Week 8
MOLECULAR SPECTROSCOPY
Part 1
-Fourier Transform Infra-Red (FT-IR)
by Tan Ching Mun (16002662)
Molecular Spectroscopy
The study of the interaction of electromagnetic radiation (energy) with matter and can be used to obtain information about the identity and structure of substances.
Molecular spectroscopy based on ultraviolet, visible and infrared radiation is widely used for the identification and determination of many inorganic, organic, and biochemical species.
Infrared absorption spectroscopy -- good for determining the structure of both organic and inorganic compounds.
Molecular UV/Vis absorption spectroscopy -- primarily for quantitative analysis & more extensively applied in chemical and clinical lab.
The study of the interaction of electromagnetic radiation (energy) with matter and can be used to obtain information about the identity and structure of substances.
Molecular spectroscopy based on ultraviolet, visible and infrared radiation is widely used for the identification and determination of many inorganic, organic, and biochemical species.
Infrared absorption spectroscopy -- good for determining the structure of both organic and inorganic compounds.
Molecular UV/Vis absorption spectroscopy -- primarily for quantitative analysis & more extensively applied in chemical and clinical lab.
Introduction to Molecular Spectroscopy
Fourier Transform Infra-Red (FT-IR)
A premier technique for qualitative analysis.
In IR region, absorption of radiation give below information:
Rotational ( very low E, microwave/ far-IR region)
Vibrational (higher E, near-IR region)
Electronic (higher energy, visible and UV regions)
Simple correlations of group vibrations to regions of infrared absorption
Factors affecting wavenumber
A premier technique for qualitative analysis.
In IR region, absorption of radiation give below information:
- Identity of compounds
- Presence/absence of functional groups (C=O, C=C, C-H, C≡O, O-H)
- Structure of molecules
Rotational ( very low E, microwave/ far-IR region)
Vibrational (higher E, near-IR region)
Electronic (higher energy, visible and UV regions)
Infrared Region
- Near IR (4000-14000/cm)
-for routine quantification of certain species
-H2O, CO2, sulfur, HCs, amine, nitrogen
- Mid IR (670-4000/cm)
-most widely used region
-determining the structure of organic and biochemical species
- Far IR (<650/cm)
-determining the structure of inorganic and metal-organic species
-rarely used ( looks like a fingerprint)
Simple correlations of group vibrations to regions of infrared absorption
wavenumber: triple bond > double bond > single bond
Abbreviation Table of Group Frequencies for Organic Functional Groups
Abbreviation Table of Group Frequencies for Organic Functional Groups
Factors affecting wavenumber
Example (Identifying Spectra)
Example 1
Example 1
Example 2
Example 3
Example 4
Example 5
Example 6
Reflection
Before attending the lecture, I felt very confused just by looking at the lecture notes. I am worried as I do not understand most of the information given based on the FTIR graph. However, after the lecture, I am able to determine the functional group and the types of bonds present in the molecule. The explanation give by the lecturer is clear and the sample questions prepared is very helpful in training the students.
Tutorial 5 - Molecular Spectroscopy
3. The logarithm of the molar absorptivity for acetone in ethanol is 2.75 at 366 nm. Calculate the range of acetone concentrations that can be used if the absorbance is to be greater than 0.100 and less than 2.000 with a1.50-cm cell.
4. What minimum requirement is needed to obtain reproducible results with a single-beam spectrophotometer?
The requirement includes wavelength accuracy, wavelength reproducibility, stray light, resolution, photometric accuracy, photometric reproducibility, noise, baseline flatness, system stability, and linearity of response.
5. A photometer with a linear response to radiation gave a reading of 690 mV with a blank in the light path and 169 mV when the blank was replaced by an absorbing solution. Calculate:
(a) The transmittance and absorbance of the absorbing solution.
(b) The transmittance to be expected if the light path through the original solution is doubled.
7. The spectrum below has a molecular formula of C3H6O. Provide a molecular structure of this component.
1. What experimental variables must be controlled to assure reproducible absorbance data?
Several experimental conditions were studied to optimize in order to obtain maximum and reproducible absorbance results. These factors include: reagent concentration, buffer (type, pH, and volume) used, diluting solvent, reaction temperature and reaction time.
2. The molar absorptivity for the complex formed between bismuth (III) and thiourea is 9.32 × 103 L cm-1 mol-1 at 470 nm. Calculate the range of permissible concentrations for the complex if the absorbance is to be no less than 0.10 nor greater than 0.90 when the measurements are made in 1.00-cm cells.
Several experimental conditions were studied to optimize in order to obtain maximum and reproducible absorbance results. These factors include: reagent concentration, buffer (type, pH, and volume) used, diluting solvent, reaction temperature and reaction time.
2. The molar absorptivity for the complex formed between bismuth (III) and thiourea is 9.32 × 103 L cm-1 mol-1 at 470 nm. Calculate the range of permissible concentrations for the complex if the absorbance is to be no less than 0.10 nor greater than 0.90 when the measurements are made in 1.00-cm cells.
3. The logarithm of the molar absorptivity for acetone in ethanol is 2.75 at 366 nm. Calculate the range of acetone concentrations that can be used if the absorbance is to be greater than 0.100 and less than 2.000 with a1.50-cm cell.
4. What minimum requirement is needed to obtain reproducible results with a single-beam spectrophotometer?
The requirement includes wavelength accuracy, wavelength reproducibility, stray light, resolution, photometric accuracy, photometric reproducibility, noise, baseline flatness, system stability, and linearity of response.
5. A photometer with a linear response to radiation gave a reading of 690 mV with a blank in the light path and 169 mV when the blank was replaced by an absorbing solution. Calculate:
(a) The transmittance and absorbance of the absorbing solution.
(b) The transmittance to be expected if the light path through the original solution is doubled.
6. A portable photometer with a linear response to radiation registered 75.5 mA with a blank solution in the light path. Replacement of the blank with an absorbing solution yielded a response of 23.7 mA. Calculate:
(a) The absorbance of the sample solution.
(b) The transmittance to be expected for a solution that has twice the concentration of the sample solution.
(a) The absorbance of the sample solution.
(b) The transmittance to be expected for a solution that has twice the concentration of the sample solution.
7. The spectrum below has a molecular formula of C3H6O. Provide a molecular structure of this component.
8. Select which the components match the IR spectra below:
