Solvent reorientation time

A fully realistic picture of solvation would recognize that there is a distribution of solvent relaxation times (for several reasons, in particular because a second dispersion is often observable in the macroscopic dielectric loss spectra [353-355], because the friction constant for various types or modes of solute motion may be quite different, and because there is a fast electronic component to the solvent response along with the slower components due to vibration and reorientation of solvent molecules) and a distribution of solute electronic relaxation times (in the orbital picture, we recognize different lowest excitation energies for different orbitals). Nevertheless we can elucidate the essential physical issues by considering the three time scales Xp, xs, and Xelec-... 

When the temperature is lowered and/or the viscosity of the solution is increased by using glycerol-water mixtures as solvent, the reorientational correlation time increases. Since the reorientational time is the correlation time for nuclear relaxation, the effects on the NMRD profile (Pig. 27) are (i) higher relaxivity values at low frequencies (ii) a shift toward lower... 

SDS micelles [188-190]. These results may be a consequence of a lack of template-induced orientation or of the orientational forces being too weak to overcome the orientational preferences between an excited and a ground state molecule. It is certainly the case in all of the micellar examples cited that the solvent relaxation times should allow molecules to reorient themselves at the interface (should they so choose) on timescales which are comparable to those necessary for an excited molecule to form its photoproducts. 

Here td is the so-called Debye dielectric relaxation time. One could view td as a phenomenological time constant which applies to dielectric relaxation measurements, or alternatively for simple causes, involving dielectric relaxation of weakly interacting dipoles, tD is related to the reorientation time constant of the solvent dipole in the laboratory frame. 

Dielectric friction is the measure of the dynamic interaction of a charged or dipolar solute molecule with the surrounding polar solvent molecules. This concept has been applied, by Hynes et al. [339] and others [486], to solvent- and time-dependent fluorescence shifts resulting from the electronic absorption by a solute in polar solvents. If the solvent molecules are strongly coupled to the charge distribution in ground- and excited-state molecules, the relatively slow solvent reorientation can lead to an observable time evolution of the fluorescence spectrum in the nano- to picosecond range. This time-dependent fluorescence (TDF) has been theoretically analysed in terms of dynamic... 

An ultrafast intermolecular electron transfer (ET) from electron donating solvent to an excited dye molecule was found. A temperature-dependent non-exponential time dependence was observed in aniline, and a temperature-independent single exponential process for Nile blue (160 fs) and oxazine 1 (260 fs) was observed in N,N-Smethylaniline. The solvation times of solvent anilines were obtained by dynamic Stokes shift measurements. The rate of ET in some systems was observed to be much greater than the solvation time of anilines. The dynamic behavior was simulated by the 2-dimen ional potential energy surface for reaction, taking into account of the effects of both solvent reorientation and nuclear motion of reactants. 

The rotational reorientation times of the sample in several solvents at room temperature were measured by picosecond time-resolved fluorescence and absorption depolarization spectroscopy. Details of our experimental setups were described elsewhere. For the time-correlated single photon counting measurement of which the response time is a ut 40 ps, the sample solution was excited with a second harmonics of a femtosecond Ti sapphire laser (370 nm) and the fluorescence polarized parallel and perpendicular to the direction of the excitation pulse polarization as well as the magic angle one were monitored. The second harmonics of the rhodamine-640 dye laser (313 nm 10 ps FWHM) was used to raesisure the polarized transient absorption spectra. The synthesis of the sample is given elsewhere. All the solvents of spectro-grade were used without further purification. 

The measured rotational reorientation times, initial value of the fluorescence anisotropy, and fluorescence lifetimes in several solvents are listed in Table 1. The initial value of anisotropy is very close to 0.4, which suggests that the directions of the transition dipole of the absorption and the fluorescence are the same and the depolarization not due to the reorientation of the molecule can be ignored. 

Figure 3 shows the plot of the rotational reorientation time versus the solvent viscosity. The straight lines show the reorientation times calculated as... 

Table I. Excited-state rotational reorientation times, fluorescence lifetimes (ps), and initial values of the fluorescence anisotropy of I in several solvents at room temperature.

Solvent reorientation and isomerization of trans-stilbene in alkane solutions has been studied by ps time scale anisotropic absorption and polarization239 Coupling of solute and solvent decreases as the size of the solvent molecules increases. The applicability of currently favoured models for the activated barrier crossing in the photoisomerization of stilbene is discussed, A method for measuring quantum yields in the photoisomerization of trans-stilbene gives high accuracy without use of a chemical actinometer . Evidence has been found for dynamic solvent effects on the photoisomerization of 4,4 -dimethoxystilbene in which the effects of temperature and hydrostatic pressure were made in n-alkane and n-alkyl alcohol. A ps laser time-resolved study fits frequency dependent solvent shifts but gives results inconsistent with the free volume model. Photophysical and theoretical studies of trans and 9-... 

On the other hand is comparable to solvent structural relaxation times and is shorter than solvent reorientation and conformational relaxation times. In this case the effective friction for motion in the vicinity of Rj will be less than the zero frequency friction which describes diffusion for R > R. ... 

Label Effect. In order to assess at least partially the effect of the label on the chain dynamics, we also performed measurements on dilute solutions of 9,10-dimethyl anthracene. The reorientation time for the free dye in cyclohexane was - 10 psec, 50 times faster than the time scale for motion of the labeled chain in cyclohexane. Hence we conclude that the observed correlation functions are not dominated by the hydrodynamic interaction of the chromophore itself with the solvent, but can be attributed to the polymer chain motions. 

Water has an essential role in living systems and is ultimately involved in the structure and function of biological polymers such as proteins. However, in this contribution we sh tll focus primarily not on what the water does for the blopolymer but rather on the effects that the biopolymer has on the water that Interacts with it. Of Interest are alterations in the structural, energetic, and dynamic properties of the water molecules. Studies of the rotational mobility of water molecules at protein surfaces have been interpreted by dividing the solvent molecules into three groups U). The most rapidly reorienting group has a characteristic rotational reorientation time (t ) of not more than about... 

Bauer et al. (1974) have studied reorientational relaxation times of a wide variety of molecules in organic solvents and find that the single particle rotational reorientation time about a given molecular axis is of the form... 

The light-scattering experiments were performed as a function of concentration at a constant viscosity of. 56 centipoise and a constant temperature of 22 °C. The solvents used (mixtures of carbon tetrachloride and isopentane) contributed very little to the depolarized scattering at high chloroform concentrations. These solvents, however, contributed approximately 10% to the intensity of the 40% chloroform solution and approximately 20% to the intensity of the 20% chloroform solution. The contribution of the solvent was subtracted for these chloroform solutions. The measured light-scattering reorientation times for the chloroform solutions are shown in Fig. 12.3.1. 

CftFe + CgH and + 1,3, S-MeaCgHg. Both total-intensity and line-width measurements were made and no evidence of long-lived complexes was obtained. The results could be rationalized by assuming a strong unlike intermolecular interaction that manifested itself both in the static and in the dynamic correlation functions and in the individual molecule reorientation times. The single particle reorientation times of each anisotropic component in both binary mixtures were found to be slowed considerably when compared with their values in neutral solvents such as CCI4. 

---