Molecular Symmetry - Physical Chemistry - Lecture Notes, Study notes of Physical Chemistry

Download Molecular Symmetry - Physical Chemistry - Lecture Notes and more Study notes Physical Chemistry in PDF only on Docsity! Molecular Symmetry A set of nuclear coordinates defines a particular symmetry for a molecule which is given by its symmetry operations. These act to exchange equivalent atoms. The symmetry operations present in a given molecule are elements of a group. There are only five types of symmetry operations possible: E the do nothing operator - present in all groups reflection through a plane i inversion through a center point Ci rotation about an axis Si improper rotation about an axis Water as an example symmetry operations: E C2 xz yz Need to carefully define x, y, and z coordinates. There is a convention which can be followed: o z is defined to coincide with the principle axis of rotation o x is defined as the out-of-plane axis Docsity.com A Gaussian calculation will reorient the molecule in this standard orientation even if you do not initially provide it as such. Results must be understood in terms of this definition of coordinates. GaussView can quickly show the Cartesian axes. Now let's work out the symmetry operations present in each of these: Ethane Eclipsed Ethane Staggered Allene The Group of Symmetry operations define a point group for a molecule. Point groups have properties that are contained in a character table. Use a flow diagram to identify the point group of the following molecules. Then verify by examining the character table for that point group. eclipsed ethane nitrogen monoxide staggered ethane hydrogen peroxide (cis) allene hydrogen peroxide (trans) difluoro-bromomethane hydrogen peroxide (gauche) methylene chloride phosphorus pentafluoride Docsity.com