Decomposition of the Experimental Raman and Infrared Spectra of

Subscriber access provided by Technical University of Munich University Library

Letter

Decomposition of the Experimental Raman and Infrared Spectra of Acidic Water into Proton, Special Pair, and Counter-Ion Contributions Clyde A Daly, Louis M. Streacker, Yuchen Sun, Shannon R. Pattenaude, Ali A. Hassanali, Poul Bering Petersen, Steven A. Corcelli, and Dor Ben-Amotz J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02435 • Publication Date (Web): 04 Oct 2017 Downloaded from http://pubs.acs.org on October 5, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry Letters

Decomposition of the Experimental Raman and Infrared Spectra of Acidic Water into Proton, Special Pair, and Counter-Ion Contributions Clyde A. Daly Jr.,#1 Louis M. Streacker,#2 Yuchen Sun,3 Shannon R. Pattenaude,2 Ali Hassanali,4 Poul B. Petersen,3 Steven A. Corcelli,*1 and Dor Ben-Amotz*2 1. U. of Notre Dame, Department of Chemistry and Biochemistry, Notre Dame, IN 46556 2. Purdue U., Department of Chemistry, West Lafayette, IN 47907 3. Cornell U., Department of Chemistry and Chemical Biology, Ithaca, NY 14853 4. Condensed Matter and Statistical Physics, International Centre for Theoretical Physics, Strada Costiera, 11 I - 34151 Trieste, Italy # First authors * Corresponding authors Abstract Textbooks describe excess protons in liquid water as hydronium (H3O+) ions, although their true structure remains lively debated. To address this question, we have combined Raman and IR multivariate curve resolution spectroscopy with ab initio molecular dynamics and anharmonic vibrational spectroscopic calculations. Our results are used to resolve, for the first time, the vibrational spectra of hydrated protons and counterions and reveal that there is little ion-pairing below 2 M. Moreover, we find that isolated excess protons are strongly IR active and nearly Raman inactive (with vibrational frequencies of ~1500 ± 500 cm-1), while flanking water OH vibrations are both IR and Raman active (with higher frequencies of ~2500 ± 500 cm-1). The emerging picture is consistent with Georg Zundel's seminal work, as well as recent ultrafast dynamics studies, leading to the conclusion that protons in liquid water are primarily hydrated by two flanking water molecules, with a broad range of proton hydrogen bond lengths and asymmetries.

TOC image

H+ Raman and IR

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 20

The hydrated proton – a fundamental charge carrier and arbiter of acidity in aqueous solutions – has been the subject of seminal experimental1-2 and theoretical3-5 investigations,6 as well as detailed studies of cold clusters,7-9 solid hydrates,10-11 and aqueous solutions.11-27 Important open questions remain regarding the concentration at which counterion pairing begins to influence hydrated proton structure22, 28 and the assignment of aqueous acid vibrational features in relation to high-symmetry "Zundel" (H5O2+)1 and "Eigen" (H9O4+)29 structures observed in gasphase clusters.12, 17, 22, 28 We address these questions by utilizing Raman and IR multivariate curve resolution (MCR) measurements and theoretical calculations to resolve, for the first time, the vibrational spectra of an isolated proton, its flanking waters and hydrated counter-ion. Here we show that there is no significant ion-pairing below a concentration of 2 M and find that hydrated protons are quantum mechanically delocalized between two nearest neighbor oxygen atoms, with a broad range of O…H+…O distances and asymmetries, consistent with Georg Zundel's prescient conclusion1-2, 18-19 (although he incorrectly assumed that proton delocalization is primarily mediated by tunneling16). The importance of Zundel-like configurations is further supported by recent 2D-IR vibrational dynamics results indicating that "a relatively long-lived Zundel complex represents an important part of the proton-transfer mechanism"22 and "suggesting a central role of Zundel-like geometries in aqueous proton solvation and transport."25 Our results further indicate that, independent of whether a given asymmetric hydrated proton structure is classified as Zundel-like or Eigen-like, the hydrated proton itself invariably has a low vibrational frequency . 2200 cm-1, while its flanking water molecules have higher OH stretch vibrational frequencies & 2000 cm-1. Unlike the high-symmetry structures found in cold clusters, thermal fluctuations in liquid water give rise to a broad continuum of hydrated proton configurations, all of which involve a proton closely associated with two water molecules (as exemplified in Figure 1).


