Linear and Nonlinear Infrared Spectroscopies Reveal Detailed Solute

Subscriber access provided by University of Sunderland

B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution

Linear and Nonlinear Infrared Spectroscopies Reveal Detailed SoluteSolvent Dynamic Interactions of a Nitrosyl Ruthenium Complex in Solution Minjun Feng, Juan Zhao, Pengyun Yu, and Jianping Wang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b07247 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 13, 2018

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 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 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.

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 34 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

Linear and Nonlinear Infrared Spectroscopies Reveal Detailed Solute-Solvent Dynamic Interactions of a Nitrosyl Ruthenium Complex in Solution

Minjun Feng†,‡, Juan Zhao†,‡, Pengyun Yu†,‡*, and Jianping Wang†,‡* †

Beijing National Laboratory for Molecular Sciences; Molecular Reaction Dynamics

Laboratory, Institute of Chemistry, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China ‡

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

*

Authors to whom correspondence should be addressed. Tel: (+86)-010-62656806; Fax:

(+86)-010-62563167; E-mails: [email protected]; [email protected]; ORCID: 00000001-7127-869X.

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

Abstract In this work, the solvation of a nitrosyl ruthenium complex, [(CH3)4N][RuCl3(qn)(NO)] (with qn = deprotonated 8-hydroxyquinoline), which is a potential NO-releasing molecule in bio-environment, was studied in two bio-friendly solvent, namely deuterated dimethylsulfoxide (dDMSO) and water (D2O). A blue-shifted NO stretching frequency was observed in water with respect to that in dDMSO, which was believed to be due to ligandsolvent hydrogen-bonding interactions, one N=O⋯D and particularly three Ru-Cl⋯D, that show competing effects on the NO bond length. The dynamic differences of the NO stretch in these two solvents were further revealed by transient pump-probe IR and two-dimensional IR results: faster vibrational relaxation and faster spectral diffusion (SD) were observed in D2O, confirming stronger solvent-solute interaction and also faster solvent structural dynamics in D2O than in DMSO. Further, a significant non-decaying residual in the SD dynamics was observed in D2O but not in DMSO, suggesting the formation of a stable solvation shell in water due to strong multi-site ligand-solvent hydrogen-bonding interactions, which is in agreement with the observed blue-shifted NO stretching frequency. This work demonstrates that small solvent molecules such as water can form a relatively rigid solvation shell for certain transition metal complex due to cooperative ligand-solvent interactions and show slower dynamics.

2


Page 2 of 34

Page 3 of 34 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. Introduction Nitric oxide (NO) plays a crucial role in various biological processes, such as neurotransmission and immunity.1-2 Certain diseases have been related to NO deficiency, which aroused a great interest in searching for exogenous NO donor. Nitrosyl ruthenium complexes are believed to be one class of potentially useful compounds for in-vivo NO delivery besides other organic nitrites and inorganic nitrates, for example, amyl nitrite and sodium nitroprusside,3 that have been reported to be effective as NO donors in clinical settings. It has been shown that nitrosyl ruthenium compounds can release NO by biological reduction agent or more conveniently by light excitation,4 with the latter being one of the advantages particularly for such nitrosyl-containing complexes. Further, a previous study5 suggested that certain ruthenium complexes can have a long-lasting NO releasing process, hence enabling a potentially long-period maintenance of efficacy, which has not been reported yet for inorganic nitrites. In addition, the ruthenium compounds may also be generally useful as drug candidates for their tunable activity (toxicity) by the oxidation state and/or by varying their coordinating ligands.6 Further, in a typical nitrosyl ruthenium complex, it is usually the charged NO+ species (referred to as NO hereafter for simplification) that binds to a Ru (Ⅱ) center and functions as the photo-releasing source of nitric oxide. With the aid of density functional theory (DFT) computations,7-11 it was found that the Ru-NO bond strength was largely determined by the π-backbonding donation, which, however, may also be influenced by other coordinating ligands of Ru(II), as suggested by these studies. As shown in this work, when chloride ion serves as ligand, the property of NO indeed changes significantly. This is because the strongly 3



polarized character of the Ru-Cl bond can result in an enhanced negative partial atomic charge of the halogen, allowing it to effectively accept a hydrogen bond.12-13 This is also known as central metal-assisted hydrogen bond formation, and the indirect participation of a metal atom under such circumstances has been previously reported by X-ray crystallography.14-15 Because ligand releasing can be both solvent and chemical environment dependent, acquiring a detailed picture of immediate solute-solvent interaction for the NOcontaining molecular systems in the condensed phases, particularly in bio-friendly solvents, is hence of great importance. Time-resolved experiments have been reported to understand such Ru complexes.16-18 However, a detailed dynamical solvation picture of the nitrosyl ruthenium complexes mentioned above has not been reported. Herein, two important bio-friendly solvents, namely deuterated water (D2O) and fully deuterated dimethylsulfoxide (dDMSO), were chosen to study the dynamical solvation of a synthesized canonical hexacoordinate nitrosyl ruthenium complex that is in the form of [(CH3)4N][RuCl3(qn)(NO)] (qn = deprotonated 8-hydroxyquinoline) and can work as a potential NO-releasing molecule. The [RuCl3(qn)(NO)]- complex (simply denoted as Ru-NO), is negatively charged upon solvation. Furthermore, three chloride ions may intervene the solvation and even influence the NO-releasing ability of such Ru-NO complex. Besides the solvent-solute structural dynamics, it is also important to understand inter- and intramolecular energy transfer pathways that are relevant to the structural properties of such Ru-NO complex. Here, it should be mentioned that D2O is often used as an equivalent solvent of H2O in studying peptide conformational dynamics19-20 and protein folding21-22 using the amide I mode. The structural dynamics of water itself act as solvent have been extensively studied in recent years.23-31 Less-polar dDMSO has been widely known as bio-cosolvent, whose aqueous 4


Page 4 of 34

Page 5 of 34 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


solution is of fundamental importance in a diversified fields such as pharmacology, cryoprotection, and cell biology. The cosolvent effect of DMSO on water hydrogen bond dynamics and on carbonyl and water interactions has been examined recently by the twodimensional infrared (2D IR) method.32-33 Detailed intermolecular interaction between DMSO and water has also been investigated very recently.34 In our recent works,35-37 a similar Ru-NO complex with cis and trans configurations has been examined using ultrafast infrared spectroscopies. It was found that nonlinear IR signatures (spectral line width, dynamical parameters) especially those extracted from 2D IR spectra, can be used to differentiate the cis and trans configurations.36 The 2D IR method has been well developed in recent years, with a broad range of application, as demonstrated in recent works and reviews.38-48 The time window of this technique is on the femtosecond to picosecond time scales, which is suitable for monitoring chemical-bond breaking and reforming processes, as well as molecular structural dynamics.49-51 For example, it has been known that the frequency time-correlation function (FTCF) can be extracted from dynamicaltime dependent 2D IR measurements, using the inverse of the center-line slope of the timedependent 2D IR signal in the frequency domain.52 On the other hand, while the solutesolvent structural dynamics of the NO group-containing compound has been examined recently using vibrational spectroscopy,53-54 the structural dynamics of an enzyme active site has also been investigated using the NO stretching vibration as a vibrational probe.55 Further, in a recent 2D IR study of sodium nitroprusside,56 vibrational solvatochromism of the NO stretching mode has been investigated in detail.

2. Materials and Methods 5



2.1 Sample and 1D IR Experiments [(CH3)4N][RuCl3(qn)(NO)] was synthesized using a previously described procedure57 and structurally confirmed by proton NMR spectrometry. Fully deuterated dimethylsulfoxide (dDMSO) and water (D2O), non-deuterated DMSO and non-deuterated ethylene glycol (EG) were purchased and used as received. Sample solutions were prepared at a concentration of 15 mM. Linear IR (1D IR) spectra were taken using a commercial FTIR spectrometer equipped with a liquid nitrogen-cooled mercury cadmium telluride (MCT) infrared detector. To prevent the interference of water moisture and CO2, sample chamber of the spectrometer was constantly purged by dry air. A liquid IR sample cell is constructed by pressing two CaF2 IR optical windows (2 mm in thickness each) together in a stainless-steel holder, with the aid of a Teflon ring spacer (50 m in thickness). Weak solvent background IR absorption in the NO stretching frequency region was subtracted from sample spectra. The absorbance of the NO mode was determined to be 0.08 optical density (OD) in dDMSO, 0.12 OD in D2O, and 0.11 OD in EG at the same sample concentration and IR-cell thickness. The FTIR spectra were measured with 16-scan average and 1-cm-1 resolution, at laboratory temperature (22 ºC).

