Article pubs.acs.org/JPCB
Alignment of N@C60 Derivatives in a Liquid Crystal Matrix Guoquan Liu,†,‡ Maria del Carmen Gimenez-Lopez,§ Martyn Jevric,†,∥ Andrei N. Khlobystov,§ G. Andrew D. Briggs,† and Kyriakos Porfyrakis*,† †
Department of Materials, University of Oxford, Oxford OX1 3PH, United Kingdom Research Group ESR Spectroscopy, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Göttingen, Germany § School of Chemistry, University of Nottingham, Nottingham NG7 2RD, United Kingdom ‡
S Supporting Information *
ABSTRACT: The orientation of different N-substituted [N@ C60]fulleropyrrolidine derivatives hosted in a nematic liquid crystal matrix has been studied by electron spin resonance spectroscopy. In this study, variations on the zero field splitting parameter of the guest species are employed as a means of determining their degree of orientation. Fulleropyrrolidines with more rigid N-substituents are preferentially oriented in the liquid crystal matrix, whereas those with less rigid substituents are almost randomly distributed. Additionally, the orientation of a C60 fullerene cage bearing a nitroxide radical has also been investigated in comparison. where A is the nuclear hyperfine interaction tensor and D⃗ is the ZFS tensor. The tensor D⃗ could be converted into two parameters, D and E (D = 3z/2 and E = (y − x)/2, where x, y, and z are the eigenvalues of D⃗ ). In a high external magnetic field where the Zeeman splitting is larger than the hyperfine splitting, parameter E is negligible for an axial molecule like N@C70, and the effective D observed in the ESR spectrum could be represented by
1. INTRODUCTION Endohedral fullerenes have been proposed as future building blocks for an electron-spin-based quantum computer.1,2 Of all the known spin-active endohedral fullerenes, N@C60 exhibits the longest decoherence time (T2 = 240 μs at 170 K).3 High performance liquid chromatography (HPLC) has been used to obtain N@C60 of high purity,4 and high-fidelity single qubit operations on N@C60 have been demonstrated using pulsed electron spin resonance (ESR).5 To realize two-qubit quantum operations in ensembles, it is necessary that the spin−spin couplings are all the same, and for classical dipole coupling, this means that all the pairs of spins must be aligned at the same angle with the magnetic field. Control of spin−spin coupling is not essential, but if required, it can be varied by controlling the relative orientation of the molecules and the field.1 In either case, it is important to devise systems with an ordered structure of fullerenes and their derivatives.6 Alignment of endohedral fullerenes has been investigated in several anisotropic matrixes including liquid crystals,7,8 single crystals,9 and carbon nanotubes.10,11 Among these matrixes, liquid crystals in their nematic phase, such as N-(4methoxybenzylidene)-4-butylaniline (MBBA), can provide a readily accessible one-dimensional environment.12−14 The ability to form a nematic phase in ambient conditions is crucial for endohedral nitrogen fullerenes, which are sensitive to both heat and light.15 The molecular orientation of N@C70 in MBBA was previously investigated via the zero field splitting (ZFS) effect.7,8 The spin Hamiltonian of N@C70 can be represented by the following equation: H = ωeSz − ωI Iz + S ⃗·A ·I ⃗ + S ⃗·D⃗ ·S ⃗ © 2013 American Chemical Society
Deff =
1 Δν = D·OZZ , 2
OZZ = (3·cos2 θ − 1)/2
(2)
where OZZ is the ordering parameter that defines the orientational order of the principal axis of N@C70 in the liquid crystal, Δν is the separation between two successive peaks in each hyperfine multiplet, and θ is the angle between axis Z and the applied magnetic field, as suggested by Meyer et al.8 When Δν and Deff are derived from the linesplitting in ESR spectra, the ordering parameter can be readily deduced from eq 2.7 In contrast, the splitting of ESR lines of N@C60 in the same liquid crystal is less significant because of the icosahedral symmetry (Ih ) of the C60 cage.8 This prompted our investigation in chemically functionalized N@C60 derivatives, which are of lower symmetry. In these derivatives, ZFS is observed as a result of the functionalization, with the ZFS parameter D ranging from 6 to 16.5 MHz.16−18 According to eq 2, the larger D (compared with 2.34 MHz for N@C70) indicates a more significant effect of orientation on the ESR spectra. Received: February 13, 2013 Revised: April 11, 2013 Published: April 16, 2013
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Figure 1. Schematic representation of C60-TEMPO (1) and four N-substituted pyrrolidine derivatives of N@C60 (2−5).
