Rigid Single Carbon−Carbon Bond That Does Not Rotate in Water

Article pubs.acs.org/JPCB Rigid Single Carbon−Carbon Bond That Does Not Rotate in Water Manuel Gadogbe,† Yadong Zhou,‡ Shengli Zou,*,‡ and Dongmao Zhang*,† † Department of Chemistry, Mississippi State University, Mississippi State, Mississippi 39762, United States Department of Chemistry, University of Central Florida, Orlando, Florida 32816, United States ‡ S Supporting Information * ABSTRACT: Carbon−carbon bond is one of the most ubiquitous molecular building blocks for natural and man-made materials. Rotational isomerization is fundamentally important for understanding the structure and reactivity of chemical and biological molecules. Reported herein is the first demonstration that a single C−C bond does not rotate in water. The two distal C−S bonds in both 1,2-ethanedithiolate (−S−CH2−CH2−S−, 1,2-EDT2−) and 2,3butanedithiolate (2,3-BuDT2−) are exclusively in the trans conformer with reference to their respective center single C−C bond. In contrast, both trans and gauche conformers are observed in neutral 1,2-ethanedithiol (1,2-EDT) and 2, 3-butanedithiol (2,3-BuDT). The insight from this work should be important for understanding the charge effect on the molecular conformation in aqueous solutions. concentrations we explored. This is to our knowledge the first demonstration that a single carbon−carbon bond does not undergo rotational isomerization. 1,2-EDT2− and 2, 3-BuDT2− are ideal model molecules for studying the charge effect on the C−C rotation. First, they are the simplest molecules that contain a monatomic ion directly connected to each side of a single C−C bond. The singly charged sulfides are separated only by three covalent bonds, which is the shortest charge separation one can imagine for studying the effect of the intramolecular electrostatic interaction on the rotational isomerization of a chemical bond. Second, as an alkanethiol derivative, the 1,2-EDT and 2,3-BuDT structure and conformation can be readily determined using Raman spectroscopic measurements. This is because S−H stretching Raman intensity serves as a measure of the degree of dithiol ionization, while the peak position of C−S stretching Raman feature is sensitive to the dithiol conformation. INTRODUCTION The carbon−carbon single bond is likely the most common molecular building block in chemical and biological molecules. The C−C rotation produces large number of rotational isomers that differ in their molecular conformations. As an example, the rotations of single C−C and C−N bonds in combination with other intramolecular forces are responsible for the protein folding and unfolding.1,2 Numerous experimental and computational works have been dedicated to the fundamental understanding of the C−C rotation. Dian et al. used broadband Fourier transform microwave spectroscopy to study the isomerization kinetics of cyclopropane carboxaldehyde after exciting it above the C−C isomerization barrier. Isomerization between the two conformers (syn and anti) was observed to occur in ∼180 ps time scale.3 While it has been generally accepted that the rotational energy barrier of the C−C bond in ethane, the simplest C−C bond-containing molecule in the gas phase, is ∼3 kcal/mol,4 the origin of this energy barrier in ethane remains controversial. Several groups believe the dominant contributor to this barrier in ethane is the steric hindrance or repulsion,5−8 while others proposed that hyperconjugation predominates.9−13 However, all the existing studies use ethane or its derivative in organic solvents or in gas phase as the model molecules. Information on the charge effect on the rotational isomerization along the C−C single bonds has, to our knowledge, been totally lacking. Reported herein is the finding that the single C−C bonds in both 1, 2-ethanedithiolate (−S−CH2−CH2−S−, 1,2-EDT2−) and 2,3-butanedithiolate (−S(CH3)−CH−CH−(CH3)S−, 2,3BuDT2−) are extraordinarily rigid against rotational isomerization in water. The two C−S groups in each side of the C−C bond remain exclusively in the trans conformer relative to the central single C−C bond regardless of the temperature and salt ■ © 2016 American Chemical Society ■ EXPERIMENTAL SECTION Chemicals and Equipment. All chemicals were purchased from Sigma-Aldrich. 