2.2 Pump-Probe IR and 2D IR Spectroscopies Transient IR pump-probe and 2D IR experiments were performed by a mid-IR pulse shaper based spectrometer.58 Briefly, an ultrafast Ti:sapphire laser amplifier with 3-mJ pulse energy, 25-fs pulse width, 800-nm central wavelength, 1-kHz repetition rate was used to pump a combined optical parametric amplifier and difference frequency generator. The initially obtained mid-IR pulse has typically 12-μJ pulse energy, 1850-cm-1 central frequency, and 270-cm-1 spectral width (in full width at half maximum, FWHM), and is attenuated gradually 6


Page 6 of 34

Page 7 of 34 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


by IR optics such as IR telescope, gold mirrors, ZnSe beam splitters, and an IR acoustic-optic modulator (AOM). The final total pulse energy of the pump pair and that of the probe was typically ca. 1.0 μJ at the focusing point for sample. A 64-element MCT IR array detector was used to collect 2D IR signal after monochromator. Using a 150 line/mm IR grating, the detection frequency resolution was ca. 2 cm−1 for the pump−probe experiment. Here for simplification the zero delay-time is set to when maximum pump-probe signal was observed, and instrumental response function (IRF) deconvolution was not carried out. In this work, the lifetime of the Ru-NO complex was on the order of several picoseconds, which will not be significantly affected by neglecting the IRF contribution. The nonlinear IR experiments were also carried out at the laboratory temperature. FTIR spectra of the samples were measured before and after the 2D IR experiment, and no spectral change was observed, indicating a stable sample condition in the experimental time window. Polarization-controlled pump-probe experiment with the polarization of the pump and probe pulses setting to parallel and perpendicular conditions was conducted to derive the population and anisotropy dynamics. Single MCT pixel was used to probe at the vibrational fundamental transition (i. e., the ground-state bleaching and stimulated emission from v = 0 to v = 1, where v is vibrational quantum number) frequency of the NO stretching mode in dDMSO, in D2O, and in EG respectively, with more than 3000 laser shots for signal averaging. Anisotropy was computed using the well-known equation:59-61 r(t) =

∥(

)

∥( )

( ) ( )

,

(1)

where 𝐼∥ (t) and 𝐼 (t) are pump-probe signals at parallel and perpendicular polarizations respectively.

7



2.3 Potential Energy Distributions Analysis Geometry optimization of the ionic Ru-NO complex ([RuCl3(qn)(NO)]-) in gas phase was carried out by using the DFT method at the B3LYP level, with 6-31++G (d, p) basis set for C, O, N, H, and Cl atoms, and lanl2dz pseudo potential basis for Ru atom. Harmonic vibrational frequency calculation was followed. All the computations were performed using Gaussian.62 To evaluate the vibrational properties of the NO vibration in this complex, the potential energy distributions (PEDs) analysis on the basis of internal coordinates was carried out on the basis of the DFT calculation. The PEDs can be used to describe the relative contributions of each atom or chemical group to the total potential energy of a specific vibrational mode,6364

which, in turn, can be used to identify the coupled normal-mode vibrations that are

potentially vibrational energy acceptors for a specific vibrationally excited state.

3. Results and Discussion 3.1 Linear IR Spectra The linear IR absorption spectra of the Ru-NO complex in dDMSO and D2O in the NO stretching vibration frequency region are shown in Figure 1 with their peak intensity maxima normalized for comparison. The fitting results are given in Table 1, in which Voigt function fitting parameters of the NO stretching IR spectra of the Ru-NO complex in dDMSO and D2O were listed. The fitting results show that inhomogeneous (Gaussian) broadening appears to be dominant in dDMSO, while more homogeneous (Lorentzian) component appears in D2O. The molar extinction coefficient of the NO stretching mode was determined to be 28.2×103 M1

∙cm-1 in dDMSO and 39.7×103 M-1∙cm-1 in D2O respectively, suggesting an enhanced

transition dipole moment in the latter and thus implying stronger solute-solvent interaction. 8


Page 8 of 34

Page 9 of 34 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. Linear IR spectra of the NO stretching in the Ru-NO complex solvated in dDMSO (red) and in D2O (blue). The molecular structure of the Ru-NO complex is shown as insert. A weak high-frequency component in the case of dDMSO is due to ligand replacement by dDMSO (see text for detail).

We shall first look into the fitting result in dDMSO, in which the major peak is located at 1841.2 cm-1 and a subordinate peak is discerned at the high-frequency site. The major peak is attributed to the NO stretching mode of [RuCl3(qn)(NO)]- whereas the minor peak (ca. 10 % in integrated area) is believed to be due to the substitution of Cl with a solvent dDMSO molecule in the solution phase, which has been observed previously in similar ruthenium nitrosyl complexes.36-37 The NO stretching vibrational spectrum in D2O solvent has a relatively simple profile with only one peak located at 1874.2 cm-1. In addition, one sees that the peak width of the NO mode in dDMSO is significantly broader than that in D2O, indicating an apparent overall decreased solvent inhomogeneity in the latter. As we shall emphasize in following content, the hydrogen-bonding effect is a major contribution to the distinctive spectroscopic behavior of the NO mode in the two different solvents studied here. 9



Page 10 of 34

Table 1. Steady-state infrared vibrational parameters of the NO stretching mode of the Ru-NO complex in dDMSO and D2O.

Solvent

Peak

FWHM a

Gaussian

Lorentzian

Integrated

Molar extinction

position

/ cm-1

width /

width /

area / arb. u.

coefficient /

cm-1

cm-1

/ cm-1

103∙M-1∙cm-1

1841.2

23.2

23.2

0

24.4

1863.7

20.3

20.3

0

2.7

1874.2

18.8

12.8

9.8

24.7

28.2

dDMSO

D2O

39.7

a. Full width at half maximum.

As can be seen from Figure 1, the NO stretching frequency is blue shifted by ca. 33 cm-1 when solvent changes from dDMSO (1841.2 cm-1) to D2O (1874.2 cm-1). The NO absorption peak in dDMSO is fairly close to what was observed in a similar compound ([Ru(OAc)(2mqn)2NO] (H2mqn = 2-methyl-8-quinolinol) 35, 1845.7 cm-1) measured in KBr pallet

(data not shown). This implies that in dDMSO the solute-solvent interaction is weak. We have also measured FTIR spectra of the Ru-NO complex in H2O, DMSO and EG. The results are given in Figure S1 in the Supporting Information (SI) and the fittings parameters are listed in Table S2. The results show that the spectral properties (peak position and line width) of the NO stretching in H2O and DMSO generally agree with those in D2O and dDMSO respectively, indicating a minor solvent isotope effect. Thus this work focuses on the study of the Ru-NO complex in dDMSO and D2O. In the case of water as solvent, due to strong 10


Page 11 of 34 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


hydrogen bond (deuterium bonds in this case) formed between NO and water, a red-shifted NO peak should appear when one only considers the hydrogen-bonding effect between NO and solvent. However, a blue shifted peak in D2O with respect to that in dDMSO is observed instead in Figure 1. As a matter of fact, there are two different hydrogen bonds formed between solute, the Ru-NO (Cl) complex, and solvent, namely the N=O⋯D and Ru-Cl⋯D bonds, as illustrated in Figure 2.

Figure 2. Schematic illustration of the NO stretching frequency shift under competing effect of the NO hydrogen (deuterium) bond (top) and chlorine hydrogen bond (bottom). See Text for details.

