Infrared absorption spectroscopy: instrumentation, FTIR, advantages ofFTIR, applications of IR, qualitative and quantitative analysis, advantagesand limitations of quantitative IR methods.
Infrared SpectroscopyIR Spectroscopy is the interaction between matter and IR radiation.All molecular species except homonuclear diatomics (e.g., O2, H2, N2, etc.) are detectableIR light absorption due to changes in rotational and vibrational energy in moleculeAbsorbed energy causes molecular motions which create a net change in the dipole moment.
Regioncm-1 mNear IR12000 4000 0.8 2.5 Mid IR4000 400 2.5 25 Far IR 400 10 25 1000
Vibrations of Molecules
Theory of Infrared Absorption Spectroscopy
IR photons have low energy. The only transitions that have comparable energy differences are molecular vibrations and rotations.
Theory of Infrared Absorption Spectroscopy
In order for IR absorbance to occur two conditions must be met: 1.There must be a change in the dipole moment of the molecule as a result of a molecular vibration (or rotation). The change (or oscillation) in the dipole moment allows interaction with the alternating electrical component of the IR radiation wave. Symmetric molecules (or bonds) do not absorb IR radiation since there is no dipole moment. 2.If the frequency of the radiation matches the natural frequency of the vibration (or rotation), the IR photon is absorbed and the amplitude of the vibration increases.
DE = hnThere are three types of molecular transitions that occur in IR a)Rotational transitions When an asymmetric molecule rotates about its center of mass, the dipole moment seems to fluctuate.DE for these transitions correspond to n < 100 cm-1
Quite low energy, show up as sharp lines that subdivide vibrational peaks in gas phase spectra.b)Vibrational-rotational transitions complex transitions that arise from changes in the molecular dipole moment due to the combination of a bond vibration and molecular rotation.c)Vibrational transitions The most important transitions observed in qualitative mid-IR spectroscopy. n = 13,000 675 cm-1 (0.78 15 mM)
IntroductionThe IR Spectroscopic ProcessThe quantum mechanical energy levels observed in IR spectroscopy are those of molecular vibration
We perceive this vibration as heat
When we say a covalent bond between two atoms is of a certain length, we are citing an average because the bond behaves as if it were a vibrating spring connecting the two atoms
For a simple diatomic molecule, this model is easy to visualize:
IntroductionThe IR Spectroscopic ProcessThere are two types of bond vibration:Stretch Vibration or oscillation along the line of the bond
Bend Vibration or oscillation not along the line of the bond
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The IR Spectroscopic ProcessAs a covalent bond oscillates due to the oscillation of the dipole of the molecule a varying electromagnetic field is produced
The greater the dipole moment change through the vibration, the more intense the EM field that is generated
The IR Spectroscopic ProcessWhen a wave of infrared light encounters this oscillating EM field generated by the oscillating dipole of the same frequency, the two waves couple, and IR light is absorbed
The coupled wave now vibrates with twice the amplitude
IR beam from spectrometer
EM oscillating wavefrom bond vibration
The IR SpectrumEach stretching and bending vibration occurs with a characteristic frequency as the atoms and charges involved are different for different bonds
The y-axis on an IR spectrum is in units of % transmittance
In regions where the EM field of an osc. bond interacts with IR light of the same n transmittance is low (light is absorbed)In regions where no osc. bond is interacting with IR light, transmittance nears 100%
The IR SpectrumThe x-axis of the IR spectrum is in units of wavenumbers, n, which is the number of waves per centimeter in units of cm-1 (Remember E = hn or E = hc/l)
The IR SpectrumThis unit is used rather than wavelength (microns) because wavenumbers are directly proportional to the energy of transition being observed chemists like this, physicists hate it
High frequencies and high wavenumbers equate higher energyis quicker to understand thanShort wavelengths equate higher energy
This unit is used rather than frequency as the numbers are more real than the exponential units of frequency
IR spectra are observed for the mid-infrared: 600-4000 cm-1
The peaks are Gaussian distributions of the average energy of a transition
The IR SpectrumIn general:Lighter atoms will allow the oscillation to be faster higher energyThis is especially true of bonds to hydrogen C-H, N-H and O-H
Stronger bonds will have higher energy oscillationsTriple bonds > double bonds > single bonds in energy
Energy/n of oscillation
The Vibrational Modes of Water
Mechanical Model of Stretching Vibrations1.Simple harmonic oscillator. Hookes Law (restoring force of a spring is proportional to the displacement) F = -ky Where: F = Forcek = Force Constant(stiffness of spring)y = Displacement
Natural oscillation frequency of a mechanical oscillator depends on: a)mass of the object b)force constant of the spring (bond) The oscillation frequency is independent of the amount of energy imparted to the spring.
