Correlated High-Frequency Molecular Motions in Neat Liquid Probed

Letter pubs.acs.org/JPCL

Correlated High-Frequency Molecular Motions in Neat Liquid Probed with Ultrafast Overtone Two-Dimensional Infrared Spectroscopy Donghai Li,† Fan Yang,† Chen Han, Juan Zhao, and Jianping Wang* Beijing National Laboratory for Molecular Sciences; State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China S Supporting Information *

ABSTRACT: In this work, an overtone two-dimensional infrared (2D IR) method is shown to allow correlated molecular motions at the frequencies of overtone transitions to be studied. Waiting-time-dependent overtone 2D IR results of the C−O stretching in neat liquid methanol reveal that the autocorrelation of the v = 0 → 2 transition and the cross correlation of the v = 0 → 2/v = 2 → 4 transitions differ considerably (relaxation time being 700 fs and 2 ps, respectively), suggesting different spectral diffusion dynamics. Quantumchemical computations in combination with ab initio molecular dynamics simulations show that the overtone transition frequency of the C−O stretching mode in liquid methanol is of more structural sensitivity than the fundamental frequency. This work demonstrates a new 2D IR approach to examining the structural sensitivities of the anharmonic potential parameters of higher vibrational states, which can be used to gain new insight into the ultrafast structural dynamics particularly for neat liquids. SECTION: Liquids; Chemical and Dynamical Processes in Solution

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emitting time (or detecting time) that starts from the third pulse. A double Fourier transform along the coherence and detecting time axes yields a frequency-domain 2D IR spectrum. The structural and dynamical sensitivity of the 2D IR method lies in its power to reveal correlated molecular motions. The intrinsic nature of a frequency-domain 2D IR spectrum is a collection of 2D joint distributions of frequency fluctuations. For example, under equilibrium condition, in a 2D IR spectrum at a specific waiting time, the 2D diagonal signal due to the v = 0 to v = 1 (or simply 0−1) transition reflects the autocorrelation of this transition frequency distributions at the specific moment, whereas the diagonal signal due to the 1− 2 transition actually reflects the cross-correlation of the 0−1 and 1−2 transition frequency distributions at that moment. The frequency-fluctuation correlation function (FFCF) and spectral diffusion can be extracted in a number of ways23−33 from the waiting-time dependence of 2D IR line shape. Thus accessing more vibrational levels by 2D IR would allow more correlated motions to be addressed and their structural sensitivities to be exploited. Recent advances of the 2D IR as well as 3D IR methods34−36 have allowed the transition frequencies between the four lowest vibrational levels (v = 0, 1, 2, and 3 for the ground, first excited, and first and second overtone states, respectively) of a set of vibrational modes and their correlations to be measured. In the two-quantum 2D IR method,37,38 a third-order pulse sequence is used to access the v = 1, v = 2, as

nderstanding the structure and dynamics of neat liquid at the molecular level is of great importance.1,2 Besides water, methanol is another most studied neat liquid that possesses a hydrogen bond network. Equilibrium structures of methanol have been studied by neutron diffraction,3 X-ray diffraction 4,5 and emission, 6 and by nuclear magnetic resonance,7−9 and a picture of hydrogen-bonded chain-like oligomeric aggregates is generally believed. However, so far, only a handful ultrafast studies has been reported on the structural dynamics of solvated methanol, from which the hydrogen bonding population dynamics10,11 and the vibrational dynamics of the C−O stretch12 have been elucidated. The structural dynamics of neat liquid methanol has not been examined experimentally, only a few molecular dynamics simulations studies have been reported.13,14 Here we report on an overtone two-dimensional infrared (2D IR) experimental method that allows correlated molecular motions at the frequencies of overtone transitions to be monitored with ultrafast time resolution for the first time. The method is demonstrated to be quite useful in gaining new insight into the structural dynamics of neat liquids. Over the past decade, the 2D IR method has proven to be very useful for examining ultrafast structures and dynamics of condensed-phase molecular systems,15−22 by utilizing their intrinsic anharmonic vibrations as probes. It is a coherent thirdorder nonlinear optical method in the IR regime in which three femtosecond IR laser pulses are utilized to interact with a molecular system in controlled polarizations and incident directions. There are three time variables involved here, namely, coherence time (between the first two pulses), waiting time (between the second and third pulses), and echo signal © XXXX American Chemical Society

