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Anion Binding of One‑, Two‑, and Three-Armed Thiourea Receptors Examined via Photoelectron Spectroscopy and Quantum Computations Published as part of The Journal of Physical Chemistry virtual special issue “Mark S. Gordon Festschrift”. Evgeny V. Beletskiy,†,§ Xue-Bin Wang,*,‡ and Steven R. Kass*,† †

Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States Physical Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, MS K8-88, Richland, Washington 99352, United States

‡

S Supporting Information *

ABSTRACT: Benzene rings substituted with 1−3 thiourea containing arms (1−3) were examined by photoelectron spectroscopy and density functional theory computations. Their conjugate bases and chloride, acetate, and dihydrogen phosphate anion clusters are reported. The resulting vertical and adiabatic detachment energies span 3.93−5.82 eV (VDE) and 3.65−5.10 (ADE) for the deprotonated species and 4.88−5.97 eV (VDE) and 4.45−5.60 eV (ADE) for the anion complexes. These results reveal the stabilizing effects of multiple hydrogen bonds and anionic host−guest interactions in the gas phase. Previously measured equilibrium binding constants in aqueous dimethyl sulfoxide for all three thioureas are compared to the present results, and cooperative binding is uniformly observed in the gas phase but only for one case (i.e., 3·H2PO4−) in solution.

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INTRODUCTION

more lipophilic and basic than dihydrogen phosphate, respectively.18−22 To develop this class of thiourea receptors further, it would be helpful to know the bound complex structures and the number of hydrogen bonds that are formed in each of them. These issues are addressed in this paper by characterizing the conjugate bases of 1−3 (1a−3a) as well as the Cl−, OAc−, and H2PO4− cluster ions of all three thioureas via negative ion photoelectron spectroscopy and density functional theory calculations.

Molecular recognition plays an essential role in life processes and has been exploited in many diverse fields ranging from drug development and phase transfer catalysis to water purification and chemical analyses.1−3 Phosphate receptors are an especially important target as this anion in its different ionization states (i.e., H2PO4−, HPO42−, and PO43−) is a pervasive environmental pollutant resulting from common agricultural and industrial practices.4−11 It is also an indicator of various diseases and general well being, making the development of selective phosphate anion sensors a priority research area.12−16 Recently, one-, two-, and three-armed thioureas 1−3 (Figure 1) were prepared and each of them was found to bind H2PO4−.17 The smaller two hosts use a single thiourea arm to coordinate this guest ion whereas the larger receptor makes use of all three thiourea substituents in a cooperative manner. As a result, the association equilibrium constant for 3 is large (i.e., ≥∼5 × 106 M−1) even in an unfavorable environment such as wet dimethyl sulfoxide (DMSO) with 0.5% water. This strong host−guest interaction is big enough that 3 can be used to extract phosphate from water into a nonpolar organic solvent (i.e., chloroform) despite the high heat of hydration and low basicity of this ion. This three-armed thiourea is also selective with respect to chloride (Cl−) and acetate (CH3CO2− or OAc−) even though these common phosphate interferents are © XXXX American Chemical Society

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EXPERIMENTAL SECTION Photoelectron Spectroscopy. Thioureas 1−3 were prepared as previously described.17 Compound 1 was dissolved in methanol whereas a dimethyl sulfoxide/methanol mixture was used for 2 and 3. These solutions were individually added to separate aqueous methanol mixtures containing NaX, where X = Cl, OAc, and H2PO4. Electrospray ionization of these ∼10−3 M solutions afforded the conjugate bases of all three thioureas and the M·X− cluster ions, where M = 1−3. Mass isolation of these species enabled photoelectron spectra of all 12 anions to be recorded at 20 K with a previously described Received: August 20, 2016 Revised: October 5, 2016

A

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Figure 1. One-, two-, and three-armed thiourea anion receptors 1−3.

