  We analyze the vibrational properties of diamondoid compounds via Raman
spectroscopy. The compounds are interconnected with carbon-carbon single bonds
that exhibit exceptionally large bond lengths up to 1.71 A. Attractive
dispersion interactions caused by well-aligned intramolecular H--H contact
surfaces determine the overall structures of the diamondoid derivatives. The
strong van-der-Waals interactions alter the vibrational properties of the
compounds in comparison to pristine diamondoids. Supported by
dispersion-corrected density functional theory (DFT) computations, we analyze
and explain their experimental Raman spectra with respect to unfunctionalized
diamondoids. We find a new set of dispersion-induced vibrational modes
comprising characteristic CH/CH$_{2}$ vibrations with exceptionally high
energies. Further, we find structure-induced dimer modes that are indicative of
the size of the dimers.