Metal-bound chlorine can accept hydrogen bond, such phenomenon has been described previously.14, 65-66 Because three Cl ions serve as ligands simultaneously in this Ru complex (Figure 2), their collective effect exerts an important and significant influence on the electron density of the metal center, which further changes the bond character of the NO group. This is explained below. On one hand, the hydrogen bond formed between NO and solvent will lengthen the NO bond and cause a red shift in the NO stretching frequency (Figure 2, top). On 11



the other hand, the hydrogen bond formed between three coordinated chlorines and solvent will lower the electron density of the metal center so that its back electron donating ability decreases, leading to a lengthened N-Ru bond and subsequently shortened NO bond length and hence a blue shifted NO stretching frequency (Figure 2, low). Because there are three RuCl⋯ D interactions that are relatively strong in the case of D2O (with OD) than the case of dDMSO (with CD3), and only one N=O⋯D interaction that is also relatively strong in the case D2O than the case of dDMSO, the three collective former interactions is more significant in D2O, which is the reason of the observed blue shift of the NO stretching frequency. However, this does not necessarily mean that deuterium bonding interaction in Ru-Cl⋯ D is stronger than that in N=O⋯D. A similar protonic-solvent dependent carbonyl vibrational stretching mode in a [RuCl2(CO)3]2 complex67 was also reported, where a blue shift of the C≡O stretching frequency was observed in D2O with respect to that in methanol, but no explicit explanation was given. Here, [RuCl2(CO)3]2 is also a chlorine-containing transition metal compound and the two solvents (D2O and methanol) have different polarities and acceptor numbers (AN, with AN ≈ 54.8 for D2O and AN = 41.5 for methanol).68 As a comparison, we measured FTIR spectra of the C≡O stretching mode of W(CO)6 in two solvents, namely DMSO and CCl4. The spectra are given in Figure S2 in the SI. As can be seen, without chlorine ligand, a red shifted C≡O frequency is observed in a solvent with large AN (DMSO, with AN = 19.3) with respect to the case of solvent with a small AN number (such as CCl4, with AN = 8.6). Thus, it is reasonable to conclude that it is the presence of chlorine ligand that causes the blue shift of the C≡O stretching frequency in this transition metal carbonyl compound in a solvent with high acceptor number. These arguments can reasonably explain our FTIR observation of the 12


Page 12 of 34

Page 13 of 34 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


Ru-NO complex shown in Figure 1, and additional data shown in Figure S1. Several common solvents with their AN numbers are listed in Table S3. Similarly, a blue shift of the NO stretching from DMSO to D2O solvent was also observed previously in [Fe(CN)5NO]2- complex.69-70 In this molecule, the axial CN group was believed to significantly contribute to the vibrational solvatochromism of the NO stretching mode, which is quite similar to what we observed in the Ru-NO complex studied here. Further, because the CN group has a stronger electron withdrawing ability than chlorine,71 a larger blue shift was expected in the NO frequency in [Fe(CN)5NO]2- (51.1 cm-1)56 than in our RuNO complex (34.1 cm-1, Table 1), when changing solvent from dDMSO to D2O. In addition, the infrared spectrum of the Ru-NO complex in EG was also measured and the result is shown for comparison in Figure S1 in the SI. The NO stretching band in EG also shows a blue shift with respect to that in dDMSO, which can also be explained as the hydrogen-bonding effect (between NO and the OH group of EG). However, the blue shift in EG is not as significant as that in D2O due to either hydrogen-bonding strength difference or size effect of EG, or both. Without the collective hydrogen-bonding effect and only considering the AN number (AN = 44.9 for EG vs. DMSO AN = 19.3 for DMSO) the NO stretching frequency of the Ru-NO complex would be otherwise red-shifted with respect to the case in dDMSO, as discussed above.

3.2 Transient IR Pump-Probe Spectroscopy Transient IR pump-probe experiment can potentially map the population evolution pathway of the vibrationally excited IR chromophore.72 For the relaxation to occur, basic vibrational energy conservation rule has to be applied. The vibrational energy can dissipate through 13



intramolecular vibrational redistribution (IVR), in which the initially excited vibration will pass its energy to a nearby IR chromophores, with or without the help of low-frequency modes of the solute, or through an intermolecular vibrational energy transfer (VER).72-74 The angle between the electric-field polarizations of the pump and probe pulses was set to the so-called magic-angle (54.7 degree) so that the effect of the molecular orientation can be excluded and the true vibrational relaxation dynamics can be obtained.75 In Figure 3, in each solvent, two types of signals are given, one negative (black circle) and one positive (blue circle) signals. The former is due to vibrational ground-state bleaching and stimulated emission (from v = 0 to v = 1), and the latter is due to the first vibrational excited-state absorption (from v = 1 to v = 2). The positive and negative signals are separated in probing frequency due to anharmonicity. Each relaxation curve can be fitted reasonably using exponential function. In both solvents, the positive curves were fitted by double exponential functions, while the negative curves were fitted roughly by single exponential function. Fitting parameters are listed in Table 2. The positive relaxation process is a direct measurement of the vibrational energy dissipation dynamics of the v = 1 state. The decay time constants of the fast and slow components are found to be 0.12 ps and 7.37 ps in dDMSO, 0.10 ps and 5.67 ps in D2O respectively. The time constant of the fast component in the two different solvents is quite similar (see Table 2), but with a small amplitude (ca. 10 %). Thus, it is not a major population relaxation pathway. Note there is also a very small portion of fast component in the ground-state recovering process in both solvents, which is not considered in fitting.

14


Page 14 of 34

Page 15 of 34 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 3. Population relaxation dynamics of the NO stretching mode of the Ru-NO complex in dDMSO (left) and D2O (right). Experimental data (circle) and exponential fitting (solid line) are given, the signal rising phase (red circles) is excluded in each case during fitting.

Table 2. Fitting parameters of the transient pump-probe spectra of the NO stretching mode of the Ru-NO complex in dDMSO and D2O.

Peak position

Band

Solvent

𝑇

/ cm

-1

/ ps

𝑇

/ ps

assignment

1814.4

1→2

1841.2

0→1

1846.9

1→2

1871.2

0→1

0.12 ± 0.02 (11 %)

7.37 ± 0.03 (89 %)

dDMSO 7.72 ± 0.03 0.10 ± 0.01 (12 %)

5.67 ± 0.02 (88 %)

D2O 5.64 ± 0.02

The ruthenium complex studied here has several internal degrees of freedom, some of which may be coupled, and their combination bands may serve as a bath of accepting modes to facilitate the IVR processes of the vibrationally excited NO stretching mode, which is the 15



known IVR process.76 To account for the 10-% fast amplitude in the energy relaxation dynamics, we have carried out DFT computations and normal-mode analysis, and the results are given in Table S1 and Figure S3, and Figure S4 in the SI. The results in Table S1 suggest that the excited NO vibration may dissipate its energy through the combination mode of the C-H in-plane bending vibration (for example, 1166.5 cm-1, 1137.5 cm-1, and 1132.4 cm-1), and the Ru-N bending vibrations (759.5 cm-1 and 656.3 cm-1), due to their approximately matched energy levels, even possibly with the aid of thermal fluctuation energy (kT = 200 cm-1 at room temperature). The identification of these vibrational modes is also listed in Table S1 in the form of PEDs. In order for the excited NO to relax its vibrational energy completely, solutesolvent interaction must play an important role. We believe this is directly related to the N=O⋯D interaction that is much stronger in the case of D2O than in the case of dDMSO, as one of the simplest explanations. Stronger solute-solvent interaction in D2O than in dDMSO is also evidenced by larger molar extinction coefficient of the NO stretching mode in the former solvent than in the latter (see Section 3.1). At this moment, we do not have further experimental evidence to clearly identify the intermolecular vibrational energy relaxation pathway. However, a recent work67 studied the vibrational relaxation of metal carbonyl and suggested that the relaxing modes may experience an enhanced intramolecular coupling in the presence of rapidly fluctuating local electric field produced by water’s large dipole and its fast, abrupt reorientation dynamics (water jumping model). One could anticipate that the ubiquitously distributed hydrogen bonded network of water will be anharmonically coupled to various intermolecular combination modes and provide such energy-relaxation channels. The strong hydrogen bond formed in D2O will result in a relatively efficient intermolecular energy relaxation channel. Other possible energy relaxation pathway may involve chlorine and 16


Page 16 of 34

Page 17 of 34 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


solvent interaction, which may be also effective to an undetermined extent. The relaxation of nitrosyl ligand was studied in other complex such as sodium nitroprussside,70 with a relaxation time of 39 ps in DMSO and 23.8 ps in D2O, which are much slower than what we observed in the Ru-NO complex. However, the speedup of the vibrational relaxation time was also found in other nitrosyl complexes with ligand environment similar to our case,35-37 i. e., the organic ligands, that were not present in earlier studied nitrosyl-containing systems. This indicates that the lifetime of the NO stretching excited-state may be case dependent. Anisotropy dynamics of the Ru-NO complex in dDMSO and D2O were also examined by carrying out polarization-dependent IR pump-probe experiments. The anisotropy dynamics is a good complement of vibrational energy relaxation dynamics, the two processes have an inverse relationship in terms of time constant when there are strong solute-solvent interactions.60, 77-78 The results are shown in Figure 4. The anisotropy decays from an initial value of ca. 0.4 in each case and is fitted using a single exponential function with time constant of Tr, which is 56.6 ps in dDMSO and 17.7 ps in D2O. The polarization dependent relaxation dynamics were presented in Figure S5, where the anisotropy dynamics in EG with time constant of 220.7 ps was also measured and compared on the same time scale. The anisotropy decay can be attributed to the overall molecular rotation and the stronger hydrogen bond in D2O was supposed to show a slower reorientation dynamics. The similar results were also reported in the ref. 70 for the NO stretching of sodium nitroprusside complex with more solvents. It has found that the constant of Tr in the solvent of H2O, DMSO and EG is 16 ps, 49 ps and 300 ps with their viscosity of 0.890 mPas, 1.987 mPas and 16.1 mPas at 25 ºC, respectively, in which a correlation between viscosity and reorientation time was observed. Here, we believe the dominant factor influencing the reorientation time is the “local” viscosity 17



in the classical macroscopic perspective, but actually the solute-solvent interaction in microscopic perspective. This conclusion is supported by the result in EG (Figure S5), which shows an even slower anisotropic relaxation.

