Wetting Camphor: Multi-Isotopic Substitution Identifies the

Letter pubs.acs.org/JPCL

Wetting Camphor: Multi-Isotopic Substitution Identifies the Complementary Roles of Hydrogen Bonding and Dispersive Forces Cristóbal Pérez,†,‡ Anna Krin,†,‡ Amanda L. Steber,†,‡,§ Juan C. López,†,∥ Zbigniew Kisiel,⊥ and Melanie Schnell*,†,‡,§ †

Max Planck Institute for the Structure and Dynamics of Matter, D-22761 Hamburg, Germany The Center for Free-Electron Laser Science, D-22761 Hamburg, Germany § The Hamburg Centre for Ultrafast Imaging at the University of Hamburg, D-22761 Hamburg, Germany ∥ Departamento de Quimica Fisica y Quimica Inorganica, Facultad de Ciencias, Universidad de Valladolid, 47011 Valladolid, Spain ⊥ Institute of Physics, Polish Academy of Sciences, 02-668 Warszawa, Poland ‡

S Supporting Information *

ABSTRACT: Using broadband rotational spectroscopy, we report here on the delicate interplay between hydrogen bonds and dispersive forces when an unprecedentedly large organic molecule (camphor, C10H16O) is microsolvated with up to three molecules of water. Unambiguous assignment was achieved by performing multi H218O isotopic substitution of clustered water molecules. The observation of all possible mono- and multiH218O insertions in the cluster structure yielded accurate structural information that is not otherwise achievable with single-substitution experiments. The observed clusters exhibit water chains starting with a strong hydrogen bond to the CO group and terminated by a mainly van der Waals (dispersive) contact to one of the available sites at the monomer moiety. The effect of hydrogen bond cooperativity is noticeable, and the O···O distances between the clustered water subunits decrease with the number of attached water molecules. The results reported here will further contribute to reveal the hydrophobic and hydrophilic interactions in systems of increasing size. favorable binding sites and building up the first solvation shell. For this reason, many structural and dynamical studies are undertaken in the gas phase, usually with the sample entrained in supersonic jets.5−11 When coupled with high-resolution techniques, such as rotational spectroscopy, accurate structural information about the generated weakly bound complexes can be obtained.6,10,12−17 However, due to the experimental challenges of generating solute−water aggregates of increasing size, it has thus far been limited to the study of relatively small molecules and complexes. Recent advances in chirp-pulse excitation techniques are rapidly changing the applicability of rotational spectroscopy for the study of large clusters. Chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy18−21 offers, among other advantageous features, a high dynamic range, and fast acquisition of spectra over broad frequency ranges. Quantitative molecular structures can be experimentally determined from the measurable changes in the moments of inertia of the corresponding isotopically substituted species. This allows for the building up of the substitution structure atom-by-atom through the Kraitchman equations22 and/or least-squares fits23,24 of internal coordinates as it has been recently

H

ydrogen bonding is responsible for many of the properties of water including its unrivaled capacity as a solvent in systems of biological relevance. The vast majority of biological processes such as chemical recognition,1 protein folding,2 ion channel protein mechanisms,3 and others are largely controlled by a subtle equilibrium of hydrogen bond (HB) interactions between the active organic molecule and the first solvation shell water molecules. The so-called hydrophobic effect4 is known to be one of the main factors altering structurally how this solvation shell is formed and one of the driving forces in protein folding. This effect disrupts the HB network between water molecules in a way that solute−solute interactions become preferred over the water−solute interactions, while the water molecules then interact with each other. In this case, weak or moderate HBs and dispersive forces are the main contributors. It is thus essential to gain a better knowledge of the preferred water binding sites in solvated organic molecules and of how the structure of the solvated complex changes with increasing number of surrounding water molecules. Quantifying these specific hydrophobic and/or hydrophilic solute−water interactions from a molecular point of view is a valuable strategy for understanding increasingly more complex systems in a bottom-up approach. Under isolated conditions, water molecules can be sequentially added to the microsolvated molecule in a controlled and measurable fashion, identifying the most © XXXX American Chemical Society

Received: November 12, 2015 Accepted: December 21, 2015

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DOI: 10.1021/acs.jpclett.5b02541 J. Phys. Chem. Lett. 2016, 7, 154−160