instrument using a F2 excimer laser at 157 nm (7.867 eV).23 Each acquired spectrum was obtained at 20 Hz and shot-toshot background corrected. The resulting photoelectrons were collected and analyzed with a 5.2 m flight tube that was calibrated with I− and Cu(CN)2−. This led to spectra with a resolution of ∼50 meV for ∼2.5 eV electrons. Computational Methods. Initial AM1 structures were explored with Spartan24 and subsequently M06-2X geometry optimizations and vibrational frequencies were computed using either the 6-31+G(d,p) or aug-cc-pVDZ basis set.25−28 These calculations were carried out with the Gaussian 09 suite of programs at the Minnesota Supercomputer Institute for Advanced Computational Research (MSI).29 Single-point energies of the lowest energy conformers with the larger maug-cc-pVT(+d)Z basis set were also computed to obtain adiabatic and vertical detachment energies (i.e., ADEs and VDEs) at 0 K as well as cluster enthalpies of dissociation at 298 K.30 Unscaled vibrational frequencies were used for computing zero-point energies (ZPEs) and thermal corrections to 298 K. Geometries and energies of the resulting species are provided in the Supporting Information. Optimized anion structures were used as the starting geometries for the radicals, and in general, additional neutral structure searches were not carried out. Given the large size of the three-armed derivatives, analytical vibrational frequency computations on the corresponding doublet species was beyond the allocated resources at MSI and so numerical values were obtained. To facilitate the geometry optimizations of the three-armed thiourea cluster anions and neutrals, p-functions were initially used only for the NH hydrogens but thereafter the full 6-31+G(d,p) basis set was employed.

Figure 2. Photoelectron spectra at 157 nm (7.867 eV) of the conjugate bases of 1−3 at 20 K.

Sodium salts of chloride, acetate and dihydrogen phosphate were added to the aqueous methanol solutions of 1−3 to afford all nine cluster anions (i.e., 1·X−, 2·X−, and 3·X−, where X = Cl, OAc, and H2PO4). Their 20 K photoelectron spectra were also obtained at 157 nm (Figures 3−5), and the resulting ADEs and VDEs are provided in Table 1. M06-2X optimizations and vibrational frequencies were computed with the 6-31+G(d,p) or aug-cc-pVDZ basis set. The smaller of these was used for the larger two- and threearmed thioureas whereas the bigger Dunning basis set was employed for the one-armed thiourea and ancillary species (e.g., Cl−, OAc−, and H2PO4−).31 The lowest energy structures that were located for 1a−3a and the corresponding cluster ions of 1−3 are illustrated in Figures 6−9. Their Cartesian coordinates are provided in the Supporting Information along with images of 1−3, the corresponding radicals of 1a−3a (1r− 3r), and the nine neutral clusters (Figures S1−S5). Because the M06-2X functional typically gives more reliable thermochemical predictions with larger and more flexible basis sets, maug-

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RESULTS Deprotonated thioureas 1−3 were produced by electrospray ionization and their resulting photoelectron spectra at 20 K were recorded at 157 nm (Figure 2). Broad bands that correspond to formation of the radicals in their ground and excited states are observed in all three spectra, and the peak maxima of the lowest energy features were used to obtain the vertical detachment energies (VDEs). Upper limits for the adiabatic detachment energies (ADEs) were obtained from the onset region of the spectra and are given as the sums of the electron binding energy at the point where the signal is reliably above the background and the instrumental resolutions. The results are given in Table 1 and have estimated uncertainties of ±0.1 eV. B

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The Journal of Physical Chemistry A Table 1. Experimental and Computed ADEs and VDEs (eV) for the Conjugate Bases of 1−3 (i.e., 1a−3a) and Their Cl−, OAc−, and H2PO4− Cluster Ions M06-2Xa

expt cmpd 1a 2a 3a 1·Cl− 2·Cl− 3·Cl− 1·OAc− 2·OAc− 3·OAc− 1·H2PO4− 2·H2PO4− 3·H2PO4−

ADE 3.65 4.60 5.10 4.73 5.22 5.60 4.45 5.07 5.48 4.61 5.20 5.33

± ± ± ± ± ± ± ± ± ± ± ±

0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10

VDE 3.93 4.95 5.82 5.10 5.61 5.97 4.88 5.57 5.92 5.04 5.65 5.88

± ± ± ± ± ± ± ± ± ± ± ±

0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10

ADE 3.74 4.41 5.04 4.68 5.02 5.90 4.10 4.80 5.33 4.38 5.11 5.35

(3.78) (4.39) (5.08) (4.68) (5.08) (5.85) (4.13) (4.88) (5.32) (4.39) (5.11) (5.30)

VDE 4.15 4.79 5.48 5.20 5.68 6.04 4.85 5.57 5.92 5.03 5.71 5.90

(4.19) (4.77) (5.68) (5.27) (5.70) (6.06) (4.89) (5.55) (5.89) (5.06) (5.68) (5.87)

a

M06-2X/aug-cc-pVDZ or M06-2X/6-31+G(d,p) and M06-2X/maugcc-pVT(+d)Z single-point energies, where the latter values are given in parentheses. The smaller 6-31+G(d,p) basis set was used for 2a, 3a, and all of the two- and three-armed cluster ions (i.e., 2·X− and 3·X−).

