Article pubs.acs.org/JPCA
Quantifying Intrinsic Ion-Driven Conformational Changes in Diphenylacetylene Supramolecular Switches with Cryogenic Ion Vibrational Spectroscopy Arron B. Wolk,† Etienne Garand,‡ Ian M. Jones,§ Andrew D. Hamilton,# and Mark A. Johnson*,† †
Sterling Chemistry Laboratory, Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States ‡ Department of Chemistry, University of Wisconsin, 1101 University Avenue, Madison, Wisconsin 53706, United States § Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University Station A5300, Austin, Texas 78712, United States # Department of Chemistry, University of Oxford, 12 Mansfield Road, Oxford OX1 3TA, Great Britain S Supporting Information *
ABSTRACT: We report how two flexible diphenylacetylene (DPA) derivatives distort to accommodate both cationic and anionic partners in the binary X±·DPA series with X = TMA+ (tetramethylammonium), Na+, Cl−, Br−, and I−. This is accomplished through theoretical analysis of X±·DPA·2D2 vibrational spectra, acquired by predissociation of the weakly bound D2 adducts formed in a 10 K ion trap. DPA binds the weakly coordinating TMA+ ion with an arrangement similar to that of the neutral compound, whereas the smaller Na+ ion breaks all intramolecular H-bonds yielding a structure akin to the transition state for interconversion of the two conformations in neutral DPA. Halides coordinate to the urea NH donors in a bidentate H-bonded configuration analogous to the single intramolecular H-bonded motif identified at high chloride concentrations in solution. Three positions of the “switch” are thus identified in the intrinsic ion accommodation profile that differ by the number of intramolecular H-bonds (0, 1, or 2) at play. induced in the organic scaffold when it flexes to optimize these electrostatic interactions. For example, NMR characterization of the DPA switch in CDCl3 revealed a 9:1 preference for formation of A in the absence of an ion, whereas introduction of the chloride anion (via tert-butylammonium chloride) inverted the conformer populations such that arrangements derived from B, with the chloride attached to the urea motif, became dominant by a factor of 5.13 Such bulk measurements represent the statistical average of all conformations present in solution at room temperature, and local interactions are often masked by solvent molecules that compete with internal Hbonding combinations. Here we exploit recent advances in gasphase cryogenic ion vibrational predissociation (CIVP)14−18 spectroscopy to structurally characterize the individual conformations and noncovalent interactions at play in isolated (gas-phase) complexes of DPA derivatives with several representative ions (TMA+ (tetramethylammonium), Na+, Cl−, Br−, and I−). This survey reveals a new conformation that occurs upon accommodation of the tightly binding sodium
R
ational control of electrostatic interactions plays a central design role in supramolecular chemistry.1−6 Here we are concerned with the elementary mechanics underlying the function of molecular “switches,” a class of molecules that can adopt two or more conformations whose populations depend on the ionic composition of the solution.7−9 An important application of this behavior involves their use as chemical sensors based on the conformation dependence of optical or electrical properties.10 In this report, we focus on an archetypal molecular switch based on the diphenylacetylene (DPA) scaffold depicted in Figure 1.11,12 This recently developed variation13 has been found to adopt two locally stable, planar H-bonded environments (denoted A and B in Figure 1) that can be interconverted by 180° rotation about the acetylene axis through the out-of-plane transition-state (TS) configuration in which all internal H-bonds are broken. In isolation, A is calculated to lie about 20 kJ/mol lower in energy at the B3LYP/6-31+G(d,p) level, qualitatively consistent with the ability of its lactone-type carbonyl to form asymmetrical, bifurcated H-bonds with the two amide NHs (NHa and NHb) of the embedded urea group. Key issues controlling the DPA conformation adopted upon ion complexation are the relative strengths of the intramolecular H-bond(s), the net energies associated with ion attachment to the various functional groups, and the strain © 2013 American Chemical Society
Special Issue: Prof. John C. Wright Festschrift Received: November 12, 2012 Revised: January 6, 2013 Published: January 10, 2013 5962
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Figure 1. Two locally stable conformations, A and B, exhibited by the isolated diphenylacetylene scaffold before it is attached to an ion, which can be interconverted by rotation about the CC linkage through the transition state (TS). The CO docking sites for cation attachment are highlighted in blue, and the NH groups (denoted a, b, and c) are indicated in red. Two derivatives are explored in this work, which involve replacement of the H atom in the R position with the NO2 group.
