Extended Förster theory of donor-donor energy migration in bifluorophoric macromolecules. Part I. A new approach to quantitative analyses of the time-resolved fluorescence anisotropy

The applicability of an extended Förster theory (EFT) is explored with respect to donor-donor energy migration within a pair of identical fluorophores. The EFT accounts for reorienting motions and orientational restrictions of donor groups, which are attached to a macromolecule. Because EFT involves averaging over stochastic functions, it is inappropriate for the conventional methods used for analysing fluorescence depolarization experiments. For this reason approximations of the EFT were derived. To examine the validity of these different approximations, depolarization data were generated and re-analysed. To create the depolarization data, the EFT was used together with a Brownian dynamics simulation. Limitations of the approximate EFT are ascribed to the handling of secondarily excited donors (i.e. donors excited through energy migration). Finally on the basis of the EFT, a simulation-deconvolution method is presented which enables one to analyse the fluorescence anisotropy, without introducing any approximation.     

The ammonia dimer equilibrium dissociation energy: convergence to the basis set limit at the correlated level

The low-energy region of the intermolecular potential energy hypersurface (PES) of the ammonia dimer was studied at the level of second-order Moller-Plesset perturbation theory (MP2) using a very large basis set. Individual minima were located on the PES employing the counterpoise (CP) correction to account for the basis set superposition error (BSSE). Apart from these canonical MP2 calculations local MP2 (LMP2) calculations were performed. For the latter the BSSE at the correlated level is inherently absent by virtue of the local truncation of the virtual space. Results from canonical and local MP2 calculations are compared and the reliability of the LMP2 method for intermolecular complexes and clusters is discussed. The canonical MP2 calculations predicted five minimum structures, the four most stable ones lying energetically very close. For these four structures single point MP2 energy calculations with a further extended basis set (1024 functions for the ammonia dimer) were performed. The equilibrium dissociation energies so obtained are close to the one-particle basis set limit, as illustrated by a remaining BSSE of less than 0.2 kJ mol^?1. The geometry optimizations at the LMP2 level, using the three most stable canonical MP2 structures as initial geometries, all collapsed to a single minimum corresponding to an asymmetric structural arrangement. A canonical MP2 single point calculation, at that geometry, revealed that the LMP2 minimum structure is virtually as stable as the lowest minima on the canonical MP2 PES. Based on these calculations the global minimum of the ammonia dimer was assigned to a part of the PES represented by an asymmetric structure with an equilibrium dissociation energy of 13.5±0.3 kJ mol^?1     

A comparison of effective and polarizable intermolecular potentials in simulations: liquid water as a test case

A comparison of polarizable and effective intermolecular potentials has been carried out by employing simulated properties of liquid water at different temperatures. The effective potentials were obtained by adding a fixed fraction (~80%) of the induced dipole moments of the polarizable potential to the permanent dipole moment of the water molecule. The fraction was fitted to reproduce one structural (the height of the first peak of the oxygen-oxygen radial distribution function) and one dynamic (the self-diffusion coefficient) liquid property predicted by the polarizable potential. The two properties were well reproduced simultaneously by the effective potential at 273 K and 303 K, but less accurately at 373 K. The effective dipole moments were 2.79, 2.75, and 2.68 D at the three respective temperatures. In order to examine the effective potentials further, other liquid properties have been considered, and we found that the molecular rotational relaxation times and the hydrogen bonding properties are reproduced well by the effective potentials, whereas the velocity autocorrelation function, the pressure, the dielectric constant, and the Debye relaxation time are reproduced less accurately.