Figure 4. Photoelectron spectra at 157 nm (7.867 eV) of 1·OAc−−3· OAc− at 20 K.

Figure 3. Photoelectron spectra at 157 nm (7.867 eV) of 1·Cl−−3·Cl− at 20 K.

cc-pVT(+d)Z single-point energies were also computed.30 Reference data along with predicted deprotonation enthalpies (ΔH°acid), bond dissociation energies (BDEs), ADEs and VDEs are provided in Table 2.32−38

Figure 5. Photoelectron spectra at 157 nm (7.867 eV) of 1·H2PO4−− 3·H2PO4− at 20 K.

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DISCUSSION All three thioureas that were examined afforded their conjugate bases upon electrospray ionization. The ADEs of these anions are 3.65 (1a), 4.60 (2a), and 5.10 (3a) eV whereas their corresponding VDEs are 3.93, 4.95, and 5.82 eV. These values are well reproduced by M06-2X/aug-cc-pVDZ or M06-2X/631+G(d,p) and M06-2X/maug-cc-pVT(+d)Z calculations.

They have average unsigned errors of 0.11 and 0.12 eV (ADE) and 0.24 and 0.19 eV (VDE), respectively. The ADEs and VDEs are also larger than the electron binding energies of strongly bound anions such as OAc− and Cl− indicating that 1a−3a are highly stabilized anions with respect to their corresponding radicals. The sequential ADE increases of 1.0 C

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Figure 7. M06-2X/aug-cc-pVDZ (1·Cl−) and 6-31+G(d,p) (2 and 3· Cl−) cluster ions. For clarity, hydrogen atoms attached to carbon are not displayed in 3·Cl−.

Figure 6. Computed M06-2X/aug-cc-pVDZ (1a) and 6-31+G(d,p) (2a and 3a) conjugate bases. For clarity, hydrogen atoms attached to carbon are not displayed in 3a.

hydrogen bond donating thiourea groups can stabilize both charged centers with two hydrogen bonds. Thioureas 1−3 are predicted to be surprisingly acidic with M06-2X/maug-cc-pVT(+d)Z deprotonation enthalpies of 317.8, 305.8, and 296.6 kcal mol−1, respectively (Table 2). These values indicate that all three compounds are more acidic than acetic acid, hydrochloric acid, and phosphoric acid, and that the larger two derivatives are also stronger than sulfuric acid (ΔH°acid = 309.6 kcal mol−1).32 This is undoubtedly due to the presence of two and four intramolecular hydrogen bonds in 2a and 3a, respectively, as interactions of this sort previously have been shown to be capable of providing very large stabilizations.39−45 The predicted gas-phase acidity of 3 appears

and 0.5 eV upon incorporation of a second and third thiourea arm strongly suggests that 2a and 3a are stabilized by intramolecular hydrogen bonds. This deduction is in accord with their computed structures (Figure 6). Both ions have two NH···N− interactions with hydrogen bond distances of 1.751 and 2.141 Å (2a) and 1.781 and 2.191 Å (3a). The deprotonated three-armed thiourea also has two NH···S− interactions with bond lengths of 2.314 and 2.594 Å. These additional hydrogen bonds arise because a deprotonated thiourea has two major resonance structures in which the formal charge resides on the deprotonated nitrogen or the sulfur atom (Figure 10). Consequently, 3a with its two D

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Figure 9. M06-2X/aug-cc-pVDZ (1·H2PO4−) and 6-31+G(d,p) (2 and 3·H2PO4−) cluster ions. For clarity, hydrogen atoms attached to carbon are not displayed in 3·H2PO4−.

