Picosecond Proton Transfer Kinetics in Water Revealed with Ultrafast

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Picosecond Proton Transfer Kinetics in Water Revealed with Ultrafast IR Spectroscopy William B. Carpenter, Joseph A. Fournier, Nicholas H.C. Lewis, and Andrei Tokmakoff J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b00118 • Publication Date (Web): 16 Feb 2018 Downloaded from http://pubs.acs.org on February 17, 2018

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The Journal of Physical Chemistry

Picosecond Proton Transfer Kinetics in Water Revealed with Ultrafast IR Spectroscopy William B. Carpenter, Joseph A. Fournier, Nicholas H.C. Lewis, and Andrei Tokmakoff*a Department of Chemistry, James Franck Institute, and Institute for Biophysical Dynamics, The University of Chicago, Chicago, Illinois 60637, USA

Abstract Aqueous proton transport involves the ultrafast interconversion of hydrated proton species that are closely linked to the hydrogen bond dynamics of water, which has been a long-standing challenge to experiments. In this paper, we use ultrafast IR spectroscopy to investigate the distinct vibrational transition centered at 1750 cm-1 in strong acid solutions, which arises from bending vibrations of the hydrated proton complex. Broadband ultrafast 2D IR spectroscopy and transient absorption are used to measure vibrational relaxation, spectral diffusion, and orientational relaxation dynamics. The hydrated proton bend displays fast vibrational relaxation and spectral diffusion timescales of 200-300 fs; however, the transient absorption anisotropy decays on a remarkably long 2.5 ps timescale, which matches the timescale for hydrogen bond reorganization in liquid water. These observations are indications that the bending vibration of the aqueous proton complex is relatively localized, with an orientation that is insensitive to fast hydrogen-bonding fluctuations, and dependent on collective structural relaxation of the liquid in order to reorient. We conclude that the orientational relaxation is a result of proton transfer between configurations that are well described by a Zundel-like proton shared between two flanking water molecules.

*

Author to whom correspondence should be addressed. Electronic mail: [email protected]. Telephone: (773) 834-7696

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1. Introduction The aqueous excess proton is a ubiquitous chemical species in acid/base chemistry whose structure and motions are intimately tied to the rapid fluctuations of water’s hydrogen bond (Hbond) environment. Despite the ubiquity of the aqueous proton and its prominent role in environmental chemistry,1–3 redox chemistry,4,5 fuel cell technology,6–9 and biological proton channels,10–12 there is still active research investigating the true solvation structure and dynamics of the proton in solution.13–18 Aqueous proton transfer is often described by the Grotthuss mechanism whereby the proton is guided by the water H-bond network resulting in long-range transport of the positive charge via minimal displacements of individual water molecules.13,19,20 This diffusion mechanism is responsible for the unusually fast proton diffusion in both aqueous21 and solid phases.22 The solvation structures of the aqueous proton in this process are most often discussed in the context of two limiting cases: the Eigen complex H3O+(H2O)3, a three-fold symmetric hydronium ion tightly solvated by three water molecules,23 and the Zundel complex H5O2+, an excess proton equally shared between two water molecules.24 These aqueous proton complexes are thought to distort and interchange on femtosecond to picosecond timescales due to the low energetic barrier for proton transport and the fast fluctuations of the aqueous H-bond environment. While molecular dynamics (MD) studies25–30 have provided a detailed description of the factors influencing aqueous proton transfer, experiments have lagged behind in their ability to verify predictions and uncover new aspects of proton diffusion at the molecular level. Infrared (IR) spectroscopy has been particularly useful for studying the aqueous proton since its vibrations are sensitive to molecular structure and solvation environment. The broad distribution of local environments around the excess proton results in an IR absorption continuum in strong acid solutions spanning 1000 cm-1 to 3000 cm-1 (Fig. 1). Upon subtracting the water FTIR spectrum, the continuum shows three broad features centered at 1200 cm-1, 1750 cm-1, and 3100 cm-1.31–33 The molecular origin of these features in solution remains an active area of research, but gas-phase experiments and calculations often assign these vibrations to the stretch of a Zundel-like proton stretching motion, bending motions of the hydrated proton complex, and OH stretching motions of waters flanking the excess proton, respectively.34–36

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Figure 1. The FTIR spectrum of neat H2O (blue), 2.5M HCl (green), and their difference (red), using data from Ref. 34. The hydrated proton bend δH+ is centered at 1750 cm-1.

