8,170 research outputs found
A structural systematic study of three isomers of difluoro-N-(4-pyridyl)benzamide
The isomers 2,3-, (I), 2,4-, (II), and 2,5-difluoro-N-(4-pyridyl)benzamide, (III), all with formula CââHâFâNâO, all exhibit intramolecular C-H...O=C and N-H...F contacts [both with S(6) motifs]. In (I), intermolecular N-H...O=C interactions form one-dimensional chains along [010] [N...O = 3.0181 (16) Ă
], with weaker C-H...N interactions linking the chains into sheets parallel to the [001] plane, further linked into pairs via C-H...F contacts about inversion centres; a three-dimensional herring-bone network forms via C-H...Ï(py) (py is pyridyl) interactions. In (II), weak aromatic C-H...N(py) interactions form one-dimensional zigzag chains along [001]; no other interactions with H...N/O/F < 2.50 Ă
are present, apart from long N/C-H...O=C and C-H...F contacts. In (III), N-H...N(py) interactions form one-dimensional zigzag chains [as C(6) chains] along [010] augmented by a myriad of weak C-H...Ï(arene) and O=C...O=C interactions and C-H...O/N/F contacts. Compound (III) is isomorphous with the parent N-(4-pyridyl)benzamide [Noveron, Lah, Del Sesto, Arif, Miller & Stang (2002). J. Am. Chem. Soc. 124, 6613-6625] and the three 2/3/4-fluoro-N-(4-pyridyl)benzamides [Donnelly, Gallagher & Lough (2008). Acta Cryst. C64, o335-o340]. The study expands our series of fluoro(pyridyl)benzamides and augments our understanding of the competition between strong hydrogen-bond formation and weaker influences on crystal packing
A structural systematic study of four isomers of difluoro-N-(3-pyridyl)benzamide
The four isomers 2,4-, (I), 2,5-, (II), 3,4-, (III), and 3,5-difluoro-N-(3-pyridyl)benzamide, (IV), all with formula C12H8F2N2O, display molecular similarity, with interplanar angles between the C6/C5N rings ranging from 2.94 (11)° in (IV) to 4.48 (18)° in (I), although the amide group is twisted from either plane by 18.0 (2)-27.3 (3)°. Compounds (I) and (II) are isostructural but are not isomorphous. Intermolecular N-H...O=C interactions form one-dimensional C(4) chains along [010]. The only other significant interaction is C-H...F. The pyridyl (py) N atom does not participate in hydrogen bonding; the closest H...Npy contact is 2.71 Ă
in (I) and 2.69 Ă
in (II). Packing of pairs of one-dimensional chains in a herring-bone fashion occurs via [pi]-stacking interactions. Compounds (III) and (IV) are essentially isomorphous (their a and b unit-cell lengths differ by 9%, due mainly to 3,4-F2 and 3,5-F2 substitution patterns in the arene ring) and are quasi-isostructural. In (III), benzene rotational disorder is present, with the meta F atom occupying both 3- and 5-F positions with site occupancies of 0.809 (4) and 0.191 (4), respectively. The N-H...Npy intermolecular interactions dominate as C(5) chains in tandem with C-H...Npy interactions. C-H...O=C interactions form R22(8) rings about inversion centres, and there are [pi]-[pi] stacks about inversion centres, all combining to form a three-dimensional network. By contrast, (IV) has no strong hydrogen bonds; the N-H...Npy interaction is 0.3 Ă
longer than in (III). The carbonyl O atom participates only in weak interactions and is surrounded in a square-pyramidal contact geometry with two intramolecular and three intermolecular C-H...O=C interactions. Compounds (III) and (IV) are interesting examples of two isomers with similar unit-cell parameters and gross packing but which display quite different intermolecular interactions at the primary level due to subtle packing differences at the atom/group/ring level arising from differences in the peripheral ring-substitution patterns
Fractal Cosmology in an Open Universe
The clustering of galaxies is well characterized by fractal properties, with
the presence of an eventual cross-over to homogeneity still a matter of
considerable debate. In this letter we discuss the cosmological implications of
a fractal distribution of matter, with a possible cross-over to homogeneity at
an undetermined scale R_{homo}. Contrary to what is generally assumed, we show
that, even when R_{homo} -> \infty, this possibility can be treated
consistently within the framework of the expanding universe solutions of
Friedmann. The fractal is a perturbation to an open cosmology in which the
leading homogeneous component is the cosmic background radiation (CBR). This
cosmology, inspired by the observed galaxy distributions, provides a simple
explanation for the recent data which indicate the absence of deceleration in
the expansion (q_o \approx 0). Correspondingly the `age problem' is also
resolved. Further we show that the model can be extended back from the
curvature dominated arbitrarily deep into the radiation dominated era, and we
discuss qualitatively the modifications to the physics of the anisotropy of the
CBR, nucleosynthesis and structure formation.Comment: 7 pages, no figures, to appear in Europhysics Letter
Integrating Dynamics and Wear Modelling to Predict Railway Wheel Profile Evolution
The aim of the work described was to predict wheel
profile evolution by integrating multi-body dynamics
simulations of a wheelset with a wear model.