Figure 8. M06-2X/aug-cc-pVDZ (1·OAc−) and 6-31+G(d,p) (2 and 3·OAc−) cluster ions. For clarity, hydrogen atoms attached to carbon are not displayed in 3·OAc−.

to explain why it protonates OAc− in DMSO and helps rationalize why monohydrogen phosphate (HPO42−) is extracted into chloroform from an aqueous solution of dihydrogen phosphate (eq 1).17

number of hydrogen bonds. The computed geometries (Figures 7−9) are in accord with this view and account for why X− does not appear to abstract a proton in these complexes even though it is a stronger base than the deprotonated thioureas by 13−52 kcal mol−1 (Table 2). That is, the presence of two to six hydrogen bonds lead to unusually large M·X− cluster energies (Table 3) that more than offset the acidity differences. For example, ΔH°acid(HOAc) − ΔH°acid(1) = 30 kcal mol−1 and given a typical cluster energy of 15 kcal mol−1 for 1a·HOAc; then this complex is 45 kcal mol−1 lower in energy than 1 and OAc− but still 6−9 kcal mol−1 less stable than 1·OAc−.

3(CHCl3) + 2H 2PO4 −(aq) → 3 ·HPO4 2 −(CHCl ) + H3PO4 (aq) 3

(1)

Both the adiabatic and vertical detachment energies increase sequentially with the number of arms in the thiourea for all three series of cluster ions. The experimental values are bigger than for the nonsolvated (free) anions as expected for M·X− clusters, where M = 1−3, that are stabilized by an increasing E

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The Journal of Physical Chemistry A Table 2. Experimental and Computed Acidities, Bond Dissociation Energies, and Electron Affinities for HX (X = Cl, OAc, and H2PO4) and Thioureas 1−3a

Table 3. Computed 298 K Cluster Dissociation Enthalpies (i.e., M·X− → M + X−) (kcal mol−1) M06-2X

M06-2Xb cmpd

aug-cc-pVDZ

ΔH°acid BDE ADE

328. 6 100.1 3.64

ΔH°acid BDE ADE VDE

347.3 112.0 3.34 3.87

ΔH°acid BDE ADE VDE

328.0 115.9 4.37 5.16

ΔH°acid BDE ADE VDE

317.1 90.9 3.74 4.15

ΔH°acid BDEj ADE VDE

305.8 87.8 4.41 4.79

ΔH°acid BDEj ADE VDE

296.8 91.8 5.07 5.48

maug-cc-pVT(+d)Z HCl 330.4 101.9 3.64 HOAc 346.7 113.3 3.43 3.97 H3PO4 328.1 116.4 4.34 5.29 1 317.8 92.4 3.78 4.19 2 305.8 87.4 4.39 4.77 3 296.6 90.9 5.04 5.68

expt 333.383 ± 0.002c 103.15 ± 0.03d 3.61272 ± 0.00003e 348.2 ± 1.4f 112 ± 3d 3.38 ± 0.05g 3.52 ± 0.05g

a

330.5 ± 5h 122 ± 6i 4.55 ± 0.10i 5.10 ± 0.10i

cluster

aug-cc-pVDZa

maug-cc-pVT(+d)Z

1·Cl− 1·OAc− 1·H2PO4− 2·Cl− 2·OAc− 2·H2PO4− 3·Cl− 3·OAc− 3·H2PO4−

45.9 53.5 45.6 58.1 63.5 61.1 71.8 75.4 70.9

44.7 51.2 44.7 59.9 63.6 60.5 73.0 74.8 68.5

The 6-31+G(d,p) basis set was used for the clusters with 2 and 3.

relative to the anion cluster and are 1.102 and 1.104 Å long whereas the (HO)2P(O)O−H distances are reduced by 0.305 and 0.292 Å and are now a short 1.458 and 1.455 Å. This results in delocalization of the unpaired electron on to the thiourea sulfur atom as indicated by its computed spin density of 0.89 e−. The ADEs of 2·H2PO4− and 3·H2PO4− also stand out in that they are the same within their experimental uncertainties (i.e., ΔADE = 0.13 ± 0.14 eV). Given that the cluster energy of 3· H2PO4− is larger than that for the corresponding two-armed thiourea derivative by 9.8 (M06-2X/aug-cc-pVDZ) and 8.0 (M06-2X/maug-cc-pVT(+d)Z) kcal mol−1, there must be a similar differential stabilization for the neutral complexes. Computations indicate that this is the case and that H2PO4• interacts more strongly with 3 than 2 by 5.5 (M06-2X/aug-ccpVDZ) and 4.8 (M06-2X/maug-cc-pVT(+d)Z) kcal mol−1. These results can be accounted for by the additional hydrogen bonds in the anionic and neutral complexes with the threearmed thiourea derivative. Our gas-phase results indicate that the anion affinities of thioureas 1−3 increase with the number of hydrogen bond donors in the host, and that they are related to the proton affinity of the guest ion even though other factors play a role too. These findings indicate that positive cooperativity occurs in the gas phase for the binding of Cl−, OAc−, and H2PO4−. This is different than what is observed in aqueous DMSO solutions where only the binding of H2PO4− by 3 operates in a cooperative manner and makes use of all three thiourea arms.17 A single thiourea moiety is used in the other cases, and this indicates that the solvent and/or counterion play a key role and must be responsible for this difference. Dynamics calculations exploring this effect are warranted and would be useful.

