Spectroscopy Recall

de Broglie relationship

λ=h/p h= planck constant

speed of light frequeny and wavelength

Co = νλ.

energy of photon

E=hv=hco/λ =hcv~ v~ =wavenumber

Relationship between energy and quantity of material

E=kBT (molecular)

E=RT (molar)

Boltzmann distribution

N1/N0 = e^(- ΔE /k BT)

Beer Lambert Law

A=-log10 (It/Io)=εcl

Chromophore

colour bringer e.g. a carbonyl group that undergoes electronic transitions between bonding and antibonding orbitals. They usually have a carbonyl of conjugated pi system.

Conjugation

alternating pi bonds, the more conjugation the greater the absorbtion maximum difference between HOMO and LUMO decreased.

Frequency of vibration

ve=1/2pi sqrt (k/m) m=μ for 2 connected particles

Potential energy

V(r) = ½ k (r-re)2

Molecular potential enegry curve

lowest is not at minimum only certain values of energy allowed

Energy of molecular vibration

E(v) = hνe(v +½) v=vibrational quantum number

What molecules are IR active

if there is a molecular dipole e.g. must be diatomic, electron density not equally shared, assymetric

Vibrational modes

n atoms in molecule 3n-6 vibrational modes (3n-5) if linear

O-H vibrational frequency

3500

C-H vibrational frequency

3000

C≡N vibrational frequency

2250

C=C aromatic vibrational frequency

1500 and 1600

C=O vibrational frequency

1750

C=C vibrational frequency

1630

NMR theory

• Nucleus with magnetic dipole put into applied magnetic field

• Electrons in nucleus shield from applied magnetic field

• Causes electrons to move => current

• Current generates an induced magnetic field

• Effective magnetif cield = applies x (1xS) S = shielding constant

Chemical shift also affected by aromatic rings and pi bonding —> causes ring current which has a shielding effecr hence absorbs at higher frequencies

Standards in NMR

TMS has lots of electron density, inert, hard to measure meagnetic field in NMR machine so measure frequency relative to TMS instead

R=OH chemical shift

9.0-9.8

aromatics chemical shift

6.2-8.0

amide chemical shift

5-8

C=CH2 chemical shift

4.6-5.4

HOCH alcohol chemical shift

3.2-4.1

ester RCO2CH chemical shift

3.3-5.0

halide Cl/Br-CH chemical shift

2.9-4.0

R=COCH ester chemical shift

2-3.7

Coupling

If magnetic field of HA is affected by that of HB it will be split into 2 equal lines and vice versa. This depends on wether HA sees Hb in +1/2 or -1/2 spinn state this leads to spin spin coupling (peak heights follow pascals triangle)