Traditional spin labels (nitroxide radicals)19,20 have also been used to confirm the alignment of fullerene derivatives in MBBA. An example of this technique has been demonstrated in the probe of orientations of fullerene derivatives in nanotubes.21 In our work, a fulleroid derivative bearing the nitroxide radical was first examined in MBBA. On the basis of the experimental observation that functionalized fullerenes could be aligned in a liquid crystal matrix, a series of novel N-substituted pyrrolidine derivatives of N@C60 were dispersed in MBBA and their orientations were studied via the ZFS effect.
Prior to the synthesis of N@C60 derivatives 2−5, the corresponding C60 derivatives were synthesized and characterized by following the Prato reaction procedure.25 Due to the undistinguishable reactivity between N@C60 and C60,26 the N@ C60 derivatives were synthesized under the same conditions using N@C60/C60 (1/1000) as the starting material. These N@ C60 derivatives exhibit the same retention time in HPLC to their corresponding C60 derivatives and were confirmed by ESR without any further characterization. Amino Acid Precursors. 2-{[(4-tert-Butylphenyl)methyl]amino}acetic acid (4b) is commercially available from Aurora Fine Chemicals. 2-{[4-(Methylsulfanyl)phenyl]amino}acetic acid (3b) and 2-({[4-(hexyloxy)phenyl]methyl}amino)acetic acid (5b) were synthesized according to procedures described in the Supporting Information. N-[4-(Methylsulfanyl)phenyl]-fullero[6,6]pyrrolidine (Corresponding to N@C60 Derivative 3). A mixture consisting of C60 (5 mg, 0.007 mmol), paraformaldehyde (1.35 mg, 0.042 mmol), and 3b (4.2 mg, 0.021 mmol) in degassed toluene (10 mL) was immersed in a preheated oil bath (110 °C) and stirred for 40 min. The solution was allowed to cool to room temperature and filtered. Purification of the resulting toluene solution by HPLC afforded the pure title compound as a brown solid (1.9 mg, 30%). MALDI-MS 885 m/z [M]−. 1H NMR (500 MHz, CS2/C6D6) δ/ppm: 2.25 (s, 3H), 4.72 (s, 2H), 6.93 (d, J = 8.8 Hz, 2H), 7.34 (d, J = 8.8 Hz, 2H). 13C NMR (500 MHz, CS2/C6D6, insert) δ/ppm: 191.90, 153.53, 146.75, 145.71, 145.51, 145.26, 145.20, 145.03, 144.72, 143.97, 142.57, 142.11, 141.63, 141.52, 141.36, 139.74, 135.62, 129.36, 129.09, 128.47, 127.76, 124.89, 116.68, 69.11, 62.65, 17.45. Elemental analysis found (expected) %: C 93.6 (93.55), H 1.1 (1.25), N 1.5 (1.6). UV−vis (toluene) λmax: 432 nm. HPLC retention time: 6.95 min.
