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Exploring Gas-Phase Ion-Ionophore Interactions: Infrared Spectroscopy of Argon-Tagged Alkali Ion-Crown Ether Complexes† Jason D. Rodriguez,‡ Dongwook Kim,§ Pillarisetty Tarakeshwar,| and James M. Lisy*,‡ Department of Chemistry, UniVersity of Illinois at Urbana-Champaign, Urbana, Illinois 61801, School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, and Department of Chemistry, UniVersity of NeVada Las Vegas, 4505 Maryland Parkway, Las Vegas, NeVada 89154 ReceiVed: August 13, 2009; ReVised Manuscript ReceiVed: NoVember 5, 2009
Argon-tagged alkali metal ion-crown ether complexes were generated in the gas phase and investigated using a combination of infrared predissociation (IRPD) spectroscopy, density functional, and symmetry-adapted perturbation theory (SAPT). The IRPD spectra of M+(12-crown-4 ether)Ar and M+(18-crown-6 ether)Ar, where M ) Li, Na, K, Rb, and Cs, were collected in the CH stretching region, using the argon-messenger technique. The gas-phase neutral vibrations for each crown ether shift to higher frequency when complexed to an alkali metal ion. Similarities in the experimental IRPD spectra of Li+(12-crown-4 ether)Ar and Li+(18crown-6 ether)Ar indicate that the binding of Li+ is similar for both crown ethers. In the 12-crown-4 ether systems, there are trackable changes with ion size in the IRPD spectra until the point is reached where the ion is too large to bind to the interior of the macrocycle. Starting with Na+, the 18-crown-6 spectra vary only slightly as the ion size is increased. The overall profiles of the IRPD spectra indicate that a similar configuration of the complexes is adopted for the ions larger than Na+. The calculated SAPT interaction energies mirror the trends exhibited by the IRPD spectra and provide interesting insights on the roles of the different interaction energy components in binding the cation to the crown-ether. Introduction Ionophores are specialized molecules that bind and transport ions selectively. Thus, the study of ionophores is important in gaining insight into the mechanism of ion binding and transport.1,2 While ionophores have been the topic of several condensedphase studies, little is known about the microscopic-level interactions present in these systems. Since many naturally occurring ionophores are large, complex systems, studying them directly via conventional gas-phase techniques and quantum chemical calculations is challenging. One useful alternative is to use smaller model ionophores such as the crown ethers.1 Since their discovery by Pederson3,4 in the 1960s, crown ethers have become one of the most important classes of compounds in macrocyclic chemistry. Since crown ethers have the ability to bind certain ions preferentially, they are amenable to a wide variety of applications such as drug delivery,5,6 nanotechnology,7 and environmental applications.8 Insight gained from studying crown ether systems at the molecular level may prove useful in both expanding the applications of crown ethers and the design and development of new, more efficient ionophores and crown ethers.9 The selectivity trends associated with certain crown ethers in solution was noted by Pedersen10 and Izatt.11-13 Based on initial measurements, one of the most common models used to explain condensed-phase crown ether selectivity is the “bestfit” model.10 According to this model, crown ethers are predicted to preferentially bind ions with sizes that match well with the †
Part of the “W. Carl Lineberger Festschrift”. * Corresponding author. Tel: (217) 333-2898. Fax: (217) 244-3186. E-mail:
[email protected]. ‡ University of Illinois at Urbana-Champaign. § Georgia Institute of Technology. | University of Nevada Las Vegas.
