1 Flashcards
The Born-Oppenheimer Approximation
Electrons are much lighter than nuclei and therefore much faster. This allows the nuclei motion to be separated from the electron motion. Nuclei motion is effectively 0.
Potential Energy Surface (PES)
A PES is the relationship - mathematical or graphical - between the energy of molecules (or a collection of molecules) and its geometry.
Molecular Mechanics (MM)
Key Principle:
Express the energy of a molecule as a function of its resistance towards bond stretching, bending and atom crowding, and then minimise this energy-function to get the minimum-energy geometry.
MM Advantages
ery Fast Calculations
Can be accurate with relevant parametrisation.
Undemanding Hardware Requirements
Can be applied to large molecules eg drug design
MM Disadvantages
Doesn’t treat electrons therefore can’t desrcibe bond formation / breaking or where electronic effects dominate (no reaction pathway)
Transferring parameters to another molecule is unlikely to perform well.
Semi-Empirical Methods
Use parameters derived from experimental data to simplify computation.
Ab Initio Methods
No experimental parameters, based purely on laws of QM.
Restricted and Unrestricted Calculations
Restricted: Each spatial orbital holds 2 electrons
Unrestricted: Separate orbitals for α and β electrons.
Single Point Energy Calculations (SPE)
Obtains basic info
Starting point for geometry optimisation
Minima and Transition States on the PES
A global minima occurs at the optimised geometry.
A maxima in one direction and a minima in all others is a first-order saddle point and corresponds to a T.S
Frequency Calculations
Give:
Force Constants, Vibrational Frequencies, Intensities, Thermochemical Analysis, Nature of the Stationary Point
Nature of the Stationary Point
Minimum: All vibrational frequencies are real.
First-order saddle point: There is one imaginary frequency, all others are real.
nth-order saddle point: there are n imaginary frequencies.