Figure 4. Anisotropy dynamics probed at the 0-1 transition of the NO stretching mode of the Ru-NO complex in dDMSO (a) and in D2O (b). Experimental data (open circle) and exponential fitting (red line) are shown.

3.3 Waiting-Time-Dependent 2D IR Spectra The 2D IR spectroscopy is useful to characterize molecular structural dynamics and vibrational energy transfers on the ultrafast time scales through a network of waiting-time dependent diagonal and off-diagonal signals. It is most common to present 2D IR spectra in the purely absorptive way, in which the 2D line-shape is phased and best frequencyresovled.79-83 In a pump-probe geometry based 2D IR spectrometer, the rephasing and nonrephasing signals have the same phase matching condition and are emitted along the probe beam, hence the probe self-heterodyned and purely absorptive signals in this dimension can be measured directly in the frequency domain using a monochromator, whereas the coherence 18


Page 18 of 34

Page 19 of 34 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


dimension can be measured in the time domain using a fast scanning algorithm.58, 84 2D IR spectra of the NO stretching mode of the Ru-NO complex in dDMSO and D2O at several waiting times are shown in Figure 5. Each 2D IR spectrum contains a pair of peaks arising from the 0-1 vibrational transition roughly on the diagonal and the 1-2 transition that is leftshifted along the horizontal frequency axis due to anharmonicity. The 2D IR spectra at additional waiting times are given in Figure S6.

Figure 5. Real-part of purely absorptive 2D IR spectra of the NO stretching mode of the RuNO complex in dDMSO (upper panels) and D2O (lower panels) at various waiting times. Diagonal line was shown as a thin blue line across the positive signal (red). The estimated homogeneous linewidth (FWHM) of 10 cm-1 is shown as thin blue arrows in two left panels. The nodal line was shown in each panel by a short thick blue line in between the positive (red) and negative (blue) 2D signals. Spectra are normalized by the intensity of the positive peak.

To understand the origin of the 2D IR line shape,85 one may use Kubo’s stochastic theory of line shapes.86 According to the theory, the microscopic broadening mechanisms of 2D IR spectral line shape were ascribed to the dephasing of vibrational transition during the 19



Page 20 of 34

coherence time t1 and detection time t3 periods. Population relaxation and pure dephasing as well as inhomogeneous dephasing contribute to the overall dephasing process. These dephasing processes are sensitive to the solute-solvent interaction. As a result, the line shape can be a very sensitive indication of the system’s environmental structure changes. Within certain approximation, the theoretical model of dephasing is formulated by frequency fluctuation time-correlation function (FTCF).87 When considering both the homogeneous and inhomogeneous components, the FTCF can be written in the form of C(t) = =

( )

+ ∑ ∆ exp(−𝑡/𝜏 ) .

(2)

Here ∆ is the inhomogeneous frequency fluctuation amplitude and 𝜏 is the correlation time of the ith component. 𝑇 is the homogeneous dephasing time, which includes pure dephasing time 𝑇 ∗ , population relaxation time 𝑇 and reorientation relaxation time 𝑇 . The homogeneous line width (∆𝜈) can be formulated as ∆𝜈 =

=

∗

+

,

(3)

where the 𝑇 term was neglected because this time constant is rather long (56.6 ps in dDMSO and 17.7 ps in D2O) and hence only has a minor contribution to the homogeneous time in the present case. The vibrational population time (7.37 ps, i. e., 1.44 cm-1 in line width in dDMSO, and 5.67 ps, i. e., 1.87 cm-1 in line width in D2O) is also of minor contribution to the homogeneous line width as can be simply seen from Eq. (3). At 0-fs waiting time, the 2D IR line shapes are elongated along diagonal, suggesting a significant contribution of the inhomogeneous broadening to the NO stretching mode in both dDMSO and D2O. The diagonal elongation of a 2D IR diagonal peak contains also homogeneous broadening contribution that can be estimated from the anti-diagonal 2D IR spectral width. It is found that the anti-diagonal width is very similar in two solvents, both giving the homogeneous 20


Page 21 of 34 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


linewidth of approximately 10 cm-1 (full width at half maximum, FWHM, see the two left panels in Figure 5, fitting not shown) which corresponds to a value of T2 of ca.1 ps. Several parameters obtained from 2D IR spectra have been demonstrated to associate with FTCF in certain limits. For instance, the reciprocal of the center line slope (CLS) of the 0-1 transtion,52, 88-90 or the reciprocal of the nodal line slope (NLS) between the 0-1 and 1-2 transitions,91-92 has been used to extract the dynamical change of 2D IR signal as a function of waiting time, that is, vibrational spectral diffusion (SD) that is used to describe the FTCF dynamics. In this work, we use the nodal-line approach, and the nodal line was shown in Figure 5 as a short thick blue line in each 2D IR spectrum. The NLS dynamics of the NO stretching vibration in the Ru-NO complex in the two solvents are plotted against the waiting time and are shown in Figure 6. Clearly, a single exponential decay with time constant of 1.0 ps plus a significant unrelaxed residue is observed in the NLS dynamics in D2O (Figure 6, right panel). As contrary, the spectral diffusion in the dDMSO shows a bi-exponential process: a fast component (1.2 ps) and a slow component (14. 3 ps), with similar amplitude (40 % for the former and 60 % for the latter), as shown in Figure 6 (left panel). Further, to evaluate the uncertainties of the NLS at later times, more than five sets of 2D IR data were used to calculate the standard deviation of the NLS at several waiting times (0.2 ps, 3.5 ps, 20 ps in dDMSO and 0.2 ps, 3.5 ps, 7 ps in D2O), and the error bars are also shown in Figure 6. In addition, we also examined the CLS dynamics in each solvent, which was found to be in general agreement with the corresponding NLS dynamics (see Figure S7). Figure 6 shows that the initial value of the NLS curves in dDMSO and in D2O are somewhat similar. At longer waiting time, the NLS dynamics shows a tendency of 21



approaching zero in the case of dDMSO as solvent, suggesting that the NO group can well samples the randomly distributed neighboring DMSO molecular interaction in the time window of 30 ps. However, as can be seen quite surprisingly in Figure 6, this is not the case in D2O: there is a nearly static component (ca. 0.23) shown in the NLS dynamics even in the first 15-ps time window. 0.8 dDMSO

D2O

 = 1.2 ps

0.6 NLS

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 22 of 34

= 1.0 ps

 = 14.3 ps

0.4 0.2 0.0 0

10

20 30 0 10 Waiting time / ps

20

30

Figure 6. Frequency time-correlation function of the NO stretching vibration in the Ru-NO complex in terms of NLS dynamics (black dash lines with circles) extracted from the real part of purely absorptive 2D IR spectra. The red lines are their fittings, with bi-exponential decay for the case in dDMSO (left panel, with time constant (amplitude) of 1.2 ps (40 %) and 14.6 ps (60 %) respectively) and single-exponential decay for the case in D2O (right panel, 1.0 ps, with a static residue of ca. 0.23). The uncertainty of the NLS value at certain waiting-times has been evaluated in each case and shown in error bars (see text for detail).

