Quantification of Aromaticity in Oxocarbons: The Problem of the Fictitious “Nonaromatic” Reference System

Using three major criteria to evaluate aromaticity of five-member C-containing rings and their Si-, N-, and P-substituted aromatic heterocyclics

“…NICS(0), calculated at the center of the rings, revealed the opposite qualitative order to the homodesmotic ASE values; such NICS values computed at the ring centers were due to the nearly complete diatropic and paratropic compensation, which was not so precise. To reflect the π -electron effects better, NICS(0.6) [20] and NICS(0.8) [21], computed 0.6 and 0.8 Å above ring centers respectively, were taken here. And NICS(0.6), NICS(0.8) revealed the same qualitative order as the homodesmotic ASE values.…”

“…In this index, the NICS value of studied molecule is computed as the negative magnetic shielding at some selected point in space, e.g. at a ring or cage center, 0.5 Å [19], 0.6 Å [20], 0.8 Å [21], 1.0 Å [22], and etc, above the plane of the rings. Comparing with other criteria, NICS does not require reference standards, increment schemes and calibrating (homodesmic) equations for evaluation; and more important, in several sets of related molecules, NICS correlates well with other aromaticity indexes based on geometric, energetic, and other magnetic criteria [23].…”

Vibrational dephasing of the croconate dianion in different environments

“…2 -4 Ab initio quantum chemistry calculations have been used in quantifying aromaticity in oxocarbon species. 5,6 Vibrational spectra and electronic structure of oxocarbon ions have been the subject of resonance Raman and optical spectroscopy investigations. 7 -10 In a continued investigation of microscopic structure and dynamics of oxocarbon species in solution, we considered the combined use of Raman and molecular dynamics (MD) simulation techniques.…”

The adsorption of squaric acid and its derived species on silver and gold surfaces studied by SERS

“…To a great extent, such interest originates from the peculiar structure of the oxocarbon anions and their derived species, where an extensive electronic delocalization is present. Recently this question has been addressed in detail on the basis of new X-ray and NMR data, which shed new light on the controversial issue of the aromaticity of the oxocarbon [3,4]. In particular, squaric acid, and its deprotonated species, hydrogenosquarate (HSq À ) and squarate (Sq 2À ), whose structures are shown in Scheme 1, have been the source of considerable interest as synthons for supramolecular structures [5], starting materials for highly electron-delocalized structures, such as squaraines [6], and charge transfer complexes, such as that involving squaric acid and tetrathiafulvalene, which shows remarkable optical and electronic behavior [7].…”