Received: October 13, 2012 Accepted: November 21, 2012

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well as v = 1 + 1 (combination) states, whereas a dualfrequency 2D IR method can specifically measure the correlation between two very different transition frequencies.39,40 In the present work, we report a novel way to access even higher vibrational state in the frame of the third-order 2D IR method (Figure 1). By tuning the center frequency of the three

In comparison with the conventional 2D IR experiment probing the Δv = 1 transitions, the overtone 2D IR signal is much weaker due to the usually very small extinction coefficient of an overtone transition even for a strong IR absorber such as the C−O stretch. However, we find that using a 100 μm thick liquid methanol sample, the time- and frequency-domain overtone 2D IR signals can become quite substantial. The sum of the rephrasing and nonrephasing signals from the overtone C−O stretch, along the τ axis for a given detecting frequency axis (ωt), is shown in Figure 1c as a typical timedomain absorptive signal. Figure 1d shows the FTIR spectra of the first overtone (right) and fundamental (left) transitions of the C−O stretch in methanol. The peak of the overtone transition is centered at 2044.6 cm−1, with full width at half-maximum (fwhm) of 40 cm−1. The peak of the fundamental transition is centered at 1028.2 cm−1 (fwhm = 20 cm−1). These results agree with previous IR measurements.45,46 Note that the overtone transition is usually forbidden in the harmonic approximation; however, it becomes allowed because of anharmonicity, which is determined to be Δ012 = 2 ω01 − ω02 = 11.8 cm−1. The ratio of the integrated areas of the overtone and fundamental absorption peaks is found to be 1/41.3, meaning the ratio of their transition dipole magnitudes is 1/9.06; and their extinction coefficients at peak positions are determined to be 1.547 and 117.4 M−1 cm−1, respectively (see the Supporting Information). Figure 2 shows the overtone 2D IR spectra of the C−O stretching vibration in liquid methanol at different waiting

Figure 1. (a) Double-sided Feynman diagrams showing possible pathways of the overtone 2D IR response. First row: rephasing; second row: nonrephasing. (b) Anharmonic potential energy surface illustrating one of the pathways of third-order overtone 2D IR signal (diagram 3 or 6). (c) Trace of absorptive time-domain overtone 2D IR signal along the τ-axis by adding the rephasing and nonrephasing signals (see the Supporting Information) for details). (d) 1D IR spectrum (blue) of the first overtone (ω02) and fundamental (ω01) transitions of the C−O stretch in neat liquid methanol at ca. 2 μm sample thickness. The spectrum of ω02 at 100 μm thickness (red) with simulation (green dashed) is also shown. Figure 2. Experimental overtone 2D IR spectra of the C−O stretching mode of liquid methanol at different waiting times. The spectra are intensity-normalized at different waiting times to visualize the 2D line shape evolution. 2D IR signal center lines (blue and red) and nodal line (short black) are shown to illustrate the spectral diffusion.

laser pulses to the first overtone transition of a molecular vibration, thus referred to as the overtone 2D IR method throughout the text, one can access vibrational states for v = 0, 2, and 4 and obtain the 2D IR spectra containing the 0−2 and 2−4 transitions. In this method, the double-sided Feynman diagrams (Figure 1a) are the same as those in a conventional third-order 2D IR method; however, the involved vibrational quantum number change is Δv = 2. Shown in Figure 1b is a schematic diagram of an anharmonic potential energy surface, where the transition processes of the second excited-state absorption having echo signal emit from the v = 4 vibrational state are illustrated. Studying the overtone (and higher vibrational) transitions allows us to explore more anharmonic regions of the potential energy surfaces, which is potentially useful because the overtone transitions are believed to be more sensitive to molecular geometrics such as bond length.41 Note that the double quantum transitions have indeed been little studied in all-IR 2D method, and they can be accessed using mixed IR-Raman techniques.42−44

times. Each 2D IR spectrum consists of two peaks, the positive peak is due to bleaching of the v = 0 state and stimulated emission of the v = 2 state (together assigned as the 0−2 transition); while the negative peak is due to absorption of the v = 2 state (i.e., the 2−4 transition). (The possibility of having the 1−3 transition contribution to the anharmonically shifted negative 2D IR signal can be ruled out, as shown in the Supporting Information.) Because of the nonzero anharmonicity (Δ024 = ω02 − ω24 > 0), the negative peak is red-shifted relative to the positive peak along the ωt axis. The projection of a 2D IR spectrum with ωτ = 2000 − 2090 cm−1 on the ωt axis can be fitted with one positive and one negative peak, from which the anharmonicity Δ024 is found to have a mean value of 41.6 cm−1 for different waiting times (Table S1 of the Supporting Information). This suggests that the mean value 3666