crown ether cavity. The two crown ethers used in this study, 12-crown-4 ether (12c4) and 18-crown-6 ether (18c6), are thus predicted to be selective for Li+ and K+, respectively. Subsequent work in the condensed phase seemed to contradict14 this model and found it to be deficient in certain instances where the crown ether systems are flexible.15 Gas-phase experiments by Brodbelt16,17 Dearden,18-21 and Armentrout22-26 on binary alkali metal ion crown ether systems also did not replicate the “best-fit” selectivity trends for these systems. Rather, it was found that the crown ethers bound strongest to the smaller metal ions that are able to fit inside the cavity and maximize the overall crown ether · · · M+ interaction. Theoretical binding enthalpies calculated by Glendening et al.27 also found that the smaller ions tended to bind crown ethers strongly. In the case of 18c6, which is known to be selective for K+, the calculations indicated that order of gas-phase 18c6 binding energies for the alkali metal family decreased according to size:27 Li+ > Na+> K+ > Rb+ > Cs+. Recently, there have been further efforts to study alkali metal ion-crown ether systems using both theoretical27-30 and experimental techniques.31-34 In this paper, we report our efforts to study M+(12c4) and M+(18c6) systems in the gas phase, where M ) Li, Na, K, Rb, and Cs, using a combination of infrared predissociation spectroscopy (IRPD) and quantum chemical calculations to track size-selective behavior in the two crown ethers. Reports on hydrated alkali metal ion-crown ether are reported elsewhere.31,32 Experimental Section Since our experimental apparatus has been reported in detail elsewhere,35 only a brief overview is given here. Ion-host complexes of M+(12c4) and M+(18c6) are generated in the gas
10.1021/jp907838r 2010 American Chemical Society Published on Web 12/02/2009
Spectroscopy of Alkali Metal Ion Crown Ether Complexes phase by the collision of an alkali metal ion, ejected from a custom ion gun, with a fully expanded neutral molecular beam containing 12c4Arn or 18c6Arm clusters. The clusters are formed by expanding a mixture of Ar/12c4 or Ar/18c6 through a 180 µm diameter conical nozzle (backing pressure ∼550-620 Torr). Both crown ethers are heated (30-35 °C for 12c4 and 80-100 °C for 18c6) in a small holding cell just before the nozzle where they are mixed and collected by a stream of argon carrier gas (∼50 ppm of 12c4 in Ar). Nascent cluster ions stabilize solely via evaporation36 of Ar. The cluster ions are then passed through a 2.0 mm skimmer into a differentially pumped intermediate chamber. While in the intermediate chamber, ions are collected and guided by a 9.5 cm octapole ion guide and a series of electrostatic aperture lenses before entering the detection chamber that contains a triple-quadrupole mass spectrometer. In the first quadrupole, the parent cluster ion of interest is mass selected. The mass-selected parent cluster is then irradiated by a tunable IR laser (Laser Vision OPO/OPA pumped by a 10 Hz Nd:YAG laser) in the second, RF-only, quadrupole. When on resonance with a vibrational mode in the cluster, a photon is absorbed and photodissociation occurs if the combination of photon energy and internal energy of the cluster exceeds the binding energy of the most labile ligand, which in this case is argon with a binding energy estimated to be less than 5 kJ/mol based on our calculations. The third quadrupole mass-analyzes the fragment ions generated via photodissociation and the IR action spectrum is collected as a function of laser frequency. Absolute frequency calibration ((4 cm-1) is achieved in the C-H stretching region by simultaneously collecting the IR spectrum of gaseous HCl, which is held inside a custom reference cell. Where noted, the IRPD spectra have been smoothed using a three-point adjacent averaging algorithm. Calculations Quantum chemical calculations were carried out on M+(12c4) and M+(18c6) systems to help characterize our experimental results. Starting geometries were generated with Spartan37 and full geometry optimizations were done at the B3LYP/6-31+G* level of theory using Gaussian 03.38 Harmonic vibrational frequency calculations were also done at the B3LYP/6-31+G* level of theory using the fully optimized geometries. In both the geometry and vibrational frequency calculations, LANL2DZ effective-core potentials39-41 were used for K, Rb, and Cs atoms. As in our previous study on the K+(18c6)(H2O)1Ar1-4 system,31 argon is not included in the structures reported here. Calculations including the Ar atom were performed to primarily determine binding energies and indicated that argon does not have a significant impact on the optimized geometries or vibrational frequencies of the M+(crown-ether) complexes. Zero-point binding energies (the inverse of interaction energies) were calculated using the supermolecular method42 (SM), wherein the interaction energy is evaluated as the difference of the energy of the complex and the energy of the isolated monomers. Simulated IR spectra were generated with SWizard43 using 15 cm-1 fwhm Lorentzian line shapes to model the experimental spectra and are based on the harmonic vibrational frequencies that have been scaled by 0.955 and 0.969 for 12c4 and 18c6 systems, respectively. The scaling factors for each system were determined by comparing the calculations to the experimental results and are consistent with scaling factors reported in our previous studies.31,44 Visual representations of M+(12c4) and M+(18c6) optimized geometries were generated using Molden.45 Even though conventional supermolecular quantum-chemical calculations27-30 have been useful in obtaining the lowest energy
