Article pubs.acs.org/JPCA
C vs N: Which End of the Cyanide Anion Is a Better Hydrogen Bond Acceptor? Raghunath O. Ramabhadran, Yuran Hua, Amar H. Flood, and Krishnan Raghavachari* Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States S Supporting Information *
ABSTRACT: The ability of the C and N ends of the cyanide anion (CN−) as acceptors of hydrogen bonds, an experimentally difficult problem, has been computationally examined in this study. Structures obtained in our previous work involving cyanide binding within the cavity of a triazolophane macrocycle (Chem.Eur. J. 2011, 17, 9123−9129) were used to analyze the problem. Three different approaches involving (a) breakdown of the triazolophane into smaller components, (b) population analyses, and (c) ion−dipole analyses helped demonstrate that the N terminus of cyanide is a slightly better hydrogen bond acceptor than the C terminus even though it is not the site of protonation or covalent bond formation. This outcome reflects a competition between the preference for noncovalent interactions at the nitrogen and covalent bond formation at the carbon.
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INTRODUCTION Cyanide is a highly toxic anion that inhibits the enzyme cytochrome c oxidase,1 located at the end of the respiratory chain. Cyanide is also a byproduct of the chemical industry,2 potentially leading to the risk of acute and chronic environmental exposure.3 Consequently, different strategies have been investigated to sense cyanide by exploiting its well-known chemical properties. For example, cyanide’s basicity has been utilized in its carbon-centered complexation4,5,7 with boronyl4 and other Lewis acidic metal centers,5 and its nucleophilicity has been employed in reversible C−C covalent bond formation.6,7 In contrast, the binding of the cyanide anion, CN−, to molecular receptors that take advantage of hydrogen bonding has been largely overlooked, perhaps arising from cyanide’s tendency to favor full hydrogen transfer, i.e., deprotonation of an acidic hydrogen in the receptor.8 The few known examples of receptor binding9,10 only considered cyanide as one of a series of anions, rather than focusing uniquely on its binding preferences. Even in the instances when cyanide was tightly bound to the receptor9b,d (rather than deprotonating it),9h,11 the details of binding were never investigated. This scenario prompted us to investigate exactly how cyanide binds to hydrogen-bonding receptors wherein we employed a combined theoretical and experimental study12 that took advantage of rigidly preorganized macrocyclic triazolophanes (Figure 1). These receptors direct non-traditional hydrogenbonding CH donors from 1,2,3-triazoles into the well-defined shape and size (r ≈ 1.9 Å) of the triazolophane’s cavity.13 CH donors14 also resist deprotonation13f on account of their low basicity. We investigated12 the different modes of cyanide binding (Figure 2) and noticed that, in contrast to the dominance of carbon’s basicity, it is instead the N terminus of © XXXX American Chemical Society
Figure 1. Macrocyclic triazolophane receptors, which employ CH hydrogen bonds to bind cyanide. R = H, in our computationally expedient model, and R is either an alkoxy-linked, methyl-ether terminated triethylene glycol (OTg) group or a tert-butyl group in the experimental studies (ref 12).
cyanide that formed shorter hydrogen bonds. Consequently, we were intrigued by an interesting question: Which end of the cyanide anion is a better hydrogen bond acceptor? A priori, based on the Lewis structure of cyanide (Figure 3), in which the formal negative charge resides on the C end, it may appear easy and intuitive to predict that the C end is the stronger hydrogen-bond receptor. However, cyanide has an almost equal split of its negative charge (Figure 3); while our calculations based on the electrostatic potential (ESP) give a slightly greater portion of the charge (−0.54) on the carbon Special Issue: Kenneth D. Jordan Festschrift Received: December 31, 2013 Revised: March 17, 2014
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Figure 2. CN− binding modes from ref 12. On top are the possible in-plane modes: (a) binding along the north−south axis (minimum), NS·CN−, (b) binding along the east−west axis, EW·CN− (transition state for the precessional interconversion between two degenerate NS modes), and (c) binding along the diagonal, D·CN− (not a stationary point). At the bottom is the out-of plane, perpendicular mode (second order saddle point), P· CN−. In all of these complexes, the N end of CN− predominantly forms shorter hydrogen bonds. Other views of these structures can be found in the Supporting Information of ref 12.
