14028
J. Phys. Chem. B 2007, 111, 14028-14033
Correlation of the Vibrations of the Aqueous Azide Ion with the O-H Modes of Bound Water Molecules Chun-Hung Kuo, Dmitriy Yu. Vorobyev, Jianxin Chen, and Robin M. Hochstrasser* Department of Chemistry, UniVersity of PennsylVania, Philadelphia, PennsylVania, 19104-6323 ReceiVed: August 13, 2007; In Final Form: September 26, 2007
Dual frequency two-dimensional infrared spectroscopy (2D-IR) has been used to investigate the dynamics of the azide-water solvation shell. The memory of the azide transition frequencies is detected in the echo emitted by the OH stretching mode of the ion-bound water molecules. There is a significant positive correlation of the two frequency distributions that decays on a 140 fs time scale. The result confirms that the O-H bond of water molecules in the solvent shell have frequency fluctuations that are considerably slowed from those that are known in bulk water. The positive correlation is attributed to cooperative interactions of coordinated water molecules with an azide ion.
Introduction
made predictions regarding the dephasing of the CN- vibrational transition. Another view of the effect of small ions on water structure was obtained from recent work by Bakker and coworkers,20,21 which focused on direct observations of the water vibrational modes. Our objective is to combine the concept of these different types of experiments by obtaining information both from the viewpoint of the ion vibrator and the water dynamics in the presence of ions using the methods of dual frequency 2D-IR.22-24 Experimentally, dual frequency 2D-IR has been proven to be a useful tool in extracting information on coupled systems.24,25 The strong coupling between molecular ions and water yields ion-water clusters having relatively well-defined structures. The clustering slows down the reorientation and accelerates energy relaxation of the ion. Ultrafast, IR pump/probe measurements have established an inverse relationship between energy relaxation times and the reorientation of azide ions,15,26 by directly measuring their vibrational transitions. This picture is further evidenced by recent reports of the slow anisotropy decay of the OH stretch mode of water nearby to atomic ions.27 More detailed information of the solvation shell of ions can be gained by three pulse photon echo experiments, and indeed, the first IR experiment of this type involved the azide ion.13 The frequency-frequency time correlation function of the asymmetric stretching mode of azide yielded direct information on the fluctuation of the solvation shell. The same experimental methods have been applied to the aqueous cyanide ion, which also shows ultrafast spectral diffusion of the cyanide vibration.28 The azide ion was chosen for several reasons. It is useful as a vibrational probe for protein dynamics11,29 because its strong absorption in the mid-IR range makes it suitable for both linear and nonlinear infrared experiments free from protein absorption. Experimentally, the dynamics of vibrational-relaxation of azide has been characterized.14 Furthermore, there are numerous theoretical studies of the aqueous azide structures,16,17,30 and there have been high level computations of its energy relaxation.18,31
While the hydrogen-bond network of neat water has been widely studied by theory and experiment,1-3 the interaction between water and solutes presents a less studied but equally important class of related questions. The structural dynamics of water in the neighborhood of solutes is of great interest, particularly in cases where key physical properties of the solute are critically dependent on hydration, as in the examples of polar or charged molecules and proteins. For instance, water molecules in the vicinity of proteinssthe so-called biological waters differentiate themselves from the bulk and influence the protein structure and functionality.4,5 Therefore, the development of new methods of exposing the properties of the associated water represents an important goal. The motion of water molecules of hydration like those in the liquid can be extremely fast at ambient temperatures, so real-time studies of them requires special laser techniques. The ultrafast motions in neat and isotopically mixed liquid water have been examined by a variety of approaches using pulsed laser methods including the recent two-dimensional infrared spectroscopy (2D-IR).6-9 Just as the unusual structure and dynamics of liquid water show up clearly in these nonlinear infrared experiments, so the ultrafast structural changes of water around solutes present tractable questions for such experiments and theory. The ultrafast motions of small molecules or ions in aqueous solutions10-13 are particularly interesting because of the strong interactions and relatively welldefined hydrogen bond structures. Studies of photon echoes of molecular ions such as N3- in water13 and relaxation studies of pseudohalogen ions14,15 and cyanide12 have provided baseline data on some of the ultrafast dynamical parameters and vibrational frequency fluctuations of simple aqueous ions. The dynamical aspects of water structures around small ions have also received considerable theoretical attention.16-19 In particular, Skinner and co-workers have given a quantitative description of the population and spectral relaxation of the azide ion17,18 from first principles calculations of the three pulse photon echo responses. The simulations by Hynes and co-workers on cyanide19 have found energy relaxations in encouraging agreement with experiment, and they have
Experimental Methods
* To whom correspondence should be addressed. E-Mail: hochstra@ sas.upenn.edu.