Frequency of absorption of radiation can be predicted with a modified Hookes Law.Where: n = wavenumber of the abs. peak (cm-1)c = speed of light (3 x 1010 cm/s) k = force constantm = reduced mass of the atoms
Where: Mx = mass of atom x in kgMy = mass of atom y in kgForce constants are expressed in N/m (N = kgm/s2)-Range from 3 x 102 to 8 x 102 N/m for single bonds -500 N/m is a good average force constant for single bonds when predicting k. -k = n(500 N/m) for multiple bonds where n is the bond order
IR Sources and DetectorsSources -inert solids that heat electrically to 1500 2200 K.Emit blackbody radiation produced by atomic and molecular oscillations excited in the solid by thermal energy. The inert solid glows when heated.Common sources: 1.Nernst glower -constructed of a rod of a rare earth oxide (lanthanide) with platinum leads. 2.Globar -Silicon carbide rod with water cooled contacts to prevent arcing. 3.Incandescent wire -tightly wound wire heated electrically. Longer life but lower intensity.
Detectors measure minute changes in temperature.1.Thermal transducer Constructed of a bimetal junction, which has a temperature dependant potential (V). (similar to a thermocouple)Have a slow response time, so they are not well suited to FT-IR.2.Pyroelectric transducer Constructed of crystalline wafers of triglycine sulfate (TGS) that have a strong temperature dependent polarization. Have a fast response time and are well suited for FT-IR. 3.Photoconducting transducer Constructed of a semiconducting material (lead sulfide, mercury/cadmium telluride, or indium antimonide) deposited on a glass surface and sealed in an evacuated envelope to protect the semiconducting material from the environment. Absorption of radiation promotes nonconducting valence electrons to a conducting state, thus decreasing the resistance (W) of the semiconductor. Fast response time, but require cooling by liquid N2.
FTIR BackgroundFTIR is a modern spectroscopic method which operates in the IR region (molecular vibrations and rotations)The FT in FTIR gives the wavelength selection method (Fourier Transformation)Prior to FTIR, grating and prism spectrometers were used
What is FTIRFourier-transform infrared spectroscopy is a vibrational spectroscopic technique, meaning it takes advantage of asymmetric molecular stretching, vibration, and rotation of chemical bonds as they are exposed to designated wavelengths of light.Fourier transform is to transform the signal from the time domain to its representation in the frequency domain
He-Ne gas laser
Fixed mirrorMovable mirrorSample chamberLight source (ceramic)Detector(DTGS)
FT Optical System Diagram
Chapter 2 Principles of FTIR 2.4 Structure of an Interferometer
Fourier spectroscopy used in FT-IR is the general term for the use of a two-beam interferometer (primarily Michelson interferometers) in spectroscopy. A Michelson interferometer consists of a half-mirror (beam splitter) and two reflecting mirrors. One of the reflecting mirrors is fixed in place (fixed mirror) and the other has a mechanism for moving parallel to the optical axis (movable mirror).
Light from the light source is collimated and directed into the interferometer, striking the beam splitter at an angle, thereby separating the light into transmitted light and reflected light. These two beams of light are each reflected by the fixed mirror and movable mirror, and then returned to the beam splitter where they are recombined into a single beam.
Simplest interferometer designBeamsplitter for dividing the incoming IR beam into two partsTwo plane mirrors for reflecting the two b