Accepted Manuscript Determination of long-range scalar 1H-1H coupling constants responsible for polarization transfer in SABRE Nan Eshuis, Ruud L.E.G. Aspers, Bram J.A. van Weerdenburg, Martin C. Feiters, Floris P.J.T. Rutjes, Sybren S. Wijmenga, Marco Tessari PII: DOI: Reference:

S1090-7807(16)00053-7 http://dx.doi.org/10.1016/j.jmr.2016.01.012 YJMRE 5806

To appear in:

Journal of Magnetic Resonance

Received Date: Revised Date:

18 December 2015 18 January 2016

Please cite this article as: N. Eshuis, R.L.E. Aspers, B.J.A. van Weerdenburg, M.C. Feiters, F.P.J. Rutjes, S.S. Wijmenga, M. Tessari, Determination of long-range scalar 1H-1H coupling constants responsible for polarization transfer in SABRE, Journal of Magnetic Resonance (2016), doi: http://dx.doi.org/10.1016/j.jmr.2016.01.012

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Title:   Determination   of   long-­‐range   scalar   1H-­‐1H   coupling   constants  responsible  for  polarization  transfer  in  SABRE      

Author  names  and  affiliations:  

Nan   Eshuis,   Ruud   L.   E.   G.   Aspers,   Bram   J.   A.   van   Weerdenburg,   Martin   C.   Feiters,   Floris  P.  J.  T.  Rutjes,  Sybren  S.  Wijmenga,  and  Marco  Tessari     Radboud  University,  Institute  for  Molecules  and  Materials,  Heyendaalseweg  135,   6525  AJ  Nijmegen,  The  Netherlands     [email protected]   [email protected]   [email protected]   [email protected]   [email protected]   [email protected]   [email protected]     Corresponding  Author:     Marco  Tessari   Radboud  University,  Institute  for  Molecules  and  Materials,  Heyendaalseweg  135,   6525  AJ  Nijmegen,  The  Netherlands   [email protected]      

Determination of long-range scalar 1H-1H coupling constants responsible for polarization transfer in SABRE Nan Eshuis, Ruud L. E. G. Aspers, Bram J. A. van Weerdenburg, Martin C. Feiters, Floris P. J. T. Rutjes, Sybren S. Wijmenga, and Marco Tessari*

1

ABSTRACT

SABRE (Signal Amplification By Reversible Exchange) nuclear spin hyperpolarization method can provide strongly enhanced NMR signals as a result of the reversible association of small molecules with parahydrogen (p-H2) at an iridium metal complex. The conversion of p-H2 singlet order to enhanced substrate proton magnetization within such complex is driven by the scalar coupling interactions between the p-H2 derived hydrides and substrate nuclear spins. In the present study these long-range homonuclear couplings are experimentally determined for several SABRE substrates using an NMR pulse sequence for coherent hyperpolarization transfer at high magnetic field. Pyridine and pyrazine derivatives appear to have a similar ~1.2 Hz 4J coupling to p-H2 derived hydrides for their ortho protons, and a much lower 5J coupling for their meta protons. Interestingly, the 4J hydride-substrate coupling for five-membered Nheterocyclic substrates is well below 1 Hz. Keywords: Hyperpolarization; SABRE; J-modulation 2

INTRODUCTION

The inherent low sensitivity of NMR has driven the development of nuclear spin hyperpolarization techniques over the past few decades.1-3 A major advancement was introduced in 2009 in the field of Para-Hydrogen Induced Polarization (PHIP4,5), when a non-hydrogenative variant was introduced6 which greatly expands the applicability of PHIP - until then almost exclusively limited to irreversible hydrogenation reactions. This so-called Signal Amplification By Reversible Exchange (SABRE)7 method enhances the NMR signals of small molecules based on their reversible interaction with para-hydrogen (p-H2) at an iridium metal center.8-19 When such transient metal complex is formed at low magnetic field (Btransfer), spontaneous conversion of p-H2 spin order to enhanced longitudinal magnetization of the nuclear spins of the substrate occurs. Subsequent complex dissociation results in hyperpolarized substrate molecules in solution. Theoretical work of several groups has provided useful insight into SABRE mechanisms. An initial description based on density matrix theory by Adams et al. indicated the importance of complex lifetime and Btransfer for hyperpolarization transfer efficiency.20 An alternative approach based on level anticrossings (LACs) developed by Ivanov and co-workers provides simulated field-dependent curves that are in good agreement with experimental data,21 and was recently applied in a high-field SABRE method in which strong RF fields were used to match LAC conditions at high magnetic field (RF-SABRE).22,23 In a similar approach, Warren and co-workers used low-power CW irradiation to obtain in situ 15N hyperpolarization (LIGHT-SABRE).24

From these theoretical descriptions it appears that for polarization transfer to substrate protons, the hydride-hydride scalar coupling determines the position of a level anti-crossing point - i.e. the value(s) of Btransfer at which maximum polarization transfer between the hydride and substrate proton occurs - while the long-range hydride-substrate coupling drives this polarization transfer within the metal complex.18,2026 However, such descriptions are based on unsubstantiated values of the 4J coupling between pyridine ortho-protons and their trans-oriented hydride, that range from 126,27 to 320,21,23,25 Hz. Note that in the case of 15N labeled pyridines18,28-30 and phosphines31 heteronuclear 2J couplings drive SABRE polarization transfer to the substrate. Here, we provide experimental evidence that the homonuclear hydride-substrate 4J coupling is ~1.2 Hz for pyridine-like substrates. We have accurately determined long-range hydride-substrate homonuclear coupling constants using an in situ hyperpolarization approach based on bubbling p-H2 at high magnetic field. Enhanced signals are obtained after a forth-and-back coherence transfer between hyperpolarized hydrides and substrate protons. In order to measure the long-range coupling constants, we have followed a J-modulation approach, implemented along the lines of the homonuclear INEPT block.32-36 Accurate values of the couplings were obtained by fitting the signal integrals in a series of J-modulated spectra to a known transfer function. This approach requires the proton coherence to survive sufficiently long to observe at least once the sign inversion of the signals in the J-modulation series. A screening of the experimental conditions has led to an optimal sample temperature of 268 K, in which the complex dissociation rate is strongly slowed down, while the hydrides T2 relaxation time is still sufficiently long.

