Solvation dynamics function measurements

Solvation dynamics are measured using the more reliable energy relaxation method after a local perturbation [83-85], typically using a femtosecond-resolved fluorescence technique. Experimentally, the wavelength-resolved transients are obtained using the fluorescence upconversion method [85], The observed fluorescence dynamics, decay at the blue side and rise at the red side (Fig. 3a), reflecting typical solvation processes. The molecular mechanism is schematically shown in Fig. 5. Typically, by following the standard procedures [35], we can construct the femtosecond-resolved emission spectra (FRES, Stokes shifts with time) and then the correlation function (solvent response curve) ... 

Investigation of water motion in AOT reverse micelles determining the solvent correlation function, C i), was first reported by Sarkar et al. [29]. They obtained time-resolved fluorescence measurements of C480 in an AOT reverse micellar solution with time resolution of > 50 ps and observed solvent relaxation rates with time constants ranging from 1.7 to 12 ns. They also attributed these dynamical changes to relaxation processes of water molecules in various environments of the water pool. In a similar study investigating the deuterium isotope effect on solvent motion in AOT reverse micelles. Das et al. [37] reported that the solvation dynamics of D2O is 1.5 times slower than H2O motion. 

To measure solvation dynamics in a liquid we follow the rearrangement of surrounding solvent molecules after photo-excitation of a dissolved dye by recording its fluorescence as a function of time. The Stokes shift response function S(t) = (v(t), v(0), and v(oo) re-... 

A number of analogous compounds to BA have been reported, including 5,5 -dibenzo-[a]-pyrenyl (BBPY) [116]. These compounds exhibit emission spectra similar to BA. It would be interesting to explore the ultrafast dynamics of BBPY in order to test the generality of the GLE model. It would also be interesting to study the femtosecond dynamics of BA as a function of applied pressure. Static experiments on the emission of BA, reported by Hara et al. [123], demonstrate that in low viscosity solvents an increase of pressure affects the emission similarly to an increase of solvent polarity. As the pressure is increased, however, the LE/CT interconversion is slowed down. It would be interesting to measure C(r) in these environments and compare the solvation dynamics with LE/CT dynamics, in order to test the generality of the GLE dielectric friction model. 

Strikingly, the solvation dynamics for all mutants are nearly the same. All correlation functions can be best described by a double exponential decay with time constants of 0.67 ps with 68% of the total amplitude and 13.2 ps (32%) for D60, 0.47 ps (67%) and 12.7 (33%) for D60G, and 0.53 ps (69%) and 10.8 ps (31%) for D60N. Relative to SNase above, the solvation dynamics are fast, which reflects the neighboring hydrophobic environment. We also measured the anisotropy dynamics and, as shown in the inset of Fig. 33, the local structure is very rigid in the time window of 800 ps. This observation is consistent with the inflexible turn (-T30W31-) in the transition from the second /1-sheet and the second x-helix (Fig. 31). Thus, the three mutants, with a charged, polar, or hydrophobic reside around the probe (Fig. 34) but with the similar time scales of... 

A time-resolved fluorescence measurement collects the emission spectra at regular time intervals after the excitation, defined at t=0, from which one constructs the normalized solvation dynamics response function, S(t) = [hv(t)—hv(oo)]/[hv(0)— hv(oo)] [55], In our simulations, hundreds of uncorrelated equilibrium molecular configurations with the electron in its ground state were selected as initial configurations (t=0). From each of these initial configurations, the electronic state is adiabatically promoted to the first excited state, the system is then propagated in... 

Emission decays were obtained from a fs Tl sapphire laser uorescence upconversion spectrometer whose construction is reported elsewhere [8]. Here we only note that the overall temporal response used in these studies was between 112-125 fs as measured by the FWHM of the cross correlation between the pump and gate pulses. Decays (0-200 ps with a variable step size) were collected at a series of ten emission wavelengths (8 nm bandpass) which were then used to reconstruct time-evolving emission spectra in the manner described in Refs. 8 and 9. From these spectra the solvation dynamics was extracted in the form of the spectral response function,... 

Detailed measurements of the s-tetrazine gas-phase spectrum were made. With these data, measurement of the absolute Stokes shift S(t) is possible. Because the Stokes shift is zero in the absence of solvent nuclear dynamics, the magnitude of the Stokes shift at the earliest times represents the amount of relaxation within the experimental time resolution. The steady-state absorption and fluorescence spectra were also measured to provide an independent value of the equilibrium Stokes shift S< With this data, the absolute solvation response function... 