Sample Preparation. Sodium azide (Aldrich, 99% purity) at 0.25 M in H2O was used for all experiments (Figure 1a).
10.1021/jp076503+ CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2007
Vibrations of the Aqueous Azide Ion with O-H Modes
J. Phys. Chem. B, Vol. 111, No. 50, 2007 14029
Figure 2. Spectral profiles of k1, k2 (red), and k3 (green) pulses. The center frequency for k1 is ca. 2100 cm-1 and that for k3 is ∼3530 cm-1. The dip at ∼2340 cm-1 is from CO2 absorption. Inset: interference of kSH and k3 (blue), kSH (red), k3 (green) as functions of delay between k1 and k3.
Figure 1. (a) FT-IR absorption spectrum of sodium azide (0.25 M) in H2O. (b) Difference absorption spectrum. This is obtained by subtraction of the H2O spectrum from a spectrum of N3- in H2O.
The solution is squeezed between two IR-grade CaF2 windows (1.4 mm thickness) without any spacer. This is to reduce the optical density at the OH stretch mode of water. A fresh sample is needed every few hours because of degradation. Laser Setup and Data Processing. The dual frequency echo experiment was based on the generation of femtosecond laser pulses in the 3 and 5 µm regions from a Ti:sapphire regenerative amplifier and two homemade IR optical parametric amplifiers (OPAs). The IR pulses, k1, k2, k3, are arranged to have almost equal intensities but different wavelengths (k1 and k2 are in the 5 µm range, k3 is in the 3 µm range). They generate the IR echo and a fourth weak local oscillator beam (kLO) heterodynes the echo field. The excitation beams were focused on the sample in the geometry of a box configuration. The generated thirdorder field in the -k1 + k2 + k3 phase-matched direction was combined with the kLO beam and sent to a liquid N2 cooled 32-element HgCdTe array detector after dispersion by a monochromator (TRIAX-190, HORIBA Jobin Yvon). The local oscillator pulse was advanced on the echo signals by ∼400 fs to optimize the spectral resolution and facilitate the data processing. The delays of the k1, k3, and kLO pulses were varied separately by computer-controlled mechanical stages. The output of the regenerative amplifier operating at 1 kHz consisted of 800 µJ pulses with widths of ∼50 fs. A singlefilament, white-light continuum was generated and divided into the seeds for two independent OPAs by a customized beam-
splitter. For the first OPA, the continuum was amplified through a two-stage KTiOPO4 optical parametric amplifier (OPA: KTiOPO4, type II, 3 mm crystal thickness) using ∼390 µJ of the available energy. The output beam of the OPA: KTiOPO4 was collimated by a curved mirror and filtered by a long-pass germanium filter. The resulting 2.4 µJ pulse duration of ∼75 fs had a fwhm ≈ 250 cm-1 and was centered at ∼3525 cm-1. Its energy was reduced and used to generate the k3 and local oscillator (kLO) pulses by means of an AR-coated wedged CaF 2 window (9:1 ratio). For the second OPA, the continuum was amplified using two stages of β-BaB2O4 (BBO, type II, 4 mm crystal thickness). The signal and idler were collimated onto AgGaS2 (Type I, 0.9 mm) to generate a beam in the 5 µm region. The resulting 2.1 µJ pulse duration of ∼70 fs (fwhm ≈ 270 cm-1), centered at 2100 cm-1 was split into the k1 and k2 pulses by an AR-coated ZnSe window. The spectra of the pulses that were used in the experiment are shown in Figure 2. In order to check the relative stability of the OPA’s outputs, the k1 beam was frequency-doubled in another AgGaS2 (Type I, 1 mm) to generate kSH and tuned to the frequency of k3. Time-delayed kSH and k3 pulses were then focused into a