3

TH HEO ORY Y

3 3.1

Sp pin system m

TThee mo ost com mm monly used d SA ABREE co omp plexx [Irr(IM Mes)(H)2(s1 1)3]C Cl (IIMees-1 1) is forrmeed upon n hyydro ogeenattion n of ccom mpleex preecurrsorr [Irr(IM Mes))(CO OD)Cl] (IM Mes-p) in thee p pressencce o of an exccesss of su ubsstratte (s1)). TThe o octaaheedraal sttruccturre o of IM Mess-1 p posssessses a ssym mmeetricc eq quattorial p plan ne, with tw wo cheemiccally equivvaleent hyd drides (Figguree 1)). D Due to diffferrentt sccalar coup plinggs with the equ uato oriaal liigan nds (4JAM ≠ 4JA’M the M), t maggneetic equivaalen nce wiithin p p-H2 is brrokeen upo on asso ociaatio on tto tthiss metaal co omplexx, aallow win ng tthe ccon nverrsion off p--H2 ssingglett ord der into enhaanceed longgitudinal m maggnettization n on n th he subsstratte p prottonss. TThis ccoh hereent ttran nsfeer m mechan nism m is onlly effecctive undeer prop per ‘‘maatch hingg’ co onditions, i.e. wh hen Btraansfer ~ m mT 2 20 aas in thee orrigin nal low w-field SA ABR RE eexpeerim men nts, o or u und der RF irrradiatio on in thee hiigh--fielld SSAB BRE aapp proaachees o of Ivvano ov233 an nd W Warren n24. A Alteernaativvely,, we h havee reecen ntlyy sh how wn thatt a COSSY-ttypee polarizaation n trranssferr to o su ubsttrate p protonss is 37 7 posssible aat high maagneeticc fieeld w with hin asyymm metric SSAB BRE com mpllexees. Such h assymmetric complexees ccan be form med d upon n meta m al biindiing of a seeco ond sub bstrratee (s2 2), rresu ultin ng iin aan aasym mmetriic eequaatorrial plaane (Figguree 1; IMes-2 2 an nd IIMees-3 3) with thee hyydrid des NM MR ssign nalss consissting off tw wo d doublets ssplitt byy 2JAX. TThis hyd dride in nequivaalen nce allo owss th he 4JAMii coupling to be pro obed d by seelecctivee exxcittatio ons of hyd drid de HA aand ssub bstraate pro oton n Mi, ussingg a sstraaigh htforwaard hom mon nuclearr HSSQC C-typ pe N NMR p pulsee seequencce (vvidee inffra).

Figu ure 1. SSym mmetric (IM Mes--1) aand asyymm metrric ((IMees-2 2, IM Mes-3) SAB BRE acttive meetal com mpleexess (to op) and d th heir ccorrresp pond dingg pro oton n spin syysteems (bo ottom m) w with h ‘acctivee’ an nd ‘p passsive’’ spiins Mi and Mj.

3 3.2

NMR R

P P-H H2 spin orderr caan b be con nverrted d in nto enh han nced d in n-ph hasee hydrride siggnals u usin ng a SEEPP P38 ((selective eexcitattion off po olarizattion ussingg PA ASADEN NA339) p pulsse sscheemee, w wheere hyd drid de HA ((spiin IA) is seelecctively eexcited d with w a sshap ped 90 0-deegreee p pulsse fo ollo owed b by a peeriod d of τH (Figurre 2 2A) to refo ocus th he aantiphaase x ccoh hereencee with resp pecct to o hyydrid de HX (spin n I ) produ ucin ng in n-ph hasee en nhaanceed m maggnettization n.

W We have previouslyy sh how wn th hat it is sttraigghtfforw ward to o trranssfer this en nhaanceed h hydridee HA magn netizatiion 37 tto tthe substratee proto onss M. Herre, a d diffeeren nt h hom monucleear sin nglee-qu uanttum m co orreelation experrimeent (SEP PP-HoSSQC C) is p pressentted,, displayeed in Figu ure 2B B. TThiss pulsee scheemee reealizzes a forrth--and d-baack ccoh hereencee trranssfer pathw way,, sim milaar to a conveentionaal H HSQC C expeerim mentt.40 Sin nce at high h field hyd drid des aand d substratee prroto ons ressonaate in ttwo o weell-ssepaaratted reggion ns of th he specctrum ((25 – 35 ppm disstan nce)), it is posssiblee to driive tthe coh herencce trransferr ussing shaapeed seelecctive pulsees. A Afteer b bubb blin ng p-H2 thrrouggh tthe ssolu utio on aat h high h magn netic field,, th he ccoherencee flo ow can n bee describeed by meanss off prrodu uct opeerattor fform malism m as: ⎛ π ⎞I A ⎜ ⎟ ⎝2⎠

⎛ π ⎞I A ⎛ π ⎞ I A ⎜ ⎟ ,⎜ ⎟ ,δ1 ⎝ 2 ⎠ y ⎝ 2 ⎠x

(

τH 2 IˆzA IˆzX ⎯ ⎯⎯x⎯ → →− −2 IˆyA IˆzX ⎯ ⎯⎯ ⎯→ IˆxA ⎯⎯ ⎯ ⎯⎯ ⎯⎯ ⎯⎯ ⎯→ −22 IˆxA IˆzM sinn π J AMM δ1 ⎛ π ⎞I A ⎛ π ⎞IM ⎜ ⎟ ,⎜ ⎟ ,t1 ⎝ 2 ⎠ y ⎝ 2 ⎠x

(

)

(

⎯ ⎯ ⎯⎯ ⎯⎯ ⎯→ →−22 IˆzA IˆyM sinn π J AM δ1 ccos((ω M t1 )∏ coos π J AAM δ1 A ⎛ π ⎞I A ⎛ π ⎞IM ⎜ ⎟ ,⎜ ⎟ ⎝ 2 ⎠x ⎝ 2 ⎠x

(

i

)

j≠ii

(

j

⎯ ⎯ ⎯⎯ ⎯⎯ ⎯ → 2 IˆyA IˆzM sinn π J AAM δ1 ccoss(ω M t1 )∏ coos π J AM δ1

(

) (

i

)

j≠ ≠i

j

(

i

)∏ ccos (π J jj≠i

)

i

j≠ ≠i

τ ISI

) (1)

) ) (

2 ⎯δ⎯ → IˆxA sin →−I s π J AMM δ1 sinn π J AAM δ2 ccoss(ω M t1 )∏ coos π J AM δ1 coos π J AM δ2 A i

AM Mj

j

j

)

TThee shapeed πpulsess on n the hyydridess du urin ng b both h ho omo onucleaar IN NEP PT transferrs (d dep picteed in gray in Figu ure 2 2B) sh hould b be sseleectivve for HA to o prreveent thee evvolu utio on d duee to o JAAX d duriing δ1 and d δ2, reesultingg in n a modulatio on o of th he d deteecteed ssignal aas a fun nction o of o onlyy thee acctive an nd p passivee long-range sscallar ccou uplin ngs JAMii and JAAMj (Figu ure 1). .

