The IR Region Just below red in the visible region. Wavelengths usually 2.5-25 um. More common units are wavenumbers, or cm-1,the reciprocal of the wavelength in centimeters. Wavenumbers are proportional to frequency and energy. Chapter 12 6
Chapter 12 6 The IR Region • Just below red in the visible region. • Wavelengths usually 2.5-25 mm. • More common units are wavenumbers, or cm-1 , the reciprocal of the wavelength in centimeters. • Wavenumbers are proportional to frequency and energy. =>
Molecular Vibrations Covalent bonds vibrate at only certain allowable frequencies. spring force spring force ○y00 stretched compressed equilibrium bond length Copyright2005 Pearson Prentice Hall,Inc. Chapter 12
Chapter 12 7 Molecular Vibrations Covalent bonds vibrate at only certain allowable frequencies. =>
Stretching Frequencies TABLE 12-1 Bond Stretching Frequencies. Bond Bond Energy [kJ(kcal)] Stretching Frequency(cm1) Frequency decreases with increasing atomic mass C一H 420(100)<note these are trs> 3000 C-D heavier 420(100) 2100 atoms v decreases C-C 350(83) 1200 Frequency increases with bond energy C-C 350(83) 1200 C=C 611(146) stronger 1660 bond 7 increases C=C 840(200) 2200 Frequency decreases with increasing atomic mass. Frequency increases with increasing bond energy. Chapter 12 8
Chapter 12 8 Stretching Frequencies • Frequency decreases with increasing atomic mass. • Frequency increases with increasing bond energy. =>
Vibrational Modes Nonlinear molecule with n atoms usually has 3n-6 fundamental vibrational modes. H symmetric stretching antisymmetric stretching bending (scissoring) Copyright 2005 Pearson Prentice Hall,Inc. Chapter 12
Chapter 12 9 Vibrational Modes Nonlinear molecule with n atoms usually has 3n - 6 fundamental vibrational modes
Fingerprint of Molecule Whole-molecule vibrations and bending vibrations are also quantized. No two molecules will give exactly the same IR spectrum (except enantiomers) Simple stretching:1600-3500 cm-1. Complex vibrations:600-1400 cm-1, called the“fingerprint region.” > Chapter 12 10
Chapter 12 10 Fingerprint of Molecule • Whole-molecule vibrations and bending vibrations are also quantized. • No two molecules will give exactly the same IR spectrum (except enantiomers). • Simple stretching: 1600-3500 cm-1 . • Complex vibrations: 600-1400 cm-1 , called the “fingerprint region.” =>