3.65 ± 0.10 3.93 ± 0.10

4.60 ± 0.10 4.95 ± 0.10

5.10 ± 0.10 5.82 ± 0.10

a Acidities and bond energies are in kcal mol−1, and ADEs and VDEs are in eV. bM06-2X/aug-cc-pVDZ or M06-2X/6-31+G(d,p) and M062X/maug-cc-pVT(+d)Z single-point energies are provided. The smaller basis set was used for the two- and three-armed thioureas. c Reference 32. dReference 33. eReference 34. fReference 35. g Reference 36. hReference 37. iReference 38. jMore stable radicals with different structures (conformers) than for 2a and 3a were used.

Figure 10. Resonance hybrids of a deprotonated thiourea.

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Dihydrogen phosphate is anomalous in that the ADE of 1· H2PO4− is within the experimental uncertainty of the value for H2PO4− (i.e., 4.61 ± 0.10 vs 4.55 ± 0.10 eV, respectively).38 This can be rationalized by invoking a hydrogen atom shift from a nitrogen to an oxygen atom upon photoejection of an electron from the cluster ion. Such a process is energetically more favorable than for the Cl• and OAc• complexes because the BDEs in kcal mol−1 follow the order: H−Cl (103.2) < H− OAc (112) < H−OPO3H2 (122).33,38 It is also predicted to be exothermic by 21 kcal mol−1, so it is not surprising that a hydrogen-shifted radical structure was located. A slightly more stable (∼1 kcal mol−1) nonrearranged complex was also found (Figure S5) and interestingly this species can take advantage of the large ΔBDE without transferring the hydrogen atom. Instead, the two N−H bond lengths are elongated by 0.064 Å

CONCLUSIONS Aromatic substrates with 1−3 thiourea groups were examined, and their conjugate bases and anionic clusters with Cl−, OAc−, and H2PO4− were found to be stabilized by intramolecular hydrogen bonds. This leads to large electron binding and cluster energies and surprisingly acidic compounds, two of which are predicted to have acidities that exceed sulfuric acid. Positive cooperativity is observed as the stability of these species increases with the number of thiourea substituents and hydrogen bonds. This contrasts with the anion binding behavior observed in solution where cooperativity is only observed in the molecular recognition of dihydrogen phosphate by the three-armed thiourea. Consequently, at least for these compounds, cooperative binding is dependent on the presence F