Figu ure 2. P Pulsse seequencces tthatt prrovid de eenhanced in-p phasse h hydride sign nalss: A) Th he SSEPP P exxperrimeent, witth τH = 2 1/(2 2∙ JAAX). B B) TThe SEP PP-H HoSQ QC eexpeerim mentt, w wherre n can n bee im mpleemented d fo or evvolu ution n in n thee indireect d dimension (increm mentting t1), or aacqu uirin ng a serries of J-mo odulated d 1D D sp pectrra b by vaaryin ng δ1. Ph hasee cycclingg: φ1 = x,-x; φ2 = 2(yy),2(--y); φ3 = 4(x),4(--x); φ4 = 8(xx),8((-x); φrec = x,, -x, -x, x,2(-x, x, xx, -x)), x, -x, -x, x. Shap ped pulsses are ebu urp--1 fo or exxcitatio on, aand rebu urp for refo ocussing/invversion.. Squ uareed b blockks d depiccted d in greyy ind dicaate p pulse-fieeld grad dien nts.

Four situations can be distinguished, concerning the active spin(s) (Mi) and the passive spin(s) (Mj): a) Two spins Mi are chemically equivalent and can be selectively excited/refocused, while excluding the passive proton spins Mj. The signal modulation due to scalar couplings reduces to sin(2π∙JAMi∙δ1). This is typically the case for ortho-protons in symmetrical substituted pyridines or pyrazines. b) A single active spin (Mi ) can be selectively excited/refocused, while excluding the passive proton spins Mj. The signal modulation due to scalar couplings reduces to: sin(π∙JAMi∙δ1). c) At least one of the passive proton spins Mj impedes a reliable selective excitation/refocusing of active spin Mi because of close proximity in chemical shift. The signal modulation due to scalar couplings reduces to sin(π∙JAMi∙δ1)∙cos(π∙JAMj∙δ1) in case of a single active and passive coupling. This is often the case for ortho-protons in meta-substituted pyridines or pyrazines. d) Two spins Mi are chemically equivalent, while at least one of the passive proton spins Mj impedes a reliable selective excitation/refocusing of spins Mi because of close proximity in chemical shift. The signal modulation due to scalar couplings reduces to sin(2π∙JAMi∙δ1)∙cos(π∙JAMj∙δ1) in case of a single passive coupling. Note that for chemically equivalent spins Mi (case (a) and (d)) cosine and sine factors in Equation (1) combine to sin(2π∙JAMi∙δ1), while in case (b) through selective refocusing of only spin Mi the cosine term in Equation (1) disappears. In the (a) – (b) cases, a 1D version of the SEPP-HoSQC experiment suffices to determine the JAMi coupling constant. In case (c) and (d) the experiment should be performed in 2D fashion to independently follow the signal modulations for both Mi and Mj so that both JAMi and JAMj coupling constants can be determined. 1D SEPP-HoSQC p-H2 lifetime at 268 K amounts to ca. 30 – 90 s for the samples examined in this study. Therefore, after bubbling p-H2 through the solution for ca. 2 s, N consecutive 1D SEPP-HoSQC spectra can be acquired corresponding to N different durations of the period δ1. Such an arrayed experiment (p-H2 bubbling followed by multiple acquisitions) is repeated several times to implement the phase cycling described in the legend to Figure 2B so that selection of the desired coherence transfer pathway can be achieved. 2D SEPP-HoSQC For non-equivalent Mi and Mj spins resonating close to each other, a 2D correlation spectrum is necessary to separate the Mi and Mj resonances in the indirect dimension. Modulation of the signal integrals for both Mi and Mj is obtained by recording a series of 2D correlation spectra at different duration of δ1. After bubbling p-H2 through the sample, a full 2D dataset can be acquired for a given δ1 value by recording the complete set of t1 increments before consumption of p-H2 in solution. The slow decay of p-H2 concentration appears as a contribution to the signal linewidth in the indirect dimension. The whole operation (bubbling and complete 2D acquisition) is performed a second time to achieve quadrature detection in the indirect dimension via States (thus incrementing by 90 degree the phase φ1)

or echo/anti-echo (inverting the sign of gradient G1). This is then repeated several times to implement phase cycling for selection of the desired coherence transfer pathway. 3.3

Analysis

1D SEPP-HoSQC The scalar coupling modulation of the hydride signals acquired in the 1D SEPP-HoSQC experiment (Figure 2B) is described in Equation (1). The signal dependence on δ1 is further influenced by two additional processes: transverse relaxation/chemical exchange and p-H2 decay with rate constant Tp-1. These additional contributions are described in Equation (2) and (3), respectively: ⎡ −δ ⎤ S ∝ exp ⎢ 1 ⎥ ⎣ Trelax ⎦

(2)

⎡ −t ⎤ S ∝ exp ⎢ p ⎥ ⎣⎢ Tp ⎦⎥

(3)

where (Trelax)-1 = (T2)-1 + (Texchange)-1 and tp represents a generic time interval following bubbling. Combining these exponential dependencies (Equations (2) and (3)) with the scalar coupling modulation leads to Equations (4a) and (4b), which apply in previously discussed cases (a) and (b), respectively: ⎡ t ⎤ ⎡ δ ⎤ S (δ1 ) = c1 ⋅ sin 2π J AMi δ1 + φ ⋅ exp ⎢− 1 ⎥ ⋅ exp ⎢− p ⎥ + c2 ⎣ Trelax ⎦ ⎣⎢ Tp ⎥⎦

(

)

⎡ t ⎤ ⎡ δ ⎤ S (δ1 ) = c1 ⋅ sin π J AMi δ1 + φ ⋅ exp ⎢− 1 ⎥ ⋅ exp ⎢− p ⎥ + c2 ⎢⎣ Tp ⎥⎦ ⎣ Trelax ⎦

(

)

(4a)

