Article pubs.acs.org/Organometallics
Neutron Diffraction Characterization of C−H···Li Interactions in a Lithium Aluminate Polymer Jacqueline M. Cole,*,†,‡,§ Paul G. Waddell,†,∥ Andrew E. H. Wheatley,⊥ Garry J. McIntyre,# Andrew J. Peel,⊥ Christopher W. Tate,⊥ and David J. Linton⊥ †
Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, U.K. Department of Chemistry, University of New Brunswick, P.O. Box 4400, Fredericton, New Brunswick E3B 5A3, Canada § Argonne National Laboratory, 9700 S Cass Avenue, Argonne, Illinois 60439, United States ∥ Australian Nuclear Science and Technology Organization, Lucas Heights, New South Wales 2234, Australia ⊥ Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K. # Institut Laue-Langevin, 6 Rue Jules Horowitz, BP 156, 38042 Grenoble, France ‡
S Supporting Information *
ABSTRACT: The reaction of AlMe3 with tBuLi in the presence of trimethylacetonitrile affords the bimetallic complex [tBu(Me)Al(μMe)2Li·NC(tBu)]∞ (1). Pseudotetrahedral Al centers form by the nucelophilic addition of tBuLi to AlMe3.The alkali-metal center is stabilized through coordination of the unreacted nitrile and polymer formation via the construction of Al(μ-Me)nLi (n = 1, 2) motifs. Neutron diffraction evidences agostic interactions in the bridging methyl group to give further stabilization. There is only one previous report of a neutron structure of a lithium aluminate compound. This work therefore offers an important structural example of agostic interactions and the precise nature of Al(μ-Me)2Li bridging.
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rganoaluminum compounds1 have been employed extensively in catalysis2−7 and synthetic organic chem8−10 Their promotion of carbon−carbon and carbon− istry. heteroatom bond formation in aliphatic systems has been probed,11,12 and the ability of aluminate complexes to generate arylaluminum intermediates has also been established.13−15 The option of using bimetallic reagents incorporating aluminum16,17 to achieve smoothly the directed ortho metalation (DoM)18−21 of aromatic substrates has promised to circumvent limitations associated with the transmetalation methods hitherto employed in preparing arylaluminum species.22 Recently, the highly regioselective functionalization of asymmetric ketones, using aluminates both under mild conditions and also at elevated temperatures, has also been developed.23 The search for new and more highly tunable reactivity in the field of directed deprotonation has furnished a variety of heterometallic or synergic organylamido bases that benefit from low nucleophilicity.24,25 In recent years, these new reagents have collectively revolutionized the area of directed aromatic elaboration, rendering mono-26,27 and dideprotonated28 aryls that were hitherto unachievable. While fine tuning the reactivity of synergic bases has generally focused on modulating the combination of metals, changes to the kinetically active Ndonor ligand have been less well studied. The use of HMDS29 and DMP (cis-2,6-dimethylpiperidide)30 has recently been reported; these promise reagent preparation significantly © 2014 American Chemical Society
cheaper than the original generation of 2,2,6,6-tetramethylpiperidide (TMP)-based bases. While such work has been predicated on the deprotonation of amines, little has been attempted by way of generating synergic amido- or imidometallate complexes through nucleophilic addition across either an imido or nitrile π system. This led us to investigate the treatment of a simple nitrile with an organoaluminum prior to the addition of an alkali-metal reagent in an attempt to yield the corresponding lithium alkyl(imido)aluminate. One facet of the new field of synergic base chemistry that requires further attention is the exact nature of ligand bridging between the two metals. Whereas bridging of the N-donor ligand has received significant attention,27,31 a recurrent structural feature of, for example, lithium aluminates is the suggestion of bridging by alkyl groups. While the precise nature of this bonding mode is far from clear, the phenomenon itself is considered to be highly significant. Thus, compounds related to lithium aluminum hydrides have attracted attention for the nature of the role that methyl hydrogen atoms play in terms of interaction with the lithium ions.32 It is generally understood that C−H moieties and lithium ions in organolithium complexes form a variety of agostic bonds,33−38 three-center− Received: March 13, 2014 Published: May 1, 2014 3919
