HCl Dissociation in Methanol Clusters from Ab Initio Molecular

Article pubs.acs.org/JPCA

HCl Dissociation in Methanol Clusters from Ab Initio Molecular Dynamics Simulations and Inner-Shell Photoelectron Spectroscopy Mahmoud Abu-samha*,† and Knut J. Børve Department of Chemistry, University of Bergen, Allégaten 41, NO-5007 Bergen, Norway S Supporting Information *

ABSTRACT: HCl dissociation in methanol clusters is studied by ab initio molecular dynamics simulations and experimentally by X-ray photoelectron spectroscopy. From theoretical simulations of HCl in oligomers and medium-sized clusters of methanol, two states of solvation are identified for HCl: an intermediate proton-sharing (ion pair) state and a fully dissociated state. Lowering the temperature from 150 to 100 K is found to promote full dissociation over the proton-sharing state. The dissociation of HCl is well reflected in the experimental chlorine 2p photoelectron spectrum recorded for a beam of clusters formed by adiabatic expansion of the vapor over a solution of HCl in methanol. In order to reproduce the observed Cl 2p spectrum by means of theoretical line-shape modeling, one needs to take into account both the intermediate proton-sharing state and the fully dissociated state.

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INTRODUCTION Solvation and dissociation of simple acids (e.g., HCl and HBr) on ice have attracted considerable attention as important steps in the depletion of stratospheric ozone.1−4 Of particular interest is whether these acids dissociate or stay intact on ice surface, and just how many solvent molecules it takes to break up the H−Cl (H−Br) bond. For HCl, several solvation and dissociation states were identified using reactive ion scattering5 and infrared spectroscopy with the help of ab initio calculations.6 Ab initio investigations of HCl dissociation in small water oligomers indicate that a minimum of three Hbonds (two to Cl and one to H) are needed to break up HCl. Moreover, a strong temperature dependence is found for HCl dissociation on ice:6,5 while HCl stays essentially intact on ice surfaces at 50 K, it dissociates upon increasing the temperature to 90 K. Solvation and dissociation of HCl in other protic solvents such as alcohols, ammonia, and solvent mixtures7−12 have also been investigated. Solvation of HCl in methanol provides an interesting contrast to the HCl/ice system, since HCl is a weaker acid in methanol than in water (HCl has a pKa value of −8 in water compared to 1.23 in methanol at 25 °C13). This allows for closer investigation of intermediate dissociation states. For HCl adsorbed on methanol surfaces, infrared spectroscopy in conjunction with ab initio molecular dynamics simulations8,14 led to identification of several intermediate dissociation states including single-donor slightly stretched HCl, a proton-sharing state (partial proton transfer),15,16 and the fully dissociated state (i.e., complete proton transfer from HCl to methanol). It turned out that at least three methanol molecules (two bonded to Cl and one to H) are needed to break the HCl bond, as in the HCl/water system. Moreover, investigation of HCl solvation in methanol revealed a new © 2014 American Chemical Society

phenomenon, namely the formation of extended proton wires.12,14,17 This is in contrast to the typical proton solvation state: either localized on a single solvent molecule or shared between two molecules. In the past decade, researchers turned to electron spectroscopy for studying solvation and dissociation on surfaces, in nanodroplets, and in liquid jets.18 This class of techniques includes X-ray absorption spectroscopy,19−21 Auger electron spectroscopy,22 and X-ray photoelectron spectroscopy. Adsorption and dissociation of HCl on ice surfaces (at 20 and 90 K) were investigated by X-ray absorption spectroscopy,20,21 and it was found that HCl dissociates at temperatures as low as 20 K. This new insight challenges the understanding from infrared spectroscopy and reactive ion scattering and led to much debate.23−25 So far and to the best of our knowledge, there has been no electron spectroscopy studies of HCl solvation/ dissociation in methanol. In this contribution, we report on investigations of HCl solvation and dissociation in methanol clusters by means of ab initio Car−Parrinello molecular dynamics (CPMD26) simulations and state-of-the-art synchrotron-based core-level photoelectron spectroscopy. CPMD is used to identify and characterize different states of solvation and dissociation and to explore how the degree of dissociation varies with radial position in the clusters, that is, surface versus bulk sites, as well as cooperativity between multiple HCl molecules in the same cluster. Chlorine 2p photoelectron spectroscopy is used to characterize HCl species in methanol clusters, drawing on the sensitivity of this spectroscopy to the intra- and extramolecular Received: May 18, 2014 Revised: July 30, 2014 Published: July 31, 2014 6900

