Rotating fast kinetics

The excitation (absorption of a photon) and the red-shifted emission are two distinct events that are separated by a time window ranging from units to hundreds of nanoseconds depending on the fluorophore and the host system. This enables monitoring fast kinetics, because a number of molecular processes proceed on this timescale in small volumes delimited by distances comparable with the range of intermolecular interactions and affect the time-dependent emission characteristics. They include translational and rotational diffusion of the fluorophore, reorientation of molecules in the solvation shell, segmental dynamics of flexible macromolecules, and nonradiative excitation energy transfer, etc. 

Under conditions where the rotation about the C-N bond of dimethylformamide is slow relative to the NMR time scale, the two methyl resonances will be separate singlets. Conversely, if the rotation is made to be very fast, the two methyl groups will be chemically equivalent. Their resonance will then appear as a sharp singlet. In between these extremes, kinetic information can be extracted from the line shapes. In most systems the parameter that is changed to go between these limits is the temperature. In some systems, pH or pressure has the same effect. 

The RHSE has the same limitation as the rotating disk that it cannot be used to study very fast electrochemical reactions. Since the evaluation of kinetic data with a RHSE requires a potential sweep to gradually change the reaction rate from the state of charge-transfer control to the state of mass transport control, the reaction rate constant thus determined can never exceed the rate of mass transfer to the electrode surface. An upper limit can be estimated by using Eq. (44). If one uses a typical Schmidt number of Sc 1000, a diffusivity D 10 5 cm/s, a nominal hemisphere radius a 0.3 cm, and a practically achievable rotational speed of 10000 rpm (Re 104), the mass transfer coefficient in laminar flow may be estimated to be ... 

This value represents the upper limit of a first order reaction rate constant, k, which may be determined by the RHSE. This limit is approximately one order of magnitude smaller that of a rotating electrode. One way to extend the upper limit is to combine the RHSE with an AC electrochemical technique, such as the AC impedance and faradaic rectification metods. Since the AC current distribution is uniform on a RHSE, accurate kinetic data may be obtained for the fast electrochemical reactions with a RHSE. 

As with solution experiments, flash photolysis in the gas phase has produced evidence for the existence of intermediates but no information about their structure. In principle gas phase IR spectra can provide much more information, although the small rotational B value of gaseous carbonyls and low lying vibrational excited states preclude the observation of rotational fine structure. As described in Section II, time-resolved IR experiments in the gas phase do not suffer from problems of solvent absorption, but they do require very fast detection systems. This requirement arises because gas-kinetic reactions in the gas phase are usually one... 

To learn that the rotated ring-disc electrode (RRDE) is one of the most powerful analytical tools for following the kinetics of fast homogeneous reactions. 

The mass transport rate coefficient, kd, for a RDE at the maximum practical rotation speed of 10000 per min"1 is approximately 2 x 10-2 cms-1 [28], which sets a limit of about 10 3 cms 1 for the electrode reaction kinetics. For the study of very fast electrode processes, such as some outer sphere redox reactions on noble metal electrodes under stationary conditions, higher mass transport rates in the solution adjacent to the electrode must be employed. 

Recent studies describe the use of cyclic voltammetry in conjunction with controlled-potential coulometry to study the oxidative reaction mechanisms of benzofuran derivatives [115] and bamipine hydrochloride [116]. The use of fast-scan cyclic voltammetry and linear sweep voltammetry to study the reduction kinetic and thermodynamic parameters of cefazolin and cefmetazole has also been described [117]. Determinations of vitamins have been studied with voltammetric techniques, such as differential pulse voltammetry for vitamin D3 with a rotating glassy carbon electrode [118,119], and cyclic voltammetry and square-wave adsorptive stripping voltammetry for vitamin K3 (menadione) [120]. 

---