Letter

The Journal of Physical Chemistry Letters

Figure 1. Portion of the 2−8 GHz spectrum (1 million acquisitions, 9 h of measurement time) of camphor−water. For both panels, the black trace shows the experimental spectrum, while the lower traces represent simulations based on fitted parameters for camphor−(H2O) (1w(I) (red), 1w(II) (blue)) and camphor−(H2O)2 (2w(I) (bottom panel, green)) at a temperature of 1.5 K. All four water isotopologues of the 2w(I) complex arising from multi-isotopic substitution are shown in the bottom panel. As depicted, the 18O isotopologues (magenta, orange, and cyan) are well-resolved in frequency, with the double 18O−18O insertion (cyan) as the most red-shifted from the parent spectrum (green trace). The rest of the strong lines shown in the figure belong to the 2w(II) isotopologues.

Table 1. Experimentally Determined Rotational and Centrifugal Distortion Constants for the Normal Species of the Observed Camphor−Water Complexes

a

1w(I)

1w(II)

2w(I)

2w(II)

3w

A (MHz) B (MHz) C (MHz)

1344.654(66) 662.98812(73) 637.90564(72)

1339.83(15) 689.03465(94) 672.95812(73)

1065.040(15) 514.70479(25) 472.43647(25)

1134.2098(40) 528.38221(28) 477.44192(25)

893.24443(30) 401.10981(12) 356.27688(12)

ΔJ (kHz) ΔJK (kHz) ΔK (kHz) δJ (kHz)

0.1482(28) 2.163(16) − −0.0197(47)

0.7309(44) 1.273(18) − −0.1234(61)

0.0526(16) 0.2025(76) − −

0.0629(21) 0.169(16) − −

0.09431(71) −0.1721(72) − −

Na σb (kHz)

38 4.64

28 3.96

42 3.85

39 4.46

63 3.45

Number of lines in the fit. bStandard deviation of the fit.

imental structures of camphor−(H2O)1−3 complexes, determined from the first multi-isotopic substitution experiments in clusters of this size with up to a total of 14 heavy atoms. The rotational spectrum of camphor−(H2O)n clusters with n = 1−3 was recorded with our CP-FTMW spectrometer in Hamburg operating at 2−8 GHz. Experimental details are given elsewhere.19 Two separate rotational broadband measurements were performed. To add water to the monomer, a CP-FTMW spectrum using H216O water was measured and is shown in the top part of Figure 1. The predominant spectrum was identified as belonging to the previously studied camphor monomer,26 not shown in Figure 1, with a signal-to-noise ratio (SNR) of roughly 1500:1. Two a-type spectra corresponding to complexes containing one water were observed. The strongest

exemplified for molecular complexes, such as in the structural studies of water clusters,8,12,18 sevoflurane-benzene,25 and β-propiolactone (BPL).16 This last study provided unprecedented information on the preferred binding sites for water molecules on an organic molecule with multiple polar moieties. In this study we examine the first steps in the formation of the solvation shell of a conformationally rigid bicyclic molecule, which has only a single polar group (−CO) and is endowed with considerable steric hindrance elsewhere. This allows us to accurately investigate the influence of weak HBs and how those HBs shape the preferred active sites of an amphiphilic molecule. The molecule chosen was the terpenoid camphor, (C10H16O, 1,7,7-trimethylbicyclo[2.2.1]hepta-2-one). The rotational spectrum of camphor and its structure were previously reported by Kisiel and co-workers.26 Herein we present accurate exper155


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Figure 2. Experimental structures of the most stable 1w(I) and the less stable 1w(II) conformers of the cluster between camphor with one water molecule obtained from the r0 least-squares fits to six moments of inertia of the parent and the H218O isotopologue. The relevant experimental structural parameters are compared with results of quantum chemistry calculations (lower values). In both cases the water molecule is bound by a well-defined O−H···O hydrogen bond to the carbonyl group, and it also touches the camphor molecule by means of a peripheral van der Waals interaction. The relevant experimental structural parameters are compared with results from MP2/6-311++g(d,p) and B3LYP-D3/aug-cc-pVTZ quantum chemical calculations (lower values at MP2 level when not stated otherwise). Energy differences are at the MP2/6-311++g(d,p) level of theory.