2

Page 3 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Hydrated proton structures, potentials (black curves), and probability distributions (blue curves), obtained from AIMD simulations and proton vibrational spectral calculations. The AIMD snap-shots highlight the excess proton (blue), its two nearest oxygen atoms (red), and flanking hydrogen atoms (white). The proton positions are referenced to the mid-point of the O...H+...O coordinate (and the proton probability distributions are normalized to the same area). The predicted proton vibrational frequencies, shown above each graph, correspond to the difference between the first two vibrational energies (red-dashed horizontal lines). The virtue of combining Raman and IR measurements of hydrated protons is linked to the expectation that a proton in an idealized centrosymmetric Zundel structure should be Raman forbidden and IR allowed, while the symmetric stretch vibration of an idealized Eigen structure should be Raman allowed and IR forbidden.15 These expectations, combined with our Raman-MCR and IR-MCR results (described below), confirm that Zundel-like proton vibrations occur over a broad frequency range, below ~2200 cm-1, and further imply that there is no vibrational spectral region that is dominated by symmetric Eigen-like vibrations. Moreover, the broad range of both IR and Raman active OH stretch vibrations that are red-shifted relative to pure water imply that water molecules surrounding a proton tend to be more strongly hydrogen bonded than pure water. We have corroborated and extended these experimental implications by combining ab initio molecular dynamics (AIMD) simulations with anharmonic local-mode

vibrational spectroscopic calculations that include nuclear quantum effects such as proton delocalization and zero point energy (ZPE)16 as exemplified by the simulation snap-shots shown in Figure 1. Note that the configuration shown in Figure 1a, which is one of the rare examples of double-well proton potentials, is also one that is classified as Eigen-like when the


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 20

proton is treated classically but Zundel-like when the proton is treated quantum mechanically,30 as a result of the quantum mechanical delocalization of the proton's ground vibrational probability density, as indicated by the blue curve in Figure 1a. Figure 2 shows our experimentally measured Raman (left-hand column) and IR (righthand column) spectra obtained from aqueous acid solutions, revealing the characteristic redshifted OH intensity of the acid solutions.1, 18, 31 The upper panels (a and d) in Figure 2 compare pure water and concentrated HCl solution spectra. The middle panels (b and e) in Figure 2 compare the spectra of pure water with 1M HCl and 1M NaCl solutions. The lower two panels in Figure 2 show the resulting Raman-MCR (c) and IR-MCR (f) spectra of hydrated H+ (green) and Cl- (orange). The hydrated Cl- spectra are equivalent to the MCR solute-correlated spectra of aqueous NaCl, as previous studies have shown that water molecules in the first hydration-shell of Na+ (and other relatively low charge density cations) have little influence on the OH stretch band of the surrounding water molecules.32-34 The hydrated H+ solute-correlated spectra are obtained by applying MCR to the measured spectra of 1 M NaCl (treated as the solvent) and 1 M HCl (treated as the solution) much in the same way that MCR has previously been used to separate counterion35 and head-group36 contributions to hydration-shell spectra. More specifically, the resulting hydrated proton spectra are equivalent to the minimum-area (nonnegative) difference between the measured spectra of 1 M HCl and 1 M NaCl; since hydrated Cl- is present in both solutions, the resulting difference spectra necessarily arise primarily from vibrations of the hydrated proton itself and any of its surrounding water molecules whose vibrations are been significantly perturbed by the proton. The above results are further supported by essentially identical hydrated proton Raman-MCR spectra obtained from aqueous HNO3 and NaNO3 solutions (see SI Figure S7). Additional concentration dependent measurements confirm that there is little or no ion-pairing between H+ and either Cl- or NO3- at a concentration below ~2 M (see SI Figures S7, S11, and S12), and thus the hydrated H+ spectra


4

Page 5 of 20

shown in Figure 2c and 2f represent the first experimentally measured vibrational spectra of an isolated (non-ion-paired) proton in water.

6

x10 25

IR-ATR

d 4M HCl

0.4

H2O

H2O

0.2 0.0

6

0

1000

2000

3000

4000 1000

b

Raman

10

2000

3000

4000

e

IR-ATR 1M HCl 1M NaCl H2O

1M HCl 1M NaCl H2O

0.6 0.4 0.2

0

0.0 0

1000

2000

Raman-MCR

3000 +

H

4000 1000

c

2000

IR-MCR

1000

2000

4000

+

f

3000

4000

H -

Cl x 0.5

-

Cl x 0.2

0

3000

3000

4000 1000

2000

Intensity (normalized)


0.6

12M HCl

0

x10

Intensity (counts)

a

Raman

Intensity (absorbance)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1

Vibrational Frequency (cm ) Figure 2. Hydration-shell spectra of isolated hydrated H+ and Cl- ions are obtained from Raman (left) and IR-ATR (right) spectra of water and aqueous solutions of HCl and NaCl. a, Raman spectra of concentrated HCl and pure water. b, Raman spectra of 1 M HCl and NaCl, and pure water. c, Raman-MCR hydration-shell spectra of dilute H+ and Cl- in water. All these spectra are obtained from un-polarized (total) Raman scattering measurements. d, IR-ATR spectra of concentrated HCl and pure water. e, IR-ATR spectra of 1 M HCl and NaCl, and pure water. f, IR-MCR hydration-shell spectra of 1 M H+ and Cl- in water. The prominent Raman band at very low frequencies (below 200 cm-1) in Figure 2a-2c has previously been attributed to the "anisotropic proton polarizability of the hydrogen bonds in H5O2+ groupings".

