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
What bonds, if any, give no IR peaks. Why?
Symmetrical bonds (N2, O2) have no dipole moment and thus give no IR peaks at all. Symmetrical stretches (CH2, CO) give no peaks either, BUT asymmetrical stretches and bending modes that give rise to dipole moment change gives peaks.
What 3 core rules do vibrations obey?
- Bonds vibrate all the time so there is no fixed bond length
- frequency at which a given bond vibrates is constant
- vibrations are discrete so certain bonds only absorb certain frequencies
- in the mid IR region you can only go from V0–>V1 and V1–>V2, not from V0 to V2 in a single step directly.
Is IR spectrum reliable if done properly? Why?
Yes because different bonds vibrate at different natural frequency and also this vibration is dependent on different environments in different molecules.
So no 2 molecules will have exactly the same IR spectrum - it is like a fingerprint or barcode
Rate of molecular vibrations
High energy end
Low energy end
High energy end (4000cm^-1): vibration every 8 femto seconds
Low energy end (666cm^-1): vibration every 50 femto seconds
NB: 1 femto second = 10^-15 of 1 second
C=O present at 1650-1550
When is it likely to be a carboxylic acid?
If there’s a broad O-H too (3400-2400)
C=O present at 1650-1550
When is it likely to be an amide?
If N-H also there (3500)
C=O present at 1650-1550
When is it likely to be an ester?
If there’s also a C-O bond (1300-1000)
C=O present at 1650-1550
When is it likely to be an anhydride?
RCO-O-COR
If two close C=O togehter (1810 and 1760)
C=O present at 1650-1550
When is it likely to be an aldehyde?
If there’s a weak C-H (2850, 2750)
C=O absent
When is it likely to be an alcohol?
If OH is there (3600-3300)
C=O absent
When is it likely to be an ether?
R-O-R
If there is a C-O bond/ether linkage (1300-1000)
C=O absent
When is it likely to be an amine?
Medium intensity ~3500
Nitro groups 2 strong: R-N-O2
1600-1500 and 1390-1300
After initial deductions, what else can you do to analyse the IR spectra?
Use other resources, such as tables or correlation charts for specific frequencies and shapes (to identify functional groups)
Uses of IR Spectra?
- BP
- Identify drugs by matching spectrum
- Obtain structural information about functional groups on a drug molecule (functional group analysis)
- Detect polymorphs of drugs (Near-IR)
Can you assign a complete structure with just IR?
Not easily.
Possible to ascertain which functional groups are present by comparing data with tables that give experimentally observed frequency range for typical vibrations of different functional groups
Describe how a reference is used and the importance of calibration
-care taken to calibrate the machine so it has similar resolution and wavelength accuracy as that used to obtain a reference (spectrum from an official record)
What does BP say about the reference drug/substance?
BP states the sample preparation and operation conditions
A reference that complies with BP conditions should be used for sample and reference
What are the two types of instrumentation used for IR?
- Dispersive Instrument
- Fourier Transform
Describe the instrumentation used for a dispersive instrument
Source–>Cell–>Monochromator–>Detector–>Output
The frequencies of IR radiation absorbed are determined by passing through a rotating prism to focus one frequency at a time onto the detector
Why is a reference used?
It ensures peaks due to water/carbon dioxide etc. Can be cancelled out.
Describe Fourier Transform Instrumentation
- Modern instrumentation
- Similar to dispersive instrument but monochromator is replaced by an interferometer
- Interferometer produces a pattern of frequencies that are possible to resolve via Fourier Transform
What is Fourier Transform?
What is the main advantage of using it?
It’s a mathematical transformation
The main advantage of using it is high data acquisition speed (1sec vs minutes)
Thus many scans per minute averaged to improve the S/N ratio
(signal/noise ratio)
To identify a drug via IR by just using the spectra, what is needed?
Positions and relative sizes of peaks in spectra of substance to be examined and the reference sample/spectrum must be concordant
Describe the peaks when N-H stretching occurs
Primary amine: RNH2 - broad with 2 sharp spikes
Secondary amine R2NH - broad with 1 sharp spike
Describe the peaks that occur due to O-H stretching and hydrogen bonding
OH: rounded peak:
- Sharp peak ~3600 if no H bonding
- Broad peak 3400 if intramolecular H bonding
- Very broad peak if intermolecular H bonding
How do you prepare a liquid sample for IR?
- Fill a cell that can be held in position of IR beam
- Prepare a liquid film between NaCl, KCl or KBr discs
- If sample is solid then grind to form a paste in liquid paraffin
- place a drop of sample in between 2 circular plates to produce a thin capillary film
- place the plates into a holder ready for analysis
Why is KBr used in sample preparation?
Used in sample preparation because KBr has no IR absorption in the region 4000-650cm^-1
How do you prepare a solid sample for IR analysis?
Disk preparation
- Use a powdered sample (~2mg)
- with dry KBr (~200mg)
- grind finely
- compress under a high pressure (30tons/square inch) using a spanner and nuts and bolts
- this forms a disk which can be placed in the IR machine
Describe how a specifically designed apparatus can help with solid sample preparation
This apparatus named a disk die can do similar pressing as with spanner and nuts and bolts, but under vacuum to avoid moisture
What is the main thing you want of a cell used for IR?
IR inactive in the interested wavelength range
What are suitable cell materials for non-aqueous samples?
KBr NaCl CaF2 CsI (These materials are not resistant to water)
Aqueous sample use what kind of cell materials for sample preparation?
AgCl
ZnS
Ge
Diamond
Applications of near IR (NIR)
- Identification of polymorphic drugs
- determination of moisture
- blend uniformity
- particle size
- quality control
NIR range
Low energy or high wavelength range
700-2500nm
Downside of NIR?
Useful to determine physical properties of formulation, but it requires a skilled person with computing knowledge to extract information from a spectrum obtained
How to tell whether an IR spectrum was made using a dispersive instrument or a fourier transform instrument?
You can’t tell from the spectrum.
