J. Phys. Chem. B 1999, 103, 1821-1825
1821
Programming a Gold Nanocrystal to Recognize and Selectively Bind a Molecular Substrate in Solution Damian Aherne, S. Nagaraja Rao, and Donald Fitzmaurice* Department of Chemistry, UniVersity College Dublin, Belfield, Dublin 4, Ireland ReceiVed: August 6, 1998; In Final Form: NoVember 4, 1998
Gold nanocrystals stabilized by a chemisorbed mixture of a long-chain alkane thiol, namely dodecanethiol (95%), and a long-chain dodecanethiol incorporating a receptor, namely 12-mercaptododecyl-1-uracil (5%), have been prepared. In solution, these nanocrystals recognize and selectively bind a long-chain alkane incorporating a complementary substrate, namely, N,N′-2,6-pyridinediylbis[undecamide].
Introduction
SCHEME 1
The size dependence of the electronic and optical properties of metal and semiconductor nanocrystals have been studied in detail.1 Such studies have been facilitated by the preparation of size monodisperse colloids of essentially defect-free nanocrystals stabilized by chemisorbed long-chain alkanes.2-8 Fundamental insights into the evolution of bulk electronic and optical properties in metals and semiconductors have resulted.3,9 Increasingly, however, it is the collective electronic and optical properties of metal and semiconductor nanocrystals that are being studied.3,10,11 Such studies are being facilitated by the preparation of organized nanocrystal assemblies.1,5,10-14 Of particular interest is how the size dependence of the electronic and optical properties of the constituent nanocrystals may be exploited to tune their collective properties. Generally, nanocrystal assemblies are prepared at an airwater interface using Langmuir-Blodgett techniques (twodimensional)12 or on a suitable substrate by controlled solvent evaporation (three-dimensional).10 Both approaches, however, are limited by the fact that only relatively simple nanocrystal architectures may be realized. For this reason, approaches that may permit the assembly of complex nanocrystal architectures are of particular interest. One possible approach is to adsorb stabilizer molecules incorporating a receptor at the surface of a nanocrystal. It is expected that this nanocrystal will recognize and selectively bind another at which are adsorbed stabilizer molecules incorporating a complementary substrate. By this means, the assembly of complex nanocrystal architectures in solution may be programmed.15 To date, TiO2 nanocrystals stabilized by physisorbed longchain alkanes incorporating a uracil moiety (receptor) have been programmed to recognize and selectively bind in solution TiO2 nanocrystals stabilized by physisorbed long-chain alkanes incorporating an diaminopyridine moiety (substrate).16 Ordering of the constituent nanocrystals is apparent in the mesoaggregates formed as a result (Scheme 1). While encouraging, the specific approach outlined above has a number of limitations. Among these is the fact that the stabilizer molecules are not covalently adsorbed and cannot be assumed to remain at the surface of the nanocrystal at which they are initially adsorbed. To address this limitation, a * To whom correspondence should be addressed.
methodology has been developed that permits controlled chemisorption of a stabilizer, incorporating the desired receptor or substrate, at the surface of the nanocrystals of a stable dispersion. This methodology has been used to prepare gold nanocrystals stabilized by a chemisorbed mixture of a long-chain alkane thiol, namely dodecanethiol (95%), and a long-chain dodecanethiol incorporating a receptor, namely 12-mercaptododecyl-1-uracil (5%). It has been established, as reported below, that these nanocrystals recognize and selectively bind in solution a longchain alkane incorporating a complementary substrate (Scheme 2), namely, N,N′-2,6-pyridinediylbis[undecamide]. Some im-
10.1021/jp9832950 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/02/1999
1822 J. Phys. Chem. B, Vol. 103, No. 11, 1999 SCHEME 2
SCHEME 3. Reagents and conditions: (i) thiourea (1.3 equiv), H2O, reflux (6 h); (ii) NaOH (2 equiv), H2O, reflux (3 h); (iii) HBr/H2SO4, reflux (7 h); (iv) uracil (10 equiv), K2CO3/DMSO, reflux (24 h at 80°C; (v) tributylphosphine (2 equiv), MeOH, reflux (3 h).