capillary. The resulting complexes were guided through four differentially pumped stages in radio frequency ion guides to a nominal chamber pressure of 3 × 10−7 Torr. After redirection in a DC turning quadrupole, the ions were focused and collected in a cryogenically cooled (Sumitomo closed-cycle helium cryostat) quadrupole ion trap (QIT, Jordan). A mixture of 20% of D2 in He was pulsed (1 ms) into the trap with a subsequent time delay (30−90 ms), allowing for a significant drop in buffer gas pressure before ion extraction. This has been shown21 to increase both the abundance of tagged species and the mass resolution in the following analysis. The ions were then injected into the extraction region of a Wiley−McLaren time-of-flight mass spectrometer and intersected with the output of an OPO/OPA laser system (Laservision). The bandwidth of the tabletop laser system increases from approximately 3 cm−1 in the energy range above 2500 wavenumbers to 6 cm−1 below this energy, a result of the additional mixing in a AgGaSe2 crystal. Predissociation spectra were measured by monitoring the photoproduction of bare complex from the doubly tagged X±·DPA·(D2)2 species. D2 molecules are photodissociated upon resonant excitation energies as low as 800 cm−1. To ensure all spectra are collected in the linear regime, the 1064 nm beam pump in the OPA was lowered until a linear response of photoproduct yield to laser fluence was observed. That behavior is critical because it ensures that the D2 molecules are indeed weakly bound as required for faithful recovery (i.e., with minimal solvent shifts) of the band patterns. The molecular switch samples were obtained from the Hamilton lab without further purification.13 Theoretical Procedures. Conformational analyses were initiated from the structures given by a previous molecular mechanics analysis in the MOE package. With their relatively rigid backbones and obvious hydrogen-bond contact points, the possible structures of the switches were narrowed down to simple rotations around the amide and acetylene bonds. From these structures, a body of possible conformers was constructed by placing the charge center in a grid around all functionalities
cation, which is distinct from the two structural classes previously reported in the context of anion sensing. We discuss factors controlling the DPA switch conformational changes, including the relative strengths of its intramolecular H-bonds and the binding energies of an ion to its various functionalities, both of which are qualitatively encoded in the frequencies of sharp transitions arising from stretching displacements of particular CO and NH groups.
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EXPERIMENTAL AND THEORETICAL METHODS Experimental Procedures. Cryogenic ion vibrational predissociation (CIVP) spectroscopy, a refinement of the more commonly used room temperature infrared multiplephoton dissociation technique,19,20 extracts species from room temperature solutions into the gas phase where they are cooled close to 10 K in an ion trap before spectroscopic interrogation in a mass-selective photofragmentation spectrometer.16,21 Vibrational spectra are obtained over the fingerprint region by first condensing molecular hydrogen (or D2) onto the cold ions so that vibrational resonances can be detected by mass-loss upon excitation with a scanned infrared laser. This infrared predissociation or “messenger” spectroscopy yields linear action spectra closely related the traditional absorption spectra obtained with FTIR, with line widths as small as 6 cm−1 over a wide range of energies (600−4200 cm−1) for species frozen close to their vibrational zero-point energies. As such, these spectra are directly comparable to theoretical harmonic spectra of vibrational fundamentals for various minimum energy structures without complications from rotational or vibrational hot bands. In short, millimolar solutions of the molecular switches were prepared in 50/50 CHCl3/methanol (v/v) with a large access of the tetramethylammonium (TMA) salt of chloride, bromide, or iodide. A high concentration of the TMA salt (over 20 times the concentration of the molecular switch) was required to produce appreciable amounts of the complexes of interest. These solutions were electrosprayed from a 15 μm ground capillary tip (New Objective) into a heated stainless steel 5963
