4103FSBMOL - Lecture 2 - Introduction to Spectroscopy. Flashcards
Electromagnetic Readation/ Spectrum, UV/Visible Spectroscopy and Infrared Spectroscopy.
What is the definition of Spectroscopy?
The interaction between light and matter as a function of a wavelength (λ).
What is the definition of a Spectrum?
Plot (graph) of the response as a function of the wavelength. (x-axis = wavenumber, y-axis = % absorption).
What is the definition of Spectrometry?
The measurement of the responses.
What is Electromagnetic Radiation (EM)?
It is radiation which contains bundles of energy known as photons. Photons are really unstable, move very fast and don’t have a mass. They can be in the form of waves or particles and as a wave, electromagnetic energy is determined as a wavelength, wavenumber or frequency.
What is Planck’s Equation?
E = h v.
What does ‘E’ stand for in E = h v?
Energy.
What does ‘h’ stand for in E = h v?
Planck’s Constant.
What does ‘v’ stand for in E = h v?
Frequency.
What version of Planck’s Equation can you use if you don’t have a frequency?
E = hc / λ.
What does ‘E’ stand for in E = hc / λ?
Energy.
What does ‘h’ stand for in E = hc / λ?
Planck’s Constant.
What does ‘c’ stand for in E = hc / λ?
Speed of light.
What does ‘λ’ stand for in E = hc / λ?
Wavelength.
What units is Wavelength measured in?
cm-1.
What is the name for the top part of a wave?
Peak.
What is the name for the bottom bit of a wave?
Trough.
What is the name for the distance between 2 peaks of a wave?
Wavelength.
What is the name for the distance between the peak and the trough?
Amplitude.
What happens to the energy if the wavelength increases?
It also increases.
What equation can you use to be the frequency from speed of light and wavelength?
v = c/λ.
v = Frequency.
c = Speed of Light.
λ = Wavelength.
How do you describe the wavelength?
The distance travelled over a complete wave cycle.
How do you describe the Wavenumber?
The number of waves per centimetre (cm).
How do you describe the Frequency?
The number of wavelengths which pass through a certain point every second.
What type of radiation is consisted of in the Electromagnetic Spectrum?
Polychromatic Radiation (multiple colours).
This makes it less dangerous.
What colours make up the Visible Spectrum?
- Ultra Violet.
- Violet.
- Indigo.
- Blue.
- Green.
- Yellow.
- Orange.
- Red.
- Infrared.
What wavelength region can Ultraviolet Light be found?
100 - 400nm.
What wavelength region can Violet Light be found?
400 - 420nm.
What wavelength region can Indigo Light be found?
420 - 440nm.
What wavelength region can Blue Light be found?
440 - 490nm.
What wavelength region can Green Light be found?
490 - 570nm.
What wavelength region can Yellow Light be found?
570 - 585nm.
What wavelength region can Orange Light be found?
585 - 620nm.
What wavelength region can Red Light be found?
620 - 780nm.
What wavelength region can Infrared Light be found?
780 - 1000nm (1mm).
What different types of Spectroscopy are there?
- Absorption (e.g. infrared).
- Emission (e.g. spectrofluorometry).
- Scattering/ Raman (amount of light a substance scatters).
What are the different types of Common Spectroscopic Techniques?
- Flame Spectroscopy.
- Fluorescence Spectroscopy.
- Infrared Spectroscopy.
- Plasma Emission Spectroscopy.
- Raman Spectroscopy.
- UV/Vis Spectroscopy.
- X-ray Spectroscopy.
What is UV/Visible Spectroscopy?
The spectroscopy of photons in which absorption happens at a single wavelength or over a range of different wavelengths (in the range 160-780 nm).
What is UV/Visible Sepctroscopy able to detect?
Functional groups, Impurities, Chromophores and also drugs containing chromophores.
What Law does a UV/Vis Spectrophotometer use to measure Absorbance and Concentration?
Beer-Lambert Law.
What does the Beer-Lambert Law state?
Absorbance is directly proportional to the Concentration.
What is the important feature you have to remeber with the Beer-Lambert Law?
It only works for DILUTE Solutions!
What is the equation for the Beer-Lambert Law?
A = εcl.
What does the ‘A’ stand for in A = εcl?
What are the units of it?
Absorbance.
No units.
What does the ‘ε’ stand for in A = εcl?
What are the units of it?
Molar Absoption Coefficient.
M-1cm-1.
What does the ‘c’ stand for in A = εcl?
What are the units of it?
