EXP. 5: Interpreting FTIR Spectra

Interpreting FTIR Spectra

Background

Infrared (IR) spectroscopy is a vital analytical technique in organic chemistry, used to identify and characterize compounds by examining how molecules interact with infrared radiation. When IR light passes through a sample, specific wavelengths are absorbed, causing molecular vibrations that correspond to the types of bonds and functional groups present. The resulting IR spectrum displays these absorptions as peaks, which serve as a unique "molecular fingerprint" for each compound.

• Vibrational Spectroscopy Principle: The energy of absorbed radiation is related to the frequency of vibration according to Planck's equation:

$E = h\nu$

• Bond Vibration Frequency: The frequency of vibration depends on bond strength and atomic masses, described mathematically by:

$\nu = \frac{1}{2\pi} \sqrt{\frac{k}{\mu}}$

• Definitions:

  • k: bond force constant

  • μ: reduced mass

  • h: Planck's constant

  • ν: frequency of absorbed IR light

• Stronger bonds and lighter atoms: Vibrate at higher frequencies, producing distinct peaks in the spectrum.

Fourier Transform Infrared (FTIR) Spectroscopy

FTIR is an advanced form of IR analysis that uses an interferometer to simultaneously measure all infrared frequencies. The recorded signal is mathematically converted into a spectrum using a Fourier transform, offering faster scanning and improved sensitivity compared to conventional dispersive IR instruments.

• FTIR Spectrum Range: Commonly spans 4000–400 cm−1.

• Functional Group Region: 4000–1000 cm−1 (characteristic group vibrations).

• Fingerprint Region: 1000–400 cm−1 (unique to each molecule).

• Peak Intensity and Shape: Influenced by molecular structure and bonding.

• Applications: Used in organic and analytical chemistry, pharmaceuticals, forensic analysis, and materials science.

Objectives of the Experiment

This experiment aims to familiarize students with the operation and handling of an FTIR spectrometer, including sample preparation, instrument setup, and spectrum analysis. Emphasis is placed on proper laboratory practices and safety.

• Operate an FTIR spectrometer safely and effectively.

• Identify and use appropriate sampling accessories for different sample types.

• Collect and analyze background and sample spectra using FTIR software.

• Follow instrument maintenance procedures, including cleaning and desiccant replacement.

• Handle samples and maintain the spectrometer according to best practices.

FTIR Spectrometer Operation

Connecting Power

• Insert the power supply connection into the instrument.

• Connect the USB communication cable.

• Ensure the power cord is connected before plugging into the outlet.

• Keep the instrument away from heat and electromagnetic interference.

Turning On and Off the Instrument

• Press and hold the power button for two seconds to turn on; wait for the LED to turn green.

• Allow a five-minute warmup period.

• To turn off, press and hold the power button for two seconds until the LED turns red.

Power Switch LED Status Table

Status	LED Color	Action (% duty cycle)
System OFF	No illumination	N/A
System ON	Red / Green	100%
System ON (Idle)	Green	100%
Firmware Update	Red / Green	50% each

Diamond ATR Sampling Accessory

Cleaning

• Rotate the top knob counterclockwise to open the sample press.

• Clean the sample press and diamond window using acetone or alcohol.

Collecting a Background Spectrum

• Follow software instructions to collect a background spectrum before each sample analysis.

Collecting a Sample Spectrum

• Place a small amount of sample on the crystal, ensuring full coverage.

• Apply cleaning method after analysis for next use.

Characteristic IR Absorption Table

The following table summarizes typical IR absorption frequencies for common organic functional groups:

Vibration	Position (cm−1)	Intensity	Notes
Alkanes (C–H stretch)	2850–2960	m	See Table 1 for detail
Alkenes (C=C stretch)	1620–1680	w	Characteristic of double bonds
Alkynes (C≡C stretch)	2100–2260	w	Triple bond region
Aromatic Compounds (C–H stretch)	3030	w	Characteristic aromatic region
Alcohols (O–H stretch)	3200–3600	s, br	Broad, strong band
Carboxylic Acids (O–H stretch)	2500–3300	s, br	Very broad
Esters (C=O stretch)	1735–1750	s	Sharp, strong band
Amides (N–H stretch)	3100–3500	m	May be split
Aldehydes (C–H stretch)	2720–2820	w	Fermi doublet
Acid Chlorides (C=O stretch)	1750–1810	s	High frequency
Nitro Compounds (N–O stretch)	1500–1600	s	Strong absorption
Thiols (S–H stretch)	2550–2700	w	Weak, sharp
Aralkyl & Aryl Halides (C–X stretch)	500–800	w	Halides in fingerprint region

Additional info: Intensity: s = strong, m = medium, w = weak, br = broad. The fingerprint region (below 1500 cm−1) is highly specific for individual compounds.

Alkene and Benzene Ring Absorptions

Vibration	Position (cm−1)	Ring Substitution	Notes
Alkene Bending	965–985	Monosubstituted	Out-of-plane bending
Benzene Bending	730–770	Disubstituted, ortho	Characteristic aromatic bending
Benzene Bending	750–810	Disubstituted, meta	Meta substitution pattern
Benzene Bending	860–900	Disubstituted, para	Para substitution pattern

Laboratory Safety and Hazard Assessment

Hazard Assessment Table

Substances Involved	Hazards and Toxicities	Precautions and First Aid
Acetone	Flammable, irritant	Use in fume hood, avoid skin contact, rinse with water if exposed
Alcohol (ethanol/isopropanol)	Flammable, irritant	Use in fume hood, avoid ingestion, rinse with water if exposed
Unknown organic samples	Varies; may be toxic or irritant	Wear gloves, avoid inhalation, seek medical attention if exposed
Diamond ATR crystal	Physical hazard (breakage)	Handle with care, avoid dropping

Additional info: Always consult the Material Safety Data Sheet (MSDS) for each chemical used.

Data Analysis and Interpretation

Treatment of Results

• Identify unknown compounds assigned to your group based on the FTIR spectra.

• Compare observed absorption bands to the characteristic frequencies in the tables above.

• Assign functional groups and deduce molecular structure.

Guide Questions

• Why is collecting a background spectrum required before every sample analysis? To account for atmospheric and instrumental absorptions, ensuring accurate sample measurement.

• What are possible sources of error if FTIR readings are irregular or too low? Poor sample contact, dirty crystal, instrument malfunction, or incorrect background collection.

References

• Agilent Technologies. (2021). Agilent Cary 630 FTIR Spectrometer User Guide.

• Ashamust, J. (2019). Interpreting IR Spectra: A Quick Guide. Master Organic Chemistry.

• California State University. (2014). Interpretation of Infrared Spectra.

• FTIR User Guide. (2024). Fourier Transform Infrared Spectrometer user guide.

• The University of Texas. (n.d.). Infrared (IR) Spectroscopy Lecture Notes.

• Wade, L. G. (2006). Organic Chemistry (6th ed.). Pearson Prentice Hall.