The structural dynamics of D2O have already been well investigated with HDO : D2O mixed solution.93-96 These studies demonstrated the presence of a spectral diffusion process with a time constant of ca. 1 ps, which is very similar to what is probed by the NO stretching in D2O in the present work. Such a fast spectral diffusion time constant is believed to be 22


Page 23 of 34 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


associated with the collective reorganization of water hydrogen-bonding network, as also revealed in recent study of anion-water systems.97 On the other hand, the reorganization process of DMSO molecules is known to be much slower,32-33, 98 which is reflected by the slow spectral diffusion time of the NO stretching mode in dDMSO (Figure 6, left panel). The slow spectral diffusion process in the dDMSO is found to be roughly 14 times slower than the single-exponential component found in the case of D2O, which is clearly associated with the slow solvent dynamics of DMSO. The fast component found in dDMSO (1.2 ps) is also significant in amplitude, which may be related to certain fast solvent fluctuation that is sensed by the NO group through the weak NO⋯D (CD3) hydrogen bond in dDMSO. Such motion is likely to be associated with the weak hydrogen bond. The NO stretching vibration dynamics was studied in nitroprusside using three-pulse vibrational echo peak shift spectroscopy,54 which yielded a spectral diffusion time of 1.1 ps in D2O along with a small static component. A recent comprehensive 2D IR study of nitroprusside was carried out in a series of solvents,56 which showed also a spectral diffusion time of 1.4 ps in D2O but without static component, and a spectral diffusion time of 4.4 ps in DMSO. Thus the spectral diffusion time in D2O found in this study (1.0 ps) generally agrees with previous results in terms of time scale. However, a significant static slow component was seen in our Ru-NO complex solvated in D2O, which differs considerably from previous results. This may be contributed to the structural effect: In this work, the solute structure is more asymmetric and bulkier, with several ligands forming hydrogen bonds with solvent water molecules. This may result in a compact and tight solvent layer that may exhibit slower solvent dynamics, which is believed to be associated with the observed very slow spectraldiffusion component (i. e., the static offset). In EG, we also observed a nonzero offset in the 23



spectral diffusion dynamics in the 30-ps time window (Figure S8 and Figure S9), which is due to much sticker and relatively bulkier EG molecule, yet still has hydrogen-bond forming ability. On the other hand, the previously reported spectral diffusion time of the NO stretch in DMSO (4.4 ps),56 is actually not too much different from what we extracted from our 2D IR data. As shown in Figure 6, the two exponential relaxation processes can be fitted roughly by a single exponential decay with 5-7 ps time constant (data not shown). In addition, it should be noted that the weak high-frequency component observed in fitting linear IR spectrum of the Ru-NO complex in dDMSO (Figure 1) was neglected during the NLS analysis because we are mainly concerned with the strong peak, whose slope is unlikely to be significantly influenced by the high-frequency component. Based on above discussion, we believe a structurally stabilized solvent shell is formed around the Ru-NO complex in D2O, but not in dDMSO. To further illustrate this point, a simply cartoon is drawn and shown in Figure 7, in which the two solvation cases are compared. As can be seen, because there are three Ru-Cl⋯D interactions and one NO⋯D interaction, that are relatively strong in the case of D2O (right side), a quite stable and coordinated solvent structure (perhaps in the form of a solvent cage) is formed in the case of D2O, which results in a relatively slow solvent structural motion that prohibits NO group from completely sampling the solvent configurations within the time window of our 2D IR experiment. On the other hand, these hydrogen-bonding interactions in DMSO are all relatively weaker, hence the NO species has more chance to sample all the possible solvent configurations in a relatively shorter time. The picture illustrated in Figure 7, which is in agreement with computational predictions to some extent (Figure S4), explains the observed 24


Page 24 of 34

Page 25 of 34 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


fast SD component (1.2 ps) in DMSO, and this picture can also explain the observed blueshifted NO stretching frequency from dDMSO to D2O in the FTIR spectra, as we discussed above. Thus, our result suggests that even though H2O can form a more homogeneous bulky environment for polar solute in a steady-state view, a relative rigid solute-solvent configuration, which is solvent-dependent, may form, and can be seen dynamically at the chemical bond level using non-steady-state spectroscopic methods.

Figure 7. Illustration of the solvation layer (dashed circle) in the vicinity of NO group and Cl ions for the Ru-NO complex in dDMSO (left) and in D2O (right). A rigid and tight solvent cluster forms in the latter case.

Univalent ions solvated in a series of alcohols has been studied previously,99 in which a rigid solvent-berg model was proposed to interpret the polar solvation dynamics of small ions particularly in the case of sticky solvent. This result, however, somewhat differs from what we observed here for the Ru-NO complex that has multiple sites for solvent interactions and has a much larger molecular size. As a matter of fact, we observed slow solvent dynamics in the spectral diffusion experiment for water (Figure 6), which can also be explained in terms of the rigid solvent-berg (i.e., a compact and structured solvent layer) formed due to hydrogen25



bonding interactions between ligands and solvent molecules (Figure 7).

4. Conclusion In this work, the NO ligand of the [(CH3)4N][RuCl3(qn)(NO)] (qn = deprotonated 8hydroxyquinoline) complex was used as an effective vibrational probe in the investigation of the solute-solvent structural contact and solvent dynamics in solutions. Two common and important solvents, fully deuterated DMSO and deuterated water (D2O) were used. Linear and nonlinear IR experiments were performed, and solvent influence on the steady-state IR absorption line shape, and on transient vibrational excited-state relaxations, and on the NO stretching FTCF dynamics, was examined. A blue-shifted NO stretching absorption peak in D2O with respect to that in dDMSO was observed, accompanied by an overall narrowed spectral width in the former case. The frequency shift between the two solvents can be reasonably explained as the result of competing influences of two different hydrogen bonds formed between solute (the Ru-NO complex) and neighboring solvent molecules (in the first solvation layer), namely the N=O⋯D and Ru-Cl⋯D bonds, where the presence of three latter interactions plays a very important role in blue-shifting the NO stretching mode in D2O. The strength of solute-solvent interaction can also reasonably explain the observed faster relaxation of the excited-state energy of the NO stretching vibration in water than in DMSO. The anisotropy dynamics, on the other hand, can be understood based on solvent viscosity, rather than solute-solvent interactions. Further, the significantly non-zero offset of the spectral diffusion dynamics in the case of D2O is most likely due to the formation of a structured solvent shell, also owing to strong 26


Page 26 of 34

Page 27 of 34 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


solute-solvent hydrogen bonds for the entire Ru-NO complex (mainly due to three chlorine ligands), so that the environmental solvent molecules cannot be fully sampled by the NO group in the 15-ps time window. Apart from this relatively rigid solvation shell, bulk water is more or less similarly inhomogeneous as DMSO, which, quantitatively speaking, has been demonstrated by their similar initial NLS values. These observations are in general agreement with the known properties of these two solvents, for example, fast spectral diffusion of the NO stretching mode (with time constant of ca. 1.0 ps) in D2O is well known and can be attributed to the collective reorganization of the water hydrogen-bonding network. However, a fast spectral diffusion process was also observed in the case of dDMSO (1.2 ps) with appreciable weight in amplitude, which is related to certain fast solvent fluctuation that is sensed by the NO group through the weak NO⋯D (CD3) hydrogen bond in dDMSO. To conclude, our work provides a chemical-bond level description of a polar group (NO+) as a ligand of a transition metal complex, and its interaction with solvent that can be mediated by the solute chemical composition, that is, the transition metal center and chlorine ligands, to be more specific. Our work shows that H2O can form a relatively rigid solute-solvent configuration for polar transition metal complex, which hinders a complete sampling of solvent configurations on the picosecond time scale by a vibrational probe provided by the solute.

Acknowledgement This work was supported by the National Natural Science Foundation of China (21327802 and 21573243). The author thanks Prof. H. Wang for providing sample. 27



Supporting Information PED analysis, additional FTIR spectra, 2D IR spectra and extracted NLS dynamics for comparison.

Notes The authors declare no competing financial interest.