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with a time constant of 1.0 ps, differing from both CLS results. These are the waiting-time-dependent parts of the true FFCF, which is the inhomogeneous contribution. Such contributions to the FFCFs are not influenced by the vibrational relaxation because they are derived from the shape change of 2D IR spectra. A homogeneous contribution, including the contribution of the vibrational lifetime (T1), can be measured by the drop from 1 to the initial value of the CLS or NLS, which is found to be ca. 0.73. The microscopic origin of frequency fluctuation correlations between different vibrational transitions has been examined theoretically.48 In the case of a harmonic oscillator or a strongly anharmonic oscillator, the normalized FFCFs for the transitions associated with the three consecutive vibrational levels (e.g., v = 0, 1, and 2) are identical,48 that is, = = . This implies that the 0−1 transition frequency absolutely correlates to the 1−2 transition frequency. If this were true, then the center and nodal lines shown in Figure 2 should be parallel to one another. In the situation intermediate between the harmonic extreme and the strongly anharmonic extreme, the FFCFs of different transitions would be different. Our results demonstrate that this is the case in the overtone 2D IR spectroscopy, meaning the v = 0, 2, and 4 states are not highly anharmonic. Thus the overtone 2D IR line shapes are very sensitive to the correlations between different overtone transition frequencies. Note that in previous 2D IR experiments probing the Δv = 1 vibrational transitions, usually little attention was paid to the difference between and , and the nodal slope was often used to extract a FFCF.23−25 However, in a previous work,49 it was found that the tilting of the 0−1 and 1−2 transitions of the N−H stretching mode in a model dipeptide AcAlaOMe are very different, suggesting different and correlations for the N−H stretching mode. Further, one important piece of information from Figure 3a is that the cross-correlation of the 0−2 and the 2−4 transition frequencies decays slower than the autocorrelation of the 0−2 transition frequency. This implies that the 2−4 transition frequency has slower autocorrelation dynamics and the intraand intermolecular forces modulate different overtone vibrational states differently so that the 2−4 transition is closely correlated with the 0−2 transition. This correlation is even more persistent than the 0−2 autocorrelation. Such a surprising result has not been observed for the transitions among the v = 0, 1, and 2 states. Furthermore, the weight of homogeneous and inhomogeneous contributions to the line width is also reflected by the CLS dynamics of the 0−2 transition. The initial value of the CLS is only roughly 0.27, suggesting that ∼73% of the line shape is contributed from the homogeneous broadening. The FFCF can be assumed to be in the following form:

of the 0−4 transition frequency should be centered at 4047.6 cm−1. Thus the transition frequencies of the 0−1, 0−2, 2−4, and 0−4 of the C−O stretching mode of liquid methanol are determined from the linear IR and 2D IR measurements. Quantum chemical computations (see the Supporting Information) yield Δ012 = 8.5 cm−1 and Δ024 = 38.5 cm−1 for the C−O stretching mode; both values are in good agreement with experimental results. These are direct measures of the lower overtone-state energy parameters for the anharmonic potential surface. There are several key features in Figure 2. First, at short waiting time, the 2D IR signal partially elongates along the diagonal for both 0−2 and 2−4 transitions, indicating some degree of inhomogeneous broadening. Second, as the waiting time increases, the 2D IR line shape becomes more vertically tilted because of the spectral diffusion, which can be characterized by the FFCF. The center-line-slope (CLS) method33 is used to characterize the FFCF dynamics. The CLS is the inverse of the slope of the line that connects the maxima (or minima) of the peaks of a series of horizontal (parallel to the ωt axis) cuts through a 2D spectrum. It yields the waiting-time-dependent contribution of the FFCF, and its usefulness has been previously discussed.47 The center lines of the positive peak and the negative peak are shown in Figure 2 for each waiting time. The two obtained CLS dynamics are plotted in Figure 3a. A single exponential fits each slope curve

Figure 3. (a) CLS dynamics for the v = 0 to v = 2 transition (red) and the v = 2 to v = 4 transition (blue). NLS dynamics is also given (green). (b) Computed FFCF for the 0−2 transition of the C−O stretch from the MD trajectory, using a modulated Morse potential protocol. (c) Transient grating signal. Each curve includes a single or biexponential fit (solid line).