2. EXPERIMENTAL SECTION Raw C60 was supplied by MER Corporation (>99.5%). N@C60 was produced by ion implantation,22 and the product was enriched using HPLC until the purity was approximately 1/ 1000 (mol/mol).4 All other reagents and solvents were purchased from Aldrich and used without further purification. NMR spectra were obtained on a Bruker AV(III)500 spectrometer. Mass spectrometry was carried out on a Bruker Ultraflex III MALDI-TOF spectrometer using DCTB as a matrix (355 nm) in negative ion mode. Infrared spectra were measured on a Nicolet Avatar 380 FT-IR spectrometer as KBr discs unless specified otherwise. Elemental analyses (C, H, N) were performed by the Elemental Analysis Service of London Metropolitan University. HPLC for fullerene derivatives was carried out using a Cosmosil BuckyPrep-M column (20 × 250 mm2, toluene as eluent, 17 mL/min flow rate, λ = 312 nm). Xband CW ESR measurements were performed on a Magnettech Miniscope MS200, and spectra were simulated using the EASYSPIN software package.23 C60-TEMPO (1) was prepared according to the literature.24 5926
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Figure 2. (a) ESR spectra of C60-TEMPO (red trace) and TEMPO (black trace) in the liquid crystal matrix MBBA; (b) spin density distribution in C60-TEMPO.
N-[(4-tert-Butylphenyl) Methyl]-fullero[6,6]pyrrolidine (Corresponding to N@C60 Derivative 4). Following the procedure for the synthesis of 2, 4b was reacted with C60 and paraformaldehyde to afford the title compound as a brown solid (yield 35%). MALDI-MS 909 m/z [M]−. 1H NMR (500 MHz, CS2/toluene-d8 = 5:1) δ/ppm: 1.21 (s, 9H), 4.03 (s, 2H), 4.20 (s, 4H), 7.28(d, J = 8.5 Hz, 2H), 7.42 (d, J = 8.4 Hz, 2H). 13C NMR (500 MHz, CS2/DMSO-d6, insert) δ/ppm: 154.18, 149.38, 146.56, 145.56, 145.36, 145.22, 144.98, 144.82, 144.58, 143.88, 142.43, 141.96, 141.57, 141.40, 141.22, 139.55, 135.68, 134.12, 128.41, 127.97, 127.70, 125.07, 124.83, 69.93, 66.93, 58.10, 33.59, 30.96, 30.82. Elemental analysis found (expected) %: C 96.3 (96.3), H 2.0 (2.1), N 1.5 (1.5). UV−vis (toluene) λmax: 432 nm. HPLC retention time: 5.65 min. N- {[4-(Hexy lo xy )ph en yl]meth yl}-f ull ero[6,6]pyrrolidine (Corresponding to N@C60 Derivative 5). Following the procedure for the synthesis of 2, 5b was reacted with C60 and paraformaldehyde to afford the title compound as a brown solid (yield 38%). MALDI-MS 953 m/z [M]−. 1H NMR (500 MHz, CS2/toluene-d8 = 5:1) δ/ppm: 0.78 (t, J = 5.9 Hz, 3H), 1.10−1.37 (m, 6H), 1.61 (m, 2H), 3.77 (t, J = 6.0 Hz, 2H), 4.0 (s, 2H), 4.20 (s, 4H), 6.75 (d, J = 8.7, 2H), 7.37 (d, J = 8.8 Hz, 2H). 13C NMR (500 MHz, CS2/DMSO-d6, insert) δ/ ppm: 158.04, 154.18, 146.53, 145.52, 145.34, 144.95, 144.78, 144.55, 143.85, 142.40, 141.93, 141.54, 141.37, 141.18, 139.51, 135.64, 129.20, 128.72, 114.04, 69.89, 67.14, 66.82, 57.89, 31.52, 29.58, 29.21, 25.74, 22.75, 14.06. Elemental analysis found (expected) %: C, 94.4; (94.4), H, 2.5; (2.4), N, 1.4 (1.5). UV−vis (toluene) λmax: 432 nm. HPLC retention time: 5.46 min. General Procedure for the Dispersion of Spin-Active Fullerene Derivatives in MBBA. A solution of C60(TEMPO) 1 or N@C60 derivatives 2−5 (approximately 10−4 mmol) in toluene (0.1 mL) was dried in a standard ESR tube under high vacuum. MBBA (0.1 mL) was then added to the powder, and the tube was placed in an ultrasonic bath for 1 h at 325 K until the sample became homogeneous. After cooling down to room temperature (295 K), the ESR spectra of the prepared samples were recorded immediately. Density Functional Theory (DFT) Calculations. Hybrid DFT calculations were carried out with the Gaussian 03 program27 at the B3LYP level. The split-valence 6-31G(d,p)
basis set was used for all elements unless specified. Full geometry optimization was carried out by means of energy gradient techniques. Spin density and electron density distribution were calculated with SCF density and visualized using GaussView.28 All calculations were set to be in the gas phase regardless of solvent effects.
