Glossary – E
E (Entgegen): “E” stands for “entgegen” which is German for “opposite”. This term is used to describe the geometry of species (for example those attached to a double bond) if they are on opposite sides – i.e. trans.
EXAFS: See Extended X-ray Absorption Fine Structure.
Eclipsed: A term used in describing conformations. This means that if viewed as a Newman projection, the position of the atoms at the front obscure the positions of those at the rear completely.
Eigenfunction: If the application of an operator to a function returns the original function multiplied by some constant, then the function is termed an eigenfunction of that operator.
Eigenvalue: If the application of an operator to a function gives back the original function multiplied by some constant, then this constant is known as the eigenvalue of that function. In quantum mechanics, eigenvalues often correspond to observables.
Elastic Collision: One in which no energy is lost to the surroundings – the total energy of the colliding bodies remains the same before and after the collision.
Electric Dipole: Produced in a molecule when the centers of positive and negative charge do not coincide.
Electrochemical Series: The list of compounds when arranged in order of their standard reduction potentials, from most oxidizing to most reducing.
Electromagnetic Radiation: A wave with both an electric and a magnetic component, at right angles to each other. The electric field will interact with any charged particles, the magnetic field will only interact with charged particles that are motion.
Electron Affinity: The energy change accompanying the addition of an electron to a species.
Electron Donating: An electron donating group is one which through either mesomeric or inductive mechanisms forces electron density away from itself and towards other areas.
Electron Shielding: This is the reduction of the of the nuclear charge experienced by an electron due to the presence of electrons closer to the nucleus.
Electron Transfer: A reaction involving the transfer of an electron from one complex ion to another.
Electron Withdrawing: An electron withdrawing group is one which through either mesomeric or inductive mechanisms brings electron density towards itself and away from other areas.
Electronegativity: The ability of an atom to inductively pull electrons towards itself. The more electronegative an atom, the tighter it pulls the electrons. Commonly measured on the Pauling scale, the higher the electronegativity, the more strongly an atom attracts electrons. Note electronegativity is a characteristic property of a given atom.
Electrophilic: Literally: “electron loving”. This describes a species, and is a useful way to determine where a particular reagent might attack. i.e. electrophilic species will attack areas of high electron density.
Electropositivity: This is the opposite of electronegativity. The more electropositive an atom is, the less tightly it holds onto electrons.
Elimination: A class of reaction. An elimination reaction is essentially an addition reaction in reverse. e.g. one parent molecule splits into two or more molecules. Normally, the smaller fraction is said to have been eliminated.
Emission Spectroscopy: Generally, spectroscopy that relies upon exciting a sample and then measuring the radiation emitted as the sample falls down to lower energy levels.
Empirical: Based upon experimental observation, not calculated from a theoretical basis.
Enantiomer: An enantiomer is one of a pair of non-superimposable mirror image molecules. Two molecules are enantiomers if they are mirror images of each other that cannot be superimposed by any rotation or translation.
Endothermic: A reaction or process that is endothermic takes in heat from the surroundings.
Energy Term: In spectroscopy, energy levels are commonly referred to as their energy term, which is the energy expressed as a wavenumber. It is derived very simply from the energy, by division by hc.
Energy: The capacity of a system to do work.
Enolate: A deprotonated enol.
Enol: Molecule/functional group as shown here;
Generally only observed as a minor tautomer of a ketone, in keto-enol tautomerism.
Entgegen: see “E”
Enthalpy: A thermodynamic property of a system given the symbol H and defined as H = U + pV (U the internal energy, p the pressure and V the volume). It is a state function. It is a State Function
Entropy: A thermodynamic quantity (S), it is a measure of disorder of a system, i.e. the higher the disorder, the higher the entropy value.
Epimerism: A type of isomerism shown by substances which contain several asymmetric centres, but differ in the configuration of only one.
Eqm: Short for equilibrium.
Equatorial: With reference to planes of molecules, equatorial applies to any substituent that lies equatorially (or approximately so) in the plane of the molecule.
Equilibrium Constant: For any reversible reaction, the equilibrium constant is equal to the product of the concentrations of products divided by the product of the concentrations of reactants.
Equilibrium: A point in a chemical reaction at which there is no driving force in either direction; the point of minimum Gibbs energy for a process.
Equipartition theorem: States that the average energy associated with a degree of freedom is �kT.
Ester: A carbonyl containing compound – see here for picture.
Ether: A molecule of the generic type R-O-R, where R is a carbon chain of some sort.
Et: Short for ethyl – denotes a C2H5 group.
Excited State: This is a state where one or more electrons have been promoted from orbitals higher than those they occupy in the ground state.
Exothermic: A reaction which is exothermic releases heat.
Extended X-ray Absorption Fine Structure (EXAFS): This is a method whereby the band structure of a material may be determined by recording the spectrum from the excitations of core electrons to vacant bands.
Extrinsic Defect: This is a defect in a solid brought about by the introduction of species foreign to the host lattice.
Extrinsic Semiconductor: A semiconductor whose conduction behaviour is brought about by the introduction of extra electrons or positive holes by dopant species.