Beschrijving

Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A semi-rigid molecule is a molecule which has a potential energy surface with a well-defined minimum corresponding to a stable structure of the molecule. The only (quantum mechanical) motions that a semi-rigid molecule makes are (small) internal vibrations around its equilibrium geometry and overall translations and rotations.A molecule consists of atoms held together by chemical bonding forces. The potential, derived from these forces, is a function of the Cartesian nuclear coordinates R1, …, RN. These coordinates are expressed with respect to a frame attached to the molecule. The potential function is known as force field or potential energy surface written as V(R1, …, RN). Often a more accurate representation of the potential V is obtained by the use of internal curvilinear coordinates, so-called valence coordinates. We mention bond stretch, valence angle bending, out-of-plane-rotation angles, and dihedral(torsion) angles. Although the curvilinear internal coordinates can give a good description of the molecular potential, it is difficult to express the kinetic energy of nuclear vibrations in these coordinates.