridge
2
2
University Press: Cambridge, U.K., 1935; Chapter II and Chapter IV, p 76.
7
2
(2) Saunders, M.; Cross, R. J.; Jimenez-Vazquez, H. A.; Shimshi, R.; Khong,
A. Science 1996, 271, 1693.
(
(
3) Komatsu, K.; Murata, M.; Murata, Y. Science 2005, 307, 238.
4) Turro, N. J.; Chen, J. Y.-C.; Sartori, E.; Ruzzi, M.; Marti, A. A.; Lawler,
R. G.; Jockusch, S.; L o´ pez-Gejo, J.; Komatsu, K.; Murata, Y. Acc. Chem.
Res. 2010, 43, 335.
*
2
2
(
5) Turro, N. J.; Marti, A. A.; Chen, J. Y.-C.; Jockusch, S.; Lawler, R. G.;
Ruzzi, M.; Sartori, E.; Chuang, S.-C.; Komatsu, K.; Murata, Y. J. Am.
Chem. Soc. 2008, 130, 10506.
6) Arena, F.; Bullo, F.; Conti, F.; Corvaja, C.; Maggini, M.; Prato, M.;
Scorrano, G. J. Am. Chem. Soc. 1997, 119, 789.
7) In these experiments, HD@1 served as an internal reference.
8) Lee, T. D.; Keana, J. F. W. J. Org. Chem. 1975, 40, 3145.
2
2
2
(
2
(
(
2
2
(9) The observation of separate signals from the two forms implies a relatively
low rate of hydrogen atom transfer between nitroxides and hydroxylamines.
See: Wu, A.; Mader, E. A.; Datta, A.; Hrovat, D. A.; Borden, W. T.; Mayer,
J. M. J. Am. Chem. Soc. 2009, 131, 11985.
*
2
2
2
1
(10) Aroulanda, C.; Starovoytova, L.; Canet, D. J. Phys. Chem. A 2007, 111, 10615.
2
JA910282P
J. AM. CHEM. SOC. 9 VOL. 132, NO. 12, 2010 4043