Interstitial carbon, C 
, defects in Si exhibit a number of
unexplained features. The C defect in the neutral charge state
of gives rise to two almost degenerate vibrational modes at 920 and
931 cm 
whose 2:1 absorption intensity ratio naturally suggests
a trigonal defect in conflict with uniaxial stress measurements. The
di-carbon, C 
-C , defect is bi-stable, and the energy difference
between its A and B forms is surprisingly small even though the
bonding is very different. In the B-form appropriate to the neutral
charge state, a silicon interstitial is believed to be located near a
bond centered site between two C atoms. This must give rise to
vibrational modes which involve the motion of both C atoms in apparent
conflict with the results of photoluminescence experiments. We use an
ab initio LDF cluster method, AIMPRO, to calculate the structure
and vibrational modes of these defects and find that the ratio of the
absorption intensities of the local modes of C is in reasonable
agreement with experiment even though the structure of the defect is
not trigonal. We also show that modes in the vicinity of those
detected by PL for the B-form of the di-carbon center involve
independent movements of the two C atoms. Finally, the trends in the
relative energies of the A and B-forms in three charge states are
investigated.