Dynamics of Weak, Bifurcated, and Strong Hydrogen Bonds in Lithium

Jun 13, 2011 - (30, 31) In this study, we report time-resolved measurements of isotopically diluted aqueous lithium nitrate trihydrate, a system that ...
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LETTER pubs.acs.org/JPCL

Dynamics of Weak, Bifurcated, and Strong Hydrogen Bonds in Lithium Nitrate Trihydrate Jasper C. Werhahn,† Stanislav Pandelov,† Sotiris S. Xantheas,*,‡ and Hristo Iglev*,† † ‡

Physik-Department E11, Technische Universit€at M€unchen, D-85748 Garching, Germany Chemical & Materials Sciences Division, Pacific Northwest National Laboratory, 902 Battelle Boulevard, P.O. Box 999, MS K1-83, Richland, Washington 99352, United States

bS Supporting Information ABSTRACT: The properties of three distinct types of hydrogen bonds, namely a weak, a bifurcated, and a strong one, all present in the LiNO3 3 (HDO)(D2O)2 hydrate lattice unit cell are studied using steady-state and time-resolved spectroscopy. The lifetimes of the OH stretching vibrations for the three individual bonds are 2.2 ps (weak), 1.7 ps (bifurcated), and 1.2 ps (strong). For the first time, the properties of bifurcated H bonds can thus be unambiguously directly compared to those of weak and strong H bonds in the same system. The values of their OH stretching vibration lifetime, anharmonicity, red shift, and bond strength lie between those for the strong and weak H bonds. The experimentally observed inhomogeneous broadening of their spectral signature can be partly attributed to the coupling with a low frequency intermolecular wagging vibration. SECTION: Kinetics, Spectroscopy

W

ater with its hydrogen-bonding (H-bonding) network remains an active field of research as evidenced by a plethora of recent studies aimed at understanding water’s structure, dynamics, and spectral and transport properties.1 The relaxation of this connected network dominates the water dynamics, and it is responsible for water’s anomalous properties. Infrared (IR) spectroscopy serves as a sensitive probe of the dynamics of this H-bonding network based on the premise that different strengths of H bonds are associated with different spectral signatures that can be probed experimentally. It has by now been well ascertained that the frequency of the OH stretching vibration of the H bond decreases, or, equivalently, the red shift from the free (non-H-bonded) OH vibration increases, as the underlying H bond becomes stronger.2,3 This correlation allows for the indirect probing of the H bonding network by time-resolved IR spectroscopy.4 7 In liquid water, the H-bonding network has a wide range of H-bond distances and angles and in addition some broken and/or bifurcated H bonds. This assignment does, however, depend on the actual definition of a hydrogen bond and varies depending on the underlying criterion used. Several criteria for defining a hydrogen bond have been introduced based on geometric,8,9 density partitioning10 or electronic structure analysis, such as the occupation number of the σ*OH antibonding orbital of the OH donor.11 The large ensemble of H bonds of different types and corresponding strengths results in the broad, structureless OH stretching band in liquid water. It is widely accepted that the contributions to the red (low frequency) side of this band arise from strong H bonds, whereas the blue (high frequency) r 2011 American Chemical Society

side is due to OH oscillators engaged in weak H bonds.2,3,12,13 However, the direct relation between a particular OH frequency or a frequency range and a specific underlying local molecular structure, as well as the origin of the center of the band and the switching between different H bonding partners are still subject to discussion.6,14,15 To date, the most detailed information on the transient structure of liquid water originates from molecular dynamics simulations that rely on a particular model that describes the underlying interactions.16 22 For these intermolecular interactions, the combination of theoretical22 24 and experimental data25,26 has led to a broader understanding of the properties of finite aqueous clusters including the ones involving the solvation of charged particles and the structure of their primary solvation shells. Recently, Laage and Hynes27,28 proposed that switching of H-bond partners in bulk water involves a transition state, where the donor H bond is bifurcated between two acceptors. Two-dimensional IR spectroscopy has suggested that broken or strained H-bonding configurations do not persist in the liquid but reform a hydrogen bond within 150 fs.29 However, the properties of these bifurcated H bonds are still subject to debate, especially since experimental probes are hindered due to the short lifetime of these fleeting H-bond configurations. Aqueous salt hydrates have emerged as promising model systems for confined water molecules with a well-defined geometrical arrangement.30,31 In this study, we report time-resolved Received: May 4, 2011 Accepted: June 13, 2011 Published: June 13, 2011 1633

