Rovibrational Calculations of Bent Triatomic Helide Ions
Ellak I. Von Nagy-Felsobuki
Department of Chemistry,
The University of Newcastle
|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.
g19 - VPP.
What are the result 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. Principally, 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 support the work of a number of PhD candidates and Honours students. In 1997 it supported the work of two PhD students (Mr. Hughes and Mr. Sudarko) and one Honours student (Mr. Wilson). It has resulted in a total of seven publications (six in international
Appendix B -
refereed Journals and one a local conference).
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 helides 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 VPP 300 (ANU, supercomputer), The other computer codes are resident on the 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 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 devised by von Nagy-Felsobuki and coworkers 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 + Uw+ 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, the approach used for the variational solution of equation (1), 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 VPP. It is important to use property surfaces in such calculations due to the possibility of electrical anharmonicity. Von Nagy-Felsobuki and coworkers have already detailed the approach that will be used with respect to triatomic systems
J. M. Hughes and E. I. von Nagy-Felsobuki, Ab Initio Calculations of the Rovibrational States of He2O2+, J. Mol. Struct. (Theochem.), 389 (1997) 1-11.
F. Wang, F. R. W. McCourt and E. I. von Nagy-Felsobuki, Ab Initio Potential Energy Surfaces and Vibrational Energies of Li3-, Chem. Phys. Lett., 269 (1997) 138-144.
J. M. Hughes and E. I. von Nagy-Felsobuki, Ab Initio Investigations of the Electronic Structure of HeHO+ and HeHO2+, J. Phys. Chem. A, 101 (1997) 3995-3997.
J. M. Hughes and E. I. von Nagy-Felsobuki, Ab Initio Investigations of the Electronic Structure of HeHC+ and HeHC2+, Chem. Phys. Lett., 272 (1997) 313-318.
J. M. Hughes and E. I. von Nagy-Felsobuki, Ab Initio Calculations of the Rotational States of He2C2+, J. Mol. Struct. (Theochem.), 398 (1997) 347-358.
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., 92 (1997) 177-186.
Appendix B -