Nuclear spin isomers of ethylene: Enrichment by chemical synthesis and application for NMR signal enhancement

Article abstract of DOI:10.1002/anie.201307389

Taking CH₂CH₂ for a spin: Enrichment of the nuclear spin isomers of ethylene was achieved by a catalytic reaction of acetylene with parahydrogen (see scheme). The spin isomers were used for NMR signal enhancement, both with and without subjecting them to a chemical reaction. The interconversion times of the nuclear spin isomers were evaluated at ambient pressure, thus revealing extremely long-lived spin states with lifetimes of (1000±400)s.

Full text of DOI:10.1002/anie.201307389

Angewandte
Chemie
DOI: 10.1002/anie.201307389
Spin Isomers
Nuclear Spin Isomers of Ethylene: Enrichment by Chemical Synthesis
and Application for NMR Signal Enhancement**
Vladimir V. Zhivonitko,* Kirill V. Kovtunov, Pavel L. Chapovsky, and Igor V. Koptyug*
The existence of distinct nuclear spin isomers of molecules
(NSIMs) is the direct manifestation of the laws of quantum
mechanics and thus has fundamental significance in science.[1]
In addition, the use of NSIMs is highly promising for the
development of powerful research tools in various scientific
disciplines. In astrophysics, measurements of the ortho/para
ratio (OPR) for molecules such as H2, H2O, H2CO, and NH3 in
objects of the interstellar medium are used to gain knowledge
about when and how these objects were formed.[2] In NMR
spectroscopy, the use of parahydrogen in homogeneous[3] and
heterogeneous[4,5] catalytic hydrogenations can provide an
enhancement of observed signals by three to five orders of
magnitude, thus advancing significantly the studies of the
mechanisms of such reactions, the development of novel
applications of MRI,[6] NMR spectroscopy in ultra-low and
zero magnetic fields,[7] and more.
However, the only NSIMs that can be readily produced in
arbitrary quantities and with maximum enrichment is para-
hydrogen.[8] Owing to the unusually large rotational constant
of H2, the enrichment can be done by simply cooling normal
H2 to cryogenic temperatures over a paramagnetic material.
For larger molecules, different enrichment techniques (by
synthesis, separation, destruction, or conversion) are
needed.[9] One of the most efficient methods is the light-
induced-drift (LID) approach,[9,10] which was successfully
used to separate the NSIMs of CH3F and 13CH3F,[10,11] as well
as ethylene[12] and 13C-labeled ethylene.[13] Separation in cold
molecular beams was used recently to produce spin-polarized
focused beams of NSIMs for water and methane.[14] Unfortu-
nately, all production approaches for NSIMs demonstrated so
far provide enriched NSIMs in very limited quantities, which
are insufficient for many scientific and practical applications.
In particular, their use for NMR signal enhancement will be
practical only when much larger amounts of enriched NSIMs
become available. In addition, a facile and abundant produc-
tion of enriched NSIMs will undoubtedly have a dramatic
impact on the studies of their fundamental properties and
behavior in various processes such as interconversion,[9,15,16]
chemical reactions,[17] scattering of particles, interaction with
external fields, etc.
In this study, the enrichment NSIMs of ethylene is
achieved by chemical synthesis in quantities sufficient to
successfully utilize it for NMR signal enhancement and to
interrogate interconversion of NSIMs at ambient pressure. To
=
perform the enrichment of NSIMs of ethylene (H2C CH2),
ꢀ
acetylene (HC CH) is catalytically hydrogenated with para-
hydrogen-enriched H2 (pH2). The reaction is performed
continuously by flowing the acetylene/pH2 mixture over
a Pd/TiO2 catalyst (Figure 1a). It was established earlier[18]
that hydrogenation of alkynes and alkenes over such catalysts
proceeds to some extent in a pairwise manner, that is, that
both H atoms of pH2 can end up in the same ethylene
molecule and the correlation of their nuclear spins is
preserved in the reaction, at least partially. Such a reaction
should yield a nonstatistical mixture of NSIMs of ethylene.
Indeed, similar to H2, normal acetylene is a 3:1 mixture of
ortho (with the total nuclear spin I = 1) and para (I = 0)
NSIMs. Thus, the reaction of normal acetylene with para-
hydrogen (I = 0) can only produce ethylene molecules with
I = 0 or 1, while ethylene molecules with I = 2 will be missing
in the reaction product. The four NSIMs of ethylene and their
relative quantities produced in this reaction are discussed in
more detail later.