A key objective for this study is contributing to the understanding of the origin of the hydrated proton’s spectral features in terms of its underlying structural and dynamic properties, and to use time-resolved IR experiments to understand the mechanism of proton transport in water. We recently measured two-dimensional infrared (2D IR) spectra of 2-4M HCl, exciting the OH stretches of the hydrated proton complex at 3200 cm-1 and probing the cross peak to the hydrated proton bend at 1750 cm-1.31 The observation of coupled stretching and bending modes at these frequencies indicated a persistent Zundel-like aqueous proton complex. Additionally, a timeresolved vibrational frequency shift of this cross peak on a 480 fs timescale was used to set a lower bound for kinetics of irreversible proton transfer. More recently, ultrafast IR experiments of 0.61.0 M HI in water and perchloric acid/water mixtures in acetonitrile have provided evidence for the proton residing in a broad, flat Zundel-like potential.37,38 As an additional challenge, the comparison of experiments and simulations are complicated by different definitions for the Eigen or Zundel complex. IR spectroscopy of the proton in solution accesses information on the vibrational potential energy of the aqueous proton complex, whereas “Eigen” and “Zundel” are more commonly defined in simulation as a function of geometric coordinates (such as O-O distances, the O···H···O proton sharing parameter δ, or coordination number). To adopt a language that is useful for both perspectives, here we use the terms “Zundellike” or “special pair” to refer to a pair of coupled water molecules sharing a proton through a single unique and persistent short H-bond, whereas we use “Eigen” to refer to a solvated complex 3 ACS Paragon Plus Environment


where the excess proton is covalently bonded to one water molecule. Within this definition, a Zundel-like complex could adopt instantaneous configurations that could be described geometrically as Eigen-like.18,39 To extend our insight into the spectroscopy of the aqueous proton and the kinetics of proton transfer, we report on time-resolved spectroscopy of the 1750 cm-1 band associated with HOH bending vibrations of the aqueous proton complex, here designated δH+ (Fig. 1). The hydrated proton bend is blue shifted compared to the water bend, consistent with stronger H-bonding, and overlaps with the continuum of protonated O-H stretch modes. The center frequency corresponds well to the asymmetric bending motion of the gas-phase Zundel complex,35,36 but the line shape broadens substantially in solution and other modes and configurations likely contribute. From recent computational analysis,40 we concluded that the hydrated proton bend shows significantly mixed bend, OH stretch, and proton stretch character. Overall, calling this transition the “hydrated proton bend” likely oversimplifies the nature of the vibration, yet this name will be useful due to the high degree of bend character. Here we take advantage of a 50 fs, 350 cm-1 bandwidth pump and a broadband IR probe to accurately measure the fast dynamics of the hydrated proton bend using 2D IR and transient absorption (TA) spectroscopy. We show that the hydrated proton bend has ultrafast lifetime and spectral diffusion relaxation timescales, but an unusually long orientational anisotropy decay of 2.5 ps. Based on the wealth of MD simulations, irreversible proton transfer is expected to involve H-bond reorganization and reorientation of the protonated species on a 1-2 ps timescale. We argue that the 2.5 ps timescale for the anisotropy decay of the hydrated proton bend is closely related to the timescale for irreversible proton transfer between configurations in which the proton is shared by two flanking waters in a Zundel-like arrangement.

2. Methods We performed 2D IR spectroscopy, transient absorption, and polarization anisotropy experiments using methods that have been described in detail recently.41 The spectrometer uses a 1 kHz Ti:Sapphire regenerative amplifier to drive two IR generation sources. Pump pulses are obtained from difference frequency generation in AgGaS2 resulting in 5 μJ, 50 fs pulses with a center frequency of 1685 cm-1 (~6 μm) and a bandwidth of 350 cm-1 FWHM. The probe pulses are generated from a broadband IR plasma source,42 which generates 250 fs, the measurements were identical with all types of sample windows. Control experiments for long time relaxation behavior, such as the concentration- and anion-dependent rotational anisotropy data, were collected with 1 mm thick CaF2 windows and a 6 μm Mylar spacer.

