Ab Initio Calculations on Highly Unsaturated Interstellar Molecules


Principal Investigator

Simon Petrie

School of Chemistry,

University of New South Wales,

Australian Defence Force Academy

The field of investigation is that of the chemical evolution of dense interstellar clouds and
circumstellar envelopes. These are environments typified by low temperatures and very low pressures, and in consequence contain many different molecules, radicals, and molecular ions which would be extremely unstable under 'conventional' laboratory conditions. This particular project is concerned with two aspects of interstellar cloud chemistry: firstly, mechanisms for the formation of species of the general formulae M(CN) and M(CCCN), where M is a main-group metal atom such as Na, Mg, or Al; and secondly, an investigation of the relative energies, and barriers to isomerization, of linear and cyclic isomers of highly unsaturated carbon chain molecules (bare, or derivatized with H, N, or S atoms). The motivations for this research are, firstly, the recent detection of three different metal cyanide species M(CN) in a circumstellar envelope known as IRC +10216, and the presence within this same envelope of highly unsaturated linear species (the largest known of which is HCCCCCCCCCCCN, but larger species are probably also present). The computational connection is that these species, being in general highly reactive, are more easily studied by theory than by experiment, and to this end high-level ab initio quantum chemical calculations are used to determine details of molecular structure, stability, and thermochemistry.



Rodney Blanch

School of Chemistry,

University of New South Wales,

Australian Defence Force Academy

Leo Radom

Research School of Chemistry



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What are the results to date and the future of this work?

Work on M(CCCN) formation and isomerism is essentially complete. A paper on M(CCCN) structures and thermochemistry has been submitted (to Monthly Notices of the Royal Astronomical Society); details of the analogous M(CCCN)H+ surfaces (energetic and spectroscopic information) has been supplied to Professor Robert Dunbar (Case Western Reserve University, Ohio), with whom I am collaborating on a study of radiative association rates between metal ions and small interstellar molecules: it is anticipated that this collaborative study will be completed in the near future. A further manuscript, concerning some implications for the success or failure of metal-containing-molecule formation via dissociative recombination of metal ion adducts, is in preparation and will be submitted for publication shortly. Additional

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studies, on the potential energy surface for magnesium dicyanide and for several related species XMg(CN) [X = F, Cl, OH, SH, NH2, CH3], have been completed and are also to be submitted for publication in the near future. Studies on the cyclization of highly unsaturated carbon-chain species have been conducted, but this part of the project has not yet been completed, with much data analysis and subsequent calculations of remaining species still to be performed. It is anticipated that the existing results will provide a sufficient indication of the reliability of semi-empirical techniques (AM1 and PM3) to indicate whether such procedures should be supplemented by ab initio calculations.

What computational techniques are used?

The calculations involved have been performed using the Gaissian94 and Molpro96.1 programming suites. These are both very widely implemented quantum chemical computational packages. Many of these calculations have involved the application of 'model chemistries' such as the Gaussian-2 (G2) computational procedure, to provide accurate enthalpies of formation for species which have not been studied experimentally. In other calculations, geometry optimizations have been performed at various levels of ab initio theory (for the M(CCCN) species, to derive molecular geometries to as great an accuracy as possible; and for the highly unsaturated carbon chain species, to assess the dependence of calculated relative energies upon the level of theory employed).

Appendix B -