References (1) Toledo, J. C.; Augusto, O. Connecting the Chemical and Biological Properties of Nitric Oxide. Chem. Res. Toxicol. 2012, 25, 975-989. (2) Hayton, T. W.; Legzdins, P.; Sharp, W. B. Coordination and Organometallic Chemistry of Metal−NO Complexes. Chem. Rev. 2002, 102, 935-992. (3) Ignarro, L. J.; Lippton, H.; Edwards, J. C.; Baricos, W. H.; Hyman, A. L.; Kadowitz, P. J.; Gruetter, C. A. Mechanism of Vascular Smooth Muscle Relaxation by Organic Nitrates, Nitrites, Nitroprusside and Nitric Oxide: Evidence for the Involvement of S-Nitrosothiols as Active Intermediates. J. Pharmacol. Exp. Ther. 1981, 218, 739749. (4) Pereira, A. C.; Paulo, M.; Araujo, A. V.; Rodrigues, G. J.; Bendhack, L. M. Nitric Oxide Synthesis and Biological Functions of Nitric Oxide Released from Ruthenium Compounds. Braz. J. Med. Biol. Res. 2011, 44, 947-957. (5) de Gaitani, C. M.; de Melo, M. C.; Lunardi, C. N.; de, S. O. F.; da Silva, R. S.; Bendhack, L. M. Hypotensive Effect of the Nitrosyl Ruthenium Complex Nitric Oxide Donor in Renal Hypertensive Rats. Nitric Oxide 2009, 20, 195-199. (6) Allardyce, C. S.; Dyson, P. J. Ruthenium in Medicine: Current Clinical Uses and Future Prospects. Platinum Met. Rev. 2001, 45, 62-69. (7) Orenha, R. P.; Rocha, M. V. J.; Poater, J.; Galembeck, S. E.; Bickelhaupt, F. M. Nature of the Ru-NO Coordination Bond: Kohn-Sham Molecular Orbital and Energy Decomposition Analysis. Chemistryopen 2017, 6, 410-416. (8) Batista, A. P. D.; de Oliveira, A. G. S.; Galembeck, S. E. Photophysical Properties and the NO Photorelease Mechanism of a Ruthenium Nitrosyl Model Complex Investigated using the CASSCF-in-DFT Embedding Approach. Phys. Chem. Chem. Phys. 2017, 19, 13860-13867. (9) Tfouni, E.; Krieger, M.; McGarvey, B. R.; Franco, D. W. Structure, Chemical and Photochemical Reactivity and Biological Activity of Some Ruthenium Amine Nitrosyl Complexes. Coord. Chem. Rev. 2003, 236, 57-69. (10) Pan, H.; Wang, W.; Ma, Z.; Xu, L.; Zhang, Z.; Zhao, Y.; Zhao, X.; Wang, H. Structures and Spectroscopic Properties of Three [RuCl(2mqn) 2 NO] (H2mqn = 2-Methyl-8-Quinolinol) Isomers: An Experimental and Density Functional Theoretical Study. Polyhedron 2016, 118, 61-69. (11) Da Silva, A. C. H.; Da Silva, J. L. F.; Franco, D. W. Nitroxyl as a Ligand in Ruthenium Tetraammine Systems: 28


Page 28 of 34

Page 29 of 34 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


A Density Functional Theory Study. Dalton Trans. 2016, 45, 4907-4915. (12) Kovács, A.; Varga, Z. Halogen Acceptors in Hydrogen Bonding. Coord. Chem. Rev. 2006, 250, 710-727. (13) Brammer, L. Metals and Hydrogen Bonds. Dalton Trans. 2003,

3145-3157.

(14) Aullon, G.; Bellamy, D.; Brammer, L.; Bruton, E. A.; Orpen, A. G. Metal-Bound Chlorine Often Accepts Hydrogen Bonds. Chem. Commun. 1998,

653-654.

(15) Neve, F.; Crispini, A. C−H···Br-M Interactions at Work: Tetrabromometalates of the Bolaamphiphilic N,N‘Dodecamethylenedipyridinium Cation. Cryst. Growth Des. 2001, 1, 387-393. (16) Sun, Q.; Dereka, B.; Vauthey, E.; Lawson Daku, L. M.; Hauser, A. Ultrafast Transient IR Spectroscopy and DFT Calculations of Ruthenium(II) Polypyridyl Complexes. Chem. Sci. 2017, 8, 223-230. (17) Horvath, R.; Fraser, M. G.; Clark, C. A.; Sun, X.-Z.; George, M. W.; Gordon, K. C. Nature of Excited States of Ruthenium-Based Solar Cell Dyes in Solution: A Comprehensive Spectroscopic Study. Inorg. Chem. 2015, 54, 11697-11708. (18) Mukuta, T.; Fukazawa, N.; Murata, K.; Inagaki, A.; Akita, M.; Tanaka, S. i.; Koshihara, S.-y.; Onda, K. Infrared Vibrational Spectroscopy of [Ru(bpy)2(bpm)]2+ and [Ru(bpy)3]2+ in the Excited Triplet State. Inorg. Chem. 2014, 53, 2481-2490. (19) DeFlores, L. P.; Ganim, Z.; Ackley, S. F.; Chung, H. S.; Tokmakoff, A. The Anharmonic Vibrational Potential and Relaxation Pathways of the Amide I and II Modes of N-Methylacetamide. J. Phys. Chem. B 2006, 110, 1897318980. (20) Wang, J.; Chen, J.; Hochstrasser, R. M. Local Structure of β-Hairpin Isotopomers by FTIR, 2D IR, and Ab Initio Theory. J. Phys. Chem. B 2006, 110, 7545-7555. (21) Serrano, A. L.; Waegele, M. M.; Gai, F. Spectroscopic Studies of Protein Folding: Linear and Nonlinear Methods. Protein Sci. 2012, 21, 157-170. (22) Montalvo, G.; Waegele, M. M.; Shandler, S.; Gai, F.; DeGrado, W. F. Infrared Signature and Folding Dynamics of a Helical β-Peptide. J. Am. Chem. Soc. 2010, 132, 5616-5618. (23) Nihonyanagi, S.; Yamaguchi, S.; Tahara, T. Ultrafast Dynamics at Water Interfaces Studied by Vibrational Sum Frequency Generation Spectroscopy. Chem. Rev. 2017, 117, 10665-10693. (24) Laage, D.; Elsaesser, T.; Hynes, J. T. Water Dynamics in the Hydration Shells of Biomolecules. Chem. Rev. 2017, 117, 10694-10725. (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, 491-495. (26) Perakis, F.; Marco, L. D.; Shalit, A.; Tang, F.; Kann, Z. R.; Kühne, T. D.; Torre, R.; Bonn, M.; Nagata, Y. Vibrational Spectroscopy and Dynamics of Water. Chem. Rev. 2016, 116, 7590-7607. (27) Ramasesha, K.; De Marco, L.; Mandal, A.; Tokmakoff, A. Water Vibrations Have Strongly Mixed Intra- and Intermolecular Character. Nat. Chem. 2013, 5, 935-940. (28) Roy, S.; Skoff, D.; Perroni, D. V.; Mondal, J.; Yethiraj, A.; Mahanthappa, M. K.; Zanni, M. T.; Skinner, J. L. Water Dynamics in Gyroid Phases of Self-Assembled Gemini Surfactants. J. Am. Chem. Soc. 2016, 138, 24722475. (29) Thämer, 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. (30) Chuntonov, L. 2D-IR Spectroscopy of Hydrogen-Bond-Mediated Vibrational Excitation Transfer. Phys. Chem. Chem. Phys. 2016, 18, 13852-13860. (31) Cui, Y.; Fulfer, K. D.; Ma, J.; Weldeghiorghis, T. K.; Kuroda, D. G. Solvation Dynamics of an Ionic Probe in Choline Chloride-Based Deep Eutectic Solvents. Phys. Chem. Chem. Phys. 2016, 18, 31471-31479. (32) Kashid, S. M.; Jin, G. Y.; Chakrabarty, S.; Kim, Y. S.; Bagchi, S. Two-Dimensional Infrared Spectroscopy Reveals Cosolvent-Composition-Dependent Crossover in Intermolecular Hydrogen Bond Dynamics. J. Phys. 29



