Computation of Molecular Properties

Question 1

Ionization potential (IP)
Electron affinity (EA)

Answer 1

IP = M –> M+ + e-
IP = |E(M+) - E(M)|
EA = M + e- –> M-
EA = |E(M-) - E(M)|

E(e-) = 0

EA is harder to do

Question 2

What is needed to predict wavenumber (rotational spectra)

Answer 2

Geometry
(opt run on Gaussian)

Question 3

What sources of error do polyatomic wavenumber (peak location) predictions have?

Answer 3

Approximation used for energy level expression
Approximation for A,B,C used

Question 4

What is ‘freq’ run for in Gaussian?

Answer 4

Needed to calculate K(q) and u(q) and second derivative calculation for K’s

Question 5

What does normal mode determination involve?

Answer 5

Hessian matrix, H
Then diagonalized to find the normal modes

Question 6

Accuracy for wavenumber predictions

Answer 6

HF (+10%)
MP2 (+5%)
BLYP (+/- 1%)
- lucky cancellation of errors
BP86 (+1%)
B3LYP (+3%)

Question 7

What are errors in wavenumber due to?

Answer 7

Approximation energy level expression and approximations in K(q) and u(q)

Question 8

Chemical shift

Answer 8

The shift in a nucleus Larmor frequency of precession in a magnetic field external to the molecule, due to local molecular magnetic field

Question 9

What do absolute shifts require

Answer 9

Large basis sets

Question 10

Accuracy for absolute shift values

Answer 10

HF (17 ppm)
MP2 (4 ppm)
BLYP (15 ppm)
B3LYP (16 ppm)

Almost too large for H nmr but good for C nmr

Question 11

Spin-spin coupling constants …

Answer 11

Can be done, but slow
GGA DFT is okay