3. RESULTS AND DISCUSSION 3.1. Alignment of C60-TEMPO Derivative. The C60TEMPO derivative exhibits a similar g tensor and hyperfine interaction tensor to the TEMPO radical. Their similarity in ESR parameters is explained by the fact that in C60-TEMPO the spin density is distributed on the TEMPO moiety rather than the carbon cage (Figure 2b). The molecular tumbling rate of TEMPO presents itself as the mI-dependence of the line width in the ESR spectrum. This phenomenon is characteristic for nitroxide radicals that have anisotropic Zeeman and hyperfine interactions.29 As shown in Figure 2a, the ESR spectra of both C60-TEMPO and TEMPO in MBBA are different from their isotropic equivalent in toluene with broader line width in the highest field component. The correlation times (reverse of the tumbling rate) derived from the simulation of the spectra are listed in Table 1. The Table 1. Best Fit Simulation ESR Parameters for TEMPO and C60-TEMPO in Toluene and the Liquid Crystal Matrix MBBA at Room Temperature toluene
TEMPO C60-TEMPO
MBBA
g
A (MHz)
A (MHz)
correlation time (ns)
2.0062 (3) 2.0061 (3)
43.1 (1) 41.7 (1)
42.7 (1) 38.7 (1)
0.42 (2) 1.58 (4)
correlation time found for C60-TEMPO is ca. 4 times longer than the one found for TEMPO. This result indicates a stronger interaction between the C60-TEMPO and the liquid crystal matrix, which is in agreement with its bigger molecular size compared with TEMPO. The TEMPO radical itself exhibited negligible alignment in MBBA, based on its similar hyperfine coupling constant A in toluene and in MBBA (Table 1). In contrast, C60-TEMPO 5927
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showed preferable orientation in the liquid crystal. The A of C60-TEMPO is significantly decreased after changing the medium from a toluene solution to a liquid crystal matrix. Due to its fast molecular tumbling, C60-TEMPO could still be considered as an axial system in the MBBA. The deviation of its A from the isotropic state is therefore related to its orientation by19 ΔA = −8.6 × OXX
derivatives demonstrates negligible effects of the functional groups on the endohedral nitrogen electron spins. This finding is consistent with previous reports on other pyrrolidine derivatives of N@C60 where similar D and E values were observed.17,18 Dispersion of the four derivatives in MBBA led to two different types of ESR spectra: powder-like spectra with ZFS features for derivatives 2 and 3 and solution-like spectra for derivatives 4 and 5. The difference in ESR spectra reveals their different orientational distributions in the liquid crystal. As shown in Figures 3a and 4, derivative 2 exhibits a powderlike spectrum in MBBA that differs markedly from its spectra in
(3)
where ΔA is the deviation of A in the unit of gauss and OXX is the ordering parameter of axis X (defined along the N−O bond of C60-TEMPO in Figure 1) along the magnetic field. Consequently, OXX is calculated to be 0.11(2) for C60TEMPO dispersed in MBBA. The positive sign of OXX indicates that the principal axis of C60-TEMPO is preferentially parallel along the magnetic field. At this point, two important conclusions could be drawn from the investigations related to the alignment of C60-TEMPO in MBBA. First, the fullerene derivatives dispersed in MBBA tumble quickly above the rigid lattice regime. Second, the preferential parallel orientation of C60-TEMPO to the applied magnetic field confirms the alignment of fullerene derivatives in the liquid crystal. As we show below, these findings prove to be very instructive to further understanding the orientation of N@ C60 derivatives in MBBA. 3.2. Alignment of N@C60 Derivatives. Four pyrrolidine N@C60 derivatives bearing groups of various length and different degrees of flexibility at the α position to the N substituent have been synthesized (Figure 1). As a consequence of the functionalization of the fullerene cage, ZFS features of these N@C60 derivatives are observed in their ESR spectra measured in a frozen solution. The intrinsic ZFS parameters are listed in Table 2. The small difference in D and E for all four
Figure 4. The absorption ESR spectrum of derivative 2 in MBBA at 298 K.