cyanide when acting as hydrogen bond acceptors and seek to better understand the hydrogen bonding properties of cyanide. Herein, we use three different approaches: (a) breakdown of the triazolophane into smaller components, (b) population analyses, and (c) ion−dipole analyses to demonstrate the marginal superiority of the N terminus of cyanide in being a CH hydrogen bond acceptor, over the C terminus. The ion− dipole analysis is also instructive in illuminating the binding differences noted between the perpendicular mode of binding and the north−south mode of binding (Figure 2) observed by us previously.
Figure 3. Cyanide’s Lewis structure, a space-filling model, calculated ESP charges (B3LYP/6-31+G(d,p)), and a portrayal of the carbon lone-pair, indicating that the C and N ends of the diatomic anion are not significantly dissimilar.
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end (B3LYP/6-31+G(d,p)), the calculated distribution depends on the level of theory employed,15 and it does not deviate much from 50:50. Additionally, Larsen et al.’s and Mautner et al.’s gas-phase studies16 on hydrogen bonding with cyanide revealed that this diatomic anion behaves like the spherical chloride anion. This behavior was attributed to a counterbalance between carbon’s greater basicity (more able to donate electrons) and nitrogen’s greater electronegativity (has more electron density) along with rapid rotations in the gas phase to average out any orientations, i.e., making it pseudospherical. It is in fact this pseudospherical nature of cyanide that makes it hard to experimentally determine the hydrogen bonding abilities of the two termini in cyanide. Finally, the experimental study of Kuhn et al.17 using the imidazolium cation to form C+−H···(cyanide) hydrogen bonds, containing examples of collinear and bifurcated hydrogen bonds (Figure S1, Supporting Information), and our own observation of shorter hydrogen bonds with the N terminus (vide supra, and ref 12) appeared to favor the nitrogen end. Addressing this interesting issue in this study, we compare and contrast the abilities of the C and N termini of
COMPUTATIONAL DETAILS Density functional theory (DFT) calculations in the gas phase have been carried out using the Gaussian 09 suite of programs.18 On the basis of our previous successes,12,13e all the geometries have been optimized using the standard B3LYP density functional19 with the 6-31+G(d,p) basis set. Vibrational frequencies, zero-point corrections, and thermal corrections have also been evaluated using the 6-31+G(d,p) basis set. Single-point energy calculations were then carried out using the significantly larger 6-311++G(3df,2p) basis set to obtain the anion binding energies. The binding energies were corrected for basis set superposition errors (BSSE) using the standard counterpoise method.20 Additionally, Grimme’s dispersion corrections21 with (a) the Becke-Johnson (BJ) damping function and (b) no damping functions are added as singlepoint energies to the computed electronic energies. The addition of dispersion corrections does not quantitatively affect the results presented in this article; hence, they are mentioned in the Supporting Information. B
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RESULTS AND DISCUSSIONS (a). Triad Approach. The triazolophane macrocycle (Figure 1) consists of three different types of moieties: The triazoles, the N-phenylenes along the north−south axis that are connected to the triazoles via nitrogens, and the C-phenylenes along the east−west axis that are connected to the triazoles via carbons. Following the work of Bandyopadhyay et al.,13e it is instructive to break the triazolophane macrocycle into three triads: the N-triad (containing the N-phenylene moiety flanked by two triazoles), the C-triad (containing the C-phenylene moiety flanked by two triazoles, connected via the carbons), and the diagonal triad (containing the triazole flanked by two phenylenes, connected via a carbon and a nitrogen, respectively); and then independently study cyanide binding and isocyanide binding, to compare binding via the C terminus with binding via the N terminus, respectively. Figure 4 contains the different triads.