⎛ t ⎞ ⎛ t⎞ ⟨δω02(t )δω02(0)⟩ = Δ12exp⎜ − ⎟ + Δ22exp⎜ − ⎟ ⎝ τ2 ⎠ ⎝ τ1 ⎠

(1)

Here the first term is the contribution of a fast decay component, and the second term is the inhomogeneous contribution estimated from the CLS. The 0−2 absorption line shape is simulated (see the Supporting Information) using the experimentally determined slow FFCF component (τ2 = 700 fs), incorporating the vibrational lifetime (T1 = 790 fs, Figure 3c) obtained from transient grating measurement. To fit

reasonably. A clear difference is observed between the two CLSs; the decay time constants are found to be 700 fs for the 0−2 transition and 2.0 ps for the 2−4 transition. This indicates that the relaxation times for the autocorrelation and cross-correlation are significantly different. In addition, Figure 3a shows the result using a nodal-line-slope (NLS) method,23−25 where it decays 3667

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constants and amplitudes: τ1 = 50 fs (with Δ12 = 370 cm−2) and τ2 = 1.6 ps (Δ22 = 100 cm−2). This suggests the frequencyfluctuation amplitude for the 0−2 transition to be 22 cm−1, which is in reasonable agreement with the FFCF result (25 cm−1). The fast FFCF component is not resolved in our 2D IR results, whereas the slow component is comparable to the time constant obtained from the CLS dynamics shown in Figure 3a. Taking together the results from both the quantum-chemical computations and ab initio MD simulations, one sees that the overtone transition frequency is influenced by both intra- and intermolecular forces, and the short decay time τ 1 of the FFCF can be associated with the delocalized molecular motions that involve only two or three hydrogen-bonded methanol molecules, for example, the hindered translation or rotation about a hydrogen-bond structure; whereas the slow one may be related to collective molecular motions. The amplitude of the fast component is about twice that of the slow component, indicating that although the hindered translation or rotation of a hydrogen-bonded methanol pair may not break the hydrogen bond it modulates the overtone C−O stretching frequency more significantly. The picture is also in agreement with a previous work where nonpolar solvent CCl4 solvated methanol clusters also exhibited a biphasic FFCF (0.1 ps and 1.6 ps) for the O−H stretch.11 The midpoint distance between two C−O bonds of a pair of hydrogen-bonded methanol is found to be 3.480 Å on average over 7 ps MD simulations (Figure 4d). Therefore, the anharmonic coupling (see the Supporting Information) between two nearby C−O stretches is estimated to be 3.2 cm−1 on average (Figure 4e). This small coupling suggests that in the liquid phase the C−O stretches can be viewed as weakly coupled vibrational chromophore. Potential energy distribution computation (Table S2 in the Supporting Information) also reveals that the C−O stretch normal mode in methanol clusters is 90% (on average) localized on the C−O bond. Moreover, because of the anharmonicity, overtone transitions are more local than the corresponding fundamental transitions.50 Together, this makes the overtone 2D IR method a useful tool for studying the local structural dynamics of liquid alcohols. In addition, the use of a thick sample can become an advantage of the overtone 2D IR method in examining neat liquids because in which case the optical density of the fundamental absorption bands may be too high to work with in either 1D IR or 2D IR method. For instance, a submicrometer thick sample was specially made to collect the 2D IR spectra of the fundamental O−H stretch in pure water.22,29 Partially due to this reason, there is little 2D IR spectroscopic study for neat liquid. With our method, overtone 2D IR spectra of neat liquid methanol can be acquired with 30−100 μm spacer. In summary, the proposed overtone 2D IR method allows one to experimentally control and rephase the overtone coherences with Δv = 2 and measure the correlated vibrational transition frequencies for up to the fifth vibrational state (v = 4), which greatly enlarges the scope of the conventional thirdorder 2D IR spectroscopy. It also allows us to measure anharmonic potential parameters that are not available from either previously reported 2D IR or 3D IR methods. Because the overtone transition is more anharmonic, the overtone 2D IR method can be a more sensitive tool to the factors that influence the anharmonic potential surface. In fact, the overtone 2D IR method is not limited to the low-energy overtone states; it can be applied to many other modes in condensed-phase

