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Nuclear spin isomers of guest
molecules in H2@C60, H2O@C60
and other endofullerenes
Judy Y.-C. Chen1 , Yongjun Li1 , Michael Frunzi1 ,
rsta.royalsocietypublishing.org
Xuegong Lei1 , Yasujiro Murata2 , Ronald G. Lawler3
and Nicholas J. Turro1,†
Research
Cite this article: Chen JY-C, Li Y, Frunzi M,
Lei X, Murata Y, Lawler RG, Turro NJ. 2013
Nuclear spin isomers of guest molecules in
H2 @C60 , H2 O@C60 and other endofullerenes.
Phil Trans R Soc A 371: 20110628.
http://dx.doi.org/10.1098/rsta.2011.0628
One contribution of 13 to a Theo Murphy
Meeting Issue ‘Nanolaboratories: physics and
chemistry of small-molecule endofullerenes’.
Subject Areas:
physical chemistry, nanotechnology,
spectroscopy
Keywords:
spin isomers, endofullerenes, para-hydrogen,
nuclear magnetic resonance, spin catalyst
Author for correspondence:
Ronald G. Lawler
e-mail: [email protected]
†
Deceased 24 November 2012.
1 Department of Chemistry, Columbia University, New York,
NY 10027, USA
2 Institute for Chemical Research, Kyoto University,
Kyoto 611-0011, Japan
3 Department of Chemistry, Brown University, Providence,
RI 02912, USA
Spectroscopic studies of recently synthesized endofullerenes, in which H2 , H2 O and other atoms and
small molecules are trapped in cages of carbon
atoms, have shown that although the trapped
molecules interact relatively weakly with the internal
environment they are nevertheless susceptible to
appropriately applied external perturbations. These
properties have been exploited to isolate and study
samples of H2 in C60 and other fullerenes that are
highly enriched in the para spin isomer. Several
strategies for spin-isomer enrichment, potential
extensions to other endofullerenes and possible
applications of these materials are discussed.
1. Introduction
(a) Macroscopic quantum effects
Quantum effects on the macroscopic properties of
chemical substances at ordinary temperatures are
rare. A well-known, easily demonstrated example
is the influence of deuterium substitution on the
elevation of the melting point and changes in other
thermodynamic properties of water [1]. Shortly after the
first preparation of D2 O, this was attributed by Bernal &
Tamm [2] to mass-dependent, zero-point energy effects
on intermolecular forces. Their original idea has been
extended to more generalized treatments of quantum
fluctuations [3].
2013 The Author(s) Published by the Royal Society. All rights reserved.
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yrot (Q,F)
ynuclear spin (1,2)
×
180∞
+
–
–
+ +
H
+
S
A
J=0
2
H
+ +
S
A
Figure 1. Schematic of the effect of rotation about the H–H axis on the symmetry (A or S) of (left) the rotational wave function
and (right) the spin wave function for the lowest energy para-H2 (J = 0) and ortho-H2 (J = 1) rotational states. The resulting
product spin–rotation wave function satisfies the antisymmetry properties required of fermions such as protons. (Online version
in colour.)
ynuclear spin
Erot
aa
ortho-H2
J=1
NMR
active
+
bb
120 cm–1
para-H2
J=0
ab + ba
NMR
inactive
ab – ba
–
Figure 2. Energy diagram for the two lowest spin-rotational states of H2 . Para-H2 (I = 0) is 120 cm−1 (0.3 kcal mol−1 ; 172 K)
more stable than ortho-H2 (I = 1). (Online version in colour.)
A more widely studied quantum effect, of increasing practical importance, gives rise to the
existence of isolable ortho and para allotropes of molecular hydrogen that differ only in their
nuclear spin and rotational wave functions [4,5] in order to satisfy the parity requirements of
fermions and bosons. (It is interesting to note that symmetry and nuclear spin effects were also
mentioned, but dismissed as unimportant, in the original explanation of Bernal & Tamm [2] for the
D2 O/H2 O melting point difference and other isotope effects in water.) Protons (IH = 1/2) obey the
rule for fermions requiring that the molecular wave function be antisymmetric under exchange
of equivalent particles. This leads to the result shown in figure 1 that the lowest energy rotational
state (J = 0) of H2 is symmetric and must be combined with an antisymmetric (I = 0; para) nuclear
spin state, whereas the asymmetric rotational wave function of the first excited rotational state
(J = 1) corresponds to the symmetric I = 1 (ortho) spin state. A more explicit description of the
nuclear spin wave functions and the corresponding energies of the two lowest energy states of
H2 is shown in figure 2.
Transitions between the ortho and para states require highly forbidden simultaneous changes in
the nuclear and rotational quantum numbers. As a consequence, a sample of molecular hydrogen
often behaves as a mixture of two separate chemical species, each with its own set of properties.
......................................................
J=1
1
H
rsta.royalsocietypublishing.org Phil Trans R Soc A 371: 20110628
H
2
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80
3
no catalyst
catalyst
E
50
E
E
100 150 200 250 300 350 400
temperature (K)
20 K
77 K
(liquid H2) (liquid N2)
290 K
(room temperature)
Figure 3. Temperature dependence of ortho-H2 composition. In the absence of a catalyst, a large metastable excess of the ortho
form may develop. (Online version in colour.)
At room temperature, the equilibrium ortho : para ratio is 3 : 1, reflecting the statistical ratio of the
I = 1 and I = 0 nuclear spin states. At easily achievable lower temperatures, however, as shown in
figure 3, the equilibrium favours the para form, as implied by the energy gap of 120 cm−1 (172 K)
[5] between the two forms.
(b) Para-hydrogen enrichment
Exploitation of the temperature dependence of the ortho : para ratio is the basis of the most
commonly used methods for producing samples enriched in para-H2 [5]. For example, at 20 K,
the boiling point of liquid H2 at one atmosphere, the equilibrium sample is more than 99%
para-H2 . Under conditions where the temperature of a sample of H2 is lowered more rapidly than
the rate of ortho–para conversion, however, a metastable state is reached in which the ortho form
is many times more abundant than the equilibrium value. To avoid the build-up of potentially
dangerous levels of latent energy, large-scale users of liquid hydrogen routinely use a catalyst to
ensure that equilibrium is maintained as liquefaction proceeds or as hydrogen boil off occurs [6].
(c) Enrichment of less stable spin isomers
(i) Chromatography and selective adsorption
The distinctly different thermodynamic properties of the two forms [5] and their mode of
interaction with surfaces make possible separation of the hydrogen allotropes by selective
adsorption or chromatography [7]. The separation of the allotropes of hydrogen and deuterium
illustrated in figure 4 [8] might even be described as a striking macro-scale manifestation of the
Pauli exclusion principle! The parity of the deuterium nucleus (ID = 1) is that of a boson and
requires that the spin–rotation wave function be symmetric to exchange of two equivalent nuclei.
As with H2 , this gives rise to ortho and para states for D2 , except that the lowest energy, J = 0,
state corresponds to symmetric nuclear spin functions I = 0 and 2 (ortho), and the J = 1 state to
I = 1 (para).
By selectively trapping the eluent from the stream of separated hydrogen allotropes, it becomes
possible to isolate the metastable form in useful quantities [9]. Selective adsorption of the
metastable (para) form of D2 had also been accomplished by selective adsorption on γ-alumina
near the liquefaction point [10].
Selective excitation of one spin isomer with electromagnetic radiation followed by an
irreversible process is a potential method of isomer separation analogous to methods applied
for isotope enrichment. Laser-induced drift, which relies on selective excitation of a well-resolved
......................................................
70
60
50
40
30
20
10
0
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equilibrium
% ortho-H2
E
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column: 80 m × 0.25–0.3 mm, 30 µm silica coating
oven: 77.6 K (liquid nitrogen)
carrier gas: 2 ml min–1, neon
sample: 1.5 µl gas mixture
4
p-D2
o-H2
p-H2
0
9 (min)
Figure 4. Separation of the isotopologues and allotropes of hydrogen and deuterium by gas chromatography. The conditions
used are as shown. Adapted from Mohnke [8].
transition of one isomer, has been used, for example, for selective enrichment of spin isomers
of CH3 F [11]. State-selective photolysis of CH2 O has been used to selectively remove one spin
isomer [12], enriching the sample in the other. It was also shown that one nearly pure allotrope
of H2 could be formed by this state-selective photolysis [13]. It remains to be seen whether
such methods can be applied in condensed medium or to encapsulated molecules in fullerenes.