J. Phys. Chem. A, Vol. 114, No. 3, 2010 1515 structures, and corresponding vibrational frequencies, they do not offer insights into the relative magnitudes of the various forces (electrostatic, inductive, dispersive) responsible for the interaction between the metal cation and the crown-ether. The perturbation method enables one to obtain a more conceptual picture because the interaction energy is obtained as a sum of the electrostatic, exchange, dispersion, and induction energies. In the present study, we employed the symmetry adapted perturbation theory (SAPT) method to evaluate the magnitude of each interaction energy component. All the calculations were carried out using the optimized geometries (obtained from SM calculations) of the complexes. The SAPT interaction (Eint) is given by eq 1 (1) (2) (2) (2) (2) HF Eint)Eelst +E(1) exch+Eind+Eexch-ind+Edisp+Eexch-disp + δint (1) (1) where Eelst is the electrostatic energy of the monomers with the unperturbed electron distribution, E(1) exch is their first-order valence repulsion energy due to the Pauli exclusion principle, E(2) ind stands for the second-order energy gain resulting from the induction (2) represents the repulsion change due to the interaction, Eexch-ind electronic cloud deformation, E(2) disp is the second-order dispersion (2) denotes the second-order correction for a energy, Eexch-disp coupling between the exchange repulsion and the dispersion HF includes the higher order induction and interaction, and δint exchange corrections. Since BSSE effects are explicitly included when evaluating the SAPT interaction energies, a comparison of the BSSE corrected supermolecular interaction energy and the SAPT interaction energy (Eint) is appropriate. To aid the discussion of the results, we have collected all similar interaction energy terms and represent the SAPT interaction energy (Eint) as a sum of electrostatic (Ees), induction (Eind), dispersion (Edisp), and exchange (Eexch) terms. Given the size of the systems investigated and the level of theory employed in this study to evaluate the various energy components, it was not feasible to evaluate the computationally demanding higher order components. A detailed description of SAPT and some of its applications are described in refs 46-50.
Results and Discussion The M+(12c4) and M+(18c6) optimized geometries using the space-filling representations are shown in Figure 1. The gasphase neutral geometries for free 12c4 and 18c6 are also shown for comparison. Both 12c430 and 18c627 complexes with the alkali metal ion series have been studied in detail from a theoretical perspective, so only a brief overview of our computational results is given here. Since the cavity diameter51 of 12c4 is only 1.2-1.5 Å, it is only able to accommodate Li+ (ionic diameter51 1.36 Å) within its cavity. This is shown in both top and side views, with Li+ adopting a symmetric orientation inside the 12c4 cavity. The Li+(18c6) case is different, adopting an asymmetric orientation, as Li+ is too small to fill the 18c6 cavity, which has a diameter51 of 2.6-3.2 Å. Starting with Na+, the remaining M+(12c4) complexes feature the M+ lying above the macrocycle since the ions are too large to fit inside the cavity. Although for Li+(12c4) the crown ether is slightly distorted from a planar configuration, the 12c4 structure begins to mirror the gas-phase neutral conformation as the ion size is increased. This indicates that the 12c4 · · · M+ interaction is weakening with increasing cation size. This is shown in Figure 2, where the zero-point binding energies decrease as the size of the ion is increased. The magnitude of the SAPT interaction energies (Table 1) indicate that the
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Figure 1. Space-filling representation of gas-phase neutral 12c4 and 18c6 and M+(12c4) and M+(18c6) complexes based on the B3LYP/6-31+G* geometry optimizations. The structures show how the crown ether configurations change as the ion size is increased.
Figure 2. Zero-point binding energies (the inverse of interaction energies) of the crown ether in M+(12c4) and M+(18c6) complexes. The binding energies are calculated using the supermolecule method (ref 42) and based on the harmonic vibrational frequency calculations carried out at the B3LYP/6-31+G* level of theory.