Using fs resolution, two residence times of water at the surface of two proteins have been reported (Fig. 7.6) [21]. The natural probe tryptophan amino acid was used to follow the dynamics of water at the protein surface. For comparison, the behavior in bulk water was also studied. The experimental result together with the theoretical simulation-dynamical equilibrium in the hydration shell, show the direct relationship between the residence time of water molecules at the surface of proteins and the observed slow component in solvation dynamics. For the two biological systems studied, a bimodal decay for the hydration correlation function, with two primary relaxation times was observed an ultrafast time, typically 1 ps or less, and a longer one typically 15-40 ps (Fig. 7.7) [21]. Both times are related to the residence period of water at the protein surface, and their values depend on the binding energy. Measurement of the OH librational band corresponding to intermolecular motion in nanoscopic pools of water and methanol... 

Two points should be mentioned here. First, the effect of solutes on the solvent dielectric response can be important in solvents with nonlocal dielectric properties. In principle, this problem can be handled by measuring the spectrum of the whole system, the solvent plus the solutes. Theoretically, the spatial dependence of the dielectric response function, s(r, co), which includes the molecular nature of the solvent, is often treated by using the dynamical mean spherical approximation [28, 36a, 147a, 193-195]. A more advanced approach is based on a molecular hydrodynamic theory [104,191, 196, 197]. These theoretical developments have provided much physical insight into solvation dynamics. However, reasonable agreement between the experimentally measured Stokes shift and emission line shape can be... 

The structure of the adsorbed ion coordination shell is determined by the competition between the water-ion and the metal-ion interactions, and by the constraints imposed on the water by the metal surface. This structure can be characterized by water-ion radial distribution functions and water-ion orientational probability distribution functions. Much is known about this structure from X-ray and neutron scattering measurements performed in bulk solutions, and these are generally in agreement with computer simulations. The goal of molecular dynamics simulations of ions at the metal/water interface has been to examine to what degree the structure of the ion solvation shell is modified at the interface. 

In conclusion, these gas-phase measurements provide new elues to the role of solvation in ion-moleeule reaetions. For the first time, it is possible to study intrinsie reactivities and the extent to which the properties of gas-phase ion-moleeule reaetions relate to those of the eorresponding reactions in solution. It is clear, however, that gas-phase solvated-ion/moleeule reaetions in which solvent moleeules are transferred into the intermediate elusters by the nucleophile cannot be exaet duplieates of solvated-ion/ molecule reactions in solution in which solvated reactants exchange solvent molecules with the surrounding bulk solvent [743]. For a selection of more recent experimental [772] and theoretical studies of Sn2 reactions in gas phase and solution by classical trajectory simulations [773], molecular dynamics simulations [774, 775], ab initio molecular orbital calculations [776, 777], and density functional theory calculations [778, 779], see the references given. For studies of reactions other than Sn2 ion-molecule processes in the gas phase and in solution, see reviews [780, 781]. 

The main purpose of this section is to give the basis of how measurements of the dielectric constants of ionic solutions can give information on solvation, particularly primary hydration numbers. However, dielectric measurements as a function of frequency also give information on the dynamic behavior of water by allowing us to determine the relaxation time of water in ionic solutions and expressing the changes in terms of the number of water molecules bound to the ion. 

The surrogate Hamiltonian is expressed in terms of renormalized solute-solvent interactions, a feature that leads to a simple and natural linear response description of the solvent dynamics in the vicinity of the solute. In addition to the measurable solvation time correlation function (tcf), we can also calculate observables needed to elucidate the detailed mechanism of solvation response, such as the evolution of the solvent polarization charge density around the solute. 

A promising method, developed in recent years, is the use of first principles molecular dynamics as exemplified by the Car-Parrinello technique (8]. In these calculations the interatomic potentials are explicitly derived from the electronic ground-state within the density functional theory in local or non-local approximation. It combines quantum mechanical calculations with molecular dynamics simulations and, therefore, overcomes the limitations of both methods. Actual computers allow only simulations of aqueous solutions of about 60 water molecules for several ps (10 s). This limit is still at least one order of magnitude shorter than the fastest directly measured water exchange rate, k = 3.5 x 10 s for [Eu(H20)8], i.e. one exchange event every (8 x 3.5 x lO s ) = 36 ps [9]. Nevertheless, several publications appeared in the late 1990s on solvated Be [10], K+ [11] and Cu + [12] presenting mainly structural results. 

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