A Strong cis‐Effect in an Imidazole‐Imidazolium‐Substituted Alkene

Abstract

Abstract:We report the first example of an alkene with two carbon-bound substituents (imidazole and imidazolium rings) where the Z-isomer has agreater thermodynamic stability than the E-isomer which persists in both the gas phase and in solution. Theoretical calculations,s olution fluorescence spectroscopyand gas-phase ion mobility mass spectrometry studies confirm the preference for the Z-isomer,the stability of which is traced to an on-covalent interaction between the imidazole lone pair and the imidazolium ring.The greater thermodynamic stability of E-stilbene 1 (and Edisubstituted alkenes in general) over the Z-isomer, and the thermal/photoisomerism between the two forms,are concepts taught to virtually every chemistry student. Cases where the cis-orZ-isomer has greater thermodynamic stability are rare, mostly limited to di-halo (e.g. 2)ordimethoxy alkenes (e.g. 3), monohalo-propenes and 2-cyanopropene with strong electron-withdrawing atoms/groups are directly bound to the alkene.The phenomenon has been coined the "cis-effect" and the underlying rationale for the effect has been the subject of study over the last fifty years.[1] Allyl anions also adopt a Zconfiguration, particularly when coordinated to am etal counter-cation, however calculations suggest that the Zconfiguration is also preferred in the gas-phase anion. [2] Ford ifluoroethylene the Z-isomer is calculated to be more stable than the E-isomer by 4.5 kJ mol À1 at the CCSD-(T) level, while the effect is somewhat weaker in the other cases.[1i] Them ost recent proposals suggest the origin of the cis-effect in difluoroethylene arises from agreater correlation energy of the Z-isomer in combination with steric attractions between the s-and p-type lone pairs on Fatoms present in the Z-isomer.[1j]Here we report on the first example of anon-cyclic alkene bearing two carbon-atom-bound groups (a benzimidazole and benzimidazolium ring) where the Z-isomer is more thermodynamically stable than the E-isomer.Itisdemonstrated that non-covalent interactions between the benzimidazole and benzimidazolium ring is the most important factor in stabilizing the Z-isomer.All geometry optimizations were carried out in the gasphase with the B3LYP density functional [3] and 6-311+ ++ +G-(d,p) basis set [4] within Gaussian 09.[5] Stationary points were characterized as minima by calculating the Hessian matrix analytically at this level of theory.For comparison, geometries were optimized with B3LYP-D3(BJ), [6] yielding deviations of less than 0.005 in calculated bond distances to the B3LYP geometry.Molecular orbitals (MO) and natural bond orbitals (NBO) were calculated at the B3LYP/6-311+ ++ +G(d,p) level of theory.N BOs were calculated with the NBO 5.9 program. [7] Single-point energies were calculated at the B3LYP/6-311+ ++ +G(d,p) gas phase geometries.D FT (including TD-DFT) and HF calculations were carried out within Gaussian 09, [5] with solvent effects employing aP CM approach [8] with Tr uhlarsS MD model with acetonitrile.[9] DLPNO-CCSD(T) [10] calculations with the...

Tautomerism and electronic spectroscopy of protonated 1- and 2-aminonaphthalene

“…Such conditions correspond to a diffusion regime in which elastic collisions between the molecules of interest and buffer gas (air) occur, without activation. This effect has already been observed when investigating in-source dissociation processes 51,59 . Here it is assumed that most of the activation of the ions occurs in the high pressure region of the ESI, with very weak increase of the internal energy in the octopole where the pressure is reduced by three orders of magnitude.…”

“…Provided that ESI tends to conserve the liquid phase structure, one has to be careful in the assignment of the most stable forms of protonated species when using electrospray sources and photofragmentation spectroscopy. It seems that ion mobility may be a better method 60,59 as long as one can reach thermal equilibrium and if the collisional cross sections between tautomers are different enough 27,28,30,56 .…”