Lecture Notes: Practical Guide to IR Spectroscopy

Introduction to IR Spectroscopy

• IR Spectroscopy (Infrared Spectroscopy)
  • Type of vibrational spectroscopy
  • All bonds vibrate, visible in the IR spectrum
  • Functional groups absorb light at different frequencies
  • Allows identification of molecular components

Regions of the IR Spectrum

• Analytical Region: Above 1500 reciprocal cm
  • Focus of this lecture
• Fingerprint Region: Below 1500 reciprocal cm
  • Unique for each molecule but difficult to interpret
  • Useful for identifying molecules by comparison

Key Concepts in IR Spectroscopy

• Wave Number
  • Expressed in reciprocal cm
  • Relates to vibrational frequency and bond energy
  • Higher wave number = higher vibrational energy
• Signal Shape and Strength
  • Broad, smooth signals versus spiky signals
  • Signal strength varies, least important factor
  • Strength affected by method of spectrum acquisition

Practical Interpretation of IR Spectrum

• Using Reference Tables
  • Match wave numbers and shapes to a reference table
  • Visual and tabular reference tables available
  • Memorize common numbers for efficiency during tests

Solving IR Spectra

• Approach
  • Analyze spectrum left to right
  • Identify functional groups by wave numbers and shapes
  • Differentiating between similar signals (e.g., OH vs. NH)

Important Regions to Analyze

• Around 3500 reciprocal cm
  • OH groups: Broad and smooth signals
  • NH groups: Weaker and sharper signals
• 3000 reciprocal cm Line
  • Carbon-hydrogen stretches
  • SP3 hybridized (under 3000), SP2 hybridized (over 3000)
• 2500 to 2000 reciprocal cm
  • Triple bonds, terminal triple bonds identified by sharp spikes
• 1800 to 1500 reciprocal cm
  • Double bonds (e.g., C=O at 1700, C=C at 1650)

Practice and Application

• Importance of Practice
  • Solve multiple problems to internalize concepts
  • Recognizing patterns and functional groups through repetition
• Example Analysis
  • Assess presence of OH, NH, C=O, and carbon-hydrogen stretches
  • Apply combined data from IR, NMR to determine molecular structure

Conclusion

• Summary of Skills
  • Interpretation of wave numbers, signal shapes, and strengths
  • Efficient use of reference tables
• Call to Action
  • Practice solving spectroscopy problems daily
  • Develop proficiency before exams
• Engagement
  • Encourage communication, further queries, and learning resources.

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