(4b)

where c1, c2, and φare constants. Equations (4) indicate that for long durations of the variable delay δ1, a strong signal decrease due to chemical exchange as well as T2 relaxation (i.e. Trelax) is to be expected. In order to compensate for this attenuation, the series of 1D spectra is measured starting from the longest duration of the delay δ1, and progressively decrementing it. In this way, the largest attenuation due to Trelax is partially balanced by the highest p-H2 concentration in solution (and therefore the highest signal enhancement). Both equations can be rewritten by expressing δ1 and tp in terms of an increment counter n: ⎡ ( nT0 + 12 n ( 2 N - n + 1) Δ ) ⎤ ⎡ ( N - n) Δ ⎤ S ( n ) = c1 ⋅ sin 2π J AM i ( N - n)Δ + φ ⋅ exp ⎢ − ⎥ + c2 ⎥ ⋅ exp ⎢ − Trelax ⎦ Tp ⎣ ⎣⎢ ⎦⎥

(

)

⎡ ( nT0 + 12 n ( 2 N - n + 1) Δ ) ⎤ ⎡ ( N - n) Δ ⎤ exp ⋅ S ( n ) = c1 ⋅ sin π J AM i ( N - n)Δ + φ ⋅ exp ⎢ − ⎢− ⎥ + c2 ⎥ Trelax ⎦ Tp ⎣ ⎣⎢ ⎦⎥

(

(5a)

)

(5b)

in which Δis the (constant) increment in the series of δ1 values, N is the total number of increments, and T0 is the duration of the pulse scheme between the square brackets (see Figure 2B), excluding the variable delay δ1. A non-linear regression analysis of hydride 1D signal integrals with respect to the independent variable n allows the determination of the JAMi coupling constant. 2D SEPP-HoSQC For case (c) and (d) previously discussed, the signal dependence on the delay δ1 is given by Equations 4c and 4d, respectively: ⎡ δ ⎤ S (δ1 ) = c1 ⋅ sin π J AM i δ1 + φ ⋅ cos π J AM j δ1 + φ ⋅ exp ⎢ − 1 ⎥ + c2 ⎣ Trelax ⎦

(4c)

⎡ δ ⎤ S (δ1 ) = c1 ⋅ sin 2π J AM i δ1 + φ ⋅ cos π J AM j δ1 + φ ⋅ exp ⎢ − 1 ⎥ + c2 ⎣ Trelax ⎦

(4d)

(

(

(

)

)

)

(

)

Note that the decay of p-H2 does not contribute to the signal dependence on δ1. Non-linear regression of the signal integrals from the series of 2D spectra with respect to the independent variable δ1 allows the determination of the coupling constants. Note that a simultaneous fit of the two 2D signals is necessary in order to extract reliable values of JAMi and JAMj.

4

RESULTS and DISCUSSION

The present study provides experimentally determined values of the long-range homonuclear hydridesubstrate coupling constants (JAM) in SABRE complexes. The SEPP-HoSQC experiment is employed for this purpose and illustrated by means of model substrate pyridine, providing couplings for the ortho- (4JAM), and meta-protons (5JAM). Subsequently, 4JAM couplings of other six-membered N-heterocyclic substrates are determined, which generally requires a 2D SEPP-HoSQC approach. Finally, it is shown that fivemembered N-heterocyclic substrates have significantly smaller 4JAM couplings than the six-membered analogues. 4.1 Pyridine Asymmetric SABRE complexes with pyridine (py) are obtained by dissolving complex precursor [Ir(IMes)(COD)Cl] (IMes-p) in methanol-d4 in the presence of an excess of equal amounts of the substrates py (s1) and acetonitrile (mecn; s2). This results in the almost exclusive formation of complex IMes-2 upon dissolution of molecular hydrogen at 5 bar, as demonstrated by the thermal 1H spectrum of the hydride region of this sample in Figure 3A (black trace). A signal enhancement of two orders of magnitude is obtained by selective excitation of hydride HA, after bubbling 51% enriched p-H2 through the sample at high magnetic field (Figure 3A, red trace). At 298 K, this enhanced coherence does not survive a subsequent forth-and-back transfer to the two orthoprotons of py (equivalent spins Mi) in a 1D SEPP-HoSQC experiment (Figure 3A, green trace), due to the relatively short lifetime of IMes-2 at this temperature (~0.1 s).27 However, extended complex lifetimes can be obtained by operating at a lower temperature (268 K), which allows efficient coherence transfer in the 1D SEPP-HoSQC experiment, as evidenced by the enhanced NMR signal for hydride HA in Figure 3B. Furthermore, this slowed chemical exchange ensures a slow decay of the p-H2 concentration in solution as shown in Figure 3B, providing significantly enhanced hydride signals over a complete array of δ1 increments (1D SEPP-HoSQC), or t1 increments (2D SEPP-HoSQC). Note the sudden drop in signal in Figure 3B after the first scan (n = 0). This is a result of incomplete refreshment of p-H2 at the metal complex during the SEPP-HoSQC block (~1 s) for all n ≥ 1, compared to the first scan which benefits from a maximal build-up of longitudinal spin order during bubbling (2 s). The first increment (n = 0) is therefore always discarded in a series of 1D SEPP-HoSQC J-modulation spectra.

1

Figu ure 3 3. H Hydrride region of H N NMR R speectrra accquiired d at 500 0 MH Hz o of a sam mplee con ntainingg 4 m mM M IMes aactivvateed w with 5 baar H2 in n the preseencee off sub bstrratess pyy an nd m mecn n, 4 40 m mM each. A A) In n black:: Hyydrid des of aasym mmetricc co omp plex IMees-2 (HA and HX) an nd sym mmeetric co omp plex IMes-1 1 (H HAA’ at -22.8 ppm) at theermal eequilibrrium m. In red: Hyp perp polarrized d sp pectrum m ob btain ned by sseleectivve exxcitaation off HA in a SEPP eexpeerim mentt (τH = 6 62.5 ms)). In greeen: No HA ssign nal eenhaanceemeent is o obseerveed in n a 1D SEP PP-H HoSQ QC exp perim men nt (τH = 62..5 m ms, δ1 = δ2 = 160 ms) at 298 8 K. B) Enhancced hydridee HA ressonaancees obtaiined d with tthe sam me 1D SEPP P-Ho oSQC C exxperrimeent yyet at lo ow ttem mperrature (T = peated n tim mes afteer a singgle b bubblin ng evventt. 268 K), rep