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The Journal of Physical Chemistry Letters measurements of isotopically diluted aqueous lithium nitrate trihydrate, a system that incorporates all three distinct kinds of hydrogen bonds (strong, weak, bifurcated) in a well characterized, static (not dynamically interconverting) environment. The sample is first characterized with conventional IR absorption spectroscopy that reveals an interesting polarization effect stemming from the geometric arrangement of the water molecules in the crystal. This allows for a complete assignment of the observed individual OH stretching bands and their mapping onto three distinct local bonding environments corresponding to a weak, a bifurcated, and a strong H bond. We found that the bifurcated H bond displays an intermediate behavior when compared to the weak and strong ones. We observe a correlation between the red shift of the different vibrations, the anharmonicity, the relative strengths of the respective H bonds, and the corresponding relaxation times. The time-resolved data in combination with theoretical simulations provide novel information on the properties of bifurcated H bonds, which can be used to decipher the properties of hydrogen bonds in more complex environments such as water and ice. The used pump probe laser setup has been described before.30,32 We generate tunable pulses in the range 1700 3700 cm 1 (2300 3700 cm 1) through laser-pumping of two independent parametric oscillator amplifier devices. The pulses have duration of 0.8 ps (1 ps), a spectral width of 18 cm 1 (15 cm 1), and energies of 10 nJ (6 μJ), with the numbers in parentheses corresponding to the pump pulses. The normal vector of the plane of the optical table has been used for the definition of the polarization planes of the pump and probe beams. We changed the polarization and the frequency of the pump pulse to reach a selective excitation of the different OH stretching vibrations in our anisotropic sample (for more details, see Figure 2). In contrast, the polarization plane of the probe beam is set to 45 relative to the normal vector. Using this alignment, it is possible to measure the absorbance of the probe beam in parallel (||) and perpendicular (^) polarization (relative to the normal vector of the plane of the optical table) simultaneously, by simply decomposing the probe beam into its parallel and perpendicular components with the help of a beam-splitting polarizer. The energy transmittance T(ν) of the probing pulse through the excited sample is compared with the probe transmittance T0(ν) for the blocked excitation beam. The resulting relative transmission change ln(T/T0)||,^ for variable probe frequency ν and probe delay times tD are used in the following as a direct measure of the underlying dynamics. We used 15 M HDO in D2O samples to spectrally separate the probed OH stretches from the adjacent OD groups of the water molecules. The solvent was prepared by isotopic exchange in a mixture of appropriate amounts of D2O (>99.9 atom % D) and tridistilled H2O. An aqueous solution of LiNO3 at a concentration of 18.5 M was used to ensure the proper ratio for forming the trihydrate. The hydrate crystals are grown by slowly cooling the salt solution between two CaF2 windows in a cryostat at 200 K and warming it up later on to the desired temperature at ambient pressure. Three-micrometer-thick spacers were used to ensure a constant sample thickness. Steady-state IR absorption spectra of the sample were obtained from a commercial VECTOR 22 Fourier-transform infrared spectrometer (FTIR, Bruker Optics) with a spectral resolution of 1 cm 1. The potential energy scans and optimizations were performed at the second order Moller Plesset (MP2) level of theory with the aug-cc-pVTZ basis set33,34 using the MOLPRO quantum chemical package.35 This level of theory has been shown to produce accurate descriptions of hydrogen-bonded

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Figure 1. FTIR spectra of LiNO3 3 (HDO)(D2O)2. (a) The absorption spectrum of the liquid sample at 305 K (black line) is compared to the one of the crystalline sample at 220 K (green line). Three distinct peaks appear upon crystallization, pointing toward dicrete, distinguishable H-bonding environments of the OH groups. (b) Comparison of the polarization-resolved and FTIR spectra. The polarization-resolved measurements resulted in parallel (blue line) and perpendicular (red line) signals. For the definition of the parallel and perpendicular planes of polarization, refer to Figure 2 and the description of the used polarization-resolving laser setup. The sum of these signals (green line) reproduces the FTIR spectrum quite accurately.

