CHM A Look at the Dissociation Pathways of Acetone and Tri-fluoroacetone

Transcription

1 CHM 7440 COMPUTATIONAL CHEMISTRY PROJECT A Look at the Dissociation Pathways of Acetone and Tri-fluoroacetone By Smriti Anand & Muhannad Zamari April 26 th / 2001

2 Introduction: One of the papers that was recently submitted by DrLevis s group 1 discussed the possibility of controlling the outcome of dissociation when a molecule is exposed to an intense laser pulse (~10 13 W cm -2 ) This is a tremendous and very high amount of energy since a billion photons are introduced to the system in an extremely short duration The molecules studies were acetone, trifluoroacetone, and acetophenone Previous work by Zewail 2 done on acetone showed that at relatively low energies (λ > 266nm), the major dissociation channel is the cleavage of one C-C bond producing CH 3 CO and CH 3 radicals However, at high energies (λ < 193nm), both the C-C bonds adjacent to the carbonyl group of acetone dissociate Two CH 3 radicals and CO are formed as the final products with a quantum yield of nearly unity In this report, we will look at the energies of all possible products resulting from the dissociation of acetone and trifluoroacetone when subjected to high ionizing energies and indicate the most favorable dissociation path for each Acetophenone was not studied for now, as we wanted to study simple systems first and get a feel of the chemistry involved 1GMenkir, HRabitz, RJLevis (Submitted to Science) 2 SK Kim; SPederson; AHZewail, J Chem Phys 1995, 103,477

3 Calculations: The energies of all the possible dissociation products were calculated for acetone and trifluoroacetone using AM1, HF/3-21G, and the higher, more accurate CBS-QB3 and CBS-APNO methods All values obtained are given in Table 1 From these values, we were able to determine the enthalpies of all possible dissociation reactions Again, the different reactions and their enthalpies are shown in Table 2 All calculations were done using the development version of the Gaussian 99 program Results and Discussions: The parent molecule dissociating into a cation and an anion seemed almost impossible since it requires a very high energy and therefore this possibility was eliminated The only other possibility was that the parent molecule in the strong laser field loses an electron and is left as a radical cation, which is just ready to dissociate further In order to have an idea of where the unpaired electron is located on the radical cation, we looked at the spins (in the population analysis) and we were able to conclude that it was located on the oxygen In trifluoroacetone, the C-CF 3 bond was longer than the C-CH 3 bond This can be explained in terms of strong electron withdrawing by the fluorine atoms that elongates the C-CF 3 bond Moreover, this would also indicate that the dissociation along the C-CF 3 bond is more probable which agrees with our energy calculations

4 In order to get a clearer picture of what was happening, it became necessary to explore the potential energy surface The scans are shown in Figures 1,2 &3 We ran a rigid scan at the HF/ 3-21G level along the C-CF 3 and C-CH 3 bonds in trifluoroacetone To check for any errors, we ran a scan in the opposite direction where the fragments (separated at 40 Å) approached each other and formed a bond Both, the forward and reverse scans, were essentially the same Next, we ran a relaxed scan where a geometry optimization would occur at each step while scanning from 12 Å to 40 Å For the scan along the C-CH 3 bond, we only observed an overall decrease in energy, which was a normal response However, along the C-CF 3 bond, other than the overall decrease in energy, a transition state was observed when the C-CF 3 bond distance was 204 Å! To be certain, we optimized the structure of this transition state at the HF/3-21G level The frequency calculation on this optimized geometry returned an imaginary frequency A similar scan carried out along the C-CH 3 bond in Acetone revealed the presence of a transition state (also confirmed by a frequency calculation on the optimized structure) For this transition state, the C-CH 3 distance was 217 Å The next question was, what did these transition states correspond to? To get an answer, in trifluoroacetone, we ran a rigid scan in the reverse direction starting form the optimized geometry where the C-CF 3 distance was 253 Å The picture that revealed itself was very exciting!!! There seemed to be two curves that crossed at the Transition state!!!! The forward relaxed scan was thus an avoided curve crossing!!! Acetone also showed a similar behavior!!!

5 Future Work: There are still many questions that remain to be answered What is the state that corresponds to the last point on the relaxed and rigid scans? Is there any change in the electronic structure that is taking place during the dissociation? A population analysis of some of the structures might give us a hint We hope to get an insight as to what exactly is going on in the system by doing trajectory calculations Furthermore, we look forward to understand the various pathways of dissociation and ultimately be able to calculate the branching ratios for the pathways The possibility of rearrangement in the aforementioned systems is still to be explored There is also the question of the formation of some other fragments We plan to carry out the calculations at a higher level of theory and also be able to study other systems (like acetophenone) in the future Acknowledgments: Special thanks to: -Dr Schlegel -Dr Levis -Xiaosong Li -Noel Moore for their guidance, help, and support that made our project a success Thanks are also due to all our friends who helped us in this project