Probing Systems in Solution by NMR Using Sulfur Hexafluoride as a

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Probing Systems in Solution by NMR Using Sulfur Hexafluoride as a Spy Molecule Luca Fusaro,†,‡ Emanuela Locci,† Adolfo Lai,† and Michel Luhmer*,‡ Dipartimento di Scienze Chimiche, UniVersita` di Cagliari, Cittadella UniVersitaria di Monserrato, 09042 Monserrato (CA), Italy, and Laboratoire de RMN haute re´solution CP 160/08, UniVersite´ Libre de Bruxelles, 50 AV. F.-D. RooseVelt, 1050 Brussels, Belgium ReceiVed: January 27, 2009; ReVised Manuscript ReceiVed: March 27, 2009

The use of SF6 as a spy molecule in solution-state NMR is investigated as an alternative to 129Xe NMR. 19F chemical shift, longitudinal relaxation time, and integral measurements, as well as intermolecular nuclear Overhauser effects, are reported for SF6 dissolved in simple deuterated solvents and/or in various solutions among which are aqueous solutions of cucurbit[6]uril (CB) and R-cyclodextrin (RCD). Both CB and RCD form a 1:1 inclusion complex with SF6. In a 0.2 M D2SO4-D2O solution, the affinity of CB for SF6 is 2.9 × 105 L mol-1 at 298 K; it is the largest value ever found for the inclusion of a neutral guest into the CB cavity. It is one order of magnitude larger than in a 0.2 M Na2SO4-D2O solution, and the role of the cation is evidenced. In D2O, the affinity of RCD for SF6 is about 25 L mol-1. With CB, the kinetics is slow on both the 1H and 19F chemical shift time scales, while with RCD fast exchange is observed. The 19F chemical shift of SF6 is much less sensitive to medium effects than the chemical shift of 129Xe. This might be a limitation if minute structural changes are to be investigated but, in the present study, turned out to be an advantage for determining the affinity of RCD for SF6. With CB, the 19F chemical shift of included SF6 is found to be sensitive to the nature of the cation bound at the portals. The 19F relaxation time of SF6 dissolved in deuterated solvents is dominated by the spin-rotation mechanism and is orders of magnitude shorter that the 129Xe relaxation time. It is found to be rather sensitive to the environment and was used to determine the affinity of RCD for SF6. The detection limit of SF6 by 19F NMR is lower than the NMR detection limit of thermally polarized 129 Xe by more than three orders of magnitude. It lies in the micromolar range, and SF6 concentrations of the order of 10 µM were determined by 19F NMR signal integral measurements and used to quantify the affinity of CB for SF6. Integrals, which are usually not reliable for quantitative measurements in 129Xe NMR, were proven to be highly valuable for determining the affinity of RCD for SF6 as well. Most interestingly, heteronuclear Overhauser effect measurements allow to selectively highlight the 1H of a SF6 binding site according to the 19F-1H proximity and were used to characterize the inclusion complex formed with RCD. Introduction The use of SF6 as a spy molecule in solution-state NMR is investigated as an alternative to 129Xe NMR. Xenon is a hydrophobic and rather inert atom which has a diameter of 4.2 Å. It possesses a large and highly polarizable electron cloud, and consequently, its local environment strongly affects the chemical shift and/or the relaxation rates of 129Xe (I ) 1/2) and 131 Xe (I ) 3/2). Therefore, monatomic xenon is used as a “spinspy” to probe the physicochemical properties of solid, liquid, and gaseous systems.1-7 129Xe NMR has been extensively used to distinguish and characterize systems and to study a large variety of processes. Most xenon NMR studies deal with the chemical shift of 129Xe which has been shown to be exquisitely sensitive to the surrounding of the xenon atom and allows highlighting minute structural changes involved, for instance, in conformational or configurational equilibriums.8-12 The chemical shift of 129Xe is easily measured, but signal broadening due to chemical exchange, long longitudinal relaxation times (typically of the order of 102 s in solution), and the rather low NMR receptivity (5.7 × 10-3 with respect to 1H) may be responsible for long measurement times and poor signal-to-noise * To whom correspondence should be addressed. E-mail: [email protected]. Phone: +32 2 650 6637. Fax: +32 2 650 6642. † Universita` di Cagliari. ‡ Universite´ Libre de Bruxelles.

