Rovibrational Calculations of Bent Triatomic Molecules
E. von Nagy-Felsobuki
Department of Chemistry
The University of Newcastle
The aim of this project is to carry out a thorough and systematic investigation of the electronic and structural properties of the ground and excited states of the XY and XY ions (where X is a first, second or third row atom and Y is a first, second or row atom).
The specific aim is to calculate potential energy surfaces of ground and excited states of the XY and XY ions in order to determine their rotationally resolved infrared spectrum. Attention is also on calculating ab initio vibrational-rotational eigenenergies as well as evaluating rovibrational transitional probabilities. This will enable an assessment of the dominant transitions in these ions.
The proposed development of theory and calculations aims to give a greater understanding of the spectroscopy of helides ions and the chemistry of helium. The theoretical calculations proposed will utilise state-of-the-art theories (eg. CCSD(T)) in order to give a systematic account of the changes in potential energy surfaces as the binding partners become more complex. The resulting studies of the potential energy surfaces of the helide ions and the subsequent calculated spectroscopic properties (transitions and transition strengths) will be of benefit to experimentalists and astronomers. This will be of particular interest to the latter group since the chemistry of helium with dimeric ion fragments in the chemistry of interstellar gas clouds has been largely unexplored. Moreover, such an investigation will give further insights into the chemical reactivity and the unusual properties of the helide ions.
The proposed modelling will be tested and/or validated by experiment by a number of overseas experimental and theoretical groups currently interested in the vibrational and rovibrational modelling of small molecules. This will further assist in constructing benchmarks for spectroscopically accurate calculations.
g19 - VPP
- Appendix B
What are the results to date and the future of the work?
The project was initially funded in 1992 and is progressing well with the potential energy surfaces and rovibtational states of a number of alkali metal ions being characterised (as well as the rovibrational states of the water cation). The results arising from access to the ANU supercomputer have led to almost 30 papers published in international journals, numerous conference papers and a number of successful post-graduate research projects as well as a monograph.
Most recently, the chief investigator (CI) and coworkers have calculated the rovibrational states of He2X using the CCSD(T)/cc-pCVTZ electronic potential within their normal coordinate rovibrational Hamiltonian. They have also determined the optimised structures of diatomic and triatomic molecules at this level of theory. Their most recent calculations indicate that HeCH+ is bound, which significantly extends the chemistry of helium.
The specific aim in 1996/1997 is to calculate spectroscopic properties of helide molecules (XHe+n and XHe2+n ions, where X represents a foreign atom such as carbon, nitrogen or oxygen, and n=1..4) using ab initio techniques in order to enable the spectroscopic characterisation and so their experimental detection. Moreover, it is hoped that such an investigation will give further insights into the chemical reactivity and the unusual properties of the helide ions.
What computational techniques are used?
Solving the electronic Schrodinger eigenvalue problem for electron dense systems is at present an open-ended problem. To that end GAUSSIAN suite of programmes enable "effective" truncations to be made to force tractable solutions. Using the GAUSSIAN programmes with valence double zeta (+polarisation) basis sets coupled with CCD, SDCI , MP4 or CCSD(T) level of theory enables computation of ab initio discrete potential energy surfaces of spectroscopic quality. For example, the computation time required to produce just one surface with a total of 50 points consumes at least 60 SU. Therefore five discrete electronic surfaces would require c.a. 300 SU.
B. Smart, C. Marsden, J.M. Hughes, F. Wang and and E. I. von Nagy-Felsobuki, Ab Initio and Analytical Potential Energy Functions of NaK, J. Mol. Struct. (Theochem.), 376 (1996) 449.
B. Smart, C. Marsden, J.M. Hughes, F. Wang and and E. I. von Nagy-Felsobuki, Variational Calculations of the Rovibrational Energy Levels of NaK, Chem. Phys. Lett., 261 (1996) 51.
F. Wang and E. I. von Nagy-Felsobuki, Variational Calculations for the Rovibrational States of SiC and SiC, Spectrochimica Acta Part A, 52 (1996) 1581.
J. Haywood, Sudarko, J.M. Hughes, E. I. von Nagy-Felsobuki and L.P. Alderidge, Rovibrational States of the 1A1 Ground Electronic State of Si3, Mol. Phys., accepted for publication (1997).
J. Hughes and E. I. von Nagy-Felsobuki, Chem. Phys, 211 (1996) 135.
J.M. Hughes and E.I. von Nagy-Felsobuki, Variational Calculations Of Rovibrational States Of He2C2+,He2 N2+ and He2O2+, Fourth World Congress of Theoretically Oriented Chemists, Proceedings Page A-39, Jerusalem, Israel (1996).