IR

ELECTROMAGNETIC SPECTROSCOPY

ORGANIC STRUCTURE DETERMINATION

  • Key Questions in Organic Structure Determination:

    • How do we know:

    • How are atoms connected together?

    • Which bonds are single, double, or triple?

    • What functional groups exist in the molecule?

    • If we have a specific stereoisomer?

  • The field of organic structure determination attempts to answer these questions.

INSTRUMENTAL METHODS OF STRUCTURE DETERMINATION

  1. Nuclear Magnetic Resonance (NMR)

    • Excitation of the nucleus of atoms through radiofrequency irradiation.

    • Provides extensive information about molecular structure and atom connectivity.

  2. Infrared Spectroscopy (IR)

    • Triggering molecular vibrations through irradiation with infrared light.

    • Primarily provides information about the presence or absence of certain functional groups.

  3. Mass Spectrometry

    • Bombardment of the sample with electrons and detection of resulting molecular fragments.

    • Provides information about molecular mass and atom connectivity.

  4. Ultraviolet Spectroscopy (UV)

    • Promotion of electrons to higher energy levels through irradiation of the molecule with ultraviolet light.

    • Primarily provides information about the presence of conjugated π systems and the presence of double and triple bonds.

SPECTRUM INTERPRETATION PROCESS

  1. Recognize a pattern.

  2. Associate patterns with physical parameters.

  3. Identify possible meanings, i.e., propose explanations.

    • From spectrum, extract the information it contains in abstract or hidden form.

    • Requires recognition of certain patterns, association with physical parameters, and interpretation in terms of meaningful and logical explanations.

EFFECT OF ELECTROMAGNETIC RADIATION ON MOLECULES

  • Higher frequency corresponds with shorter wavelength and greater energy.

  • Energy levels:

    • Gamma rays: $10^9$ Hz

    • X rays: $10^6$ Hz

    • Vacuum UV: $10^2$ Hz

    • Near UV: $10^5$ Hz

    • Visible: $10^4$ Hz

    • Infrared: $10^3$ Hz

    • Microwave: $10^{-2}$ Hz

    • Radio: $10^{-4}$ Hz

ELECTROMAGNETIC SPECTRUM

  • Most organic spectroscopy uses electromagnetic energy, or radiation, as the physical stimulus.

  • Important parameters associated with electromagnetic radiation are:

    • Energy (E):

    • Energy is directly proportional to frequency and inversely proportional to wavelength, as indicated by the equation:
      E = heta

    • Frequency (μ)

    • Wavelength (λ)

INFRARED SPECTROSCOPY

Theory and Interpretation of IR Spectrum

  • Infrared (IR) or vibrational spectroscopy is the measurement of the interaction of IR radiation with matter by absorption, emission, or reflection.

  • Used to study and identify chemical substances or functional groups in solid, liquid, or gaseous forms.

  • IR spectroscopy is conducted with an instrument called an IR spectrometer which produces an IR spectrum.

Theory

  • An IR spectrum visualized in a graph of IR light absorbance (or transmittance) on the vertical axis vs. frequency or wavenumber on the horizontal axis.

  • Units of wavenumber used in IR spectra is cm$^{-1}$.

  • Wavenumber ($n$) is defined as:
    n=rac10000extwavelength(extcm)n = rac{10000}{ ext{wavelength} ( ext{cm})}

  • Common instrument using this technique is a Fourier Transform Infrared (FTIR) spectrometer.

Regions of the Infrared Spectrum

  • Divided into three regions:

    • Near IR: 15000 cm$^{-1}$ to 3000 cm$^{-1}$

    • Mid IR: 4000 cm$^{-1}$ to 400 cm$^{-1}$

    • Far IR: 200 cm$^{-1}$ to 10 cm$^{-1}$

  • Most commonly used region is 4000 cm$^{-1}$ to 670 cm$^{-1}$.

  • Energies associated with infrared (1 to 15 kcal/mole) are normally not high enough to excite electrons but may induce vibrational excitation of covalently bonded atoms and groups.

  • IR spectroscopy exploits the fact that molecules absorb frequencies that are characteristic of their structure at resonant frequencies, where the frequency of the absorbed radiation matches the vibrational frequency.

Number of Vibrational Modes

  • Molecule can vibrate in many ways, called vibrational modes.

  • For linear molecules, the vibrational degrees of freedom (modes) are given by:
    3N−53N - 5

  • For non-linear molecules: 3N−63N - 6

    • Example: H$_2$O (non-linear) has 3 degrees of vibrational freedom:

    • $3 imes 3 - 6 = 3$ modes.

Types of Molecular Vibrations

  • Vibrational modes include:

    • Stretching:

    • Asymmetric Stretch

    • Symmetric Stretch

    • Scissoring

    • Rocking

    • Wagging

Transmission and Absorbance

  • When a chemical sample is exposed to IR radiation:

    • Possible outcomes:

    • Reflection

    • Absorption

    • Transmission

    • Transmitted light occurs when the sample absorbs specific frequencies and allows the rest to pass through.

    • The detector detects the transmitted frequencies, revealing the values of absorbed frequencies.

Interpretation of IR Spectrum

  • The IR spectrum is essentially a plot of transmitted (or absorbed) frequencies versus intensity of the transmission (or absorption).

    • Frequencies: x-axis (units of inverse centimeters - wavenumbers)

    • Intensities: y-axis (percentage units).