plications of these findings for the assembly of complex nanocrystal architectures are considered. Experimental Section Preparation and Characterization of Molecules. Dodecanethiol (I) was used as supplied by Aldrich. 12-mercaptododecyl-1-uracil (II) was prepared as shown in Scheme 3. N,N′-2,6-Pyridinediylbis[undecamide] (III) was prepared as described in detail elsewhere.17 Characterization of these compounds by elemental analysis and 1H NMR is summarized below. Calculated for I (C12H26S): C, 71.21; H, 12.95; S, 15.84. Found: C, 71.19; H, 13.12; S, 15.55. 1H NMR (chloroform-d): ∂ 0.88 (t, 3H, J ) 7.0 Hz); ∂ 1.26-1.36 (m, 16H); ∂ 1.40 (t, 2H, J ) 7.7 Hz); ∂ 1.58 (m, 2H); ∂ 2.52 (q, 2H, J ) 7.3 Hz). Calculated for II (C16H28N2O2S): C, 61.50; H, 9.03; N, 8.97; S, 10.26. Found: C, 61.28; H, 8.93; N, 8.81; S, 9.61. 1H NMR (chloroform-d): ∂ 1.25-1.45 (m, 18H); ∂ 1.37 (t, 1H, J ) 7.7 Hz); ∂ 1.64-1.73 (m, 2H); ∂ 2.56 (q, 2H, J ) 7.3 Hz); ∂ 3.75 (t, 2H, J ) 7.3 Hz); ∂ 5.72 (dd, 1H, J ) 2.3, 7.9 Hz); ∂ 7.18 (d, 1H, J ) 7.9 Hz); ∂ 8.20 (bs, 1H, -NH imidic). Calculated for III (C29H51N3O2): C, 73.53; H, 10.85; N, 8.87. Found: C, 73.60; H, 10.64; N, 8.91. 1H NMR (chloroform-d) ∂ 0.92 (t, 6H, J ) 7.0 Hz); ∂ 1.20-1.82 (m, 36H); ∂ 2.40 (t, 4H, J ) 7.5 Hz); ∂ 7.56 (s, 2H, -NH amidic); ∂ 7.73 (t, 1H, J ) 8.0 Hz); ∂ 7.93 (d, 2H, J ) 7.9 Hz).
Aherne et al. Preparation and Characterization of Nanocrystals. Dodecanethiol-stabilized gold nanocrystals were prepared following the method reported by Brust et al.18 Briefly, the gold precursor (HAuCl4, 0.58 g) in deionized water (57 mL) was added to a phase-transfer catalyst ([CH3(CH2)7]4NBr, 1.04 g) in chloroform (45 mL) and the resulting mixture stirred vigorously for 10 min. The organic layer was recovered, the stabilizer (I, 0.09 g) in chloroform (15 mL) followed by the reducing agent (NaBH4, 0.45 g) in water (51 mL) were added, and the resulting mixture was stirred overnight at room temp. The nanocrystals were precipitated by concentrating the reaction mixture (to 15 mL) and adding it to ethanol (400 mL). The precipitated nanocrystals were isolated by centrifugation (10 000 rpm) and filtration (0.2 µm, Schleicher and Schuell). The isolated nanocrystals were then redissolved in chloroform (10 mL) and reprecipitated by addition of acetone (200 mL). Gold nanocrystals stabilized by chemisorbed I, denoted Au-I, were recovered by centrifugation and filtration and dried in air. The modified stabilizer II was chemisorbed at the surface of Au-I as follows. Au-I (0.100 g) was added to a solution of I (0.090 g) and II (0.015 g) in chloroform (50 mL) and refluxed for 24 h. The nanocrystals were precipitated by concentrating the reaction mixture (to 5 mL) and adding it to acetone (200 mL). The precipitated nanocrystals were recovered by centrifugation and filtration before, once again, being redispersed in chloroform (5 mL) and precipitated by adding to acetone (200 mL). Gold nanocrystals stabilized by a chemisorbed mixture of I and II, denoted Au-(I+II), were recovered by centrifugation and dried in air. Characterization Techniques. Transmission electron micrographs (TEMs) were obtained using a JEOL TEMSCAN 2000 for samples prepared by evaporation on a carbon-coated copper grid. 1H NMR spectra were recorded using either a JEOL JNM-GX270 FT or Varian 500 FT spectrometer at 25 °C. FTIR spectra were recorded using a Mattson Galaxy 3000 FT spectrometer (CaF2 windows, 0.20 mm path length) at 25 °C. 1H NMR and FT-IR studies on I, II, and III employed 8 × 10-3 mol dm-3 chloroform-d solutions. Similar studies on equimolar mixtures of II and III employed 4 × 10-3 mol dm-3 (each component) chloroform-d solutions. 