ratios, prevent quantitative interpretation of the signal integrals, and drastically restrict the range of feasible NMR experiments. The use of hyperpolarized 129Xe, also referred to as laserpolarized 129Xe, provides remarkable sensitivity enhancement for a variety of NMR and magnetic resonance imaging (MRI) experiments. Signal-to-noise ratios may be increased by 4-5 orders of magnitude offering new possibilities, notably for the characterization of biological systems.5-7,11,13-16 Most interestingly, the 129Xe polarization can be transferred via crossrelaxation to other nuclear spins: for instance, to the 1H covering the internal surface of a xenon-binding cavity. SPINOEs (spin polarization-induced nuclear Overhauser effect) originating from encapsulated 129Xe were first observed using R-cyclodextrin (RCD) and cryptophane-A as molecular containers.17,18 SPINOEs are now used for the detection and the characterization of cavities in proteins.19-24 Hyperpolarized 129Xe NMR requires optical-pumping equipment for 129Xe polarization. In addition, the transient nature of the 129Xe hyperpolarization imposes additional experimental constraints. Indeed, repeated shaking of the NMR tube or a specific experimental setup is needed to bring fresh hyperpolarized gas into the sample. This may be at the origin of temperature and magnetic field inhomogeneities that significantly affect line widths and line shapes and consequently deteriorate resolution and the signal-to-noise ratio. 1 H SPINOE spectra may therefore be of rather poor quality.

10.1021/jp9008042 CCC: $40.75  2009 American Chemical Society Published on Web 05/05/2009

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Sulfur hexafluoride (SF6) is nontoxic, hydrophobic, and, at room temperature, rather chemically inert. The SF6 molecule has an octahedral geometry and a diameter of 5.3 Å.25 SF6 possesses six chemically equivalent 19F nuclei, and the NMR receptivity of 19F, 0.83 with respect to 1H, is two orders of magnitude larger than the 129Xe NMR receptivity. Consequently, for thermal equilibrium polarizations and identical line widths, the detection limit of SF6 by 19F NMR is at least three orders of magnitude lower that the detection limit of natural abundance xenon by 129Xe NMR. The chemical shift of 19F ranges over hundreds of parts per million. 19F gas-to-solution shifts of SF6 were determined by Jackowski and Wilczek26 and compared to the corresponding values for 129Xe. Both nuclei experience deshielding on gas dissolution, and a very good correlation was found between the 129Xe and the 19F data. The observed range of solvent effects is about 8 ppm on the 19F chemical shift of SF6 and 250 ppm on the 129Xe chemical shift. The 19F longitudinal relaxation time of gaseous SF6 ranges between a few milliseconds and hundreds of milliseconds;27,28 it is orders of magnitude shorter that the 129Xe longitudinal relaxation time. 19 F NMR of gaseous SF6 has deserved much attention29-31 and remains a subject of investigations, notably because applications for lung imaging are being developed.28,32 Kuethe et al. have pioneered the technique of void space imaging using highly fluorinated inert gases in lieu of noble gases; short 19F longitudinal relaxation time, high 19F NMR receptivity, and the presence of multiple equivalent 19F nuclei were highlighted as major advantages.33,34 19F NMR of SF6 in condensed phases is rather rare. A few works deal with 19F NMR of SF6 in the pure liquid phase,35 in simple solvents,26,36 and for probing zeolites,37-39 Vycor glass,40 polymers,41,42 liquid crystals,43,44 and clathrate hydrates.45-49 Recently, we reported on the encapsulation of SF6 by cucurbit[6]uril (CB).50 This study led us to conclude that 19 F NMR of SF6 is a promising approach for probing systems in solution and urged us to pursue investigations on the use of SF6 as a spy molecule in solution-state NMR. The present work reports on 19F chemical shift and longitudinal relaxation time measurements for SF6 dissolved in simple deuterated NMR solvents, on SF6 concentration measurements, and on the characterization of SF6 inclusion processes in the slow or fast exchange regime as well as on probing local environments by 19 F-1H incoherent longitudinal magnetization transfers. Experimental Methods SF6 gas (99.97%) was purchased from Alpha gas. Deuterated solvents were purchased from various companies (SigmaAldrich, Merck, Euriso-top), Na2SO4 and D2SO4 (96-98 wt % in D2O) from Sigma-Aldrich, cucurbit[6]uril dodecahydrate and RCD from Fluka, and 1H,1H,8H,8H-perfluoro-3,6-dioxaoctan1,8-diol (FTEG, >98%) from Fluorochem. RCD was dehydrated by heating at 110 °C for 48 h; other chemicals were used as received. Solutions of FTEG dissolved in DMSO-d6 and D2O were used for 19F NMR signal integral calibration (see Supporting Information). Typically, NMR samples saturated in SF6 were prepared in 5 mm J. Young valve tubes (Wilmad 528-JY-7) by gently bubbling SF6 gas (see Supporting Information). For these samples, the partial pressure of SF6 is the atmospheric pressure minus the vapor pressure of the solvent. The CB samples, approximately 600 µL of solution in 5 mm J. Young valve tubes, as well as the samples used for HOESY (heteronuclear Overhauser effect spectroscopy) experiments, approximately 300 µL of solution in a 5 mm Wilmad valve tube (507-PV-7), were prepared on a pressure/vacuum line. These samples were