To verify experimentally the enrichment of the NSIMs of
ethylene produced in the reaction, we exploit the ability of
NSIMs to provide dramatic signal enhancements in NMR
spectra when the symmetry of the molecule is broken. This
has been convincingly demonstrated by using parahydrogen
[*] Dr. V. V. Zhivonitko, Dr. K. V. Kovtunov, Prof. I. V. Koptyug
Laboratory of Magnetic Resonance Microimaging
International Tomography Center, SB RAS
3 A Institutskaya St., Novosibirsk 630090 (Russia)
and
[**] This work was partially supported by the grants from RFBR (no. 11-
03-00248-a, 12-03-00403-a, 12-03-31386-mol-a), RAS (no. 5.1.1), SB
RAS (no. 60, 61, 57, 122), the program of support of Leading
Scientific Schools (no. NSh-2429.2012.3), the Council on Grants of
the President of the Russian Federation (MK-4391.2013.3), and the
program of the Russian Government to support leading scientists
(no. 11.G34.31.0045). We thank Prof. V. I. Bukhtiyarov and his group
(Boreskov Institute of Catalysis) for providing the heterogeneous
catalysts used in this work as well as Prof. A. V. Zibarev and Dr.
A. Yu. Makarov (Novosibirsk Institute of Organic Chemistry) for
providing PTSC and 5CB nematic liquid crystal samples, and Prof. T.
Oka (University of Chicago) for commenting on the manuscript.
Novosibirsk State University
2 Pirogova St., Novosibirsk 630090 (Russia)
E-mail: v_zhivonitko@tomo.nsc.ru
koptyug@tomo.nsc.ru
Homepage: http://www.tomo.nsc.ru/structure/departments/
mrmg/
Prof. P. L. Chapovsky
Institute of Automation and Electrometry, SB RAS
Novosibirsk 630090 (Russia)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201307389.
and
Novosibirsk State University, Novosibirsk 630090 (Russia)
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Figure 2. 1H NMR spectra observed immediately after bubbling the
produced ethylene through the PTSC solution (a,c) and 20 s after the
bubbling was stopped (b). The spectrum in (c) was acquired using
double-quantum filtering. The signal labels in (b) refer to the adduct
structure in Figure 1.
Figure 1. Experimental setup and sequence of events. a) The catalytic
reactor for hydrogenation of acetylene with parahydrogen is made of
copper tubing and consists of a spiral heat exchanger and a wider
straight section packed with the Pd/TiO2 solid catalyst. The reaction is
performed by passing the acetylene/parahydrogen mixture heated to
1508C through the catalyst layer. b) The product ethylene gas is
collected and optionally stored for various time periods in a syringe.
c) The ethylene gas produced in the reaction is bubbled through the
PTSC solution in the NMR sample tube residing inside the NMR
magnet at 7 T, thus leading to the formation of the ethylene/PTSC
adduct and breaking the symmetry of ethylene.
variable time intervals (Figure 1b) before it was bubbled
through the PTSC solution, and then monitored by 1H NMR
signal detection (see the Supporting Information for details).
As the repetitive bubbling leads to a progressive accumu-
lation of PTSCA, its thermal equilibrium signals rapidly
become much stronger than the hyperpolarized signals of the
freshly produced PTSCA, thus masking the antiphase con-
tribution and making it difficult to quantify. Therefore, the
double-quantum filtering (DQF) signal detection scheme[20,21]
was employed, which suppresses the conventional equilibri-
um NMR signals and preserves only the antiphase signals in
the spectrum (see Figure 2c and the Supporting Information
for details). This filtering made it possible to measure the
equilibration curve for NSIMs of ethylene by monitoring the
intensity of the antiphase signal of PTSCA as the function of
the ethylene storage time, and is shown in Figure 3. It is
important to stress that this curve does not reflect the
relaxation of the polarized PTSCA signal, which is rapid once
the asymmetric adduct is formed. Rather, the polarized signal
in hydrogenation reactions in the context of parahydrogen-
induced polarization (PHIP) studies.[3–5] For ethylene, we
have chosen an efficient electrophilic addition reaction of
arenesulfenyl chloride to an alkene.[19] This approach can
provide hyperpolarization of nuclear spins leading to NMR
signal enhancements of about 104 and more. Therefore, even
the enrichments of approximately 0.1% should be detectable,
and the enrichments for the catalytic system used here are
expected to be on the order of 1%.[18] It is also essential that
the use of NSIMs in NMR spectroscopy generates signals with
the characteristic features (e.g., antiphase multiplets) which
make them easily distinguishable from the conventional
NMR signals of other gases, liquids, or solutes.[3–5]
In the experiment, the produced ethylene gas was bubbled
through a CCl4 solution of perfluoro(para-tolylsulfenyl)
chloride (PTSC) in an NMR sample tube residing in the
probe of an NMR instrument. This reaction yields an
asymmetric PTSC/ethylene adduct (PTSCA; Figure 1c).
The 1H NMR spectra detected immediately after the bubbling
and 20 seconds later are shown in Figures 2a and b, respec-
tively. Their comparison clearly confirms the presence of the
contributions with the characteristic antiphase character in
1
the H NMR signals of PTSCA in the spectrum (Figure 2a),
with emission and absorption components being present
simultaneously. Control experiments performed with normal
ethylene gave the usual thermal equilibrium NMR signals
only, with no antiphase contributions. Therefore, the results
indicate that before entering the reaction with PTSC,
ethylene produced in the reaction of acetylene with pH2
represented a nonstatistical mixture of its NSIMs.
The methodology presented above was further used to
study the important issue of the interconversion of NSIMs as
the ratio of different ethylene NSIMs returns to equilibrium.
The produced ethylene gas was collected and stored for
Figure 3. The dependence of the intensity of the polarized signals of
the ethylene/PTSC adduct on the ethylene storage time. a) Selected
absolute value DQF 1H NMR spectra corresponding to various storage
times. b) Kinetics of the equilibration of nuclear spin isomers in
produced ethylene extracted from the set of spectra of the adduct,
along with the double-exponential fitting curve. The t=0 point in this
graph corresponds to the end of the syringe filling with ethylene. Note
that the NMR signals in a) are polarized; the antiphase structure (see
Figure 2) is apparently missing here because the spectra are presented
in the absolute value (or magnitude) mode to simplify their integra-
tion.
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of the adduct is measured repeatedly, using the same batch of
ethylene for repetitive bubbling events and detecting the
NMR signal right after the bubbling is stopped, and therefore
reflects the interconversion of the NSIMs of ethylene as the
function of ethylene storage time.
103 s, which matches the value of Tlong obtained here. No
interconversion between the g and u manifolds could be
observed experimentally at 1 torr.[12] Such transitions in
ethylene imply a change in parity (symmetry of the full
molecular wavefunction with respect to an inversion in space)
and thus cannot be caused by intramolecular interac-
tions.[1,12,24,25] Therefore, at present Tlong can be tentatively
assigned to the interconversion of B1u and B2u isomers.
To further demonstrate the key role of the high molecular
The equilibration curve of the produced mixture of
NSIMs of ethylene is characterized by a rapid initial decay
followed by a much slower decline. It was fitted with
a biexponential function (solid line in Figure 3), yielding
two time constants, Tshort = (10 ꢁ 5) s and Tlong = (1000 ꢁ
400) s. The relatively large uncertainties reflect the substantial
scatter of the data points because all procedures were
performed manually. This may have led to the variations in
the ethylene yield in the hydrogenation reaction, bubbling
conditions, dissolution of ethylene in the PTSC solution, etc.,
from one measurement to another. It is important to note,
however, that complete equilibration of the NSIMs of
ethylene results in the complete disappearance of the DQF
signal of PTSCA, whereas a nonzero signal is clearly observed
in the experiments with the storage time as long as 15 minutes.
The measurement of the nuclear spin lattice relaxation time
of ethylene under experimental conditions at 7 Tyielded T1 =
(270 ꢁ 20) ms. Therefore, as even the lower bounds of the
estimated Tlong and Tshort times are much longer than T1, we
are clearly dealing with the long-lived nuclear spin states.[22]
Further analysis of the equilibration process requires
a closer look at the properties of nuclear spin isomers of
ethylene.[1,12,23–25] Ethylene in its ground electronic and vibra-
tional state is known to have four nuclear spin isomers. They
can be classified using the irreducible representations of the
D2h(M) symmetry group as Ag, B1u, B2u, and B3g species with
the nuclear multiplicities (degeneracies) of Ag:B1u:B2u:B3g =
7:3:3:3. The Ag isomer includes the quintet (I = 2) and the two
singlet (I = 0) spin states of the four hydrogen nuclei in
ethylene, whereas the other three isomers all have I = 1. Upon
heterogeneous hydrogenation of acetylene with pH2, the two
H atoms added to the two different carbon atoms can end up
either on the same (Z ethylene) or on the opposite sides
(E ethylene) with respect to the double bond of ethylene
(Figure 1). The production of Z ethylene leads to the
population of the two Ag singlet states and the B1u and B3g
triplet states with the relative population ratio of
Ag:B1u:B2u:B3g = 2:3:0:3. In contrast, the formation of E ethy-
lene yields Ag:B1u:B2u:B3g = 2:3:3:0 (see the Supporting
Information).
