Theoretical Studies of Negative Ions

               

Principal Investigator

John Bowie

Department of Chemistry,

University of Adelaide

Ab initio Molecular Orbital calculations carried outon the PC provide us with complementary
information to that provided by our Negative Ion Mass Spectrometric experiments. Sophisticated algorithms allow us to obtain the accurate structures, energies and thermochemical properties of gas phase anions. Calculations allow us to understand and even predict rearrangements of the anions that we observe experimentally. Exciting new experimental techniques are allowing us to explore the structure and chemistry of transient gas phase neutrals and we plan to expand our theoretical focus to encompass these novel systems.
   

Co-Investigators

     

Stephen Blanksby

Mark Taylor

Department of Chemistry,

Uiversity of Adelaide

Leo Radom

Research School of Chemistry

     

     
             
               

Projects

g77 - PC

           
               

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

1. The Homoallyl Anion - Structure and Reactivity

We have investigated the geometrical and electronic structure of the homoallyl anion (1) using ab initio Molecular Orbital Theory. We have shown that the anion's unusual stability toward electron loss is not simply explained by qualitative molecular orbital pictures such as negative hyperconjugation or homoconjugation. The mechanism of isomerism of the homoallyl anion to the 1-methylallyl anion (2) has been studied with a focus on comparing the energetics of a direct 1,2-hydrogen shift and a dissociative-addition mechanism (involving hydride ion-butadiene complexes (3) Both pathways have been fully characterised at the MP2(full)/6-31+G(d,p) level of theory and have been shown to be energetically competitive. Current work will involves applying high level single point energy calculations to the structures obtained so as to refine the reaction energetics.

The interconversion chemistry of other C4H7- isomers (2), (4), (5), (6) and (7) has also been investigated theoretically and experimentally. Theoretical calculations have proven to be of vital import in this regard as our experimental technique for probing the anion interconversion chemistry (Charge Reversal Tandem Mass Spectrometry) is unable to distinguish between isomeric C4H7- anions.

               
- Appendix B

 
               

           

The final part of the project (to be carried out during February-April 1998) will be the application of G2 techniques in order to determine accurate gas phase acidities (DHoacid) of selected C4H8 hydrocarbons and electron affinities (EA) of selected C4H7 radicals. These thermochemical properties have recently been measured for the aforementioned species, and will serve as an excellent calibration for our theoretical models.

 

 

 

2. Negative Ion Studies of Interstellar Cumulenes

Theoretical investigations into the structures and relative stabilities of isomers of the C5H anion have continued in the last term. We have shown that a third isomer containing a three membered ring (3) is also stable relative to the two species that have been experimentally and theoretically characterised (1,2). This has proven to be an exciting example of a computational result directing further experimental studies.

Recent interest in the literature in C5H2 species has led us to probe minima on the anion potential surface. The computational studies have shown that the three structural isomers arising from our experimental investigation are indeed stable species. We hope that this work will prompt some interest from the wider research community.

Finally, we are commencing investigations of the anions of other cumulenes and heterocumulenes in order to coincide with a recent upgrade in our experimental facilities. This study will therefore focus not only on anions, but also their corresponding neutrals, which we generate experimentally by the technique of Neutralisation Reionisation Mass Spectrometry.

           
 
           
Appendix B -

           

     

3. The Gas Phase Wittig Rearrangement

The gas phase Wittig rearrangement of deprotonated ethers (Scheme 1) has been studied previously by our group through calculation of the potential energy surface (PES) belonging to the model system -CH2OCH3. The PC was used to calculate high level single point energies for selected points on the PES that would have been too computationally intensive for the platforms that we had used previously.

 

 

 

What computational techniques are used?

The program packages which have been used for the work described are Gaussian 94 and Molpro, both of which are implemented on the PC.

Publications

S. J. Blanksby, S. Dua, J. H. Bowie, and J. C. Sheldon, The Synthesis and Structure of the Symmetrical Isomer of C5H-, Chemical Communications, 19, 1997, 1833-1834.

     
- Appendix B