room temperature and frozen solutions. The ESR line splitting Δν is 7.6 MHz (taken as the average of the two outer separations Δνa,b and Δνf,g in Figure 4, which cancels out the second-order hfi-shifts). According to eq 2, Deff is calculated to be 3.8(2) MHz and OZZ is 0.23(1). A positive sign for the ordering parameter is assumed on the basis of both the results of derivative 1 and N@C70 dispersed in MBBA.7 Similarly, Deff and OZZ for derivative 3 dispersed in MBBA are calculated to be 3.9(2) MHz and 0.24(1), respectively. The introduction of a rigid 4-(methylsulfanyl)phenyl group in derivative 3, which increases the molecular aspect ratio, does not enhance significantly the ordering parameter with respect to derivative 2. The ordering parameters for derivatives 2 and 3 are twice larger than the 0.11(2) for derivative 1 in MBBA. The main
Table 2. N@C60 Fulleropyrrolidine Derivatives and Their Best Fit ZFS Parameters in Solid Statea 2 3 4 5 a
D (MHz)
E (MHz)
16.2 16.3 16.3 16.3
0.5 0.4 0.6 0.3
The estimated uncertainty is 0.2 MHz.
Figure 3. The first derivative ESR spectra of derivative 2 (a) and derivative 4 (b): in toluene at 298 K (1), in MBBA at 298 K (2), and in toluene at 77 K (3). * = impurity. 5928
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reason is that the N−O bond in C60-TEMPO (the X axis of the ESR tensor) does not coincide with the principal molecular axis and its conformational flexibility gives rise to extra orientational fluctuation. Therefore, the N@C60 derivatives 2 and 3, one of whose ZFS axis (Z) coincides with their principal molecular axis, are advantageous over the traditional nitroxide radicals to investigate the orientational distribution of fullerenes in liquid crystals. Further analysis results of the ESR peaks of derivative 2 in MBBA are listed in Table 3. The peaks c and e correspond to
of its lowered molecular symmetry) in MBBA. If their difference in cos4 θ and |cos2 θ| is not significant (their ordering parameters are close), the line width difference for derivative 2 is about 2 orders of magnitude larger that that for N@C70. ΔLW ∝ D2τC(cos 4 θ − |cos2 θ|2 )/gβ ℏ
In contrast to derivatives 2 and 3, derivatives 4 and 5 show solution-like ESR spectra in MBBA (Figure 4). The absence of ZFS features in their ESR spectra highlights three key points with respect to the dispersion. First, the distribution of derivatives 4 and 5 is nearly random. Second, tumbling rates for both derivatives 4 and 5 in MBBA are high enough to average out the ZFS effect, consistent with the finding on C60TEMPO. Finally, the matrix-induced ZFS effect, which was previously observed in the mixture of pristine N@C60 and MBBA,7 is negligible compared to the peak line width (about 0.6 G). These three points provide further support to the oriented distribution of derivatives 2 and 3 in MBBA. To explain the two different orientational distribution behaviors in MBBA, we propose that it is the π−π interaction between the fullerene cage and MBBA that determines the orientation of fullerene derivatives. MBBA and the fullerene derivatives exhibit different charge distributions in the electrostatic potential maps. The π electron clouds in both phenyl ring centers of MBBA are negatively charged, and the σ frames as well as the protons are positively charged (Figure 5a). The hexagonal and pentagonal ring centers of the fullerene cage in derivative 2 are positively charged, whereas the carbon−carbon bonds are negatively charged (Figure 5b). A face-to-face alignment, where the positively charged fullerene ring centers point to the negatively charged MBBA phenyl centers, is favored according to the Hunter−Sanders model.31 For derivative 3, although there is a phenyl group in the pyrrolidine moiety, its flexibility is limited by the relatively rigid structure. The fullerene cage still dominates the interactions with MBBA molecules as a result of its larger surface area. Therefore, derivative 3 shows a similar degree of orientation to derivative 2.