constants is the so-called substitution, rs , method using Kraitchman equations.22 This method exploits mono-isotopic substitution information and provides a straightforward way to use the isotopic changes in the moments of inertia to determine the atom coordinates in the principal axis frame. This purely experimental approach allowed us to “locate” the water units in the complexes with camphor and to generate plausible candidates that were further optimized using ab initio (MP2) and density functional theory (DFT) methods (M06-2X and B3LYP-D3 dispersion-corrected density functionals). The relevant atomic coordinates are collected in the Supporting Information (Tables S3, S7, S11, S17, and S23). Although the Kraitchman rs method is valuable when trying to build the experimental structure atom-by-atom, it poses limitations such as problems for small rs coordinates of atoms located near the three principal axes, it is only applicable to using mono-isotopic substitution data, and delivers only the magnitudes of the atomic rs coordinates. This is particularly limiting in a conformationally rich system such as camphor− water. When choosing the sign of the resulting experimental coordinates, chemical intuition or knowledge must be brought in when reliable theoretical structures are not available. For large systems, this could become very time-consuming. Therefore, additional methods to determine the molecular structure need to be applied in order to avoid a misassignment of the observed spectra. An alternative, complementary tool to the Kraitchman analysis is to least-squares fit all of the available data, which allows exploiting multi-isotopic information.23,24 The simplest type of such a fit involves fitting the effective ground state, r0 , structural parameters. The r0 geometry and computational chemistry allow one to assign the coordinate signs to the rs coordinates and allow for the eventual unambiguous identification of the observed clusters. The experimental information available from 18O substitution in the water for the cluster with a single water molecule is limited to only 6 available moments of inertia (three each for the H216O and H218O species). The two-water clusters have already 12 moments of inertia, while the three-water complex has 24. We used the available literature

of the two (1w(I) in red in Figure 1) exhibits a SNR of about 300:1, whereas the next one in decreasing order of intensity appeared at a SNR of 90:1 (1w(II) in blue in Figure 1). We also identified two complexes with two water molecules (2w(I) and 2w(II)) as well as camphor surrounded by three water molecules (3w). The corresponding rotational parameters are reported in Table 1, and the rotational transitions are tabulated in the Supporting Information (Tables S4, S8, S12, S18, and S24). No internal rotation splittings attributable to the camphor methyl groups and/or water motions were observed. All of the spectra were first fit using the JB9527 program; then, the measured data sets and the fits were further refined with the AABS program suite.28−30 To confirm that the observed rotational spectra belonged to water-containing complexes, we performed a second experiment using an isotopically enriched sample of water with 50% H218O. This experiment aimed to observe the singly substituted species as well as all the possible 16O−18O combinations incorporated in the clusters with multiple water molecules. The effect of the multi-isotopic substitution in the 2w(I) complex is illustrated in the bottom panel of Figure 1. It was possible to observe the spectra corresponding to all possible 16O/18O water combinations, i.e., 2w(I)-16O-16O, 2w(I)-16O-18O, 2w(I)-18O-16O and even 2w(I)-18O-18O, without any theoretical input. We used the recently introduced AUTOFIT31 program that requires only parameters extracted from the spectra of the normal species. Our methodology consisted of searches over the necessarily red-shifted frequency ranges as the mass of the complex increases upon single or multiple H218O incorporation. All possible singly and multisubstituted H218O isotopologues, i.e., a total of two, six, and seven additional spectra for camphor− (H2O) (two conformers), camphor−(H2O)2 (two conformers), and camphor−(H2O)3 (one conformer) complexes, respectively, were identified in the enriched sample. Hamiltonian parameters and frequencies can be found in the Supporting Information (Tables S2, S5, S6, S9, S10, S13− S16, S19−S22, and S24−S31). An established approach beyond the simple identification of molecules by comparison of measured and calculated rotational 156


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Figure 3. Experimental structures of the most stable 2w(I) and the less stable 2w(II) clusters consisting of a camphor molecule with two water molecules. The r0 experimental parameters (upper values) are obtained in each case from a fit to 12 moments of inertia of 4 different isotopologues: the parent, the two single H218O species, and the (H218O)2 species. The relevant experimental structural parameters are compared with results from MP2/6-311++g(d,p) ab initio calculations (lower values). The experimental O···O distances are shorter in both cases compared to the isolated water dimer, 2.98 Å,33 due to the effect of σ-bond cooperativity of the hydrogen bonds.