1,2-EDT, 2,3-BuDT, 1,4-butanedithiol (1,4-BuDT), 1,6-hexanedithiol (1,6-HDT) and 1,8-octanedithiol (1,8-ODT) were all purchased within the past 7 months and they were used as received. The reagent purity are all 95% or above. Nanopure water was used throughout the experiments. The normal Raman spectra were obtained with a Horiba LabRam HR800 confocal Raman microscope system and a 633 Received: December 12, 2015 Revised: February 16, 2016 Published: February 16, 2016 2418 DOI: 10.1021/acs.jpcb.5b12166 J. Phys. Chem. B 2016, 120, 2418−2422 The Journal of Physical Chemistry B Article Figure 1. Raman spectra of intact and dithiolated (A) 1,2-EDT, (B) 2,3-BuDT, and (C) 1,4-BuDT. The dashed lines from left to right represent the peak positions for gauche C−S, trans C−S, C−H bending, S−H stretch, and C−H stretch, respectively. The Raman spectra of the intact molecules were acquired using the neat samples. The nominal concentration of the molecules in the 1 M NaOH samples is 117 mM. so that only the SCCS dihedral angle is fixed and all other internal coordinates are relaxed during the calculations. nm Raman excitation laser. The laser power used in the Raman acquisition is ∼13 mW for all the Raman spectra taken. Normal Raman Analysis. All the normal Raman spectra for neat alkanedithiols as well as the dithiols in 1 M NaOH were taken with an Olympus 10× objective (NA = 0.25) and spectrograph grating of 300 grooves/mm or 1800 grooves/mm. The spectral integration time was 50 s and when necessary ∼20 accumulations were taken to improve the signal-to-noise ratio. The Raman shift was calibrated with a neon lamp, and the Raman shift accuracy was ∼0.5 cm−1. The normal Raman spectra of all dithiols were acquired using the neat liquid samples and ∼117 mM of the samples in 1 M NaOH. The temperature dependent Raman spectra were taken after heating the mixture of the dithiols in 1 M NaOH to near boiling. pH Titration of 1,2-EDT and 2,3-BuDT. 1,2-EDT (20 μL) and 2,3-BuDT (30 μL) were each dissolved in different pH of NaOH solution (2 mL). The nominal concentration of 1,2EDT and 2,3-BuDT were kept the same in each mixture. After mixing thoroughly, the pH of the resulting mixture was taken and the Raman spectra acquired for the mixtures. Charge Screening and Neutralization Experiments. Charge screening studies were performed using ∼117 mM of 1,2-EDT or 2,3-BuDT in 1 M NaOH mixed with solid NaCl or LiCl to saturation concentrations. The Raman spectra were obtained before and after addition of NaCl or LiCl, and after heating the same mixtures to near boiling. In the charge neutralization experiments, 1 mL of 1,2-EDT or 2,3-BuDT (117 mM) in 1 M NaOH was mixed with 1 mL of 1 M AgNO3. Upon sample mixing, precipitates were formed. Raman spectra were obtained after washing the resulting precipitates with copious amount of water to remove the excess AgNO3. Computational Calculation of Raman Spectra and Potential Energy Surface (PES). Gaussian 09 was used to calculate the Raman spectra and energies of the dithiols. The calculations are based on bp86 method with the 6-311+G** basis functions. The solvent (water) effect was included by the keyword scrf = (solvent = water). In the initial calculations, we started from many different possible configurations to make sure that all the local energy minima of different configurations are located in the calculations. The potential energy surface (PES) calculations for 1,2-EDT were done using the same bp86 method and 6-311+G** basis functions as in all other calculations. The multiplicity of the dianion (1,2-EDT2−) and neutral molecule (1,2-EDT) are one (1) and three (3), respectively. PES was calculated as a function of the SCCS dihedral angle. At each dihedral angle, we used opt = Z-matrix ■ RESULTS AND DISCUSSION Figure 1 shows the Raman spectra of the intact and dithiolated 1,2-EDT, 2,3-BuDT, and 1,4-BuDT. The dithiolated molecules were prepared by adding the dithiols into 1 M NaOH. The total absence of the characteristic S−H stretching Raman feature in the ∼2600 cm−1 region indicates that two thiols in 1,2-EDT, 2,3-BuDT, and 1,4-BuDT dissolved in