Fusaro et al. degassed by three cycles of evacuation and sonication in an ultrasound bath prior to SF6 gas loading. NMR spectra were recorded on Varian VNMRSystem spectrometers equipped with temperature regulation operating at 9.4 T (399.9 MHz for 1H and 376.3 MHz for 19F), using a 5 mm automated triple broadband probe simultaneously tuned to 1 H and 19F, or at 14.1 T (600.0 MHz for 1H and 564.6 MHz for 19 F), using a 5 mm dual broadband probe that was tuned either to 1H or 19F. The samples were left to reach equilibrium at the desired temperature within the magnet for at least 30 min before the NMR measurements. 19F spin-lattice relaxation times were measured using the inversion-recovery pulse sequence with 16 transients, 5.5 s repetition time, and 17 recovery delays ranging between 0.1 and 5 s. T1 data were determined using integrated intensities and a three parameter nonlinear fit. 19F-1H 1D HOESY experiments were carried out using the NMR pulse sequence proposed by Gerig.51 Results and Discussion 19 F NMR of SF6 Dissolved in Simple Deuterated Solvents. F spectra were recorded at 9.4 T and 298 K for SF6 dissolved to saturation in a series of usual deuterated solvents. Sulfur possesses four stable natural isotopes, and consequently, various lines are observed in the 19F NMR spectrum of SF6 (Figure S1, Supporting Information).52 The singlet signal of 32SF6 (95.06% of natural abundance) dominates the spectrum and was used for chemical shift measurements. Typically, the singlet signal of 34SF6 (4.18%) was observed with a single transient, and the 1:1:1:1 quadruplet of 33SF6 (0.74%; I33S ) 3/2) was detected with a few transients. The singlet signal of 36SF6 (0.014%) was not detected. No significant medium effect was observed on the isotope shifts (-0.027 ppm for 33SF6 and -0.053 ppm for 34 SF6), but small variations of the 1J(33S,19F) scalar coupling constant were measured (Table 1). In the studied series of solvents, the range of chemical shift variation is found to be 3 ppm (Table 1). For the corresponding series of undeuterated solvents, the range of 129Xe chemical shift variation is about 100 ppm.53-56 This indicates, in agreement with the work of Jackowski et al.,26 that the sensitivity of 19F chemical shift of SF6 to medium effects is approximately 30 times weaker than the sensitivity of 129Xe chemical shift. The 19F longitudinal relaxation time of SF6 dissolved in the selected deuterated solvents ranges between 0.66 s and about 1 s (Table 1). It is more than two orders of magnitude shorter than the longitudinal relaxation time of monatomic 129Xe dissolved in simple solvents.18,57,58 For SF6 dissolved in 1,1,2,2tetrachloroethane-d2 and in DMSO-d6, longitudinal relaxation time measurements were also carried out at various temperatures at 9.4 and 14.1 T (Table S1, Supporting Information). In both solvents, the 19F longitudinal relaxation time of SF6 decreases with temperature and it is significantly shorter at a higher static magnetic field. As it is the case in the pure gas and liquid phases,25,35 the spin-rotation mechanism is dominating the 19F longitudinal relaxation of dissolved SF6. These data also show that the chemical shift anisotropy mechanism of relaxation plays a role in solution; this contribution was found to be similar in both solvents and is about 12% at 14.1 T and 298 K (Table S2, Supporting Information). SF6 Concentration Measurements. In 129Xe NMR studies, the knowledge of the amount of xenon in solution may be a challenge. Indirect determination via 129Xe chemical shift measurements is possible, but integral data cannot be exploited as a consequence of extremely slow longitudinal relaxation.56 Since the 19F longitudinal relaxation time of dissolved SF6 is 19

