Computational Quantum Chemical Studies of Electron Transfer and Diels-Alder Facial Stereoselectivity

Significant Achievements, Anticipated Outcomes and Future Work

(I) An intensive computational study of the intramolecular Diels-Alder reaction (see scheme below) was undertaken in an attempt to explain and predict the stereoselectivity of this reaction. Particular issues addressed were the effect of substituents on the cis/trans product selectivity and whether the composition of the tether could affect this stereochemical outcome. Thus, well over 300 transition structures were calculated for a variety of substituents, a fraction of which is indicated in the scheme. The level of theory B3LYP/6-31+G(d) was used throughout the study, although some very large MP4/6-31G+G(d) single point energies were calculated for selected molecules (each calculation consumed 300 SU’s).

Important outcomes from the calculations to date are as follows:

1. An understanding of the greater reactivity of E substituents over Z substituents.
   
2. An understanding of the intrinsic cis selectivity of the unsubstituted.
   
3. Knowledge from (1) and (2) above has led to a prediction that cis/trans selectivity may be controlled by tether composition.
   
4. Calculations also predicted a strategy for controlling stereoselectivity, which was subsequently confirmed experimentally.
   
5. We have achieved excellent agreement between experimental activation energies and computed ones (to within 1%), but the computational level required to reach this degree of agreement is very high (MP4(SDTQ)/6-31+G(D)).

Six manuscripts on this work are in active preparation. One publication has appeared (see publication # 4 below).

(II) A DFT study of mechanistic aspects of the function of cytochrome c oxidase has been completed and published (see publication # 5 below).

(III) Detailed calculations of the geometries and vibrational frequencies of triplet charge-separated states have been carried out, together with calculations of singlet-triplet energy gaps. These calculations were carried out using DFT.

Computational Techniques Used

We mainly use standard quantum chemical packages, particularly GAUSSIAN 03 and GAMESS. All calculations employ some form of electron correlation in their final stages, such as DFT, MP2, QCI and CC methods. For molecular dynamics with quantum transitions, we use our in-house developed software. Most of our jobs take approximately 100 - 200 SUs (four cpus for large-scale electron correlation calculations). In principle, we could run these calculations on workstations but there is no way we could get the same speed. Using the APAC National Facility, we have achieved more in one year than we would in three - four years using workstations (publication # 3). Publications 1 and 2 review electron transfer, including computational studies carried out by Paddon-Row using the APAC National Facility and the ac3 facilities.

Publications, Awards and External Funding

External Funding and Awards

(1) ARC Discovery-Project DP0208012 2002-2004 $530,000. A new approach to the generation of long-lived charge-separated states in rigid donor-bridge-acceptor systems.

(2) ARC Discovery-Project DP0344445 (with Sherburn) 2003-2005 $500,000. New horizons in Diels-Alder chemistry.

Publications

(1) M. N. Paddon-Row, Orbital Interactions and Long-Range Electron Transfer, Adv. Phys. Org. Chem., 38, 2003, 1-85.

(2) M. N. Paddon-Row, Superexchange-mediated Charge Separation and Charge Recombination in Covalently Linked Donor-Bridge-Acceptor Systems, Aust. J. Chem., 56, 2003, 729-748.

(3) L. Hviid, W. Bouwman, M. N. Paddon-Row, H. J. van Ramesdonk, J. W. Verhoeven, A. M. Brouwer, Spin control of the lifetime of an intramolecular charge-transfer excited state, Photochem. Photobiol. Sci., 2, 2003, 995-1001.

(4) T. N. Cayzer, M. N. Paddon-Row, M. S. Sherburn, Stereocontrol of the Intramolecular Diels–Alder Reaction by Internal Hydrogen Bonding, Eur. J. Org. Chem., 2003, 4059-4068.

(5) S. B. Colbran, M. N. Paddon-Row, Could the tyrosine–histidine ligand to CuB in cytochrome c oxidase be hemilabile? Implications from a quantum chemical model study of histidine substitutional lability and the effects of the covalent tyrosine–histidine cross-link, J. Biol. Inorg. Chem., 8, 2003, 855–865.