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With a focus on the bridging action in 1, the Li−C distances involved (2.357(19) and 2.282(19) Å) are reminiscent of the equivalent distances in the bis(pyrazolyl) Me2Al(μ-Me)(μ-L)Li structure, where L = bis(2-(3,5-dimethyl-1-pyrazolyl)ethyl)amine (2.364(5) and 2.281(5) Å).44 This bridging helps hold 1 in a crystallographic pseudomirror (Figure 1, bottom inset) that is offset by methyl groups and a slight puckering of the N1− Li1−Al1 chain. The lithium ion inhabits an essentially tetrahedral environment; the nitrile nitrogen of the trimethylacetonitrile (tBuCN) molecule, N1, coordinates to Li1 and three agostic interactions complete the coordination sphere (Figure 1). These agostic interactions (C−H···Li) all involve the methyl groups of the alkylaluminum anion. As the lithium ion and the carbon atoms involved in these agostic interactions do not comprise the same species, these interactions are best described as intermolecular agostic bonds.54,55 These agostic interactions can be described as belonging to two different environments. The first is the result of the close contact of one hydrogen atom on each of C1 and C2 in a pincer formation about Li1, the bite angle allowing for the close approach of two hydrogen atoms (C1−Al1−C2 = 113.6(3)°). In the case of H1C, the interaction distance falls outside the range of 1.80−2.20 Å proposed by Braga et al.56 and exhibits a reduced C−H···Li angle, though the agostic interaction can still be assumed by the close approach of C1 to Li1. The second agostic environment manifests as a bifurcated interaction between Li1 and two hydrogens on C7 on the aluminate anion of the adjacent monomeric component of the observed polymer chain, where the Al1−C7···Li1 angle of 161.7(2)° allows for the close approach of two methyl hydrogens. Both agostic interactions relating to hydrogen atoms on C1 and C2 are observed to be longer than those relating to C7. This can be attributed to a distortion of the tetrahedral geometry of both methyl groups with respect to Al1, such that in each case the CH3 moiety appears to be tilted away from Li1. This can be attributed to repulsive close H···H contacts (H7A··· H2C = 2.14(2) Å and H7C···H1B = 2.21(2) Å) with hydrogen atoms on C7, which also prevent the close approach of H1B and H2C to Li1, such that these distances are too long to be considered agostic interactions. The proximity of the hydrogen atoms to the lithium ion is a direct result of the close approach of their parent carbon atoms to the lithium ion. The C···Li distances are very similar: within 1 standard deviation (Table 1). Due to this consistency between the two C−H···Li environments, the C−H···Li angle correlates well with the H···Li distance (see the Supporting Information). However, no apparent trend in the C−H distance is observed in relation to either the C−H···Li angle
two-electron interactions which form as a result of both electrostatics and the overlap of lithium orbitals with the carbon lone pair and the σ-C−H orbitals.39,40 Hitherto, studies on the bridging action of the AlMe groups in lithium aluminates have only used X-ray diffraction, where the direct observation of the H atoms during Fourier synthesis is absent. Information on the role of H in such bridges has been inferred from short Li−C(alkyl) bonds that have been noted to complete four-membered metallacycles in several aluminate structures.41−46 However, while the negatively hyperconjugated dimer of (2-C5H4N)(Me3Si)2CLi has been probed thoroughly,47 a survey of the Cambridge Structural Database48 reveals that only one single-crystal neutron diffraction study of a lithium aluminate has ever been reported.49 In fact, the current volume of literature devoted to a detailed structural analysis of any type of metal−methyl agostic interaction remains modest, as the positions of methyl hydrogen atoms are notoriously difficult to locate using any method other than neutron diffraction.50−53 As yet, only one comprehensive neutron study focusing on agostic interactions to lithium ions has been reported,47 though interactions of this type had been identified before the formal classification of agostic interactions.35,45 In addition, few compounds incorporating intermolecular C−H···Li agostic interactions have been reported; rarer still are those involving di- and trifurcated interactions, for which only a handful of structures are known, few being neutron-derived structures. We therefore conducted a single-crystal neutron diffraction study on the product obtained by mixing AlMe3 and tBuLi in the presence of trimethylacetonitrile to evidence putative agostic interactions between the Li+ ion and the methyl hydrogen atoms. The neutron structure of 1 (Figure 1) reveals a polymeric form of the tBu(Me)Al(μ-Me)2Li·NC(tBu) monomer, where
Figure 1. Crystal structure of 1 from neutron diffraction showing two bridging methyl groups that stabilize the Al(μ-Me)2Li moiety and the Li+ coordination sphere including three C−H···Li agostic interactions. Top inset: conformational overlay between X-ray and neutron diffraction data sets, revealing the inaccuracies in the H position placement via X-ray diffraction. Bottom inset: pseudomirror present in 1 (ADPs drawn at the 50% probability level).
