Rovibrational Calculations of Helide Ions
Ellak I. Von Nagy-Felsobuki
g19 - PC, VPP
|Prior to the 1980s helium was thought to be inert,
capable of only forming simple charged species
such as HeH+. Recent ab initio calculations and experimental investigations have confirmed that helium is more reactive, capable of forming a number of polymeric species. The aim of this project is to calculate spectroscopic properties of helide ions (eg. XmHez+n, where X represents a foreign atom such as carbon, m=1,2, n=1..4 and z=1,2) using ab initio methods, thereby enabling their experimental detection. Moreover, such an investigation will provide further insights into the chemical reactivity and the unusual properties of these helide ions.
What are the results to date and the future of the work?
The specific aim of this project is to thoroughly and systematically calculate spectroscopic properties of helide ions of form XmHenz+ (where X represents a foreign atom such as carbon, nitrogen or oxygen and m=1,2 , n=1..4 and z=1,2) using state-of-the-art ab initio methods in order to enable their experimental detection. Moreover, such an investigation will give further insights into the chemical reactivity and the unusual properties of these helide ions.
There has been considerable theoretical interest in structures, stabilities and bonding in helide molecules. Principly, this is because of recent theoretical investigations by three groups: Frenking, Koch and coworkers, Radom and coworkers and von Nagy-Felsobuki and coworkers. They have shown that helium can form strong bonds in ions and even in some cases, in neutral compounds. Whilst the electrostatic interactions were postulated to be responsible for the binding force of most of the He-X+ dimers (where X = Li to Ne), covalent bonding is thought to prevail for their dication counterparts.
While the former two groups have concerned themselves mainly with the nature of helide bonding, only von Nagy-Felsobuki and coworkers have recently calculated ab initio electronic structure, discrete potential energy surfaces, potential energy functions, vibrational and rotational spectroscopic constants and the rovibrational structure of He2X2+ and HHeX+ (where X=C, N and O) These calculations will hopefully assist experimentalists to spectroscopically detect these helide ions not only in a laboratory but also in interstellar gas clouds.
The project has thus far supported the work of a number of PhD candidates and Honours students. In 1999 it will support the work of three PhD students (Mr. Sudarko, Mr. Wilson and Mr. Sky). Moreover, Dr. J.M. Hughes PhD candidature was supported from this project. His thesis was examined in 1998 and he was awarded a PhD in 1999 from the University of Newcastle.
Appendix B -
In summary the future aims of this project are to improve the understanding of:
(i) the chemistry of helium and unusual properties of helide ions.
(ii) the potential energy surfaces for helide ions.
(iii) rovibrational Hamiltonians and appropriate solution algorithms for molecules containing up to five atoms.
(iv) spectroscopic constants of helides, thus enabling their spectroscopic detection.
What computational techniques are used?
The discrete potential energy and property surfaces are generated using the SGI and VPP 300. The other computer codes are resident on the SGI and VPP but can be used on workstations and PCs. The calculation of the discrete potential energy and property (eg. dipole moment) surfaces using state-of-the-art ab initio codes requires access to the SGI and VPP.
Discrete Potential Energy Surfaces.
All electronic calculations are carried out using the GAUSSIAN 94 suite of programs. The basis set employed for both helium and foreign atoms are the correlation consistent polarised core-valence quadruple zeta set of Dunning and coworkers (labelled cc-pCVQZ). Woon and Dunning have designed this basis set to treat core and core-valence correlation effects.
The construction of an electronic grid on the potential energy hypersurface is not trivial. It is desirable to calculate ab initio electronic points coincident with the quadrature points required by the potential energy integrator, which in the solution algorithm we devised is due to Harris, Engerholm and Gwinn (ie. HEG scheme).
Analytical Potential Energy Function.
The approach used centres on a multi-dimensional least-squares regression of a power series expansion and Padé approximants to generate analytical potential energy functions. A number of simple expansion variables are employed to ensure that the fits are free of "artefacts" (such as singularities) which may occur even for small c2 values.
The normal coordinate Hamiltonian is used which has form,
H=Tv+Tl+U w+V (1)
where the first term is the vibrational kinetic energy contribution, the second term is the vibrational angular momentum contribution and the third term is the Watson term, which is a mass dependent contribution to the potential energy operator.
For triatomic helide systems, we have detailed the approach used for the variational solution of equation (1), which has been detailed by the von Nagy-Felsobuki and coworkers for bent and linear systems.
- Appendix B
Dipole Moment Surfaces.
Transition strengths will be calculated using dipole moment surfaces calculated at the CCSD(T)/aug-cc-pCVQZ level of theory using the SGI and VPP. It is important to use property surfaces in such calculations due to the possibility of electrical anharmonicity. we have also detailed the approach that will be used with respect to triatomic systems .
J. M. Hughes and E.I. von Nagy-Felsobuki, Ab Initio Investigations of the Electronic Structure of HeHN+ and HeHN2+, Aust. J. Phys., 51 (1998) 57-66
J. M. Hughes and E.I. von Nagy-Felsobuki, Ab Initio Investigation of the Electronic Structure and Stabilities of Triatomic Helide Ions: He2Xn+ Clusters (where X = B-Ne, Al-Ar and n=1-2), J. Mol. Struct. (Theochem.), accepted for publication (1999).
J. M. Hughes and E.I. von Nagy-Felsobuki, Ab Initio Structures and Stabilities of Helide Cations: HeXn+ (X=B-Ne, Al-Ar and n =1-3), The European Physical Journal D, accepted for publication (1999).
Sudarko, D.J. Wilson, J. M. Hughes and E.I. von Nagy-Felsobuki, Ab Initio Structures and Stabilities of the 3B1, The European Physical Journal D, submitted for publication (1999).
Sudarko and E.I. von Nagy-Felsobuki, Radiative Properties of Triatomic Molecules, The University of Newcastle Postgraduate Student Conference, ISBN 0 7259 10100, Newcastle, Australia (1997).
J.M. Hughes and E. I. von Nagy-Felsobuki, Ab Initio Study of Helium Clusters, 2nd Australian Conference on Physical Chemistry, Abstract Page C14, The University of Queensland, Brisbane (1998).
D. J. Wilson, Sudarko, J. M. Hughes and E. I. von Nagy-Felsobuki, Ab initio potential energy surface and vibrational energies of the 3Sg- state of He2O2+, 2nd Australian Conference on Physical Chemistry, Abstract Page PO61, The University of Queensland, Brisbane (1998).
Sudarko, David J. Wilson, J.M. Hughes and E. I. von Nagy-Felsobuki, Infrared Properties of the 2A1 Ground Electronic State of He2C3+, 2nd Australian Conference on Physical Chemistry, Abstract Page PO56, The University of Queensland, Brisbane (1998).
Sudarko and E. I. von Nagy-Felsobuki, Rovibrational Spectroscopy of Triatomic Molecules, 2nd Postgraduate Student Conference, Abstract Page 23, The University of Newcastle, Newcastle (1998).
D. J. Wilson, L.P. Aldridge and E.I. von Nagy-Felsobuki, Anionic Polysulfides, 2nd Postgraduate Student Conference, Abstract Page 33, The University of Newcastle, Newcastle (1998).
D.J. Wilson, Rovibrational Hamiltonians, H1, PhD thesis, The University of Newcastle, Newcastle (1998)
J.M. Hughes, The Electronic Structure and Rovibrational States of Helide Cations, PhD thesis, The University of Newcastle, Newcastle (1999).
Appendix B -