19

However, our results shown in Figure 2c, as well as those of previous

studies,37-38 indicate that NaCl solutions also produced enhanced intensity in this region, and Raman thus both the hydrated proton and its counterion contribute to low frequency Raman scattering that is enhanced relative to pure water. With regard to the high frequency OH stretch region (at


5


~3300±300 cm-1), our additional polarized Raman-MCR measurements reveal a rather abrupt increase in depolarization ratio above ~3200 cm-1 (see SI Figure S8), indicating that the low and high frequency portions of the hydrated proton OH stretch band arise from structurally distinct configurations. Moreover, the high frequency edge of the hydrated proton spectrum, near 3500 cm-1, resembles that arising from the hydration-shells of high charge density cations such as Li+ and Mg2+ (see SI Figure S10).

Experimental +


a

Zundel Eigen Double-Well Figure 1

IR Raman

c

3000

2000 1000

1500

2000

2500

3000

Calculated +

H

3500 Water

IR

b

Raman

1500

2000

2500

3000

3500 -1

Vibrational Frequency (cm )

-0.6

-1

1000

-1

1000

Proton Vibrational Frequency (cm )

H

Water

Proton Vibrational Frequency (cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 20

0.0

Proton Asymmetry δ (Å)

Figure 3. Comparison of experimental (a) and theoretical (b), IR (solid) and Raman (dotted) spectra of the hydrated proton (green) and pure water (blue), all scaled to the same maximum intensity. The right hand panel (c) shows the calculated distribution of proton frequencies as a function of the proton asymmetry parameter, d, and the resulting classification of the proton as either Eigen-like or Zundel-like.28 The black points in c correspond to the snap-shots highlighted in Figure 1 (black dots) and the protons with double-well potential functions (black X points), including the potential shown in Figure 1a. Figure 3a compares the experimental IR and Raman spectra of pure water and hydrated protons. In the OH stretch region (~3300±300 cm-1) the IR spectra are red-shifted relative to the corresponding Raman bands. This red-shift, is due primarily to the well-established non-Condon


6

Page 7 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


effect,39-40 as the Raman OH intensity is relatively insensitive to OH frequency (and hydrogen bond strength), while the IR OH intensity increases with decreasing OH frequency (and increasing hydrogen bond strength, as further shown in SI Figure S4). The substantially different frequencies of the OH stretch bands of pure water and the flanking water molecules confirm the strong proton-induced perturbations of the flanking water molecules. The assignment of the hydrated H+ IR absorbance at low frequencies (of ~1500 ± 500 cm-1) to the vibration of an excess proton oscillating between its two nearest neighbor oxygen atoms is consistent with our observation that these bands are strongly IR active and weakly Raman active (as expected from an approximately centrosymmetric Zundel-like proton structure). This assignment is confirmed and extended by our theoretical predictions shown in Figures 3b and 3c (as well as Figure 4, as further discussed below). Note that the theoretically predicted spectra pertain only to proton and OH stretch vibrations, and thus do not include the HOH bend band of water (at ~1730 ± 50 cm-1). Our theoretical results indicate that the small Raman intensity of the excess proton band (at ~1500 ± 500 cm-1) is due to the small Raman transition polarizability matrix element of the proton (see SI Figure S5), as well as its low concentration compared to the flanking OH groups. In contrast, the strong IR intensity of the excess proton arises from its exceptionally large transition dipole moment matrix element, which is over five times larger than that of the OH stretch of pure water and the flanking water molecules (see SI Figure S4). To elucidate the IR and Raman spectral contributions arising from Eigen-like and Zundel-like species we employ a previously described classification scheme.30 Briefly, oxygen atoms are considered as nodes connected by hydrogen-bonded edges, identified by finding the proton that minimizes the sum of the O-H distances to a pair of neighboring oxygen atoms, d(OH) + d(O’-H). Each proton is then assigned a asymmetry parameter (proton transfer coordinate) d = d(O-H) – d(O’-H), and a depth-first search clustering algorithm with a cut-off value of |d| < 0.2 is used to identify Eigen-like and Zundel-like species. This clustering procedure leads to 60% Eigen-like and 40% Zundel-like species, consistent with previous studies.41 Decreasing (or


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 20

increasing) the |d| cut-off value by 0.05 decreases (or increases) the Zundel-like population by 10%..30 Since the Eigen-like and Zundel-like species have similar proton vibrational frequencies distributions the corresponding proton spectra are insensitive the precise |d| cut-off value (as shown in Figure 3c). Figure 3c shows a scatter plot of our calculated proton frequencies as a function of the proton asymmetry parameter d. These results confirm that the majority of the protons are asymmetrically distributed along the O...H+...O coordinate, forming a hydrogen bond between two nearest neighbor oxygen atoms. Although one may designate particular hydrated proton structures as either Eigen-like or Zundel-like30 these do not correspond to separate stable species but rather are members of a continuum of structures. The classification of configurations as either Eigen-like or Zundel-like depends on whether d is calculated (and the classification clustering is performed) classically or quantum mechanically.30 Quantum delocalization increases the number of Zundel-like assignments, and blurs the distinction between the Zundel-like and Eigen-like species (to produce an approximately equal number of each).30 More importantly, since virtually none of the protons in liquid water have perfectly symmetrical Eigen or Zundel structures, one can designate most of the population as either Zundel-like or Eigen-like by altering the way in which the boundary between the two categories is defined. The results shown in Figure 3c further reveal that the proton potential is predominantly single-welled, and in the rare cases where it is double-welled (X points), the associated barrier it is invariably smaller than the proton's ZPE, in agreement with previous predictions4,