weakening of the 12c4 · · · M+ interaction results from a decrease in both the electrostatic and induction energies. The presence of the metal cation outside the crown cavity also results in a decrease in the repulsive exchange energies. These trends are consistent with previous calculations of alkali metal ion-12c430 and alkali metal ion-18c6 complexes.27 The last two columns of Table 1 pose the hypothetical situation where two of the larger ions, K+ and Cs+, are constrained to the center of the 12c4 cavity. While there are large increases in the attractive electrostatic and induction energies, the rapid increase in the repulsive
exchange energy easily overwhelms these two, leading to a repulsive total interaction energy. Thus the overall geometries are driven more by a reduction in the repulsive exchange interaction, than by an increase in the attractive electrostatic and induction interactions. The situation for the M+(18c6) complexes is slightly different since the crown ether is able to accommodate Na+ and K+ as well as Li+ within its cavity. The smaller ions, Li+ and Na+, prefer to bind inside the 18c6 cavity in an asymmetric fashion, while K+ prefers to occupy a symmetric binding site inside 18c6. Starting with Rb+, the ion becomes too large to fit inside the 18c6. The zero-point binding energy trends for 18c6, also shown in Figure 2, reveal that as the ion size increases the 18c6 · · · M+interaction is also weakened. As expected, the reduction in the 18c6 · · · M+interaction is not as drastic as in the 12c4 · · · M+ interaction since the larger crown ether has additional macrocyclic oxygens that serve to increase its flexibility and to optimize the favorable interactions between the ion and the etheric oxygens. This leads to greater crown ether binding energies for 18c6 as compared to 12c4. The one exception is Li+, where both 12c4 and 18c6 have essentially identical binding energies. This is because the small size of Li+ limits the extent that it can interact with 18c6 oxygens. In this sense the binding of Li+(18c6) is very similar to that for +(12c4), being able to coordinate with only 3 or 4 macrocyclic oxygens. The IRPD spectra for Li+(12c4)Ar and Li+(18c6)Ar are shown in Figure 3A. Since the binding energies for the crown ethers, shown in Figure 2, exceed the available photon energy (∼35 kJ/mol) plus internal energy in the clusters, the spectra for all complexes in this study were acquired monitoring the
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TABLE 1: SAPT Interaction Energy Components (kJ/mol) of the Lowest-Energy Conformers of the (12c4)-M+ Complexes Obtained Using the 6-31+G* Basis Seta Eint Ees Eexch Eind Edisp a
Li
Na
K
Rb
Cs
K/center
Cs/center
-439.66 -374.84 96.16 -156.08 -4.90
-324.52 -311.44 90.50 -99.18 -4.36
-237.03 -238.79 75.96 -60.21 -13.95
-194.12 -202.96 63.35 -46.97 -7.58
-167.73 -177.49 58.37 -38.42 -10.18
256.41 -751.10 1313.72 -251.91 -54.30
289.88 -1718.86 3933.38 -1798.97 -125.66
Eint ) Ees + Eexch + Eind + Edisp.
loss of Ar, which has a binding energy of ∼0.5-5 kJ/mol in these complexes. The spectra for Li+(12c4)Ar and Li+(18c6)Ar share the same overall spectral profile, indicating that the bindings of Li+ by both crown ethers are similar. A recent comprehensive theoretical analysis found that neutral free 12c422,52 and 18c653 have S4 and S6 symmetries, respectively. Upon binding Li+, the configuration of 12c4 is altered and the
Figure 3. (A) IRPD comparison of Li+(12c4)Ar and Li+(18c6)Ar. The common spectral features between the two systems reveals that the bindings of Li+ by the crown ethers are similar despite the differences in macrocyclic ring size. (B) Top and side views of Li+(12c4) and Li+(18c6) conformers optimized at the B3LYP/6-31+G* level of theory.
Li+(12c4) system is reported to have Cs symmetry.30 The Li+(18c6) system is reported to retain its S6 symmetry27 with the Li+ fully enclosed by the etheric oxygens. As shown in the structures in Figure 3B, which have been labeled for easy identification for our discussion, the predicted conformers show that Li+ prefers to occupy an almost central binding site within the 12c4 cavity in the Li+(12c4) complex with two of the macrocyclic oxygens slightly puckered toward Li+ (O atoms between methylene groups 2,3 and 6,7 in Figure 3B). This structure is consistent with the previously reported lowest-lying energy configuration for Li+(12c4).30 In the Li+(18c6) complex, the Li+ prefers to bind in an asymmetric site inside of the 18c6 cavity, closely coordinated to three etheric oxygens. This causes a slight bending in the 18c6 backbone. The structure found for Li+(18c6) is similar to the previously reported configuration for