Polymorphism
is known to show a significant effect on the crystal
packing structures and physical properties of molecular complexes.
On the basis of our previous works on the spin-crossover (SCO) complex
[Fe(tpa)(NCS)2] (tpa = tris(2-pyridylmethyl)amine), we
have prepared the analogous complex [Fe(tpa)(NCSe)2] using
a NCSe– coligand and obtained a series of crystalline
polymorphs, namely SeI–SeIV, respectively,
by controlled synthetic processes. Single-crystal X-ray diffraction
studies reveal that these polymorphs show different packing modes
and different intermolecular interactions. Various SCO behaviors are
thus observed. Moreover, an obvious coligand effect on SCO behaviors
is found by a comparison of the present study and the reported complex
[Fe(tpa)(NCS)2]. The NCSe– complexes
show a critical shift of T
1/2 values to
a higher temperature range and a more gradual transition process in
comparison to those for the isostructural complexes using NCS–, indicating a stronger ligand-field strength and a
weaker cooperativity for the NCSe– coligand.
In the past few decades, numerical models were proposed to study car-following behavior, in order to resolve the controversy about the fundamental relation between speed and spacing under various traffic conditions. Jiang et al (2014 Plos one 9 e94351, 2015 Transp. Res. B 80 338–54) carried out a series of field experiments, revealing that the vehicle speed and its spacing span a 2D region in the speed-spacing plane and standard deviations of the velocities of cars fit to a concave curve. To describe this instability of car-following behavior, we proposed a new NaSch type model, in which a speed-dependent variable is introduced. The flow rate, spatiotemporal patterns and the velocity standard deviation can be in basic agreement with the experiment. Moreover, transitions of the traffic phases are reproduced and the fundamental relationship between flow rate, speed and density is presented.
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