Chapter 12 Infrared Spectroscopy and Mass Spectrometry.ppt

Chapter 12 Infrared Spectroscopy and Mass Spectrometry Introduction Spectroscopy is an analytical technique which helps determine structure. It destroys little or no sample. The amount of light absorbed by the sample is measured as wavelength is varied.  = Types of Spectroscopy Infrared (IR) spectroscopy measures the bond vibration frequencies in a molecule and is used to determine the functional group. Mass spectrometry (MS) fragments the molecule and measures the masses. Nuclear magnetic resonance (NMR) spectroscopy detects signals from hydrogen atoms and can be used to distinguish isomers. Ultraviolet (UV) spectroscopy uses electron transitions to determine bonding patterns. = Electromagnetic Spectrum Examples: X rays, microwaves, radio waves, visible light, IR, and UV. Frequency and wavelength are inversely proportional. c = ln, where c is the speed of light. Energy per photon = hn, where h is Planck’s constant. = The Spectrum and Molecular Effects 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. 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. 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.”