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TABLE 1: 19F Chemical Shift (δ), Longitudinal Relaxation Time (T1) at 9.4 T, 1J(33S,19F) Scalar Coupling Constant, and Concentration Measured at 298 K for SF6 Dissolved by Bubbling in Deuterated Solventsa solvent

δ 32SF6 (ppm)

T1 (s)

methanol-d4 dichloromethane-d2 acetonitrile-d3 benzene-d6 water-d2 acetone-d6 1,1,2,2-tetrachloroethane-d2 chloroforme-d1 dimethylsulfoxide-d6

56.94 57.84 58.00 58.04 58.07 58.12 58.42 58.47 59.95

0.660 0.846 0.683 0.761 1.03 0.677 0.961 0.839 0.894

1

J(33S,19F) (Hz) 253.4 254.2 253.4 254.8 253.6 254.5 254.2

[SF6]0 (M), this work 2.5 × 10-2 1.5 × 10-2 2.3 × 10-2 2.8 × 10-2 2.6 × 10-4 3.3 × 10-2 1.7 × 10-2 3.6 × 10-2 2.5 × 10-3

[SF6]0 (M), literatureb 2.41 × 10-2 2.36 × 10-2 2.58 × 10-2 2.57 × 10-4 3.20 × 10-2 2.80 × 10-3

a

Chemical shift data are reported with respect to the chemical shift of the major signal of pure CFCl3 (signal of 12CF 35Cl3 set to 0.00 ppm) used as an external reference in a coaxial insert (Wilmad WGS 5BL). The absolute error on T1 provided by the best-fit analysis of the inversion-recovery experiments is lower than (0.005 s for measurements in the organic solvents and is of the order of (0.01 s in D2O. The digital resolution of the spectra used for the coupling constant measurements was 0.09 Hz/pt. 1J(33S,19F) values measured in the present study by 19F NMR are in very good agreement with reported data obtained by 33S NMR.76 b Calculated as the solubility of SF6 in the corresponding undeuterated solvent at 298 K and 1 - Pvap atm, where Pvap is the vapor pressure of the solvent.60,75

Figure 1. Structure of CB.

on the order of 1 s, integral data can be used to determine the amount of SF6 in solution. The SF6 concentration reached in solution by bubbling SF6 gas at 298 K up to saturation was determined from 19F NMR signal integrations using 1H,1H,8H,8Hperfluoro-3,6-dioxaoctan-1,8-diol as an external standard for integral calibration (see Supporting Information). The results obtained for SF6 dissolved in deuterated solvents are given in Table 1. The agreement with data available in literature for SF6 in the corresponding undeuterated solvents is very good.59,60 Quantitative integral measurements were also carried out for SF6 dissolved to saturation at about 1 atm and 298 K in acidic and saline aqueous (D2O) solutions. The concentration of SF6 measured in a 0.2 M D2SO4 solution and in a 0.2 M Na2SO4 solution was measured to be, respectively, 3% and 18% lower than in D2O. These results agree with estimations based on the approach of Hermann et al.61 which yields salting out effects of 5.7% and 15.4%, respectively. Inclusion of SF6 by CB in Aqueous Solution. CB is a macrocyclic methylene-bridged glycoluril hexamer (Figure 1).62 It is a cavitand with a hollow core of approximately 5.8 Å in diameter.63 The cavity is accessible from the exterior through two carbonyl-fringed portals of 3.9 Å in diameter. The portals are cation-binding regions, while the cavity is a hydrophobic binding region. CB is essentially insoluble in pure water and in common organic solvents, but it is soluble in the range of 10-2 M in neutral saline aqueous solutions and in acidic aqueous solutions. The encapsulation of SF6 by CB was investigated in aqueous solutions of D2SO4 (0.2 M D2SO4-D2O) in a manner similar

