Applications II: Bond Orders, Isodesmic Reactions, IRC Scans Lecture CompChem 6 Chemistry 347 Hope College

Bond Orders No unique definition of assigning electrons to bonds Density matrix analysis: Mulliken density analysis allows crude calculation of bond orders Atoms In Molecules (AIM) theory: electrons are assigned to atoms based on the gradient of the electron density Natural Bonding Orbitals (NBO): single and double bonds, lone pairs

Isodesmic Reaction No change in number of each bond type between reactants and products Ex: CO2 + CH4 ® 2 H2CO (2 C=O and 4 C-H bonds on each side) Good results at low theory levels because of cancellation of errors Can use isodesmic reaction results plus other experimental data to calculate energetics of non-isodesmic reactions

Intrinsic Reaction Coordinate Scans Follows a reaction coordinate “downhill” (shedding kinetic energy or “momentum”) Starting from a transition state, forward and reverse IRC calculations go forward and backward along the reaction coordinate Calculation stops before equilibrium is reached; default is 6 steps

Verifying Transition States Obtain starting point from literature, symmetry, or “morph” (QST2) calculation; minimize gradient (Opt=TS) Calculate vibrational spectrum (Freq); look for single negative (imaginary) frequency Perform forward and reverse IRC calculations; look for reactants and products

Software Packages EHT, YAEHMOP, ICON-EDiT Tinker, MOSCITO Dalton MolPro ….