Chem. Lett. 2017, 8, 1604-1609. (33) Kashid, S. M.; Jin, G. Y.; Bagchi, S.; Kim, Y. S. Cosolvent Effects on Solute–Solvent Hydrogen-Bond Dynamics: Ultrafast 2D IR Investigations. J. Phys. Chem. B 2015, 119, 15334-15343. (34) Oh, K. I.; Rajesh, K.; Stanton, J. F.; Baiz, C. R. Quantifying Hydrogen-Bond Populations in Dimethyl Sulfoxide/Water Mixtures. Angew. Chem. Int. Ed. Engl. 2017, 56, 11375-11379. (35) Zhao, Y.; Yang, F.; Wang, J.; Yu, P.; Pan, H.; Wang, H.; Wang, J. Structural Dynamics of Nitrosylruthenium Isomeric Complexes Studied with Steady-State and Transient Pump-Probe Infrared Spectroscopies. Spectrochim. Acta, Part A 2016, 166, 62-67. (36) Yu, P.; Zhao, Y.; Yang, F.; Pan, H.; Wang, J.; Zhao, J.; Wang, W.; Wang, H.; Wang, J. Differentiating Two Nitrosylruthenium Isomeric Complexes by Steady-State and Ultrafast Infrared Spectroscopies. J. Phys. Chem. B 2016, 120, 11502-11509. (37) Wang, J.; Yang, F.; Zhao, Y.; Yu, P.; Qiao, X.; Wang, J.; Wang, H. Photoisomerization and Structural Dynamics of Two Nitrosylruthenium Complexes: A Joint Study by NMR and Nonlinear IR Spectroscopies. Phys. Chem. Chem. Phys. 2014, 16, 24045-24054. (38) Błasiak, B.; Londergan, C. H.; Webb, L. J.; Cho, M. Vibrational Probes: From Small Molecule Solvatochromism Theory and Experiments to Applications in Complex Systems. Acc. Chem. Res. 2017, 50, 968976. (39) Feng, Y.; Huang, J.; Kim, S.; Shim, J. H.; MacKerell, A. D.; Ge, N.-H. Structure of Penta-Alanine Investigated by Two-Dimensional Infrared Spectroscopy and Molecular Dynamics Simulation. J. Phys. Chem. B 2016, 120, 5325-5339. (40) Hunt, N. T. 2D-IR Spectroscopy: Ultrafast Insights into Biomolecule Structure and Function. Chem. Soc. Rev. 2009, 38, 1837-1848. (41) Ramos, S.; Scott, K. J.; Horness, R. E.; Le Sueur, A. L.; Thielges, M. C. Extended Timescale 2D IR Probes of Proteins: p-Cyanoselenophenylalanine. Phys. Chem. Chem. Phys. 2017, 19, 10081-10086. (42) Slenkamp, K. M.; Lynch, M. S.; Kuiken, B. E. V.; Brookes, J. F.; Bannan, C. C.; Daifuku, S. L.; Khalil, M. Investigating Vibrational Anharmonic Couplings in Cyanide-Bridged Transition Metal Mixed Valence Complexes Using Two-Dimensional Infrared Spectroscopy. J. Chem. Phys. 2014, 140, 084505. (43) Tracy, K. M.; Barich, M. V.; Carver, C. L.; Luther, B. M.; Krummel, A. T. High-Throughput Two-Dimensional Infrared (2D IR) Spectroscopy Achieved by Interfacing Microfluidic Technology with a High Repetition Rate 2D IR Spectrometer. J. Phys. Chem. Lett. 2016, 7, 4865-4870. (44) van Wilderen, L. J. G. W.; Kern-Michler, D.; Muller-Werkmeister, H. M.; Bredenbeck, J. Vibrational Dynamics and Solvatochromism of the Label SCN in Various Solvents and Hemoglobin by Time Dependent IR and 2D-IR Spectroscopy. Phys. Chem. Chem. Phys. 2014, 16, 19643-19653. (45) Zhang, W.; Markiewicz, B. N.; Doerksen, R. S.; Smith, I. I. I. A. B.; Gai, F. CN Stretching Vibration of 5Cyanotryptophan as an Infrared Probe of Protein Local Environment: What Determines Its Frequency? Phys. Chem. Chem. Phys. 2016, 18, 7027-7034. (46) Li, D.; Yang, F.; Han, C.; Zhao, J.; Wang, J. Correlated High-Frequency Molecular Motions in Neat Liquid Probed with Ultrafast Overtone Two-Dimensional Infrared Spectroscopy. J. Phys. Chem. Lett. 2012, 3, 3665-70. (47) Wang, J. Ultrafast Two-Dimensional Infrared Spectroscopy for Molecular Structures and Dynamics with Expanding Wavelength Range and Increasing Sensitivities: from Experimental and Computational Perspectives. Int. Rev. Phys. Chem. 2017, 36, 377-431. (48) Anna, J. M.; King, J. T.; Kubarych, K. J. Multiple Structures and Dynamics of [CpRu(CO)2]2 and [CpFe(CO)2]2 in Solution Revealed with Two-Dimensional Infrared Spectroscopy. Inorg. Chem. 2011, 50, 92739283. (49) Kraack, J. P.; Hamm, P. Surface-Sensitive and Surface-Specific Ultrafast Two-Dimensional Vibrational 30


Page 30 of 34

Page 31 of 34 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


Spectroscopy. Chem. Rev. 2017, 117, 10623-10664. (50) Kowalewski, M.; Fingerhut, B. P.; Dorfman, K. E.; Bennett, K.; Mukamel, S. Simulating Coherent Multidimensional Spectroscopy of Nonadiabatic Molecular Processes: From the Infrared to the X-ray Regime. Chem. Rev. 2017, 117, 12165-12226. (51) Kratochvil, H. T.; Carr, J. K.; Matulef, K.; Annen, A. W.; Li, H.; Maj, M.; Ostmeyer, J.; Serrano, A. L.; Raghuraman, H.; Moran, S. D., et al. Instantaneous Ion Configurations in the K+ Ion Channel Selectivity Filter Revealed by 2D IR Spectroscopy. Science 2016, 353, 1040-1044. (52) Kwak, K.; Park, S.; Finkelstein, I. J.; Fayer, M. D. Frequency-Frequency Correlation Functions and Apodization in Two-Dimensional Infrared Vibrational Echo Spectroscopy: A New Approach. J. Chem. Phys. 2007, 127, 124503. (53) Lynch, M. S.; Cheng, M.; Van Kuiken, B. E.; Khalil, M. Probing the Photoinduced Metal−Nitrosyl Linkage Isomerism of Sodium Nitroprusside in Solution Using Transient Infrared Spectroscopy. J. Am. Chem. Soc. 2011, 133, 5255-5262. (54) Jumpei, T.; Kaoru, O.; Keisuke, T. Vibrational Transition Frequency Fluctuation of the NO Stretching Mode of Sodium Nitroprusside in Aqueous Solutions. Chem. Lett. 2012, 41, 366-368. (55) Adamczyk, K.; Simpson, N.; Greetham, G. M.; Gumiero, A.; Walsh, M. A.; Towrie, M.; Parker, A. W.; Hunt, N. T. Ultrafast Infrared Spectroscopy Reveals Water-Mediated Coherent Dynamics in an Enzyme Active Site. Chem. Sci. 2015, 6, 505-516. (56) Brookes, J. F.; Slenkamp, K. M.; Lynch, M. S.; Khalil, M. Effect of Solvent Polarity on the Vibrational Dephasing Dynamics of the Nitrosyl Stretch in an FeII Complex Revealed by 2D IR Spectroscopy. J. Phys. Chem. A 2013, 117, 6234-6243. (57) Wang, H.; Hagihara, T.; Ikezawa, H.; Tomizawa, H.; Miki, E. Electronic Effects of the Substituent Group in 8-Quinolinolato Ligand on Geometrical Isomerism for Nitrosylruthenium(II) Complexes. Inorg. Chim. Acta 2000, 299, 80-90. (58) Shim, S. H.; Zanni, M. T. How to Turn Your Pump-Probe Instrument into a Multidimensional Spectrometer: 2D IR and Vis Spectroscopies via Pulse Shaping. Phys. Chem. Chem. Phys. 2009, 11, 748-761. (59) Cross, A. J.; Waldeck, D. H.; Fleming, G. R. Time resolved Polarization Spectroscopy: Level Kinetics and Rotational Diffusion. J. Chem. Phys. 1983, 78, 6455-6467. (60) Li, M.; Owrutsky, J.; Sarisky, M.; Culver, J. P.; Yodh, A.; Hochstrasser, R. M. Vibrational and Rotational Relaxation Times of Solvated Molecular Ions. J. Chem. Phys. 1993, 98, 5499-5507. (61) Moog, R. S.; Ediger, M. D.; Boxer, S. G.; Fayer, M. D. Viscosity Dependence of the Rotational Reorientation of Rhodamine B in Mono- and Polyalcohols. Picosecond Transient Grating Experiments. J. Phys. Chem. 1982, 86, 4694-4700. (62) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A., et al., Gaussian 09, Revision A.02,. Gaussian, Inc., Pittsburgh PA, 2009. (63) Zhao, J.; Wang, J. Chain-length and Mode-Delocalization Dependent Amide-I Anharmonicity in Peptide Oligomers. J. Chem. Phys. 2012, 136, 214112. (64) Krimm, S.; Bandekar, J. Vibrational Spectroscopy and Conformation of Peptides, Polypeptides, and Proteins. Adv. Protein Chem. 1986, 38, 181-364. (65) Gutmann, V. Solvent Effects on Metal-Carbonyls and Related Compounds. Monatsh. Chem. 1977, 108, 429435. (66) Gilli, G.; Gilli, P., The Nature of the Hydrogen Bond: Outline of a Comprehensive Hydrogen Bond Theory. Oxford University Press: New York, U.S., 2009. (67) King, J. T.; Ross, M. R.; Kubarych, K. J. Water-Assisted Vibrational Relaxation of a Metal Carbonyl Complex 31