The wear modelling approach is based on a wear
index commonly used in rail wear predictions. This
assumes wear is proportional to TÎł, where T is tractive
force and Îł is slip at the wheel/rail interface. Twin disc
testing of rail and wheel materials was carried out to
generate wear coefficients for use in the model.
The modelling code is interfaced with
ADAMS/Rail, which produces multi-body dynamics
simulations of a railway wheelset and contact conditions
at the wheel/rail interface. Simplified theory of rolling
contact is used to discretise the contact patches
produced by ADAMS/Rail and calculate traction and
slip within each.
The wear model combines the simplified theory of
rolling contact, ADAMS/Rail output and the wear
coefficients to predict the wear and hence the change of
wheel profile for given track layouts
On the geometry of closed G2-structure
We give an answer to a question posed recently by R.Bryant, namely we show
that a compact 7-dimensional manifold equipped with a G2-structure with closed
fundamental form is Einstein if and only if the Riemannian holonomy of the
induced metric is contained in G2. This could be considered to be a G2 analogue
of the Goldberg conjecture in almost Kahler geometry. The result was
generalized by R.L.Bryant to closed G2-structures with too tightly pinched
Ricci tensor. We extend it in another direction proving that a compact
G2-manifold with closed fundamental form and divergence-free Weyl tensor is a
G2-manifold with parallel fundamental form. We introduce a second symmetric
Ricci-type tensor and show that Einstein conditions applied to the two Ricci
tensors on a closed G2-structure again imply that the induced metric has
holonomy group contained in G2.Comment: 14 pages, the Einstein condition in the assumptions of the Main
theorem is generalized to the assumption that the Weyl tensor is
divergence-free, clarity improved, typos correcte
The role of initial conditions in the ageing of the long-range spherical model
The kinetics of the long-range spherical model evolving from various initial
states is studied. In particular, the large-time auto-correlation and -response
functions are obtained, for classes of long-range correlated initial states,
and for magnetized initial states. The ageing exponents can depend on certain
qualitative features of initial states. We explicitly find the conditions for
the system to cross over from ageing classes that depend on initial conditions
to those that do not.Comment: 15 pages; corrected some typo
Discrete Matter, Far Fields, and Dark Matter
We show that in cosmology the gravitational action of the far away matter has
quite relevant effects, if retardation of the forces and discreteness of matter
(with its spatial correlation) are taken into account. The expansion rate is
found to be determined by the density of the far away matter, i.e., by the
density of matter at remote times. This leads to the introduction of an
effective density, which has to be five times larger than the present one, if
the present expansion rate is to be accounted for. The force per unit mass on a
test particle is found to be of the order of 0.2cH_0. The corresponding
contribution to the virial of the forces for a cluster of galaxies is also
discussed, and it is shown that it fits the observations if a decorrelation
property of the forces at two separated points is assumed. So it appears that
the gravitational effects of the far away matter may have the same order of
magnitude as the corresponding local effects of dark matter.Comment: 16 pages, 1 figure. LaTex documen
Universality of power law correlations in gravitational clustering
We present an analysis of different sets of gravitational N-body simulations,
all describing the dynamics of discrete particles with a small initial velocity
dispersion. They encompass very different initial particle configurations,
different numerical algorithms for the computation of the force, with or
without the space expansion of cosmological models. Despite these differences
we find in all cases that the non-linear clustering which results is
essentially the same, with a well-defined simple power-law behaviour in the
two-point correlations in the range from a few times the lower cut-off in the
gravitational force to the scale at which fluctuations are of order one. We
argue, presenting quantitative evidence, that this apparently universal
behaviour can be understood by the domination of the small scale contribution
to the gravitational force, coming initially from nearest neighbor particles.Comment: 7 pages, latex, 3 postscript figures. Revised version to be published
in Europhysics Letters. Contains additional analysis showing more directly
the central role of nearest neighbour interactions in the development of
power-law correlation
1-d gravity in infinite point distributions
The dynamics of infinite, asymptotically uniform, distributions of
self-gravitating particles in one spatial dimension provides a simple toy model
for the analogous three dimensional problem. We focus here on a limitation of
such models as treated so far in the literature: the force, as it has been
specified, is well defined in infinite point distributions only if there is a
centre of symmetry (i.e. the definition requires explicitly the breaking of
statistical translational invariance). The problem arises because naive
background subtraction (due to expansion, or by "Jeans' swindle" for the static
case), applied as in three dimensions, leaves an unregulated contribution to
the force due to surface mass fluctuations. Following a discussion by
Kiessling, we show that the problem may be resolved by defining the force in
infinite point distributions as the limit of an exponentially screened pair
interaction. We show that this prescription gives a well defined (finite) force
acting on particles in a class of perturbed infinite lattices, which are the
point processes relevant to cosmological N-body simulations. For identical
particles the dynamics of the simplest toy model is equivalent to that of an
infinite set of points with inverted harmonic oscillator potentials which
bounce elastically when they collide. We discuss previous results in the
literature, and present new results for the specific case of this simplest
(static) model starting from "shuffled lattice" initial conditions. These show
qualitative properties (notably its "self-similarity") of the evolution very
similar to those in the analogous simulations in three dimensions, which in
turn resemble those in the expanding universe.Comment: 20 pages, 8 figures, small changes (section II shortened, added
discussion in section IV), matches final version to appear in PR
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