Glossary – S
SALC: See Symmetry Adapted Linear Combination.
SEM: See Scanning Electron Microscopy.
Salt Bridge: A glass tube containing an ionic electrolyte which provides electrical connection between two half cells.
Saturated: This applies to carbon chains – they are saturated if there are no multiple bonds – i.e. the chain is saturated with hydrogen.
Scalar Coupling Constant: A measure of the strength of the spin-spin coupling between two specific nuclei. This constant can be measured by the gaps between split lines in NMR spectra.
Scalar Coupling: See Spin-spin coupling.
Scanning Electron Microscopy: (SEM) This is a method for determining the surface structure of a solid by measuring the angle and energies of electrons scattered by the atoms on the surface of a sample.
Schottky Defect: This a defect consisting of a vacancy on a cationic or anionic lattice site. Equal numbers of anionic and cationic vacancies ensure charge neutrality.
Schrodinger Equation: A fundamental postulate of quantum mechanics, which states HΨ = EΨ. i.e. that the application of the hamiltonian operator to the wavefunction of a system will give the energy of that system as the eigenvalue of the function. Thus it relates the wavefunction to the allowed energies of the wavefunction.
Second Law Of Thermodynamics: ΔG = ΔH – TΔS where ΔG is the change in Gibbs’ Free-Energy and ΔS is the change in entropy. The second law of thermodynamics states that for a reaction to be ‘spontaneous‘, ΔG must be negative overall. Another way of stating this is that for a reaction to be spontaneous, the overall entropy increase must be positive.
Secondary: If a carbon centre is described as secondary, it has one functional group, two carbons and one hydrogen bound to it – e.g. a secondary alcohol;
A secondary nitrogen centre is one with two carbons and a hydrogen attached.
Selection Rule: A statement about whether a particular transition is allowed or not.
Semiconductor: A compound which has a conductivity intermediate between that of a metal and an insulator, and whose conductivity increases with temperature.
Semimetal: A compound with a zero band gap between the filled valence band and the empty conduction band.
Shielding Constant: The coefficient of proportionality between the applied magnetic field and the additional local fields generated by interaction of the applied field with electrons. The induced fields oppose the applied fields, hence the name shielding constant.
Shielding Parameter, σ: This is a measure of the difference between the effective nuclear charge and the actual nuclear charge (Zeff = Z – σ).
Sigma Bond: A bond between two atoms that consists of two electrons occupying a bonding molecular orbital with cylindrical symmetry about the internuclear axis. (Note this means that there are no nodes in the wavefunction as you rotate about the internuclear axis.)
Single Bond: See σ bond.
Singlet State: This a state which has only paired electron spins.
Sodalite Cage: One of the basic structural units which make up the extended structure of zeolites.
Soft: See hard. An element will be considered ‘soft‘ if it is large and polarisable e.g. S, Hg, Cu.
Solubility: The amount of a substance that will dissolve per unit volume of a particular solvent.
Solute: A substance dissolved in a solvent. Specifically, in a liquid-liquid mixture, the name given to the minor component.
Solvation Enthalpy: ΔH when ions are solvated – that is, taken from gaseous state and surrounded by solvent.
Solvation: The reaction of a molecule with the solvent molecules when it enters a solution.
Solvent leveling: The ability of a solvent to limit the range of attainable pH values to the pKw value of the solvent, where Kw is the solvent autoprotolysis constant.
Solvent parameters: A set of thermodynamic quantities, the donor number and acceptor number, which quantify the ability of a solvent to act as an acid or a base.
Solvent: A substance used to dissolve another. Specifically, in liquid-liquid mixtures, the name given to the major component.
Spectroscopy: The study of the interaction of electromagnetic radiation with matter.
Spin angular momentum: A type of angular momentum possessed by some nuclei, characterised by a quantum number I. The value of I is characteristic of a particular isotope.
Spin relaxation: In general, the reversion of nuclear spins that have been somehow excited back to their equilibrium state.
Spin-Lattice Relaxation: A specific case of spin relaxation in which the relaxation is permitted due to the fluctuating magnetic fields that arise from the random motion of molecules containing magnetic nuclei.
Spin-Orbit Coupling: The interaction of the spin angular momentum and orbital angular momentum, of individual electrons or of the system as a whole.
Spin-Spin Coupling (aka scalar coupling): Interactions between the nuclear spins of nearby nuclei, that give rise to characteristic splittings of resonance lines in NMR spectra.
Spinels: These are a range of metal oxides of formula AB2O4.
Spiro: When two rings are fused through the same atom.
Spontaneous: A spontaneous process is defined as one which will either occur immediately at room temperature, or which, once initiated by heating, will sustain itself until the start materials are exhausted.
Standard Gibbs energy of reaction: The difference between the standard molar Gibbs energies of the products and the reactants.
Standard Hydrogen Electrode: This is the electrode made up of the H+/H2 redox couple under standard conditions, against which all standard reduction potentials are measured.
Standard Reduction Potential: The reduction potential for a redox couple when measured in a cell with the standard hydrogen electrode and with all species having unit activities.
Standard conditions: Standard conditions are 298 K, 1 atmosphere’s pressure, all solutions at unit activity and all elements in their reference states.
Starting Materials: The molecules at the start of an organic reaction, which have reagents applied to them, and become products.
State function: A function whose value depends solely upon the current state of the system, and is totally independent of how that state was reached. As such, it is determined by the variables that determine the state of the system (eg pressure, temperature, etc.)
Step-Wise: Applied to a reaction mechanism, if it is step-wise, the steps occur sequentially. See also Concerted.
Stereocentre: See chiral.
Stereochemistry: Chemistry involving stereoisomers, ie: the shape dependent chemistry of that molecule.
Stereoisomer: Isomers that are related to each other by either rotations of double bonds, or alterations of chiral centres are stereoisomers.
Stereoselectivity: If a reaction displays stereoselectivity, it will create a specific stereoisomer in greater yield than other possible stereoisomers.
Steric Hindrance: Physical hindrance to something.
Steric Relief: Energetic stabilisation afforded when steric strain is relieved during the course of a reaction.
Steric Strain: A molecule will be subject to steric strain if there are large bulky pieces of molecule forced together – this is an unfavourable state to be in.
Sterics: General physical properties of molecules.
Stoichiometric Coefficient: The coefficient that occurs before a species in a chemical equation to ensure that the equation balances.
Stoichiometric Point: In the titration of an acid against a base, the stoichiometric point is that when equal volumes of acid and base have been mixed.
Stoichiometry: The number of species of each type of element in the formula for a compound or reaction.
Strong Acid: An acid with pKa < 0.
Strong Field: This is when the effect of the field created by the ligand field is a large separation in energy of the d-orbitals.
Structure Maps: These show the structure adopted by a range of compounds of a given stoichiometry as a function of the degree of ionic character and the hardness of the spheres representing the species involved.
Substituent: A group on a molecule.
Substitution: Replacing one thing with another – see the substitution chapter.
Superconductor: A compound which conducts with zero resistivity. This often occurs only below a certain temperature, the Superconductivity transition temperature, Tc.
Supercritical fluid: The phase of matter that exists above the critical temperature.
Surface Acid: A compound which has groups at its surface which can act as acids.
Surroundings: The rest of the universe, excepting the system.
Symmetry Adapted Linear Combination: This is a combination of molecular or atomic orbitals, which allows for the fact that only orbitals of the same symmetry may overlap.
Symmetry Element: The point, line or plane about which a symmetry operation is carried out.
Symmetry Operation: Any operation that can be carried out upon an object that leaves it looking unchanged. Common examples are rotations, reflections and inversions.
Synergic Bond: This is a bond formed by a ligand to a metal ion which has a σ-bond from the ligand to the metal ion, and the back donation of electron density from the metal to the π* orbitals on the ligand.
Systematic Absences: The absence of x-ray scattering from planes of certain symmetry due to destructive interference of x-rays scattered from different atoms within those planes is known as systematic absences, and these can be used to determine the crystal type.
System: The entire physical object(s) or area in which we are interested. Systems can be classified as open, closed, or isolated.
s Orbital: An electron with no angular momentum is contained in a (spherical) s orbital.