Studied with Ultrafast 2D-IR. J. Phys. Chem. B 2012, 116, 3754-3759. (68) Mayer, U.; Gutmann, V.; Gerger, W. The Acceptor Number — A Quantitative Empirical Parameter for the Electrophilic Properties of Solvents. Monatsh. Chem. 1975, 106, 1235-1257. (69) Estrin, D. A.; Baraldo, L. M.; Slep, L. D.; Barja, B. C.; Olabe, J. A.; Paglieri, L.; Corongiu, G. Theoretical and Experimental Study of Medium Effects on the Structure and Spectroscopy of the [Fe(CN)5NO]2- Ion. Inorg. Chem. 1996, 35, 3897-3903. (70) Sando, G. M.; Zhong, Q.; Owrutsky, J. C. Vibrational and Rotational Dynamics of Cyanoferrates in Solution. J. Chem. Phys. 2004, 121, 2158-2168. (71) Politzer, P.; Murray, J. S.; Clark, T. Halogen Bonding and Other [Sigma]-Hole Interactions: a Perspective. Phys. Chem. Chem. Phys. 2013, 15, 11178-11189. (72) Kenkre, V. M.; Tokmakoff, A.; Fayer, M. D. Theory of Vibrational Relaxation of Polyatomic Molecules in Liquids. J. Chem. Phys. 1994, 101, 10618-10629. (73) Zimdars, D.; Tokmakoff, A.; Chen, S.; Greenfield, S. R.; Fayer, M. D.; Smith, T. I.; Schwettman, H. A. Picosecond Infrared Vibrational Photon Echoes in a Liquid and Glass Using a Free Electron Laser. Phys. Rev. Lett. 1993, 70, 2718-2721. (74) Tokmakoff, A.; Sauter, B.; Fayer, M. D. Temperature‐Dependent Vibrational Relaxation in Polyatomic Liquids: Picosecond Infrared Pump–Probe Experiments. J. Chem. Phys. 1994, 100, 9035-9043. (75) Fleming, G., Chemical Applications of Ultrafast Spectroscopy. 1st edn. ed.; Oxford University Press: Oxford, 1986; p Medium: X; Size: Pages: 262. (76) Fedoseeva, M.; Delor, M.; Parker, S. C.; Sazanovich, I. V.; Towrie, M.; Parker, A. W.; Weinstein, J. A. Vibrational Energy Transfer Dynamics in Ruthenium Polypyridine Transition Metal Complexes. Phys. Chem. Chem. Phys. 2015, 17, 1688-1696. (77) Dutt, G. B.; Ghanty, T. K. Rotational Dynamics of Nondipolar Probes in Ethanols: How Does the Strength of the Solute–Solvent Hydrogen Bond Impede Molecular Rotation? J. Chem. Phys. 2003, 119, 4768-4774. (78) Lee, C.; Son, H.; Park, S. Effect of Hydrogen Bonds on the Vibrational Relaxation and Orientational Relaxation Dynamics of HN3 and N3– in Solutions. J. Phys. Chem. B 2016, 120, 9723-9731. (79) Khalil, M.; Demirdoven, N.; Tokmakoff, A. Obtaining Absorptive Line Shapes in Two-Dimensional Infrared Vibrational Correlation Spectra. Phys. Rev. Lett. 2003, 90, 047401. (80) Ghosh, A.; Ostrander, J. S.; Zanni, M. T. Watching Proteins Wiggle: Mapping Structures with TwoDimensional Infrared Spectroscopy. Chem. Rev. 2017, 117, 10726-10759. (81) Guo, Q.; Pagano, P.; Li, Y.-L.; Kohen, A.; Cheatum, C. M. Line Shape Analysis of Two-Dimensional Infrared Spectra. J. Chem. Phys. 2015, 142, 212427. (82) He, X.; Yu, P.; Zhao, J.; Wang, J. Efficient Vibrational Energy Transfer through Covalent Bond in Indigo Carmine Revealed by Nonlinear IR Spectroscopy. J. Phys. Chem. B 2017, 121, 9411-9421. (83) He, X.; Xu, F.; Yu, P.; Wu, Y.; Wang, F.; Zhao, Y.; Wang, J. Solvent-Dependent Structural Dynamics of an Azido-Platinum Complex Revealed by Linear and Nonlinear Infrared Spectroscopy. Phys. Chem. Chem. Phys. 2018, 20, 9984-9996. (84) Skoff, D. R.; Laaser, J. E.; Mukherjee, S. S.; Middleton, C. T.; Zanni, M. T. Simplified and Economical 2D IR Spectrometer Design using a Dual Acousto-Optic Modulator. Chem. Phys. 2013, 422, 8-15. (85) Hamm, P.; Zanni, M. T., Concepts and Methods of 2D Infrared Spectroscopy. Cambridge University Press: Cambridge, U.K., 2011. (86) Faid, K.; Fox, R. F. Stochastic Theory of Line Shape and Relaxation. Phys. Rev. A 1986, 34, 4286-4302. (87) Mukamel, S., Principles of Nonlinear Optical Spectroscopy. Oxford University Press on Demand: 1999. (88) Roberts, S. T.; Loparo, J. J.; Tokmakoff, A. Characterization of Spectral Diffusion from Two-Dimensional Line Shapes. J. Chem. Phys. 2006, 125, 084502. 32


Page 32 of 34

Page 33 of 34 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


(89) Khalil, M.; Demirdöven, N.; Tokmakoff, A. Coherent 2D IR Spectroscopy: Molecular Structure and Dynamics in Solution. J. Phys. Chem. A 2003, 107, 5258-5279. (90) Fenn, E. E.; Fayer, M. D. Extracting 2D IR Frequency-Frequency Correlation Functions From Two Component Systems. J. Chem. Phys. 2011, 135, 074502. (91) Kwac, K.; Cho, M. Molecular Dynamics Simulation Study of N-Methylacetamide in Water. II. TwoDimensional Infrared Pump–Probe Spectra. J. Chem. Phys. 2003, 119, 2256-2263. (92) Singh, P. K.; Kuroda, D. G.; Hochstrasser, R. M. An Ion’s Perspective on the Molecular Motions of Nanoconfined Water: A Two-Dimensional Infrared Spectroscopy Study. J. Phys. Chem. B 2013, 117, 9775-9784. (93) Fayer, M. D., Ultrafast Infrared Vibrational Spectroscopy. CRC Press: Boca Raton, FL, 2013. (94) Fecko, C. J.; Loparo, J. J.; Roberts, S. T.; Tokmakoff, A. Local Hydrogen Bonding Dynamics and Collective Reorganization in Water: Ultrafast Infrared Spectroscopy of HOD/D2O. J. Chem. Phys. 2005, 122, 54506. (95) Stenger, J.; Madsen, D.; Hamm, P.; Nibbering, E. T. J.; Elsaesser, T. A Photon Echo Peak Shift Study of Liquid Water. J. Phys. Chem. A 2002, 106, 2341-2350. (96) Fecko, C. J.; Eaves, J. D.; Loparo, J. J.; Tokmakoff, A.; Geissler, P. L. Ultrafast Hydrogen-Bond Dynamics in the Infrared Spectroscopy of Water. Science 2003, 301, 1698-1702. (97) Yamada, S. A.; Thompson, W. H.; Fayer, M. D. Water-Anion Hydrogen Bonding Dynamics: Ultrafast IR Experiments and Simulations. J. Chem. Phys. 2017, 146, 234501. (98) Wong, D. B.; Sokolowsky, K. P.; El-Barghouthi, M. I.; Fenn, E. E.; Giammanco, C. H.; Sturlaugson, A. L.; Fayer, M. D. Water Dynamics in Water/DMSO Binary Mixtures. J. Phys. Chem. B 2012, 116, 5479-5490. (99) Biswas, R.; Bagchi, B. Ionic Mobility in Alcohols: From Dielectric Friction to the Solvent–Berg Model. J. Chem. Phys. 1997, 106, 5587-5598.

33



Table of Contents Graphic

34


Page 34 of 34

Linear and Nonlinear Infrared Spectroscopies Reveal Detailed Solute

Recommend Documents