Na+(18c6) with C1 symmetry.27 Overlap between the symmetric and asymmetric vibrations of individual methylene groups in the CH stretching region makes assignment of the IR features to particular methylene units difficult. However, with the aid of DFT calculations we can assign some of the features in the IRPD spectra. The comparison between experiment and calculation is shown in Figure 4 for Li+(12c4)Ar and Li+(18c6)Ar systems. Figure 4A shows the Li+(12c4)Ar experimental spectrum along with the simulated IR spectra based on the harmonic vibrational frequency calculations for deuterated and nondeuterated Li+(12c4). In the deuterated systems, the hydrogens of the methylene groups closest to Li+ (2, 3, 6, 7) have been replaced with deuterium. The simulated spectra in the CH stretching region of the partially deuterated d8-12c4 (d-Calc.) are then expected to readily reproduce the features of the lesser-perturbed methylene groups (from carbons 1, 4, 5, and 8) since the vibrations of the CD2-methylene groups closest to the ion have been shifted to the CD stretching region, ∼2100 cm-1. This lifts some of the ambiguity in the nondeuterated calculation and allows us to assign peaks to particular groups of methylene units. The feature at ∼2895 cm-1 is present in both nondeuterated and deuterated calculations and is due to the symmetric stretches of methylene groups that are more weakly impacted by Li+, although these bands are shifted to higher frequency, when compared to the gas phase,54 by 34 cm-1. These bands are associated with the four 12c4 methylene groups (1, 4, 5, 8) nearest to the ether oxygens that are not puckered toward Li+. The symmetric vibrations of the methylene units most perturbed by the ion binding (2, 3, 6, 7) are shifted even higher in frequency and are responsible for the feature observed in the experimental spectrum at 2935 cm-1. The asymmetric stretches of the lesser perturbed methylene groups (2, 3, 6, 7) give rise to the feature at 2956 cm-1, while the band at 2988 cm-1 is due to the 12c4 methylene groups closest to Li+(2, 3, 6, 7). So while all of the CH stretching modes are shifted to higher frequency, those modes associated with methylene units adjacent to the most tightly binding oxygens have the greatest shifts.
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Figure 4. Experiment and computational comparison for (A) Li+(12c4)Ar and (B) Li+(18c6)Ar systems. The calculations were done at the B3LYP/6-31+G* level of theory and the harmonic vibrational frequencies scaled with a factor of 0.955 and 0.969 for the 12c4 and 18c6 systems, respectively. To lift some of the overlap between the methylene groups in the calculation (Calc.), a second calculation was performed (d-Calc.) for each system, where the hydrogens of methylene groups closest to Li+ were replaced with deuterium.
The same methodology applied to the Li+(18c6)Ar system is shown in Figure 4B. However, the Li+(18c6)Ar system, with four additional methylene groups, is a bit harder to characterize since the experimental spectra are broader. Based on comparisons between the nondeuterated and deuterated calculations, the only straightforward assignment belongs to the feature located at ∼2980 cm-1, which is due to the asymmetric stretching vibrations of the methylene groups adjacent to the oxygens positioned nearest to Li+ (1, 2, 3, 8, 9, 10, 11, 12). Assigning the remaining features in the Li+(18c6)Ar spectrum is less certain since the simulated IR spectrum based on the d16-18c6 calculation is not significantly different from that of the nondeuterated calculation except for a less-pronounced shoulder ∼2900 cm-1 in the deuterated spectrum. However, based on the results for Li+(12c4)Ar discussed above, the feature at ∼2895 cm-1 in the Li+(18c6)Ar spectrum is likely due to the symmetric stretches of the lesser-perturbed methylene groups. Those that are influenced more by the presence of the ion such as 1, 10, 11, 12 would be expected to be shifted to higherfrequency into the broad feature at ∼2940 cm-1, which likely has significant contributions from both symmetric and asymmetric methylene vibrations. The remaining M+(12c4)Ar and M+(18c6)Ar spectra are shown in Figure 5A,B, respectively. The gas-phase neutral