Molar Concentration.
(M) - Molar.
What does the ‘l’ stand for in A = εcl?
What are the units of it?
Optical Path Length.
Centimetres (cm).
What other version of Beer-Lamberts Law can be stated involving logs?
A = -log(l) / l0.
l = intensity of light after the sample.
l0 = intensity of incident light on the sample.
What is the definition of Absorbance?
The measurement of changes of the frequency of incident light as it passes through a sample. It occurs as a result of changes of vibrational and rotational levels.
Vibration happens at ______ energy.
1. high.
2. low.
High.
Rotation happens at ______ energy.
1. high.
2. low.
Low.
What factors does Vibrational Energy depend on?
- Arrangement of atoms.
- Masses of atoms.
- Strengths if bonds.
- Bond distances.
Why do substances absorb energy?
When molecules are exposed to light they get excited from the ground to the excited state (via absorption - movement from ground to excited state).
What equation can be used to represent the Electronic Transition (Energy difference in the energy gap)?
E1 - E0 = hv.
h = Planck’s Constant.
v = frequency.
What equation can be used to describe the total energy in the system?
Etotal = Eelectronic + Evibrational + Erotational.
Why can’t saturated hydrocarbons be determined using the near UV region?
Because they are saturated, they only contain sigma bonds. Sigma bonds are stronger than pi bonds and require more radiant energy to break the bonds. Because of this, a shorter length of radiation (far UV) is needed.
Non-bonding electrons (n) - such as Lone Pair electrons - are ______ to excite.
- easy.
1. hard.
Easy.
Do pi bonds require more/less energy to break? Why?
They require less energy to break compared to sigma bonds and so need a longer wavelength (near UV region) to break.
What can we use to represent a sigma bond?
σ – σ*.
What can we use to represent a pi bond?
π – π*.
What are the 4 types of Bonding Transition?
σ – σ, n – σ, n – π* and π – π*.
What does σ – σ* represent?
Give an example.
σ bonding in the Ground State to σ* anti-bonding in the Excited State. It requires a large amount of Energy.
Methane - Maximum absorbance is at 125nm (lower = more energy)
What does n – σ* represent?
Give an example.
Non-bonding in the Ground State to σ* anti-bonding in the Excited State. It requires less Energy than saturated compounds (σ – σ) with unshared electrons.*
Methyl Chloride at 173nm - mainly compounds below 200nm.
In saturated alkyl halides, Energy decreases as the size of halogen increases.
What does n – π* represent?
Give an example.
Non-bonding in the Ground State to π* anti-bonding in the Excited State. There is prescence of unsaturated functional groups (double and triple bonds).
Carbonyl compounds with double bonds. (Ketone, Carboxyllic Acid etc.)
Minimum Energy and absorption band above 300 nm.
What does π – π* represent?
Give an example.
π bonding in the Ground State to π* anti-bonding in the Excited State. There is prescence of unsaturated functional groups (double and triple bonds). Least Energy Required.
Compounds with Multiple Bonds - Alkenes, alkynes, carbonyls, aromatics.
How does a UV/Vis Spectrophotometer work to give a reading?
A Sample of light is put into a Monochromator which then it passes through the sample and hits a PMT (Photomultiplier Detector) the other side. This converts a signal into a graph which is then amplified and gives a readout on the screen.
What does a Monochromator do?
It filters radiation out of light and turns it from Polychromatic Light to Monochromatic Light.
Why do you need to do a blank for a UV/Vis Machine?
To remove any noise from the readings. A ratio is used to work out the calculations.
What is Infrared Spectroscopy?
It is the most common type of absorption spectroscopy which helps you to identify the composition and functional groups in samples. It handles the Infrared Region of the Spectrum (from 4000-400cm-1).
How do you calculate the wavenumber?
Wavenumber = 1/ wavelength.
What Law is Harmonic Potential Based off of?
Hooke’s Law.
What equation can be used for Vibrational Frequency?
v = (1/2π)(√k/m).
What does ‘v’ stand for in v = (1/2π)(√k/m)?
Vibrational Frequency.
What does ‘k’ stand for in v = (1/2π)(√k/m)?
Bond Stiffness (Force Constant).
What does ‘m’ stand for in v = (1/2π)(√k/m)?
Mass (reduced).
What 2 things is Absorption due to?
- Stretching.
- Bending.
The Absorption of radiation by a sample is due to changes in the vibrational energy.
What different types of Stretching are there?
What do each of them mean?
- Symmetric Stretching (same bond length either side).
