Barrier widths, barrier heights, and the origins of anomalous kinetic H/D isotope effects
- Queen's Univ., Kingston, Ontario (Canada)
Proton transfer between MeO{sup {minus}} and HOMe has been studied using ab initio molecular orbital theory. At the highest level employed (MP2/6-31+G(d)//6-31G(d)+ZPE), {minus}{Delta}H{sup 298} and {minus}{Delta}G{sup 298} for the formation of the ion-molecule complex MeO{sup {minus}}{hor ellipsis}HOMe from the separated reactants are 26.3 and 15.2 kcal/mol, respectively. At the 6-31G(d)//6-31G(d) level of theory, the (MeO-H-OMe){sup {minus}} transition structure is 2.19 kcal/mol higher in energy than the ion-molecule complex ({Delta}E{sup {double dagger}} = 2.19), but this barrier disappears when zero-point energies are taken into account. The performance of AM1 on this system is quantitatively different ({minus}{Delta}H{sup 298} = 13.3; {minus}{Delta}G{sup 298} = 6.9; {Delta}E{sup {double dagger}} = 4.91; k{sub H}/k{sub D} = 5.13, increasing to 5.79 when quantum mechanical tunneling is invoked) but appears to be acceptable for the research envisaged in the title. The effect of an enforced separation of the heavy atoms upon proton transfer barriers and isotope effects (which simulates steric effects) has been studied briefly at the 6-31G(d) level and in some detail using AM1.
- OSTI ID:
- 6879765
- Journal Information:
- Journal of the American Chemical Society; (USA), Journal Name: Journal of the American Chemical Society; (USA) Vol. 112:11; ISSN 0002-7863; ISSN JACSA
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
400202* -- Isotope Effects
Isotope Exchange
& Isotope Separation
CALCULATION METHODS
CHARGED-PARTICLE TRANSPORT
DATA ANALYSIS
ISOTOPE EFFECTS
MATHEMATICAL MODELS
MOLECULAR ORBITAL METHOD
ORGANIC COMPOUNDS
ORIGIN
PROTON TRANSPORT
RADIATION TRANSPORT