2-Naphthyl(carbomethoxy)carbene Revisited: Combination of Ultrafast UV−vis and Infrared Spectroscopic Study

Ultrafast laser flash photolysis (310 nm) of methyl 2-napthyldiazoacetate (2-NpCN₂CO₂CH₃) in acetonitrile or cyclohexane produces a diazo excited state which absorbs broadly in the visible region (τ = 300 fs). The decay of the excited diazo compound is accompanied by growth of the vibrationally excited singlet 2-naphthyl(carbomethoxy)carbene (¹NpCCO₂CH₃). The singlet carbene absorbs at 360 and 470 nm. In acetonitrile these bands do not decay over 3 ns, but they do decay by ∼50% of their original intensity in cyclohexane in 3 ns. It is concluded that ¹NpCCO₂CH₃ has a singlet ground state in acetonitrile but a triplet ground state in cyclohexane. Related experiments reveal a singlet ground state in Freon-113 and chloroform. This interpretation is supported by ultrafast IR spectroscopy, which confirms that only ¹NpCCO₂CH₃ is formed within 50 ps of the laser pulse rather than a singlet−triplet equilibrium mixture of carbene. The planar singlet relaxes to the preferred perpendicular singlet over a few tens of picoseconds, as evidenced by a red shift of the carbonyl stretching vibration. Although our data agrees with previous studies, its interpretation is somewhat altered.