Rodriguez et al. values for free 12c4 and 18c654 are given as a reference in both sets of spectra. The reference values listed are adapted from the NIST data54 by fitting the experimental reference spectra, which contain only relatively broad features, with Lorentzian line shapes in Origin 7.5 (OriginLab, Northampton, MA) for both 12c4 and 18c6. The entire M+(12c4)Ar series appears to have IR features that are, in general, shifted to higher frequencies compared to the gas-phase neutral values.54 However, as the size of the ion is increased, the M+(12c4)Ar features in our IRPD experiments trend back toward the gas-phase neutral values. As shown in Figure 5, starting with K+, two new features begin to appear, a weak feature at 2785 cm-1 and a more prominent feature at 2844 cm-1. Both of the bands are due to the symmetric stretch of the methylene groups adjacent to the 12c4 oxygen lying furthest (3.052 Å) from K+, with H-K+ distances of 2.987 and 3.251 Å. This direct interaction is only possible when the ion is above the plane of the crown ether. The simulated spectrum and corresponding calculated structure for K+(12c4) are shown in Figure 6 alongside the experimental spectrum. Note that the two calculated bands labeled “a” and “b”, at 2815 and 2850 cm-1, have no counterpart in the Li+(12c4) calculated spectrum (in Figure 4A) and are shifted to lower frequency when compared to the symmetric stretch of gas-phase 12c4. This shift to lower frequency of CH stretching vibrations has been previously observed55 for M+(cyclohexane), with M ) Li, Na, and K, where the ions sit above the carbon framework and directly interact with three of the axial CH groups. The calculated H-K+ distance of 2.75 Å, in K+(cyclohexane) is close to that in K+(12c4), and shifts to lower frequency of ∼25 and 50 cm-1 were reported for the CH stretching modes, relative to neutral cyclohexane. In a similar manner, experimental CH stretches observed at 2844 and 2785 cm-1 for K+(12c4) are shifted to lower frequencies of 17 and 76 cm-1, respectively relative to neutral 12c4. Although the lower frequency experimental band is not well-resolved for Rb+ and Cs+, the higher frequency CH stretch is readily observed and less shifted with increasing ion size. This suggests that the ion-H interaction weakens with increasing ion size, as the M+-H distance increases. The M+(18c6)Ar spectra shown in Figure 5B are much broader than the M+(12c4)Ar spectra. The feature previously attributed mostly to the symmetric stretch of the less-perturbed 18c6 methylene groups (located at 2895 cm-1 for Li+(18c6)Ar) drops significantly in relative intensity starting with Na+(18c6)Ar and shifts toward the gas-phase value54 of 2872 cm-1 as the size of the ion increases. This indicates that the Li+(18c6) conformation is unique in comparison with the other alkali metal ions. As shown in Figure 1, the ions prefer to lie close to the central site of the 18c6 macrocycle as the ion size increases with less distortion of the ether. This is in contrast to the case of Li+, where the crown ether “wraps” the ion. For Na+ and K+, the ions are able to lie inside the cavity without significant distortion of the neutral 18c6 geometry. For Rb+ and Cs+, they are simply too large to fit inside the cavity, and lie directly above the cavity. There is also a significant reduction in the intensity of the highest frequency bands observed in Li+(18c6)Ar, which appear at 2977 and 2985 cm-1, providing further support that the Li+(18c6) configuration is unique and the remaining M+(18c6)Ar complexes share similar 18c6 configurations. The experimental and computational comparison for K+(18c6) is shown in Figure 7. The calculation predicts that K+ binds in a symmetric position inside the 18c6 cavity with identical K+sO18c6 distances (2.827 Å). The two features in the simulated harmonic IR spectrum for K+(18c6) located at
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Figure 5. IRPD spectra for (A) M+(12c4)Ar and (B) M+(18c6)Ar complexes in the 2700-3100 cm-1 region. The M+(18c6)Ar spectra have been treated using a three-point adjacent averaging smoothing algorithm. The gas-phase neutral values for the unperturbed 12c4 and 18c6 vibrations are shown (red dotted line).
with CH2 bend overtones via Fermi resonances. The M+(18c6)Ar spectra for Rb+ and Cs+ are similar to the experimental spectrum for K+(18c6)Ar although there are slight differences in the relative intensities of some features. Conclusions
Figure 6. Experiment and computational comparison for K+(12c4)Ar showing the optimized structure and corresponding simulated frequency calculation based on the harmonic vibrational frequency calculation (B3LYP/6-31+G*). The calculated frequencies are scaled by 0.955. The structure and simulated spectrum show two CH oscillators that give rise to bands at 2785 and 2844 cm-1 in the experimental spectrum.
Figure 7. Experiment and computational comparison for K+(18c6)Ar showing the optimized structure and corresponding simulated frequency calculation based on the harmonic vibrational frequency calculation (B3LYP/6-31+G*). Calculated frequencies are scaled by 0.969.