16, 24

and thus leads to the associated low proton vibrational frequencies, as

illustrated in Figure 1a. Figure 4 shows a more detailed decomposition of our calculated Raman and IR hydrated proton spectra into contributions arising from the excess proton itself (red) and the OH groups on the closer (green) and farther (blue) flanking water molecules, as illustrated in the diagram in


8

Page 9 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4a. More specifically, the green Raman and IR bands are obtained from OH groups on the oxygen atom that is nearest to the proton, and thus the red and green bands together may be viewed as arising from an over-coordinated oxygen atom, resembling a distorted hydronium (Eigen-like) structure. The blue flanking water bands are obtained from OH groups on the special pair oxygen atom that is farther from the proton. This spectrum is assumed to also approximate that of the water molecules in the first hydration-shell of the special pair (light blue protons in the diagram in Figure 4a), and thus the blue band is scaled to reflect the total number of blue protons (both dark blue and light blue). The latter approximation is supported by previous calculations showing that all such flanking and first-hydration-shell water molecules are approximately equally perturbed by the proton.7 Figure 4a reveals that the Raman spectrum of a hydrated proton arises predominantly from such flanking water molecules, while Figure 4b shows that the IR spectra contain more significant contributions from the proton itself, and the OH groups on the oxygen atom nearest to the proton.


9


Raman

IR a

Hydrated Proton Flanking Near Flanking Far

1000

2000

3000

1000

2000

2000

3000 d

1000 e

2000

g

2000

3000

2000

3000 f

Zundel

1000

3000

Eigen

1000

1000

3000

Zundel

1000

b

c

Zundel Eigen

Intensity (relative)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 20

2000

3000 h

Eigen

1000

2000

3000

-1

Vibrational Frequency (cm )

Figure 4. Calculated Raman and IR spectral components arising from vibrations of the proton (red), closest flanking water molecules (green), and next nearest neighbor flanking water molecules (blue), as well as the Zundel (purple) and Eigen (orange) configurations. The lower six panels in Figure 4 show spectra obtained from structures that are classically classified as either Eigen-like (orange) or Zundel-like (purple).30 The results in panels c and d reveal that both species give rise to similar spectra, although the Eigen-like species have a greater IR intensity peaked near ~2200 cm-1. The lower four panels of Figure 4 show our predicted decomposition of the Eigen and Zundel spectra into proton, and flanking water


10

Page 11 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


components. The results in Figure 4h make it clear that the broad green IR absorption band peaked near 2200 cm-1 of Eigen-like species arises from OH groups on the oxygen nearest to the proton. The low frequency of this green band makes sense as it pertains to OH groups of the distorted hydronium ion whose delocalized positive charge leads to stronger hydrogen bonding and a greater OH red-shift. On the other hand, the results shown in Figure 4f indicate that for Zundel-like species the green and blue OH populations have similar spectra, as they all resemble flanking water species (that are more weakly hydrogen bonded than the Eigen-like OH groups, but more strongly hydrogen bonded that pure water). Although the approach we utilized to calculate the hydrated proton IR and Raman spectra relies on several approximations, the generally good agreement between the calculated and measured spectra implies that our calculations faithfully capture the basic physics of the hydrated proton. More specifically, we have treated the proton and flanking water vibrations as uncoupled local modes and have neglected dynamical effects (i.e. motional narrowing) and Fermi resonances (including that between the proton and bends of the flanking water molecules). The methods we employ to compute both the IR and Raman spectra rely on sampling of the immediate environment of the excess proton, between its two flanking water molecules. Since the interaction between the proton and surrounding water is effectively a strong charge-dipole interaction, it is not too surprising that GGA functionals like BLYP succeed in capturing the local solvation environment of the proton as observed in previous theoretical and experimental studies.41-43 It is also worth stressing that, while the AIMD configurations we used are sampled from classical simulations, some nuclear quantum effects (such as proton delocalization and ZPE) are included in the subsequent IR and Raman spectral calculations. The neglect of water bending vibrations, Fermi resonance, and dynamical coupling between OH vibrational modes (as well as the fact that the calculated flanking water spectra are obtained from the nearest, non-over-coordinated, water molecule), are all expected to contribute to the relatively small differences between the shapes of the calculated and measured IR spectra.44