to that described in our previous study in which the solvent was an aqueous solution of Na2SO4 (0.2 M Na2SO4-D2O).50 19 F and 1H NMR spectra were recorded at 14.1 T using highly diluted CB solutions (5.0 × 10-5 M) and low SF6 gas pressures (about 1.3 × 10-1 atm).64 In the whole investigated temperature range, 288-338 K, the exchange between free and encapsulated SF6 was found to be slow on both the 19F and 1H spectral time scales, and no important signal broadening was observed (Figure S5, Supporting Information). This contrasts with the encapsulation of xenon by CB for which, at 298 K and 8.5 T, (i) infinitely fast exchange is observed on the 1H spectral time scale, (ii) slow exchange is observed on the 129Xe spectral time scale, but (iii) a degenerate exchange process corresponding to the displacement of xenon by xenon is responsible for important broadening of the 129Xe NMR signals and may prevent the observation of encapsulated 129Xe.65,66 At 298 K, the 19F signal of 32SF6 included in the CB cavity (32SF6,in) is observed at -0.214 ppm with respect to the signal of 32SF6 free in solution (32SF6,out). This complexation induced shift (CIS) is significantly larger than the value observed with samples of CB dissolved in 0.2 M Na2SO4-D2O (-0.070 ppm at 298 K).50 The 19F chemical shift of 32SF6 dissolved in 0.2 M D2SO4-D2O is almost identical to the value measured in 0.2 M Na2SO4-D2O (+0.008 ppm and +0.006 ppm, respectively, with respect to the value in D2O).67 Therefore, it can be concluded that the 19F chemical shift of SF6 included within the cavity of CB is sensitive to the nature of the cation bound at the portals. The 19F CIS decreases for increasing temperatures, and from extrapolations, it is estimated that in 0.2 M D2SO4-D2O the signals of 32SF6,out and 32 SF6,in are superimposed at about 342 K (Figure S6, Supporting Information). For both 32SF6,out and 32SF6,in, the chemical shift temperature variation observed in 0.2 M D2SO4-D2O is essentially identical to the variation observed in 0.2 M Na2SO4-D2O (Figure S6, Supporting Information). The equilibrium constant, Keq, characterizing the inclusion of SF6 by CB in 0.2 M D2SO4-D2O solutions was determined from 19F and 1 H NMR signal integrations (see Supporting Information). The overall SF6 encapsulation process is exothermic as indicated by the decrease of Keq with increasing temperatures (see Supporting Information). At 298 K, the inclusion constant is determined to be 2.9 × 105 L mol-1 (relative error estimated as 10%), that is, one order of magnitude larger than the value measured in 0.2 M Na2SO4-D2O (3.1 × 104 L mol-1).50 To the best of our knowledge, it is the largest value ever found for the inclusion of a neutral guest into the CB cavity. The solubility

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Figure 2. 19F NMR spectra recorded at 298 K and 9.4 T (a) for SF6 dissolved to saturation in D2O and (b) for SF6 dissolved to saturation in a 2.0 × 10-2 M aqueous (D2O) solution of RCD. These spectra were recorded using 200 transients (45° pulse, 3 s relaxation delay, and 2 s acquisition time). The processing comprises zero-filling and exponential multiplication of the free induction decay; the resulting full line width at half height is 2 Hz in both spectra. Without apodization, the line width is 0.7 Hz in (a) and 0.9 Hz in (b). The chemical shift scales are calibrated with respect to the signal of 32SF6 in D2O (set to zero).

of SF6 in 0.2 M D2SO4-D2O is similar to the value in 0.2 M Na2SO4-D2O (see SF6 concentration measurements) suggesting that the thermodynamics of SF6 desolvation is not responsible for this Keq variation. Interestingly, in the presence of both D2SO4 (0.2 M) and Na2SO4 (0.2 M), Keq was measured to be 2.6 × 104 L mol-1 at 298 K, that is, of the same order of magnitude as the value measured in 0.2 M Na2SO4-D2O. It is thus the presence of sodium ions, presumably bound at the portals, that mainly affects the SF6 inclusion process. These results agree with data relative to the inclusion of xenon. Indeed, in 0.2 M Na2SO4-D2O solutions, the affinity of CB for xenon is not significantly affected by the presence of 0.5 M D2SO4.65 Furthermore, a water-soluble CB derivative was recently found to exhibit an increased affinity for xenon in the absence of

