Water spectra hydrogen bond

Another case is where the chemistry is known, but the spectroscopy is unknown, because the pure material is not available. Protein, for example, cannot be extracted from wheat (or at least not and still remain protein), so the spectrum of pure protein as it exists in wheat is unknown. Even simpler molecules are subject to this effect we can measure the spectrum of pure water easily enough, for example, but that is not the same spectrum as water has when it is present as an intimate mixture in a natural product -the changes in the hydrogen bonding completely change the nature of the spectrum. 

The infrared spectrum of erythromycin is commonly used for its identification. Figure 2 shows the spectrum of a 75 mg./ml. chloroform solution. The bands at 1685 and 1730 cm- are due to the ketone carbonyl and the lactone carbonyl, respectively. The absorption peaks between 1000 and 1200 cm-1 are due to the ethers and amine functions. The CH2 bending is evidenced by peaks between 1340 and 1460 cm-, and alkane stretching peaks appear between 2780 and 3020 cm-. Hydrogen bonded OH and water appear as bands between 3400 and 3700 cm-1. 

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