10 The infrared spectrum for HCN (a linear molecule) was obtained using an infra
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10 The infrared spectrum for HCN (a linear molecule) was obtained using an infrared spectrophotometer which operates in the range: 4000-600cmil. The spectrum shows three absorption bands at 3311 cm i, 2097 cm i and 712 cm 1. a. Explain how the number of normal vibrational modes can be determined for any molecule. b. For HCN, the types of vibrational motion are: i. C-N stretching ii. H-C-N bending iii. C-H stretching Complete the following Table by identifying the type of vibrational motion associated with each absorption band in the infrared spectrum for HCN. wavenumber, cm' type of vibrational motion 712 2097 3311 c. HCN has four normal vibrational modes. Explain why the infrared spectrum shows only three absorptions (instead of four)Explanation / Answer
a) The required number of “normal modes” can be determined with the help of a vibrational degree of freedom i.e., the number of modes for a nonlinear molecule is 3N63 and for a linear molecule is 3N5.
b) 712 is due to H-C-N bending,
2097 is due to C-N stretch
3311 is due to C-H stretch
c) Althugh there are four normal vibration modes, but only three are IR active. Hence, infrared spectrum shows only three absorptions.
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