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room temperature FTIR of compound 1 in CDCl3, displayed as an inset below panel 2a. The solution spectrum does not exhibit any activity near the free NH transition and features much more diffuse bands in the upper energy range. For both the TMA+ and the Cl− complexes, the nominally forbidden D2 transition at 2992 cm−1 is embedded in the CH stretching group of bands and is not strong if indeed present. The resonances from the complexes are generally grouped into two classes: a dense suite of transitions in the lower energy, fingerprint region (1200−1700 cm−1) and the C−H and N−H stretches much higher in energy (2700−3400 cm−1). The CC stretch of the acetylene linker around 2200 cm−1 is only evident in the 1-Cl− complex, and there is generally a large gap in the middle of the energy range. The transitions arising from the CO stretching fundamentals appear as two distinct peaks at approximately 1700 cm−1. Closer inspection of the 1-TMA+ spectrum (Figure 2a) reveals three peaks in the NH region, consistent with three distinct NH environments within a single conformer. As such, the locations of these three resonances immediately signal the crude conformation at play through the red shifts anticipated to occur when NH groups are involved in H-bonding. To establish the location of a nonbonded NH group, the dominant NH −1 25 fundamental in N-methylacetamide (vNH is nma at 3480 cm ) indicated by the broken line at the right in Figure 2a, which indeed falls quite close to the highest observed 1-TMA+ band at 3467 cm−1. In folded peptides, it is well-known that neutral NH···OC linkages yield red-shifts in the N−H stretches of about 100 cm−1,17,18,26 which correspond to the region where the two other NH peaks appear in Figure 2a. The inset in Figure 2a presents an expanded view of the CO region and to highlight the qualitative implication of the peak positions, the free CO frequencies of the urea, methyl ester, and NNH CO methylacetamide (vCO urea , vnma, and vester , resepectively) are included as broken lines as indicated. The highest energy (1730 cm−1) peak in 1-TMA+ thus appears too high to be an amide or urea CO, and can therefore be assigned to the lactone CO that is red-shifted by 55 cm−1 due to its H-bonding interactions. Taken together, these observations suggest a structure based on conformer A, and indeed the lowest energy minimum calculated at the B3LYP/6-31+G(d,p) level, presented in Figure 3a, retains the general character of the H-bonding arrangement in A. Comparison between traces (a) and (b) of Figure 2 indicates that all the signature CO and NH bands respond when the complexing ion is changed from TMA+ to Cl−, suggesting that there are distinctly intermolecular bonding motifs at play. First, the CO stretching fundamentals blue-shift by about 15 cm−1, indicating these groups are more weakly interacting or even free. In addition, the 1-Cl− spectrum does not display any activity in the region of nonbonded NH groups. Instead, only one peak appears in the neutral H-bonded NH region whereas a congested series of features, overlapping with the CH region, appears in the 1-Cl− spectrum and extends over the range 2800−3300 cm−1. Note that the anionic H-bond to an NH group is known to be quite strong, as evidenced by redshifts in the NH stretch of hundreds of cm−1 in a manner highly correlated with the proton affinities of the anions.27−29 The strong, red-shifted bands in the 1-Cl− spectrum near 2800 cm−1 are therefore consistent with ionic H-bonded NH activity, but the complexity of the band structure relative to that of the cationic species indicates that either multiple conformers are in play or anharmonic effects significantly perturb the spectrum of
in the molecule (NH, CO, and phenyl ring). Though this represents a large number of possible conformations, most initial geometries at the B3LYP/6-31+G(d,p) level optimized (using the Gaussian 09 package)22 to two arrangements from 180° rotation around the central triple bond. The prohibitive size of the complexes limited optimization and frequency calculations to the B3LYP/6-31+G(d,p) level. All calculated spectra were empirically scaled by 0.957 to match the highest energy NH mode above 2000 cm−1 and by 0.987 to match the highest energy CO fundamental below 2000 cm−1 in the 1Na+(D2)2 complex.
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RESULTS AND DISCUSSION Spectroscopic Survey of DPA Cation and Anion Complexes. Figure 2 contrasts the vibrational predissociation
Figure 2. Vibrational predissociation spectra of (a) 1-TMA+(D2)2 and (b) 1-Cl−(D2)2. The N−H stretch transition in neutral Nmethylacetamide (vNH nma) is indicated in (a) to establish the location of a non-H-bonded NH group, and the positions of nonbonded CO groups are indicated by the carbonyl stretching fundamentals of NH CO diphenylurea, nma, and methyl acetate (vCO urea , vnma, and vester ). The inverted trace at the bottom right of (a) is the baseline-corrected FTIR spectrum of compound 1 in neutral CDCl3 for comparison with those of the cold, isolated ions. The stretching transition near 2200 cm−1, which is derived from the acetylene linker, is only evident in the anionic complex and denoted as CC in (b).