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 20

The neglect of coupling is expected to have a quantitative – but not a qualitative – influence on the overall spectrum, as the strength of the neglected coupling is expected to be substantially less than the ~500 cm-1 widths of the excess proton and flanking water spectra. Note that in pure liquid water the maximum value of the intra- and intermolecular coupling between OH vibrations is about 50 cm-1.45 The observed change in Raman depolarization ratio above ~3200 cm-1 indicates the presence of other water molecules whose vibrational spectra are only slightly perturbed relative to pure water. Previous definitive protonated water cluster studies have led Johnson and co-workers7-9 to beautifully elucidate the IR spectra of the size-selected clusters, whose vibrational features are significantly sharper than those of the hydrated proton in liquid water. High symmetry and magic number cluster spectra have previously been assigned to the corresponding Eigen and Zundel normal modes,7-8 and a recent study has shown how the Eigen cation can convert to a Zundel-like structure when interacting with strong hydrogen bond acceptor molecules.9 In liquid water at room temperature, we have found that Eigen-like and Zundel-like species give rise to nearly identical, and exceedingly broad, proton vibrational spectra associated with the broad range of liquid O...H+...O distances and asymmetries. Comparisons of our experimental and theoretical IR and Raman spectra establish that a hydrated proton vibrates between its two nearest neighbor (flanking) water molecules with frequencies of ~1500 ± 500 cm-1, while the flanking water OH groups have higher frequencies of ~2500 ± 500 cm-1, with Eigen-like OH configurations giving rise to a broad IR absorption peaked near 2200 cm-1, in general agreement with previous cluster spectra9 and liquid theoretical predictions.12 More specifically, our experimental and theoretical observations that both Eigen and Zundel species produce broad and overlapping IR absorption features is in agreement with previous theoretical predictions.12, 15, 22 However, previous assignment of the aqueous HCl IR absorption near 2750 ± 100 cm-1 primarily to a symmetric Eigen stretch vibration22 is inconsistent with our


12

Page 13 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


observations that vibrations in this region are strongly IR active and nearly Raman inactive, since a perfectly symmetric Eigen stretch is expected to be Raman allowed and IR forbidden. Raman spectra were collected using a home-built micro-Raman instrument46-47 with a 514.5 nm argon ion excitation laser with 15-20 mW power at the sample, a thermo-electrically cooled CCD detector (Princeton Instruments Inc., Pixis 400, 1340x400 pixel), a 300 mm imaging spectrograph (SpectraPro300i, Acton Research Inc.), with a 300 g/mm grating (5 cm-1 per CCD pixel dispersion), and slit width of ~ 70 µm (resolution ~12 cm-1, half width of neon line). Raman scattered light is collected using 7x100 μm core fiber optic bundle, with a round (6 around 1) arrangement at the collection end and linear (1x7) vertical stack at the spectrograph entrance slit. A temperature controlled spectroscopic cell holder (LC600, Quantum Northwest) was used to maintain the temperature of sample solutions in 1 cm glass cuvettes (Starna, Inc.) at 20.00±0.01 ˚C. Two spectra were collected from every sample and reference solution, each integrated for five minutes. Unless noted otherwise, all Raman spectra represent the total (not polarized) Raman scattering intensity. The neon and helium emission lamps are used for wavelength calibration as previously described.46-47 IR-ATR spectra were obtained using a Nicolet 8700 FTIR spectrometer (Thermo Scientific) with a diamond attenuated total reflection (ATR) Smart Orbit™ accessory (Thermo Electron Corp.), KBr beam splitter, and a liquid nitrogen cooled MCT detector.

Data were

collected using OMNIC software with 2 cm-1 spectral resolution and averaged over 512 scans, requiring about 9 minutes of data collection per spectrum. The IR spectra measured using a single-bounce diamond ATR crystal were corrected by the frequency-dependent penetration depth (dp) using frequency-dependent refractive indexes for diamond and water (n1 and n2) and incidence angle (ø) according to the following equation 48: 𝑑" =

𝜆 /

2𝜋𝑛( 𝑠𝑖𝑛+ ø − ( 0 )+ /1


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 20

Raman-MCR and IR-ATR-MCR results were obtained using the Self Modeling Curve Resolution (SMCR) algorithm49 to decompose measured Raman and IR-ATR spectra into solvent (reference) and minimum area solute-correlated (SC) components. The SMCR code was written in-house using Igor Pro 6.37 (Wavemetrics, Inc.) and utilized as previously described.46-47, 50-51 Hydrated anion A- spectra were obtained using a pure water reference and aqueous sodium salt (NaA) solution. Hydrated H+ spectra were obtained using a sodium salt (NaA) reference and an equimolar aqueous acidic (HA) solution. A constant background was subtracted from the SC spectra to produce a zero baseline intensity in the vicinity of ~3800 cm-1. Our theoretically predicted proton and flanking water spectra were obtained using a one dimensional discrete variable representation (DVR) of the corresponding proton motions, using configurations sampled from ab initio molecular dynamics (AIMD) simulations23 (see SI for details). The calculated IR and Raman spectra shown in Figure 3b and Figure 4 are histograms of the excess proton and flanking water vibrational frequencies, weighted by the square of either the transition dipole moment (IR) or the transition polarizability (Raman). The pure water IR and Raman OH stretch bands were computed in the same DVR procedure, using configurations obtained from a separate molecular dynamics (MD) simulation of neat SPC/E water.52 The protocol by which we calculated the inhomogeneous hydrated proton IR and Raman spectra mirrors the approach of Skinner and coworkers39,