1H NMR and FT-IR studies on Au-(I+II) employed 6 × 10-4 mol dm-3 (nanocrystal concentration) chloroform-d solutions corresponding to 8 × 10-2 mol dm-3 and 4 × 10-3 mol dm-3 solutions of I and II, respectively. Similar studies on equimolar mixtures of Au-(I+II) and III employed the above Au-(I+II) solution to which was added the quantity of III required to obtain a 4 × 10-3 mol dm-3 solution. Results and Discussion Nanocrystal Modification. TEM establishes that the average diameter of a gold nanocrystal in Au-I is 32 ( 8 Å and that the polydispersity is 1.08 (Figure 1a).19 Elemental analysis establishes the weight percent of gold and organic in the dry nanocrystal powder and permits an accurate determination of the average number of stabilizer molecules adsorbed at the surface of each nanocrystal.20 It is possible, therefore, to calculate that there are 163 stabilizer molecules adsorbed at the surface of each nanocrystal and that the average surface area occupied by each is 19 Å2. This value, while smaller than that for a monolayer of I self-assembled at a planar gold substrate (typically 21 Å2),21 agrees well with previously reported values for alkane thiols adsorbed at the surface of a gold nanocrystal.22 This finding is accounted for by the extreme curvature of the nanocrystal substrate.
Programming a Gold Nanocrystal
Figure 1. TEM of (a) Au-I and (b) Au-(I+II).
Figure 2. (a) 1H NMR and (b) FT-IR spectra of I, Au-I and Au(I+II) in chloroform-d.
A comparison of the 1H NMR spectra of I in solution and adsorbed at the surface of a gold nanocrystal in Au-I reveals that the resonances assigned to the R, β, and γ methylene protons of the latter are significantly broadened (Figure 2a). In fact, the resonance assigned to the R methylene protons is broadened to the extent that it is no longer visible. The resonances assigned
J. Phys. Chem. B, Vol. 103, No. 11, 1999 1823 to the C4-C12 methylene protons are largely unchanged. The above broadening is a result of a discontinuity in the diamagnetic susceptibility at the gold-hydrocarbon interface and residual dipolar interactions due to spatial constraints in the adsorbed layer.22 On this basis, it is concluded that all I are adsorbed at a nanocrystal surface. It is also concluded that while the motions of I are constrained close to the nanocrystal substrate the chemisorbed stabilizers extend into the solvent where they move freely. It is these molecular motions that provide the steric stabilization necessary to prevent aggregation. The FT-IR spectra of I in solution and adsorbed at the surface of a gold nanocrystal in Au-I have also been compared (Figure 2b). For I, the asymmetric and symmetric methylene stretches are at 2928 and 2855 cm-1, respectively, while the asymmetric (in-plane) and symmetric (Fermi resonance) stretches of the terminal methyl group are at 2954 and 2871 cm-1, respectively.23,24 For I in Au-I, the asymmetric and symmetric methylene stretches are at 2926 and 2854 cm-1, respectively. It should be noted that these shifts to lower frequencies are relatively small compared to those measured for self-assembled monolayers of alkane thiols adsorbed at a planar gold substrate.24 On this basis and consistent with the 1H NMR spectra in Figure 2a, it is again concluded that the motions of I are constrained close to the nanocrystal substrate but that the chemisorbed stabilizers extend into the solvent where they move freely. A dispersion of Au-I was refluxed in the presence of excess I and II (∼9:1 ratio) for 24 h. It was expected that this would promote exchange of I adsorbed at the surface of Au-I with I and II in solution and that the composition of the alkane thiols adsorbed at the nanocrystal surface would increasingly reflect that in solution. This expectation, as it transpired, was well founded, and gold Au-(I+II) nanocrystals stabilized by a mixture adsorbed I and II (95% I and 5% II) were obtained. It should be noted that while the above gold nanocrystal dispersion remained stable throughout, this is not the case for Au-I refluxed in the absence of any added thiol. These findings illustrate a general approach to the