Structures and Vibrational Frequencies of Pure Halocarbon Clusters, and Mixed Clusters with Water

               

Principal Investigator

Gad Fischer

Department of Chemistry,

The Faculties

The adverse environmental effects of the halocarbons are well documented. Not only are they destructive
of the stratospheric ozone layer but they are also powerful greenhouse gases. Although considerable data on the infrared absorption by these molecules has been reported in the last years, little is known about the role played by dimers and larger clusters of these molecules, and their concentrations in the atmosphere. The low temperatures encountered at high altitudes favour the formation of clusters. In this project investigations are carried out on dimers and cross dimers of the halocarbons and cross dimers with water. The work has also been extended to the study of clusters of acetylene like molecules. This latter work is in collaboration with the experimental investigations of the Maier group at Basel, Switzerland.
   

Co-Investigators

     

Xiaolin Cao

Department of Chemistry,

The Faculties

 

 

Projects

u54 - VPP, PC

     
             

     
               

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

Investigations have been carried out on a number of related topics. (1) The spectral structure arising from the puckering vibration of perfluorocyclobutane has been modelled with a quadratic-quartic potential and using the inversion barrier height , and the vibrational frequency determined in ab initio molecular orbital calculations. Side-band intensity is largely concentrated in a narrow range displaced some 60 cm-1 from the fundamentals. The molecular structure and vibrational frequencies have been determined by ab initio molecular orbital computations. Good agreement has been achieved with experiment. The infra-red spectrum has been remeasured in a jet-cooled beam. (2) Ground state calculations of the optimized geometries and vibrational frequencies of the carbon cumulenes, H2Cn n=3-15, have been variously carried out at HF/6-31G(d), MP2/6-31G(d), MP2/6-31G(d,p) and CASSCF/6-31G(d) levels, both with and without the constraint of linearity of the carbon chains. Substantial reductions in the A rotational constant are obtained when the constraint to linearity is removed, in agreement with the trend observed for the short cumulenes for which experimental results are available. Excited state calculations at the CIS/6-31G(d) level have focussed mainly on the intense 1A1 excited state, (the third 1A1 for n > 5). Also, in these calculations the tendency for non-linearity of the carbon chain has been noted, although energy differences are smaller. CASSCF calculations have been restricted to the two smallest cumulenes. They are in good agreement with the CIS calculations for the first excited 1A1 and 1A2 states, but differ for the higher 1A1 states. Empirical corrections to the CIS calculated transition energies for the intense state have been introduced through comparisons with the linear carbon chain molecules, Cn, for which both

               
- Appendix A

 
               
       

observed results and equivalent accuracy ab initio computations are available. Cumulenes with sizes H2C13 to about H2C34 have their first strong electronic transition in the 400-860 nm DIB range. (3) Calculations on the structures and potential energy surfaces for the hydrogen-bonded complexes between the fluorocarbons, CH3F and CH2F2, and water have been completed, and are waiting for experimental confirmation, using our recently constructed pulsed-slit nozzle interfaced with an FTIR spectrometer.

What computational techniques are used?

Molecular orbital calculations were carried out using the Gaussian 94 suite of programs. Excited state calculations were undertaken at the CIS/6-31G* and CASSCF/6-31G* levels. Optimized geometries and vibrational frequencies were determined. Ground state calculations were carried out at the HF, MP2 and B3LYP (density functional) levels, all using the 6-31G* basis. Optimized geometries and vibrational frequencies were calculated.

Publications

G. Fischer, R. L. Purchase, D. M. Smith, The ring-puckering motion in perfluorocyclobutane, J. Molecular Structure, 405, 159-167 (1997).

G. Fischer, J. P. Maier, Ab initio prediction of the spectra of carbon cumulenes, Chemical Physics, 223, 149-158 (1997).

       
Appendix A -