A seeriees of 1D D SEEPP-Ho oSQC sp pecttra is aacqu uireed fo or this sam mplee att 26 68 K K, in wh hich coherrencce iss traansfferrred ffrom mh hydrridee HA to thee orrtho o-prroto ons of pyrridin ne aand back w while d decrrem menttingg δ1 (Figguree 4A A). From tthe 4 resu ultin ng plo ot o of signaal integrals vverssus n (Figguree 4A A, iinseet), a JAM ouplingg co onsttantt off 1.13 Hzz is M co d determ mineed u usin ng Equaatio on (4 4a). W While thee J-m mod dulaated d co oup plingg prrofile ffor orth ho-py pro oton ns ccrosses thee x--axis tw wicee before ccom mpleete relaaxattion n byy Treelax, onlly a sin ngle abscisssa inteerceept (δ1 ~ 1.35 s) ccan bee ideentiified d fo or th he 1 1D SSEP PP-H HoSQC 5 sspeectraa modu m ulatted by thee JAM co oup plingg to o thee meta m a-pyy proto ons (Figguree 4B B). TThe ressultiing cou uplin ng p proffile 5 (Figguree 4B B, in nset) aallow ws thee deeterrmination of JAAM (0 0.37 7H Hz). An estimatio on of tthe even smalleer 6JAM ccou uplin ng tto th he ssinggle p para a-prroto on u usin ng th his SEP PP-H HoSQC app proaach waas not p possible. A As sshown in TTab ble 1 1, th he 4JAM coupliing for pyrridine aapp pearrs in nsen nsitive to tthe varriou us ligan nds at tthe meetal ccen nter, ass sim milaar valuees aare fou und forr pyyridiine in ccom mpleex w with h diffferrentt su ubsttratees ss2 aand difffereent Nhetero ocycclic ccarb ben ne (N NHC C) ligan nds IMees, SSIM Mes, and IP Pr.

Figu ure 4 4. Seriees off J-m modulatted 1H N NMR sp pecttra aand asso ociaated plo ots (inseets) of h hydrride HA in ccomplexx IM Mes-2 (s1 = py, s2 = meecn)), accquirred at 2 268 K byy selecttively exxciting/refo ocussing eith her tthe orth ho (A A) o or m meta a (B) pyrridin ne p proto ons usin ng th he 1 1D SSEPP P-Ho oSQC C seequeencee. δ1 is d decrremeenteed ffrom m 1.1 15 to 0..05 s (Δ= 0..05 ss, (A A)), or ffrom m 1.6 65 to 0..05 (Δ= 0 0.1 s, (B B)), resp pecttively, aafter bu ubbling p-H H2 fo ollow wed by a du umm my sscan n (n = 0). Pllots were fitted d with Equatio on 5.4a 2 2 961 ((A), R = 0.9 9993 35 (B)). (R = 0.999

s1 1

s2 2

NH HC-lligand

JAM (Hzz)

pyy (orrtho)) pyy (meta))

m meccn m meccn

IMees-2 IMees-2

1..13 ± 0.01 0..37 ± 0.02

pyy (orrtho)) d dmsso IMees-2 1..19 ± 0.02 pyy (orrtho)) fp IMees-2 1..06 ± 0.01 pyy (orrtho)) m meccn IPrr-2 1..15 ± 0.01 pyy (orrtho)) m meccn SSIMes-2 2 1..19 ± 0.02 TTab ble 1 1. JAM oupling con nstan nts obtaaineed w with NHC-2 com mpleexess, wheree: N NHC = IM Mes, IPrr, orr SIM Mes;; s2 = aceto onitrile M co (meecn),, dim methyl ssulfoxid de (d dmsso), or 3 3-flu uoro opyrridin ne (ffp); aand s1 = pyyridiine ((py)).

4 4.2 six--meemb bereed N N-heteroccyclees bstittuteed p pyrid dinee-derivvativves, ineequ uivalentt orrtho o-prroto ons Mi aand d Mj oftten ressonaate too o clo ose For meeta--sub tto eeach otheer fo or sselectivve eexcittation, neecesssitaating a 2D D SEEPP--HoSQC C ap pproacch (‘casse ((c)’). A serries of ssuch 2 2D sspectraa, sshow wing the corrrelaatio on b betw weeen hyd dridee HA aand orttho-pro otons M1 and M2 of (-)nico otin ne in ccom mpleex IM Mess-2 (s2 2 = meecn)), iss sh how wn iin FFigu ure 5A.. No ote that ttwo o ep pim mericc co omp plexxes (denotted as ‘‘I’ aand ‘II’)) aree fo orm med upo on b bind dingg off (-)--niccotine, ressultiing in ffourr sim milaar coup plingg profiles 4 4 modulated by siin( JAMii∙δ1)ccos(( JAM Figu ure 5B). A sim multtaneeous fit off the tw wo pro ofilees d derivved forr eaach Mj∙δ1) (F 4 ccom mpleex yyields a JAM co onsttantt off 1..2 H Hz ffor both orth ho-pro oton ns o of (-)-n nico otinee, irresspectivve o of tthe ccom mpleex issom mer..

Figu ure 5: A A) J-mod dulaated d 2D D SEPP-H HoSSQC speectraa accquired by seleectivve eexcittatio on o of bo oth orth ho-p prottonss (deenotted aas M1 aand M2) of ((-)-n nicottinee in eepim meric co omp plexes I and d II ((s2 = mecn). B)) The reesultting plo ots fo or ccomplexxes I and II 2 2 w werre fittted d sim multtaneeoussly u usingg Eq quattion 5.4c (R R =0 0.99 9777 7 (I), R = 0.998 896 (II))). 4

5

A similaar 2D ap ppro oach h waas u used d forr thee deeterm minatio on of the JAAM aand JAM pyraazine (p pn) in assym mmettric M couplings of p ccom mpleex IM Mes--2 (ss1 = pn,, s2 = m mecn n), in n wh hich h thee tw wo equivvaleent o orth ho-proto ons (M1) reeson nate too o clo ose tto th he ttwo eequivalent meeta-p prottonss (M M2) for sselecctivee exxcitaation (‘ccasee (d)’). TThe ressulting ccoupling prrofilles o of th he o orth ho- aand 4 5 m metta-proto ons are sim multaaneously fit (Figurre 6), reesultting in JAM and d JAAM ccoup plingg constaantss of 1.26 6 Hzz and 0..54 Hz, resp pecttivelly. N Notee thaat d due to tthe p of tw wo eequiivaleent passivee spiins Mj, the mo odulatio on in n Eq quattion (4d) is pressencce o 2 ggiveen by sin n(2∙JAMii∙δ1)ccos (JAM Mj∙δ1).

Figu ure 6 6. A A) J-m mod dulaated 2D SEP PP-H HoSQ QC sspecctra (siggnals with n negaative siggn in reed) aacqu uired byy seelecttive exccitation o of the o orth ho- (M1) and d meta-- (M M 2) p protons of p pyraazine (p pn) in co omp plex IMees-2 2 (s1 1=p pn, ss2 = meecn). B) Thee resultting 2 were fitted ssimu ultan neouslyy usiing Equatio on 5.4d (R = 0..999 992)). plotts w

4 4.3 fivee-m mem mberred d N-h heteerocyccles JJ-m modu ulatted speectra off 1-methyyl-1,2,3-triaazole, pyrazo ole aand iso oxazzole do o no ot sh how w an n ab bscisssa inteerceept 4 ffor δ1 < 1..5 s (S ∝ ssin(( JAM Thiss indicaatess a significan nt sm maller hyd drid de-ssubsstraate scalar cou upling for Mi∙δ1)). T ffivee-meemberred N-h heteeroccycllic ssubstraatess co omp pareed tto ssix-m mem mbeered analogu ues succh aas p pyriidine aand pyrazin ne, whosee J-m mod dulaated d sp pecttra (Figgurees 4 4-6) sho ow a siign invversion at δ1 ~ 0.4 4 s (S ∝ siin(2 2∙4JAAMi∙δ1)). O d fivve-m mem mbeered N N-heeterrocyyclees, o onlyy issoxaazole sshow ws a sign invverssion n beeforre tthe Of thee invvestigaated ssign nal disaapp pearrs att high δ1 vvalues d duee to Trelax. From tthe ressultiing plo ot off siggnal integral verrsuss n, a 4JAM ccou uplin ng o of 0.61 Hz is o obtaaineed (Figu ure 7).

1

Figu ure 7 7. Seeriees off H NM MR specttra o of h hydrride HA iin co omp plexx IMes-2 2 (s1 1 = ix, s2 2 = meccn), acq quireed aat 26 68 K usiing tthe 1D SSEPP-HoSQ QC sequ uencce w wherre δ1 is d decrrem mentted ffrom m 2..1 to o 0.0 05 s (Δ= 0.1 s)) aftter b bubbling p--H2 ffollo oweed b by a 2 d dum mmyy scaan (n n=0 0). FFitting w was perrform med d witth EEquaation n 5.4 4b ((R = 0.9 9987 79).

5

CONCLUSION

We have presented an experimental approach for the determination of the long-range homonuclear JAM coupling responsible for polarization transfer in SABRE complexes. The SEPP-HoSQC pulse sequence exploits the chemical inequivalence of hydrides in asymmetric SABRE complexes to probe the hydridesubstrate scalar couplings, enabling determination of their values through the resulting J-modulated spectra. Employing pyridine as model substrate and iridium-IMes as metal complex we have found a value of ~1.2 Hz for the 4JAM coupling between hydride HA and the pyridine ortho-protons, which is in agreement with a recently reported estimation based on hydride splitting.27 The 4JAM coupling to ortho-pyridine was found to be independent of the type of NHC-ligand and cosubstrate (s2) employed (section 4.1; Table 1). Likewise, the same value was found for other sixmembered N-heterocyclic substrates (section 4.2). It is therefore safe to assume very similar 4JAM couplings for pyridine and pyrazine derivatives bound to SABRE complexes. Long-range 5JAM couplings were also determined between hydride HA and the meta-protons in pyridine and pyrazine. Not surprisingly, much lower values were found, suggesting that these interactions are less important in the hyperpolarization transfer process. Interestingly, five-membered N-heterocyclic substrates show a 4JAM coupling that is well below 1 Hz (section 4.3), presumably close to the value of 0.6 Hz that was found for isoxazole in the present study. This suggests that polarization transfer to five-membered N-heterocycles in SABRE generally is less efficient than to six-membered N-heterocycles, indicating a difference in optimal complex lifetime for these two classes of SABRE substrates.25,26

6

EXPERIMENTAL

Chemicals [Ir(IMes)(COD)Cl] (IMes-p), [Ir(SIMes)(COD)Cl] (SIMes-p), [Ir(IPr)(COD)Cl] (IPr-p), and 1-methyl-1,2,3triazole (mtz) were synthesized according to published methods.41,42 All other chemicals were purchased from Sigma-Aldrich (pyridine (py), acetonitrile (mecn), pyrazine (pn), pyrazole (pz) and (-)-nicotine (ni)), Janssen Pharmaceutica (isoxazole (ix)), Acros Organics (3-fluoropyridine (fp)), Merck Schuchardt OHG (dimethyl sulfoxide (dmso)), or Cambridge Isotope Laboratories (methanol-d4) and used as supplied. Para-hydrogen (p-H2) was prepared by cooling normal hydrogen gas (purity 5.0) down to 77 K in the presence of activated charcoal. The resulting 51% enriched p-H2 was transferred to an aluminum cylinder with an adjustable output-pressure valve. Sample preparation Active SABRE complexes [Ir(IMes)(H)2(s1)3]Cl (IMes-1), [Ir(IMes)(H)2(s1)2(s2)]Cl (IMes-2), and [Ir(IMes)(H)2(s1)(s2)2]Cl (IMes-3) (see Figure 1) are formed upon hydrogenation of IMes-p, dissolved in methanol-d4 to a concentration of 4 mM, in the presence of a 20-fold excess of a s1/s2 mixture. The two other metal complexes (SIMes-2 and IPr-2) used in this study are formed in a similar manner upon hydrogenation of complex precursors SIMes-p and IPr-p. Bubbling setup Samples are transferred to a 5 mm Wilmad quick pressure valve NMR tube sealed with an in-house built headpiece to which three PEEK tube lines are connected. The headspace above the sample is depressurized to 4 bar through a vent line (250 ms), allowing efficient bubbling of p-H2 (2 s) through the solution to a final pressure up to 5 bar. A backpressure is applied to quickly stop the bubbling (100 ms), followed by a recovery delay of 500 ms prior to acquisition. The vent-, bubble-, and backpressure delays are spectrometer-controlled through solenoid valves connected to the console. NMR experiments NMR experiments were carried out on a Varian Unity INOVA spectrometer, operating at a proton resonance frequency of 500 MHz. After full activation of the complex precursor, monitored by the decreasing hydride signal intensities of hydrogenation intermediate [Ir(IMes)(H)2(COD)(s1)]Cl,43 SEPP and SEPP-HoSQC pulse sequences are applied as described in section 3.2. Signal integral plots were fitted with Equations 5a-b and 4c-d using the Origin 9.1 (OriginLab, Northampton, USA) software.

