Vibrational and Rotational Spectra. IR Experimental Considerations. Molecular Symmetry. The Vibrational Origin of Group Frequencies. Methyl and Methylene Groups. Triple Bonds and Cumulated Double Bonds. Olefin Groups. Aromatic and Heteroaromatic Rings. Carbonyl Compounds. Ethers, Alcohols, and Phenols. Amines, C=N, and N=O Compounds. Compounds Conking Boron, Silicon, Phosphorus, Sulfur, or Halogen. Major Spectra-Structure Correlations by Spectral Regions. The Theoretical Analysis of Molecular Vibrations.