dx.doi.org/10.1021/jp504883r | J. Phys. Chem. A 2014, 118, 6900−6907

The Journal of Physical Chemistry A

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chemical environment of the site of ionization27 and in particular to the number of H-bonds that the probed molecule exchanges with its chemical surrounding.28,29 The present study allows for comparing the solvation state of HCl near a curved methanol surface to that of a flat surface,8,14 as well as exploring both bulk and surface states using the same computational and experimental techniques.

cluster. The simulations were carried out at the BLYP-D level of density functional theory (DFT), that is, with an exchangecorrelation functional in the generalized gradient approximation as composed of Becke’s 1988 exchange functional31 and the correlation functional of Lee, Yang, and Parr,32 augmented with the empirical atom-pairwise additive dispersion correction (D2) devised by Grimme.33 The inclusion of dispersion is important to obtain a realistic density of the molecular cluster. The simulations were conducted using the CPMD-3.13.2 computer program,26 at a temperature of 150 K (chosen based on previous combined experimental study and MD simulations of pure methanol clusters38), time step of about 0.1 fs, and a simulation time of approximately 10 ps. The fictitious electron mass is set to 400 au. The atomic cores of C, H, and O were described by means of Troullier−Martins34 pseudopotentials whereas the Goedecker pseudopotential35 was used for Cl. A plane-wave energy cutoff of 35 au was used, with vacuum boundary conditions imposed using the symmetry keyword. To test for convergence, the calculations were repeated for the oligomers using a plane-wave energy cutoff value of 75 au (see SI). Moreover, oligomer structures from the CPMD simulations were optimized using the Gaussian-03 program36 with the B3LYP hybrid density functional and 6-311++G(d,p) basis sets. Shifts in Cl 2p ionization energy were computed for the oligomers using the B3LYP hybrid density functional and the 6311++G(d,p) basis set. For the N = 40 cluster, a two-layer QM/QM model is used, in which the HCl molecule and its solvation shell are described by the B3LYP functional in conjunction with 6-311++G(d,p) basis sets, whereas the remaining molecules in the cluster are described at the same level of theory yet with a smaller basis set, namely the 6-31+G. Figure 1 shows a snapshot of the N = 40 cluster partitioned into two QM layers. The core-ionized Cl is modeled by Ar+, i.e., the corresponding equivalent core atom.

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EXPERIMENT A heated liquid reservoir (“an oven“) was filled with ≈1.25 M solution of HCl in methanol (≈5 mole % HCl) and maintained at 49(1) °C to evaporate the liquid gradually. Helium was used as backing gas and the total pressure (helium+vapor) in the stagnation chamber was in the range 1.6−1.7 bar. The gas mixture was expanded adiabatically into vacuum through a conical nozzle of diameter 150 μm and a total opening angle of 20°, with subsequent rapid cooling and formation of binary HCl/methanol clusters. The temperature of the nozzle was kept at 83(1) °C. A skimmer is used to separate out a directional beam of clusters and uncondensed molecules. The pressure in the analyzer chamber was 1.6 × 10−5 mbar. The experiment was carried out in 2008 at the I411 undulator beamline of the Swedish synchrotron facility MAXLab, and the cluster beam was characterized by means of chlorine 2p and carbon 1s photoelectron spectroscopy recorded at photon energies of about 250 and 330 eV, respectively. The monochromator exit slit and electron pass energy were set to 15 μm and 50 eV, respectively. Spectral broadening due to instrumental settings is represented by a Gaussian distribution with full width at half-maximum (fwhm) of ≈0.09 eV for the Cl 2p spectrum and ≈0.11 eV for the C 1s spectrum. Alcohols are known to react with HCl to form alkyl chloride and water. On the basis of ref 30, one may estimate that at the concentration (1.25 M HCl) and temperature (49(1)°C) used in the present work, HCl and CH3OH react to form methyl chloride and water at an initial rate of 0.022 M/h. The spectra presented here were recorded ≈3 h after the oven was filled and show no clear sign of methyl chloride. However, 30 min later into the experiment an additional doublet of lines became discernible in the Cl 2p photoelectron spectrum. This additional signal is assigned to methyl chloride in the gas phase. The observed time evolution appears to be consistent with the stated rate constant and sensitivity of our spectroscopic technique. In subsequent experiments using fresh samples and the same oven, the methyl chloride signal is apparent once the working temperature was stabilized (∼1 h). A possible explanation for the accelerated generation of methyl chloride in all but the first filling of the heated stainlesssteel reservoir may be corrosion by HCl to define sites that catalyze the reaction between methanol and HCl. See Supporting Information (SI) for a detailed account of these experiments. It is noteworthy that in order to avoid the possible contribution from water that is coproduced with methyl chloride, in the present work we only discuss and analyze the initial set of spectra, i.e., data void of methyl chloride (and by inference, void of water).