Table 3. Peaks in the Absorption ESR Spectrum of Derivative 2 Dispersed in MBBAa peak
center (mT)
FWHM (G)
intensity (a.u.)
a b c
334.6510 334.9218 335.2063
1.75 0.38 2.02
3.2 3.8 6.3
d e
335.4722 335.7422
0.38 1.70
3.7 6.2
f g
336.0218 336.2934
0.38 1.90
3.7 4.4
a
(4)
transitions (MI, MS) (+1, +1/2): (+1, +3/2) (+1, −1/2): (+1, +1/2) (+1, −3/2): (+1, −1/2); (0, +1/2): (0, +3/2) (0, −1/2): (0, +1/2) (−1, +1/2): (−1, +3/2); (0, −3/2): (0, −1/2) (−1, −1/2): (−1, +1/2) (−1, −3/2): (−1, −1/2)
FWHM stands for the full width at half-maximum.
two degenerate transitions due to line width broadening. The linewidth of the “outer” peaks (MS = ±1/2: ±3/2) is much larger than that of the “inner” peaks (MS = −1/2: +1/2), which accounts for the shorter “outer” peaks in the first derivative ESR spectrum (Figure 3a). This line width difference is approximately 1.4 G, which is much larger than the 0.012 G (about 34 kHz) for N@C70 dispersed in MBBA.7 In these S = 3/2 systems, coupling of the anisotropic ZFS to the molecular motion is expected to be the main spin relaxation process. According to eq 4,30 the line width differece ΔLW of the “outer” and “inner” peaks is decided by three parameters: the ZFS parameter D, the molecular correlation time τC, and the orientational distribution related to cos4 θ and |cos2 θ|. Compared with N@C70, derivative 2 has a much larger ZFS parameter D and probably longer correlation time τC (because
Figure 5. Computed electrostatic potentials (with a density isosurface of 0.0004 electrons au−3) of MBBA (a) and derivative 2 (b) (the scale bars are in atomic units). 5929
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Figure 6. (a) A conformation of derivative 4; (b) the computed electrostatic potentials with a density isosurface of 0.0004 electrons au−3. The scale bar is in atomic units.
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The major structural difference of derivatives 4 and 5 from derivative 3 is the presence of an sp3 carbon, which links the pyrrolidine ring and the phenyl group (Figure 1). More conformations exist for derivatives 4 and 5 than for derivative 3, where the phenyl group orientates differently to the fullerene cage. Particularly, a conformation of derivative 4 is identified with the phenyl group separated from the fullerene cage by just 3.6 Å (Figure 6a). An intramolecular interaction is supported by the electrostatic potentials (Figure 6b). Such interaction changes the molecular symmetry and may subsequently affect the intermolecular interaction with MBBA. Therefore, more diversified orientations of derivatives 4 and 5 in MBBA are expected, resulting in broader orientational distribution and solution-like ESR spectra with fast molecular tumbling.
