CH 16-Infrared Spectroscopy Flashcards

CH 16: An Introduction to Infrared Spectroscopy

Uses of IR spectroscopy

• Qualitative Analysis: Identification of compounds (must be pure as all compounds are IR absorptive)
• Quantitation is difficult but can be done

Instrument Types

• Originally, all instruments were dispersive instruments.
• FTIR has become the major instrument used since the 1990s.
• Interferometric instruments produce an order of magnitude in improvements over dispersive instruments.
• Using FTIR, quantitative analysis can now be done as well as the usual qualitative analysis.

Infrared Radiation

Infrared radiation is categorized into three regions based on wavenumber, wavelength, and energy:

• Near IR: Wavenumber: 12000 – 4000 cm^{-1}, Wavelength: 0.8 – 2.5 μm, Energy: 1.55 – 0.5 eV
• Mid IR: Wavenumber: 4000 – 400 cm^{-1}, Wavelength: 2.5 – 25 μm, Energy: 0.5 – 0.05 eV
• Far IR: Wavenumber: 400 – 10 cm^{-1}, Wavelength: 25 – 1000 μm, Energy: 0.05 – 0.0012 eV

Note: Wavelength in cm is not nm. For Mid IR, it's 2500 to 25000 nm.

Application of each IR type

• Near IR (NIR) – primarily used for quantitative analysis.
• Mid IR – is most often used for qualitative analysis and can analyze both liquids and solids (ATR, Diffuse Reflectance, and even photoacoustic measurements).
• Far IR – potentially useful but has had limited use due to experimental difficulties.

Definition and origin of Mid IR spectroscopy

Infrared spectroscopy measures the wavelength and intensity of the absorption of mid-infrared light by a sample. Mid-infrared light is energetic enough to excite molecular vibrations to higher energy levels.

The wavelength of infrared absorption bands is characteristic of specific types of chemical bonds, which makes infrared spectroscopy highly useful for identifying organic and organometallic molecules. The high selectivity of the method makes the estimation of an analyte in a complex matrix possible.

Theory of Infrared Absorption Spectroscopy

For a molecule to absorb IR, the vibrations or rotations within a molecule must cause a net change in the dipole moment of the molecule. The alternating electrical field of the radiation interacts with fluctuations in the dipole moment of the molecule.

If the frequency of the radiation matches the vibrational frequency of the molecule, radiation will be absorbed, causing a change in the amplitude of molecular vibration. These transitions are discrete and quantized, just as are the electronic transitions in the UV-VIS.

Wavenumbers

Due to the very long wavelengths in mid IR, instead of nm, we use wavenumbers (cm^{-1}).

Conversion formula: ν (cm^{-1}) = \frac{1}{λ(nm)} × 10^7 \frac{nm}{cm}

Example: Carbonyl band at around 1700 cm^{-1} = 5882.4 nm

Wavenumbers represent frequency instead of wavelength.

Spectrum

Typical Mid IR spectrum (polystyrene film)

The spectrum shows % Transmittance vs. Wavenumber (cm^{-1}).

Functional group region and fingerprint region are labeled.

Functional Group Wavenumbers

• O-H: 3650-3200 cm^{-1}, strong, broad
• C-H: 3300-2700 cm^{-1}, medium
• N-H: 3500-3300 cm^{-1}, medium, broad
• C≡N: 2260-2220 cm^{-1}, medium
• C≡C: 2260-2100 cm^{-1}, weak-medium
• C=C: 1680-1600 cm^{-1}, medium
• C=N: 1650-1550 cm^{-1}, medium
• C=O: 1780-1650 cm^{-1}, strong
• C-O: 1250-1050 cm^{-1}, strong

Molecular Processes in Light Absorption

• Electronic excitation
• Vibration
• Rotation

These are the energy levels excited by Mid IR light.

Electronic and Vibrational Transitions

Vibrational transitions are much smaller than the electronic ones.

Dipole Moment

To absorb IR radiation, a molecule must undergo a net change of Dipole Moment as it vibrates or rotates. Only under these circumstances can the oscillating electric field vector interact with the molecule to cause changes in the amplitude of its motions.

• H-H has no dipole and is “IR transparent.”
• H-Cl does and absorbs IR light.

Molecular Rotations

Rotational transitions are of little use to the spectroscopist. Rotational levels are quantized, and absorption of IR by gases yields line spectra.

