Search results for "Torsional potential"
showing 4 items of 4 documents
Coarse Graining of Short Polythylene Chains for Studying Polymer Crystallization.
2015
We derive coarse-grained models of polyethylene in the melt state with the aim to study polymer crystallization. This requires a low level of coarse-graining: We use a mapping of two CH2 groups onto one bead. The coarse-grained beads are connected with harmonic springs, an optimized angular potential, and an optional torsional potential. Coarse-grained potentials are derived from detailed all-atom simulations, and an optimized form of the force field is then derived which achieves a good accuracy in reproducing the static properties of the chains. We address the question over which temperature range such models can be used, and in particular if the model is capable of reproducing the phase…
Molecular orbital study of conformational isomers and rotational barriers of methyl substituted hydroquinone cation radicals
1998
Abstract The torsional potential energy curve of the hydroxyl group of hydroquinone and tetramethyl-hydroquinone cation radicals were explored with various ab initio methods. The minimum and the torsional transition state geometries and energies were computed by using high accuracy density functional methods yielding the rotation barrier height and the energy difference between the cis- and trans-isomers. The obtained minimum energy geometry for the hydroquinone cation radical indicates that the CO bond has shortened when compared to the neutral species. We attribute this to the increased double-bond character of this bond. The energy minima were located for methyl-hydroquinone, 2,3-dimeth…
Geometric Structure and Torsional Potential of Biisothianaphthene. A Comparative DFT and ab Initio Study
1997
We present a study of the torsional potential of biisothianaphthene and compare it to that of bithiophene. The calculations are performed at the ab initio and semiempirical Hartree−Fock (HF), ab initio post-Hartree−Fock, and density functional theory (DFT) levels. Our study has two major aims: (i) on the physico-chemical side, to asses the optimal conformation of biisothianaphthene and evaluate the rotational barriers toward coplanar structures and (ii) on the methodological side, to asses the usefulness of DFT approaches. In contrast to previous estimates, the torsional potential of biisothianaphthene is found to differ markedly from that of bithiophene. For biisothianaphthene, strongly r…
Difficulties of density functional theory in predicting the torsional potential of 2,2?-bithiophene
1998
The internal rotation of 2,2′-bithiophene was investigated within the density functional theory (DFT) approach. Fully optimized DFT torsional potentials are compared with Moller–Plesset (MP2) results which predict a fourfold potential with s-cis- and s-trans-gauche minima. DFT calculations fail in describing the energetics of the internal rotation because they favor planar vs. perpendicular conformers. Gradient-corrected functionals provide torsional potentials where the gauche minima have almost vanished and the s-ciss-trans interconversion barriers are twice as high as the barriers obtained at the MP2 level. The use of local functionals augments the shortcomings of the DFT approach. The g…