Molecular Vibrations A covalent bond between two atoms can be envisaged as a spring holding them together. If the bond is compressed,there is a restoring force which pushes the atoms apart,back to the equilibrium bond length. Similarly,if the bond is stretched,there is a restoring force that forces the atoms back closer together,again restoring the equilibrium bond length. O/OOOO题O OTN The frequency of the stretching vibration depends on two factors: (1)The mass of the atoms (2)The stiffness of the bond Heavier atoms vibrate more slowly than lighter ones,so a C-D bond will vibrate at a lower frequency than a C-H bond. Stronger bonds are stiffer than weaker bonds,and therefore require more force to stretch or compress them. Thus,stronger bonds generally vibrate faster than weaker bonds. So O-H bonds which are stronger than C-H bonds vibrate at higher frequencies. Table 12-1(SLIDE) The IR spectra of even simple molecules contain many different absorptions (not just one for each bond). These absorptions result from not only the stretching of the molecule as a whole,but also from bending vibrations. In general for a non-linear molecule containing n atoms,there are 3n-6 fundamental vibrational modes. Ch12 IR and MS Page4 Molecular Vibrations A covalent bond between two atoms can be envisaged as a spring holding them together. If the bond is compressed, there is a restoring force which pushes the atoms apart, back to the equilibrium bond length. Similarly, if the bond is stretched, there is a restoring force that forces the atoms back closer together, again restoring the equilibrium bond length. The frequency of the stretching vibration depends on two factors: (1) The mass of the atoms (2) The stiffness of the bond Heavier atoms vibrate more slowly than lighter ones, so a C-D bond will vibrate at a lower frequency than a C-H bond. Stronger bonds are stiffer than weaker bonds, and therefore require more force to stretch or compress them. Thus, stronger bonds generally vibrate faster than weaker bonds. So O-H bonds which are stronger than C-H bonds vibrate at higher frequencies. Table 12-1 (SLIDE) The IR spectra of even simple molecules contain many different absorptions (not just one for each bond). These absorptions result from not only the stretching of the molecule as a whole, but also from bending vibrations. In general for a non-linear molecule containing n atoms, there are 3n-6 fundamental vibrational modes. Ch12 IR and MS Page4