It is encouraging, however, that the far infrared (FIR) spectrum of H2 O@C60 displays wellresolved peaks of the ortho and para isomers at cryogenic temperatures [14] that would allow for
selective excitation of rotational transitions. Whether such excitation would be possible at higher
temperatures is not presently known.
(d) NMR applications of para-hydrogen and other long-lived nuclear spin species
There has been a resurgence of interest in para-H2 in recent years following the prediction [15] and
accidental discovery [16] of strongly enhanced NMR signals in the products of hydrogenation
reactions carried out using H2 enriched in that allotrope. The phenomenon is commonly referred
to as para-hydrogen-induced polarization (PHIP). The same methodology has been used to
produce enhanced 2 H NMR signals by using enriched ortho-D2 [17]. PHIP was first observed in
organometallic compounds and continues to be used as a tool for mechanistic studies of a variety
of hydrogenation processes [18]. The use of signal enhancement via PHIP is especially promising
[19] as a complement to hyperpolarization methods for improving the sensitivity of magnetic
resonance imaging (MRI) and analytical NMR using optical polarization of Xe [20] and dynamic
nuclear polarization [21].
Interest in nuclear spin intersystem crossing processes has also been stimulated by the
discovery that surprisingly long-lived nuclear spin singlet states may be prepared in polyatomic
molecules by proper manipulation of the spins using magnetic resonance methods [22].
Possible applications of selected nuclear spin states in quantum computing have not escaped
notice [23–25].
2. Spin isomers of molecules encapsulated in fullerenes
The discovery that some atoms and small molecules may be captured in fullerene cages in
detectable amounts [26] or placed there synthetically in high yield, including H2 [27], H2 O
......................................................
HD
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o-D2
He
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Our strategy for preparing and preserving endo fullerene samples enriched in the spin isomer,
such as para-H2 , favoured at low temperatures has required development of the following
methodologies [34]:
(i) An accurate and reliable analytical method is developed to determine and monitor the
degree of enrichment.
(ii) The endo fullerene is subjected to intimate contact with a catalyst at a temperature
sufficiently low, and for a sufficiently long time, to achieve the desired degree of
enrichment of the equilibrium mixture in the more stable spin isomer.
(iii) The catalytic activity is removed sufficiently rapidly to preserve the enrichment, and the
enriched sample is returned to the temperature where it will be studied or used, most
commonly room temperature.
(a) Analysis
To determine the ortho : para ratio of H2 in fullerenes, we have exploited the fact that only
the ortho isomer gives rise to a proton NMR signal. Fortunately, the NMR peaks from endo
molecules are shifted into a unique region free from interference by other signals. It then becomes
possible to monitor the change in allotrope content by determining the intensity of the orthoH2 peak. For convenience and improved accuracy, the sample also includes a portion of the same
fullerene containing hydrogen deuteride, HD, incorporated at the time of endofullerene synthesis.
Because endo H2 and HD have essentially the same effects on the properties of the fullerene cage
(a notable exception being the differing abilities of the isotopologues of H2 @C60 to quench singlet
oxygen [38]), the ratio of endo H2 : endo HD should be unchanged by the chemical or physical
manipulations needed to isolate and prepare samples for analysis. Furthermore, HD has only one
spin isomer and is not subject to magnetic catalysis. Changes in the ratio of the H2 : HD NMR
signals should therefore be an accurate proxy for the change in fraction of ortho-H2 in the sample.
A spectrum of H2 /HD@C60 before and after enrichment in para-H2 using method (i) (described
below) is shown in figure 5.
The high-resolution NMR strategy outlined above to monitor changes in the nuclear spin
allotrope composition should be applicable to other small molecules such as D2 , H2 O, NH3 and
CH4 . In all these cases, the intrinsic NMR intensity is different for each spin isomer so that changes
in composition result in changes in intensity of the mixed sample. The use of the corresponding
......................................................
3. Methodologies for endofullerene spin-isomer enrichment
5
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[28,29], NH3 [30] and CH4 [31], all of which have spin isomers, leads one naturally to ask if the
environment sensed by the endo molecule resembles that of the isolated molecule in the gas phase.
Based on a variety of spectroscopic studies, the emerging answer is a qualified ‘yes’ [14,32–34, and
references therein] although some coupling of rotational and internal translation modes of the
endo H2 has been predicted theoretically [35] and detected spectroscopically [32]. (It has not,
however, been possible to resolve by NMR the spin–rotation fine structure in H2 @C60 that is
resolved in gas phase molecular beam measurements [36].) Similarly, the low-temperature FIR
spectrum of H2 O@C60 resembles that of water in the gas phase [14]. Magic angle spinning (MAS)
NMR measurements and FIR reveal splitting of the rotational energy states of the ortho form at
low temperature indicating breaking of strictly spherical symmetry in the H2 O environment [14]
similar to what has been observed for isolated water molecules in rare gas matrices [37].
The observations described above have prompted us to investigate the extent to which the
relative isolation of H2 and other small molecules encapsulated in the fullerenes might make
it possible to prepare, store and ultimately make use of pure samples of the encapsulated
spin allotropes in condensed phases. We report and review below recent developments in the
methodology for interconverting the nuclear spin isomers of small molecules in endofullerenes
and suggest some promising directions for future efforts.
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(a)
6
......................................................
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HD@C60
H2@C60
1
H NMR
(b)
–1.35
–1.40
–1.45
–1.50
–1.55
ppm
Figure 5. 500 MHz 1 H NMR, room temperature, of H2 /HD@C60 mixture (a) before and (b) after enrichment in the para, NMR
silent, allotrope of H2 (see text). (Online version in colour.)
partly deuterated endo molecules as intensity standards should also be applicable to other small
molecules because of the isotope shift of the 1 H resonance induced by deuterium substitution.
For example, it has been shown that the 1 H NMR peaks from H2 O@C60 and HDO@C60 are well
resolved [29].
Another magnetic resonance method that shows some promise is to monitor relative spinisomer concentrations through changes in the intensities of peaks within multiplets arising from
coupling of other nuclei to the spin species of interest. It may be possible, for example, to detect
changes in the intensity of the centre peak of the 17 O resonance in H17
2 O@C60 which should
possess ortho and para spin isomers analogous to the 16 O isotopologue. Figure 6 shows a 17 O
NMR spectrum of H2 O@C60 containing 90% H17
2 O. The centre peak that is clearly resolved from
the two outer peaks, separated from the centre by JHO ≈ ±80 Hz, arises from overlap of signals
from the I = 0 (para) and I = 1 (ortho) states of the two protons.
Spin isomers may, of course, be distinguished by a number of other spectroscopic methods.
For example, low-temperature mid-infrared spectroscopy and inelastic neutron scattering (INS)
are able to distinguish the ortho and para forms of H2 @C60 [33], and the isomers of H2 O@C60 have
been distinguished by FIR and INS [14]. In principle, low-temperature MAS NMR of H2 O@C60
[14] and H2 @C60 [39] should also detect conversion to the NMR-silent para form in solid samples
as loss of signal intensity as the fraction of the ortho form decreases. Decrease in the intensity of
the electron–nuclear double resonance (ENDOR) signal from the radical ion of an endofullerene
containing H2 with decreasing temperature has also been interpreted as loss of the ENDOR-active
ortho isomer that is coupled to the unpaired electron via contact and dipolar interactions [40].
To date, these methods have proven useful for in situ monitoring of the progress of ortho–para
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7
–33
–34
–35
–36
–37
–38
–39
chemical shift (ppm)
Figure 6. 17 O NMR of H172 O@C60 (90%), 54.2 MHz, CDCl3 /CS2 (1/1), room temperature. (Online version in colour.)
conversion. There are also reports that ortho- and para-H2 O have been detected in the liquid phase
using four-wave mixing spectroscopy [41]. It is not clear to what extent this method could be
applied to fullerene-encapsulated H2 O.
(b) Paramagnetic catalysis of intersystem crossing in spin allotropes
The basic mechanism by which nuclear spin isomers may be interconverted by a magnetic catalyst
was first described by Wigner [42]. This mechanism may be summarized as follows:
Transitions between nuclear spin states differing in the orientation of two (or more)
geometrically equivalent spins may be achieved through magnetic symmetry breaking
via a field gradient across the spins applied by a suitably placed magnetic moment for a
sufficiently long time.
In Wigner’s theory, a dipolar field from a magnetic catalyst molecule colliding with H2 was
assumed. Expansions of the basic idea have included a variety of other interactions, many of
which pertain primarily to magnetic surfaces [43,44].
Our strategy for developing spin catalysts is based on Wigner’s mechanism. Three different
approaches have so far been used:
(i) the endofullerene is dispersed by adsorption on a zeolite and exposed to liquid oxygen
at 77 K [45,46]. This is in a sense the inverse of the commonly used method of para-H2
enrichment by passing a stream of gaseous H2 over a surface-bound paramagnet such as
activated carbon [5]. In our case, the catalyst diffuses to the surface-bound H2 substrate;
(ii) a paramagnetic auxiliary group consisting of a nitroxide radical is fixed to the outside
of the fullerene cage, and a frozen solution of the modified endofullerene is cooled to a
feasibly low temperature [47,48]; and
(iii) the small molecule is incorporated in a fullerene cage that forms a photo-accessible triplet
state of sufficiently long lifetime to allow interaction of the triplet magnetic moment with
the nuclei of the endo molecule [49].
A fourth proposed method which has so far not been successfully implemented for ortho–para
conversion but has been studied spectroscopically [40] is
......................................................
–32
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H2O17@C60
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Each of the catalytic methods outlined above has a corresponding means of rapid removal or
inactivation of the catalyst once equilibrium has been attained:
(i) the O2 is removed by evaporation with pumping at 77 K. The sample is then warmed to
room temperature, and the adsorbed endofullerene extracted for further study [45,46];
(ii) as noted above, if necessary, the nitroxide may be rapidly converted to the corresponding
diamagnetic hydroxylamine using a reducing agent such as phenylhydrazine. Methods
of removing the auxiliary group, such as flash pyrolysis, to form the parent endofullerene
are under development [47,48];
(iii) removal of the triplet state catalyst occurs spontaneously in milliseconds or less after
irradiation ceases [49]; and
(iv) the radical ion intermediate might similarly be removed almost instantaneously by back
electron transfer from a photo-generated radical ion pair, or more slowly by chemical
electron transfer.
We have coined the term ‘magnetic switch’ to describe methods (ii) and (iii) for allotrope
conversion [47].
4. Discussion of enrichment methods and results
(a) Wigner theory
It seems likely that in all of the methods so far used with the endofullerenes the unpaired
electron(s) of the catalyst provide the field gradient necessary for intersystem crossing via the
magnetic dipole–dipole interaction with the nuclei in the endo molecule. Method (ii), the use of
a paramagnetic auxiliary at a fixed distance from the centre of the fullerene cage, provides an
especially well-defined model for catalysis. Using the Wigner theory or a later formulation based
on the theory of spin relaxation [50], it can be shown [48] that the first-order rate coefficient,
kpo (s−1 ), for a transition from the para (p) to ortho (o) states of H2 should vary as the inverse eighth
power of the distance, d, between the paramagnetic site, expressed in atomic units (0.529 Å):
kpo = Ω 2 d−8 J(ωop , τr ).
(4.1a)
The prefactor Ω 2 depends on the magnetic properties of the proton and paramagnet and
fundamental constants (SI units):
μ 2 γ 2 g2 μ2 S(S + 1) r 2
0
0
2
P
B
,
(4.1b)
Ω =6
4π
a0
a60
where r0 = 0.74 Å is the distance between the protons in the hydrogen molecule, S is the spin of the
unpaired electron(s) of the paramagnet, gμB its magnetic moment and γP is the magnetogyric ratio
of the proton. The spectral density, J(ωop , τr ), depends on ωop = 2.3 × 1013 rad s−1 , the frequency
corresponding to the 120 cm−1 energy separation of the ortho and para states, and τr , an average
lifetime during which the field gradient across the nuclei acts:
J(ωop , τr ) =
τr
2 τ2
1 + ωop
r
.
(4.1c)
......................................................