geometry differences between the anion-bound triads, and the anion-bound macrocycle also play a role.13e Overall, the triad approach indicates that the N terminus of cyanide is slightly better than the C terminus in terms of acting as a hydrogen bond acceptor. (b). Population Analyses and Redistribution of Charges. The crystal structures reported by Kuhn et al.17 as well as our theoretical calculations (vide supra) seem to suggest that the N terminus of cyanide is a slightly stronger hydrogen bond acceptor than the C terminus. To further support this claim, a population analysis was conducted on the NS mode of cyanide binding with the triazolophane (Figure 2). It is important to note here that there is no unique way to carry out a population analysis and many different methods can be used. Our objective here is not to exhaustively perform all of them, but rather to only demonstrate the proof-of-concept using popular methods. The population analysis was thus investigated using ESP, Mulliken, and Hirshfeld charges. Remarkably, all of them show that the same redistribution of cyanide’s charges occur upon binding to triazolophane. Even though the numerical values of the charges on both the ends of cyanide are very different when computed by these three methods (Table 1), they consistently show that the C Table 1. Computed Charges on CN− before and after Binding with Triazolophanea
Figure 4. Different triads binding with cyanide (top) and isocyanide (bottom). The numbers are the binding energies (kcal/mol).
method
terminus
before binding
after binding
ESP ESP Mulliken Mulliken Hirshfeldb Hirshfeldb
C N C N C N
−0.536 −0.464 −0.520 −0.480 −0.513 −0.487
−0.281 −0.425 −0.464 −0.473 −0.205 −0.299
a
All charges computed at the B3LYP/6-31+G(d,p) level of theory. Hirshfeld charges were computed using the Gaussian keyword HirshfeldEE.
b
In the case of both cyanide binding with the triads (the top row in Figure 4) and isocyanide binding with the triads (bottom row in Figure 4), it is immediately noticed that the binding energy with the N-triad (first column) is greater than the binding energy with the C-triad (second column), which in turn is greater than the binding energy obtained with the diagonal triad (third column). This trend is consistent with the expectation13e,22 that the triazole CH groups are stronger hydrogen bond donors than the N-phenylene and C-phenylene CH groups. Glancing at the third column reveals that cyanide (or isocyanide) is slightly tilted toward the N-phenylene (resulting in a Cs symmetry), as opposed to a more symmetric disposition (C2v symmetry) in the first two columns. Such a tilt reflects the previously observed13e,22 superiority of the Nphenylene CH groups over the C-phenylenes, in being CH hydrogen bond donors. By comparing the cyanide binding energies (all the three columns in the top row) with isocyanide binding energies (the corresponding three columns in the bottom row), it is clear that with the N-triad, C-triad, and the diagonal triad, isocyanide binding is always favored over cyanide binding by about 1−2 kcal/mol.22 This difference is of the same magnitude as the difference obtained between the NS mode and the EW mode of cyanide binding noticed in ref 12. An additional point to note is that the binding energies obtained by the triad approach are not additive but tend to overestimate the net binding energy.13e This is mostly due to anion polarization effects though the
terminus loses more charge to the triazolophane (Table 1) than the N terminus. The greater polarizability of carbon and the higher electronegativity of nitrogen provide possible explanations for this pattern of charge redistribution. There is greater charge transferred from the carbon end to the receptor, presumably through a mechanism classically described as an acid−base interaction between the lone pair and the CH group’s empty σ* orbital. Nevertheless, it is entirely consistent that the shorter hydrogen bonds (noticed in ref 12) follow from the greater negative charge (Table 1) that exists on the nitrogen end in the triazolophane complex with cyanide. (c). Electrostatic Ion−Dipole Analysis of the Bisected Cyanide. While population analyses may provide a deeper understanding of the hydrogen bonding, chemists often employ ion−dipole analyses in an intuitive manner to predict the outcomes of binding events. This intuition helps guide the ultimate design of the receptor that is synthesized. It is important, therefore, to examine its utility as a tool for prediction even if we do so ex post facto. Consider, for example, our surprising results from ref 12: (a) in the perpendicular binding mode P·CN−, the C end and N end are not equidistant from the triazolophane CH donors, and (b) the perpendicular binding mode is calculated to be only 5 kcal/ mol less (∼10%) than the cyanide binding energy for the lowest-energy complex (NS binding mode). To help rationalize C
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Table 3. Detailed Ion−Dipole Analysis of North−South Mode of Cyanide Binding, NS•CN−a
Figure 5. Representations of (a) the perpendicular mode and (b) the NS mode of cyanide binding abstracted as a system of ion−dipole interactions.
