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D


Dative bond A dipolar bond.
   
Diamondoid As used in this volume, this term describes structures that resemble diamond in a broad sense: strong, stiff structures containing dense, three-dimensional networks of covalent bonds, formed chiefly from first and second row atoms with a valence of three or more. Many of the most useful diamondoid structures will in fact be rich in tetrahedrally coordinated carbon. Diamondoid is used more narrowly elsewhere in the literature.
   
Dipolar bond A covalent bond in which one atom supplies both bonding electrons, and the other atom supplies an empty orbital in which to share them. Also termed a dative bond.
   
Dissociation constant For systems in which ligands of a particular kind bind to a receptor in a solvent there will be a characteristic frequency with which existing ligand-receptor complexes dissociate as a result of thermal excitation, and a characteristic frequency with which empty receptors bind ligands as a result of Brownian encounters, forming new complexes. The frequency of binding is proportional to the concentration of the ligand in solution. The dissociation constant is the magnitude of the ligand concentration at which the probability that the receptor will be found occupied is 1/2.
   
Double bond Two atoms sharing electrons as in a single bond (that is, a sigma bond) may also share electrons in an orbital with a node passing through the two atoms. This adds a second, weaker bonding interaction (a pi bond); the combination is termed a double bond. A twisting motion that forces the nodal plane at one atom to become perpendicular to the nodal plane on the other atom eliminates the (signed) overlap between the atomic orbitals, destroying the pi bond. The energy required to do this creates a large barrier to rotation about the bond (see triple bond).
   
Doublet The electronic state of a molecule having one unpaired spin is termed a doublet (see radical). This term is derived from spectroscopy: an unpaired spin can be either up or down with respect to a magnetic field, and these states have different energy, resulting in field-dependent pairs, or doublets, of spectral lines. (See triplet, singlet.)

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