Medicine - Infared Spectroscopy Flashcards
Middle IR range and use?
2500nm-5000nm
Used for characterisation
Far IR range and use
50,000nm-1,000,000nm
Used:
-determine functional groups
-study purity and degradation
IR Spectra y axis and x axis
Y axis: %Transmittance
X axis: Wavenumbers (cm-1)
What is a wavenumber?
Wavenumber = 1/wavelength
Units for wavelength: cm^-1
Frequency is equal to…
F=c/wavelength
Where c=the speed of light
Wavelength range most used in IR spectrum analysis
4000-600cm^-1
Describe the origin of IR bands
- IR radiation is absorbed and converted to molecular vibrations with higher amplitude
- energy corresponding to natural vibrational frequencies of bonds are absorbed
What is the wavelength range 780-2500nm used for?
Physical properties of compounds (role in drug formulation)
Why do broad bands of absorption appear on IR spectrum and not single lines?
Because each vibrational level is associated with several rotational levels
Does IR cause electronic transitions?
No. IR energy is to low to cause this.
Dipole moment is equal to what?
Dipole moment=q x r
- where q=charge
- and r=separation
What allows molecules to absorb IR?
If the centre of positive charge is different to the centre of negative charge, on vibration (e.g. Stretch), the separation changes, this the dipole moment changes and this allows the molecule to absorb IR
What causes higher intensity of absorption in IR Spectra?
Bigger the electronegativity difference,
Bigger the dipole moment therefore higher the intensity
Electronegativity: C
O-H
N-H
C-H
Bond wavenumber range
4000-2500
C(triple bond)N
C(triple bond)C
Bond wavenumber range
2500-2000
What range has not many bands?
2000-1800
C=O
Bond wavenumber range
1800-1650
C=N
C=C
Bond wavenumber range
1650-1550
C-O
C-N
C-C
Bond wavenumber range
1550-0
What are the 2 types of vibrations?
What are the vibrations within these?
1) Stretching - Symmetrical or asymmetrical
2) Bending - Scissoring, Rocking, Wagging, Twisting
Order of frequency (energy for wavenumber) at which vibrations occur?
Asymmetric>Symmetric>Bending
Are all vibrations IR active?
No! Need charge separation (dipole moment)
Only vibrations that result in a net change in polarity of a bond will give rise to IR bands, this is the only way the photon from IR radiation can transfer its energy to the molecule
What would result in a bigger dipole moment?
Bigger difference in electronegativity
I.e. O-H absorbs stronger than C-H