(c) Rapid catalyst removal
8
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(iv) attachment of one or more unpaired electrons to the fullerene cage by chemical or
photochemical electron transfer to form a radical ion to provide an electron spin
interaction analogous to that of a triplet state.
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80
9
percentage of ortho-H2
1
40
ON
O O
O
O
N
O
20
2
0
0
20
40
60
time (days)
80
100
Figure 7. Ortho-H2 composition of a sample of two endofullerenes with paramagnetic auxiliaries cooled in liquid He for
varying times and analysed by rapid rewarming, NMR analysis and recooling before appreciable back conversion. (Online version
in colour.)
(b) Comparison of Wigner theory with experiment: method (ii)
We have measured values of kpo for several H2 @C60 nitroxides which were enriched in the para
allotrope using a variation of method (i) on the diamagnetic hydroxylamine precursors of the
nitroxide catalysts followed by rapid chemical oxidation to the nitroxide and measurement of
the back conversion rate in solution at room temperature using NMR [48]. As predicted by
equations (4.1), an excellent correlation is found between kpo and values of d−8 estimated from
molecular modelling. Rates for mono- and bis-nitroxides with the same value of d also confirm
the dependence on S. Comparison with calculated values based on estimates of d from molecular
models, or more recent density functional theory calculations [51], yields estimates of τr of 0.2–
0.3 ps which approximates the value of the correlation time for reorientation of the H2 molecule
in C60 obtained from 1 H relaxation times in H2 @C60 [52,53].
(c) Enrichment at ultra low temperature
We have recently extended method (ii) to a sample of a H2 @C60 nitroxide and bis-nitroxide cooled
in liquid He. Over a period of 26 days, approximately 95% enrichment in the para spin isomer is
achieved as shown in figure 7.
(d) Catalysis by photoexcited triplet
Photolysis, method (iii), has been successfully used to produce enriched para-H2 @C70 but for
photophysical reasons has not been successful for H2 @C60 [49]. The method is quite sensitive
to the photodynamics of triplet formation and decay. The effect is enhanced in H2 @C70 in frozen
solution because of the long natural lifetime of the triplet and isolation of the molecules in a dilute
matrix. Enrichment becomes undetectable in liquid solution, however, because of bimolecular
quenching of the triplet.
......................................................
95% para-H2
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O O
Et
O
O
ON
60
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Py N
Ph
Py
O NO Ph
Ph
Ph
Ph
Py N
Ph
O
O
10
S
1
–2
HD@C60
H2@C60-K13
H NMR
–4
–5
HD@C60-K8
–6
HD@C60-K12
ppm
HD@C60-K13
Figure 8. 500 MHz 1 H NMR spectrum of isotopologue mixture H2 /HD inside C60 , C60 -K8, C60 -K12 and C60 -K13 in
o-dichlorobenzene-d4 at 300 K. The singlet in red corresponds to H2 and the triplet in black corresponds to HD. (Online version
in colour.)
Table 1. Lifetimes, τpo = 1/kpo , of H2 , H2 @C60 and H2 @open-C60 as a function of concentration of the nitroxide TEMPO;
o-dichlorobenzene-d4 , 300 K.
5 mM
10 mM
15 mM
20 mM
H2 a
lifetimes, τpo (h)
0.6
0.3
0.2
0.15
H2 @C60
130 ± 7
140 ± 12
118 ± 7
86 ± 10
H2 @C60 -K8
82 ± 5
94 ± 10
91 ± 7
91 ± 7
H2 @C60 -K12
58 ± 3
70 ± 5
55 ± 3
67 ± 5
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
H2 @C60 -K13
72 ± 3
89 ± 7
70 ± 5
79 ± 5
..........................................................................................................................................................................................................
a Values for H are estimated from k measured in toluene-d [34].
po
2
8
(e) Enrichment in open cage fullerenes
The remarkable sensitivity of the chemical shift of endo H2 to the cage environment has made
it possible to measure simultaneously in a single sample the rates of back conversion of the
para-enriched sample to equilibrium at room temperature for three open endofullerenes that
were prepared as synthetic intermediates on the way to H2 @C60 . Their structures and the NMR
spectrum of the mixture are shown in figure 8. The method used was bimolecular catalysis,
induced by adding varying concentrations of the nitroxide 2,2,6,6-tetramethylpiperidine-1-oxyl
(TEMPO) to a mixture in which each of the four fullerenes was enriched in the para isomer.