monochromator. The resulting time domain interferogram at frequency 4134 cm-1 averaged over 400 shots is shown on the inset of Figure 2. This result proves that the pulses with different colors were phase-locked even though widely different frequency components of the seed were used to generate the different mid-IR pulses. The heterodyned vibrational photon echo signal S(τ,T,λ) was measured as a function of the detected wavelengths, λ, and the two time intervals τ, between the first and second pulses, and T, between the second and third pulses. The echoes in the direction -k1 + k2 + k3, with k1 arriving at the sample before/ after k2 are referred to as rephasing (R) and nonrephasing (NR) signals. The three-time signal S(τ,T,t) is defined by numerical Fourier transformation on the monochromator frequency axis.32 The inverse Fourier transformation on the τ axis of S(τ,T,t) defines S(ωτ,T,t). A further Fourier transformation versus t gives the complex 2D-IR spectra ∼ S (ωτ,T,ωt). The relative timing of the k1 and k2 pulses was determined by maximizing the thermal grating signal.28 The timing of k2 and k3 was determined by
14030 J. Phys. Chem. B, Vol. 111, No. 50, 2007
Kuo et al.
Figure 3. Integrated heterodyned photon echo signal of N3- in H2O for some representative T values (dash line). Also shown are the Gaussian line fittings by putting a mask between -150 fs < τ < 150 fs in order to illustrate the real peak position of the interested echo signal.
maximizing the nonresonant (instantaneous) signal of CCl4. The overall accuracy of this measurement is (2 fs. The polarizations of the k1 and k2 pulses could be rotated relative to k3 and kLO, allowing the relative values of the signal tensors and to be estimated. The relative magnitude of the two-color pump/probe spectra for each polarization was obtained by matching the shape of the real part of each of the rephasing spectral projections33 to the highfrequency side of the difference FTIR as a model of the transient pump/probe spectrum. The calibration of the two polarization data sets was obtained from the nonresonant signal from the CaF2 window. The polarized background signal due to the combination mode of neat water was also considered in the analysis.28 Results Figure 1b shows the difference between the spectra of the azide solution and neat water. Azide has an absorption peak at 2049 cm-1 (fwhm ∼ 28 ( 1 cm-1). The water combination (bending/libration) mode is seen clearly in this same region of Figure 1a but is not present in the difference spectrum, which only shows changes in the OH stretch region. A series of integrated heterodyned photon echoes with different waiting times T is shown in Figure 3. The arbitrary intensity I specified in the figure corresponds to a discrete sum over the entire frequency range of ωt corresponding to the following:
IR(τ,T) )
∑ω |SR(τ,T,ωt)| t
for τ > 0 and
INR(τ,T) )
∑ω |SNR(τ,T,ωt)| t
for τ < 0
The strong signal confined within -150 fs < τ < 150 fs contains the nonresonant signal from the window and a resonant signal from the combination mode of water.28,34 At T ) 180 fs, the nonresonant signal from the window has disappeared. The thermal grating signal from water grows in over a period of a few ps, but is still confined to the region -150 fs < τ < 150 fs.28 At T values of 300 fs and 1 ps, the signal within this time range is a mixture of the thermal grating signal from neat water
Figure 4. Absolute magnitude of the rephasing 2D-IR spectra in the cross-peak region for N3- in H2O. The tilt angle which is defined in the text is also shown for T ) 30 fs.