the linear IR data shown in Figure 1d, a fast FFCF component is derived to be τ 1 = 135 fs (with frequency fluctuation amplitude Δ12 = 424.2 cm−2) and Δ22 = 200.8 cm−2 for the slow component, so the total amplitude is Δ = Δ1 + Δ2= 25 cm−1. Here Δ 1τ 1 is found to be 0.524, which is less than 1, suggesting a picture with a dominating homogeneous broadening. Similar to water, liquid methanol forms a hydrogen-bond network. The fluctuating hydrogen bonds continuously modulate the C−O stretching frequency. However, experimentally or theoretically, little is known about how intra- and intermolecular forces influence the joint frequency distributions of higher vibrational excited states of the C−O stretch in this system. To examine the structural sensitivity of the overtone C−O stretch frequency, gas-phase CH3OH and four typical hydrogen-bonded complexes of CH3OH are first considered (Figure 4a). In these cases, the target methanol molecule either

Figure 4. (a) Isolated methanol and representative hydrogen-bonded clusters in liquid methanol, where the target molecule is labeled with a yellow filled “Δ”. (b) Computed overtone and fundamental frequency sensitivities to the inverse of the C−O bond length. (c) Distribution of the overtone C−O stretching frequency. (d) Neighboring C−O bond midpoint distance. (e) Transition dipole coupling strength between neighboring C−O stretches in neat methanol.

serves as the hydrogen bond acceptor or donor or both or serves as one acceptor and two donors simultaneously. The calculated anharmonic fundamental and overtone C−O stretching frequencies (see Table S2 in the Supporting Information) are found to vary in these systems. The structural sensitivity of the two frequencies is plotted in Figure 4b, clearly showing that they are inversely proportional to the C−O bond length, and the overtone frequency is of more bond-length sensitivity. Also, as shown in Figure 1d, the fwhm of the overtone transition is nearly twice that of the fundamental transition, supporting this conclusion. Next, to examine the overtone C−O stretching vibration in methanol on the ultrafast time scale, ab initio molecular dynamics (MD) simulations are carried out. A modulated Morse potential method41 is used to compute the instantaneous overtone vibrational frequencies using the ab initio MD trajectories. Details are given in the Supporting Information. The obtained static distribution of the 0−2 transition frequency is shown in Figure 4c. It can be fitted by a Gaussian with an fwhm of 44.5 cm−1 and centered at 2059 cm−1. The computed FFCF of the 0−2 transition frequency is given in Figure 3b and fitted with a double exponential decay function with two time 3668