CH donor
r
θ
cos θ
μrel
q
E
n
Etot
north triazole with C end north triazole with N end south triazole with C end south triazole with N end north N−Ph with C end north N−Ph with N end south N−Ph with C end south N−Ph with N end C−Ph with C end C−Ph with N end
3.55
166.8
−0.97
1
−0.5
−3.8
2
−7.7
2.72
174.6
−0.99
1
−0.5
−6.7
2
−13.4
2.85
179.0
−1.00
1
−0.5
−6.1
2
−12.3
3.71
164.5
−0.96
1
−0.5
−3.5
2
−7.00
3.97
180.0
−1.00
0.5
−0.5
−1.7
1
−1.7
2.79
180.0
−1.00
0.5
−0.5
−3.2
1
−3.2
3.00
180.0
−1.00
0.5
−0.5
−2.8
1
−2.8
4.18
180.0
−1.00
0.5
−0.5
−1.5
1
−1.5
3.54
170.6
−0.99
0.4
−0.5
−1.6
2
−3.1
3.58
166.9
−0.97
0.4
−0.5
−1.5
2
−3.0 −55.7
a
this difference, and to verify whether the N terminus remains the slightly stronger hydrogen bond acceptor, the energies of the two binding modes were determined by performing an ion−dipole analysis (Figure 5) using
bond dipole, and the C-phenylene bond dipole have the ratio 1.0:0.4:0.3. While these ratios reflect the relative contributions to the stabilization of anions from the entire triazole unit,13e it is reasonable to use the simplifying approximation of the CH groups because they are the point of intermolecular contact. Also, given the ambiguity related to the distribution of charges in cyanide (vide supra), the point charges on the C and N termini of cyanide are considered to be of equal magnitude (−0.5). While this approach to the analysis has some simplifications, it does represent a reasonable starting point in the early design phase. For the perpendicular mode the last column in Table 2 informs us that it is mostly the N end of cyanide that is a slightly better hydrogen bond acceptor than the C end. Maximal differences are noted in the interactions with triazoles. Similarly, ion−dipole analysis for the north−south mode (Table 3) results in the same trends. It is worthwhile to point out here that, the ion−dipole interaction for the triazoles in the south with the C and N ends of cyanide are together smaller than between the northern triazoles and the two ends of the cyanide (Table 3). To summarize, reflecting on the relative stabilities of the perpendicular mode and the north−south mode, this ion− dipole analysis (Table S2, Supporting Information) retains the 10% difference in binding energy obtained from the fully optimized electronic structure calculations. The majority of the interaction energy stems from the triazoles. Geometric considerations (Figure 5) show that, for the perpendicular mode, both the northern and southern hemispheres interact with both ends of the cyanide point charges (−0.5 each) with equal intensity. For the in-plane binding mode along the north−south axis (Figure 5), the interactions from CH donors in the north are stronger and weaker to the nearer and farther ends of cyanide, respectively. This analysis suggests, therefore, that bisecting the cyanide allows the CH donors to maximize their interaction with both ends of the cyanide anion at the same time. This intuitively satisfying interpretation, which could just as easily have been a prediction, nevertheless misses the details of charge redistribution that impact on the structure
E = −μq cos θ /4πεrε0r 2
Here, μ refers to the dipole moment of the CH hydrogen bond donors, q is the charge on the anion’s nuclei, θ is the angle from the midpoint of the CH dipole to the location of the charge, and r is the distance from the midpoint of the CH dipole to the location of the charge. In this calculation, all CH donors were represented as bond dipoles. The relative magnitudes of the bond dipoles originating from the three different CH groups were based on Bandyopadhyay et al.’s work.13e Thus, the relative values of μ (defined as μrel in Tables 2 and 3, vide infra) between the triazole CH bond dipole, the N-phenylene CH Table 2. Detailed Ion−Dipole Analysis of the Perpendicular Mode of Cyanide Binding with Triazolophane CH donor
ra
θb
cos θ
μrelc
triazole with C end triazole with N end N−Ph with C end N−Ph with N end C−Ph with C end C−Ph with N end
3.28
158.7
−0.93
3.22
173.7
3.52
qd
Ee
nf
Etotg
1
−0.5
−4.3
4
−17.3
−0.99
1
−0.5
−4.8
4
−19.2
174.6
−1.00
0.5
−0.5
−2.0
2
−4.0
3.49
162.4
−0.95
0.5
−0.5
−1.9
2
−3.8
3.61
166.6
−0.97
0.4
−0.5
−1.9
2
−2.9
3.56
171.1
−0.99
0.4
−0.5
−1.6
2
−3.3
All the symbols are the same as those used in Table 2.