6

REFERENCES

(1)

R. E. Mewis, Developments and advances concerning the hyperpolarisation technique SABRE, Magn. Reson. Chem. 2015, 53, 789. J. H. Lee, Y. Okuno, S. Cavagnero, Sensitivity enhancement in solution NMR: Emerging ideas and new frontiers, J. Magn. Reson. 2014, 241, 18. P. Nikolaou, B. M. Goodson, E. Y. Chekmenev, NMR Hyperpolarization Techniques for Biomedicine, Chem. Eur. J. 2015, 21, 3156. C. R. Bowers, D. P. Weitekamp, Transformation of Symmetrization Order to Nuclear-Spin Magnetization by Chemical-Reaction and Nuclear-Magnetic-Resonance, Phys. Rev. Lett. 1986, 57, 2645. T. C. Eisenschmid, R. U. Kirss, P. P. Deutsch, S. I. Hommeltoft, R. Eisenberg, J. Bargon, R. G. Lawler, A. L. Balch, Para Hydrogen Induced Polarization in Hydrogenation Reactions, J. Am. Chem. Soc. 1987, 109, 8089. K. D. Atkinson, M. J. Cowley, S. B. Duckett, P. I. Elliott, G. G. Green, J. Lopez-Serrano, I. G. Khazal, A. C. Whitwood, Para-hydrogen induced polarization without incorporation of para-hydrogen into the analyte, Inorg. Chem. 2009, 48, 663. R. W. Adams, J. A. Aguilar, K. D. Atkinson, M. J. Cowley, P. I. P. Elliott, S. B. Duckett, G. G. R. Green, I. G. Khazal, J. Lopez-Serrano, D. C. Williamson, Reversible interactions with parahydrogen enhance NMR sensitivity by polarization transfer, Science 2009, 323, 1708. M. J. Cowley, R. W. Adams, K. D. Atkinson, M. C. R. Cockett, S. B. Duckett, G. G. R. Green, J. A. B. Lohman, R. Kerssebaum, D. Kilgour, R. E. Mewis, Iridium N-heterocyclic carbene complexes as efficient catalysts for magnetization transfer from para-hydrogen, J. Am. Chem. Soc. 2011, 133, 6134. S. Glöggler, R. Müller, J. Colell, M. Emondts, M. Dabrowski, B. Blümich, S. Appelt, Para-hydrogen induced polarization of amino acids, peptides and deuterium-hydrogen gas, Phys. Chem. Chem. Phys. 2011, 13, 13759. E. B. Dücker, L. T. Kuhn, K. Münnemann, C. Griesinger, Similarity of SABRE field dependence in chemically different substrates, J. Magn. Reson. 2012, 214, 159. L. S. Lloyd, R. W. Adams, M. Bernstein, S. Coombes, S. B. Duckett, G. G. R. Green, R. J. Lewis, R. E. Mewis, C. J. Sleigh, Utilization of SABRE-derived hyperpolarization to detect low-concentration analytes via 1D and 2D NMR methods, J. Am. Chem. Soc. 2012, 134, 12904. B. J. A. van Weerdenburg, S. Glöggler, N. Eshuis, A. H. J. Engwerda, J. M. M. Smits, R. de Gelder, S. Appelt, S. S. Wymenga, M. Tessari, M. C. Feiters, B. Blumich, F. P. J. T. Rutjes, Ligand effects of NHC-iridium catalysts for signal amplification by reversible exchange (SABRE), Chem. Commun. 2013, 49, 7388. J. B. Hövener, N. Schwaderlapp, T. Lickert, S. B. Duckett, R. E. Mewis, L. A. R. Highton, S. M. Kenny, G. G. R. Green, D. Leibfritz, J. G. Korvink, J. Hennig, D. von Elverfeldt, A hyperpolarized equilibrium for magnetic resonance, Nat. Commun. 2013, 4, 2946. N. Eshuis, N. Hermkens, B. J. A. van Weerdenburg, M. C. Feiters, F. P. J. T. Rutjes, S. S. Wijmenga, M. Tessari, Toward nanomolar detection by NMR through SABRE hyperpolarization, J. Am. Chem. Soc. 2014, 136, 2695. F. Shi, A. M. Coffey, K. W. Waddell, E. Y. Chekmenev, B. M. Goodson, Heterogeneous solution NMR signal amplification by reversible exchange, Angew. Chem. Int. Ed. 2014, 53, 7495. H. Zeng, J. Xu, M. T. McMahon, J. A. B. Lohman, P. C. M. van Zijl, Achieving 1% NMR polarization in water in less than 1min using SABRE, J. Magn. Reson. 2014, 246, 119.

(2) (3) (4) (5) (6) (7) (8)

(9) (10) (11) (12)

(13) (14) (15) (16)

(17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34)