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RESULTS AND DISCUSSION HCl Dissociation in Methanol Clusters from CPMD Simulations. Onset of Dissociation and Effect of Temper-

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COMPUTATIONAL DETAILS Ab initio Car−Parrinello molecular dynamics (CPMD26) simulations were carried out to study HCl solvation in methanol oligomers (N = 5, 6, and 7) and in a (CH3OH)≈40

Figure 1. Snapshot of a HCl·(MeOH)40 cluster model indicating the two-layer QM/QM model used in computing shifts in Cl 2p ionization energy. 6901



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Figure 2. Snapshots of (a) HCl·(CH3OH)5, (b) HCl·(CH3OH)6, and (c) HCl·(CH3OH)7.

ature. We begin the presentation of our theoretical results for HCl dissociation in methanol by considering methanol oligomers: the pentamer, hexamer, and heptamer. The respective initial structures are shown in Figure 2a−c. We investigate how the dissociation of HCl evolves with oligomer size, based on CPMD simulations at 150 K and energy cutoff of 35 au. Figure 3a−c shows the time evolution of the HCl bond length in the oligomers. In the pentamer, Figure 3a, the HCl bond varies mainly in the range 1.6−1.8 Å. In ref 14 it was suggested that a Cl···H distance in this range corresponds to an intermediate ion-pair state where HCl shares the proton with its nearest neighbor, and we will adopt this criterion for classification of HCl solvation states. Turning to the hexamer, Figure 3b, the HCl bond fluctuates between proton-sharing (at about 1.8 Å) and full dissociation (at about 2.0 Å), although HCl spends more time in a proton-sharing state than in the fully dissociated state. Regarding the heptamer, Figure 3c, we observe the same trend as for the hexamer. However, full dissociation of HCl is now the dominant event. In Figure 3d−f we present distributions of H−Cl bond lengths as accumulated in MD simulations of the oligomers. At 150 K, shown in red in the figure, the distribution of H−Cl lengths of HCl as solvated in a pentamer of methanol has a maximum at Cl···H distance of about 1.65 Å. In addition, a small shoulder at Cl···H distance of about 1.4 Å corresponds to structures where HCl acts as a single donor of hydrogen bond.12 Compared to the pentamer, HCl in a methanol hexamer has a broader bond-length distribution that is shifted toward longer Cl···H distance with a maximum at Cl···H separation of 1.75 Å and a shoulder at R(Cl···H) = 2.0 Å. In the heptamer, the distribution of Cl···H bond distances is rather bimodal with peaks at 1.7 and 2.0 Å, with the latter being significantly higher. To test the CPMD results for convergence, simulations were repeated at an energy cutoff of 75 au. The general effect of increasing the energy cutoff is to reduce the degree of dissociation somewhat, in particular for HCl in the pentamer (the results are detailed in the SI). The strong size dependency of HCl dissociation in the oligomers is a result of the variation in coordination number of chlorine; see Figure 2. In the cyclic pentamer, HCl is involved in two H-bonds only and the distribution function shows the population of the proton-sharing state and, to a lower extent, of the single-donor slightly stretched solvation state. In this case, the polarization effect of the environment is not sufficient to outweigh the attraction between the proton and the chlorine. Turning to the hexamer, HCl makes three H-bonds and the proton-sharing state dominates over the fully dissociated state. It is only when HCl is involved in four H-bonds, e.g., in the heptamer, that the fully dissociated state becomes more abundant than the proton-sharing state. These results indicate a gradual shift of population from the intermediate ion-pair