controlled adsorption of stabilizers incorporating a recognition group at the surface of a nanocrystal. TEM establishes that the average diameter of a gold nanocrystal in Au-(I+II) is 29 ( 5 Å and that the polydispersity is 1.06 (Figure 1b).19 As a consequence of Au-(I+II) being precipitated from solution, these nanocrystals possess a smaller average radius and are more size monodisperse (Figure 1b). It is noted, however, that not all these nanocrystals are spherical. Elemental analysis establishes that there are an average of 151 molecules at the surface of each nanocrystal, comprised of 144 molecules of I (95%) and 7 molecules of II (5%).20 It is calculated that the surface area occupied by each thiol is 17 Å2. It appears (see below), therefore, that refluxing and subsequently cooling Au-I in a solution of I and II to form Au-(I+II) results in an increase in the packing density of the long-chain alkane thiols adsorbed at the surface of a nanocrystal. A comparison of the 1H NMR spectra of I and II (not shown) and Au-(I+II), reveals that the resonances assigned to the R, β, and γ methylene protons of both I and II are broadened to a greater extent in Au-(I+II) (Figure 2a). As was the case for Au-I, the resonance assigned to the R methylene protons is broadened to an extent that it is no longer visible. The resonances assigned to the C4-C12 methylene protons and the uracil moiety are largely unchanged. On this basis and consistent with the increase in packing density noted above, it is concluded that the molecular motions of I and II are more constrained in Au-(I+II) than in Au-I. It is also concluded that while the
1824 J. Phys. Chem. B, Vol. 103, No. 11, 1999
Aherne et al.
Figure 3. (a) 1H NMR and (b) FT-IR spectra of II, III, and a 1:1 mixture of II and III in chloroform-d.
Figure 4. (a) 1H NMR and (b) FT-IR spectra of Au-(I+II), III, and a 1:1 mixture of Au-(I+II) and III in chloroform-d.
motions of I and II are constrained close to the nanocrystal substrate, the chemisorbed stabilizers extend into the solvent where they move freely. The FT-IR spectra of I and II (not shown) and of a dispersion of Au-(I+II) have also been compared (Figure 2b). The bands at 2924 and 2852 cm-1 in Au-(I+II) are assigned to the asymmetric and symmetric methylene stretches, respectively. While these shifts to lower frequencies are larger than those observed for Au-I, they are still small compared to those measured for a monolayer of alkane thiol self-assembled on a planar gold substrate.24 On this basis and consistent with the increase in packing density noted above and the 1H NMR spectra in Figure 2a, it is concluded that the molecular motions of I and II are more constrained in Au-(I+II) than in Au-I, but that the chemisorbed stabilizers extend into the solvent where they move freely. Having prepared and characterized Au-(I+II), it is clear that it is possible to prepare relatively size monodisperse Au nanocrystals stabilized by alkane thiol molecules, a predetermined fraction of which incorporate a receptor, namely, uracil, and that Scheme 2 is essentially an accurate representation of such nanocrystals. Nanocrystal-Molecule Recognition. 1H NMR and FT-IR spectroscopies were used to characterize the receptor-substrate complexes formed by equimolar mixtures of II and III and of Au-(I+II) and III in solution. The 1H NMR spectra of II, III, and an equimolar mixture of II and III, denoted (II+III), have been measured (Figure 3a). The modified dodecanethiol II incorporates a uracil moiety, and a one-proton resonance characteristic of an imidic proton is observed at ∂ 8.20.15-17 The modified alkane III incorporates a diaminopyridine moiety, and a two-proton resonance charac-