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Studied by Nuclear Magnetic Resonance. J. Phys. Chem. 1990, 94, 7308−7312. (8) Wallen, S. L.; Palmer, B. J.; Garrett, B. C.; Yonker, C. R. Density and Temperature Effects on the Hydrogen Bond Structure of Liquid Methanol. J. Phys. Chem. 1996, 100, 3959−3964. (9) Asahi, N.; Nakamura, Y. Nuclear Magnetic Resonance and Molecular Dynamics Study of Methanol up to the Supercritical Region. J. Chem. Phys. 1998, 109, 9879−9887. (10) Asbury, J. B.; Steinel, T.; Stromberg, C.; Gaffney, K. J.; Piletic, I. R.; Goun, A.; Fayer, M. D. Hydrogen Bond Dynamics Probed with Ultrafast Infrared Heterodyne-Detected Multidimensional Vibrational Stimulated Echoes. Phys. Rev. Lett. 2003, 91, 237402. (11) Piletic, I. R.; Gaffney, K. J.; Fayer, M. D. Structural Dynamics of Hydrogen Bonded Methanol Oligomers: Vibrational Transient Hole Burning Studies of Spectral Diffusion. J. Chem. Phys. 2003, 119, 423− 434. (12) van den Broek, M. A. F. H.; Nienhuys, H. K.; Bakker, H. J. Vibrational Dynamics of the C-O Stretch Vibration in Alcohols. J. Chem. Phys. 2001, 114, 3182−3186. (13) Pagliai, M.; Cardini, G.; Righini, R.; Schettino, V. Hydrogen Bond Dynamics in Liquid Methanol. J. Chem. Phys. 2003, 119, 6655− 6662. (14) Vrhovsek, A.; Gereben, O.; Jamnik, A.; Pusztai, L. Hydrogen Bonding and Molecular Aggregates in Liquid Methanol, Ethanol, and 1-Propanol. J. Phys. Chem. B. 2011, 115, 13473−13488. (15) Remorino, A.; Korendovych, I. V.; Wu, Y.; DeGrado, W. F.; Hochstrasser, R. M. Residue-Specific Vibrational Echoes Yield 3D Structures of a Transmembrane Helix Dimer. Science 2011, 332, 1206−1209. (16) Moran, S. D.; Woys, A. M.; Buchanan, L. E.; Bixby, E.; Decatur, S. M.; Zanni, M. T. Two-Dimensional IR Spectroscopy and Segmental 13C Labeling Reveals the Domain Structure of Human Gamma DCrystallin Amyloid Fibrils. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 3329−3334. (17) Rosenfeld, D. E.; Gengeliczki, Z.; Smith, B. J.; Stack, T. D. P.; Fayer, M. D. Structural Dynamics of a Catalytic Monolayer Probed by Ultrafast 2D IR Vibrational Echoes. Science 2011, 334, 634−639. (18) Kolano, C.; Helbing, J.; Kozinski, M.; Sander, W.; Hamm, P. Watching Hydrogen-Bond Dynamics in a β-turn by Transient TwoDimensional Infrared Spectroscopy. Nature 2006, 444, 469−472. (19) Tokmakoff, A. Shining Light on the Rapidly Evolving Structure of Water. Science 2007, 317, 54−55. (20) Maekawa, H.; De Poli, M.; Toniolo, C.; Ge, N.-H. Couplings between Peptide Linkages across a 310-Helical Hydrogen Bond Revealed by Two-Dimensional Infrared Spectroscopy. J. Am. Chem. Soc. 2009, 131, 2042−2043. (21) Elsaesser, T. Two-Dimensional Infrared Spectroscopy of Intermolecular Hydrogen Bonds in the Condensed Phase. Acc. Chem. Res. 2009, 42, 1220−1228. (22) Cowan, M. L.; Bruner, B. D.; Huse, N.; Dwyer, J. R.; Chugh, B.; Nibbering, E. T. J.; Elsaesser, T.; Miller, R. J. D. Ultrafast Memory Loss and Energy Redistribution in the Hydrogen Bond Network of Liquid H2O. Nature 2005, 434, 199−202. (23) Kwac, K.; Cho, M. Molecular Dynamics Simulation Study of NMethylacetamide in Water. II. Two-Dimensional Infrared Pump-Probe Spectra. J. Chem. Phys. 2003, 119, 2256−2263. (24) Eaves, J. D.; Loparo, J. J.; Fecko, C. J.; Roberts, S. T.; Tokmakoff, A.; Geissler, P. L. Hydrogen Bonds in Liquid Water Are Broken Only Fleetingly. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 13019−13022. (25) Kuroda, D. G.; Hochstrasser, R. M. Dynamic Structures of Aqueous Oxalate and the Effects of Counterions Seen by 2D IR. Phys. Chem. Chem. Phys. 2012, 14, 6212−6217. (26) Roberts, S. T.; Loparo, J. J.; Tokmakoff, A. Characterization of Spectral Diffusion from Two-Dimensional Line Shapes. J. Chem. Phys. 2006, 125, 084502−084508. (27) Finkelstein, I. J.; Ishikawa, H.; Kim, S.; Massari, A. M.; Fayer, M. D. Substrate Binding and Protein Conformational Dynamics Measured

