lineshapes and scalar relaxation of the water - endofullerene

The O isotopomer of the water-endofullerene H2O@C60 displays a remarkable proton NMR spectrum, with six well resolved peaks. These peaks are due to the J-coupling between the water protons and the O nucleus, which has spin-5/2. The resolution of these peaks is enabled by the suppression of water proton exchange by the fullerene cage. The six peaks display an unusual pattern of linewidths, which we model by a Liouville-space treatment of scalar relaxation due to quadrupolar relaxation of the O nuclei. The data are consistent with rotational diffusion of the water molecules on the sub-picosecond timescale. The synthetic procedure known as “molecular surgery” consists of opening fullerene cages by chemical reactions, impregnating the cages with guest molecules, and resealing the fullerene cages by further chemical reactions. This method has led to a variety of caged molecular systems, including H2@C60, H2O@C60 and HF@C60 [1–3]. The confined molecules behave as free rotors with a well defined energy level structure [4] and have been probed by numerous spectroscopic methods including infrared spectroscopy, inelastic neutron scattering, and nuclear magnetic resonance (NMR) [3, 5–9]. Nuclear spin isomer interconversion has been studied [5, 8, 10], and the dielectric constant of H2O@C60 was found to depend on the ortho/para ratio of the confined molecules [11]. Endofullerenes provide a sheltered environment for the inserted small molecules, with the confining arena limiting interactions with other molecules. In the case of H2O@C60, proton exchange between water molecules is completely suppressed by the fullerene encapsulation. Molecular dynamics (MD) simulations of quantum rotors inside C60 suggest that the guest molecules reorient on a sub-picosecond timescale [12]. In this communication, we report the observed NMR lineshapes of single O-labelled water molecules encapsulated in C60, see figure 1. The suppression of water proton exchange by the fullerene cage allows the observation of well-resolved H-O scalar couplings. The H peak splits into a well-resolved sextet through coupling to the spin-5/2 O nucleus. The sextet components display an unusual linewidth pattern, which is explained by scalar relaxation of the second kind, associated with O 1Mr. S. J. Elliott, Mr. C. Bengs, Dr. K. Kouřil, Dr. B. Meier, Mr. S. Alom, Prof. Dr. R. J. Whitby, Prof. Dr. M. H. Levitt School of Chemistry University of Southampton Southampton, SO17 1BJ (UK) E-mail: [email protected] * -��� -��� -��� -��� -��� �� �������� ����� (���) + 52 + 32 + 12 − 1 2 − 32 − 52 Figure 1: Relevant portion of the experimental proton spectrum for 36.2 mM H2 O@C60 in degassed ODCB-d4 solvent acquired at 11.7 T (H Larmor frequency = 500 MHz) and 25◦C with 1024 transients. See the Supporting Information (SI) for the full H spectrum. Black line: experimental spectrum; blue line: simulated spectrum. The simulation assumes extreme-narrowing O relaxation, with the following parameters: JOH = -77.9 Hz, T1( O) = T2( O) = 81 ms. The peak of the H2 O@C60 impurity is denoted by an asterisk. The labels MS ∈ {±5/2,±3/2,±1/2} refer to the magnetic quantum number of the O nucleus assuming a negative H-O J-coupling. Inset: schematic representation of H2 O@C60. Red sphere denotes O atom, grey spheres denote H atoms. quadrupolar relaxation. An estimate of the rotational correlation time for H2 O encapsulated in C60 is obtained from O quadrupole relaxation. H2 O@C60 was synthesised as in references [11, 13] with 90% O labelled H2O starting material. See the Supporting Information (SI) for details regarding preservation of the O labelling level. 26.77 mg of H2 O@C60 was dissolved in 1 mL of orthodichlorobenzene-d4 (ODCB-d4) leading to a concentration of 36.2 mM. All samples were subjected to thorough degassing using 4 standard freezepump-cycles in a Wilmad low pressure/vacuum NMR tube (5 mm outer diameter) to remove the majority of dissolved molecular oxygen. The relevant portion of the proton NMR spectrum of H2 O@C60 is shown in figure 1 (for the full spectrum, see the Supporting Information). A similar spectrum was previously obtained in the laboratory of the late Prof. Nick Turro (Columbia University, New York) but was not published. The spectrum shows a sextet splitting due to the H-O scalar coupling, with |JOH| = 77.9 ± 