and the signal from azide. The azide echo signal is asymmetric with respect to τ at early T. Its maximum (∼50 on the scale of Figure 3) is masked by the strong background signal. The maximum of the cross-peak signal is shifted to positive values of τ for T < 0.12 ps, and it is centered within the range -150 fs < τ < 150 fs for T > 0.12 ps. The 2D-IR spectra can be used to separate the azide signal from that of neat water and the CaF2 window. In the heterodyned 2D-IR photon echo experiment a cross-peak is observed in the frequency range ωt ) 3300-3700 cm-1 and ωτ ) 2010-2070 cm-1. Figure 4 shows results for the absolute value of the rephasing photon echo signal for T varying up to 1 ps. At early times, up to 150 fs, the spectral cross-peak is distorted by the nonresonant spectrum of the CaF2 window, which is apparent on the left side (lower frequency) of all of the spectra. In addition, there is a weak background over the whole explored region from a broad water signal that underlies the cross-peak and stretches across ωτ. Despite these additional signals, the cross-peak from the azide/OH dual color experiment is easily seen. For T up to 0.3 ps, the cross-peak is tilted away from being parallel to ωt, as indicated in Figure 4. This tilt angle is the angle between the major axis of the elliptical outline of the signal and the ωt axis. The procedure to obtain the angle was as follows: the contour plot of the cross-peak was least-squares fit by an ellipse which gave the major and minor axes and their tilt angle (θ). At delay times where the cross-peak is distorted by the window signal, only the non-affected, higher frequency part of the signal was used in the processing. In the 2D-IR spectra the cross-peak has a pronounced tilt angle for waiting times up to T ) 0.15 ps but tends to 0° for T ≈ 0.3 ps. The integrated signal amplitude of the cross-peak decreases with time up to T ) 0.4 ps and then increases to reach a plateau at T )
Vibrations of the Aqueous Azide Ion with O-H Modes
Figure 5. Variation of the tilt angle on waiting time T. An exponential decay with time constant 137 fs is drawn through the data.
0.9 ps. The kinetics of the cross-peak is explained by scattering of k3 by the grating induced in water by energy relaxation from the azide excitation (see below). Thus, by T ) 0.4 ps, the crosspeak of the photon echo is beginning to be contaminated by the grating signal, which soon dominates. It is important to emphasize that the slow formation of the grating signal does not influence the early time behavior of the ellipse evolution. The ratio / was found to be 1.17 ( 0.07 for T near 0 and became 1.00 ( 0.04 for T exceeding 200 fs. The small positive value of this ratio indicates that the mean angle between the OH and N3- transition dipoles is less than the magic angle.35 Discussion Linear IR. The interaction of the ion and its solvation shell is clearly seen from the difference spectrum in Figure 1b, which demonstrates that the OH stretch band loses intensity at the lowfrequency side and is enhanced at the high-frequency side on addition of ions. Thus, there is an overall weakening of the hydrogen bonds.36,37 This equilibrium change in the OH stretch band suggests that hydrogen bonds between the azide ion and water molecules in the solvation shell are weaker than hydrogen bonds in neat water. Such shifts have been observed in other ionic solutions37,38 and attributed to water bound to anions. The fraction of water molecules that are affected by the anions is estimated from the magnitude of difference spectra to be ∼3.2%. This corresponds to ∼7 water molecules per anion, which is consistent with results of simulations.16,30 Echo Experiment. Figure 3 shows that the heterodyned echo signal integrated along ωt exhibits maxima at finite τ values for small enough values of T. The existence of this peak shift indicates that a vibrational frequency distribution persists for the very early waiting times. However, the echo peak is no longer visible when T > 120 fs because it moves into the region of the water and window responses, where it cannot be directly measured. However, the result shows that there is a component of the spectral diffusion that is ∼120 fs. For longer waiting times, the total signal increases, but is dominated in the time range of -150 fs < τ < 150 fs by a thermal grating. The part of the spatial modulation and energy dissipation contributed by the azide excitation is easily seen from the traces in Figure 4. For example, at T ) 1 ps, the signal decay along τ corresponds to the absolute value of the free induction decay of azide transition. Unlike the thermal grating from neat water, which is only detectable when k1 and k2 are overlapped in time and position because of the very short coherence time of the water