N. Eshuis, B. J. A. van Weerdenburg, M. C. Feiters, F. P. J. T. Rutjes, S. S. Wijmenga, M. Tessari, Quantitative trace analysis of complex mixtures using SABRE hyperpolarization, Angew. Chem. Int. Ed. 2015, 54, 1481. T. Theis, M. L. Truong, A. M. Coffey, R. V. Shchepin, K. W. Waddell, F. Shi, B. M. Goodson, W. S. Warren, E. Y. Chekmenev, Microtesla SABRE enables 10% nitrogen-15 nuclear spin polarization, J. Am. Chem. Soc. 2015, 137, 1404. V. Daniele, F. X. Legrand, P. Berthault, J. N. Dumez, G. Huber, Single-scan multidimensional NMR analysis of mixtures at sub-millimolar concentrations using SABRE hyperpolarization, ChemPhysChem 2015, 16, 3413. R. W. Adams, S. B. Duckett, R. A. Green, D. C. Williamson, G. G. R. Green, A theoretical basis for spontaneous polarization transfer in non-hydrogenative parahydrogen-induced polarization, J. Chem. Phys. 2009, 131, 194505. A. N. Pravdivtsev, A. V. Yurkovskaya, H. M. Vieth, K. L. Ivanov, R. Kaptein, Level Anti-Crossings are a Key Factor for Understanding para-Hydrogen-Induced Hyperpolarization in SABRE Experiments, ChemPhysChem 2013, 14, 3327. A. N. Pravdivtsev, A. V. Yurkovskaya, H. M. Vieth, K. L. Ivanov, Spin mixing at level anti-crossings in the rotating frame makes high-field SABRE feasible, Phys. Chem. Chem. Phys. 2014, 16, 24672. A. N. Pravdivtsev, A. V. Yurkovskaya, H. M. Vieth, K. L. Ivanov, RF-SABRE: A Way to Continuous Spin Hyperpolarization at High Magnetic Fields, J. Phys. Chem. B 2015, 119, 13619. T. Theis, M. Truong, A. M. Coffey, E. Y. Chekmenev, W. S. Warren, LIGHT-SABRE enables efficient in-magnet catalytic hyperpolarization, J. Magn. Reson. 2014, 248, 23. J. B. Hövener, S. Knecht, N. Schwaderlapp, J. Hennig, D. von Elverfeldt, Continuous rehyperpolarization of nuclear spins using parahydrogen: theory and experiment, ChemPhysChem 2014, 15, 2451. D. A. Barskiy, A. N. Pravdivtsev, K. L. Ivanov, K. V. Kovtunov, I. V. Koptyug, Simple analytical model for Signal Amplification by Reversible Exchange (SABRE) process, Phys. Chem. Chem. Phys. 2016, 18, 89. R. E. Mewis, R. A. Green, M. C. Cockett, M. J. Cowley, S. B. Duckett, G. G. Green, R. O. John, P. J. Rayner, D. C. Williamson, Strategies for the hyperpolarization of acetonitrile and related ligands by SABRE, J. Phys. Chem. B 2015, 119, 1416. A. N. Pravdivtsev, A. V. Yurkovskaya, H. Zimmermann, H.-M. Vieth, K. L. Ivanov, Transfer of SABRE-derived hyperpolarization to spin-1/2 heteronuclei, RSC Advances 2015, 5, 63615. R. V. Shchepin, M. L. Truong, T. Theis, A. M. Coffey, F. Shi, K. W. Waddell, W. S. Warren, B. M. Goodson, E. Y. Chekmenev, Hyperpolarization of "Neat" Liquids by NMR Signal Amplification by Reversible Exchange, J. Phys. Chem. Lett. 2015, 6, 1961. M. L. Truong, T. Theis, A. M. Coffey, R. V. Shchepin, K. W. Waddell, F. Shi, B. M. Goodson, W. S. Warren, E. Y. Chekmenev, N Hyperpolarization by Reversible Exchange Using SABRE-SHEATH, J. Phys. Chem. C 2015, 119, 8786. V. V. Zhivonitko, I. V. Skovpin, I. V. Koptyug, Strong 31P nuclear spin hyperpolarization produced via reversible chemical interaction with parahydrogen, Chem. Commun. 2014. X. J. Miao, R. Freeman, A Homonuclear Equivalent of the Inept Experiment - Simplification of Crowded Proton Spectra and Measurement of Unresolved Couplings, J. Magn. Reson., Ser A 1995, 117, 128. X. J. Miao, R. Freeman, Spin-echo modulation experiments with soft Gaussian pulses, J. Magn. Reson., Ser A 1996, 119, 90. T. N. Pham, T. Liptaj, P. N. Barlow, D. Uhrin, J-modulated 1D directed COSY for precise measurement of proton-proton residual dipolar coupling constants of oligosaccharides, Magn. Reson. Chem. 2002, 40, 729.

(35) (36) (37) (38) (39) (40) (41) (42) (43)

N. Tjandra, S. Grzesiek, A. Bax, Magnetic field dependence of nitrogen-proton J splittings in N-15enriched human ubiquitin resulting from relaxation interference and residual dipolar coupling, J. Am. Chem. Soc. 1996, 118, 6264. D. Neri, G. Otting, K. Wuthrich, New Nuclear-Magnetic-Resonance Experiment for Measurements of the Vicinal Coupling-Constants 3jhn-Alpha in Proteins, J. Am. Chem. Soc. 1990, 112, 3663. N. Eshuis, R. L. E. G. Aspers, B. J. A. van Weerdenburg, M. C. Feiters, F. P. J. T. Rutjes, S. S. Wijmenga, M. Tessari, 2D NMR Trace Analysis by Continuous Hyperpolarization at High Magnetic Field, Angew. Chem. Int. Ed. 2015, 54, 14527. H. Sengstschmid, R. Freeman, J. Barkemeyer, J. Bargon, A New Excitation Sequence to Observe the PASADENA Effect, Journal of Magnetic Resonance, Series A 1996, 120, 249. C. R. Bowers, D. P. Weitekamp, Parahydrogen and synthesis allow dramatically enhanced nuclear alignment, J. Am. Chem. Soc. 1987, 109, 5541. G. Bodenhausen, D. J. Ruben, Natural Abundance N-15 Nmr by Enhanced Heteronuclear Spectroscopy, Chem. Phys. Lett. 1980, 69, 185. M. A. Seefeld, M. B. Rouse, H. D. A., S. Peace, Y. D., K. C. McNulty, Inhibitors of AKT activity, WO 2008/098104 A1, 14 August 2008. R. A. Kelly III, H. Clavier, S. Giudice, N. M. Scott, E. D. Stevens, J. Bordner, I. Samardjiev, C. D. Hoff, L. Cavallo, S. P. Nolan, Determination of N-Heterocyclic Carbene (NHC) Steric and Electronic Parameters using the [(NHC)Ir(CO)2Cl] System, Organometallics 2008, 27, 202. M. L. Truong, F. Shi, P. He, B. Yuan, K. N. Plunkett, A. M. Coffey, R. V. Shchepin, D. A. Barskiy, K. V. Kovtunov, I. V. Koptyug, K. W. Waddell, B. M. Goodson, E. Y. Chekmenev, Irreversible catalyst activation enables hyperpolarization and water solubility for NMR signal amplification by reversible exchange, J. Phys. Chem. B 2014, 118, 13882.

Highlights

- High field p-H2 hyperpolarization experiment - J-modulation approach allows determination of important coupling constants for SABRE - Values of coupling constants are determined for different SABRE substrates and complexes