Effects of the β‐heteroatom and leaving group on neighbouring group participation in the gas phase: A density functional theory study

Abstract

AbstractObservation of the [M + H]+ ion in electrospray ionisation mass spectrometry (ESI‐MS) for either product identification or further unimolecular reactivity studies is not always guaranteed. When a suitable donor is β to a good leaving group such as protonated alcohols, neighbouring group participation (NGP) is one process that can lead to facile in‐source decomposition leading to three‐membered ring product ions. In this study, lone pair rich heteroatoms are explored computationally as donors for the intramolecular displacement of either water or ammonia in protonated 2‐heteroethanols and ethanamines, respectively. Density functional theory (DFT) calculations showed that for the halogens (Cl, Br and I) both the loss of water and ammonia are overall endothermic processes with modest barriers for the 2‐haloethanol fragmentation by NGP. The chalcogens (S, Se and Te) displayed a significant shift with the loss of water becoming exothermic and the enthalpic barrier approximately zero in most cases leading to little [M + H]+ ion observation. The loss of ammonia remained endothermic for both the chalcogens (S, Se and Te) and pnictogens (P, As and Sb), but the 2‐pnictoethanol fragmentation showed mixed results as two pathways were accessible due to the increased s character of the pnictogen lone pair. Examination of the donor–acceptor interactions by natural bond orbital (NBO) theory reflected an increase in the heteroatom lone pair participation at the transition state, but that stabilisation due to the polarisable CX bond dominated and should not be discounted in processes believed to proceed by nonvertical neighbouring group participation.