2925 and 2962 cm-1 adequately reproduce the two broad features at ∼2887 and 2920 cm-1 and are assigned to the symmetric and asymmetric methylene vibrations, respectively, of 18c6. The broadness in both features is likely due slight differences in the orientation of the individual methylene groups. Additional smaller features may be due to vibrational coupling
There is no indication of unique behavior for Li+ in the CH stretching region of the spectrum for Li+(12c4)Ar or K+ in K+(18c6)Ar that would seem to explain the condensed-phase affinity of 12c4 for Li+ and 18c6 for K+. However, these results do indicate that the binding of the alkali metal ions in M+(12c4) and M+(18c6) systems depend on sizesof both the ion and crown ether. When the ion is able to fit inside the crown ether, as in Li+(12c4)Ar and Li+(18c6)Ar, the IRPD spectra indicate a similar binding motif. When the ion is much too large to fit inside the macrocycle, as in the case of the 12c4 with Rb+ and Cs+, the Rb+(12c4)Ar and Cs+(12c4)Ar spectra are essentially identical. The IRPD spectra for the alkali metal ions (other than Li+) with the larger crown ether, 18c6, suggest that the complexes have similar configurations. While no spectral signature has been detected to explain condensed-phase selectivity, this study has proven useful in providing insight into the fundamental crown ether · · · M+ binding interaction. The effect of microhydration likely holds the key26 to uncovering the origin of condensed-phase selectivity. A first step in the spectroscopic characterization of this selectivity was made with the argontagged hydrated K+(18c6) system.31 More will follow. Acknowledgment. We thank the National Science Foundation (Grants CHE-0415859 and CHE-0748874) for partial support of this research. Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research. Computational work was done on NCSA Cobalt Supercomputer System (Award no. TGCHE070097). J.M.L. expresses his appreciation of the scientific contributions and leadership by W. Carl Lineberger in the field of gas-phase cluster spectroscopy and dynamics. References and Notes (1) Noskov, S. Y.; Roux, B. Biophys. Chem. 2006, 124, 279. (2) Gokel, G. W.; Daschbach, M. M. Coord. Chem. ReV. 2008, 252, 886. (3) Pedersen, C. J. J. Am. Chem. Soc. 1967, 89, 2495. (4) Izatt, R. M. Chem. Soc. ReV. 2007, 36, 143. (5) Marjanovic, M.; Kralj, M.; Supek, F.; Frkanec, L.; Piantanida, I.; Smuc, T.; Tusek-Bozic, L. J. Med. Chem. 2007, 50, 1007.
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(6) Ju, X.-J.; Liu, L.; Xie, R.; Niu, C. H.; Chu, L.-Y. Polymer 2009, 50, 922. (7) Han, W.; Lee, Y.; Jung, K.; Ly, S.; Hong, T.; Kim, M. J. Anal. Chem. 2008, 63, 987. (8) Horwitz, E. P.; Dietz, M. L.; Fisher, D. E. SolVent Extr. Ion Exch. 1991, 9, 1. (9) Singh, N. J.; Olleta, A. C.; Kumar, A.; Park, M.; Yi, H. B.; Bandyopadhyay, I.; Lee, H. M.; Tarakeshwar, P.; Kim, K. S. Theor. Chem. Acc. 2006, 115, 127. (10) Pedersen, C. J.; Frensdorff, H. K. Angew. Chem., Int. Ed. 1972, 11, 16. (11) Izatt, R. M.; Terry, R. E.; Nelson, D. P.; Chan, Y.; Eatough, D. J.; Bradshaw, J. S.; Hansen, L. D.; Christensen, J. J. J. Am. Chem. Soc. 1976, 98, 7626. (12) Izatt, R. M.; Rytting, J. H.; Nelson, D. P.; Haymore, B. L.; Christensen, J. J. Science 1969, 164, 443. (13) Izatt, R. M.; Pawlak, K.; Bradshaw, J. S.; Bruening, R. L. Chem. ReV. 1995, 95, 2529. (14) Michaux, G.; Reisse, J. J. Am. Chem. Soc. 1982, 104, 6895. (15) Gokel, G. W.; Goli, D. M.; Minganti, C.; Echegoyen, L. J. Am. Chem. Soc. 1983, 105, 6786. (16) Maleknia, S.; Brodbelt, J. J. Am. Chem. Soc. 1992, 114, 4295. (17) Brodbelt, J. S.; Liou, C.-C. Pure Appl. Chem. 1993, 65, 409. (18) Zhang, H.; Chu, I. H.; Leming, S.; Dearden, D. V. J. Am. Chem. Soc. 1991, 113, 7415. (19) Zhang, H.; Dearden, D. V. J. Am. Chem. Soc. 1992, 114, 2754. (20) Dearden, D. V.; Zhang, H.; Chu, I. H.; Wong, P.; Chen, Q. Pure Appl. Chem. 1993, 65, 423. (21) Chu, I. H.; Zhang, H.; Dearden, D. V. J. Am. Chem. Soc. 1993, 115, 5736. (22) Ray, D.; Feller, D.; More, M. B.; Glendening, E. D.; Armentrout, P. B. J. Phys. Chem. A 1996, 100, 16116. (23) More, M. B.; Ray, D.; Armentrout, P. B. J. Phys. Chem. A 1997, 101, 4254. (24) More, M. B.; Ray, D.; Armentrout, P. B. J. Phys. Chem. A 1997, 101, 7007. (25) More, M. B.; Ray, D.; Armentrout, P. B. J. Am. Chem. Soc. 1999, 121, 417. (26) Armentrout, P. B. Int. J. Mass Spectrom. 1999, 193, 227. (27) Glendening, E. D.; Feller, D.; Thompson, M. A. J. Am. Chem. Soc. 1994, 116, 10657. (28) Abdoul-Carime, H. J. Chem. Soc., Faraday Trans. 1998, 94, 2407. (29) El-Azhary, A. A.; Al-Kahtani, A. A. J. Phys. Chem A 2005, 109, 8041. (30) Al-Rusaese, S.; Al-Kahtani, A. A.; El-Azhary, A. A. J. Phys. Chem A 2006, 110, 8676.
