A comparative study of the aromaticity of pyrrole, furan, thiophene, and their aza-derivatives

Abstract

The relative aromaticity of pyrrole, furan, thiophene, and their aza-derivatives has been examined using TRE (topological resonance energy), MRE (magnetic resonance energy), ring current (RC), and ring current diamagnetic susceptibility (χG) methods. The results obtained were compared with results obtained by others who used the energetic method ASE (aromatic stabilization energy), the geometric method HOMA (harmonic oscillator model of aromaticity), and the magnetic method NICS(1) (nucleus-independent chemical shift). The impact of nitrogen atoms on the aromaticity of the aza-derivatives of pyrrole, furan, and thiophene is discussed. An excellent correlation was found between the energetic (TRE, MRE) and magnetic (RC and χG) criteria of aromaticity for all compounds. It was expected that inclusion of a heteroatom would decrease the aromaticity relative to the cyclopentadienyl anion. Our results show that the type of the first heteroatom, which donates two electrons to the system, as well as the number of nitrogen atoms and their positions in the molecule have a strong effect on aromaticity. In general, aromaticity is enhanced when the nitrogen atom is adjacent to the first heteroatom. The magnitude of aromaticity is related closely with the uniformity of distribution of π-electrons in the molecule.

New Donor-Acceptor Stenhouse Adducts as Visible and Near Infrared Light Polymerization Photoinitiators

“…When examining the nucleophilic addition of these five amines A1-A5 to PP1-PP20, no reaction was detected for any of the dyes bearing thiophenes or pyrroles as heterocycles. This is directly related to an aromaticity increase of the order furan < pyrrole < thiophene, impeding the ring-opening reaction of pyrrole and thiophene by a nucleophile [53][54][55]. A similar behavior was also observed with PP17-PP22, despite the presence of the furyl group.…”

Weak interactions in furan dimers

“…Except for the practical reasons listed above, furan is popular in the literature because it is an aromatic heteroatomic compound [11–13], and the aromatic ring with a delocalized electron cloud and a heteroatom included into the ring determines the furan intermolecular interaction as well as the interaction of furan with other molecules. Therefore, furan and its complexes with small molecules were the subject of many experimental and theoretical studies [14–18].…”

Sensitivity of high-order-harmonic generation to aromaticity

“…There are also other interesting schemes that link aromaticity to measurable observables based on molecular response properties, such as electric polarizabilities and hyperpolarizabilities and nuclear magnetic shieldings [9]. The different computational and experimental approaches of measuring aromaticity lead to different ordering of aromatic molecules [10,11]. This disagreement is due to the presence of competing effects beside electron delocalization that can influence the results of these methods.…”

“…ASE and χ G [11] measures consider pyrrole to be more aromatic than thiophene, whereas according to NICS(1) [31] measure thiophene is more aromatic. Other versions of NICS [1,22] indicate a classification that agrees with ASE and χ G .Onthe other hand, the HOMA [11] index gives comparable values for both molecules. Figure 4(a) shows the ratio of the harmonic signal of pyrrole to furan for three different intensities at 1825 nm.…”