Classification of IR Bands

  • IR bands can be classified based on their intensities as:

    • Strong (s): Covers most of the y-axis.

    • Medium (m): Falls to about half of the y-axis.

    • Weak (w): Falls to one third or less of the y-axis.

Infrared Band Shapes

  • Band forms can be:

    • Narrow: Thin and pointed (like a dagger).

    • Broad: Wide and smoother.

    • Example: Broad bands are often associated with O-H bonds in alcohols and carboxylic acids.

Absorption Bands

  • IR absorption range for covalent bonds is between 600 - 4000 cm$^{-1}$.

  • Example of a sharp band around 2200-2400 cm$^{-1}$ suggests the presence of either a C≡N or a C≡C triple bond.

The Fingerprint Region

  • The range from 600 - 1400 cm$^{-1}$ is considered the fingerprint region.

    • This area shows many complex bands, often overlapping.

    • Not very useful in analysis; focus should be on the region left of 1400 cm$^{-1}$.

Functional Groups and IR Ranges

  • Different functional groups (e.g., alkenes, alcohols, ketones) absorb at specific ranges.

IR Spectrum of Alkanes

  • Alkanes lack functional groups; their IR spectra display only C-C and C-H bond vibrations.

    • Most useful are the C-H bands around 3000 cm$^{-1}$; prevalent in most organic molecules.

IR Spectrum of Alkenes

  • In addition to C-H bonds, alkenes exhibit:

    • A medium band corresponding to C=C bond stretching vibration at about 1600-1700 cm$^{-1}$.

    • A band for the =C-H bond stretch around 3080 cm$^{-1}$ (can be obscured by bands at around 3000 cm$^{-1}$).

IR Spectrum of Alkynes

  • Most notable band for alkynes arises from the carbon-carbon triple bond:

    • Appears as a sharp, weak band around 2100 cm$^{-1}$.

    • In symmetrical alkynes, this band may not appear due to low polarity.

    • Terminal alkynes display a C-H bond involving the sp carbon, showing a sharp, weak band around 3300 cm$^{-1}$.

    • Internal alkynes lack this C-H bond and thus this band.

Comparison of Alkynes

  • Comparison shown between unsymmetrical terminal alkyne (1-octyne) and symmetrical internal alkyne (4-octyne).

IR Spectrum of a Nitrile

  • Nitriles show a band around 2250 cm$^{-1}$ from the C≡N triple bond:

    • Band is sharp and pointed but more polar giving a stronger indication than alkynes.

IR Spectrum of an Alcohol

  • The most prominent band in alcohols is due to the O-H bond:

    • Displays a strong, broad band covering approximately 3000 - 3700 cm$^{-1}$.

    • The size and broad shape dominate the IR spectrum.

IR Spectrum of Aldehydes and Ketones

  • Carbonyl function (C=O) characteristics:

    • Aldehydes: C=O bonded to a carbon and hydrogen at the chain's end.

    • Ketones: C=O bonded to two other carbons in the middle of the chain.

    • Both display a strong, stake-shaped band around 1710 - 1720 cm$^{-1}$ due to the polar C=O bond.

Differences Between Aldehydes and Ketones

  • Aldehydes have an additional =C-H bond, showing medium strength bands at about
    2700 and 2800 cm$^{-1}$.

  • Ketones lack the C-H bond resultingly lacking these bands.

IR Spectrum of a Carboxylic Acid

  • Carboxylic acids demonstrate:

    • Very strong and broad band between 2800 and 3500 cm$^{-1}$ for O-H stretch.

    • Stake-shaped band around 1710 cm$^{-1}$ corresponding to C=O stretch.

IR Spectra of Amines

  • Amines exhibit N-H bond stretch within 3200 - 3600 cm$^{-1}$:

    • Appear as weak to medium, broad bands (less broad than O-H bands in alcohols).

    • Primary amines: exhibit two N-H bonds, showing two spikes (resembling molar teeth).

    • Secondary amines: show a single spike (resembling canine teeth).

    • Tertiary amines: no N-H bonds present, hence the band is absent.

IR Spectrum of Amides

  • Amides encompass attributes of both amines and ketones with:

    • N-H stretch seen as a strong, somewhat broad band between 3100 and 3500 cm$^{-1}$.

    • A stake-shaped C=O stretch band around 1710 cm$^{-1}$ also appears.

    • Primary amides show two spikes; secondary amides present only one.

References / Bibliography

  • Fleming I., Williams D. B. (2019). Spectroscopic Methods in Organic Chemistry, 7th Edition, Springer Nature Switzerland AG: Gewerbestrasse 11, 6330 Cham, Switzerland.

  • Pedersen-Bjergaard, S., Gammelgaard, B., Halvorsen T. G. (2019). Introduction to Pharmaceutical Analytical Chemistry 2nd Edition. John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, United Kingdom.

  • Batson J., Mehta D.K., Maule C., Gotecha P., Greenfield E.S. (Eds)(1986). Clarke's Isolation and Identification of Drugs in pharmaceuticals, body fluids, and post-mortem material 2nd Edition. The Pharmaceutical Press, London.

  • Kalsi P. S. (2004). Spectroscopy of Organic Compounds, 6th Edition. New Age International (P) Limited, Publishers, 4835/24, Ansari Road, Daryaganj, New Delhi -110002.