Low temperature formation of naphthalene and its role in the synthesis of PAHs (polycyclic aromatic hydrocarbons) in the interstellar medium.

Polycyclic aromatic hydrocarbons (PAHs) are regarded as key molecules in the astrochemical evolution of the interstellar medium, but the formation mechanism of even their simplest prototype-naphthalene (C(10)H(8))-has remained an open question. Here, we show in a combined crossed beam and theoretical study that naphthalene can be formed in the gas phase via a barrierless and exoergic reaction between the phenyl radical (C(6)H(5)) and vinylacetylene (CH(2) = CH-C ≡ CH) involving a van-der-Waals complex and submerged barrier in the entrance channel. Our finding challenges conventional wisdom that PAH-formation only occurs at high temperatures such as in combustion systems and implies that low temperature chemistry can initiate the synthesis of the very first PAH in the interstellar medium. In cold molecular clouds, barrierless phenyl-type radical reactions could propagate the vinylacetylene-mediated formation of PAHs leading to more complex structures like phenanthrene and anthracene at temperatures down to 10 K.

Formation of nitric oxide and nitrous oxide in electron-irradiated H(2)18O/N2 ice mixtures--evidence for the existence of free oxygen atoms in interstellar and solar system analog ices.

We investigated the irradiation of low temperature H(2)(18)O/N(2) ice mixtures with energetic electrons in an ultrahigh vacuum chamber. The newly formed species, such as nitric oxide (N(18)O), nitrous oxide (NN(18)O), hydrogen peroxide (H(2)(18)O(2)) and hydrazine (N(2)H(4)), were identified in the experiments with infrared absorption spectroscopy and mass spectrometry. The results suggest that the unimolecular decomposition of water molecules within water ices at 10 K can lead to the formation of transient, suprathermal oxygen atoms. These oxygen atoms may play an important role in the formation of oxygen-containing biomolecules such as amino acids and sugar, as well as the decomposition of the biomolecules in the ices.

On the formation of resonantly stabilized C5H3 radicals--a crossed beam and ab initio study of the reaction of ground state carbon atoms with vinylacetylene.

The formation of polycyclic aromatic hydrocarbons in combustion environments is linked to resonance stabilized free radicals. Here, we investigated the reaction dynamics of ground state carbon atoms, C((3)P(j)), with vinylacetylene at two collision energies of 18.8 kJ mol(-1) and 26.4 kJ mol(-1) employing the crossed molecular beam technique leading to two resonantly stabilized free radicals. The reaction was found to be governed by indirect scattering dynamics and to proceed without an entrance barrier through a long-lived collision complex to reach the products, n- and i-C(5)H(3) isomers via tight exit transition states. The reaction pathway taken is dependent on whether the carbon atom attacks the π electron density of the double or triple bond, both routes have been compared to the reactions of atomic carbon with ethylene and acetylene. Electronic structure/statistical theory calculations determined the product branching ratio to be 2:3 between the n- and i-C(5)H(3) isomers.

On the interaction of methyl azide (CH3N3) ices with ionizing radiation: formation of methanimine (CH2NH), hydrogen cyanide (HCN), and hydrogen isocyanide (HNC).

Methyl azide (CH(3)N(3)) might be a potential precursor in the synthesis of prebiotic molecules via nonequilibrium reactions on interstellar ices initiated by energetic galactic cosmic rays (GCR) and photons. Here, we investigate the effects of energetic electrons as formed in the track of cosmic ray particles and 193 nm photons with solid methyl azide at 10 K and the inherent formation of methanimine (CH(2)NH), hydrogen cyanide (HCN), and hydrogen isocyanide (HNC). We present a systematic kinetic study and outline feasible reaction pathways to these molecules. These processes might be also important in solar system analogue ices.

Untangling the chemical dynamics of the reaction of boron atoms, 11B(2Pj), with diacetylene, C4H2(X1Σg+)--a crossed molecular beams and ab initio study.

A crossed molecular beams experiment with ground state boron atoms, B((2)P(j)), and diacetylene, C(4)H(2)(X(1)Σ(g)(+)), was conducted at a collision energy of 21.1 ± 0.3 kJ mol(-1) under single collision conditions and combined with electronic structure calculations on the (11)BC(4)H(2) potential energy surface. Our combined experimental and computational studies indicate that the reaction proceeds without entrance barrier and involves indirect scattering dynamics. Three initial collision complexes, in which the boron atom adds to one or two carbon atoms, were characterized computationally. These intermediates rearranged via hydrogen shifts and/or successive ring-opening/ring closure processes on the doublet surface ultimately yielding a cyclic, C(s) symmetric (11)BC(4)H(2) intermediate. The latter was found to decompose via atomic hydrogen loss to yield a cyclic (11)BC(4)H(X(1)A') isomer; to a minor amount, the cyclic intermediate isomerized via ring-opening to the linear HCCBCCH(X(2)Σ(g)(+)) molecule, which in turn emitted a hydrogen atom to yield the linear HCCBCC(X(1)Σ(+)) molecule. The overall reactions to form these isomers were found to be exoergic by 55 and 61 J mol(-1), respectively, and involved rather loose exit transition states. On the basis of the energetics, upper limits of two energetically less stable species, the linear HBCCCC(X(1)Σ(+)) and BCCCCH(X(1)Σ(+)) species, were derived to be 12 and 2.2%, respectively. The dynamics of this reaction are also compared with the reaction of ground state boron atoms with acetylene studied earlier in our group.