spectrum of compound 1 complexed with TMA+ (1-TMA+, upper trace) with that of the chloride complex (1-Cl−, lower trace). The D2 tags are most likely attached to the complexing ion.16,21 Note that the complexes exhibit relatively sharp (6 cm−1 fwhm) bands throughout the 1000−3600 cm−1 range, providing qualitatively new structural information about noncovalently bonded adducts in this size range. The performance of CIVP spectroscopy on these systems is thus similar to that recently reported in its application to protonated polypeptides.23,24 The utility of this approach in the present study is underscored by comparing the CIVP results with the 5964
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energy structure displayed at the top, which recovers the observed band positions remarkably well. Although the normal coordinates involve collective motions of the NH oscillators, the atoms contributing the dominant displacements to the normal modes are color-coded such that, for example, the NHabased band is displayed in red. The broadened and asymmetrical shape of the lower energy CO envelope is thus explained as a result of two partially overlapping amide I transitions. This assignment scheme predicts that the NH interloper involves the NHa group (red) more distant from the CO Hbond acceptor, and we therefore set out to introduce a chemical modification that would have the most effect on the strength of its H-bond to the carbonyl. It is anticipated that replacement of the para hydrogen with a −NO2 group on the phenyl ring (giving compound 2, shown in Figure 1) would have such a selective effect on the proximal NH because that substituent is known to reduce the pKa of the urea functionality from roughly 18.9 to 16.7.33 Moreover, such a modification on the exterior of the complex allows for subtle perturbation of the bands involved in the urea linkage while largely retaining the overall conformational motif. This expectation is borne out in the calculated atomic distances and harmonic shifts (table and downward bars in Figure 3d, respectively), where the red band is predicted to display over twice the red shift as that of the purple NHb group closer to the CO acceptor, whereas the amide NH (blue) is unaffected. The inverted trace in Figure 3d presents the observed CIVP spectrum for 2-TMA+, which indeed exhibits qualitatively larger shifts in the intermediate NH and highest energy CO stretching transitions relative to those of the nearby features. This allows us to unambiguously assign the peaks at 3336 cm−1 (purple) and 3396 cm−1 (red) to the urea modes and the 1730 cm−1 (green) CO stretch to the H-bonded lactone carbonyl. It is useful to note that these correlated red shifts between the donor (NH) and acceptor (CO) transitions upon modification of the donor pKa provide an excellent example where vibrational spectroscopy yields a detailed picture of a hydrogen bond linkage at play in this system. This ability is usually reserved to 2D infrared or NMR techniques.34−38 This experimental validation of predicted trends in the band pattern allows us to establish that the weakly coordinating TMA+ ion basically attaches to the type A conformers of both derivatives, which are the most stable forms of the isolated neutral DPA compounds. It is therefore of interest to explore what type of cationic interaction could induce a change in conformation of the DPA scaffold. Because the regions with the most negative electrostatic potential are the carbonyl groups, an obvious strategy is to invoke a cation that binds with sufficient strength to CO that it can break one or more of the neutral N−H···OC linkages. The sodium cation, Na+, is an attractive candidate for this purpose as its binding energy to acetone, for example, is over a factor of 2 stronger than that of TMA+ (130 kJ/mol39 versus 61 kJ/mol,40 respectively). Figure 4 compares the CIVP spectra of the 1-Na+ and 1TMA+ complexes along with their calculated harmonic spectra. With the exception of the highest energy NH band, all the transitions blue shift in the Na+ complex such that none of the NH bands occur in the H-bonded region.41 This in turn indicates that the more strongly interacting Na+ completely disrupts the intramolecular H-bonds present in DPA configurations based on either A or B. The structure of the lowest energy 1-Na+ complex identified in our optimizations is
Figure 3. (a) Schematic of 1-TMA+ and its lowest energy structure obtained at the B3LYP/6-31+G(d,p) level, along with (b) a table of calculated NH distances. (c) and (d) present vibrational predissociation spectra of 1-TMA+(D2)2 and 2-TMA+·(D2)2 species, respectively, with colored band assignments corresponding to the colors in (a). The harmonic stick spectra are calculated at the B3LYP/ 6-31+G(d,p) level and are scaled by 0.957 to match the highest energy NH stretch above 2000 cm−1, and by 0.987 below 2000 cm−1 to match the highest energy CO stretch at 1731 cm−1.