J. Phys. Chem. B, Vol. 111, No. 50, 2007 14031 combination mode,28 the thermal grating induced by energy relaxation from the azide transition can occur even when τ is longer than the pulse width. This is because the coherence decay time of the azide transition is long enough to preserve the spatial modulation of the pulse, which explains why the azide induced grating signal shape mimics the FID of azide transition. From the measurement, the coherence time of azide asymmetric stretch can be estimated by fitting the integrated photo echo signal at T ) 1 ps to a single-exponential decay function: this procedure leads to a coherence time of 320 ( 50 fs. This evolution can also be visualized in the frequency domain as traces along ωτ. For the 2D-IR spectra, the interesting dynamical feature is the change of the tilt angle which is shown in Figure 5. When fitted to an exponential decay function, it gives a decay time of 137 ( 20 fs. The simplest description of the rephasing part of the 2D-IR spectrum at T ) 0 fs is obtained from Fourier transform along τ and t of response functions of the following type:24,39,40,41 2 2
2 2
eiωAZt-iωOHt-σAZ τ /2-σOH τ /2+fσAZσOHτt in which static inhomogeneous distributions are assumed, and other types of relaxation are omitted for clarity. The ωAZ and ωOH are the transition frequencies of two modes, σAZ and σOH are the widths of their frequency distributions and f is their correlation coefficient. It is f that determines the tilt angle. For the case in which the widths are the equilibrium spectral widths of azide and the OH stretch, the tilt angle at T ) 0 fs is expected to be (6.6° for f ) (1 and it will be 0° if there is no correlation. At T ) 30 fs, which is the first waiting time for which we can extract information without contamination from background, a tilt angle of + 5° is observed which is a significant positive correlation based on the foregoing assumptions. When spectral diffusion is included, a decay of this angle should provide a general picture of the joint correlation function 〈δωAZ(t)δωOH(0)〉. The foregoing discussion is a very qualitative picture of the signal, which is all that can be justified at present. The response written above is oversimplified in comparison with what might be obtained from detailed simulations. For example, it neither incorporates the frequency dependence of the transition dipoles42 nor the fluctuations due to specific water structures. Furthermore, it greatly simplifies the effects of the electrostatic interactions of water with the ion. Independently of these approximations, the experiment shows clearly that the mode distributions are correlated. The correlation, based on a qualitative concept of the tilt angle, is positive and significant, and it decays on the 150 fs time scale. Although this discussion is based on the tilt angle present in the rephasing 2D-IR spectrum, a similar result is obtained experimentally and theoretically when the absorptive spectrum is considered as done in previous dual frequency 2D-IR experiments.24 Two key related issues are now addressed: the sign of the correlation of the frequency distributions, and the significance of the correlation time. The positive frequency shift of the azide vibrational transition from the value in vacuum to that in neat water or in coordination with metal cations is well-known. When azide ions are coordinated to Zn2+, a shift to higher frequency of 107 cm-1 is observed for the asymmetric stretching mode.11,29 Of the three resonance structures: (1) N2-sN+tN, (2) NdN+dN- and (3) NtN+sN2-, (2) is dominant in the gas phase. On coordination with Zn2+, the resonance structures 1 or 3 become more important and the triple bond contribution to the mode is enhanced, which increases its frequency. Alternatively, the frequency shift43 is +63 cm-1 on adding azide to water and although water molecules are polar, they produce
14032 J. Phys. Chem. B, Vol. 111, No. 50, 2007 a smaller charge redistribution in azide than does the metal ion.44 In our experiment, when the high-frequency side of the azide transition inhomogeneous distribution is excited during the coherence period, the OH frequency that is observed in the experiment is also on the high-frequency side of the OH band. This is not what would be expected if the formation of the H-bond were to lower the OH frequency and if the triple bond structure were slightly preferred as a result of the hydrogen bond formation. In the case of CN-, the strong Coulombic interactions that pull the water molecules close into the ion create configurations where the Lennard-Jones interactions cause an increase in the frequency of CN- compared with the gas phase.19 It is evident that a detailed simulation