3. Results 3.1 Early-time 2D IR spectrum

Figure 2. The isotropic 2D IR spectra of (a) the H2O bend and (b)-(d) the 2D IR spectra of 1M, 2M, and 3M HCl, respectively at τ2 = 100 fs. Bleaches are presented in red while induced absorptions are blue, normalized to the highest intensity bleach feature in each spectrum.


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2D IR spectra of H2O and aqueous HCl solutions at early waiting time (τ2 = 100 fs) are presented in Fig. 2. The 2D line shape of the H2O bend (δHOH) in liquid water (Fig. 2a) has recently been discussed in detail.41 The positive feature in red on the diagonal at 1650 cm-1 is the ground state bleach (GSB) of the H2O bend. It is flanked by a broad excited state absorption (ESA) in blue that extends along the detection axis from 3 < 1500 cm-1 to >1900 cm-1, which is peaked at the 1→2 transition of the HOH bend (3=1590 cm-1). The large breadth of the ESA is attributed to the anharmonic H-bond environment and the ~2:1 Fermi resonance between the bend and the O-H stretch.41 The addition of an excess proton introduces new features in the 2D IR spectrum, as seen from the concentration series of 1M, 2M, and 3M HCl in Figs 2b-d. With the H2O bend at ω1=1650 cm-1, we observe a broad bleach of the hydrated proton bend (δH+) at excitation frequency ω1 = 1750 cm-1. We also observe an off-diagonal bleach around (ω1, ω3) = (1600 cm-1, 1750 cm-1). The appearance of this off-diagonal feature at the earliest waiting time suggests strong coupling between the hydrated proton bending mode and the bulk water HOH bend; however, the offdiagonal bleach intensity extends in frequency to ω11700 cm-1, we believe this signal originates from the background acid continuum signal, and does contribute to relaxation processes in our signal for τ2>60 fs. For τ2 > 5 ps, the TA spectra of H2O and 2M HCl across the entire detection range are independent of polarization and are similar, but not identical, to their 10° temperature difference FTIR spectra (Fig. S8). The overall resemblance arises from the fact that both ultrafast vibrational excitation and increased temperature lead to growth of population in low-frequency intermolecular motions, which appears as a net weakening of the H-bond network. However, unlike previous observations in aqueous solutions, the HCl spectra at frequencies >1700 cm-1 do not match the temperature difference spectrum, which indicates that the HGS spectrum at the waiting times in this work is not reporting on a fully equilibrated state.



Figure 6. (a) Isotropic TA trace of the δH+ bleach over ω3 = 1740 cm-1 – 1790 cm-1. The TA trace was normalized to the amplitude at early time and corrected for the HGS at long times. (b) TA traces of 2M HCl in the same ω3 range in the parallel (solid blue) and perpendicular (dashed green) polarization configurations. Inset: TA traces for τ2 < 600 fs, including isotropic signal (black dot-dashed). (c) TA anisotropy decay of the acid bend mode integrated over ω3 = 1740 cm-1 – 1790 cm-1, and fit to a biexponential decay after τ2 = 200 fs, used to determine the average timescale 〈τor〉 of the decay. Inset: Comparison of the δH+ anisotropy decay (blue) with the 80 fs anisotropy decay of δHOH (black).

To isolate the orientational relaxation component of the TA spectrum, we calculate the TA anisotropy decay as a function of waiting time (Fig. 6c). The TA anisotropy starts at 0.4 ± 0.05, dips below 0.1 by 70 fs, and then rises back to 0.4 ± 0.05 by 170 fs, before decaying slowly to zero with a mean timescale of 2.5 ± 0.5 ps. The anisotropy persists over an order of magnitude longer than the 80 fs anisotropy decay of the H2O bend in neat water (Fig. 6b inset).41 As illustrated from the instrument response (Fig. S7), the anisotropy dip at

≈60 fs and the anisotropy recurrence at

= 170 fs are well within our time-resolution. The linear growth of the parallel TA signal with concentration at τ2 = 200 fs (Fig. S7b) demonstrates that it arises from a δH+ resonant transition 12 ACS Paragon Plus Environment

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rather than a nonresonant Raman or transient birefringence signal. The anisotropy decay does not fit well to a single exponential, and we cannot definitively conclude that it has a biexponential form. Therefore, to quantify the relaxation time, a biexponential decay was fit to the anisotropy for τ2 > 200 fs and used to calculate a mean relaxation timescale of 2.5 ± 0.5 ps from the first moment of the relaxation curve. The error bars reflect the range of values obtained by different fitting procedures (Fig. S9).