Initial enrichment of the mixture was accomplished using adsorption on a zeolite and liquid O2
as described above. Back conversion of H2 @C60 and solutions of H2 induced by nitroxides has
been described previously [34].
The rate data, expressed as the pseudo-first-order back conversion lifetime, τpo , are given
in table 1 for different concentrations of TEMPO [54]. For comparison, the lifetime for back
conversion of free H2 in solution under similar conditions is also given. As previously reported
[34], catalysis of back conversion in free H2 by TEMPO is approximately two orders of magnitude
more effective than conversion in C60 . This emphasizes the relative isolation of caged H2 from
external magnetic effects.
It can be seen from table 1 that catalysis of back conversion in H2 @C60 by TEMPO is clearly
shown, as previously reported. It is apparent that the open forms back convert at a somewhat
faster rate than the parent H2 @C60 . This could be due to the presence of proton magnetic moments
......................................................
H2@C60-K12
H2@C60-K8
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H2@C60
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The ability to purposefully incorporate H2 , H2 O, NH3 and CH4 , and their various isotopologues
into C60 or other fullerenes is now well established. Furthermore, the preparation of (H2 )2 @C70
[55] presages the availability of whole families of endofullerenes encapsulating several small
molecules. We list below some ways that these materials might be used to prepare and study
nuclear spin isomers.
(a) H2 O
Studies of the translation–rotation energy levels of H2 O@C60 are well underway [14]. Changes
in the ortho : para ratio have been detected at cryogenic temperatures using FIR spectroscopy,
and MAS NMR shows promise of being used in a similar fashion to monitor changes in the
concentration of ortho-H2 O@C60 .
The important question remains whether the back conversion rate in para-H2 O will prove to be
sufficiently slow that paramagnetic catalysis can be detected and used, as in the case of H2 @C60 ,
in rationally designed schemes for producing samples enriched in the para isomer. Unlike the case
of H2 , relatively rapid conversion in H2 O and other polyatomic molecules may take place in the
absence of a magnetic catalyst [56]. This prediction is based, in part, on the smaller value of ωop
and the higher density of states, i.e. the number of potential pathways for conversion, available
to polyatomic molecules. The ortho–para perturbation required for transitions is provided by a
combination of spin–rotation and nuclear dipole–dipole coupling [56]. Based on relaxation time
measurements on the endo molecules in C60 and a paramagnetic derivative [57], however, the
electron–nuclear dipole interaction and reorientation correlation time for paramagnetic catalysis
are similar for H2 O and H2 in the same fullerene environment.
(b) D2 and D2 O
It should be possible, in principle, to apply the same methods of detection and catalysis to the D2
allotropes in the fullerene environment that we have used for H2 . Because both the ortho and para
states of D2 are NMR-active, however, changes in intensity of the 2 H NMR signal will be smaller
for the same degree of enrichment. Furthermore, the dependence of the Wigner mechanism on
the square of the nuclear moment indicates that magnetic catalysis under the same conditions
should be at least an order of magnitude less effective at converting D2 spin isomers than has
been observed for H2 . This may actually prove to be an asset for catalytic systems where the
removal of catalyst by chemical methods is slow compared the back conversion rate for H2 .
Magnetic catalysis for D2 O would be subject to the same differences relative to H2 O as D2
is relative to H2 . Predicting the relative rates of the non-catalysed rates for D2 O and H2 O is
complicated by the fact that the relative values of ωop and density of states for the two molecules
should be opposite to the relative values of the spin–rotation and dipole–dipole interactions in
......................................................
5. Future directions
11
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on the groups attached to the orifice (figure 8). Surprisingly, however, the open forms in the
same sample seem to show no consistent evidence of catalysis up to the highest concentrations
of catalyst that could be used because of solubility limits [54]. Because the back conversion rate
for all of the open forms is enhanced somewhat relative to H2 @C60 , in order to be detectable
catalysis would have to produce a rate comparable with that observed with H2 @C60 . This might
be expected given that an opening in the cage could possibly allow more exposure to the catalyst
during an encounter between the two molecules. The fact that treatment with liquid oxygen leads
simultaneously to enrichment in H2 @C60 and in the open forms indicates that the opening does
not fundamentally change the ability to catalyse conversion. The data in table 1 do, however,
indicate that the dependence of bimolecular catalysis on structure is subtle.