−50.5 a
r is the distance from the midpoint of the CH dipole to the location of the charge, computed using the cosine rule in Å. bθ is the angle from the midpoint of the CH dipole to the location of the charge. cμrel is the relative dipole moments of the various CH bond dipoles. dq is the charge on both ends of the cyanide. eE is the ion−dipole interaction energy (kcal/mol). fn is the number factor, accounting for the total number of CH bond dipoles. gEtot is the net ion−dipole interaction energy (kcal/mol). D
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(5) (a) Burmeister, J. L. Ambidentate Ligands, the Schizophrenics of Coordination Chemistry. Coord. Chem. Rev. 1990, 105, 77−133. (b) Dunbar, K. R.; Heintz, R. A. Chemistry of Transition Metal Cyanide Compounds: Modern Perspectives. Prog. Inorg. Chem. 1997, 45, 283−391. (b) Kim, Y.-H.; Hong, J.-I. Ion Pair Recognition by Zn− Porphyrin/Crown Ether Conjugates: Visible Sensing of Sodium Cyanide. Chem. Commun. 2002, 512−513. (6) (a) Ros-Lis, J. V.; Martinez-Manez, R.; Soto, J. A Selective Chromogenic Reagent for Cyanide Determination. Chem. Commun. 2002, 2248−2249. (b) Tomasulo, M.; Raymo, F. M. Colorimetric Detection of Cyanide with a Chromogenic Oxazine. Org. Lett. 2005, 7, 4633−4636. (c) Yang, Y. K.; Tae, J. Acridinium Salt Based Fluorescent and Colorimetric Chemosensor for the Detection of Cyanide in Water. Org. Lett. 2006, 8, 5721−5723. (d) Chung, Y. M.; Raman, B.; Kim, D. S.; Ahn, K. H. Fluorescence Modulation in Anion Sensing by Introducing Intramolecular H-bonding Interactions in Host−Guest Adducts. Chem. Commun. 2006, 186−188. (e) Jo, J.; Lee, D. Turn-On Fluorescence Detection of Cyanide in Water: Activation of Latent Fluorophores through Remote Hydrogen Bonds That Mimic Peptide β-Turn Motif. J. Am. Chem. Soc. 2009, 131, 16283−16291. (7) Electron transfer is a new reactivity modality recently reported, see: Ajayakumar, M. R.; Mukhopadhyay, P.; Yadav, S.; Ghosh, S. Single-Electron Transfer Driven Cyanide Sensing: A New Multimodal Approach. Org. Lett. 2010, 12, 2646−2649. (8) The tendency for deprotonation also leads to a natural comparison to the basic fluoride ion; for example, see the last sentence of the Conspectus in Hundall, T. W.; Chiu, C.-W.; Gabbai, F. P. Fluoride Ion Recognition by Chelating and Cationic Boranes. Acc. Chem. Res. 2009, 42, 388−397. (9) (a) Bisson, A. P.; Lynch, V. M.; Monahan, M. K. C.; Anslyn, E. V. Recognition of Anions through NH-p-Hydrogen Bonds in a Bicyclic Cyclophane. Selectivity for Nitrate. Angew. Chem., Int. Ed. 1997, 36, 2340−2342. (b) Sun, S. S.; Less, A. J. Anion Recognition Through Hydrogen Bonding: A Simple, yet Highly Sensitive, Luminescent Metal-Complex Receptor. Chem. Commun. 2000, 1687−1688. (c) Takeuchi, M.; Shiova, T.; Swager, T. M. Allosteric Fluoride Anion Recognition by a Doubly Strapped Porphyrin. Angew. Chem., Int. Ed. 2001, 40, 3372−3376. (d) Anzenbacher, P., Jr.; Tysson, D. S.; Juriskova, K.; Castellano, F. N. Luminescence Lifetime-Based Sensor for Cyanide and Related Anions. J. Am. Chem. Soc. 2002, 124, 6232− 6233. (e) Chang, K.