molecular systems. Further, by using a two-color scheme, in principle, the frequency correlations involving the v = 3 state (for example, between the 0−1 and 1−3 transitions, or between the 0−2 and 2−3 transitions) can also be addressed. Furthermore, by utilizing fifth-order heterodyned 3D IR method, the 0−2, 2−4, and 4−6 transitions may be accessed simultaneously; however, sensitivity could be an issue because the fifth-order signal is much smaller than the third-order signal. Therefore, the present work opens the opportunity not only to explore more anharmonic vibrational potential parameters using various multidimensional IR methods but also to examine liquid structural dynamics from a novel perspective. Systems containing two or more same-type overtone modes are currently under study.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental section, determination of the extinction coefficient, 2D IR data at more waiting times, examination of the nature of the observed overtone 2D IR signal, FFCF simulation, quantum chemical computations, molecular dynamics simulations, computation of the instantaneous overtone frequencies, and the anharmonic couplings. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (+86)-010-62656806; Fax: (+86)-010-62563167. Author Contributions †

These authors contributed equally.

Notes

The authors declare no competing financial interest. All authors have given approval to the final version of the manuscript.

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ACKNOWLEDGMENTS This work was supported by the Chinese Academy of Sciences (Knowledge Innovation Program, KJCX2-EW-H01 and Hundred Talent Fund), and by the National Natural Science Foundation of China (20727001, 91121020).

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REFERENCES

(1) Nibbering, E. T. J.; Elsaesser, T. Ultrafast Vibrational Dynamics of Hydrogen Bonds in the Condensed Phase. Chem. Rev. 2004, 104, 1887−1914. (2) Bakker, H. J.; Skinner, J. L. Vibrational Spectroscopy as a Probe of Structure and Dynamics in Liquid Water. Chem. Rev. 2010, 110, 1498−1517. (3) Montague, D. G.; Gibson, I. P.; Dore, J. C. Structural Studies of Liquid Alcohols by Neutron Diffraction. Mol. Phys. 1981, 44, 1355− 1367. (4) Magini, M.; Paschina, G.; Piccaluga, G. On the Structure of Methyl Alcohol at Room Temperature. J. Chem. Phys. 1982, 77, 2051− 2056. (5) Narten, A. H.; Habenschuss, A. Hydrogen Bonding in Liquid Methanol and Ethanol Determined by X-Ray Diffraction. J. Chem. Phys. 1984, 80, 3387−3391. (6) Kashtanov, S.; Augustson, A.; Rubensson, J.-E.; Nordgren, J.; Agren, H.; Guo, J.-H.; Luo, Y. Chemical and Electronic Structures of Liquid Methanol From X-Ray Emission Spectroscopy and Density Functional Theory. Phys. Rev. B. 2005, 71, 104205. (7) Schulman, E. M.; Dwyer, D. W.; Doetschman, D. C. Temperature and Pressure Dependence of Hydrogen Bonding in Liquid Methanol 3669