Fusaro et al. sodium ions; at 295 K, the inclusion constant was determined to be 180 L mol-1 in a 0.2 M Na2SO4 aqueous solution, while it was measured to be of the order 103 L mol-1 in pure water.68 Inclusion of SF6 by rCD in Aqueous Solution. 19F spectra were recorded at 298 K and 9.4 T for SF6 dissolved to saturation (about 1 atm) in RCD solutions of concentration ranging between 2.0 × 10-2 and 1.2 × 10-1 M as well as in a concentrated solution of maltohexaose (MH, 8.4 × 10-2 M). MH is the linear analogue of RCD (six glucose units), and since it does not have a molecular cavity, there is no possibility of forming an inclusion complex with SF6. A single set of 19F signals, corresponding to the species 32SF6 and 34SF6, was observed in each of the recorded spectra, and no significant line broadening was detected (Figure 2). In the concentrated MH solution, the 19F chemical shift of SF6, the total integrated NMR signal intensity, and the longitudinal relaxation rate are found to be similar to the values measured in the solvent. A small but significant low-field shift, 0.079 ppm, is nevertheless observed,67 and the total signal integral is decreased by about 10% (not shown). In contrast, at the lowest concentration of RCD, a lowfield shift of 0.853 ppm was observed, and the total integrated intensity is increased by about 50% (Figure 2). At the highest RCD concentration, the low-field shift reaches 1.906 ppm, and the total integral is increased by about 300% with respect to the value measured in the solvent. Integral, chemical shift, and longitudinal relaxation rate variations observed for increasing RCD concentrations are shown in Figure 3 (see also Table S3, Supporting Information). These data highlight an exchange process that is infinitely fast on the 19F NMR spectral time scale and, consequently, on the 19F longitudinal relaxation time scale as well. Typically, inclusion of a molecule by RCD gives rise to 1H chemical shift variations for the H-3 and H-5 protons that are located inside the hydrophobic cavity. The 1H NMR spectrum of RCD recorded for a degassed solution and the spectrum recorded in the presence of SF6 (about 10 atm) are highly similar (Figure S8, Supporting Information). However, 19F-1H 1DHOESY experiments clearly reveal the binding of SF6 (Figure 4). Most interestingly, the observed HOEs are highly selective with outstanding effects being measured for the H-3 protons. In contrast, with hyperpolarized 129Xe, selective enhancements were reported for both the H-3 and the H-5 protons.17 These data indicate that the SF6 molecule, which is somewhat larger than a xenon atom, is not deeply included into the RCD cavity but is located on the wide rim and is only partially included (Figure 4). Assuming a 1:1 stoichiometry of binding, the equilibrium constant characterizing the inclusion of SF6 by RCD, Keq, is

Figure 3. Variation of the 19F NMR data measured at 298 K and 9.4 T for SF6 dissolved in aqueous (D2O) solutions of RCD. I/I0 is the total integral of the 19F NMR signals of SF6 divided by the corresponding value measured in pure D2O (I0). δ - δ0 is the 19F chemical shift of 32SF6 with respect to the value measured in pure D2O (δ0). R1 is the longitudinal relaxation rate. The line in (a) and the curve in (b) and (c) are the best-fit of eqs 5, 6, and 7, respectively.

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defined by eq 1 where [RCD0] stands for the molar concentration of RCD that does not contain a SF6 molecule, [SF6,out] for the concentration of SF6 free in solution, and [SF6,in] for the concentration of SF6 included in the RCD cavity.

SF6,out + RCD0 h SF6,in

Keq )

[SF6,in] [SF6,out][RCD0] (1)

[RCD] ) [RCD0] + [SF6,in]

(2a)

[SF6] ) [SF6,out] + [SF6,in]

(2b)

Using the mass balance equation for RCD (eq 2a), the concentration of SF6 included in the RCD cavity can be written as the following:

[SF6,out] ≈ [SF6]0 [SF6] 0

[SF6]

)

I ≈ 1 + κ[RCD] I0

with

(4) κ)

Keq 1 + Keq[SF6]0 (5)

Using for [SF6]0 the value determined in the present study (2.6 × 10-4 M), the best fit of eq 5 to the integral data shown in Figure 3a yields Keq ) 28 ( 1 L mol-1.70 Under fast exchange conditions, the observed chemical shift (δ) and the observed longitudinal relaxation rate (R1) are given by eqs 6 and 7, where δout, R1,out and δin, R1,in are the corresponding data for SF6 free in solution and included in the RCD cavity, respectively.

