5,957 research outputs found
Dynamical Insights into the Decomposition of 1,2-Dioxetane
Chemiluminescence in 1,2-dioxetane occurs through a thermally activated
decomposition reaction into two formaldehyde molecules. Both ground-state and
nonadiabatic dynamics (including singlet excited states) of the decomposition
reaction have been simulated, starting from the first O-O bond-breaking
transition structure. The ground-state dissociation occurs between t = 30 fs
and t = 140 fs. The so-called entropic trap leads to frustrated dissociations,
postponing the decomposition reaction. Specific geometrical conditions are
necessary for the trajectories to escape from the entropic trap and for
dissociation to be possible. The singlet excited states participate as well in
the trapping of the molecule: dissociation including the nonadiabatic
transitions to singlet excited states now occurs from t = 30 fs to t = 250 fs
and later. Specific regions of the seam of the S0/S1 conical intersections that
would "retain" the molecule for longer on the excited state have been
identified
Standard Error of Empirical Bayes Estimate in NONMEM® VI.
The pharmacokinetics/pharmacodynamics analysis software NONMEM® output provides model parameter estimates and associated standard errors. However, the standard error of empirical Bayes estimates of inter-subject variability is not available. A simple and direct method for estimating standard error of the empirical Bayes estimates of inter-subject variability using the NONMEM® VI internal matrix POSTV is developed and applied to several pharmacokinetic models using intensively or sparsely sampled data for demonstration and to evaluate performance. The computed standard error is in general similar to the results from other post-processing methods and the degree of difference, if any, depends on the employed estimation options
Calculations of Hubbard U from first-principles
The Hubbard \emph{U} of the \emph{3d} transition metal series as well as
SrVO, YTiO, Ce and Gd has been estimated using a recently proposed
scheme based on the random-phase approximation. The values obtained are
generally in good accord with the values often used in model calculations but
for some cases the estimated values are somewhat smaller than those used in the
literature. We have also calculated the frequency-dependent \emph{U} for some
of the materials. The strong frequency dependence of \emph{U} in some of the
cases considered in this paper suggests that the static value of \emph{U} may
not be the most appropriate one to use in model calculations. We have also made
comparison with the constrained LDA method and found some discrepancies in a
number of cases. We emphasize that our scheme and the constrained LDA method
theoretically ought to give similar results and the discrepancies may be
attributed to technical difficulties in performing calculations based on
currently implemented constrained LDA schemes.Comment: 24 pages, 13 figures; Submitted to Phys. Rev.
How Do Methyl Groups Enhance the Triplet Chemiexcitation Yield of Dioxetane?
Chemiluminescence is the emission of light as a result of a nonadiabatic
chemical reaction. The present work is concerned with understanding the yield
of chemiluminescence, in particular how it dramatically increases upon
methylation of 1,2-dioxetane. Both ground-state and nonadiabatic dynamics
(including singlet excited states) of the decomposition reaction of various
methyl-substituted dioxetanes have been simulated. Methyl-substitution leads to
a significant increase in the dissociation time scale. The rotation around the
O-C-C-O dihedral angle is slowed; thus, the molecular system stays longer in
the "entropic trap" region. A simple kinetic model is proposed to explain how
this leads to a higher chemiluminescence yield. These results have important
implications for the design of efficient chemiluminescent systems in medical,
environmental, and industrial applications
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