environments as recently shown by comparison with methods that include higher levels of electron correlation at the CCSD(T) level.36 Figure 1a shows the FTIR spectrum of aqueous LiNO3 at 220 and 305 K. In the following discussion, water always refers to a 15 M HDO/D2O mixture. The liquid salt spectrum at 305 K exhibits only small deviations from that of neat water, the differences arising from the interactions of the OH groups with the respective ions. Upon freezing, a hydrate crystal is formed as indicated by the appearance of three distinct peaks in the OH stretching region (green solid line in Figure 1a. A weak shoulder at 3300 cm 1 appears due to the formation of small amounts of spatially separated hexagonal ice.31 Figure 1b shows the polarization-resolved, steady-state spectra measured using our laser apparatus and the same sample just blocking the pump beam. We observe that the three peaks do not occur in all polarizations, but the middle band is polarized orthogonal to the two narrow lines on the blue and red sides. The crystal structure of LiNO3 trihydrate has been previously determined from X-ray and neutron diffraction data.37,38 A picture of its orthorhombic structure belonging to the space group Cmcm is given in Figure 2a. The Cartesian coordinates of the unit cell are given in the Supporting Information. The water molecules are arranged in parallel planes interconnecting the cations and anions, as shown in Figure 2b. These crystal planes are connected by water molecules that are aligned perpendicularly to each other, as can be seen in Figure 2c. This novel orthogonal alignment of the water molecules in the anisotropic crystal gives rise to the observed polarization-separated spectrum. 1634

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The Journal of Physical Chemistry Letters

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Figure 2. (a) Crystal structure of LiNO3 (HDO)(D2O)2. The crystal structure belongs to the orthorhombic group Cmcm with the OH groups of the water molecules all lying in planes perpendicular to each other. These planes are projected in panels b and c for a better view. (b) Plane showing the bifurcated H bond giving rise to the absorption line indicated in red in Figure 1b. (c) Plane showing two types of hydrogen bonds: a weak bond to the oxygen atom of a nitrate anion, and a strong, ice-like bond between two water molecules, which results in the blue absorption spectrum in Figure 1b.

The spatial arrangement of the water molecules in the lattice makes it easy to map the absorption peaks onto local vibrations of the OH groups in the unit cell. The data of Figure 2b indicate that the water molecules lying in the same plane with the NO3 anion exhibit only one type of H-bonding environment: a bifurcated H bond to the adjacent nitrate anions. This vibration is assigned to the middle peak of the spectra at 3475 cm 1, since the perpendicular planes exhibit two different types of hydrogen bonding environments. The blue-shifted vibration at 3536 cm 1 is assigned to the OH group involved in a water nitrate H bond, while the red-shifted peak at 3385 cm 1 is due to the OH group engaged in a water water H bond (see Figure 2c). The oxygen oxygen distance in the latter (2.85 Å) is close to the one found in ice (2.7 Å). This, along with the reduced multipole moments of the OH group due to the lower water density in the crystal, accounts for the smaller red-shift of this vibration when compared to HDO ice (the observed red-shift is 90 cm 1 less than ice32). The steady-state spectrum of the sample (Figure 1b) can be analyzed by a simple model that includes three individual Lorentzian lines. The spectral widths (full width at half-maximum (fwhm)) of those bands are 30 cm 1, 80 cm 1, and 15 cm 1 for the strong, bifurcated, and weak bonds, respectively. It should be noted here that the splitting of the three peaks into the two orthogonal planes of polarization that the probe beam is separated into after transmittance through the sample happens only if the crystal sample is aligned correctly. The bisector between the red and blue plane in Figure 2a is oriented parallel to the light propagation direction, the red plane is parallel to the surface of the optical table, so that only the perpendicular (^) component of the probe beam is absorbed here, while the blue plane contains OH groups absorbing the parallelly polarized component of the probe beam (||). This alignment of the crystal is the only possible one to yield the observed polarization resolved spectra from Figure 1a. We now turn to the time-resolved measurements obtained for LiNO3 trihydrate at 220 K. Figure 3a shows the relative transmission changes measured at different delay times after pumping of the OH groups engaged in the weak(er) water nitrate bonds. At short delay times, the 0 1 transition is depleted by the pump pulse, leading to ground state bleaching (GSB) at the excitation frequency of 3536 cm 1. Due to the anharmonicity of the OH stretching mode, the excited state absorption (ESA) corresponding to the