teristic of an amidic proton is observed at ∂ 7.56.15-17 It was expected that an equimolar mixture of these molecules would form a 1:1 complex in solution associated by a triple array of hydrogen bonds (Scheme 2).15-17,25 Consistent with this expectation, the imidic and amidic proton resonances of II and III are shifted downfield to ∂ 9.82 and ∂ 8.55, respectively, in (II+III). The FT-IR spectra of II, III, and (II+III) have also been measured (Figure 3b). The band at 3391 cm-1 is assigned to the imidic N-H stretch of the uracil moiety in II.16,17,23,26,27 The band at 3423 cm-1 is assigned to the amidic N-H stretch of the diaminopyridine moiety in III.16,17,23,26,27 As established above, II and III form a triply hydrogen bonded 1:1 complex in solution. Accordingly, upon complexation, the bands assigned to the imidic and amidic N-H stretching modes of II and III, respectively, are reduced in intensity,16,17,23,26-28 while a series of bands newly observed at lower frequencies (3278 and 3212 cm-1) are assigned to the N-H stretches of the hydrogen bonded protons in (II+III).16,17,23,26-28 Having established that II recognizes and selectively binds III in solution to form the 1:1 receptor-substrate complex (II+III), it was expected that Au-(I+II) would also recognize and selectively bind III. The 1H NMR spectra of Au-(I+II), III, and an equimolar mixture of Au-(I+II) and III have been measured (Figure 4a). The modified dodecanethiol stabilizer chemisorbed at Au(I+II) incorporates a uracil moiety, and a one-proton resonance characteristic of an imidic proton is observed at ∂ 8.00. The modified alkane III incorporates a diaminopyridine moiety and a two proton resonance characteristic of an amidic proton is observed at ∂ 7.56. It was expected that in solution Au-(I+II) and III would form a 1:1 complex associated by a triple array
Programming a Gold Nanocrystal of hydrogen bonds and denoted Au-(I+II)+III (Scheme 2).15-17,25 Consistent with this expectation, the imidic and amidic proton resonances of Au-(I+II) and III are shifted downfield to ∂ 9.99 and ∂ 8.04, respectively, in Au-(I+II)+III. The FT-IR spectra of Au-(I+II), III, and Au-(I+II)+III have also been measured (Figure 4b). The band observed at 3389 cm-1 is assigned the imidic N-H stretch of the uracil moiety of II adsorbed at Au-(I+II). The band observed at 3423 cm-1 is assigned to the N-H stretch of the amidic protons of the diaminopyridine moiety incorporated in the modified alkane III. As demonstrated above, Au-(I+II) and III form a triply hydrogen bonded 1:1 complex in solution (Scheme 2). Accordingly, in Au-(I+II)+III, the bands assigned to the imidic and amidic N-H stretching modes of Au-(I+II) and III, respectively, are reduced in intensity,16,17,23,26-28 while a series of bands newly observed at lower frequencies (3278 and 3211 cm-1) are assigned to the N-H stretches of the hydrogen bonded protons.16,17,23,26-28 Having prepared and characterized Au-(I+II)+III, it is clear that it is possible to prepare relatively size monodisperse Au nanocrystals stabilized by alkane thiol molecules, a predetermined fraction of which incorporate a receptor, namely, uracil, and that these nanocrystals recognize and selectively bind a molecular substrate incorporation and appropriate substrate, namely, diaminopyridine, as shown in Scheme 2. Conclusions A methodology has been developed that permits controlled chemisorption of a stabilizer at the surface of a nanocrystal. This methodology has been used to prepare a gold nanocrystal dispersion stabilized by a mixture of chemisorbed long-chain alkane thiol (95%) and dodecanethiol incorporating an uracil receptor (5%). Addition of a molecule incorporating a diaminopyridine substrate results in the formation of a 1:1 complex associated by a triple array of complementary hydrogen bonds. These findings have implications for the assembly of complex nanocrystal architectures in solution. Specifically, a nanocrystal has been programmed to recognize and selectively bind a molecular substrate in solution. If this molecular substrate is located at a particular site within a nanocrystal assembly, it would be predicted that an appropriately programmed nanocrystal will recognize and be selectively bound at that site. Acknowledgment. This work was supported by a grant from the Commission of the European Union under the Joule III program (Contract JOR3-CT96-0107). References and Notes (1) Alivisatos, A. P. Science 1996, 271, 933 and references therein. (2) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (3) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science