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Almost nothing is known about the behaviour of spin isomers of these molecules in condensed
phases at ordinary temperatures. However, nuclear spin conversion in CH4 , both uncatalysed
[59] and catalysed by O2 [60], has been studied in the solid at low temperatures.
(d) (H2 )2 @C70
The ability to encapsulate two, or possibly more, H2 molecules in a fullerene cage, pioneered
by the preparation of (H2 )2 @C70 [55], suggests fascinating possibilities for studying cooperative
effects in ortho–para conversion. Such effects have been studied, for example, as they affect the
NMR spectrum of solid H2 [61]. It should be possible to enrich the para-H2 form of (H2 )2 @C70
using the photoexcitation method which we have shown to work for H2 @C70 [49]. Being able to
adjust the fractions of the para : para, para : ortho and ortho : ortho forms should make it possible, for
example, to study spectroscopically at low temperatures, in condensed media, the interactions
between isolated hydrogen molecule pairs in carefully controlled rotational states.
6. Applications of isolated spin isomers
The ability to prepare samples of fullerenes that contain a substantial excess of one nuclear
spin isomer of an encapsulated small molecule is of scientific importance simply because of the
insights it provides about subtle aspects of energy exchange in small molecules and the way
that they interact with well-defined surroundings. Nevertheless, there are also some potentially
important practical aspects of research in this area. Two examples follow.
(a) NMR intensity enhancement
A sample of para-H2 or para-H2 O corresponds to selective population of only one of the four spin
states available to a pair of equivalent protons. On the other hand, the four levels in the same
sample at equilibrium at room temperature, even in the highest available magnetic field, have
populations that differ by only a few parts per million. The intensity of transitions among the
spin levels in an NMR or MRI measurement depends on the difference in population between
the levels. It is clear that an enormous increase in signal intensity can be achieved if the selective
spin state population of a pure sample of the para form can be used in an NMR experiment. As
detailed above, the basic PHIP phenomenon [15,16] accomplishes this through chemical reactions
that break the symmetry of the spin pair without changing the relative orientations of the spins. In
other words, the NMR-silent I = 0 state of two protons is converted into the corresponding NMRactive states of two non-equivalent I = 1/2 particles with the potential for enormous increase in
the intensity of the observed transitions [20].
Similar applications of encapsulated para-H2 or H2 O will require developing methods by
which the spin states of the endo protons can communicate their selective population to the
outside world. One possibility would be to design open cages, or ways of opening closed cages,
that would be able to expel enriched para molecules into the surrounding medium to be used as
chemical reagents to produce enhancement via PHIP. Indeed, such chemical exchange between
endo and exo water molecules has been measured in the NMR spectrum of an open cage fullerene
containing water [62].
......................................................
(c) NH3 and CH4
12
rsta.royalsocietypublishing.org Phil Trans R Soc A 371: 20110628
the two molecules. That is, H2 O has large ωop , smaller density of states, larger spin–rotation
and dipole interaction. D2 has smaller ωop , higher density of states, smaller spin–rotation and
dipole interaction.
It is also possible that the nuclear quadrupole moment of the deuteron may interact with local
electric field gradients in D2 or D2 O encapsulated in the fullerenes to bring about interconversion
of the spin isomers, as first proposed by Casimir [58].
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(b) Spin-selective D2 fusion
Based on the above results, we are confident that developments in methodology for generating
and using samples of small molecules encapsulated in fullerenes and enriched in spin allotropes
or other specific nuclear spin states should be of increasing importance in chemistry, medicine
and other areas of application. The possibility of measuring the energetics and dynamics of small
molecules in a well defined, nearly isolating, environment in condensed media also provides
a set of irresistibly sweet problems that should occupy experimentalists and theorists for some
time to come.
Acknowledgements. The authors thank the National Science Foundation for its generous support through grant
no. CHE-11-11392.
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