a single conformer. We therefore turn to empirical evolution of the band patterns with chemical and charged adduct variation as a means to validate theoretical interpretation of the spectra in the context of detailed structures. Analysis of Cation Complexes: TMA+ and Na+. The extraction of structural information from complicated vibrational band patterns is greatly enhanced when particular transitions can be traced to local displacements of embedded oscillators within the extended molecular scaffold. One powerful way to establish whether such transitions exist is through isotopic labeling of specific NH and CO groups to empirically establish their particular contributions to the spectrum.16,17,30−32 Although such selective isotope incorporation is quite demanding in the DPA case (as opposed to complexes involving peptides, for example), chemical modifications designed to modulate the proton pKa’s of the embedded urea NH groups can also provide a useful means with which to assess the accuracy of the theoretical assignment scheme. Figure 3 highlights the behavior of the CO and NH groups along with the harmonic spectrum obtained for the minimum 5965
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Figure 4. Infrared predissociation spectra of (a) 1-Na+·(D2)2 and (b) 1-TMA+·(D2)2, along with the minimum energy structure of 1-Na+ at the B3LYP/6-31+G(d,p) level. The associated harmonic stick spectra of the Na+ and TMA+ complexes are scaled as given in the caption of Figure 3. The stick spectra are normalized to the most intense peak in each of the two regions. The free NH transitions in gas-phase N-methylacetamide (vNH nma) and diphenylurea (vNH urea) are given in (a).
displayed at the top of the figure, which features a tricoordinated Na+ binding arrangement whose harmonic spectrum recovers the observed band pattern. Note that this structure optimizes all three Na+···OC interactions by twisting the two pendent groups out of plane, resulting in rupture of all intramolecular H-bonds. The interaction with Na+ thus induces a DPA structure close to the transition state between the A and B conformations (Figure 1), a configuration that is not stable in the absence of a strongly coordinating cation. It is interesting to note that there is a weak band in the 1TMA+·(D2)2 spectrum near 3440 cm−1 (the band labeled * in Figure 4b) that is near the free urea NH fundamentals displayed by 1-Na+(D2)2. This suggests that TMA+ may also support an open transition-state-like structure which, though higher in energy than the conformer A based form, is accessible under the ion source conditions. One implication of this scenario is that cations in solution could act to catalyize interconversion by lowing the TS energy, a speculation that would be useful to pursue with molecular dynamics simulations. Analysis of Anion Complexes: Cl−, Br−, I−. We next address the cause of the complex series of NH bands recovered in the 1-Cl− spectrum (Figure 2b), again exploiting spectral evolution under systematic chemical modifications to challenge the theoretical band assignments. Our initial inspection of the N−H stretching bands in Figure 2b revealed that all NH groups in 1-Cl− are engaged in either neutral or ionic H-bonds. This is consistent with formation of a configuration based on conformer B (Figure 1), and indeed the lowest energy
minimum identified in our calculations, displayed in the right side inset in Figure 5a, adopts this docking arrangement. The calculated spectrum for this 1-Cl− structure in the critical regions of the CO and N−H stretches is presented as inverted sticks in Figure 5c. We note that in this geometry, in contrast to the situation in 1-TMA+, the NHa group of the embedded urea is the preferred partner in the H-bonding interaction to the anion in its asymmetrical docking site. This asymmetry appears to result from the steric hindrance of the nearby CH on the phenylacetylene, which prevents full coordination with both NH groups.33,42,43 Although the CO stretching pattern is well reproduced, the observed NH (and possible CH) patterns display much more activity than predicted at the harmonic level. As expected, NHa displacement makes the largest contribution to the lowest energy harmonic NH fundamental (red downward peak in Figure 5c), whereas the companion NHb frequency (purple) occurs at much higher energy (3225 cm−1), falling 140 cm−1 below the sharp band highest in energy at 3365 cm−1 that is clearly assigned to the neutral H-bonded NHc (blue). It is interesting to note that in the 1-Cl− complex, the frequency of the lactone CO (1754 cm−1), when singly H-bonded, falls between that group found in 1-TMA+ (1731 cm−1), where it is doubly H-bonded, and a free lactone CO (1778 cm−1). This indicates that for linkages between similar partners (overall neutral NH and CO), the frequencies of the transitions reflect the overall strengths of the local interactions. To gain confidence in the theoretical assignment of the lowest energy band structure around 3000 cm−1 to activity in 5966
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Figure 5. (a) Structural schematic of the anionic DPA complexes (left) and the minimum energy structure found at the B3LYP/6-31+G(d,p) level for the 1-Cl− species (right). The Hi···Cl− H-bond distances are indicated in the minimized geometry. The CIVP spectra are presented for (b) 2Cl−(D2)2, (c) 1-Cl−(D2)2, (d) 1-Br−(D2)2, and (e) 1-I−(D2)2 in the NH and CO stretching regions. The theoretical stick spectra calculated at the B3LYP/6-31+G(d,p) level are scaled as given in the caption of Figure 3 and are presented below the corresponding experimental traces. The sticks are color-coded to reflect dominant bond displacements according to the scheme at the left side of (a). The urea CO transition (vCO urea) is indicated with an arrow in (b), and the free NH transition in 1-TMA+(D2)2 (vNH free ) is indicated in (c) and (d). The * denotes evidence for a possible second conformer with a free NH.