However, in liquids or solids, these lines broaden into a continuum due to molecular collisions and other interactions.

Rotational Spectrum of HCl

Vibration-Rotation Transitions

Transitions from the ground vibrational state to the first excited state of HCl with a change Δj = ±1 in rotational angular momentum.

• Transitions v=0,j to v=1, j-1
• Transitions v=0, j to v=1, j+1

Molecular Vibrations

Molecular vibrations are also important in understanding infrared absorption and the mechanisms and kinetics of chemical reactions. Frequencies are most commonly measured with infrared or Raman spectroscopy.

Rotational-vibrational spectroscopy, isotope substitution, and many forms of force-field modeling are used to determine characteristic atomic motions.

Vibrational Motion

Vibrational motion is subdivided into so-called normal modes of vibration, which rapidly increase with the number of atoms in the molecule. Each of these normal vibrational modes contributes RT to the average molar energy of the substance and is a primary reason why heat capacities increase with molecular complexity.

If there are X{vib} modes of vibration, then the vibrational energy contributes X{vib}(RT) to the average molar energy of the substance.

Vibrations of Formaldehyde

• Symmetric C-H stretching: 2766 cm^{-1}
• Asymmetric C-H stretching: 2843 cm^{-1}
• C-O stretching: 1746 cm^{-1}
• Symmetric bending: 1500 cm^{-1}
• Asymmetric bending: 1251 cm^{-1}
• Out-of-plane bending: 1167 cm^{-1}

Stretching modes appear from 4000-2000 cm^{-1}. Bending is found lower in the "fingerprint" region.

Potential Energy Diagrams for Vibrational Transitions

Vibrations can be modeled by Hooke’s Law: F = -ky

Where:

• k = force constant (stiffness of the spring)
• y = axis.

Stretching and Bending

Stretching Vibrations

• Symmetric
• Asymmetric

Symmetric and Asymmetric Absorption of Carbon Dioxide

The symmetric stretch of CO2 does not produce a good change of dipole and is therefore very minor.

Bending Vibrations

• In-plane rocking
• In-plane scissoring
• Out-of-plane wagging
• Out-of-plane twisting

Selection Rules

A selection rule, or transition rule, formally constrains the possible transitions of a system from one quantum state to another. The energy for a transition from energy level 1 to 2 or 2 to 3 should be identical to that of for the 0-1 transition.

Theory indicates that the only transitions that can take place are those where the VIBRATIONAL quantum number is 1. That is Δν = ± 1. And there must be a change in the dipole moment during the vibration.

Vibrational Modes

For polyatomic molecules, the number of vibrations can be calculated by finding their position around a point in 3D space. For N points, you need 3 coordinates for each point = 3N. A molecule with N atoms has 3N degrees of freedom.

• Normal Mode Vibrations for a linear molecule are 3N-5
• Non-linear molecules = 3N-6

Vibrational Coupling

A vibration in one bond of a molecule can affect (couple) the bonds around it.

Factors influencing Coupling:

• Strong coupling between stretching vibrations occurs only when an atom is common to the two vibrations.
• Interaction between bending vibrations requires a common bond between the vibrating groups.
• Coupling between a stretch and bend occurs only if the stretching bond forms one side of the angle that bends.
• Coupling is greatest when the coupled groups have individual energies that are nearly equal.
• If a bond is two or more bonds from a vibration, little or no coupling is seen.
• Coupling requires that the vibrations be of the same symmetry species.

Infrared Instruments

An infrared spectrophotometer is an instrument that passes infrared light through an organic molecule and produces a spectrum that contains a plot of the amount of light transmitted on the vertical axis against the wavelength of infrared radiation on the horizontal axis. In infrared spectra, the absorption peaks point downward because the vertical axis is the percentage transmittance of the radiation through the sample.

Absorption of radiation lowers the percentage transmittance value. Since all bonds in an organic molecule interact with infrared radiation, IR spectra provide a considerable amount of structural data.

Dispersive Spectroscopy

1. Wavelength separation
2. Slit
3. Sample
4. Detector
5. Computer

Light is separated into constituent wavelengths and passed one at a time through the sample. Throughput is tiny.