0.9 Hz, in agreement with data on very dilute solutions of H2 O in organic solvents [14]. The H signal resonance of H2 O@C60 is at -4.81 ppm referenced with respect to Preprint submitted to Angew. Chem. Int. Ed. December 12, 2017 -���� -���� -���� -���� -���� ��� �������� ����� (���) Figure 2: Relevant portion of the experimental oxygen-17 NMR spectrum for 36.2 mM H2 O@C60 in degassed ODCB-d4 solvent acquired at 11.7 T (O Larmor frequency = 67.8 MHz) and 25◦C with 32768 transients. Black line: experimental spectrum; blue line: simulated spectrum. The simulation assumes extreme-narrowing O relaxation, with the following parameters: JOH = -77.9 Hz, T1( O) = T2( O) = 81 ms. the H2O impurity peak in ODCB-d4 at 1.39 ppm [2]. The proton spectrum of H2 O@C60 was acquired with an acquisition time of 1.638 s and was processed without additional line broadening. The proton peak linewidths (full width at half maximum) are as follows: 9.1 ± 0.3 Hz (MS = ±5/2), 13.2 ± 0.3 Hz (MS = ±3/2), and 10.7 ± 0.1 Hz (MS = ±1/2). All peaks have the same integral value within experimental error (2 %). The intense narrow line at -4.81 ppm is attributed to H2 O@C60, which has a linewidth (full width at half maximum) of ∼2.5 Hz. From spectral integration the fraction of the H2 O isotopomer in the encapsulated water is ∼88.1% with the remaining ∼11.9% being the O isotopomer. This is in good agreement with the O labelling levels of the O enriched H2O starting material (90%), indicating that the synthetic procedure only changed the O enrichment by ∼1.9%, in agreement with the experimental error for the peak integrals. The relevant portion of the oxygen-17 NMR spectrum of H2 O@C60 is shown in figure 2. A similar spectrum was reported previously by Chen et al. [7]. A triplet with |JOH| = 77.9 Hz is observed. The O peak is at -36.4 ppm referenced with respect to a sample of H2 O water (signal resonance placed at 0 ppm), in agreement with the literature [7]. The oxygen-17 spectrum of H2 O@C60 was acquired with an acquisition time of 0.603 s and was processed without line broadening. The proton and oxygen-17 T1 times for H2 O@C60 are shown in table 1 for two different temperatures. The H T1 decreases with increasing temperature, while the O T1 remains approximately constant over this temperature range. The proton T1 values of H2 O@C60 and H2 O@C60 were found to be the same within experimental error. T1 was estimated by using the inversion-recovery Table 1: Proton and oxygen-17 longitudinal relaxation times of 0.36 mM H2 O@C60 in degassed ODCB-d4 solution acquired at 11.7 T (500 MHz for H and 67.8 MHz for O), for two different temperatures. H2O@C60 H16 2 O@C60 Temperature/◦C T1(H)/ms T1(O)/ms T1(H)/ms 25 755 ± 55 81 ± 7 704 ± 52 57 547 ± 56 90 ± 11 578 ± 80 pulse sequence, in all cases. As shown in the Supporting Information (SI), each component of the H NMR spectrum of H2 O@C60 recovers with the same longitudinal relaxation time, within experimental error, which is indicative of the extreme-narrowing motional regime. Details of C NMR are found in the Supporting Information (SI). The C spectrum shows a 110 ppb splitting between empty and filled fullerene cages, in agreement with the literature [9, 13]. A large variety of relaxation mechanisms may contribute to the observed spectral lineshapes and longitudinal relaxation times of H2 O@C60. As well as the dipole-dipole interactions between all three magnetic nuclei [15, 16], there is also the quadrupolar relaxation mechanism of the O nucleus [17–20], scalar relaxation of the second kind (SR2K) [21, 22], induced by the O relaxation, and spin rotation [23, 24]. In this section, we examine the mechanisms responsible for the reported NMR spectra and T1 times of H2 O@C60. The high degree of rotational freedom within the fullerene cage allows H2 O@C60 to be modelled as a spherical top undergoing rapid isotropic rotational diffusion, described by a rotational correlation time τC. We assume that τC is short enough relative to the nuclear Larmor period to invoke the extreme narrowing approximation [18]. In extreme narrowing, the quadrupolar contribution to the relaxation rate constant of a nuclear spin with quantum number I is given by [20]: T−1 1 = 1 5 (2I − 1) (2I + 3) ‖AQ‖ τC, (1) where τC is the rotational correlation time, and the norm of the quadrupolar interaction tensor is given by: ‖AQ‖ = ωQ [ 1 2 ( 3 + η )] 1 2