  The growth of single wall carbon nanotubes (SWCNT) inside host SWCNTs remains
a compelling alternative to the conventional catalyst induced growth processes.
It not only provides a catalyst free process but the ability to control the
constituents of the inner tube if appropriate starting molecules are used. We
report herein the growth of inner SWCNTs from $^{13}$C labeled toluene and
natural carbon C$_{60}$. The latter molecule is essentially a stopper which
acts to retain the smaller toluene. The Raman spectrum of the inner nanotubes
is anomalous as it contains a highly isotope shifted "tail", which cannot be
explained by assuming a homogeneous distribution of the isotopes.
{\color{black}Semi-empirical} calculations of the Raman modes indicate that
this unsual effect is explicable if small clusters of $^{13}$C are assumed.
This indicates the absence of carbon diffusion during the inner tube growth.
When combined with appropriate molecular recognition, this may enable a
molecular engineering of the atomic and isotope composition of the inner tubes.