*E-mail:
[email protected]. Present Address ∥
Department of Chemistry, Universitetsparken 5 DK- 2100 Copenhagen, Denmark. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge funding from EPSRC (EP/F028806/01 and EP/D048761/01), the European Science Foundation, and the Royal Society. G.L. is supported by the China Oxford Scholarship Fund, William Louey Educational Fund, and a Graduate Scholarship from St Anne’s College, Oxford. We thank Dr Erik M. Gauger for useful discussions. We thank Dr Nick Rees at the Department of Chemistry, University of Oxford, for the acquisition of NMR spectra and the Oxford Supercomputing Centre for the provision of computing services.
4. CONCLUSIONS To conclude, a nitroxide radical labeled fullerene derivative was initially aligned in a nematic phase liquid crystal matrix. An ordering parameter with a positive sign and a rotationcorrelation time of 1.58(4) ns was determined by simulation of the ESR spectra. Four pyrrolidine derivatives of N@C60 were synthesized and dispersed in the liquid crystal. Derivatives 2 and 3 (with rigid functional groups) are preferentially oriented in the MBBA matrix based on the ZFS features in their ESR spectra. Derivatives 4 and 5 (with a flexible bond linked between the phenyl group and the pyrrolidine ring) exhibit solution-like ESR spectra and a random distribution in the liquid crystal. These findings enable us to evaluate the feasibility of using liquid crystals to align N@C60 derivatives for potential QIP applications. The ordering parameter OZZ is 0.23(1) for derivative 2 and 0.24(1) for derivative 3. These values still need to be improved in the future to satisfy the orientational requirements in QIP applications.
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AUTHOR INFORMATION
Corresponding Author
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REFERENCES
(1) Harneit, W. Fullerene-Based Electron-Spin Quantum Computer. Phys. Rev. A 2002, 65, 032322. (2) Harneit, W.; Meyer, C.; Weidinger, A.; Suter, D.; Twamley, J. Architectures for a Spin Quantum Computer Based on Endohedral Fullerenes. Phys. Status Solidi B 2002, 233, 453−461. (3) Morton, J. J. L.; Tyryshkin, A. M.; Ardavan, A.; Porfyrakis, K.; Lyon, S. A.; Briggs, G. A. D. Electron Spin Relaxation of N@C60 in CS2. J. Chem. Phys. 2006, 124, 014508. (4) Kanai, M.; Porfyrakis, K.; Briggs, G. A. D.; Dennis, T. J. S. Purification by HPLC and the UV/Vis Absorption Spectra of the Nitrogen-Containing Incar-Fullerenes i-N@C60, and i-N@C70. Chem. Commun. 2004, 210−211. (5) Morton, J. J. L.; Tyryshkin, A. M.; Ardavan, A.; Porfyrakis, K.; Lyon, S. A.; Briggs, G. A. D. High Fidelity Single Qubit Operations Using Pulsed Electron Paramagnetic Resonance. Phys. Rev. Lett. 2005, 95, 200501. (6) Benjamin, S. C.; Ardavan, A.; Briggs, G. A. D.; Britz, D. A.; Gunlycke, D.; Jefferson, J.; Jones, M. A. G.; Leigh, D. F.; Lovett, B. W.; Khlobystov, A. N.; et al. Towards a Fullerene-Based Quantum Computer. J. Phys.: Condens. Matter 2006, 18, S867−S883. (7) Jakes, P.; Weiden, N.; Eichel, R. A.; Gembus, A.; Dinse, K. P.; Meyer, C.; Harneit, W.; Weidinger, A. Electron Paramagnetic
ASSOCIATED CONTENT
S Supporting Information *
Synthesis and characterization of amino acids 3b and 5b, ESR spectra of derivatives 3 and 5 in MBBA, as well as complete refs 6 and 27. This material is available free of charge via the Internet at http://pubs.acs.org. 5930
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The Journal of Physical Chemistry B
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