Title: Molecular vibrations Content: Molecular vibrations are one of three different kinds of motion for molecules that include translational motion (when the whole molecule goes in the same direction) and rotational motion (when the molecule spins like a top) A vibrational motion for a molecule is when the bonds between atoms within a molecule move. Think of the atoms as round balls that are attached by a spring that can stretch back and forth. An example of this motion is "stretching", the simplest example of a vibration for a molecule and occurs between just two atoms. Some examples of molecules that are like this include hydrogen H2, nitrogen N2 and oxygen O2 == Types of vibrations == If the molecule has more than two atoms, then things get more complicated. Suppose just one more atom is added so now there are three atoms like in water H2O where the two hydrogen atoms are both attached to the central oxygen atom. Remember with hydrogen there was one kind of stretching, but in water there are two kinds of stretching and four other kinds of vibration called bending vibrations as shown below. The atoms in a CH2 group or molecules like water can vibrate in six different ways: symmetric and antisymmetric stretching, scissoring, rocking, wagging and twisting:  \\!, where n is a quantum number or "energy level" that can take values of 0, 1, 2 ... The statement that energy levels can only go up or down one level at a time is known as a selection rule which states that the only allowed transitions between energy levels are: :\Delta n = \pm 1 where \Delta n is the energy transition.McQuarrie, D. A.; Simon, J. D.; Physical Chemistry A Molecular Approach,Anne McGuire, University Science Books, 1997. pg.166-167. == Applications of vibrational motions == When light of a certain frequency hits a molecule that has a vibration whose motion corresponds to the same frequency then the light gets absorbed into the molecule and the energy from the light causes the bonds to move in that specific vibrational motion. By checking for light that gets absorbed, scientists can tell if a certain kind of molecular bond is there and match it against a list of molecules that have that bond. However, some molecules like helium and argon have only one atom and don't have any bonds. This means that they won't absorb any light in the same way a molecule with more than one atom can. The specific fields of chemistry that use molecular vibrations in their studies include infrared spectroscopy (IR) and Raman spectroscopy (Raman) with IR being more widely used and having three subfields of its own. These subfields are known as Near IR, Mid IR and Far IR spectroscopy. The following is a general list of these fields and real world applications Near IR: quantitative determination of species such as proteins, fats, low low-molecular-weight hydrocarbons and water. Further use is obtained in the agricultural products, food, petroleum and chemical industries Mid IR:The most popular of the IR fields, is used in determining structure of organic and biochemical compounds. Far IR: this field is less popular though it has found uses in inorganic studies Skoog, D. A.; Holler, F. J.; Crouch, S. R. Principles of Instrumental Analysis, 6th ed.; Thomson Brooks/Cole: Belmont, CA, 2007. pg.430-495. Raman: Is used for the qualitative and quantitative study of inorganic, organic and biological systems often as a complimentary technique to IR.Grasselli, J. G.; Bulkins, B. J.; Analytical Raman Spectroscopy, eds. New York: Wiley, 1991.