Hydrogen geometry

The final geometry was obtained when the calculated forces acting on the atoms and stress on the supercell became smaller than the threshold values. To evaluate the relative expansion of the lattice induced by the interstitial hydrogen, geometry optimizations of the pure metals were also employed. 

Two other examples will sufhce. Methane physisorbs on NaCl(lOO) and an early study showed that the symmetrical, IR-inactive v mode could now be observed [97]. In more recent work, polarized FTIR rehection spectroscopy was used to determine that on being adsorbed, the three-fold degeneracies of the vs and v modes were partially removed [98]. This hnding allowed consideration of possible adsorbate-adsorbent geometries one was that of a tripod with three of the methane hydrogens on the surface. The systems were at between 4 and 40 K so that the equilibrium pressure was very low, about 10 atm. 

Gavezzotti A and Filippini G 1994 Geometry of the intermolecular XH.. . Y (X,Y = N,0) hydrogen bond and the calibration of empirical hydrogen-bond potentials J. Phys. Chem. 98 4831... 

The origin of a torsional barrier can be studied best in simple cases like ethane. Here, rotation about the central carbon-carbon bond results in three staggered and three eclipsed stationary points on the potential energy surface, at least when symmetry considerations are not taken into account. Quantum mechanically, the barrier of rotation is explained by anti-bonding interactions between the hydrogens attached to different carbon atoms. These interactions are small when the conformation of ethane is staggered, and reach a maximum value when the molecule approaches an eclipsed geometry. 

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