the 1 M NaOH solution are completely ionized. The possibility of disulfide formation, which can also lead to the disappearance of the S−H stretching features and has been observed when alkanedithiols are attached onto plasmonic gold nanoparticle surfaces,14 is excluded on the absence of the S−S stretching feature (∼520 cm−1 region) in the Raman spectra of both intact and the dithiolated dithiols. The Raman spectral feature in the C−S stretch region provides rich information about the organothiol conformation. The C−S stretching features at ∼735 cm−1 and ∼662 cm−1 regions correspond to trans and gauche conformers of the two distal C−S bonds, respectively, with reference to the Cα−Cβ bond.15−17 The Cα carbon refers to the carbon to which the sulfur atom is directly attached, while the Cβ is the carbon directly linked to the Cα carbon. The concurrent appearance of the ∼747 cm−1 (692 cm−1 for 2,3-BuDT) and ∼638 cm−1 peaks in Raman spectra of the intact 1,2-EDT, 2,3-BuDT, and 1,4-BuDT indicates that both trans and gauche conformers are present in the intact dithiols (Figure 1). This is consistent with the existing literature that trans and gauche conformer can change rapidly in solution due to their low energy barrier between these two conformers.18 However, upon dithiolation, only the C−S trans stretching peak at the ∼747 cm−1 and ∼692 cm−1 is observable in 1,2-EDT and 2,3-BuDT samples, respectively. There is no detectable Raman feature in the ∼638 cm−1 region that is associated with 1,2-EDT gauche conformer. This result indicates that the C−S bonds in both 1,2-EDT2− and 2,3-BuDT2− is predominately in a trans conformer relative to their central C−C single bonds. In other words, the single carbon−carbon bonds that link the two sulfur ion cannot rotate to generate the gauche isomer. In contrast, both trans and gauche C−S Raman features are present in 1,4-BuDT regardless of the ionization states of their distal thiols. The Raman spectral assignments are supported by the computational simulations (Figure 2). The experimental 1,2EDT2− Raman spectrum is in good agreement with the density 2419 DOI: 10.1021/acs.jpcb.5b12166 J. Phys. Chem. B 2016, 120, 2418−2422 The Journal of Physical Chemistry B Article remains entirely in trans conformer when the solution is heated from room temperature to the initiation of the boiling. Parts A and B of Figure 3 show the one-dimensional (1D) PES calculations for the 1,2-EDT2− and 1,2-EDT, respectively Figure 2. (A−C) Images of representative 1,2-EDT2− rotational isomers, and their energies. The energy differences are calculated relative to the lowest energy state conformer. (D) Comparison of the 1,2-EDT2− experimental Raman spectra with computationally simulated 1,2-EDT2− Raman spectra for the different conformations shown in parts A−C. The nominal concentration of 1,2-EDT in the sample used to acquire the experimental spectrum is 117 mM. Figure 3. 1D potential energy surface as a function of rotational dihedral angle of SCCS for the (A) 1,2-EDT2−, (B) 1,2-EDT, and (C) Coulomb repulsive energy between the two singly charged sulfide in 1,2-EDT2− and (D) the sum of the potential energy calculated for 1,2EDT (B) and the Coulomb repulsive energies (C). functional theory calculated Raman spectrum of 1,2-EDT2− trans conformer in which the two C−S bonds are in ∼180 deg to each other, but differs significantly from gauche conformer in which the angle between the two C−S bonds viewed along the C−C bond are 0 and ∼75 deg (G1 and G2) respectively. The exceptional rigidity of the C−C bond in the 1,2-EDT2− and 2,3-BuDT2− against rotational isomerization can be due to collective effects of steric hindrance of the sulfide ion and the Coulomb repulsion. However, the former is unlikely the dominant effect. Otherwise one should observe a similar rotational energy barrier for the intact 1,2-EDT and 2,3-BuDT. Instead, our computational simulations revealed that the Coulomb repulsion alone between the two negatively charge distal sulfide are adequate to impose a high energy barrier inhibiting the 1,2-EDT2− rotational isomerization. Indeed, the simulated energy difference between the two gauche conformers (G1 and G2) and trans conformer of 