Libration (molecule)
Encyclopedia
Libration is a type of reciprocating motion
in which an object with a nearly fixed orientation repeatedly rotates
slightly back and forth. In physics
and chemistry
, a molecule
(or other group of atoms) can undergo libration if it is subject to external forces or constraints that restrict its orientation.
For example, in liquid water
, any given water molecule is attracted
to neighboring molecules, so that it has a preferred orientation and cannot freely rotate. (Of course, over time, the neighboring molecules move around and the preferred orientation changes.) However, it can undergo librational motions, which are measureable in an infrared absorption spectrumand contribute to motional narrowing
of other peaks, for instance the OH stretch.
Another example is a molecular crystal: Each molecular unit has a preferred orientation due to interactions with the nearby molecules, but they have librational modes corresponding to small rotations about this preferred orientation.
Reciprocating motion
Reciprocating motion, also called reciprocation, is a repetitive up-and-down or back-and-forth motion. It is found in a wide range of mechanisms, including reciprocating engines and pumps. The two opposite motions that comprise a single reciprocation cycle are called strokes...
in which an object with a nearly fixed orientation repeatedly rotates
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...
slightly back and forth. In physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
and chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
, a molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
(or other group of atoms) can undergo libration if it is subject to external forces or constraints that restrict its orientation.
For example, in liquid water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
, any given water molecule is attracted
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
to neighboring molecules, so that it has a preferred orientation and cannot freely rotate. (Of course, over time, the neighboring molecules move around and the preferred orientation changes.) However, it can undergo librational motions, which are measureable in an infrared absorption spectrumand contribute to motional narrowing
Motional narrowing
In physics and chemistry, motional narrowing is a phenomenon where a certain resonant frequency has a smaller linewidth than might be expected, due to motion in an inhomogeneous system.-Example: NMR spectroscopy:A common example is NMR...
of other peaks, for instance the OH stretch.
Another example is a molecular crystal: Each molecular unit has a preferred orientation due to interactions with the nearby molecules, but they have librational modes corresponding to small rotations about this preferred orientation.