Table 1. C−H···Li Geometry for 1a
the bridging action of two Al-bonded Me groups stabilizes the Li+ center. The ease with which tBuLi can attack AlMe3 to give a pseudotetrahedral aluminate moiety was demonstrated in the reaction via the observation of unreacted tBuCN, which solvated the alkali metal to help afford 1.
C−H···Li
C−H
H···Li
C···Li
C−H···Li
C1−H1B···Li1 C1−H1C···Li1 C2−H2B···Li1 C2−H2C···Li1 C7−H7A···Li1 C7−H7B···Li1 C7−H7C···Li1
1.04(1) 1.09(1) 1.11(1) 1.16(1) 1.10(1) 1.086(6) 1.12(1)
2.47(2) 2.26(2) 2.17(2) 2.51(2) 2.12(1) 2.479(9) 2.14(2)
2.32(2) 2.32(2) 2.32(2) 2.32(2) 2.344(7) 2.344(7) 2.344(7)
69.5(8) 79.6(8) 83.3(8) 67.2(7) 87.6(7) 70.1(6) 86.0(7)
a
Distances are given in Å and angles in deg. The unambiguous agostic interactions shown in Figure 1 are given in bold.
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or the H···Li distance, in contrast to the findings of a study investigating a large group of similar compounds.56 The coordination sphere around the lithium ion in 1 is in many ways similar to that observed in the structure of LiBMe4 (2), a related structure for which neutron structure data are available.35 In 2, pincer formation similar to that seen in 1 is present (Scheme 1). However, in this case two bifurcated
contacts are indeed agostic interactions, although they are sterically hindered by neighboring hydrogen atoms. In summary, the isolation and full characterization of 1 has allowed us to enhance significantly our understanding of how agostic bonding stabilizes the alkali metal and influences the nature of the Al(μ-Me)2Li bridging in lithium aluminates. Interestingly, inter-hydrogen repulsion can now be invoked to rationalize hitherto inexplicable features in the corresponding X-ray diffraction structure. We are now investigating a similarly detailed study of alkyl(amido)aluminate structures in order to understand better the bonding in the AlCNLi metallacycle, and this will subsequently be expanded to other synergic reagents.
Scheme 1. Comparison of the Coordination Spheres of 1 and 2a
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a
EXPERIMENTAL SECTION
General Procedures. Reactions and manipulations were carried out under an inert atmosphere of dry nitrogen, using standard doublemanifold and glovebox techniques. Toluene was distilled off sodium immediately prior to use. Trimethylacetonitrile, trimethylaluminum, and tert-butyllithium (tBuLi) were purchased from Sigma-Aldrich; trimethylacetonitrile was stored over molecular sieves (4 Å). Purchased reagents were used without further purification. NMR data were collected on a Bruker Avance III 400 Cryo (400.140 MHz for 1H, 100.615 MHz for 13C) or Bruker Avance 500 BB ATM (500.20 MHz for 1H, 125.775 MHz for 13C, 194.397 MHz for 7Li, 130.336 MHz for 27Al) FT NMR spectrometer. Spectra were obtained at 298 K, and chemical shifts are internally referenced to C6D6 and calculated relative to TMS (1H, 13C), LiCl (7Li), and AlCl3·6H2O (27Al). Chemical shifts are expressed in δ (ppm). The following abbreviations are used for NMR spectra: s, singlet; br, broad. NMR solvents were stored over a freshly prepared sodium mirror. Synthesis. To a solution of trimethylacetonitrile (0.165 mL, 0.124 g, 1.5 mmol) in toluene (1 mL) was added dropwise AlMe3 (0.75 mL, 2.0 M in toluene, 1.5 mmol) under N2(g) at room temperature. The solution was heated to reflux for 2 min and cooled to room temperature and then to −78 °C, whereupon tBuLi (0.88 mL, 1.7 M in pentane, 1.5 mmol) was added dropwise under N2(g). The solution was warmed to room temperature, and the resulting white suspension was heated to reflux for 5 min and then cooled to room temperature, giving a pale straw-colored solution. Storage at room temperature for 24 h yielded colorless blocks of 1. Yield: 0.28 g (85%). Mp: 99−101 °C. Anal. Found: C, 65.04; H, 12.27; Li, 3.01; N, 6.39. Calcd for C12H27AlLiN: C, 65.73; H, 12.41; Li, 3.16; N, 6.39. 