δ ) δout + xin(δin - δout)

(6)

R1 ) R1,out + xin(R1,in - R1,out)

(7)

Using eqs 3-5, the mole fraction of included SF6 is given by

Keq[SF6,out] [SF6,in] ) [RCD] 1 + Keq[SF6,out]

(3)

In the concentrated MH solution, the concentration of SF6 is found to be similar to the concentration in pure D2O, [SF6]0 (see SF6 concentration measurements). Assuming that this is also the case for the concentration of free SF6 (eq 4), the total concentration of SF6 in the RCD solutions and, consequently, the integrated intensity of the 19F NMR signals of SF6 are expected to increase linearly with the RCD concentration (see eq 5 obtained using eqs 2b, 3, and 4), and this agrees with the variation observed experimentally (Figure 3a).69

xin )

[SF6,in] κ[RCD] ≈ [SF6] 1 + κ[RCD]

(8)

The best fit of eq 6 to the chemical shift data shown in Figure 3b yields Keq ) 27 ( 2 L mol-1 and δin - δout ) 2.52 ( 0.03 ppm.70 Similarly, the best fit of eq 7 to the longitudinal relaxation rate data shown in Figure 3c yields Keq ) 20 ( 4 L mol-1 and R1,in - R1,out ) 0.374 ( 0.008 s-1.70 The Keq values determined from integral and chemical shift measurements are in excellent agreement. The Keq value determined from the longitudinal relaxation rate measurements is slightly smaller, but the agreement is nevertheless very good. The equilibrium constant characterizing the inclusion of xenon by RCD in aqueous solution was measured to be 22 ( 6 L

Figure 4. 1H NMR spectra of a 0.04 M solution of RCD in D2O recorded at 278 K and 9.4 T in the presence of about 10 atm of SF6. (a) Standard 1 H spectrum, (b) 1D-HOESY control spectrum recorded without 19F excitation, and (c) 1D-HOESY spectrum recorded with a mixing time of 1.4 s (12 800 transients, 1 s acquisition time, and 4 s relaxation delay). No significant HOE was detected at 298 K, and positive HOE was observed for H-3 at 318 K (not shown).

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mol-1 at 298 K.71 Despite that SF6 is more hydrophobic than xenon, the inclusion constants are similar. This is most probably due to the fact that SF6 is only partially included in the cavity of RCD, as shown by the HOESY spectra. With respect to chemical shift and longitudinal relaxation rate measurements, integral measurements have the advantage that Keq can be estimated from spectra recorded for two samples only, for the solvent and for a single concentration of the cavitand. Indeed, considering that eq 4 is valid, Keq is the single undetermined parameter in eq 5. Using separately the integral data shown in Figure 3a, the estimation of Keq ranges between 25 and 30 L mol-1, and the standard deviation is 2 L mol-1 (Table S3, Supporting Information). Furthermore, an additional hypothesis is involved in the determination of Keq via chemical shift or longitudinal relaxation rate measurements. Indeed, it is assumed in eqs 6 and 7 that δout and R1,out are constant throughout the titration experiment. With 129Xe, this hypothesis was shown to be wrong for δout, and actually, nonspecific interactions between xenon and RCD were dominating the variation of the observed chemical shift.71 In that case, the extreme sensitivity of the 129Xe chemical shift to the environment was a disadvantage for characterizing the inclusion process. In contrast, the spectrum recorded for the concentrated solution of MH suggests that the contribution of nonspecific interactions to the 19F chemical shift of SF6 is weak in the RCD solutions.72 The response of the chemical shift to the inclusion process depends on the CIS value, that is, δin - δout in eq 6. Similarly, the response of the longitudinal relaxation rate depends on R1,in - R1,out. The 19F CIS observed in D2O because of the inclusion of SF6 by RCD, 2.52 ( 0.03 ppm, is rather large considering the range of solvent shifts (Table 1).73 In 80:20 H2O/ D2O, the corresponding 129Xe CIS corrected for the effects of nonspecific interactions is about 3 ppm71 showing that, in aqueous solution, the sensitivity of the 129Xe chemical shift to the environment change associated with the inclusion by RCD is similar to the sensitivity of the 19F chemical shift of SF6. Finally, it is worth noting that the 19F longitudinal relaxation time of SF6 is significantly affected by the inclusion process. Indeed, R1,in is determined to be about 40% larger than the value measured in the solvent. Conclusions The 19F chemical shift of SF6 is much less sensitive to medium effects than the chemical shift of 129Xe. Obviously, this is a major drawback of 19F NMR of SF6 for probing systems in solution, and it is likely that minute structural changes cannot be highlighted through this parameter. However, it must be stressed that high precision chemical shift measurements are easily achieved in solution-state NMR and that chemical shift variations of the order of 10-2 ppm can be exploited. In addition, the extreme sensitivity of the 129Xe chemical shift may complicate the interpretation since multiple contributions, arising for instance from specific and nonspecific interactions with a xenon binding solute or from xenon-xenon interactions in solution, must be taken into account.8,56,71,74 The detection limit of dissolved SF6 by 19F NMR is of the order of 10-6 M at 14.1 T for measurement times of a few minutes with a standard 5 mm NMR probe. The 19F longitudinal relaxation time of SF6 dissolved in diamagnetic solutions ranges between a few hundreds of milliseconds and about 1 s. It is much shorter than the longitudinal relaxation time of monatomic 129Xe. It is easily and precisely measured and is rather sensitive to medium effects. Obviously, these are major advantages of 19F NMR of SF6 with respect to 129Xe NMR, notably because the 19F longitudinal

