Direct Observation of Ethylidene (CH₃CH), the Elusive High-Energy Isomer of Ethylene

Abstract

Despite experimental efforts spanning more than 80 years, there has been no direct observation of free ethylidene (CH3CH), the simplest alkyl-substituted carbene. Here, we report that ethylidene is indefinitely stable in the absence of collisions if produced in the triplet ground state at energies below the threshold for intersystem crossing. Near-UV photolysis of gaseous methylketene, or propenal (followed by isomerization to methylketene), leads to CO loss producing triplet ethylidene, which is detected by photoionization mass spectrometry. Electronically excited singlet ethylidene is also produced, rapidly undergoing isomerization by a 1,2-hydrogen atom shift, producing highly vibrationally excited ethylene. The measured product translational energy distributions verify the theoretically calculated enthalpy of formation of triplet ethylidene and are consistent with a singlet–triplet energy gap of approximately 12.5 kJ/mol.

Site-Specific Carbon–Carbon Bond Fission in Photoexcited Propyl Radicals Leads to Isomer-Selective Carbene and Radical Products

“…The ratio of m / e (s) = 28:26 was found to be 2.0 ± 0.4 for the product from photodissociation of 2-propyl radicals at 248 nm. This is identical with the ratio observed for singlet ethylidene produced by 355.1 nm photodissociation of methylketene or 355 nm photodissociation of acrolein . For both propyl radical isomers, we were unable to observe any dependence of the signal levels on input laser polarization (see the Supporting Information), indicating that the angular distributions are isotropic, i.e., the anisotropy parameter (β) = 0 ± 0.2 .…”

“…The pyrolytic radical beam was collimated by two skimmers and crossed by the attenuated 248 nm output from an excimer laser (20 mJ/pulse, 20 ns, 30 Hz, 3 × 5 mm). The product time-of-flight (TOF) distributions were analyzed by a forward convolution method …”

“…The m / e = 15 products are ascribed to nascent methyl radicals (CH 3 ), produced by C–C bond fission. The counterfragment signals at m / e = 28 are assigned to ethylidene (CH 3 CH), which was characterized recently for the first time via photodissociation of methylketene and acrolein . Because of strong coupling between singlet and triplet CH 3 CH states, isomerization to ethylene on the singlet surface after departure of the methyl counterfragment is expected .…”

“…The counterfragment signals at m / e = 28 are assigned to ethylidene (CH 3 CH), which was characterized recently for the first time via photodissociation of methylketene and acrolein . Because of strong coupling between singlet and triplet CH 3 CH states, isomerization to ethylene on the singlet surface after departure of the methyl counterfragment is expected . Although the ionization energy of internally cold ethylene is 10.5 eV, since ethylidene lies ∼300 kJ/mol (3 eV) above ethylene, the high level of vibrational excitation facilitates ionization at 9.9 eV .…”

“…The product time-offlight (TOF) distributions were analyzed by a forward convolution method. 21 The TOF spectra recorded at m/e = 29 for photodissociation of 1-propyl radicals (3s state) are shown in Figure 1, with the corresponding P(E) also illustrated. We sought TOF spectra for the nascent methylene (CH 2 ) counterfragments using 9.9 eV photoionization.…”