1,2-EDT2− are ∼11.3 and 3.1 kcal/mol respectively (Figure 2) while the energy difference between the most stable gauche conformer and the trans conformer for the intact 1,2-EDT is ∼0.66 kcal/mol (Figure S1, Supporting Information). The latter is in agreement with the reported energy difference between the gauche and trans conformer for neutral 1,2-EDT.18−20 This value also explains the presence of both gauche and trans conformers of intact 1,2EDT at room temperature. The images and relative energies of the other rotational isomers of 1,2-EDT are shown in Figure S2, Supporting Information). The calculated energy difference (∼11.3 and 3.1 kcal/mol) of the 1,2-EDT2− gauche and trans conformers is significantly greater than its thermal energy at room temperature. This explains why no gauche conformer can be observed in the 1,2EDT2− Raman spectrum. According to the Boltzmann distribution law, essentially all (>99%) of the 1,2-EDT2− are in the trans conformer even when at the boiling temperature of the sample (100 °C). This conclusion is supported by the temperature-dependent Raman measurement (Figure S3, Supporting Information). The latter shows that 1,2-EDT2− as a function of the rotational dihedral angle of SCCS. Figure 3C is the PES for the Coulomb repulsive energy for the two singly charged sulfide as a function of the dihedral angle. Addition of the Coulomb repulsive energy term (Figure 3C) to the neutral 1,2-EDT (Figure 3B) yields the PES profile in Figure 3 (D) which has a similar trend as observed for the dianion (Figure 3A). This result indicates that the relative barrier between trans and gauche is due predominantly to the Coulomb repulsion between the two singly charged sulfide ions in 1,2-EDT2−. The PES calculations also show that the energy is the highest when the dihedral angle is 0° in 1,2-EDT2− and lowest when the dihedral angle is at 10° in 1,2-EDT. It is worthy to note that conformer G1 (with dihedral angle of 0°) corresponds to a saddle point instead of a local minimum. In calculating the Coulomb repulsion between two negatively charged sulfur atoms, we used classical Coulomb’s law. One issue in this calculation is the determination of the required relative permittivity. While Coulomb interaction between two charge species separated by solvent can be computed straightforwardly using the solvent relative permittivity, there is no general model for determining the relative permittivity required for the calculation of Coulomb interactions between charged species separated by only a few chemical bonds in the same molecule. We found in this work that when combined with the DFT potential energy surface calculated for the neutral molecule (Figure 3B), the Coulomb repulsion energy calculated with a relative permittivity of 2 (Figure 3C) gives a total potential energy surface (Figure 3D) comparable to the DFT potential energy surface calculated for the dianion molecule. This empirical permittivity value is very close to experimental relative permittivity of alkane molecule pentane (1.8),21 but significantly smaller than that for the relative 2420 DOI: 10.1021/acs.jpcb.5b12166 J. Phys. Chem. B 2016, 120, 2418−2422 The Journal of Physical Chemistry B Article permittivity of bulk water (78). This result suggests that one can use the relative permittivity of the molecule itself to estimate the classical Coulomb interaction between charged atoms located in the same small molecule. The normal mode vibrational frequencies calculated using Hessian matrix also show that the vibrational frequency corresponding to the torsional rotation is 126 cm−1 with a zero point energy of 0.75 kJ/mol (0.18 kcal/mol). This zero point energy is much lower than the energy barrier of ∼8 kJ/ mol for the gauche conformer (G2) in Figure 3A. The fact that the Coulomb repulsion between the distal sulfide ion (−S−) is the dominant factor in defining the 1,2EDT2− and 2,3-BuDT2− conformations is further confirmed from the pH titration studies. Complete trans conformer appears only when both two distal −SH groups in 1,2-EDT or 2,3-BDT are both deprotonated (Figure 4). Otherwise, both the gauche and trans conformers coexist in the Raman spectra obtained with the samples whenever there