1H NMR spectroscopy (500 MHz, C6D6): δ 1.41 (s, 9H; tBuCN), 0.64 (s, 9H; tBuAlMe3), −0.51 (s, 9H; tBuAlMe3). 13C NMR (100 MHz, C6D6): δ 126.9 ((Me)3CCN), 31.1 ((Me)3CCN)), 27.8 ((Me)3CCN)), 27.0 ((Me)3CAlMe3)), −10.3 (br, ((Me)3CAlMe3). 7Li NMR (194 MHz, C6D6): δ −1.07 (br, s). 27Al NMR (130 MHz, C6D6): δ 156 (br, s). Single-Crystal X-ray Diffraction. A 0.1 × 0.1 × 0.2 mm3 crystal of 1, mounted in perfluoropolyether oil, was placed onto a Nonius Kappa CCD diffractometer, equipped with graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation and employing an Oxford Cryosystems Cryostream open-flow nitrogen cooling device. Data were acquired at 180(2) K. Unit cell parameters were refined against data from all regions of reciprocal space using HKLScalepack.57 Data reduction, including Lorentz and polarization corrections, employed HKLDenzo and Scalepack,57 while SORTAV58 was used to account for absorption effects. The structure was solved with direct methods within SHELXS-97 and refined by full-matrix least-squares methods on F2 using SHELXL-97.59 Positional and anisotropic displacement parameters were refined for all non-hydrogen atoms. All hydrogens were modeled as riding atoms on their adjoining methyl carbons, with Uiso(H) = 150% Uiso(C). Full crystallographic information is provided in the Supporting Information. Single-Crystal Neutron Diffraction. A trapezoid-shaped crystal of 1 (∼3.5 × 1.0 × 1.0 mm3), wrapped in Al foil smeared with vacuum grease, was mounted onto the D19 diffractometer at the Institut LaueLangevin, Grenoble, France, via a vanadium rod glued to a wide Al base. The sample was cooled to 20 K using a closed-cycle He
Agostic interactions are denoted by dashed lines.
agostic interactions are observed, as opposed to the single H··· Li contacts observed in 1. Two trifurcated agostic contacts which complete the tetrahedron about the lithium ion are also observed in 2. In the structure of 1, one of these trifurcated contacts is replaced by the bifurcated agostic interaction and the other by the trimethylacetonitrile molecule. It is worth noting that the linear relationship between H···Li and C−H···Li parameters noted for 1 is absent from the structure of 2 and the C···Li distances in 2 are shorter for the contacts in the pincer formation and longer for the trifurcated interactions than the analogous distance in 1. When the structures of 1 and 2 are compared further, the different environments regarding the formation of agostic bonds can be attributed to crystal-packing features. The repulsive H···H interactions observed in 1 are not present in 2, and hence bifurcated contacts are possible for the methyl groups involved in pincer formation. The near 180° B−C···Li angles in 2 allow for the trifurcated agostic interactions and at the same time prevent the close approach of the associated hydrogen atoms involved to those hydrogen atoms involved in the bifurcated interactions. In the structure of 1, the Al1−C7··· Li1 angle is smaller and only allows for a bifurcated polymerizing interaction. The added steric bulk of the tBuCN molecule is most likely the cause of these differences. Given that agostic interactions are