Rodriguez et al. (31) Rodriguez, J. D.; Lisy, J. M. Int. J. Mass Spectrom. 2009, 283, 135. (32) Rodriguez, J. D.; Vaden, T. D.; Lisy, J. M. J. Am. Chem. Soc., in press. (33) Peiris, D. M.; Yang, Y.; Ramanathan, R.; Williams, K. R.; Watson, C. H.; Eyler, J. R. Int. J. Mass Spectrom. 1996, 157-158, 365. (34) Sobott, F.; Kleinekofort, W.; Brutschy, B. Anal. Chem. 1997, 69, 3587. (35) Lisy, J. M. Int. ReV. Phys. Chem. 1997, 16, 267. (36) Klots, C. E. J. Chem. Phys. 1985, 83, 5854. (37) Deppmeier, B. J., et al. SPARTAN 2002; SPARTAN 2002 SGI IRIX 64 (mips4) ed.; Irvine, CA, 2002. (38) Frisch, M. J.; et al. Gaussian 03, B.04 ed.; Gaussian, Inc.: Pittsburgh, PA, 2003. (39) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (40) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (41) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (42) Sapse, A. M.; Russell, C. S. Int. J. Quantum Chem. 1984, 26, 91. (43) Gorelsky, S. I. SWizard program; CCRI, University Of Ottawa, Ottawa, Canada, 2008; http://www.sg-chem.net/. (44) Rodriguez, J. D.; Lisy, J. M. J. Phys. Chem. A 2009, 113, 6462. (45) Schaftenaar, G.; Noordik, J. H. J. Comput.-Aided Mol. Design 2000, 123. (46) Jeziorski, B.; Moszynski, R.; Szalewicz, K. Chem. ReV. 1994, 94, 1887. (47) Szalewicz, K.; Jeziorski, B. Molecular Interactions - From Van der Waals to Strongly Bound Complexes 1997, 3. (48) Jeziorski, B.; Moszynski, R.; Ratkiewicz, A.; Rybak, S.; Szalewicz, K.; Williams, H. L. SAPT: A program for many-body symmetry-adapted perturbation theory calculations of intermolecular interactions. In Methods and Techniques in Computational Chemistry: METECC-94; STEF: Cagliari, Italy, 1993; Vol. B, p 79. (49) Tarakeshwar, P.; Kim, K. S.; Kraka, E.; Cremer, D. J. Chem. Phys. 2001, 115, 6018. (50) Kim, D.; Hu, S.; Tarakeshwar, P.; Kim, K. S.; Lisy, J. M. J. Phys. Chem A 2003, 107, 1228. (51) Frensdorff, H. K. J. Am. Chem. Soc. 1971, 93, 600. (52) El-Azhary, A. A.; Al-Kahtani, A. A. J. Phys. Chem A 2004, 108, 9601. (53) Al-Jallal, N. A.; Al-Kahtani, A. A.; El-Azhary, A. A. J. Phys. Chem. A 2005, 109, 3694. (54) Shimanouchi, T. Tables of Molecular Vibrational Frequencies; Vol. I, NIST Chemistry WebBook, NIST Standard Reference Database Number 69; National Institute of Standards and Technology: Gaithersburg, MD, 1972; http://webbook.nist.gov, accessed June 2007. (55) Patwari, G. N.; Lisy, J. M. J. Phys. Chem A 2007, 111, 7585.
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