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by 2D-IR Vibrational Echo Spectroscopy. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 2637−2642. (28) Fang, C.; Bauman, J. D.; Das, K.; Remorino, A.; Arnold, E.; Hochstrasser, R. M. Two-Dimensional Infrared Spectra Reveal Relaxation of the Nonnucleoside Inhibitor TMC278 Complexed with HIV-1 Reverse Transcriptase. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 1472−1477. (29) Kraemer, D.; Cowan, M. L.; Paarmann, A.; Huse, N.; Nibbering, E. T. J.; Elsaesser, T.; Miller, R. J. D. Temperature Dependence of the Two-Dimensional Infrared Spectrum of Liquid H2O. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 437−442. (30) Kuo, C.-H.; Hochstrasser, R. M. Two Dimensional Infrared Spectroscopy and Relaxation of Aqueous Cyanide. Chem. Phys. 2007, 341, 21−28. (31) Loparo, J. J.; Roberts, S. T.; Tokmakoff, A. Multidimensional Infrared Spectroscopy of Water. II. Hydrogen Bond Switching Dynamics. J. Chem. Phys. 2006, 125, 194522−194512. (32) Asbury, J. B.; Steinel, T.; Kwak, K.; Corcelli, S. A.; Lawrence, C. P.; Skinner, J. L.; Fayer, M. D. Dynamics of Water Probed with Vibrational Echo Correlation Spectroscopy. J. Chem. Phys. 2004, 121, 12431−12446. (33) Kwak, K.; Park, S.; Finkelstein, I. J.; Fayer, M. D. FrequencyFrequency Correlation Functions and Apodization in Two-Dimensional Infrared Vibrational Echo Spectroscopy: A New Approach. J. Chem. Phys. 2007, 127, 124503−124517. (34) Ding, F.; Zanni, M. T. Heterodyned 3D IR Spectroscopy. Chem. Phys. 2007, 341, 95−105. (35) Garrett-Roe, S.; Hamm, P. Purely Absorptive Three-Dimensional Infrared Spectroscopy. J. Chem. Phys. 2009, 130, 164510− 164519. (36) Garrett-Roe, S.; Perakis, F.; Rao, F.; Hamm, P. ThreeDimensional Infrared Spectroscopy of Isotope-Substituted Liquid Water Reveals Heterogeneous Dynamics. J. Phys. Chem. B. 2011, 115, 6976−6984. (37) Fulmer, E. C.; Mukherjee, P.; Krummel, A. T.; Zanni, M. T. A Pulse Sequence for Directly Measuring the Anharmonicities of Coupled Vibrations: Two-Quantum Two-Dimensional Infrared Spectroscopy. J. Chem. Phys. 2004, 120, 8067−8078. (38) Kim, Y. S.; Wang, J.; Hochstrasser, R. M. Two-Dimensional Infrared Spectroscopy of the Alanine Dipeptide in Aqueous Solution. J. Phys. Chem. B 2005, 109, 7511−7521. (39) Rubtsov, I. V.; Wang, J.; Hochstrasser, R. M. Dual-Frequency 2D-IR Spectroscopy Heterodyned Photon Echo of the Peptide Bond. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 5601−5606. (40) Kuo, C. H.; Vorobyev, D. Y.; Chen, J.; Hochstrasser, R. M. Correlation of the Vibrations of the Aqueous Azide Ion with the O-H Modes of Bound Water Molecules. J. Phys. Chem. B 2007, 111, 14028−14033. (41) Henry, B. R. The Frequency-Bond Length Correlation in LocalMode Overtone Spectra. J. Mol. Struct. 1989, 202, 193−201. (42) Pakoulev, A. V.; Block, S. B.; Yurs, L. A.; Mathew, N. A.; Kornau, K. M.; Wright, J. C. Multiply Resonant Coherent Multidimensional Spectroscopy: Implications for Materials Science. J. Phys. Chem. Lett. 2010, 1, 822−828. (43) Pakoulev, A. V.; Rickard, M. A.; Kornau, K. M.; Mathew, N. A.; Yurs, L. A.; Block, S. B.; Wright, J. C. Mixed Frequency-/TimeDomain Coherent Multidimensional Spectroscopy: Research Tool or Potential Analytical Method. Acc. Chem. Res. 2009, 42, 1310−1321. (44) Fournier, F.; Gardner, E. M.; Kedra, D. A.; Donaldson, P. M.; Guo, R.; Butcher, S. A.; Gould, I. R.; Willison, K. R.; Klug, D. R. Protein Identification and Quantification by Two-Dimensional Infrared Spectroscopy: Implications for an All-Optical Proteomic Platform. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 15352−15357. (45) Falk, M.; Whalley, E. Infrared Spectra of Methanol and Deuterated Methanols in Gas, Liquid, and Solid Phases. J. Chem. Phys. 1961, 34, 1554−1568. (46) Bertie, J. E.; Zhang, S. L. Infrared Intensities of Liquids XXI: Integrated Absorption Intensities of CH3OH, CH3OD, CD3OH and

CD3OD and Dipole Moment Derivatives of Methanol. J. Mol. Struct. 1997, 413, 333−363. (47) Kwak, K.; Rosenfeld, D. E.; Fayer, M. D. Taking Apart the TwoDimensional Infrared Vibrational Echo Spectra: More Information and Elimination of Distortions. J. Chem. Phys. 2008, 128, 204505−204510. (48) Piryatinski, A.; Skinner, J. L. Determining Vibrational SolvationCorrelation Functions from Three-Pulse Infrared Photon Echoes. J. Phys. Chem. B. 2002, 106, 8055−8063. (49) Rubtsov, I. V.; Wang, J.; Hochstrasser, R. M. Dual Frequency 2D-IR of Peptide Amide-A and Amide-I Modes. J. Chem. Phys. 2003, 118, 7733−7736. (50) Henry, B. R.; Kjaergaard, H. G. Local Modes. Can. J. Chem. 2002, 80, 1635−1642.

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