largely electrostatic in nature,56 and given the similarities observed in the structures of 1 and 2, the argument could be made that the longer H···Li distances correspond to sterically hindered agostic interactions (Table 1; unhighlighted values). At first glance the directionality of the interactions, where one hydrogen atom forms a much closer contact than another on the same parent carbon, points to a greater degree of orbital overlap in these interactions. However, at such a low temperature (the structure of 1 is measured at 20 K), the overlap would be less effective and the electrostatic contribution would dominate. This should be manifested as more consistent H···Li distances, similar to bifurcated hydrogen bonds, which are also primarily electrostatic. Though this is not the case in the structure of 1, the linear relationship observed, between the C−H···Li angle and the H···Li distance, convinces us that the H···H contacts produce the discrepancy in H···Li distance and that the longer 3921
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(7) Iida, T.; Yamamoto, N.; Sasai, H.; Shibasaki, M. J. Am. Chem. Soc. 1997, 119, 4783−4784. (8) Saito, S. In Main Group Metals in Organic Synthesis; Yamamoto, H., Oshima, K., Eds.; Wiley-VCH: Weinheim, Germany, 2004. (9) Taylor, M. S.; Zalatan, D. N.; Lerchner, A. M.; Jacobsen, E. N. J. Am. Chem. Soc. 2005, 127, 1313−1317. (10) Wieland, L. C.; Deng, H.; Snapper, M. L.; Hoveyda, A. H. J. Am. Chem. Soc. 2005, 127, 15453−15456. (11) Novak, T.; Tan, Z.; Liang, B.; Negishi, E.-I. J. Am. Chem. Soc. 2005, 127, 2838−2839. (12) Liang, B.; Novak, T.; Tan, Z.; Negishi, E.-I. J. Am. Chem. Soc. 2006, 128, 2770−2771. (13) Ishikawa, T.; Ogawa, A.; Hirao, T. 1998, 7863, 5124−5125. (14) Wu, K.-H.; Gau, H.-M. J. Am. Chem. Soc. 2006, 128, 14808− 14809. (15) Chen, C.-A.; Wu, K.-H.; Gau, H.-M. Angew. Chem., Int. Ed. 2007, 46, 5373−5376. (16) Uchiyama, M.; Naka, H.; Matsumoto, Y.; Ohwada, T. J. Am. Chem. Soc. 2004, 126, 10526−10527. (17) Naka, H.; Uchiyama, M.; Matsumoto, Y.; Wheatley, A. E. H.; McPartlin, M.; Morey, J. V.; Kondo, Y. J. Am. Chem. Soc. 2007, 129, 1921−1930. (18) Snieckus, V. Chem. Rev. 1990, 90, 879−933. (19) Gschwend, H. W.; Rodriguez, H. R. Org. React. 1979, 26, 1− 360. (20) Conway, B.; Hevia, E.; García-Alvarez, J.; Graham, D. V.; Kennedy, A. R.; Mulvey, R. E. Chem. Commun. 2007, 5241−5243. (21) Conway, B.; Crosbie, E.; Kennedy, A. R.; Mulvey, R. E.; Robertson, S. D. Chem. Commun. 2012, 4674−4676. (22) Clayden, J. Organolithiums: Selectivity for Synthesis; Pergamon: Oxford, U.K., 2002. (23) Naka, H.; Morey, J. V.; Haywood, J.; Eisler, D. J.; McPartlin, M.; García, F.; Kudo, H.; Kondo, Y.; Uchiyama, M.; Wheatley, A. E. H. J. Am. Chem. Soc. 2008, 130, 16193−16200. (24) Mulvey, R. E.; Mongin, F.; Uchiyama, M.; Kondo, Y. Angew. Chem., Int. Ed. 2007, 46, 3802−3824. (25) Mulvey, R. E. Acc. Chem. Res. 2009, 42, 743−755. (26) Davies, R. P. Coord. Chem. Rev. 2011, 255, 1226−1251. (27) Mongin, F.; Harrison-Marchand, A. Chem. Rev. 2013, 113, 7563−7727. (28) Armstrong, D. R.; Clegg, W.; Dale, S. H.; Graham, D. V.; Hevia, E.; Hogg, L. M.; Honeyman, G. W.; Kennedy, A. R.; Mulvey, R. E. Chem. Commun. 2007, 598−600. (29) Conway, B.; García-Á lvarez, P.; Kennedy, A. R.; Klett, J.; Mulvey, R. E.; Robertson, S. D. New J. Chem. 2010, 34, 1707. (30) Armstrong, D. R.; Garden, J. A.; Kennedy, A. R.; Leenhouts, S. M.; Mulvey, R. E.; O’Keefe, P.; O’Hara, C. T.; Steven, A. Chem. Eur. J. 2013, 19, 13492−13503. (31) Mulvey, R. E. Dalton Trans. 