Figure 3. (a) Probe spectra for three different delay times after excitation of the weakly bound OH groups at 3536 cm 1. (b,c) Transient data after excitation of the bifurcated and strongly bound OH groups, respectively. (d) Temporal evolution of the OH GSB (positive signal, experimental points) measured after excitation of the three individual vibrations shown in red, green, and blue, respectively. The signal transients obtained at the maxima of the corresponding ESA at 3140 cm 1, 3260 cm 1 and 3335 cm 1 are shown for comparison (same color code, negative signals, triangles). Three different lifetimes are calculated for the individual vibrations (solid lines). 1635

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The Journal of Physical Chemistry Letters 1 2 excitation by the probe pulse appears at 3335 cm 1. Both features decay within the first 10 ps. The spectral width of the ESA (fwhm) of 62 ( 10 cm 1 is significantly larger than that of the GSB (20 ( 10 cm 1). The values are obtained from fitting Lorentzian distributions to the transient spectral changes measured at a delay time of 1 ps (solid lines in Figures 3a c). Note that the spectral width of our probe pulse is 18 cm 1. The spectral feature around 3380 cm 1 is assigned to transient heating of the sample. The transient response of the strong H bond measured after excitation at 3385 cm 1 shows a similar behavior (see Figure 3c), except for the presence of an extremely broad ESA of 125 ( 10 cm 1 centered at 3140 cm 1. We note that a significantly higher pumping intensity was required to obtain a detectable ESA signal. The properties of this ESA are very similar to those found in isotopically diluted ice,32 a fact that provides further evidence for assigning this band to the ice-like bond. The transient spectra after pumping the bifurcated bond are shown in Figure 3b. The width of the GSB is 31 ( 10 cm 1, considerably smaller than the width of the corresponding transition measured by conventional IR spectroscopy (see Figure 1), a result that provides evidence for an inhomogeneous broadening of this band. This inhomogeneous broadening could give rise to different relaxation dynamics upon the change in the probing frequency within this band. We have not, however, observed any difference in the dynamics within our experimental resolution of 0.3 ps, which suggests a very fast spectral diffusion with dephasing time significantly shorter than 0.3 ps. The ESA measured after excitation of the bifurcated bond is located at 3260 cm 1 and has a spectral width of 110 ( 10 cm 1. It is noteworthy that the increasing strength of the observed bonds correlates not only with the red-shift of the respective OH frequencies, but also with the magnitude of their anharmonicity, which is reflected in the spectral shift between the GSB and the ESA and the width of the latter. All peaks show a nonvanishing signal at the excited frequency after long delay times due to the crystal heating via the pump pulse. Figure 3d shows the transient response of the crystal after excitation of the three individual OH stretching vibrations. The pump intensities during different measurements were adjusted in order to obtain the same amplitude of the GSB, respectively. The temporal evolution of the OH GSB measured at the same frequencies as the excitation are shown in red, green, and blue, respectively (see the positive signals and experimental points in Figure 3d). The signal transients obtained in the maxima of the corresponding ESA at 3140 cm 1, 3260 cm 1 and 3335 cm 1 are shown for comparison (same color code, negative signals, triangles). All features decay exponentially and terminate at different transmission levels due to the transient heating of the sample. The extracted lifetimes are 2.2 ( 0.3 ps for the vibration at 3536 cm 1, 1.7 ( 0.3 ps for the vibration at 3475 cm 1 and 1.2 ( 0.3 ps for the vibration at 3385 cm 1. The measured lifetimes of the OH stretching vibrations for the three individual bonds are considerably faster than those of isolated HDO monomers,30,39,40 which are on the order of 6.8 ps. By contrast, the lifetimes of the OH stretching vibrations in LiNO3 3 (HDO)(D2O)2 exceed the corresponding values measured in isotopically diluted ice Ih of just 380 fs.41 The magnitude of the red-shift of the OH stretching vibrations also correlates with the shortening of the measured lifetime. The results shown in Figures 1 and 3 suggest that the OH groups involved in strong and weak bonds appear to be quasihomogeneously broadened within our experimental accuracy. In