is intact thiol. (Figure 4). Figure 5. Raman spectra of (A) 1, 2-EDT, and (B) 2,3-BuDT in 1 M NaOH and saturated LiCl; and 1, 2-EDT, 2,3-BuDT sequentially mixed with 1 M NaOH and 1 M AgNO3. The Raman spectrum was taken after washing the precipitate formed to remove excess AgNO3. The first two dashed lines from left to right indicate the expected peak positions for the gauche C−S (ν(C−S)G) and trans C−S (ν(C−S)T), respectively, on both 1,2-EDT2− and 2,3-BuDT2− spectra. The Raman spectra were acquired using the 300 grating on the Raman instrument. AgNO3, solid precipitates form and both trans and gauche C−S Raman features for the conformers appeared in the Raman spectra acquired with these precipitates. The latter can be silver dithiolate salts or the dithiolated covered silver nanoparticles (AgNP). Our recent study revealed that the organothiol reaction with silver is a highly complicated process in which thiol can induce both AgNP formation and dissolution under ambient conditions.24 The fact that both gauche and trans C−S feature were observed in the AgNO3 treated 1,2-EDT2− and 2,3-BuDT2− solution indicates that Ag+ is highly effective in neutralizing the sulfide ion in the dithiolated molecules. The fact that both gauche and trans are observed in the 1,4BuDT (Figure 1) and other long-chain dithiolate such as 1,6hexanedithiolate (1,6-HDT) and 1,8-octanedithiolate (1,8ODT) (Figure 6) indicates that the effect of intermolecular Coulomb repulsion between the distal sulfide ion on the alkanedithiol conformation decreases rapidly as the charge separation increases. Several possible reasons can be offered for this observation. First, Coulomb’s law dictates that the larger the charge separation, the lower the Coulomb repulsion. Second, larger charge separation provides more room to Figure 4. Raman spectra of (A) 1,2-EDT and (B) 2,3-BuDT at different pH. The Raman spectra of the neat samples are also plotted for comparison. The nominal concentration of 1,2-EDT and 2,3-BuDT in the samples are both ∼117 mM. The dashed lines from left to right represent the peak positions for gauche C−S, trans C−S, and S−H stretch, respectively. The Raman spectra were acquired using the 300 grating on the Raman instrument. The possibility of the electrolyte screening of the Coulomb repulsion between the two singly charged sulfide ions in both 1,2-EDT2− and 2,3-BuDT2− was studied using LiCl and NaCl as the model electrolytes. It is possible under high salt concentration, metal ions form ion pairs with one or both sulfide ions in 1,2-EDT2− and 2,3-BuDT2−, or form a metal ion bridge between the two sulfur ions. The latter is in analogy to the salt bridge that is commonly invoked to explain the electrolyte effect on protein and DNA structure, conformation, and their bioactivity.22,23 However, both 1,2-EDT2− and 2,3BuDT2− remain exclusively as trans conformers even when saturated amount of LiCl and NaCl was added and the electrolyte-containing solution is heated to near boiling (Figure 5 and Figure S4 in Supporting Information). This result indicates that there is no significant ion pairing or salt bridge formation between the singly charged sulfide ion and the positively charged metal ion, or these intermolecular charge interactions is inadequate to induce significant conformation change in 1,2-EDT2− and 2,3-BuDT2−. One likely reason is the distances of charge separation in these two molecules are too small to accommodate a solvated metal ion as the salt bridge. However, when 1,2-EDT2− and 2,3-BuDT2− are treated with Figure 6. Raman spectra of (a) 1,6-HDT, (b) 1,6-HDT2−, (c) 1,8ODT, and (d) 1,8-ODT2−. The dithiolates were obtained after mixing neat 1,6-HDT or 1,8-ODT in 1 M NaOH. The nominal concentrations of 1,6-HDT2− and 1,8-ODT2− in the samples are both 117 mM. The dashed lines from left to right represent the peak positions for gauche C−S, trans C−S, C−H bending, S−H stretch, and C−H stretch, respectively. The Raman spectra were acquired using the 300 grating on the Raman instrument. 