Spectroscopy (RD)

Absorption Spectroscopy

1 Spectroscopy (introduction) : monitors changes in energy states of a molecule

Electromagnetic Radiation properties :

- particle (momentum); photon – particle of discrete amount of energy (quantum)

- wave (diffraction)

Energy states: nuclear spin state, vibration of a bond, electronic energy levels (orbitals)…

Quantisation of energy: only certain values are possible

hν∆E

ground state

excitedstate

for a stronger bond (larger k), ν increases

CC bonds: C-C (1000cm-1), C=C (1600cm -1) and C≡C (2200cm -1)

CH bonds: C-C-H (2900cm -1), C=C-H (3100cm -1) and C≡C-H (3300cm -1)

for heavier atoms attached (larger µ value), ν decreases

C-H (3000cm-1), C-C (1000cm-1), C-Cl (800cm-1), C-Br (550cm-1), C-I (about 500cm-1)

m1 m2

force constant – k(stiffness)

µπ=ν k21frequency of the absorption

21

21

mmmm+

=µreduced mass

3.2 Hooke’s law

Fingerprint regionfor confirming a structure; peaks are from complex deformations of the molecule; characteristic for its symmetry

Functional group regioncharacteristic of specific

kinds of bonds –to identify whether a

specific functional group is present

strong, usuallybroad due

to H-bonding

as the hybridization of C changes sp3-sp2-sp,

the frequency increases

strongcharacteristic

weak (lowpolarity)

strong“finger”

Shifts: proximal groups, conjugation, ring strain

Information: frequency, intensity, shape

3.3 Example

Spectroscopy (RD)

volts

data points (time)

4 FTIR (FourierTransform Infrared) Spectroscopy ’80slike IR – increase in vibrational energy level

FTIR spectrometer: based on Michelson interferometer: interference of IR waves

50%

50%

moving mirror

sample

all frequencies from the source are measured at one time

the sample absorbs all the different wavelengths characteristic of its spectrum, and this subtracts specific wavelengths from the interferogram

Fourier transform:

Interferogram: constructive/destructive interference

Background should be measured separately

Advantages:

quick – all spectral elements measured at a time (Fellget advantage); but – danger of spreading of noise from the source over the whole spectrum

better for kinetic work

the frequency scale is known accurately (Connes advantage)

( ) dtetfF ti∫∞

∞−

νπ−=ν 2)()(

( )∫∞

∞−

νπ−νπ= dteFtf ti 2)(2)(

Spectroscopy (RD)

5.1 Basic principles

radio frequency: 60 - 800 MHz for hydrogen

in clinical research : 15 - 80 MHz

random orientation

spin aligned or opposed

hν

ground state

excitedstate

spin flip H0

low energy

high energy

as H increases

⇒ the separation of energies of the spin states increases

⇒ the frequency required to cause the spin-flip (resonance) increases

Spectroscopy (RD)

5 Nuclear Magnetic Resonance (NMR) : absorption of energy associated with transitions of nuclei between adjacent nuclear magnetic energy levels

Spin-Spin Coupling (J coupling)

Equivalent nuclei - with the same chemical environment or chemical shift

Nuclei which are close to one another exert an influence on each other's effective magnetic field. This effect shows up for nonequivalent nuclei:

Splitting in the energy levels

Configuration Peak Ratios

A 1

AB 1:1

AB2 1:2:1

AB3 1:3:3:1

AB4 1:4:6:4:1

AB5 1:5:10:10:5:1

AB6 1:6:15:20:15:6:1

Pascal’s triangle(x+1)n

5.2 NMR spectrometer

sample tube

radio frequencyinput

radio frequencyoutput

spectrum:

excitation via radio frequency pulsation

Continuous Wave experiments

applied radio frequency vs absorption

Fourier transformation

Spectroscopy (RD)