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adjusted model, we reconsider simple 1,2-dyotropic reactions with the introduction of a
concept based on the intramolecular dynamics of a tetrahedron (van ’t Hoff modeling). In fact the
dyotropic reactions are strongly related to conversions originated from neighbouring
group participation or anchimeric assistance, defined as the interaction of a
center with a lone pair of electrons in an atom and the electrons present in aδor π bond. The researchful 1,2-dyotropic reactions, based on the
1,2-interchange of halogens, methyl and hydrogen taking place in a concerted
fashion, are in competition with the two-step reaction in which the
neighbouring group participation or anchimeric assistance comes to full
expression by ionic dissociation of the other exchangeable (halogen) atom. As
to be expected there is an essential difference between halogen or methyl
exchange regarding the number of electrons participating in the transition state.
This aspect becomes evident in the geometries of the corresponding transition
state geometries. In this paper we refer to ab
initio MO calculations and VB considerations. We consider the 1,2-halogen
exchange as a combination of two SN2 reactions each containing four
electrons. The van ’t Hoff dynamics appears a useful model in order to illustrate the
computations in a straightforward manner.
Spatially homogeneous and anisotropic Cosmological
models play a significant role in the description of the early stages of evolution
of the universe. The problem of the cosmological constant is still unsettled.
The authors recently considered time dependent G and L with Bianchi type–I Cosmological model .We considered in this paper
homogeneous Bianchi type -I space-time with variable G and L containing matter in the form of a perfect fluid assuming the cosmological
term proportional to R-2 (where R is scale factor). Initially the
model has a point type singularity, gravitational constant G (t) is decreasing and cosmological
constant L is infinite at this time. When time
increases L decreases. Unlike
in some earlier works we have neither assumed equation of state nor particular
form of G. The model does not approach isotropy, if ‘t’ is small .The model is quasi-isotropic for large value of ‘t’.
In this paper homogeneous Bianchi type -I space-time
with variable G and L containing matter in the form of a perfect fluid assuming the cosmological term
proportional to H2 (where H is Hubble Parameter). Initially the model has a
point type singularity, gravitational constant G (t) is decreasing and
cosmological constant L is
infinite at this time. When time increases,L decrease. The model does not
approach isotropy, if it is small. The model is quasi-isotropic for large value of it.
We have studied Locally Rotationally Symmetric (LRS) Bianchi type-I
cosmological model filled with anisotropic fluid in general theory of
relativity. The solutions of the field equations are obtained by using special
form of deceleration parameter which gives early deceleration and late time
accelerating cosmological model. The geometrical and physical aspect of the
model is also studied.