Helical versus linear Jahn–Teller distortions in allene and spiropentadiene radical cations

Abstract

The allene radical cation can be stabilized both by Jahn-Teller distortion of the bond lengths and by torsion of the end-groups. However, only the latter happens and the allene radical...

Jahn–Teller effects in initial and final states: high-resolution X-ray absorption, photoelectron and Auger spectroscopy of allene

“…2. [43][44][45][46] The choice of the molecular orbitals is completely arbitrary. If we adopt the two rotation axes, we end up with the so called helical molecular orbitals that span the whole length of the molecule 43 and are of interest for their role in the reactivity and physical properties of such molecules.…”

“…[43][44][45][46] The choice of the molecular orbitals is completely arbitrary. If we adopt the two rotation axes, we end up with the so called helical molecular orbitals that span the whole length of the molecule 43 and are of interest for their role in the reactivity and physical properties of such molecules. [43][44][45][46][47][48][49] Secondly, after the removal of an electron, allene exhibits a characteristic Jahn-Teller distortion along internal coordinates which can lead to bond-length changes, torsion, or both, with recent work by Garner et al 43 suggesting the torsional motion is energetically stabilising for the allene cation.…”

“…If we adopt the two rotation axes, we end up with the so called helical molecular orbitals that span the whole length of the molecule 43 and are of interest for their role in the reactivity and physical properties of such molecules. [43][44][45][46][47][48][49] Secondly, after the removal of an electron, allene exhibits a characteristic Jahn-Teller distortion along internal coordinates which can lead to bond-length changes, torsion, or both, with recent work by Garner et al 43 suggesting the torsional motion is energetically stabilising for the allene cation. By combining state-of-the-art quantum chemical approaches with the nuclear ensemble approach (NEA), 50,51 that naturally captures accurate peak positions, widths, and intensities as well as non-Condon effects, a new analysis and decomposition of the highresolution XAS spectra of allene can be performed.…”

The fundamental relation between electrohelicity and molecular optical activity

“…The even-carbon series of cumulenes are much more explored due to their increased synthetic availability relative to the odd-carbon series. [54][55][56][57] They have co-planar end-groups in the ground-state structure, 8 and achieves helical frontier MOs as its symmetry is reduced by torsion of the end-groups. As the end-groups of butatriene ([3]cumulene) are rotated out of plane, its two π-systems mix and even at small dihedral angles the MOs achieve visually clear helical character.…”

“…7 However, its experimental relevance is limited by the simultaneous presence of both helicities in the electronic structure. 8 Nonetheless, relations to single-molecule conductance, 9,10 reaction selectivity, 11 and optical properties [12][13][14][15] have recently been suggested. Is the electrohelicity effect a fundamental cause for a strong chiroptical response?…”