incorporating the charge fluctuations due to the solute vibrational motion31 and those from the solute fields will be needed to predict the correlation factor. In order to obtain a qualitative assessment of the importance of the Coulomb interactions on the observed correlations, we studied a very simple model that is intended to estimate the effect of the separation of the OH and the ion on the vibrational frequencies. The model assumes the azide ion and an O-H group are collinear with the H atom at distance R from the closest N atom. The most probable value of R is 1.88 Å, based on molecular dynamics simulations.16 The partial charges on azide were chosen as those of the second valence bond structure having a negative charge on each end and a positive charge on the middle nitrogen. Those on oxygen were obtained from the SPC/E model of water.45 The center of mass of the O-H bond was fixed and the O and H atomic displacements described by the stretching normal coordinate, QOH. An expansion of the Coulomb interaction J was carried out to second order in QOH. We found that when the OH motion was described as a Morse oscillator having the gas-phase parameters,46 the Coulombic force-induced frequency shift J11-J00 was dominated by the linear term. The results clearly show that the O-H frequency is decreased as the distance R decreases, which we interpret to imply that strengthening the H-bonding of OH to the ion will decrease the OH frequency as a result of the Coulombic interaction. The frequency shift of the azide ion was then estimated in an analogous manner by fixing the OH bond distance, calculating the perturbation on the azide asymmetric stretch coordinate and varying R. In this case, we assumed the mode was harmonic, so the quadratic term gives the shift. Although the azide and OH shifts differ by a large factor they both increase as R increases. For the OH stretch, the frequency shift spans ∼300 cm-1; however, for the azide asymmetric stretch, it is less than 1 cm-1. There are several reasons for this large difference. The potential is sensitive to the displacement of the OH stretch, but for the azide asymmetric stretch the N atoms with different charges displace in opposite directions, cancelling some of the effect. In the expansion of the potential, the contribution to the frequency shift of the OH stretch is mainly from the linear term and the effect of the quadratic term on the frequency shift is small. It is a key factor that the OH stretch mode is very anharmonic. However, for azide the frequency shift comes only from a small quadratic term. Furthermore, the shifts depend on the expectation value of the displacement during the vibrational motion which is much larger for the OH stretch than for azide mode, because of the light H atom. According to simulations,16,30 the OH bonds of the ion bound water are not, on average, aligned along the axis of azide but subtend a significant angle. This effect would decrease these estimates of the frequency shift of the water OH stretch. Furthermore, for azide there are many
Kuo et al. coordinated water molecules so the magnitude of the frequency shift for the azide should be significantly larger than the estimate from the simple pair model. The foregoing model predicts that the Coulombic interaction contribution should give a positive correlation between the OH and azide vibrational modes. However, the effect of the non-Coulomb interactions, represented by a Lennard-Jones potential, has an even larger effect on the azide frequency. The LJ effect was estimated in the same manner as above with the OH bond fixed and a vibratating azide ion. The azide frequency is substantially increased as the distance R is decreased. The simple estimates predict a frequency increase that significantly exceeds that of the Coulomb interaction, which is consistent with the mechanism suggested by Hynes and co-workers for the solvent shift to higher frequency in water of the CN- ion vibration. The same effect has been predicted from simulations of aqueous azide.30 All of these estimates predict a strong negative correlation of OH and azide frequencies. The decay of the correlation should be dependent on the motion of the OH bond, which changes the distance and angles relative to the azide axis and to energy transport between the bonded OH and nearby OH modes of the solvent. In effect, the tilt angle time evolution is determined by the azide and OH frequency autocorrelation functions and their joint correlation function. The autocorrelation function of the azide vibrational frequency fluctuation has a 1.3 ps component13 that can be considered as static on the time scale of the 140 fs decay observed in this work. However, the autocorrelation of the OH vibrational frequency in