NHa, we again turn to the −NO2 derivative as this compound should, in analogy to the situation encountered in 1-TMA+, impart the greatest shift to bands involving the NHa group. The 2-Cl− spectrum is shown in Figure 5b. Whereas the bands above 3100 cm−1 are largely unaffected, the lower energy region is much simpler in 2-Cl− such that a single dominant band now appears below the onset of the broad envelope observed in 1Cl−. Not only does the NO2-induced red shift support the harmonic assignments, but the concomitant spectral simplification raises the scenario that the complexity in 1-Cl− results from strong anharmonic coupling to background states near 3000 cm−1. It is important to emphasize that the chemical modifications designed to affect the H-bonding interaction of a particular NH donor do not directly establish that the bond in fact involves contact to the halide ion. That conclusion was based solely on the magnitude of the red shifts of the bands traced to the NHa
stretch. To challenge this assignment, we follow the spectral evolution for a series of similar anions with increasingly lower proton affinities. This variation should act to blue-shift NH bands associated with the NH···X− contact and is readily accomplished by simply replacing Cl− with the larger halides Br− and I−. The CIVP results for the D2-tagged 1-Br− and 1-I− complexes are presented in Figure 5d,e, respectively, and indeed the centroid of the band system blue-shifts for heavier halides while the spectra become increasingly complex. Note that the sharp high energy feature assigned to the lactone N−H···OC linkage is not affected by halide substitution, thus establishing that it is indeed not associated with the docking site to the ion. Regarding the origin of the rather complex series of bands traced to the NH interaction with the halides, we note that such extra bands have been observed and analyzed at length in the case of water attachment to anions.44,45 In those cases, the 5967
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frequencies of the two nominally identical H-bond donors are strongly dependent on displacement of the ion from one Hbond donor to the next. That effect provides a natural mechanism for the activation of soft modes involving ion motion upon excitation of the H-bond donors46−50 and may be in play here. This conjecture can be addressed by following evolution of the bands in the ND isotopologues, where extra bands are typically suppressed.51 These studies will allow us to sort out the intrinsic anharmonicity induced by anion binding from coupling to remote C−H stretches. We remark that with the larger halides, a very weak feature (denoted * in Figure 5d,e) appears near the free NH position, suggesting that these larger ions may bind to yet another arrangement that leaves one NH group nonbonded. In summary, cryogenic ion vibrational predissociation (CIVP) spectroscopy, when integrated with theory and systematic chemical variation, is demonstrated to provide a qualitatively new capability to obtain the nature of the docking arrangements in noncovalently bound ion−molecule complexes with extreme sensitivity and spectroscopic precision.
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ASSOCIATED CONTENT
S Supporting Information *
Table comparing experimental and calculated frequencies of the NH and CO oscillators in the molecular switch complexes characterized 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
*E-mail:
[email protected]. Author Contributions
A.B.W. and E.G. collected the experimental data. A.B.W. performed all theoretical calculations. A.B.W., E.G., and M.A.J. wrote the paper, with I.M.J. and A.D.H. contributing comments. I.M.J. and A.D.H. developed, synthesized, and characterized the switches studied in this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for thoughtful comments by a reviewer of this manuscript regarding the implications of the ion-dependent structure on the possible mechanism for conformer interconversion in solution. M.A.J. thanks the National Science Foundation under grant CHE-1213634. The data collected in this work were collected on the Yale Cryogenic Infrared Photofragmentation Spectrometer developed with support by the Air Force Office of Scientific Research under grant FA955009-1-0139. A.D.H. thanks the University of Oxford and the NSF (CHE-0750357) for funding.
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ABBREVIATIONS DPA, diphenylacetylene; CIVP, cryogenic ion vibrational predissociation REFERENCES
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