Figure 4. Experimental structure of the observed conformer of camphor with three water molecules obtained from the r0 least-squares fit of 24 moments of inertia for all 8 available isotopologues. The relevant experimental structural parameters are compared with results of MP2/6-311++g(d,p) calculations (lower values). The left panel shows how the chain of three water molecules extends from the carbonyl group and is anchored at the other end to the camphor molecule by two van der Waals contacts. In the right panel the camphor molecule is oriented as in Figures 2 and 3, and the near planar nature of the water ring is apparent. The indicated distances are for OH separations and the differences between the three hydrogen bonds and two van der Waals contacts are clear.

r0 experimental geometries for the camphor26 and the water32 subunits as initial assumptions. The smallest number of parameters providing a stable fit was 3, 7, and 10 for the 1w, 2w, and 3w clusters, respectively (Tables S32−S36). The primary parameters of the fit were the O···O distances and angles, while the other orientations of the water subunits were taken from the preferred MP2/6-311G++(d,p) ab initio calculations. The key results are depicted in Figures 2−4. Comparison with quantum chemistry computations revealed that the MP2/6-311G++(d,p) result is significantly closer to the experimental structure than those obtained with density functional (DFT, B3LYP-D3/aug-cc-pVTZ) methods (see Figure 2, left). Because of this, the former is compared with experiment in Figures 2−4, where not explicitly indicated. The Kraitchman analysis was performed using the KRA program;28 the r0 fits were carried out with the STRFIT program,23,28 and the complete results and graphical comparisons with the preferred optimized ab initio structures appear in the Supporting Information (Figures S1−S5). There is a good agreement, and theoretical methods found the experimentally observed structures when starting from our multi-isotopic fitted data. Note that just by using Kraitchman analysis several structures can be built that, after further computational

optimization, yielded reasonably good matches to the experimental ones, i.e., acceptable rotational constants and dipole moment components. In other words, without the complete set of moments of inertia from our experiment, it is fairly easy to find a “good” but mistaken answer. In fact, for the 3w cluster it was the r0 fit (used as the starting point) that guided the computation because the initial predictions missed the configuration actually observed. The emerging picture is that all of the observed clusters consist of a cyclic arrangement with an increasing number of water molecules. In each case the water cycle is anchored to the carbonyl group by a well-defined O−H···O hydrogen bond. The other end is attached to the camphor by a predominantly van der Waals interaction, as indicated by comparison of the resulting C···O and C−H···O contacts with standard van der Waals radii34 (rvdW(O+C) = 3.22 Å, rvdW(O+H) = 2.72 Å). The average O···O distance decreases steadily with cluster size and is equal to 2.854, 2.801, 2.788 Å, for the 1w(I), 2w(I), and 3w clusters, respectively. This is similar to the successively larger cyclic water clusters and is a consequence of the existence of hydrogen bond cooperativity.35 In consequence, a given O−H group becomes more polar, so that these cycles or chains are characterized by a shortening of the hydrogen bond and O···O 157


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dispersion is smaller. Assuming that the main contributor to the electrostatic energy is the O−H···O hydrogen bond, this result qualitatively reproduces the observed values for the O···O distances, 2.854(4)Å and 2.898(7)Å, respectively, showing a stronger interaction as the distance becomes shorter. In summary, we have presented here an accurate structural study for microsolvated camphor at the first stages of the formation of its solvation shell involving up to three molecules of water. Two different binding topologies for aggregates with one and two water molecules have been observed and characterized, as well as one form of the trihydrate complex of camphor. This represents the first experimental observation of a microsolvate of this size (14 heavy atoms) by rotational spectroscopy. We have also shown that multi-isotopic substitution experiments are crucial when determining structural information on clusters of this size where multiple nearly isoenergetic isomers are possible. Our results illustrate a valuable approach for unveiling the trade-off between hydrophobic and hydrophilic interactions and reveal the weak or moderate hydrogen bonds and dispersive interactions that shape the three-dimensional structure of increasingly more complex water−organic molecule complexes. The observed SNR for the trihydrate of camphor (50:1) is encouraging and leaves the door open to the observation of larger clusters in this system, as well as of larger clusters in general.

