Singlet alkyl carbenes are highly unstable and there is only one spectroscopic report on such a carbene, namely adamantylidene (1, Figure ). The majority of research into these carbenes stems from computational and theoretical studies. Beyond methylene (which has a triplet ground state), singlet ethylidene represents the simplest alkyl carbene, which, however, undergoes a very facile [1,2]H-shift; despite multiple attempts, the direct spectroscopic characterization of ethylidene has remained elusive. Analogs such as 2,2,2-trifluoroethylidene have been generated and characterized in noble gas matrices at low temperatures. Carbenes including norbornen-7-ylidene, cyclobutylidene, and tricyclooct-8-ylidene 15 have all been investigated theoretically. Such so-called foiled carbenes display a combination of through-space interactions, non-classical bonding, and hyperconjugative interaction. Apart from fast 1,2-shifts, cyclic singlet carbenes may also undergo ring expansion by heavy-atom QMT through C-C or C-H bond insertion reactions. Heavyatom QMT is less common, owing to the mass dependence of the tunneling rate, which typically results in significantly extended tunneling half-lives unless the reaction barrier is narrow. Examples of reactions involving carbon tunneling include automerizations of cyclobutadiene, 1,5-dimethylsemibullvalene, and the ring closure of cyclopentane-1,3-diyl. As QMT half-lives depend profoundly on particle mass but even more so on barrier height and width, computational predictions require a suitable potential energy hypersurface (PES). Canonical Variational Transition State Theory (CVT) corrected for small-curvature tunneling (SCT) in conjunction with density functional theory (DFT) provides results in reasonably good agreement with experiment. Carbene 1 stands alone as the first and only alkyl carbene to be isolated and characterized spectroscopically by the matrix isolation technique by Bally and Platz et al. In 2014, Kozuch et Bally and Platz et al. observed the rearrangement of 1 to 4 upon UV irradiation, they (unintentionally) did not wait long enough to also observe its reactivity in the dark. As such, we sought to experimentally examine the predicted QMT ring closure reaction of 1 to 4 in the dark, and to take this QMT reactivity as a model for possibly observing ICS with a carbene. As we will outline, 1 proved not to be suitable for this purpose, even though it does show the predicted QMT reactivity, because alternative rearrangements pathways are not competitive. We moved on to pentacyclo[5.4.0.0 2.6 .0 3.10 .0 ]undecanylidene (PCU-carbene) (2), which constitutes only the second spectroscopically characterized alkyl carbene. Surprisingly, 2 does not display observable QMT reactivity because it is unexpectedly stable under our conditions, most likely due to stabilization through a carbon-nitrogen complex, which hinders QMT. We finally arrived at protoadamantylidene (3), an isomer of 1 that has a rather short QMT half-life, but does demonstrate H/D-ICS in competing QMT reactions. Adamantane diazirine ( ) and pentacyclo[5.4.0.0 2.6 .0 3.10 .0 5.9 ]undecan-8-diazirine (PCU diazirine, 6, Figure ) were synthesized according to literature procedures from the corresponding ketones 7 and 8. Due to their high volatility 5 and 6 could easily be evaporated onto the cold matrix window using an excess of argon as the host gas.