Figure 3. Evolution of the Cl···H bond distance during CPMD simulations of HCl·(MeOH)N, where N equals (a) 5, (b) 6, and (c) 7. In (d), (e), and (f) we present histograms of R(Cl···H) based on (a), (b), and (c), respectively. The red (black) curves denote results for simulations at 150 K (100 K). The black arrows indicate the minimum-energy Cl···H separation based on geometry optimization using B3LYP; see text.

state to the fully dissociated state upon increasing the coordination number of Cl. To address the temperature effect on population of the dissociation states of HCl, CPMD simulations at energy cutoff of 35 au were repeated at 100 K and the results are compared to those at 150 K in Figure 3. For HCl solvation in the pentamer, lowering the temperature effectively depletes the single-donor state as seen in Figure 3d from the loss of the population with an essentially intact Cl···H bond at 1.4 Å. The temperature effect is much more pronounced in the hexa- and heptamers. For the hexamer, cf. Figure 3e, decreasing the temperature from 150 to 100 K led to significant shift of population from the proton-sharing state to the fully dissociated state. Our calculations of minimum-energy structures indicate that the hexamer is characterized by a global minimum at a Cl···H distance of 1.95 Å and a local minimum at 1.8 Å, with an energy difference of about 1.26 kJ/ mole taking zero-point energy into account. Hence, it is reasonable that lowering the simulation temperature results in increasing the population of the fully dissociated state. Turning to the heptamer in Figure 3f, the Cl···H distribution function changes from a bimodal to a unimodal distribution with a peak at a Cl···H distance of about 2.0 Å, indicating that the acid spends significantly longer time in the fully dissociated state at 100 K compared to at 150 K. The optimized structure has a Cl···H separation of 1.98 Å. This analysis indicates that dissociation requires fewer solvent molecules at 100 K compared to 150 K: whereas HCl dissociates fully upon contact with only three solvent molecules at 100 K, three-tofour solvent molecules are required to break up the HCl bond at 150 K. Solvation of HCl in a Medium-Sized (N ≈ 40) Methanol Cluster. CPMD simulations were carried out for a (CH3OH)N≈40 cluster with one, two, or three HCl molecules, corresponding to ≈2.5, 5, and 7.5 mole % HCl. First, CPMD simulations were carried out for the methanol N ≈ 40 cluster containing a single HCl molecule near its center, 6902



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modified by placing an additional HCl molecule in the surface layer. We chose two initial configurations for the surface HCl molecule; singly versus doubly coordinated. For each of these, the simulation ran for about 15 ps. Profiles of Cl···H distances for both bulk and surface molecules are shown in Figure 4b,c. For the cluster with singly coordinated HCl at its surface, the surface HCl molecule stays intact with somewhat relaxed bond distance (≈1.4 Å). Turning to the cluster with doubly coordinated HCl in the surface layer, the surface molecule stays at 1.4 Å during the first 4 ps of the simulation, after which point it relaxes to a Cl···H distance of about 1.7 Å, characteristic for the proton-sharing state. The HCl in the bulk stays fully dissociated during the simulation with a mean Cl···H distance of 2.01 ± 0.08 Å. In this case, one can see that the presence of HCl on the cluster surface has little effect on the Cl···H profile of the bulk molecule. Simulations were also performed for a methanol N = 40 cluster containing three HCl molecules (7.5 mole % HCl). The initial geometry is a methanol N ≈ 40 cluster with fully dissociated HCl in the bulk and two HCl molecules in the surface layer. The simulation ran for ≈16 ps. Time evolutions of Cl···H distances are presented in Figure 4e. The profiles indicate that a fully dissociated state is stabilized in the bulk from 12 ps onward. Regarding the HCl molecules in the surface layer, both molecules may be characterized by the protonsharing state. From the preceding account, one may characterize the dissociation of HCl in the methanol N ≈ 40 cluster as follows. In the cluster interior, there is complete proton transfer from Cl to a neighboring methanol molecule. It appears unlikely that the core of this rather small cluster contains more than one fully dissociated HCl. At the cluster surface, by contrast, HCl either engages in a proton-sharing state or stays intact with a somewhat elongated bond. Theoretical Shifts in Cl 2p Ionization Energy for HCl/ (MeOH)N. In order to prepare for comparison to experimental core photoelectron spectra of HCl/methanol clusters, chlorine 2p ionization energies relative to that of the free HCl molecule were computed as described in the Computational Details section, based on structures from the CPMD trajectories described above. In Figure 5, Cl 2p shifts are plotted vs the Cl···