Fusaro et al. relaxation time of SF6 might be used for characterizing the system under study; signal-to-noise ratio and longitudinal relaxation do not jeopardize the feasibility of demanding NMR experiments, and spectra suitable for quantitative interpretation of signal integrals are easily obtained. Integral data are indeed highly valuable since they are directly related to the concentration of SF6 in solution and may be used for detecting and characterizing SF6 binding. The NMR characterization of SF6 inclusion processes is by far less demanding than the NMR characterization of xenon inclusion processes. It is worth pointing out that the SF6 inclusion constants measured in this work range over four orders of magnitude and that a xenon inclusion constant of the order of 105 L mol-1 cannot be determined by NMR of thermally polarized 129Xe. Furthermore, and most interestingly, HOESY experiments can be used to selectively highlight the 1H of a SF6 binding site according to the 19F-1H proximity. In conclusion, the present study demonstrates that solutionstate NMR of SF6 is a versatile and informative approach for probing systems in solution and highlights the potential of SF6 for applications relying on the encapsulation of a NMR active gas among which applications that are presently limited to the use of hyperpolarized 129Xe. Acknowledgment. The authors gratefully acknowledge Khalid Ahrika, Naima Chibani, Gerard Fannou Tedzong, and Rita D’Orazio for their contribution to this work as well as Prof. J.T. Gerig for providing a version of the HOESY pulse sequence. The authors thank the Executive Program for Belgium-Italy Scientific Cooperation (Project 2007-2008 05618-S) and the “Communaute´ franc¸aise de Belgique” (ARC 2002-2007 no. 286) for financial support. M.L. thanks the “Fonds de la Recherche Scientifique” (FNRS-FRS), the “Bureau des relations internationales et de la coope´ration” (BRIC) of the Universite´ Libre de Bruxelles, and the “Regione Autonoma della Sardegna” for financing missions in Cagliari. Supporting Information Available: (1) 19F spectrum of SF6 dissolved to saturation at about 1 atm in tetrachloroethane-d2, (2) 19F longitudinal relaxation time of SF6 dissolved in 1,1,2,2tetrachloroethane-d2 and in DMSO-d6 measured at various temperatures at 9.4 T and at 14.1 T, (3) contribution of the chemical shift anisotropy mechanism to the 19F longitudinal relaxation rate of SF6 dissolved in 1,1,2,2-tetrachloroethane-d2 and in DMSO-d6, (4) SF6 concentration measurements, (5) 19F and 1H NMR spectra recorded at 298 K and 14.1 T for a dilute solution of in 0.2 M D2SO4-D2O and low SF6 pressure, (6) temperature variation of the 19F chemical shift of 32SF6 measured in CB solutions, (7) measurement and analysis of the inclusion constant of SF6 by CB in 0.2 M D2SO4-D2O solutions, (8) 19F NMR data measured at 298 K and 9.4 T for SF6 dissolved in aqueous solutions of RCD and the inclusion constant determined from integral data, and (9) 1H NMR spectra of a 0.04 M solution of RCD in D2O recorded at 278 K and 9.4 T for a degassed solution and in the presence of about 10 atm of SF6. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Reisse, J. New J. Chem. 1986, 10, 665–672. (2) Dybowski, C.; Bansal, N.; Duncan, T. M. Annu. ReV. Phys. Chem. 1991, 42, 433–464. (3) Barrie, P. J.; Klinowski, J. Prog. Nucl. Magn. Reson. Spectrosc. 1992, 24, 91–108. (4) Raftery, D.; Chmelka, B. F. In NMR Basic Principles and Progress; Blu¨mich, B., Ed.; Springer-Verlag: Berlin, 1994; Vol. 30, pp 111-157.

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