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
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 ○m0000③③m○ equilibrium stretched compressed bond length Chapter 12
Chapter 12 7 Molecular Vibrations Covalent bonds vibrate at only certain allowable frequencies. =>
Stretching Frequencies Bond Energy Stretching Frequency Bond [kcal (k)] cm Frequency dependence on atomic masses C-H 100(420) 3000 heavier C-D 2100v decreases atoms 100(420) C-C 83(350) 1200 F requency dependence on bond energies C-C 83(350) 1200 stronger C=C 146(611) bond 1660Dincreases C≡C 200(840) 2200 Frequency decreases with increasing atomic weight Frequency increases with increasing bond energy Chapter 12
Chapter 12 8 Stretching Frequencies • Frequency decreases with increasing atomic weight. • Frequency increases with increasing bond energy. =>
Vibrational modes Nonlinear molecule with n atoms usually has 3n-6 fundamental vibrational modes / symmetric stretching antisymmetric stretching bending(so cIssoiD 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 quantitized No two molecules will give exactly the same IR spectrum(except enantiomers Simple stretching: 1600-3500 cm Complex vibrations: 600-1400 cm-1 called the fingerprint region Chapter 12
Chapter 12 10 Fingerprint of Molecule • Whole-molecule vibrations and bending vibrations are also quantitized. • 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.” =>