Ralf I. Kaiser

A combined crossed molecular beam and ab initio investigation of C2 and C3 elementary reactions with unsaturated hydrocarbons—pathways to hydrogen deficient hydrocarbon radicals in combustion flames

Crossed molecular beam experiments on dicarbon and tricarbon reactions with unsaturated hydrocarbons acetylene, methylacetylene, and ethylene were performed to investigate the dynamics of channels leading to hydrogen-deficient hydrocarbon radicals. In the light of the results of new ab initio calculations, the experimental data suggest that these reactions are governed by an initial addition of C2/C3 to the pi molecular orbitals forming highly unsaturated cyclic structures. These intermediates are connected via various transition states and are suggested to ring open to chain isomers which decompose predominantly by displacement of atomic hydrogen, forming C4H, C5H, HCCCCCH2, HCCCCCCH3, H2CCCCH and H2CCCCCH. The C2(1 sigma g+) + C2H4 reaction has no entrance barrier and the channel leading to the H2CCCCH product is strongly exothermic. This is in strong contrast with the C3(1 sigma g+) + C2H4 reaction as this is characterized by a 26.4 kJ mol-1 threshold to form a HCCCCCH2 isomer. Analogous to the behavior with ethylene, preliminary results on the reactions of C2 and C3 with C2H2 and CH3CCH showed the H-displacement channels of these systems to share many similarities such as the absence/presence of an entrance barrier and the reaction mechanism. The explicit identification of the C2/C3 vs. hydrogen displacement demonstrates that hydrogen-deficient hydrocarbon radicals can be formed easily in environments like those of combustion processes. Our work is a first step towards a systematic database of the intermediates and the reaction products which are involved in this important class of reactions. These findings should be included in future models of PAH and soot formation in combustion flames.

Fourier transform millimeter-wave spectroscopy of the HCS radical in the 2A′ ground electronic state

The 101–000 rotational transition of the HCS radical in the X 2A′ ground electronic state has been observed with a Fourier transform millimeter-wave spectrometer in combination with a pulsed discharge nozzle. The radical is produced by discharging a mixture of CH4 and H2S diluted in Ar. Six fine and hyperfine components are detected, and the effective rotational constant, spin–rotation interaction constant, and hyperfine interaction constants are determined accurately. The Fermi contact term of the hydrogen nucleus is found to be much smaller than that for the isovalent radical, HCO, indicating that the HCS radical is more close to a linear structure than the HCO radical.

Computational study of Si2H and Si2H2 species as candidates of interstellar molecules

Disilicon hydrides as candidates of interstellar molecules have been studied employing density functional and coupled cluster quantum chemical methods. Potential energy surfaces of Si2H and Si2H2 in the electronic ground state reveal that several isomeric structures are energetically well separated. This means that these isomers can be distinguished in low‐temperature conditions such as the interstellar medium due to the disfavor of their inter‐conversions. Two nearly degenerate states exist for cyclic structures of Si2H that both can be possible isomers. In the case of Si2H2, four isomeric structures are well separated by energy barriers for isomerization processes between them. We have also examined potential energy surfaces in the lowest excited state for the Si2H and Si2H2 species and found that the energies of several isomeric species in the ground state are very close to their energies in the lowest excited states. The reactivity of Si2H and Si2H2 with hydrogen atom is examined and possible formatio...