- Asymmetric Stretching (different bond length either side).
Atoms remain in the same bond axis. The distance between 2 atoms increase/ decrease.
What different types of Bending are there?
What do each of them mean?
- Scissoring - A normal mode of vibration for polyatomic molecules.
- Rocking - A normal mode of vibration for polyatomic molecules.
- Wagging - Similar to making a “V” sign with your fingers and bending them back and forth.
- Twisting - Similar to atoms walking on a treadmill.
Changes the bonding angle. Position of atoms change with respect to the original bond axis.
What does ATR stand for?
Attenuated Total Refelectance.
What is Attenuated Total Refelectance?
It includes multiple reflections and involves light being reflected internally by a transmittance medium.
What are IR Sources?
They are are inert solids that heat electrically to 1500 – 2200 K. These sources glow when heated.
What types of IR Sources are there?
- Nemest Glower - rod of rare earth oxide (lanthanide) with platinum leads.
- Globar - Silicon Carbide rod with water cooled contacts.
- Incandescent Wire - tightly wound wire heated electrically.
- CO2 Lasers.
- Mercury Arc.
- Tungsten Lamps.
How do you run Solids for IR?
They are run in solvents and solid films. The Mull and Pressed Pellets Techniques are used.
How do you run Liquids for IR?
A drop of liquid is squeezed between 2 NaCl plates that are transparent in the 4000-625cm-1 region.
How do you run Samples for ATR?
- Minimum Sample needed.
- No Preparation required.
What is a Monochromator made up of?
A Reflective Grating and a Glass Prism which isn’t suitable as it absorbs IR Radiation.
What different types of Detector are there for IR?
- Thermal Transducer.
- Pyroelectric Transducer.
- Photoconducting Transducer.
What is a Thermal Transducer Detector made of?
For IR.
They are made of a bimetal junction with a temperature dependant potential.
Not suitable for FTIR.
What is a Pyroelectric Transducer Detector made of?
Crystalline wafers of Triglycine sulfate (TGS) with strong temperature dependant polarisation.
Fast and Suitable for FTIR.
What is a Photoconducting Transducer Detector made of?
Semiconducting Material such as Lead Sulphide, Mercurcy/Cadmium Telluride or indium antimonide is used. It is deposited on a glass surface and sealed in an envelope to protect the material.
Fast response but requires cooling by liquid nitrogen.
How do you work out the number of types of vibration for Linear Molecules?
3n-5.
n = number of atoms.
How do you work out the number of types of vibration for Non-Linear Molecules?
3n-6.
n = number of atoms.
Why can observed vibrations be less than predicted?
- Symmetry (no change in dipole).
- Energies of vibration are identical.
- Absorption intensity is too low.
- Frequency beyond range of instruments.
What are the 4 primary regions of the IR Spectrum?
- Single Bonds (Fingerprint Region) - made of C-C, C-N and C-O bonds. 600-1600cm-1nm.
- Double Bonds - made of C=O, C=N, C=C bonds. 1600-2000cm-1nm.
- Triple Bonds - made of C≡C and C≡N bonds. 2000-2700cm-1nm.
- Bonds to H - made of O-H, N-H, C-H bonds. 2700-4000cm-1nm.
What are the most important IR regions?
- 600-1600 cm-1: Fingerprint Region.
- 1680-1750 cm-1: C=O stretches feature very strongly in IR spectra.
- 2700-3100 cm-1: different types of C-H stretching vibrations.
- 3200-3700 cm-1: various types of O-H and N-H stretching vibrations.
How does an IR spectra show a Strong Detection?
A tall peak where the transmittance is between 0-35%.
How does an IR spectra show a Medium Detection?
Mid-height peak with transmittance between 35-75%.
How does an IR spectra show a Weak Detection?
Short Peak with a high transmittance between 75-90%.
What different peak shapes are there for different bonds?
- Broad U-shape: O-H bond in carboxyllic acid.
- Sharp Spike: C≡C-H bond.
- V-shape: N-H bond for Secondary (2°) Amine.
- W-shape: N-H bond for Primary (1°) Amine.
What are the Advantages of FTIR versus IR?
- Faster (seconds vs minutes).
- Collect all frequencies at once.
- Signal Averaging to increase signal-to-noise (S/N).
- High inherent S/N.
- High resolution (<0.1cm-1).
What are the Disadvantages of FTIR versus IR?
- Single Beam - needs collecting a blank.
- Can’t use Thermal Detectors - too slow.