Figure 7. TA orientational anisotropy relaxation traces of δH+ integrated over the range ω3 = 1740 - 1790 cm-1, normalized to the recurrence at τ2 = 200 fs. (a) Concentration-dependent anisotropy decays with timescale determined using an exponential fit after 1 ps. Decays are presented with offsets in steps of 0.05 for clarity. (b) Transient absorption anisotropy decays with varied halide counterion, fit to an exponential decay after 1 ps. (c) Anisotropy decay time as a function of viscosity due to increasing HCl concentration. Viscosity values are from Ref. 44.

In performing control experiments (Fig. 7), we find that the slow relaxation of the anisotropy is not strongly influenced by the identity of the counterion, varying by ~10% for HCl, HBr, and HI (Fig. 7a). The relaxation time also increases modestly with acid concentration (Fig. 7b). The results are consistent with the anisotropy decay time tracking the viscosity of the solution50 (Fig. 7c), although our error bars make it difficult to state this definitively.

4. Discussion 4.1 Anisotropy decay and irreversible proton transfer The 2.5 ps timescale observed for the TA anisotropy decay of the hydrated proton bend is unusually long when compared to other measurements of vibrational dynamics in H2O. While orientational relaxation of water in a variety of environments has been studied extensively with isotopically dilute water (HOD), revealing relaxation timescales of a few ps,48,51–54 other



mechanisms such as vibrational energy transfer and exciton delocalization have been shown to dictate the anisotropy decay of vibrational excitations in solutions with isotopically pure H2O. For example, the anisotropies of H2O bending and stretching excitations relax in 70-80 fs due to the delocalized nature of these vibrations,41,44,46,55–59 and on timescales of 200-300 fs for H2O coupled to molecular ions.45,60,61 The 2.5 ps time scale is more closely aligned with the 3 ps anisotropy decay observed for the OH stretching mode in isotopically dilute water,49,62,63 or similar orientational relaxation processes for water in ion solvation shells.48,54,60,64–66 These processes involve reorganization of the H-bond network of water. Observation of the same relaxation time and dependence on viscosity in ultrafast OKE experiments67 of HCl solution is another indication of the collective and thermally activated nature of the structural relaxation process involved. We therefore conclude that the bending vibration of the aqueous proton complex is relatively localized, with a transition dipole moment orientation that is insensitive to fast hydrogen-bonding fluctuations, and dependent on collective structural relaxation of the liquid in order to reorient. The anisotropy decay for the hydrated proton bend is also unusual since it persists much longer than the vibrational relaxation of the aqueous proton bend. The HGS has traditionally been interpreted as an isotropically thermalized state; however, here the orientational memory of the original excitation remains encoded in the HGS for at least several ps. A similar observation was made in aqueous hydroxide solutions which was attributed to a pre-thermalized heating effect that does not scramble the orientational information.68 Following this reasoning, it would appear that the intermolecular motions of the proton complex driven by relaxation of the aqueous proton bend results in a transient non-thermal intermolecular excitation that initially remains aligned with the original excitation. The spectral shift associated with the HGS retains the orientational information obtained by exciting the proton complexes aligned with the pump polarization. This HGS signal from pumping proton complexes is one component of the total HGS shifting the hydrated proton bend, the other being the isotropic HGS that arises from pumping the solvent water around the protonated complex. We account for four possibilities by which this anisotropy can relax: (1) irreversible aqueous proton transfer that results in a persistent change of alignment of the localized complex, (2) physical reorientation of the complex within the liquid without a proton transfer event, (3) rapid structural fluctuations within the complex and (4) vibrational energy transport and thermal 14 ACS Paragon Plus Environment