53

(see the SI Vibrational Frequency

Calculations for details). A closely related strategy was recently utilized by Tokmakoff, Voth, and coworkers to investigate the IR absorption spectrum of the excess proton in water.14-15 While the present theoretical methodology is similar to the work by Voth and coworkers, the application to the novel solute-correlated infrared and Raman spectra reported in this paper is different. More specifically, we obtained the potential energy experienced by the excess proton using density functional theory (DFT) calculations of protonated water cluster containing the proton and its closest 25 water molecules, with configurations obtained using AIMD simulations (see SI and Figure S2). The resulting one-dimensional Schrödinger equation was solved numerically using


14

Page 15 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the DVR approach.54 Although we used a hybrid DFT functional for all our calculations, we also compared our results for a handful of configurations with a higher level of theory, namely MP2. We find that the trends observed with B3LYP are consistent with those obtained from MP2 (Figure S6). The transition dipole and isotropic polarizability matrix elements were obtained from the DVR wavefunctions for the ground and first excited vibrational energy levels, along with calculations of the dipole moment and polarizability as a function of the location of the excess proton between its two closest oxygen atoms. The flanking water spectra were obtained using the same procedure applied to the OH bonds of the nearest non-over-coordinated oxygen to the excess proton. Acknowledgements L.M.S., S.R.P. and D.B.-A. were supported by the National Science Foundation (CHE-1464904). Y.S. and P.B.P. were supported by the National Science Foundation (CHE-1151079) and The Arnold and Mabel Beckman Foundation. C.A.D. and S.A.C were supported by the National Science Foundation (CHE-1565471). Supplementary Information Experimental and computational supplementary methods and results Figures S1-S12 References (1) Zundel, G. Hydration and Intermolecular Interaction. Infrared Investigations of Polyelectrolyte Membranes. Academic Press: New York, 1969. (2) Zundel, G. Hydrogen Bonds with Large Proton Polarizability and Proton Transfer Processes in Electrochemistry and Biology. In Adv. Chem. Phys., 2000; Vol. 111, pp 1-217. (3) Agmon, N. The Grotthuss Mechanism. Chem. Phys. Lett. 1995, 244, 456-462.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 20

(4) Marx, D. Proton Transfer 200 Years after Von Grotthuss: Insights from Ab Initio Simulations. ChemPhysChem 2006, 7, 1848-1870. (5) Laage, D.; Hynes, J. T. A Molecular Jump Mechanism of Water Reorientation. Science 2006, 311, 832-835. (6) Agmon, N.; Bakker, H. J.; Campen, R. K.; Henchman, R. H.; Pohl, P.; Roke, S.; Thamer, M.; Hassanali, A. Protons and Hydroxide Ions in Aqueous Systems. Chem. Rev. 2016, 116, 76427672. (7) Headrick, J. M.; Diken, E. G.; Walters, R. S.; Hammer, N. I.; Christie, R. A.; Cui, J.; Myshakin, E. M.; Duncan, M. A.; Johnson, M. A.; Jordan, K. D. Spectral Signatures of Hydrated Proton Vibrations in Water Clusters. Science 2005, 308, 1765-1769. (8) Fournier, J. A.; Johnson, C. J.; Wolke, C. T.; Weddle, G. H.; Wolk, A. B.; Johnson, M. A. Vibrational Spectral Signature of the Proton Defect in the Three-Dimensional H+(H2o)21 Cluster. Science 2014, 344, 1009-1012. (9) Wolke, C. T.; Fournier, J. A.; Dzugan, L. C.; Fagiani, M. R.; Odbadrakh, T. T.; Knorke, H.; Jordan, K. D.; McCoy, A. B.; Asmis, K. R.; Johnson, M. A. Spectroscopic Snapshots of the Proton-Transfer Mechanism in Water. Science 2016, 354, 1131-1135. (10) Buch, V.; Dubrovskiy, A.; Mohamed, F.; Parrinello, M.; Sadlej, J.; Hammerich, A. D.; Devlin, J. P. Hcl Hydrates as Model Systems for Protonated Water. J. Phys. Chem. A 2008, 112, 21442161. (11) Reed, C. A. Myths About the Proton. The Nature of H+ in Condensed Media. Accounts Chem. Res. 2013, 46, 2567-2575. (12) Kulig, W.; Agmon, N. A 'Clusters-in-Liquid' Method for Calculating Infrared Spectra Identifies the Proton-Transfer Mode in Acidic Aqueous Solutions. Nat. Chem. 2013, 5, 29-35. (13) Hassanali, A. A.; Cuny, J.; Verdolino, V.; Parrinello, M. Aqueous Solutions: State of the Art in Ab Initio Molecular Dynamics. Philos. Trans. R. Soc. A-Math. Phys. Eng. Sci. 2014, 372.


