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C


   
CAD Computer-aided design.
   
Cam A component that translates or rotates to move a contoured surface past a follower; the contours impose a sequence of motions (potentially complex) on the follower.
   
Carbanion A highly reactive anionic chemical species with an even number of electrons and an unshared pair of electrons on a tetravalent carbon atom.
   
Carbene A highly reactive chemical species containing an electrically neutral, divalent carbon atom with two nonbonding valence electrons; a prototype is CH2.
   
Carbonium ion A highly reactive cationic chemical species with an even number of electrons and an unoccupied orbital on a carbon atom.
   
Carbonyl A chemical moiety consisting of O with a double bond to C. If the C is bonded to N, the resulting structure is termed an amide; if it is bonded to O, it is termed a carboxylic acid or an ester linkage.
   
Carboxylic acid A molecule that includes a C having a double bond to O and a single bond to OH.
   
Catalyst A chemical species or other structure that facilitates a chemical reaction without itself undergoing a permanent change.
Cation A positively charged ion.
   
Classical mechanics Classical mechanics describes a mechanical system as a set of particles (which in a limiting case can form continuous media) having a well-defined geometry at any given time, and undergoing motions determined by applied forces and by the initial positions and velocities of the particles. The forces themselves may have electromagnetic or quantum mechanical origins. Classical statistical mechanics uses the same physical model, but treats the geometry and velocities as uncertain, statistical quantities subject to random thermally-induced fluctuations. Classical mechanics and classical statistical mechanics give a good account of many mechanical properties and behaviors of molecules; but for describing the electronic properties and behaviors of molecules, they are often useless.
   
CMOS An acronym for complementary metal-oxide-semiconductor, as in CMOS transistor and CMOS logic.
   
Col In describing landforms, a pass between two valleys is sometimes termed a col. In describing molecular potential energy functions, this term is commonly used to describe analogous features of the PES; a col is the region around a saddle point having negative curvature along one axis and positive curvature along all orthogonal axes.
   
Compliance The reciprocal of stiffness; in a linear elastic system, displacement equals force times compliance.
   
Configuration space A mathematical space describing the three-dimensional configuration of a system of particles (e.g., atoms in a nanomechanical structure) as a single point; the configuration space for an N particle system has 3N dimensions.
   
Conformation A molecular geometry that differs from other geometries chiefly by rotation about single or triple bonds; distinct conformations (termed con formers) are associated with distinct potential wells. Typical biomolecules and products of organic synthesis can interconvert among many conformations; typical diamondoid structures are locked into a single potential well, and thus lack conformational flexibility.
   
Conjugated A conjugated pi system is one in which pi bonds alternate with single bonds. The resulting electron distribution gives the intervening single bonds partial double-bond character, the pi electrons become delocalized, and the energy of the system is reduced.
   
Conservative In design and analysis, a conservative model or a conservative assumption is one that departs from accuracy in such a way that it reduces the chances of a false-positive assessment of the feasibility of the system in question. Conservative assumptions overestimate problems and underestimate capabilities.
   
Covalent bond A bond formed by sharing a pair of electrons between two atoms.
   
Covalent radius Given a set of N elements that can form covalent single bonds in molecules, with N(N - 1) possible elemental pairings, it has proved possible to define a covalent radius for each element such that the actual bond length between any two elements that form a covalent single bond is roughly equal to the sums of their covalent radii.
   
CPU The central processing unit of a computer, responsible for executing instructions to process information.
   
Cyclic A structure is termed cyclic if its covalent bonds form one or more rings.
   
Cycloaddition A reaction in which two unsaturated molecules (or moieties within a molecule) join, forming a ring.
     

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