Reorientation rotational

Loutfy and coworkers [29, 30] assumed a different mechanism of interaction between the molecular rotor molecule and the surrounding solvent. The basic assumption was a proportionality of the diffusion constant D of the rotor in a solvent and the rotational reorientation rate kOI. Deviations from the Debye-Stokes-Einstein hydrodynamic model were observed, and Loutfy and Arnold [29] found that the reorientation rate followed a behavior analogous to the Gierer-Wirtz model [31]. The Gierer-Wirtz model considers molecular free volume and leads to a power-law relationship between the reorientation rate and viscosity. The molecular free volume can be envisioned as the void space between the packed solvent molecules, and Doolittle found an empirical relationship between free volume and viscosity [32] (6),... 

In (8), the solvent-independent constants kr, kQnr, and Ax can be combined into a common dye-dependent constant C, which leads directly to (5). The radiative decay rate xr can be determined when rotational reorientation is almost completely inhibited, that is, by embedding the molecular rotor molecules in a glass-like polymer and performing time-resolved spectroscopy measurements at 77 K. In one study [33], the radiative decay rate was found to be kr = 2.78 x 108 s-1, which leads to the natural lifetime t0 = 3.6 ns. Two related studies where similar fluorophores were examined yielded values of t0 = 3.3 ns [25] and t0 = 3.6 ns [29]. It is likely that values between 3 and 4 ns for t0 are typical for molecular rotors. 

Moog RS, Ediger MD, Boxer SG, Fayer MD (1982) Viscosity dependence of the rotational reorientation of rhodamine B in mono- and polyalcohols. Picosecond transient grating experiments. J Phys Chem 86 4694-4700... 

We demonstrate that the spectral function of valence harmonic vibrations of a diatomic group that effects rotational reorientations is broadened by w. The vector of atom C displacements relative to the atom B (see Fig. A2.1) may be represented as x(t)e(t), where x(t) is the change in the length of the valence bond oriented at the time t along the unit vector e(/). Characteristic periods of valence vibrations are much shorter than periods of changes in unit vector orientations. As a consequence, the GF of the displacements defined by Eq. (4.2.1) can be expressed approximately as ... 

This effective dye relaxation time rp is the spontaneous fluorescence decay time shortened by stimulated emission which is more severe the higher the excitation and therefore the higher the population density w j. The dependence of fluorescence decay time on excitation intensity was shown in 34 35>. Thus, fluorescence decay times measured with high intensity laser excitation 3e>37> are often not the true molecular constants of the spontaneous emission rate which can only be measured under low excitation conditions. At the short time scale of modelocking the reorientation of the solvent cage after absorption has occurred plays a certain role 8 > as well as the rotational reorientation of the dye molecules 3M°)... 

The relaxation of the 13C nucleus is dominated by 13C— H dipolar interactions. For slow rotational reorientation (cocrc > 1) assuming a single correlation time, tc, the following equations for l3C spin-lattice and spin-spin relaxation times are valid ... 

The shape of the vibration-rotation bands in infrared absorption and Raman scattering experiments on diatomic molecules dissolved in a host fluid have been used to determine2,15 the autocorrelation functions unit vector pointing along the molecular axis and P2(x) is the Legendre polynomial of index 2. These correlation functions measure the rate of rotational reorientation of the molecule in the host fluid. The observed temperature- and density-dependence of these functions yields a great deal of information about reorientation in solids, liquids, and gases. These correlation functions have been successfully evaluated on the basis of molecular models.15... 

The NMR rotational reorientation time of liquid water at 25 °C is 2.5 10-12 sec and the dielectrid relaxation time of liquid water is 8 1(T12 sec173). 

In the case of rapid reactions of lanthanide complexes if Tie the electron spin relaxation time is short compared to the rotational reorientation time, rr, the electron—nuclear dipolar interaction will give rise to nuclear relaxation rates given by [27]... 

In an interesting study on the rotational reorientation of the 1-adamantyl cation in different superacid solutions, Kelly and Leslie confirmed that interactions between carbocation and the surrounding medium are also weak. 

An anisotropy of the rotational reorientation measured by NMR spectroscopy was found in SO2CIF solution indicating external stabilization of C+ whereas in SO2 no such anisotropy was observed. The anisotropy in SO2CIF solution was explained by weak electrostatic interactions between C+ and a diffuse anionic charge cloud rather than specific cation-solvent interactions. [103]... 

Dynamic fluorescence anisotropy is based on rotational reorientation of the excited dipole of a probe molecule, and its correlation time(s) should depend on local environments around the molecule. For a dye molecule in an isotropic medium, three-dimensional rotational reorientation of the excited dipole takes place freely [10]. At a water/oil interface, on the other hand, the out-of-plane motion of a probe molecule should be frozen when the dye is adsorbed on a sharp water/oil interface (i.e., two-dimensional in respect to the molecular size of a probe), while such a motion will be allowed for a relatively thick water/oil interface (i.e., three-dimensional) [11,12]. Thus, by observing rotational freedom of a dye molecule (i.e., excited dipole), one can discuss the thickness of a water/oil interface the correlation time(s) provides information about the chemi-cal/physical characteristics of the interface, including the dynamical behavioiu of the interfacial structure. Dynamic fluorescence anisotropy measurements are thus expected... 

Other spectroscopic methods have also been used to study the statics and dynamics of solvation shells of ions and molecules [351-354], In this respect, solvation dynamics refers to the solvent reorganization e.g. rotation, reorientation, and residence time of solvent molecules in the first solvation shell) in response to an abrupt change in the solute properties, e.g. by photoexcitation of the solute with ultra-short laser-light pulses. Provided that this excitation is accompanied by an electron transfer or a change in the dipole moment, then the dynamics of this process correspond to how quickly the solvent molecules rearrange around the instantaneously created charge or the new dipole. 

Redfield limit, and the values for the CH2 protons of his- N,N-diethyldithiocarbamato)iron(iii) iodide, Fe(dtc)2l, a compound for which Te r- When z, rotational reorientation dominates the nuclear relaxation and the Redfield theory can account for the experimental results. When Te Ti values do not increase with Bq as current theory predicts, and non-Redfield relaxation theory (33) has to be employed. By assuming that the spacings of the electron-nuclear spin energy levels are not dominated by Bq but depend on the value of the zero-field splitting parameter, the frequency dependence of the Tj values can be explained. Doddrell et al. (35) have examined the variable temperature and variable field nuclear spin-lattice relaxation times for the protons in Cu(acac)2 and Ru(acac)3. These complexes were chosen since, in the former complex, rotational reorientation appears to be the dominant time-dependent process (36) whereas in the latter complex other time-dependent effects, possibly dynamic Jahn-Teller effects, may be operative. Again current theory will account for the observed Ty values when rotational reorientation dominates the electron and nuclear spin relaxation processes but is inadequate in other situations. More recent studies (37) on the temperature dependence of Ty values of protons of metal acetylacetonate complexes have led to somewhat different conclusions. If rotational reorientation dominates the nuclear and/or electron spin relaxation processes, then a plot of ln( Ty ) against T should be linear with slope Er/R, where r is the activation energy for rotational reorientation. This was found to be the case for Cu, Cr, and Fe complexes with Er 9-2kJ mol" However, for V, Mn, and... 

Ru complexes much lower values of r were found, implying that a time-dependent process other than rotational reorientation is operating. Modulation of the ground state potential energy surface via a dynamic Jahn-Teller effect is suggested as the process controlling the electron spin relaxation in these compounds. 

We have studied the concentration dqtendence of the Co relaxation rate of [Co(acac)3] in acetonitrile, and concluded that the relaxaticxi is attributable to the rotational reorientation of the complex and the macroscopic viscosity of the solution is the dominant factor in this dependence. 1... 

Busse determined the Co relaxation rates of [Co(acac)3] in several solvents by using the dual spin probe technique. Busse and Abbott conclude that the relaxation rate for cobalt in such an ostoisibly symmetrical com dex can be explained by small djmamic distortion from ideal symmetry that fluctuate owing to rotational reorientation in QD, CE>3COCI>3, and diglyme. If the relaxation is through the rotational reorientation, the relaxation rate, l/Ti, is given by... 

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.

One of the central questions in the rheology of complex fluids is the molecular origin of mechanical propertie,s. Therefore, coupling of rheometry with techniques which are sensitive to molecular behaviour like molecular alignment, rotational reorientation, velocity distributions, and tramslational diffusion is required, A method which allow.s the detection of all these molecular characteristics is NMR imaging [Cal4J,... 

Figure 5. Geometrical considerations for a CH vector giving rise to T p(C) relaxation in a solid by restricted rotational reorientation.

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... 

Figure 8. Rotational reorientation of water molecular dipole direction for 1 = I (Equation 7) (a) "bulk (b) "nonpolar ...

Rotational reorientation of frans-stilbene in alkane solution at room temperature occurs in the 10 to 30-ps time domain [347]. Rare-gas complexes with trons-stilbene were studied by purely rotational coherence spectroscopy [51,364]. Moreover, the decay kinetics of excited trans-stil-bene-cyclodextrin complexes were examined [366], It is worth mentioning that great progress has also been made in high-resolution spectroscopy [52, 369-372], Resonance coherent Raman spectroscopy showed a large enhancement of the electronic hyperpolarizability of t with respect to ground state trons-stilbene [374]. Vibrational motions were observed with ps transient Raman spectroscopy [375]. 

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