16

Page 17 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14) Biswas, R.; Tse, Y. L. S.; Tokmakoff, A.; Voth, G. A. Role of Presolvation and Anharmonicity in Aqueous Phase Hydrated Proton Solvation and Transport. J. Phys. Chem. B 2016, 120, 1793-1804. (15) Biswas, R.; Carpenter, W.; Fournier, J. A.; Voth, G. A.; Tokmakoff, A. Ir Spectral Assignments for the Hydrated Excess Proton in Liquid Water. J. Chem. Phys. 2017, 146. (16) Marx, D.; Tuckerman, M. E.; Hutter, J.; Parrinello, M. The Nature of the Hydrated Excess Proton in Water. Nature 1999, 397, 601-604. (17) Agmon, N. Structure of Concentrated Hcl Solutions. J. Phys. Chem. A 1998, 102, 192-199. (18) Pernoll, I.; Maier, U.; Janoschek, R.; Zundel, G. Interpretation of Raman-Spectra of Aqueous Acid Solutions in Terms of Polarizability of Hydrogen-Bonds .1. Aqueous Hcl Solutions. J. Chem. Soc., Faraday Trans. II 1975, 71, 201-206. (19) Danninger, W.; Zundel, G. Intense Depolarized Rayleigh-Scattering in Raman-Spectra of Acids Caused by Large Proton Polarizabilities of Hydrogen-Bonds. J. Chem. Phys. 1981, 74, 2769-2777. (20) Botti, A.; Bruni, F.; Ricci, M. A.; Soper, A. K. Eigen Versus Zundel Complexes in Hcl-Water Mixtures. J. Chem. Phys. 2006, 125. (21) Tielrooij, K. J.; Timmer, R. L. A.; Bakker, H. J.; Bonn, M. Structure Dynamics of the Proton in Liquid Water Probed with Terahertz Time-Domain Spectroscopy. Phys. Rev. Lett. 2009, 102. (22) Thamer, M.; De Marco, L.; Ramasesha, K.; Mandal, A.; Tokmakoff, A. Ultrafast 2d Ir Spectroscopy of the Excess Proton in Liquid Water. Science 2015, 350, 78-82. (23) Hassanali, A.; Giberti, F.; Cuny, J.; Kühne, T. D.; Parrinello, M. Proton Transfer through the Water Gossamer. Proceedings of the National Academy of Sciences 2013, 110, 13723-13728. (24) Dahms, F.; Costard, R.; Pines, E.; Fingerhut, B. P.; Nibbering, E. T. J.; Elsaesser, T. The Hydrated Excess Proton in the Zundel Cation H5o2+: The Role of Ultrafast Solvent Fluctuations. Angew. Chem.-Int. Edit. 2016, 55, 10600-10605.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 20

(25) Dahms, F.; Fingerhut, B. P.; Nibbering, E. T. J.; Pines, E.; Elsaesser, T. Large-Amplitude Transfer Motion of Hydrated Excess Protons Mapped by Ultrafast 2d Ir Spectroscopy. Science 2017, 357, 491-494. (26) Decka, D.; Schwaab, G.; Havenith, M. A Thz/Ftir Fingerprint of the Solvated Proton: Evidence for Eigen Structure and Zundel Dynamics. Phys. Chem. Chem. Phys. 2015, 17, 11898-11907. (27) Napoli, J. A.; Marsalek, O.; Thomas E. Markland; (2017). Decoding the Spectroscopic Features and Timescales of Aqueous Proton Defects,. https://arxiv.org/abs/1709.05740 2017. (28) Baer, M. D.; Fulton, J. L.; Balasubramanian, M.; Schenter, G. K.; Mundy, C. J. Persistent Ion Pairing in Aqueous Hydrochloric Acid. J. Phys. Chem. B 2014, 118, 7211-7220. (29) Eigen, M.; Demaeyer, L. Self-Dissociation and Protonic Charge Transport in Water and Ice. Proc. R. Soc. Lond. A-Math. Phys. Sci. 1958, 247, 505-533. (30) Giberti, F.; Hassanali, A. A.; Ceriotti, M.; Parrinello, M. The Role of Quantum Effects on Structural and Electronic Fluctuations in Neat and Charged Water. J. Phys. Chem. B 2014, 118, 13226-13235. (31) Busing, W. R.; Hornig, D. F. Effect of Dissolved Kbr, Koh or Hcl on Raman Spectrum of Water. J. Phys. Chem. 1961, 65, 284-292. (32) Perera, P. N.; Browder, B.; Ben-Amotz, D. Perturbations of Water by Alkali Halide Ions Measured Using Multivariate Raman Curve Resolution. J. Phys. Chem. B 2009, 113, 1805-1809. (33) Schultz, J. W.; Hornig, D. F. Effect of Dissolved Alkali Halides on Raman Spectrum of Water. J. Phys. Chem. 1961, 65, 2131-2138. (34) Gaiduk, A. P.; Galli, G. Local and Global Effects of Dissolved Sodium Chloride on the Structure of Water. J. Phys. Chem. Lett. 2017, 8, 1496-1502. (35) Long, J. A.; Rankin, B. M.; Ben-Amotz, D. Micelle Structure and Hydrophobic Hydration. J. Am. Chem. Soc. 2015, 137, 10809-10815.