FTIR

The Michelson interferometer principle

1. example: Monochromatic light

• Detector
• Movable mirror
• Stationary Mirror
• Beamsplitter
• Interference
• δ = Optical Path Difference
• δ = (n + \frac{1}{2})λ
• δ = nλ

The laser used is the HeNe laser with its 632.8 nm wavelength coherent beam. The laser is also beamed through the interferometer and provides the “laser fringe reference system” due to its very reproducible cosine fringe or wave. It is used to regulate the Sampling Interval, the signal that is co-added together.

Nernst Glower

Made from rare earth oxides in the form of a cylinder. When powered up, it heats to 1200-2200K. Since resistance is high, it has to be heated to a dull red heat before the current can become high enough to maintain the temperature.

Spectral Energy Distribution from a Nernst Glower

Glowbar

This is a silicon carbide rod about 5 cm long and is electrically heated by resistance coils to 1300-1500 K.

Advantage: a positive coefficient of resistance

And – it provides better output below 5 μm than the Nernst Glower.

Disadvantage: you have to water cool the electrical contacts.

Incandescent Wire Source

This consists of a tightly wound spiral of nichrome wire heated electrically to 100 K. A second style, used a great deal, is the Rh wire heater sealed in a ceramic cylinder and has similar properties but is more expensive. These require no cooling and are nearly maintenance-free.

IR Transducers (Detectors)

Three Types:

1. Pyroelectric transducers (some FTIR and in dispersive instruments)
2. Thermal transducers (In older dispersive instruments but are too slow for FTIR)
3. Photoconducting transducers (Found in many FTIR instruments)

Detector Transducers

• The heating effect of radiation
• Thermal transducer- black body, small, very low heat capacity- ΔT=10^{-3} K, housed in vacuum, signal is chopped
• Thermocouples
  • Two junctions of dissimilar metals, An and Bi
  • One is IR detector, one is reference detector
  • Potential difference that develops is proportional to ΔT; detection of ΔTs of 10^{-6} K is possible

Sample Preparation

• Sample holder must be transparent to IR - not many things are transparent…salts (KBr) or ZnSe
• Liquids
  • Salt Plates
  • Neat, 1 drop
  • Samples dissolved in volatile solvents- 0.1-10%
• Solids
  • KBr pellets
  • Mulling (dispersions)
• Quantitative analysis-sealed cell with NaCl/NaBr/KBr windows

Thermal Transducers

For Infrared samples, Thermal Transducers are used. These devices are sensitive to IR radiation. The simplest of these is the Thermocouple consisting of two dissimilar metals fused together (like Cu and constantan (55% Cu/45% Ni)). A voltage develops that varies with temperature.

Pyroelectric transducers use dielectric materials such as Triglycine sulfate doped with D2 or alanine.

FTIR Sampling Techniques

• KBr Pellets
• Salt Plates
• Diffuse Reflectance
• ATR

KBr Pellet Die

This is the most common method of FTIR Analysis

Diffuse Reflectance Method

ATR System

This one uses ZnSe. The Electric Vector of the evanescent wave penetrates the surface of the crystal and interacts with the sample.

Photo-conducting Transducers

These consist of a thin film of a semiconductor such as Lead Sulfide, mercury telluride – cadmium telluride (MCT) or Indium antimonide – and are deposited on a non-conducting glass surface that is sealed in an evacuated envelope.

These work by absorbing radiation and promoting non-conducting valence electrons into a higher energy conducting state. This decreases the resistance of the semiconductor. The resistance drop is proportional to the intensity of the radiation.

IR of ALKENES

=C—H bond, “unsaturated” vinyl (sp^2)

• 3020-3080 cm^{-1}
• 675-1000 RCH=CH_2
• 910-920 & 990-1000 R2C=CH2
• 880-900 cis-RCH=CHR
• 675-730 (v) trans-RCH=CHR

C=C bond: 1640-1680 cm^{-1}

IR spectra BENZENEs

=C—H bond, “unsaturated” “aryl” (sp^2)

• 3000-3100 cm^{-1}
• 690-840 mono-substituted
• 690-710, 730-770 ortho-disubstituted
• 735-770 meta-disubstituted
• 690-710, 750-810(m) para-disubstituted
• 810-840(m)

C=C bond 1500, 1600 cm^{-1}

IR spectra ALCOHOLS & ETHERS

C—O bond 1050-1275 (b) cm^{-1}

• 1^o ROH 1050
• 2^o ROH 1100
• 3^o ROH 1150
• ethers 1060-1150

O—H bond 3200-3640 (b)