Figure 4. Time evolution of Cl···H distances for HCl solvation/ dissociation in methanol cluster with size N ≈ 40. In (a) a single HCl is solvated in the bulk (red line). In (b) and (c) two HCl molecules are solvated, one in the bulk (red lines) and one on the surface (blue lines). In (d) two HCl molecules are solvated in the bulk (red and gray lines) and in (e) three HCl molecules are solvated, one in the bulk (red line) and two on the surface (blue and black lines). The difference between (b) and (c) is in the coordination number of HCl at the surface, see text.

corresponding to 2.5 mole % HCl. The simulations ran for 25 ps at 150 K, and the time evolution of the Cl···H distance is shown in Figure 4a. The first 8 ps of the simulation are characterized by large fluctuations in the Cl···H distance between the proton-sharing and fully dissociated states, after which time HCl dissociates completely and stays dissociated for the remainder of the simulation. In the production phase (from 8 ps onward), the Cl···H distance has a mean value of 2.01 Å and a standard deviation σ = 0.06 Å. Next, CPMD simulations were performed for methanol N ≈ 40 cluster containing 2 HCl molecules (5 mole % HCl). Several choices of initial structures were explored. In the first of these, both HCl molecules were embedded in the same cyclic methanol hexamer only separated from each other by two methanols, located at the bulk-like core of the cluster. The time evolution of the Cl···H distances is presented in Figure 4d. During the first 9 ps of the simulation, the HCl molecules dissociate, as reflected in mean Cl···H distances of 1.94 ± 0.11 Å and 1.91 ± 0.09 Å, compared to 2.01 ± 0.06 Å for the single HCl at the cluster core as discussed above. From 9 ps onward, one HCl remains fully dissociated (with a mean Cl···H distance of 1.99 ± 0.09 Å) whereas the other gradually diffuses to the surface of the cluster and forms a proton-sharing state as evident from a mean Cl···H distance of 1.85 ± 0.10 Å. In order to investigate the effect of coordination number on the behavior of HCl, we ran simulations for the N ≈ 40 cluster containing two HCl molecules, one in the bulk and one on the surface. The starting point is the methanol N ≈ 40 cluster with one fully dissociated HCl at its core. The structure was

Figure 5. Theoretical Cl 2p shift vs Cl···H distance of HCl solvated in methanol pentamers (squares), hexamers (circles), and heptamers (triangles). 6903



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Figure 6. Distributions of shifts in Cl 2p ionization energy for (a) HCl·(CH3OH)40, and (b,c) (HCl)2·(CH3OH)40. In (a), the HCl is fully dissociated in the cluster bulk. In (b), the bulk HCl (red bars) is fully dissociated whereas the surface HCl (blue bars) acts as a single H-bond donor with a slightly stretched Cl−H bond. In (c), the bulk HCl (red bars) is fully dissociated whereas the surface HCl (blue bars) is forming a protonsharing state.

fwhm) is substantially larger for the proton-sharing state compared to the fully dissociated state. Experimental XPS Study of Neutral HCl/Methanol Clusters, with Comparison to Theory. In Figure 7, we present both Cl 2p and C 1s photoelectron spectra for binary HCl/methanol clusters. The C 1s photoelectron spectrum is characterized by a signal from gas-phase methanol (set at zero, ionization energy of 292.42 eV37) and a cluster signal at lower ionization energy with a cluster−monomer shift of −0.83 eV. From previous calculations of C 1s ionization energy for pure methanol clusters,38 a cluster−monomer shift of −0.83 eV corresponds to a mean cluster size of 40−50 molecules. The justification for comparison to theoretical calculations for pure methanol clusters is that the cluster composition is dominated by methanol and the perturbing effect of HCl is a minor one since the molecule may take part in the H-bonding pattern in the same way as an OH group. Turning to HCl, the Cl 2p photoelectron spectrum in Figure 7 shows two doublets corresponding to two chemically inequivalent chlorine sites. The narrow peaks at about 0.0 and 1.6 eV are the J = 3/2 and 1/2 spin−orbit components for gas-phase HCl (adiabatic ionization energies of 207.41 and 209.03 eV, respectively39). Of particular interest here is the broad doublet shifted by approximately −4.6 eV relative to the gas-phase signal. The direction of the shift and the sizable full width at half-maximum (fwhm) of about 1.0 eV shows that this signal corresponds to chlorine in a cluster. Combined with the corresponding cluster signal in the C 1s spectrum for methanol and knowledge of the constituents of the expanded vapor, this shows that we do produce mixed HCl/methanol clusters. The observed Cl 2p shift may be compared to the C 1s shift of −0.83 eV for methanol in the same cluster. The lack of a chlorine signal at an energy shift similar to the C 1s shift of methanol implies that there is no evidence of solvated, intact HCl in the present experiment. Moreover, from the theoretical calculations presented in the previous section, it was found that a single-donor slightly stretched HCl molecule has a chemical

