Can diamond nanowires grow inside carbon nano\-tubes?

We investigate the possibility of templated growth of diamond nanowires from functionalized diamondoid molecules enclosed in a carbon nanotube (CNT). Our {\em ab initio} density functional theory studies identify suitable candidate molecules and conditions, under which such molecules may fuse to narrow diamond nanowires with C$_8$H$_8$ or C$_7$H$_8$ unit cells inside a CNT. We find that the unique environment inside a narrow carbon nanotube, which can be suitably represented by a cylindrical potential, subjects enclosed molecules to a high pressure, caused by ``capillary'' forces, and orients them in a suitable way favoring fusion and constraining the resulting structure. Based on total energy calculations, we find that fusion of C$_{10}$H$_{16}$ adamantane molecules requires additional energy, whereas fusion of C$_{14}$H$_{18}$(COOH)$_2$ diamantane di-acid molecules in hydrogen atmosphere occurs as an exothermic reaction. Our canonical molecular dynamics calculations at elevated temperatures indicate likely intermediate products occurring during this reaction.