Properties calculated by molecular orbital methods:Energy (enthalpy of formation)Dipole momentOrbital energy levels (HOMO, LUMO, others)Electron distribution (electron density)Electrostatic potentialVibrational frequencies (IR spectra) Semi-Empirical Molecular Orbital TheoryUses simplifications of the Schrdinger equation to estimate the energy of a system (molecule) as a function of the geometry and electronic distribution.The simplifications require empirically derived (not theoretical) parameters (or fudge factors) to allow calculated values to agree with observed values. Molecular Mechanics ForcefieldsMM2, MM3 (Allinger)MMX (Gilbert, in PCModel)MM+ (HyperChems version of MM2)MMFF (Merck Pharm.)Amber (Kollman)OPLS (Jorgensen)BIO+ (Karplus, part of CHARMm)(others) Enthalpy of Formation (sometimes)Dipole Moment Geometry (bond lengths, bond angles, dihedral angles) of lowest energy conformation. Properties calculated by MM:Steric or Total energy = sum of various artificial energy components, depending on the program.not a real measurable energy. Molecular Mechanics.Similar calculations for other deviations from normal geometry (bond angles, dihedral angles)Based on simple, empirically derived relationships between energy and bond angles, dihedral angles, and distancesIgnores electrons and effect of p systems!Very simple, yet gives quite reasonable, though limited results, all things considered. Molecular Mechanics Employs classical (Newtonian) physicsAssumes Hookes Law forces between atoms (like a spring between two masses) Estretch = ks (l - lo)2 graph: C-C C=O Relative Computation CostSemi-empirical and ab initio molecular orbital methods.cpu time scales as the third or fourth power of the number of atomic orbitals (basis functions) in the basis set.Semi-empirical calculations on ~MW 300 compound take a few minutes on a pc, seconds on a parallel computer (cluster). Relative Computational CostMolecular mechanics.cpu time scales as square of the number of atoms.Calculations can be performed on a compound of ~MW 300 in a minute on a pc, or in a few seconds on a parallel computer.This means that larger molecules (even large peptides) and be modeled by MM methods. Molecular dynamicssolves Newtons laws of motion for atoms on a potential energy surface temperature dependent can locate minimum energy conformations.QSARused to predict properties of new structures or predict structures that should have certain properties (e.g., drugs) Levels of Calculation.Density functional theorymore efficient and often more accurate than ab initio calc. Levels of CalculationMolecular mechanics.quick, simple accuracy depends on parameterization.Semi-empirical molecular orbital putationally more demanding, but possible for moderate sized molecules, and generally more accurate.Ab initio molecular orbital methods.much more demanding computationally, generally more accurate. What is Computational Chemistry?Use of computers to aid chemical inquiry, including, but not limited to:Molecular Mechanics (Classical Newtonian Physics)Semi-Empirical Molecular Orbital TheoryAb Initio Molecular Orbital TheoryDensity Functional TheoryMolecular DynamicsQuantitative Structure-Activity RelationshipsGraphical Representation of Structures/Properties An Introduction to Computational Chemistråomputational ChemistråHM 425/525 Fall 2011Dr.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |