HOMO: See Highest Occupied Molecular Orbital.

HSAB: See Hard and Soft Acids and Bases theory.

Half angle: In a right cone, the angle between the edge of the cone and its perpendicular axis.

Half-Reaction: The reaction describing the reduction of a given species and balanced by electrons, protons, hydroxide ions and water molecules.

Half-cell: An electric cell, from which reduction potentials may be measured, is considered to be made up of two half cells, one for each of the redox couples.

Hamiltonian: The operator that gives the total energy of a system when applied to the wavefunction. This is a mathematical expression describing all the energetic interactions in a given system. As such, it is the sum of the operators for the kinetic energy and the potential energy.

Hard and Soft Acids and Bases theory: Hard acids are those which form strong bonds to first row elements, and soft acids form strong bonds to lower group elements. Hard bases react with hard acids, and soft bases react with soft acids.

Hard: Part of the hard/soft classification system for elements (and to a certain extent, functional groups) – an element will be considered ‘hard’ if it is small and non-polarisable e.g. O, F and N. Hard elements will have an affinity for other hard elements, or functional groups although these classifications are really only guidelines.

Harmonic motion: Motion in which the moving body experiences a force towards some position that is proportional to its displacement from that position.

Harmonic oscillator: A system that oscillates within a parabolic potential energy curve.

Hartree-Fock Orbitals: These are the variationally optimized, lowest energy orbitals for a given system. These depend on the set of basis orbitals.

Heat: Transfer of energy occurs in the form of heat when the transfer involves a disorderly or chaotic motion of particles.

Hess’ Law: States that the standard enthalpy of an overall reaction is equal to the sum of the standard enthalpies of the processes into which the reaction may be divided. An alternative statement is that the standard enthalpy change accompanying a reaction is independent of the route taken by the reaction. The law arises because enthalpy is a state function.

Hetero-aromatic: An aromatic ring containing a heteroatom – e.g. pyridine, furan. See aromatic.

Heteroatom: In the context of organic chemistry, a heteroatom is an atom other than carbon.

Heterogeneous: In a different state to.

Hexagonal Close Packed Array: An array of close packed spheres which stack in hexagonal unit cells.

Highest Occupied Molecular Orbital: The molecular orbital which contains an electron, and whose energy is the highest of the occupies orbitals.

Homogeneous Magnetic Field: A magnetic field that has the same strength everywhere in space.

Homogeneous: In the same state as.

Huckel’s Rule: This is a rule for determining whether or not a compound is aromatic by counting the number of electrons in a cyclic system. If the number is equal to 4n + 2 (where n is an integer), the molecule is aromatic.

Hund’s Rule: This states that when there is a choice of occupation of orbitals of the same energy, the electrons occupy those orbitals so as to maximize the number of electrons with parallel spin.

Hybrid Orbital: Theoretical orbitals that do not really exist, but are the result of a mathematical operation on the formation of molecular orbitals from atomic orbitals. The hybrids are generally called sp (when carbon triple bonds – forms a linear shape), sp2 (when carbon double bonds – forms a trigonal planar shape), sp3 (when carbon single bonds – a tetrahedral shape).

Hybridization: The process of constructive and destructive overlap between different regions of orbital wavefunctions that causes them to produce equivalent hybrid orbitals of a new shape and size.

Hydration Energy: The reaction enthalpy for the dissolution of a compound into aqueous solution.

Hydride: The H ion.

Hydrogen Bonding: Hydrogen bonding occurs when a hydrogen atom is attached to an electronegative atom (e.g. N,O,F). The bond between the two is clearly therefore polarised, and another atom with a lone pair can form a bond to this positively charged hydrogen atom. When this occurs between two separate molecules, this is called intermolecular hydrogen bonding.

Hydrogen Bond: This is a largely electrostatic interaction described as “the interaction between an H atom bound to an electronegative element and another electronegative element which has lone pairs of electrons”.

Hydrogenation: A method of reduction whereby hydrogen is added to a substrate directly from gaseous hydrogen.

Hydrolysis: Reaction with water.

Hyperconjugation: The interaction of alkyl groups with unsaturated systems, it indicates C-H conjugation. This effect was introduced to account for the ease of certain elimination reactions.

Hypervalent: An atom which has more bonds, and hence more electrons in its valence shell, than would be allowed by the octet rule is said to be hypervalent.