Muonium Reactions with Oxalic Acid and Oxalate Ions in Water:  Huge Kinetic Isotope Effects When Compared with H

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 101; no. 13; pp. 2442 - 2444
• Main Authors: Stadlbauer, John M, Barnabas, Mary V, Walker, David C
• Format: Journal Article
• Language: English
• Published: American Chemical Society 27.03.1997
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp963392y

The reactions of muonium atoms with the individual species HO2CCO2H, HO2CCO2 -, and -O2CCO2 - at ∼295 K in water have absolute bimolecular rate constants of 5.6, 0.71, and 0.054 × 108 M-1 s-1, respectively. No such 100-fold pH-dependence has hitherto been seen for muonium, although H and eaq - also show large pH effects in reactions with oxalic acid. Arrhenius plots for the muonium reactions at pH = 1, 3, and 8 show the unusually small activation energies of ≤17 kJ/mol for all three acid−base forms of oxalic acid. When the reaction rate of muonium is compared with published data on ordinary H atoms at pH = 1 (57% oxalic acid and 43% hydrogen oxalate ions), one obtains a kinetic isotope effect of 850 favoring muonium. For the dianion at pH ≥ 7, the muonium-to-H rate ratio is >300. These seem to be the largest primary kinetic isotope effects ever reported. The reaction is taken to be addition of muonium to one of the CO groups, and the large isotope effect and small E a suggest considerable contributions from quantum mechanical tunneling.