High-level ab initio mapping of the multiple H-abstraction pathways of the OH + glycine reaction

Abstract

We perform a systematic search in the transition-state (TS) and product-channel complex (MIN) regions of the multi-channel OH + glycine → H₂O + H₂N–CH–COOH (CH)/HN–CH₂–COOH (NH)/H₂N–CH₂–COO (COOH) reactions. Geometry optimizations reveal {7, 3, 3} CH-TS, {2, 2, 2} CH-MIN, {17, 10, 5} NH-TS, {35, 19, 19} NH-MIN, and {6, 5, 5} COOH-TS conformers at the {MP2/3-21G, MP2/aug-cc-pVDZ, CCSD(T)-F12b/aug-cc-pVDZ} levels of theory as well as 2 additional CH-TSs based on chemical intuition. The benchmark relative energies of the TS, MIN, and product conformers are obtained by considering basis set effects up to aug-cc-pVQZ using the explicitly-correlated CCSD(T)-F12b method, post-(T) correlation up to CCSDT(Q), core correlation, scalar relativistic effects, spin–orbit coupling, and zero-point energy corrections. All the CH [ΔE_e(ΔH₀) = −38.54(−38.61) kcal mol⁻¹], NH [ΔE_e(ΔH₀) = −16.72(−17.98) kcal mol⁻¹], and COOH [ΔE_e = −4.98 kcal mol⁻¹] reactions are exothermic and proceed via shallow, usually negative, classical(adiabatic) barriers of −0.37(−0.95), −1.91(−2.48), and 1.02(−0.57) kcal mol⁻¹, respectively. In the entrance channel MRCI/aug-cc-pVTZ computations reveal several complexes with reactive(non-reactive) arrangements and binding energies of 1.0, 1.6, 3.3, (5.2 and 5.9) kcal mol⁻¹, stabilized by CH⋯OH, NH⋯OH, COOH⋯OH, (OH⋯OC and OH⋯N) hydrogen bonds, respectively.