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Phys Chem Chem Phys ; 17(37): 23833-46, 2015 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-26304769


Product detection studies of C((3)P) atom reactions with ethylene, C2H4(X(1)Ag) and propylene, C3H6(X(1)A') are carried out in a flow tube reactor at 332 K and 4 Torr (553.3 Pa) under multiple collision conditions. Ground state carbon atoms are generated by 193 nm laser photolysis of carbon suboxide, C3O2 in a buffer of helium. Thermalized reaction products are detected using tunable VUV photoionization and time of flight mass spectrometry. For C((3)P) + ethylene, propargyl (C3H3) is detected as the only molecular product in agreement with previous studies on this reaction. The temporal profiles of the detected ions are used to discriminate C((3)P) reaction products from side reaction products. For C((3)P) + propylene, two reaction channels are identified through the detection of methyl (CH3) and propargyl (C3H3) radicals for the first channel and C4H5 for the second one. Franck-Condon Factor simulations are employed to infer the C4H5-isomer distribution. The measured 1 : 4 ratio for the i-C4H5 isomer relative to the methylpropargyl isomers is similar to the C4H5 isomer distribution observed in low-pressure flames and differs from crossed molecular beams data. The accuracy of these isomer distributions is discussed in view of large uncertainties on the photoionization spectra of the pure C4H5 isomers.

Chemistry ; 18(30): 9321-32, 2012 Jul 23.
Artigo em Inglês | MEDLINE | ID: mdl-22733693


Fragmentation of the γ-aminobutyric acid molecule (GABA, NH(2)(CH(2))(3)COOH) following collisions with slow O(6+) ions (v≈0.3 a.u.) was studied in the gas phase by a combined experimental and theoretical approach. In the experiments, a multicoincidence detection method was used to deduce the charge state of the GABA molecule before fragmentation. This is essential to unambiguously unravel the different fragmentation pathways. It was found that the molecular cations resulting from the collisions hardly survive the interaction and that the main dissociation channels correspond to formation of NH(2)CH(2)(+), HCNH(+), CH(2)CH(2)(+), and COOH(+) fragments. State-of-the-art quantum chemistry calculations allow different fragmentation mechanisms to be proposed from analysis of the relevant minima and transition states on the computed potential-energy surface. For example, the weak contribution at [M-18](+), where M is the mass of the parent ion, can be interpreted as resulting from H(2)O loss that follows molecular folding of the long carbon chain of the amino acid.

Chemphyschem ; 12(5): 930-6, 2011 Apr 04.
Artigo em Inglês | MEDLINE | ID: mdl-21370376


In general, radiation-induced fragmentation of small amino acids is governed by the cleavage of the C-C(α) bond. We present results obtained with 300 keV Xe(20+) ions that allow molecules (glycine and valine) to be ionised at large distances without appreciable energy transfer. Also in the present case, the C-C(α) bond turns out to be the weakest link and hence its scission is the dominant fragmentation channel. Intact ionised molecules are observed with very low intensities. When the molecules are embedded in a cluster of amino acids, a protective effect of the environment is observed. The fragmentation pattern changes: the C-C(α) bond becomes more protected and stable amino acid cations are observed as fragments of the molecular clusters. Evidently, the molecular cluster acts as a "buffer" for the excess energy, capable of rapidly redistributing excess energy and charge.

Glicina/química , Íons/química , Valina/química , Transferência de Energia , Ligação de Hidrogênio , Espectrometria de Massas