The fact that a molecule vibrates does not in itself insure that the molecule will exhibit an IR spectrum. For a particular vibrational mode to absorb infrared radiation, the vibrational motion associated with that mode must produce a change in the dipole moment of the molecule. HCl, for example, with a center of positive charge at the H atom and a center of negative charge at the Cl atom, has a dipole moment. The magnitude of the dipole moment changes as the HCl bond stretches, so this vibration absorbs IR radiation. We say that the vibration is IR active. The N2 molecule, on the other hand, has no dipole moment. Further, stretching the N-N bond does not produce a change in dipole moment, so the vibration is infrared inactive (i.e., cannot directly absorb IR radiation). It is important to realize that there are many molecules that, although possessing no permanent dipole moment, still undergo vibrations that cause changes in the value of the dipole moment from 0 to some non-zero value. An example is CO2, which has no permanent dipole moment because the individual bond dipoles exactly cancel. However, when CO2 undergoes a bending vibration, its dipole moment changes from zero to some non-zero value. This vibration produces a change in dipole moment and is IR active. These vibrational modes are responsible for the \"greenhouse\" effect in which heat radiated from the earth is absorbed (trapped) by CO2 molecules in the atmosphere. The arrows indicate the directions of motion. Vibrations labeled A and B represent the stretching of the chemical bonds, one in a symmetric (A) fashion, in which both C=O bonds lengthen and contract together (in-phase), and the other in an asymmetric (B) fashion, in which one bond shortens while the other lengthens. The asymmetric stretch (B) is infrared active because there is a change in the molecular dipole moment during this vibration. Solution The fact that a molecule vibrates does not in itself insure that the molecule will exhibit an IR spectrum. For a particular vibrational mode to absorb infrared radiation, the vibrational motion associated with that mode must produce a change in the dipole moment of the molecule. HCl, for example, with a center of positive charge at the H atom and a center of negative charge at the Cl atom, has a dipole moment. The magnitude of the dipole moment changes as the HCl bond stretches, so this vibration absorbs IR radiation. We say that the vibration is IR active. The N2 molecule, on the other hand, has no dipole moment. Further, stretching the N-N bond does not produce a change in dipole moment, so the vibration is infrared inactive (i.e., cannot directly absorb IR radiation). It is important to realize that there are many molecules that, although possessing no permanent dipole moment, still undergo vibrations that cause changes in the value of the dipole moment from 0 to some non-zero value. An example is CO2, which has no permanent dipole moment because the individual bond dipo.

1. The fact that a molecule vibrates does not in itself insure that the molecule will
exhibit an IR spectrum. For a particular vibrational mode to absorb infrared radiation, the
vibrational motion associated with that mode must produce a change in the dipole moment of the
molecule. HCl, for example, with a center of positive charge at the H atom and a center of
negative charge at the Cl atom, has a dipole moment. The magnitude of the dipole moment
changes as the HCl bond stretches, so this vibration absorbs IR radiation. We say that the
vibration is IR active. The N2 molecule, on the other hand, has no dipole moment. Further,
stretching the N-N bond does not produce a change in dipole moment, so the vibration is infrared
inactive (i.e., cannot directly absorb IR radiation). It is important to realize that there are many
molecules that, although possessing no permanent dipole moment, still undergo vibrations that
cause changes in the value of the dipole moment from 0 to some non-zero value. An example is
CO2, which has no permanent dipole moment because the individual bond dipoles exactly
cancel. However, when CO2 undergoes a bending vibration, its dipole moment changes from
zero to some non-zero value. This vibration produces a change in dipole moment and is IR
active. These vibrational modes are responsible for the "greenhouse" effect in which heat
radiated from the earth is absorbed (trapped) by CO2 molecules in the atmosphere. The arrows
indicate the directions of motion. Vibrations labeled A and B represent the stretching of the
chemical bonds, one in a symmetric (A) fashion, in which both C=O bonds lengthen and contract
together (in-phase), and the other in an asymmetric (B) fashion, in which one bond shortens
while the other lengthens. The asymmetric stretch (B) is infrared active because there is a change
in the molecular dipole moment during this vibration.
Solution
The fact that a molecule vibrates does not in itself insure that the molecule will
exhibit an IR spectrum. For a particular vibrational mode to absorb infrared radiation, the
vibrational motion associated with that mode must produce a change in the dipole moment of the
molecule. HCl, for example, with a center of positive charge at the H atom and a center of
negative charge at the Cl atom, has a dipole moment. The magnitude of the dipole moment
changes as the HCl bond stretches, so this vibration absorbs IR radiation. We say that the
vibration is IR active. The N2 molecule, on the other hand, has no dipole moment. Further,
stretching the N-N bond does not produce a change in dipole moment, so the vibration is infrared
inactive (i.e., cannot directly absorb IR radiation). It is important to realize that there are many
molecules that, although possessing no permanent dipole moment, still undergo vibrations that
cause changes in the value of the dipole moment from 0 to some non-zero value. An example is
CO2, which has no permanent dipole moment because the individual bond dipoles exactly
cancel. However, when CO2 undergoes a bending vibration, its dipole moment changes from

2. zero to some non-zero value. This vibration produces a change in dipole moment and is IR
active. These vibrational modes are responsible for the "greenhouse" effect in which heat
radiated from the earth is absorbed (trapped) by CO2 molecules in the atmosphere. The arrows
indicate the directions of motion. Vibrations labeled A and B represent the stretching of the
chemical bonds, one in a symmetric (A) fashion, in which both C=O bonds lengthen and contract
together (in-phase), and the other in an asymmetric (B) fashion, in which one bond shortens
while the other lengthens. The asymmetric stretch (B) is infrared active because there is a change
in the molecular dipole moment during this vibration.