distances and an increase of the binding energy for a given hydrogen bond. The 1w(I) and 2w(I) complexes have distances similar to that of the water trimer36 (2.85 Å), and those for the 3w complex are even shorter. Note that in the observed 3w complex, the water trimer is an open chain instead of a cyclic structure as observed for the isolated water trimer.36 CP-FTMW spectroscopy provides reliable intensity information that can be used to estimate relative populations. The observed population ratios of 4:1 for the complexes 1w(I):1w(II) do not correspond to those expected from theory for a sample at 298.5 K. Theory predicts a mixture composition of near 1.3:1 from the energy difference of 0.6 kJ mol−1 and by assuming that entropic differences between the two dimers are negligible. The two-water complexes appeared at a ratio of 2:1, while the energy difference at the MP2 level of theory is 0.4 kJ mol−1, roughly corresponding to a population ratio of only 1.2:1 at 298.5 K. Note that there might be lower temperatures that could resemble better the experimental populations as recently reported.31 The observed population ratios are related to the kinetics of formation of the complexes in the supersonic jet which can be frozen far from equilibrium. Thus, comparisons of experimental results to theoretically calculated structures (in equilibrium) can lead to discrepancies. In the present case both energy factors (balance of attractive/repulsive forces) and kinetic factors seem to favor the formation of the 1w(I) complex. This applies also to the formation of 2w and 3w complexes, in which the hydrogen bond between the water molecule and the CO group of camphor is placed in nearly the same position as in the most stable camphor−(H2O) conformer, 1w(I). No 2w or 3w conformers have been observed in which water is bound to the carbonyl group CO in the same orientation as in 1w(II). To obtain further insight into the differences between 1w(I) and 1w(II), a zeroth-order symmetry adapted perturbation theory (SAPT) calculation37 was performed to decompose the energetic contributions to the intermolecular binding forces. Starting from the optimized structures (B3LYP-D3/aug-ccpVTZ), we performed SAPT(0)/jun-cc-pVDZ38 calculations, which correspond to a reduced aug-cc-pVDZ basis set (without diffuse functions on hydrogen and without diffuse d functions on heavy atoms). This is part of the Psi4 electronic structure package,39 and the results are presented in Table 2. As a priori expected, the different contributions to the stabilization energy are similar for both structures. However, the relative values of the electrostatic and dispersive terms change between clusters. While in the most stable 1w(I) isomer the Coulombic contribution is larger compared to 1w(II), the

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b02541. Rotational constants of all the observed species, figures and results from Kraitchman analysis as well as complete least-squares structural fits, and line lists of the experimental transitions (PDF)

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1w(I) 1w(II) Δ(ΔE)e

ΔEindb

ΔEdispc

ΔEexchd

ΔEtot

−49.20 −46.02 −3.18

−14.56 −13.64 −0.92

−11.55 −12.60 1.05

43.56 42.05 1.51

−31.76 −30.21 −1.55

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.P. acknowledges a Research Fellowship from the Alexander von Humboldt Foundation. A.L.S. acknowledges a Louise Johnson Fellowship from the Hamburg Centre for Ultrafast Imaging (CUI). J.C.L. acknowledges the Ministerio de Educación, Cultura y Deporte (Mobility Grant PRX14/ 00695), Ministerio de Ciencia e Innovació n (Grant CTQ2013-40717-P) for financial support and Dr. M. Schnell for her kind hospitality. Z.K. acknowledges a grant from the Polish National Science Centre, decision number DEC/2011/ 02/A/ST2/00298. M.S. acknowledges funding by the Fonds der Chemischen Industrie via a Dozentenstipendium as well as financial support by the Deutsche Forschungsgemeinschaft within the Schwerpunktprogramm SPP 1807 (SCHN1280/41). This work has been supported by the excellence cluster “The Hamburg Centre for Ultrafast Imaging - Structure, Dynamics and Control of Matter at the Atomic Scale” of the Deutsche Forschungsgemeinschaft.

Table 2. Energy Decompositions (kJ mol−1) from a SAPT(0)/jun-cc-pVDZ Analysis of the Two Observed Conformers of Camphor−(H2O) Complexes (1w(I) and 1w(II)) ΔEelsta

ASSOCIATED CONTENT

ΔEelst is the electrostatic or Coulombic energy. bΔEind corresponds to the induction and charge-transfer interactions. cΔEdisp accounts for the dispersive interactions. dΔEexch represents the repulsion due to quantum mechanical exchange. eΔ(ΔE) is the energy difference ΔE(1w(I)) − ΔE(1w(II)). a

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The Journal of Physical Chemistry Letters Structure Program. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2012, 2, 556−565.

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Wetting Camphor: Multi-Isotopic Substitution Identifies the

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