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diffusion from the intermolecular excitation of the initially excited proton complex to another with different orientation. The weak increase in anisotropy decay time with acid concentration demonstrates that the anisotropy timescale is not affected by vibrational energy transfer as the average distance between complexes is reduced, as was the case for OH‒.68 Physical reorientation of the entire hydration complex without proton transfer is unlikely since this would require the simultaneous rupture of the many H-bonds around but not internal to the complex. Generally, water’s rapid fluctuations contribute to a partial anisotropy relaxation on femtosecond timescales, rather than the picosecond dynamics associated with H-bond reorganization. Furthermore, they are often reversible, as in the case of unidirectional proton “rattling” between a special pair of water molecules that does not reorient the hydrated proton bend dipole.69 Similarly motions such as umbrella inversion70 or O-O compression,25 which have been linked to proton transfer, do not significantly reorient the bend dipole. In acid solutions, a unique possibility for fluctuations that could lead to irreversible anisotropy decay is the “special pair dance” predicted in simulations,30 whereby the identity of the excess proton rapidly randomizes among the three protons of the hydronium ion core. We believe this process would reorient the dipole alignment of the hydrated proton bend in an analogous manner to the fast 300 fs anisotropy decay of the aqueous nitrate N-O stretches which results from fluctuations in the ion’s hydration shell,60 and therefore do not believe that it exists in the present case. Taken together, these arguments indicate that our 2.5 ps TA anisotropy timescale arises from proton transfer events that accompany reorganization of the H-bond connectivity of the liquid. This finding is consistent with MD simulations that predict an interdependence of proton transfer and H-bond reorganization. These simulations predict that irreversible proton transfer requires a concerted reconfiguration of the solvation shells of the proton donor and acceptor.69,71,72 In this way, proton transfer resembles Marcus charge transfer in that the excess proton only transfers once the solvent reorients to stabilize the new configuration.30,73. Indeed, simulations predict that irreversible proton transfer occurs on the order of a few ps,14,28,30,69 which is approximately the measured H-bond switching time in H2O.48 However, relating the rate of an anisotropy decay to the rate of an associated kinetic process is non-trivial. Anisotropy experiments measure an orientational correlation function for a transition dipole vector, whereas the proton transfer rates are scalar and describe population fluxes between donor and acceptor states. Even so, the similarity of the anisotropy decay of the hydrated proton 15 ACS Paragon Plus Environment


bend to the anisotropy for rearrangement of water’s H-bond network suggests that this collective process is intimately coupled with proton transfer. In our previous measurement of spectral relaxation of the aqueous proton stretch-bend crosspeak, we set the lower bound for the timescale of proton transfer at 480 fs. This is significantly faster than the timescale for H-bond reorganization, but the measurement was also expected to be strongly influenced by vibrational energy transfer processes.31 On the other end of timescales, one could imagine that a single irreversible proton transfer event only partially alters the orientation of the special pair, which then would imply that the 2.5 ps reorientation timescale would overestimate the timescale for irreversible proton transfer. As a result of these bounds, we estimate that the transfer of excess protons in water observed through the δH+ spectroscopy is an activated process with a timescale of 1-2 ps.

4.2 Low-frequency motion driven by the hydrated proton bend The 170 fs recurrence in the anisotropy of the hydrated proton bend bears similarity to the beats that have been observed in TA signals for water and strongly H-bonded systems.43,74–79 In these systems, the TA signals of O-H stretching vibrations exhibited oscillations corresponding to impulsive driving of H-bond displacement motions that modulate the O-H stretch frequency. Here the modulation is observed in the anisotropy measurement, indicating that excitation of the aqueous proton bend briefly modulates the orientation and/or magnitude of its transition dipole. Overall, the anisotropy recurrence we observe most likely originates from an impulsive stimulated Raman excitation of intermolecular motions of the aqueous proton complex enhanced by a resonant interaction with the hydrated proton bend. Our polarization dependence is consistent with the anisotropic Raman response, and is reminiscent of the recurrence seen at 200 fs in the impulsive Raman response in neat H2O.80 Water and HCl solutions both display Raman active modes at this frequency,18,81,82 which could originate from a number of intermolecular degrees of freedom that are coupled to and aligned with the hydrated proton bend transition moment. There are other explanations for an anisotropy beat that we have ruled out. Our 60 fs instrument response and a linear concentration dependence of the signal indicates it is not influenced by nonresonant contributions to the signal. Other proton transitions contributing to the 1900-2600 cm-1 continuum could overlap in this region, but our TA spectra indicate that these contribute only near τ2=0. Picosecond anisotropy recurrences have been observed in 16 ACS Paragon Plus Environment

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multicomponent systems with different orientation relaxation timescales;83 however, we view this scenario as unlikely in the present case, given the ultrafast timescale of this recurrence. Coherences involving couplings between the H2O bend and the hydrated proton bend would oscillate slower, but we cannot entirely rule out vibrational coherences involving the modes that give rise to the elongated ridges in 2D spectra. A waiting time series of parallel 2D IR spectra demonstrates that both the diagonal and off-diagonal features display a recurrence at 150-200 fs (Fig. S10).

