black. It is used as aÛotation agent,
solderÛux, sizing compound, and in
lacquers and plasticizers. It is also
used to provide ‘grip’ to violinists’
bows (when it may be called
colophony) and dancers’ and boxers’
shoes.
rotamer One of a set of *conforma-
tional isomers that differ from each
other by restricted rotation about
one or more single bonds.
rotational spectroscopy The
spectroscopic study of the rotational
motion of molecules. Rotational spec-
troscopy gives information about in-
teratomic distances. The transitions
between different rotational energy
levels in molecules correspond to the
microwave and far-infrared regions
of the electromagnetic spectrum. It is
only possible for there to be transi-
tions between rotational energy lev-
els in pure rotational spectra if the
molecule has a permanent dipole
moment. In the near-infrared region
rotational transitions are superim-
posed on vibrational transitions, re-
sulting in a vibrational–rotational
spectrum. This type of spectrum is
considerably more complicated than
a purely rotational spectrum. See vi-
brational spectroscopy.rotation group A group formed by
the set of all rotations about a point.
The rotation group is associated with
the angular momentum of a system
and is important in the theory of the
rotational motion of molecules. The
group representations of the rotation
group are closely associated with the
quantum theory of angular momen-
tum.rotaxane A type of compound that
has a dumbbell-shaped molecule
with a cyclic molecule around its
axis. The dumbbell has a chain with
large groups at each end, these being
large enough to trap the ring. Thereis no formal chemical bonding be-
tween the dumbbell and the ring. Ro-
taxanes are examples of compounds
with *mechanical bonding. A num-
ber of natural peptide rotaxanes have
been identiÜed. Synthesis of new ro-
taxanes is a matter of interest be-
cause of their possible use as
‘molecular machines’ in nanotech-
nology (e.g. as molecular switches or
information storage units).
RRK theorySee rice–ramsperger–
kassel theory.RRKM theory (Rice–Ramsperger–
Kassel–Marcus theory) A theory of
unimolecular chemical reactions in
which the *Rice–Ramsperger–Kassel
theory was improved by the US
chemists O. K. Rice and R. A. Marcus
in 1950; it was further improved sub-
sequently in several papers by Mar-
cus and associates by taking into
account the individual vibrational
frequencies, rotations, and zero-point
energies of the energized species and
activated complexes taking part. The
RRKM theory is very successful in ex-
plaining the results of experiments
for unimolecular reactions for a vari-
ety of reactions.R–S conventionSee absolute con-
figuration.
A- Information about IUPAC nomenclature
rubberA polymeric substance ob-
tained from the sap of the tree Hevea
brasiliensis. Crude natural rubber is
obtained by coagulating and drying
the sap (latex), and is then modi-
Üed by vulcanization and com-
pounding withÜllers. It is a polymer
of isoprene containing the unit
–CH 2 C(CH 3 ):CHCH 2 –. Various syn-
thetic rubbers can also be made.
See neoprene; nitrile rubber; sili-
cones.
rubidiumSymbol Rb. A soft silvery-rotamer 466r