In comparing the IRCs for the 2 → 9 and 2 → 11 rearrangements (Figure ), it is evident that the former reaction involves a much lower energy barrier of 14.8 kcal mol -1 , favoring the reaction to 9; this barrier can readily be surpassed by irradiation at 627 nm (45.6 kcal mol -1 ), leading to 9. Still, QMT reactivity was not experimentally observed for 2, even though the computations indicate a very short QMT half-life for the reaction to 9 (7.1x10 -9 h) and no QMT for the reaction to 11 (> 8x10 11 h). As noted above, we reasoned that 2 is stabilized through a complex with N2 that is trapped (from precursor 6) in the same matrix site; the effects on the spectroscopy for such complexes, which have been reported before, are small. Hence, we needed a system that has a lower and thinner barrier and would show QMT behavior in the right time interval. We turned our attention to hitherto also unreported carbene 3, which is an isomer of 1 that may show the right reactivity. According to our UB3LYP/6-31G(d) computations, the half-life for the C-H insertion reaction of 3 to 4 is 4 x 10 -9 h while the halflife for the competing [1,2]-H shift to 12 is 3 x 10 -4 h. These values suggest that the C-H insertion occurs much more readily than the [1,2]-H shift, thus favoring the formation of 4 as the dominant product in the protio-case. Note that the extremely short half-lives of 3 in either reaction means that the free carbene cannot be observed directly under our matrix isolation conditions, and that we must rely on product analysis. To generate 3, we initially irradiated the precursor diazirine (13) at 365 nm, leading to its expected and wellknown rearrangement into the diazo form, as observed in similar molecules. As this intermediate absorbs at the same wavelength (254 nm) as the carbene (as judged from the computed UV spectrum), this route was abandoned. Instead, we generated 3 via high vacuum flash pyrolysis (HVFP) -which also alleviates complexation with matrix-trapped N2-and recorded the typical carbene follow-up reactions leading to products 4 and 12, whose ratio depending on isotopic substitution should then reveal whether ICS does occur.