Rearrangements of 2-Nitrobenzyl Compounds. 1. Potential Energy Surface of 2-Nitrotoluene and Its Isomers Explored with ab Initio and Density Functional Theory Methods

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2000-08, Vol.104 (33), p.7856-7870
• Main Authors: Il'ichev, Yuri V, Wirz, Jakob
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp000261v

Reaction paths leading from 2-nitrotoluene (1) to 2-nitrosobenzyl alcohol (4) were investigated by using ab initio and density functional theory. Overall, 25 minima and 12 transition states were located for molecules of composition C7H7NO2. Nine conformational minima were found for 4, all being slightly lower in energy than 1. The lowest-energy path from 1 to 4 was predicted to have an overall activation energy of 51.7 kcal mol-1 at the B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d,p) level of theory. The rate-determining step is a 1,3-hydrogen shift between two oxygen atoms in the aci-form 2. The activation barrier of 20.3 kcal mol-1 predicted for this shift is substantially higher than that of 9.7 kcal mol-1 for the retautomerization 2 → 1. To account for reaction paths involving ionization of the moderately strong acid 2 in aqueous solution, anions obtained by deprotonation of 1−4 were studied. A common anion 2 - was found by CH-deprotonation of 1 and by OH-deprotonation of two aci-isomers 2. An anion 3 - was obtained by deprotonation of benzisoxazoline 3 and nitrone 6. A single conformer 4 - was located by deprotonation of 4. At the B3LYP/6-311+G(2d,p) level the activation barrier of 39.2 kcal mol-1 for the reaction 2 - → 3 - is much higher than that of 18.4 kcal mol-1 for the cyclization of the neutral aci-form, trans-2 → 3a. The barrier height for the latter reaction was reproduced within 1.0 kcal mol-1 by calculating QCISD(T)/6-31G(d,p) energies at the B3LYP and MP2/6-31G(d) geometries. On the other hand, the reaction 3 - → 4 - is expected to be very facile in contrast to ring opening in the neutral species, 3 → 4. These results indicate that the cyclization 2 → 3 is acid-catalyzed around neutral pH and that the subsequent ring opening 3 → 4 is base-catalyzed.