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(8) Hargrove, A. E.; Nieto, S.; Zhang, T.; Sessler, J. L.; Anslyn, E. V. Artificial Receptors for the Recognition of Phosphorylated Molecules. Chem. Rev. 2011, 111, 6603−6782. (9) See http://water.epa.gov/type/rsl/monitoring/vms56.cfm/; last updated 3/6/12. (10) Warwick, C.; Guerreiro, A.; Soares, A. Sensing and Analysis of Soluble Phosphates in Environmental Samples: A Review. Biosens. Bioelectron. 2013, 41, 1−11. (11) He, G.; Zhao, L.; Chen, K.; Liu, Y.; Zhu, H. Highly Selective and Sensitive Gold Nanoparticle-Based Colorimetric Assay for PO43− in Aqueous Solution. Talanta 2013, 106, 73−78. (12) Bansal, V. K. In Clinical Methods: The History, Physical, and Laboratory Examinations, 3rd ed.; Walker, H. K., Hall, W. D., Hurst, J. W., Eds.; Butterworths: Boston, 1990; Chapter 198, pp 895−899. (13) Bühlmann, P.; Pretsch, E.; Bakker, E. Carrier-Based IonSelective Electrodes and Bulk Optodes. 2. Ionophores for Potentiometric and Optical Sensors. Chem. Rev. 1998, 98, 1593−1687. (14) http://kidney.niddk.nih.gov/kudiseases/pubs/kustats/; NIH Publication No. 12-3895, last updated 11/15/12. (15) Malberti, F. Hyperphosphataemia: Treatment Options. Drugs 2013, 73, 673−688. (16) Food, Energy, and Water: Transformative Research Opportunities in the Mathematical and Physical Sciences; NSF: Washington, DC, July 2014; see http://www.nsf.gov/mps/advisory/mpsac_other_reports/ nsf_food_security_report_review_final_rev2.pdf). (17) Beletskiy, E. V.; Kass, S. R. Selective Binding and Extraction of Aqueous Dihydrogen-Phosphate Solutions via Three-Armed Thiourea Receptors. Org. Biomol. Chem. 2015, 13, 9844−9849. (18) For additional anion binding studies using thiourea receptors, see refs 19−22. (19) Bregovic, V. B.; Basaric, N.; Mlinaric-Majerski, K. Anion Binding with Urea and Thiourea Derivatives. Coord. Chem. Rev. 2015, 295, 80− 124. (20) Khansari, M. E.; Johnson, C. R.; Basaran, I.; Nafis, A.; Wang, J.; Leszczynski, J.; Hossain, M. A. Synthesis and Anion Binding Studies of Tris(3-aminopropyl)amine-based Tripodal Urea and Thiourea Receptors: Proton Transfer-induced Selectivity for Hydrogen Sulfate over Sulfate. RSC Adv. 2015, 5, 17606−17614. (21) Jeon, N. J.; Yeo, H. M.; Nam, K. C. Cooperative Anions Binding with Nitrophenyl Thiourea Derivative. Bull. Korean Chem. Soc. 2008, 29, 663−665. (22) Clare, J. P.; Ayling, A. J.; Joos, J.-B.; Sisson, A. L.; Magro, G.; Perez-Payan, M. N.; Lambert, T. N.; Shukla, R.; Smith, B. D.; Davis, A. P. Substrate Discrimination by Cholapod Anion Receptors: Geometric Effects and the ″Affinity-Selectivity Principle″. J. Am. Chem. Soc. 2005, 127, 10739−10746. (23) Wang, X. B.; Wang, L. S. Development of a Low-Temperature Photoelectron Spectroscopy Instrument Using an Electrospray Ion Source and a Cryogenically Controlled Ion Trap. Rev. Sci. Instrum. 2008, 79, 073108. (24) Spartan ’08 for Macintosh; Wavefunction, Inc.: Irvine, CA. (25) Zhao, Y.; Truhlar, D. G. How Well Can New-Generation Density Functionals Describe the Energetics of Bond-Dissociation Reactions Producing Radicals? J. Phys. Chem. A 2008, 112, 1095− 1099. (26) Zhao, Y.; Truhlar, D. G. The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Functionals. Theor. Chem. Acc. 2008, 120, 215−241. (27) Zhao, Y.; Truhlar, D. G. Density Functionals with Broad Applicability in Chemistry. Acc. Chem. Res. 2008, 41, 157−167. (28) Dunning, T. H., Jr. Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron Through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007−1023. (29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,

of a solvent and/or counterion. Further exploration of this conclusion is warranted as it has obvious implications in understanding biological processes and targeting new pharmaceuticals, as well as in the development of ion sensors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b08438. Computed structures (figures and xyz coordinates) and energies along with the complete citation to ref 29 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*X.-B. Wang. E-mail: [email protected]. Telephone number: 509-371-6132. *S. R. Kass. E-mail: [email protected]. Telephone number: 612625-7513. Present Address §

Scientific Design Company, 49 Industrial Ave., Little Ferry, NJ 07643. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Generous support from the National Science Foundation (CHE-1361766) and the Minnesota Supercomputer Institute for Advanced Computational Research are gratefully acknowledged. The photoelectron spectra work was supported by U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, the Division of Chemical Sciences, Geosciences, and Biosciences, and was performed at the EMSL, a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory, which is operated by Battelle Memorial Institute for DOE.

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REFERENCES

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DOI: 10.1021/acs.jpca.6b08438 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpca.6b08438 J. Phys. Chem. A XXXX, XXX, XXX−XXX