H distance for a single HCl molecule solvated in methanol pentamers, hexamers, and heptamers. The energy shift correlates strongly with the Cl···H bond distance as well as the dissociation state of HCl. In more detail, chemical shifts in the range −2.0 to −2.5 eV are associated with HCl donating a single H-bond (with a mean Cl···H ≈ 1.4 Å). For the protonsharing state with Cl···H distance in the range 1.6−1.8 Å, the corresponding shifts are in the range −3.5 to −4.5 eV. For the fully dissociated state, i.e., Cl···H in the range 1.9−2.2 Å, the corresponding shifts are −4.2 to −5.0 eV. Next, shifts in Cl 2p ionization energy were computed for the methanol N ≈ 40 cluster containing a single fully dissociated HCl in the bulk (2.5 mole % HCl). The resulting distribution of chemical shifts is presented in Figure 6a, characterized by a mean value of −4.7 eV and full width at half-maximum (fwhm) of 0.40 eV obtained as 2.35 times the standard deviation of the distribution. Finally, the chemical shifts in Cl 2p were computed for the (HCl)2·(CH3OH)40 cluster, at 5 mole % HCl, first considering a cluster with a fully dissociated HCl in the bulk and a singly coordinated HCl at its surface. The shifts are presented in Figure 6b. For the bulk site, the distribution of ionization energies has a mean value of −4.7 eV and a fwhm of 0.45 eV. It may be noted that the presence of HCl on the surface has little effect on the distribution of ionization energy for the bulk molecule; i.e., while the mean shift remains unchanged, the fwhm increases from 0.40 to 0.45 eV. For the single-donor slightly stretched HCl at the cluster surface, the distribution of shifts is very broad (fwhm = 1.0 eV) and has a significantly smaller mean shift of about −3.0 eV. Turning to the cluster with a doubly coordinated HCl on the surface, the distributions of shifts are shown in Figure 6c. For Cl at the cluster core, the distribution of ionization energies has a mean value of −4.7 eV and a fwhm of 0.6 eV. For the surface molecule, the mean shift is −4.5 eV and the fwhm is 0.8 eV. The difference in mean shifts between the proton-sharing and fully dissociated states is small yet significant, and the spread in shifts (based on the 6904



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the Auger and photoelectrons, and intramolecular vibrational excitations). Inelastic scattering of photoelectrons is neglected in the present line-shape models due to the small cluster size. In Figure 8 we compare the experimental Cl 2p spectrum and the two theoretical Cl 2p line-shape models just described. In the upper part of the figure (part a), the model based on only fully dissociated HCl seems to reproduce the experimental cluster−monomer shift rather well, although it underestimates the fwhm of the cluster peak and in particular the intensity on the high-energy side of both spin−orbit components of the cluster signal. This discrepancy between model and observations suggests that the variation in coordination number for HCl is larger than accounted for when including only fully dissociated HCl, in particular since the shift in Cl 2p ionization energy correlates strongly with the Cl···H separation as can be seen in Figure 5. Next, in Figure 8b the experimental spectrum is fit using theoretical line shapes that include both fully dissociated HCl and HCl in the intermediate proton-sharing state, with an adjustable ratio between the two model components. The new fit is in much better agreement with the experiment, in terms of both cluster−monomer shift and width of the cluster peak. This suggests that the clusters observed in experiments include HCl in both fully dissociated and intermediate proton-sharing states. The optimized relative intensity between the proton-sharing state and the fully dissociated state is approximately 2 to 1, although this ratio is affected by inelastic scattering of photoelectrons originated in the cluster interior.40,41

