lowest unoccupied molecular orbital (LUMO)
The orbital that can act as the electron acceptor, since it
is the innermost (lowest energy) orbital that has room
to accept an electron.
low spin In any COORDINATIONentity with a par-
ticular dn(1 < n< 9) configuration and a particular
geometry, if the n electrons are distributed so that
they occupy the lowest possible energy levels, the
entity is a low-spin complex. If some of the higher-
energy dorbitals are occupied before all the lower-
energy ones are completely filled, then the entity is a
high-spin complex.
luminescent Refers to the act of emitting light with-
out causing heat (called cold light). Bioluminescence is
the act of producing light by biological organisms, e.g.,
fireflies, luminescent bacteria (Photobacterium phos-
phoreum),and foxfire in the form of Clitocybeilludens
(Omphalotus olerius), Panellus stypticus, and Armil-
laria mellea.The lanthanides are a special group of ele-
ments—cerium (Ce, atomic no. 58) through lutetium
(Lu, atomic no. 71)—within the PERIODIC TABLEthat
have trivalent cations that emit light. When these ele-
ments are absorbed into materials, the materials can
become luminescent after being excited by an electrical
current (electroluminescence) or by absorbing light
(photoluminescence). As the ions relax to their
GROUND STATE, they release light.luster The appearance of a substance in reflected
light.lyase An ENZYMEof EC class 4 that catalyzes the
separation of a bond in a SUBSTRATEmolecule.
See alsoECNOMENCLATURE FOR ENZYMES.lyate ion The anion produced by HYDRONremoval
from a solvent molecule. For example, the hydroxide
ion is the lyate ion of water.Lyman series A series of lines in the hydrogen atom
spectrum that corresponds to transitions between the
GROUND STATE(principal quantum number n= 1) and
successive EXCITED STATEs.lyonium ion The cation produced by hydronation
(PROTONATION) of a solvent molecule. For example,
CH 3 OH 2 +is the lyonium ion of methanol.
See alsoONIUM ION.lyonium ion 169