2013, 42, 6676−6693. (32) Cole, J. M.; Gibson, V. C.; Howard, J. A. K.; McIntyre, G. J.; Walker, G. L. P. Chem. Commun. 1998, 1829−1830. (33) Gerteis, R. L.; Dickerson, R. E.; Brown, T. L. Inorg. Chem. 1964, 3, 872−875. (34) Zerger, R.; Rhine, W.; Stucky, G. J. Am. Chem. Soc. 1974, 96, 6048−6055. (35) Rhine, W. E.; Stucky, G.; Peterson, S. W. J. Am. Chem. Soc. 1975, 97, 6401−6406. (36) Ilsley, W. H.; Albright, M. J.; Anderson, T. J.; Glick, M. D.; Oliver, J. P. Inorg. Chem. 1980, 19, 3577−3585. (37) Hay-Motherwell, R. S.; Wilkinson, G.; Hussain, B.; Hursthouse, M. B. Chem. Commun. 1989, 1436−1437. (38) Stucky, G. D.; Eddy, M. M.; Harrison, W. H. J. Am. Chem. Soc. 1990, 112, 2425−2427. (39) Kaufmann, E.; Raghavachari, K.; Reed, A. E.; Schleyer, P. v. R. Organometallics 1988, 7, 1597−1607. (40) Novoa, J. J.; Whangbo, M.-H.; Stucky, G. D. J. Org. Chem. 1991, 56, 3181−3183. (41) Uhl, W.; Layh, M.; Massa, W. Chem. Ber. 1991, 124, 1511− 1516.
refrigerator, where data collection took place for a period of 8 days. The scanning strategy was optimized as far as possible to collect the maximum amount of data in reciprocal space, focusing mainly on the minimum quadrant of data required for orthorhombic samples. In total, 2078 reflections were collected, of which 1686 were unique reflections. Data spanned a range of 0 ≤ h ≤ 13, 0 ≤ k ≤ 22, 0 ≤ l ≤ 13 with a maximum 2θ value of 132.44°. At this time, D19 was equipped with one tall, thin vertically curved detector and one square flat multidetector. Background correction for each reflection was made using Retreat,60 considering the local count distribution in three dimensions. Corrections for absorption in both the sample and the cylindrical refrigerator heat shields were applied. The resulting merged data (Friedel opposites not merged) were in good internal agreement: R(σ) = 0.0383. Subsequent full-matrix least-squares refinement was conducted with a final data to parameter ratio of 2077:364, all atomic positions being refined together with anisotropic atomic displacement parameters (ADPs) except for Al1, Li1, and C3, which were modeled with isotropic ADPs since their 3D surrounding of substituents physically constrains them toward an isotropic state; information is therefore little compromised by rendering them isotropic while optimizing the data to parameter ratio elsewhere. Moreover, the ADP of Al1 is so small at 20 K that an anisotropic fit was futile. This yielded the structure, as predicted, with statistical agreement, R1 = 0.0527 (Fo > 4σ(Fo)) and wR2 = 0.0561. The close interactions sought were observed between the bridging CH3 groups and the Li ion.
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ASSOCIATED CONTENT
S Supporting Information *
CIFs giving structural information for 1 and a figure giving a graph of C−H···Li angles versus the H···Li distances for 1. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for J.M.C.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.M.C. thanks the UNB for The Vice-Chancellor’s Research Chair, NSERC for a Discovery Grant (355708), The Bragg Institute, ANSTO for funding (for P.G.W.), and the Fulbright Commission for a UK-US Fulbright Award hosted by Argonne National Laboratory, where work done was supported by the DOE Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The Institut LaueLangevin is thanked for facility access to neutron beam time. Thanks go also to the EPSRC for their financial assistance (A.J.P., C.W.T., and D.J.L.).
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REFERENCES
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