neat water decays very quickly and is completed in 50 fs.9 The correlation decay we measure is considerably slower than that found for neat water, which clearly shows that the OH groups around the ion are not efficiently connected into the liquid network of OH bonds. The ultrafast relaxation of neat water has been attributed to energy transfer between OH excitations,9 so it is evident that such an energy transfer between ion-bound water and the bulk or between different ion-bound molecules is considerably slowed from the bulk rate. A joint correlation should decay at an equal or faster rate than the autocorrelations. So there is a slowing down of the spectral diffusion compared with liquid water. The origin of this maybe the isolation of the OH modes around the ion, which prevents energy transport induced spectral diffusion and reduces the coupling between it and bulk water molecules. The result also suggests that the isolation of the bound OH involves a decoupling of the two OH bond motions of a single water molecule. The foregoing estimates, consistent with simulation results, illustrate that the interaction of a single OH group with the azide should lead to a negative correlation of azide and OH stretch frequency distributions. Therefore, it is concluded that the experimental result of positive correlation must be dominated by cooperative interactions. According to recent work by Skinner and co-workers,30 a coordination number of seven waters accounts for 45% of the equilibrium structure distribution while six and eight account for 31% and 21% respectively, so many water molecules need to be incorporated. However, the effect of cooperative binding through changes in the azide electronic structure can be illustrated by considering just a pair of water molecules. When they are bound symmetrically on either end of the azide, the valence structure #2 is preferred, whereas if one water becomes free, the triple bond structure becomes more dominant. It is immediately seen that the correlation between azide and OH frequencies is now positive. Another qualitative view of this situation can be visualized by estimating the OH
Vibrations of the Aqueous Azide Ion with O-H Modes vibrational frequency on bringing it up to the triple bond end of structures #1 or #3: its shift is to higher frequency, whereas the foregoing estimates based on structure #2 yielded the opposite effect. This qualitative argument can be carried over to structures with larger coordination numbers; however, a full simulation will be needed to obtain an answer that can be directly compared with experiment. Conclusion The dynamics of the solvation shell of azide ions in neat water has been examined by dual frequency 2D-IR experiment. By employing pairs of phase-locked pulses centered at the azide asymmetric stretch transition and the OH stretch transition, respectively, the interaction between the ions and water molecules has been measured directly. It has been found from a peak shift measurement that the memory of the inhomogeneity of the coupled motion is decaying slower than 120 fs. Furthermore, the time evolution of the 2D-IR spectrum shows a significant positive correlation between the transition frequencies of azide and OH frequency distributions. The correlation decays on a time scale of 140 fs, which mimics the decay of the cross frequency-frequency time correlation function. These results show that the spectral diffusion of the OH stretch is decoupled from the neat water network and also suggest that a cooperative motion in the solvation shell is necessary to produce the positive correlation. Acknowledgment. This research was supported by grants from NSFCHE and NIH with instrumentation funded by NIH RR01348. References and Notes (1) Bellissent-Funel, M.-C.; Dore, J. C. Hydrogen Bond Networks; Kluwer Academic Publishers; Dordrecht, 1993. (2) Luzar, A.; Chandler, D. Nature 1996, 379, 55. (3) Ohmine, I.; Saito, S. Acc. Chem. Res. 1999, 32, 741. (4) Nakasako, M. Philos. Trans. R. Soc. London Ser. B-Biol. Sci. 2004, 359, 1191. (5) Imai, T.; Hiraoka, R.; Kovalenko, A.; Hirata, F. J. Am. Chem. Soc. 2005, 127, 15334. (6) Loparo, J. J.; Roberts, S. T.; Tokmakoff, A. J. Chem. Phys. 2006, 125. (7) Lindner, J.; Vohringer, P.; Pshenichnikov, M. S.; Cringus, D.; Wiersma, D. A.; Mostovoy, M. Chem. Phys. Letters 2006, 421, 329. (8) Asbury, J. B.; Steinel, T.; Kwak, K.; Corcelli, S. A.; Lawrence, C. P.; Skinner, J. L.; Fayer, M. D. J. Chem. Phys. 2004, 121, 12431. (9) Cowan, M. L.; Bruner, B. D.; Huse, N.; Dwyer, J. R.; Chugh, B.; Nibbering, E. T. J.; Elsaesser, T.; Miller, R. J. D. Nature 2005, 434, 199. (10) Lim, M. H.; Gnanakaran, S.; Hochstrasser, R. M. J. Chem. Phys. 1997, 106, 3485.
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