2421 DOI: 10.1021/acs.jpcb.5b12166 J. Phys. Chem. B 2016, 120, 2418−2422 The Journal of Physical Chemistry B Article (6) Mo, Y.; Wu, W.; Song, L.; Lin, M.; Zhang, Q.; Gao, J. The Magnitude of Hyperconjugation in Ethane: A Perspective from Ab Initio Valence Bond Theory. Angew. Chem., Int. Ed. 2004, 43, 1986− 1990. (7) Bickelhaupt, F. M.; Baerends, E. J. The Case for Steric Repulsion Causing the Staggered Conformation of Ethane. Angew. Chem., Int. Ed. 2003, 42, 4183−4188. (8) Liu, S.; Govind, N. Toward Understanding the Nature of Internal Rotation Barriers with a New Energy Partition Scheme: Ethane and nButane. J. Phys. Chem. A 2008, 112, 6690−6699. (9) Weinhold, F. Chemistry: A New Twist on Molecular Shape. Nature 2001, 411, 539−541. (10) Pophristic, V.; Goodman, L. Hyperconjugation Not Steric Repulsion Leads to the Staggered Structure of Ethane. Nature 2001, 411, 565−568. (11) Reed, A. E.; Weinhold, F. Natural Bond Orbital Analysis of Internal Rotation Barriers and Related Phenomena. Isr. J. Chem. 1991, 31, 277−285. (12) Brunck, T. K.; Weinhold, F. Quantum-Mechanical Studies on the Origin of Barriers to Internal Rotation about Single Bonds. J. Am. Chem. Soc. 1979, 101, 1700−1709. (13) Goodman, L.; Gu, H.; Pophristic, V. Flexing Analysis of Ethane Internal Rotation Energetics. J. Chem. Phys. 1999, 110, 4268−4275. (14) Gadogbe, M.; Zhou, Y.; Alahakoon, S. H.; Perera, G. S.; Zou, S.; Pittman, C. U.; Zhang, D. 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S.; Collier, W. E.; Perez, F.; Zhang, D. Contradictory Dual Effects: Organothiols Can. Induce Both Silver Nanoparticle Disintegration and Formation under Ambient Conditions. J. Phys. Chem. C 2015, 119, 20975−20984. accommodate solvent or solvated counterion to form a salt bridge between the sulfur ions. This can further reduce the Coulomb repulsion between the distal sulfide ions. CONCLUSION In conclusion, we conducted a systematic study on the charge effect on the rotational isomerization of C−C bond in water and observed, for the first time a single C−C bond that cannot rotate in water under ambient conditions. The Coulomb repulsion between the two distal singly charged sulfide ions in both 1,2-EDT2− and 2,3-BuDT2− make the central single C−C bond extremely rigid that both 1,2-EDT2− and 2,3-BuDT2− are locked in trans conformers under all explored experimental conditions. Such charge effect is highly sensitive to the distance of charge separation. Both gauche and trans conformers appear in alkanedithiolate in which the two singly charged sulfide ions are separated by more than four chemical bonds. The insight from this study should be important for understanding of the charge effect on conformational isomerization. ■ ■ ASSOCIATED CONTENT * Supporting Information S The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b12166. Experimental details; images, relative energies and Raman spectra of 1,2-EDT; temperature dependent Raman spectra of 1,2-EDT2−; Raman spectra of 1,2EDT2− and 2,3-BuDT2− in the presence of NaCl; Raman-based pH titration of 1,2-EDT and 2,3-BuDT (PDF) ■ AUTHOR INFORMATION Corresponding Authors *(D.Z.) E-mail: Dongmao@chemistry.msstate.edu. *(S.Z.) E-mail: Shengli.Zou@ucf.edu. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by two NSF funds (CHE 1151057, EPS-0903787) and a USDA seed grant (under Project No. 5864022729) provided to D.Z. ■ ■ REFERENCES (1) Wedemeyer, W. J.; Welker, E.; Scheraga, H. A. Proline Cis−Trans Isomerization and Protein Folding. Biochemistry 2002, 41, 14637− 14644. (2) Pradeep, L.; Shin, H.-C.; Scheraga, H. A. Correlation of Folding Kinetics with the Number and Isomerization States of Prolines in Three Homologous Proteins of the RNase Family. FEBS Lett. 2006, 580, 5029−5032. (3) Dian, B. C.; Brown, G. G.; Douglass, K. O.; Pate, B. H. Measuring Picosecond Isomerization Kinetics via Broadband Microwave Spectroscopy. Science 2008, 320, 924−928. (4) Asturiol, D.; Salvador, P.; Mayer, I. Dissecting the Hindered Rotation of Ethane. ChemPhysChem 2009, 10, 1987−1992. (5) Mo, Y.; Gao, J. Theoretical Analysis of the Rotational Barrier of Ethane. Acc. Chem. Res. 2007, 40, 113−119. NOTE ADDED AFTER ASAP PUBLICATION This paper published ASAP on 2/29/16. The TOC/Abstract graphic was replaced and the caption for Figure 5 was corrected. The revised version reposted on 3/1/16. ■ 2422 DOI: 10.1021/acs.jpcb.5b12166 J. Phys. Chem. B 2016, 120, 2418−2422