-J.; Moon, D.; Lah, M. S.; Jeong, K.-S. IndoleBased Macrocycles as a Class of Receptors for Anions. Angew. Chem., Int. Ed. 2005, 44, 7926−7929. (f) Chen, C. L.; Chen, Y. H.; Chen, C. Y.; Sun, S. S. Dipyrrole Carboxamide Derived Selective Ratiometric Probes for Cyanide Ion. Org. Lett. 2006, 8, 5053−5056. (g) Sessler, J. L.; Barkley, N. M.; Pantos, G. D.; Lynch, V. M. Acyclic Pyrrole-Based Anion Receptors: Design, Synthesis, and Anion-Binding Properties. New J. Chem. 2007, 31, 646−654. (h) Lin, T. P.; Chen, C. Y.; Wen, Y. S.; Sun, S. S. Synthesis, Photophysical, and Anion-Sensing Properties of Quinoxalinebis(sulfonamide) Functionalized Receptors and Their Metal Complexes. Inorg. Chem. 2007, 46, 9201−9212. (i) Saha, S.; Ghosh, A.; Mahato, P.; Mishra, S.; Mishra, S. K.; Suresh, E.; Das, S.; Das, A. Specific Recognition and Sensing of CN− in Sodium Cyanide Solution. Org. Lett. 2010, 12, 3406−3409. (10) Alcade, E.; Alvarez-Rua, C.; Garcia-Granda, S.; GarciaRodriguez, E.; Mesquida, N.; Perez-Garcia, L. Hydrogen Bonded Driven Anion Binding by Dicationic [14]Imidazoliophanes. Chem. Commun. 1999, 295−296. (11) Amendola, V.; Esteban-Gomez, D.; Fabbrizzi, L.; Licchelli, M. What Anions Do to N−H-Containing Receptors. Acc. Chem. Res. 2006, 39, 343−353. (12) Ramabhadran, R. O.; Hua, Y.; Li, Y.; Flood, A. H.; Raghavachari, K. From Atomic to Molecular Anions: A Neutral Receptor Captures Cyanide Using Strong C−H Hydrogen Bonds. Chem.Eur. J. 2011, 17, 9123−9129. (13) (a) Li, Y.; Flood, A. H. Pure CH Hydrogen Bonding to Chloride Ions: A Pre-organized and Rigid Macrocyclic Receptor. Angew. Chem., Int. Ed. 2008, 47, 2649−2652. (b) Li, Y.; Flood, A. H. Strong, Sizeselective, and Electronically-Tunable C−H···Halide Binding with
of the complex. Overall, it can be seen that the ion−dipole analysis is consistent with the N terminus being the stronger hydrogen bond acceptor.
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CONCLUSIONS This study computationally probes a fundamental question regarding the hydrogen bonding properties of the cyanide anion. Using three different approaches involving: (a) breakdown of the triazolophane into smaller components, (b) population analyses, and (c) ion−dipole analyses, we show that the N terminus of cyanide is a slightly stronger CH hydrogen bond acceptor than the C terminus. Our result, in conjunction with Kuhn et al.’s17 findings, demonstrates how predictions based only on cyanide’s Lewis structure and the formal negative charge on the C end can be misleading. Furthermore, the ion− dipole analysis method also aids in rationalizing the binding differences noted between the perpendicular mode of binding and the north−south mode of binding observed when triazolophane binds the cyanide anion. Finally, we hope that future experimental studies will overcome the challenge posed by the pseudospherical nature of cyanide, in both the gas-phase and in solution, to individually determine the hydrogen bonding abilities of the C and N ends and thus shed more light on this exciting problem.