J. Phys. Chem. B, Vol. 103, No. 11, 1999 1825 1996, 273, 1690. (4) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmueller, A.; Weller, H. J. Phys. Chem. 1994, 98, 7665. (5) Harfenist, S. A.; Wang, Z. L.; Alvarez, M. A.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. 1996, 100, 13904. (6) Herron, N.; Calabrese, J. C.; Farneth, W. E.; Wang, Y. Science 1993, 259, 1426. (7) Schmidt, G. Chem. ReV. 1992, 92, 1709. (8) Moritz, T.; Reiss, K.; Diesner, K.; Su, D.; Chemeseddine, A. J. Phys. Chem. B 1997, 101, 8052. (9) Brus, L. J. Phys. Chem. 1986, 90, 2555. (10) Murray, C. B.; Kagan, C. R.; Bawandi, M. G. Science 1995, 270, 1335. (11) Kagan, C. R.; Murray, C. B.; Bawendi, M. G. Phys. ReV. B 1996, 54, 8633. (12) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189. (13) Motte, L.; Billoudet, F.; Lacaze, E.; Douin, J.; Pileni, M. P. J. Phys. Chem. B. 1997, 101, 138. (14) Elghanain, R.; Storhoff, J.; Micic, R.; Letsinger, R.; Mirkin, C. Science 1997, 277, 1078. (15) (a) Cusack, L.; Rao, S. N.; Wenger, J.; Fitzmaurice, D. Chem. Mater. 1997, 9, 624. (b) Cusack, L.; Rao, S. N.; Fitzmaurice, D. Chem. Eur. J. 1997, 3, 202. (c) Cusack, L.; Marguerettaz, X.; Rao, S. N.; Wenger, J.; Fitzmaurice, D. Chem. Mater. 1997, 9, 1765. (16) Cusack, L.; Rizza, R.; Gorelov, A.; Fitzmaurice, D. Angew. Chem., Int. Ed. Eng. 1997, 36, 848. (17) Brienne, M.-J.; Gabard, J.; Lehn, J.-M. J. Chem. Soc., Chem. Commun. 1989, 1868. (18) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (19) The reported values for both Au-I and Au-II are based on a statistical analysis of the diameter of 1000 nanocrystal cores of each as determined from TEMs similar to those shown in Figure 1. (20) Elemental analysis established that the percentage organic (by weight) in Au-I and Au-II is (C, 11.90; H, 2.03; N, 0.00; S, 2.65) and (C, 13.10; H, 2.26; N, 0.21; S, 3.40), respectively. In the case of Au-II, the percentage N (by weight) was used to infer the ratio of I to II. (21) (a) Rovida, G.; Pratesi, F. Surf. Sci. 1981, 104, 609. (b) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389-9401. (c) Ulman, A. J. Mater. Educ. 1989, 11, 205. (22) (a) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox, R. B. Chem. Eur. J. 1996, 2, 359. (b) Connolly, S.; Fullam, S.; Korgel, B.; Fitzmaurice, D. J. Am. Chem. Soc. 1998, 120, 2969. (23) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Matheun: London, 1954. (24) Porter, M.; Bright, T.; Allara, D.; Chidsey, C. J. Am. Chem. Soc. 1987, 109, 3568. (25) (a) Feibush, B.; Fiueroa, A.; Charles, R.; Onan, K.; Feibush, P.; Karger, B. J. Am. Chem. Soc. 1986, 108, 3310. (b) Feibush, B.; Saha, M.; Onan, K.; Karger, B.; Giese, R. J. Am. Chem. Soc. 1987, 109, 7531. (c) Hamilton, A.; Van Engen, D. J. Am. Chem. Soc. 1987, 109, 5035. (d) Bisson, A.; Carver, F.; Hunter, C.; Waltho, J. J. Am. Chem. Soc. 1994, 116, 10292. (26) Pimentel, G.; McClellan, A. The Hydrogen Bond; Freeman: San Francisco, 1956. (27) (a) Tsuboi, M. Appl. Spectrosc. ReV. 1969, 3, 45. (b) Susi, H.; Ard, J. Spectrochim. Acta 1971, 27A, 1549. (28) (a) Hamlin, R.; Lord, R.; Rich, A. Science 1965, 148, 1734. (b) Kyogoku, Y.; Lord, R.; Rich, A. Proc. Natl. Acad. Sci., U.S.A. 1967, 57, 250. (c) Kyogoku, Y.; Lord, R.; Rich, A. J. Am. Chem. Soc. 1967, 89, 496. (d) Kyogoku, Y.; Lord, R.; Rich, A. Biochim. Biophys. Acta 1969, 179, 10. (e) Zundel, G.; Lubos, W.; Kolkenbeck, K. Can. J. Chem. 1971, 49, 3795.