4.3 Relation to structures of the aqueous excess proton One of the main challenges of characterizing the hydrated proton bending IR transition is the ambiguity of the structures and vibrations that contribute to it. The peak at 1750 cm-1 has been considered a bending vibration of the aqueous proton due to its significant bending character,40 and it agrees in frequency with the gas-phase Zundel bend mode.84 However, there are multiple H-O-H bending degrees of freedom to any aqueous proton complex, and it has been demonstrated for gas-phase clusters that the immediate solvation environment plays a strong role in changing the frequency of the proton-related vibrations.85,86 Thus the presence of a peak at 1750 cm-1 cannot, by itself, be used to argue for a unique structure of the aqueous proton complex. Furthermore, the fluctuations of the H-bond environment rapidly dephase vibrations of the aqueous proton, which we see in the homogeneous broadening and fast frequency memory loss of the δH+ bleach. These line broadening effects mask the distribution of structures expected in aqueous solution. Finally strong mode mixing is predicted between proton stretching and bending motions.40 While structural configurations are often of primary interest, they may not be intuitively related to the spectroscopy of a species rapidly fluctuating on a multidimensional nuclear potential energy surface. Nevertheless, recent studies have pointed to the prevalence of aqueous Zundel-like arrangements where the proton is strongly bound between two flanking water molecules.18,31,37,38 Previously, we demonstrated that the highly redshifted OH stretch centered at 3100 cm-1 and the hydrated proton bend are strongly mixed, as seen in the proton stretch-bend cross peak in 4M HCl.31 In that work, we described the excess proton as Zundel-like since our excitation at 3100 cm-1 and detection at 1760 cm-1 is consistent with the coupled stretching and bending vibrations of the flanking waters in Zundel configurations. More recently, 2D IR and TA spectroscopy was used to interrogate the proton stretch at 1200 cm-1.37,87 By showing that the 0-1 17 ACS Paragon Plus Environment


transition of the proton stretching potential was lower than 1-2 transition, it was demonstrated that the potential had a low-barrier double-well shape, a hallmark for the Zundel ion. Another study recently compared IR and Raman spectra of the excess proton to anharmonic vibrational spectral calculations from ab initio simulations which included nuclear quantum effects. By combining simulation and experiment, a special pair picture for the hydrated proton was also supported, but it was emphasized that the hydration structure and proton can readily explore configurations between the traditional Zundel and Eigen geometries.18 The new observations reported here, in particular the long-lived anisotropy and 180 cm-1 recurrence, are also consistent with a localized Zundel-like pair of waters sharing a proton, and that there is a preferential alignment of the 1750 cm-1 hydrated proton bend with the H-bond axis between these two waters. A Zundel-like or special-pair perspective on proton structure and transport in water has been invoked in recent MD simulations.18,30,71,88 The simulations observe different mechanisms for proton transport, which can be broadly categorized as stepwise or concerted. In the stepwise mechanism, protons hop individually from one water to another, facilitated by H-bond rearrangements around the donor and acceptor.69,89 For concerted proton transport, the proton is displaced over multiple water molecules in a “burst” phase in a few hundred fs, followed by a rest phase lasting a few ps.28,72 Our current experiments cannot directly differentiate between these mechanisms. We believe that modeling our data and other experimental observations using MD simulation will be able to differentiate between proton transfer mechanisms.

5. Summary and Conclusions We have used 2D IR and vibrational TA spectroscopy to explore the ultrafast dynamics of the hydrated proton bending mode in 2M HCl. The hydrated proton bend vibrational excitation rapidly decays and loses frequency memory in

Picosecond Proton Transfer Kinetics in Water Revealed with Ultrafast

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