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Figure 4. Influence of the wagging mode on the width of the peak of the bifurcated H bond. (a) The molecular arrangement used to model the local bonding environment corresponding to the bifurcated H bonds. (b) The PES along the wagging angle R (solid line) and the square of the ground state wave function (dashed line). (c) The variation of the calculated OH bond length changes, ΔROH1 and ΔROH2, with the wagging angle R along the PES.

contrast, the bifurcated bond is subject to considerable inhomogeneous line broadening. This broadening is tentatively attributed to the coupling with various low-lying intermolecular vibrations. We relied on ab initio quantum chemical methods to illustrate the mechanism by which these couplings contribute to the broadening of the OH stretching vibration. In particular, the contribution of the water wagging mode has been further considered. We model the relevant local environment by considering two nitrate anions to which a water molecule is bound to via two bifurcated H bonds according to the arrangement shown in Figure 4a. The geometry of the model system is given in the Supporting Information. For each value of the wagging angle R, the two OH bond lengths of the water molecule are optimized while the nitrate anion moieties remain fixed at their crystal structure geometries. The resulting 1-D potential energy surface (PES) as a function of R is shown in Figure 4b, while the range of the changes in the two OH stretches (ΔROH) along this wagging motion is plotted in Figure 4c. These results suggest that values of R e ( 15 and corresponding changes in ΔROH e 0.0008 Å are accessible by the zero point motion along this 1-D path. These changes in the OH bond lengths will in turn induce a broadening of the OH band.42 As a calibration, changes in ROH of similar magnitude with the ones estimated here that are present in the water dimer bifurcated arrangement (0.0007 Å with respect to the isolated monomer value) result in shifts of ∼65 cm 1 in the corresponding frequency. This result correlates well with the experimentally observed broadening of 80 cm 1 and further demonstrates how the coupling to this low-lying wagging mode can contribute to the inhomogeneous broadening of the peak corresponding to the bifurcated H bond. 1636

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The Journal of Physical Chemistry Letters Bifurcated H bonds play an important role in the formation of aqueous hydrates and biomolecular structures,43,44 as well as in the dynamics of water molecules around solutes and in the bulk,27 further fueling the scientific interest for the investigation of their properties. Our results provide evidence that wagging motions lead to inhomogeneous broadening of the OH stretching vibration engaged in bifurcated H bonds. We believe that this inhomogeneity may be a more general feature for all OH groups bonded via a bifurcated motif. IR hole burning experiments on liquid HDO/D2O have previously reported the existence of three components of the OH stretching band with different relaxation dynamics.12 The spectral component closer to the frequency position of 3435 cm 1 is inhomogeneously broadened, and it is tentatively assigned to bifurcated H bonds. On the basis of this correlation, possible consequences for the interpretation of the OH stretching band in liquid water can be drawn. Our results support the notion that the contributions to the red side of this band stem from strongly bonded OH groups, while the blue side represents weakly bonded OH oscillators. The center of the OH stretching band is tentatively assigned to contributions from water molecules in bifurcated bonds, that are about to switch their bonding partners. It could be expected that the coupling of rotational and librational motions to the stretching mode leads to (partial) inhomogeneous broadening of the OH band in liquid water.

’ ASSOCIATED CONTENT

bS

Supporting Information. Dimensions and Cartesian coordinates of the unit cell of LiNO3(H2O)3, and Cartesian coordinates of the model system used in Figure 4. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (S.S.X.); [email protected] (H.I.).

’ ACKNOWLEDGMENT J.C.W. thanks the International Max Plank Research School on Advanced Photon Science for financial support. This work was partially supported by the DFG Cluster of Excellence “Munich Center for Advanced Photonics” and by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy (DOE). Computer resources were provided by the Office of Basic Energy Sciences, U.S. Department of Energy at the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy Office of Science user facility at Lawrence Berkeley National Laboratory. ’ REFERENCES (1) Ball, P. Water as an Active Constituent in Cell Biology. Chem. Rev. 2008, 108, 74–108. (2) Eisenberg, D.; Kauzmann, W. The Structure and Properties of Water; Oxford University Press: Oxford, 1969. (3) Franks, F. Water: A Comprehensive Treatise; Plenum Press: New York, 1972.

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