Figure 7. C 1s (upper) and Cl 2p (lower) photoelectron spectra of binary clusters prepared by adiabatic expansion of 1.25 M HCl in methanol. The C 1s (Cl 2p) spectrum was acquired at 330 eV (250 eV) photon energy. The ionization energy scale is given relative to the adiabatic ionization energy of the gas-phase molecule at the respective edge (in the Cl 2p spectrum, the J = 3/2 spin−orbit component of the gas-phase spectrum is set at 0).

Cl 2p shift of −2 to −2.5 eV in a small oligomer, increasing in magnitude to −3 eV at the surface of the N = 40 cluster. Focusing on the J = 1/2 component of the experimental Cl 2p signal in Figure 8, there is no evidence for such a species at −3 eV or even −3.5 eV. Hence, it appears that a candidate species responsible for the large shift of −4.6 eV must be sought among fully or partly dissociated HCl. To address this point further, we prepare two theoretical line-shape models based on the distribution of computed Cl 2p shifts as presented in Figure 6: including either (a) only the contribution from fully dissociated HCl at the cluster core, or (b) the contributions from both fully dissociated HCl and HCl in the intermediate, proton-sharing state. In either case, the theoretical line shapes were prepared by convoluting the computed shift distributions with an asymmetric Voigt function representing the monomer spectrum (and thus includes both lifetime broadening, postcollision interactions (PCI) between

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CONCLUSIONS

We report on core-level photoelectron spectra and ab initio molecular dynamics simulations of HCl solvation and dissociation in free, neutral methanol clusters. CPMD simulations of HCl in methanol oligomers reveal two solvation states of HCl: an intermediate dissociation state where the HCl shares its proton with its nearest neighbor with a mean Cl···H distance of about 1.7 Å and a fully dissociated state with a mean Cl···H separation of about 2.0 Å. The relative population of these states depends on the coordination number of chlorine, the cluster size, and simulation temperature. Regarding the size effect, a minimum of three hydrogen bonds is required for HCl to dissociate fully. The effect of temperature is addressed by comparing the simulations at 100 and 150 K. We find that the population of fully dissociated HCl is higher at 100 K than at 150 K. CPMD simulations were carried out for a methanol N ≈ 40 cluster with 1, 2, and 3 HCl molecules, corresponding to 2.5, 5, and 7.5 mole % HCl, respectively. Whereas HCl dissociates completely in the cluster interior, it prefers the proton-sharing state at the cluster surface. Theoretical shifts in Cl 2p ionization energy are found to be very sensitive to the Cl···H distance (the solvation/dissociation state of HCl). Neutral, binary HCl/methanol clusters are produced by adiabatic expansion and characterized by core photoelectron spectroscopy. The Cl 2p photoelectron spectrum is analyzed in terms of theoretical shift models as computed for the CPMDderived structures. Only when taking into account both the fully dissociated state and intermediate proton-sharing states does the line-shape model agree well with the observed Cl 2p photoelectron spectrum.

Figure 8. Least-squares fits to the experimental Cl 2p photoelectron spectrum (red circles) based on line shapes for (a) fully dissociated HCl. In (b) the line shape represents both the intermediate protonsharing state (dotted line) and the fully dissociated state of HCl (dashed thin line). 6905



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ASSOCIATED CONTENT

S Supporting Information *

Convergence tests of CPMD simulations for the methanol oligomers in addition to further experimental measurements of Cl 2p photoelectron spectra of the HCl/methanol sample. 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]. Present Address †

M.A-s.: Art and Science Unit, Fahd Bin Sultan University, Tabouk, KSA. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank J. Harnes and M. Winkler at the University of Bergen and M. Tchaplyguine at MAXLab for their help in recording the experimental data. Many thanks to Leif J. Sæthre for fruitful discussions. We thank the Norwegian Metacenter for Computational Science (NOTUR) for a generous grant of computer time (Project No. NN2506K) and the Norwegian Research Council for financial support (Grant No. 205512/F20, Nanosolvation in Hydrogen-Bonded Clusters).

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HCl Dissociation in Methanol Clusters from Ab Initio Molecular

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