18

Page 19 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(36) Davis, J. G.; Zukowski, S. R.; Rankin, B. M.; Ben-Amotz, D. Influence of a Neighboring Charged Group on Hydrophobic Hydration Shell Structure. J. Phys. Chem. B 2015, 119, 9417−9422. (37) Walrafen, G. E.; Chu, Y. C. Low-Frequency Raman-Spectra from Concentrated Aqueous Hydrochloric-Acid - Normal-Coordinate Analysis Using a 4-Atomic Model of Cs Symmetry, (H2o)2(H3o+)(Cl-H2o). J. Phys. Chem. 1992, 96, 9127-9132. (38) Walrafen, G. E.; Chu, Y. C.; Carlon, H. R. Raman Investigation of Proton Hydration and Structure in Concentrated Aqueous Hydrochloric Acid Solutions. In Proton Transfer in Hydrogen-Bonded Systems, Bountis, T., Ed. 1992; Vol. 291, pp 297-311. (39) Corcelli, S. A.; Skinner, J. L. Infrared and Raman Line Shapes of Dilute Hod in Liquid H2o and D2o from 10 to 90 Degrees C. J. Phys. Chem. A 2005, 109, 6154-6165. (40) Loparo, J. J.; Roberts, S. T.; Nicodemus, R. A.; Tokmakoff, A. Variation of the Transition Dipole Moment across the Oh Stretching Band of Water. Chem. Phys. 2007, 341, 218-229. (41) Tuckerman, M.; Laasonen, K.; Sprik, M.; Parrinello, M. Ab-Initio Molecular-Dynamics Simulation of the Solvation and Transport of Hydronium and Hydroxyl Ions in Water. J. Chem. Phys. 1995, 103, 150-161. (42) Botti, A.; Bruni, F.; Imberti, S.; Ricci, M. A.; Soper, A. K. Ions in Water: The Microscopic Structure of a Concentrated Hcl Solution. J. Chem. Phys. 2004, 121, 7840-7848. (43) Tuckerman, M.; Laasonen, K.; Sprik, M.; Parrinello, M. Ab-Initio Molecular-Dynamics Simulation of the Solvation and Transport of H3o+ and Oh- Ions in Water. J. Phys. Chem. 1995, 99, 5749-5752. (44) Van Hoozen, B. L.; Petersen, P. B. Origin of the 900 Cm(-1) Broad Double-Hump Oh Vibrational Feature of Strongly Hydrogen-Bonded Carboxylic Acids. J. Chem. Phys. 2015, 142. (45) Auer, B. M.; Skinner, J. L. Ir and Raman Spectra of Liquid Water: Theory and Interpretation. J. Chem. Phys. 2008, 128.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 20

(46) Davis, J. G.; Gierszal, K. P.; Wang, P.; Ben-Amotz, D. Water Structural Transformation at Molecular Hydrophobic Interfaces. Nature 2012, 491, 582-585. (47) Gierszal, K. P.; Davis, J. G.; Hands, M. D.; Wilcox, D. S.; Slipchenko, L. V.; Ben-Amotz, D. Pi-Hydrogen Bonding in Liquid Water. J. Phys. Chem. Lett. 2011, 2, 2930-2933. (48) Nunn [PhD], S.; Nishikida, K. Advanced Atr Correction Algorithm; Thermo Fisher Scientific: Madison, Wisconsin USA, 2008. (49) Lawton, W. H.; Sylvestre, E. A. Self Modeling Curve Resolution. Technometrics 1971, 13, 617-633. (50) Fega, K. R.; Wilcox, D. S.; Ben-Amotz, D. Application of Raman Multivariate Curve Resolution to Solvation-Shell Spectroscopy. Appl. Spectrosc. 2012, 66, 282-288. (51) Y. Sun, Y.; Petersen, P. B. Solvation Shell Structure of Small Molecules and Proteins by IR-MCR Spectroscopy. J. Phys. Chem. Lett. 2017, 8, 611-614. (52) Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P. The Missing Term in Effective Pair Potentials. J. Phys. Chem. 1987, 91, 6269-6271. (53) Schmidt, J. R.; Corcelli, S. A.; Skinner, J. L. Pronounced Non-Condon Effects in the Ultrafast Infrared Spectroscopy of Water. J. Chem. Phys. 2005, 123, 044513. (54) Colbert, D. T.; Miller, W. H. A Novel Discrete Variable Representation for QuantumMechanical Reactive Scattering Via the S-Matrix Kohn Method. J. Chem. Phys. 1992, 96, 19821991.


20

Decomposition of the Experimental Raman and Infrared Spectra of

Recommend Documents