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ASSOCIATED CONTENT
S Supporting Information *
Cartesian coordinates of all the cyanide and isocyanide bound triads used in this study. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*(K.R.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge support by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, U.S. Department of Energy (Grant: DEFG02-09ER16068).
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REFERENCES
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Flood, A. H. A Pentagonal Cyanostar Macrocycle with Cyanostilbene CH Donors Binds Anions and Forms Dialkylphosphate [3] Rotaxanes. Nat. Chem. 2013, 5, 704−710. (p) Hua, Y.; Liu, Y.; Chen, C.-H.; Flood, A. H. Hydrophobic Collapse of Foldamer Capsules Drives Picomolar-Level Chloride Binding in Aqueous Acetonitrile Solutions. J. Am. Chem. Soc. 2013, 135, 14401−14412. (15) (a) For instance, Q(C) = −0.501 and Q(N) = −0.499 as reported in Bonokorsi, R.; Petrongolo, C.; Scrocco, E.; Tomasi, J. J. SCF Minimal Basis Set Calculations and Exclusive Orbitals for CN−, HCN, N3−, HN3, NCO−, and HNCO. J. Chem. Phys. 1968, 48, 1500− 1508. (b) Q(C) = −0.409 and Q(N) = −0.591 as reported in Dogett, G.; McKendrick, A. Electronic structure of the cyanide anion. J. Chem. Soc. A 1970, 825−827. (16) (a) Larson, J. W.; McMahon, T. B. Hydrogen Bonding in GasPhase Anions. The Energetics of Interaction Between Cyanide Ion and Broensted Acids Determined from Ion Cyclotron Resonance Cyanide Exchange Equilibria. J. Am. Chem. Soc. 1987, 109, 6230−6236. (b) Mautner, M.; Cybulski, S. M.; Scheiner, S.; Liebman, J. F. Is Cyanide Significantly Anisotropic? Comparison of Cyanide vs Chloride: Clustering with Hydrogen Cyanide and Condensed-Phase Thermochemistry. J. Phys. Chem. 1988, 92, 2738−2745. (17) Kuhn, N.; Eichele, K.; Steimann, M.; Al-Sheikh, A.; Doser, B.; Ochsenfeld, C. Wasserstoffbrückenbindungen Mit Cyanidionen? Die Strukturen Von 1,3-Diisopropyl-4,5-Dimethylimidazoliumcyanid und 1-Isopropyl-3,4,5-Trimethylimidazoliumcyanid. Z. Anorg. Allg. Chem. 2006, 632, 2268−2275. (18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision A.01; Gaussian, Inc.: Wallingford, CT, 2009. (19) (a) Lee, C. T.; Yang, W. T.; Parr, R. G. Development of the Colle−Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785−789. (b) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (20) Mayer, I.; Surjan, P. R. Monomer Geometry Relaxation and the Basis Set Superposition Error. Chem. Phys. Lett. 1992, 191, 497−499. (21) Grimme, S.; Antony, J.; Ehlrich, S.; Krieg, H. A Consistent and Accurate ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104. (22) We have shown previously13e that the inherent ordering of the C−H hydrogen bonding strengths of the different structural components in the macrocycle is triazole ≫ N-phenylene > Cphenylene. The relative stability of the anion-bound triads can be understood based entirely on this ordering. The N-triad has two triazole bonds and one N-phenylene bond. The C-triad has two triazole bonds and one C-phenylene bond and is thus expected to be slightly less stable (since C-phenylene bonds are slightly weaker than N-phenylene bonds). The D-triad has only one triazole bond along with one N-phenylene and one C-phenylene bond. Thus, it is expected to be the least stable. The numerical values are all consistent with these expectations. For each of the three triad species, the binding to the Nend is stronger than that to the C-end as discussed in this article.
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