=
symmetry of ethylene (H2C CH2) in these studies, similar
experiments were performed with [D2]acetylene. Its hydro-
=
genation with pH2 produces [D2]ethylene (DHC CHD).The
equilibration curve of this molecule (see Figure S4 in the
Supporting Information) can be fitted using a single expo-
nential term with the time constant of about (8 ꢁ 3) s, which is
=
comparable to Tshort obtained for H2C CH2. Moreover, the
polarized signals for [D2]ethylene can be observed in the gas
or liquid phase directly, without the need to react it with
PTSC (see Figure S3). This observation is the direct con-
firmation that addition of pH2 to ethylene at least partially
proceeds in a pairwise manner and preserves the nuclear spin
correlation.[18] In contrast, for H2C CH2 no PHIP effects were
=
observed after the hydrogenation, as expected. This drastic
=
=
difference between H2C CH2 and DHC CHD is a conse-
quence of the differences in symmetry of the two molecules.
Finally, we demonstrate that ethylene produced from
acetylene and pH2 can itself exhibit hyperpolarized NMR
signals if placed in anisotropic environment, for example,
a nematic liquid crystalline solvent. The use of partially
oriented media leads to an incomplete averaging of aniso-
tropic interactions, with the dominating contribution of
residual dipolar couplings,[21,26] and results in a complex
splitting pattern in the 1H NMR spectrum of a solute. In
1
contrast to the H NMR spectrum of ethylene in isotropic
liquid or gas phase where a single peak is observed, normal
ethylene dissolved in nematic 4-cyano-4’-pentylbiphenyl
(5CB) gives the complex spectrum shown in Figure 4a along
with its modeling (Figure 4b). When ethylene produced in the
reaction of acetylene and pH2 is bubbled through 5CB for 3 s
1
followed by an immediate detection of the H DQF NMR
spectrum, an antiphase pattern is observed for some of the
spectral lines (Figure 4c). The observed effects can be
reproduced quite well by mathematical modeling, assuming
an equimolar mixture of Z and E ethylene (Figure 4d). For
symmetry reasons, the residual dipole–dipole interactions
only mix the quintet and the two singlet states belonging to
the same Ag isomer of ethylene but cannot mix nuclear spin
states of different NSIMs. Therefore, polarization for ethyl-
ene in the liquid crystal can be observed only if the relative
abundance of these particular states is nonstatistical, which is
indeed the case upon formation of both Z and E ethylene.
However, once the equilibration within the manifold of Ag
states is achieved, no polarization can be expected even if the
Ag:B1u:B2u:B3g ratio of the NSIMs is not at equilibrium.
Therefore, this experiment provides unique access to the
nuclear spin order associated with the imbalance of spin states
belonging to the same nuclear spin isomer in a highly
symmetric molecule. Importantly, the polarization in this
experiment was observed only if the produced and collected
The equilibration process in the gas phase is the result of
interconversion of the NSIMs induced by the mixing of states
of different NSIMs, which are accidentally close in energy, by
intramolecular interactions.[9,15,24,25] Several processes may
contribute to the observed curve. The interconversion time
within the u manifold (B1u$B2u) of ethylene was measured as
approximately 1800 s at the gas pressure of 1 torr.[12] Here, we
always refer to the labeling of spin isomers as used in
Ref. [25], which is different from that used in Refs. [12,24]
simply because of a different labeling of molecular axes (see
the Supporting Information). Calculations based on the direct
dipolar interaction of nuclear spins[25] successfully reproduced
this characteristic time, and also predicted that the time
constant at atmospheric pressure should be approximately
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Figure 4. 1H NMR spectra of ethylene dissolved in the nematic 5CB
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molecule (ethylene) by chemical synthesis from enriched
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ethylene produced in large quantities and in a continuous
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Received: August 22, 2013
Published online: && &&, &&&&
Keywords: hydrogenation · long-lived spin states ·
.
NMR spectroscopy · nuclear spin hyperpolarization ·
nuclear spin isomers
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Angewandte
Communications
Communications
Spin Isomers
V. V. Zhivonitko,* K. V. Kovtunov,
P. L. Chapovsky,
I. V. Koptyug*
&&&& — &&&&
Nuclear Spin Isomers of Ethylene:
Enrichment by Chemical Synthesis and
Application for NMR Signal
Enhancement
Taking CH2CH2 for a spin: Enrichment of
the nuclear spin isomers of ethylene was
achieved by catalytically reacting acety-
lene with parahydrogen (see scheme).
The spin isomers were used for NMR
signal enhancement, both with and with-
out subjecting them to a chemical reac-
tion. The interconversion times of the
nuclear spin isomers were evaluated at
ambient pressure, thus revealing
extremely long-lived spin states with life-
times of (1000ꢁ400) s.
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
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