| Popis: |
Bombardment of a silicon target in a high vacuum with a molecular beam (mixture of high energy H 2 +N 2 , obtained by charge exchange) and a thermal beam of O 2 produces on the target a variety of compounds. The target is then bombarded by the same molecular beams which produce, extracted by an electrostatic field at an energy of about 10 keV, molecular ions due to the compounds thus formed. These ions are analysed (electromagnet) to give a primary spectrum of ions according to their mass, which are individually selected and dissociated in a collision cell (same H 2 +N 2 mixture). Mass analysis of the dissociation fragments leads to the identification of silicon clusters (Si) n and of Si–O–N–H derivatives, the fragmentations of which permit a definitive determination of their molecular complexity. Dissociation spectra have thus been obtained for some of the most intense peaks of the primary spectrum, on the one hand, and on the other hand for some peaks of lower intensity but of special interest to us (see below). The composition of the fragments is confirmed by the study of the satellite ions derived from the natural 28, 29 and 30 isotopes of silicon, and by the use of deuterium instead of hydrogen. None of the Si–O–N–H derivatives obtained was apparently known earlier. It is shown that some of these molecules (those ‘of special interest to us’) may be identical with sila-analogues of standard amino acids and of nucleic bases: the fact that their fragmentations are identical with those of the corresponding carbon analogues speaks in favour of a structural identity. However, one cannot yet distinguish between the various possible isomeric arrangements, as none of them has been independently prepared, which excludes a direct comparison with reference samples, and as these isomers might give identical fragments; we hope to be able to resolve this ambiguity later. Anyhow, the substances formed are the most complex molecular silicon derivatives so far produced: e.g. Si 2 O 2 NH 5 , Si 3 O 2 NH 7 , Si 4 O 3 NH 9 , Si 4 O 2 N 2 H 4 , Si 4 ON 3 H 5 , Si 5 O 3 N 2 H 10 , Si 5 O 2 NH 11 , corresponding to sila-glycine, sila-alanine, sila-threonine, sila-uracile, sila-cytosine, sila-valine, sila-glutamine, – or isomers. Similar results have been obtained using a silicon dioxide target and high energy molecular beams of hydrogen and of nitrogen, without thermal oxygen, or with a carborundum target. © 2000 Academie des sciences / Editions scientifiques et medicales Elsevier SAS molecular impact / silicon derivatives / amino-acid silicon analogues |
