Which is more aromatic

Again, this result does not concur with the greater stability of the linear isomer. Figure 5. Atoms-in-Molecules AIM hydrogen atomic energies in a. In italics, values for dicationic species. The main conclusion of this work is that the relative stability of kinked vs. So, the answer to the question whether kinked is more stable than straight depends on the aromaticity of the two isomers. When two electrons are removed, i. The larger stability of the former also correlates with its larger aromaticity, despite the difference in aromaticity between the isomers is now smaller.

Finally, tetramethylated-pyrano-chromene isomers almost present the same aromatic character, whereas pyrilium derived systems show divergent aromaticities between central and terminal rings, i. In these cases, when no isomer is clearly more aromatic, the relative stability is determined by the balance between the lower steric repulsion of the kinked vs.

Just to conclude, our study together with other works Poater et al. This interaction is more stable for the kinked isomer in the three sets of isomers analyzed; however only in one case, the kinked system is more stable. JP and MS conceived and designed research. JP performed the calculations. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

We are very grateful for the advice given by Dr. Alkorta, I. A , — Anusooya, Y. Ring currents in condensed ring systems. Quantum Chem. Bader, R. Atoms in Molecules: A Quantum Theory. Clarendon, Oxford. A quantum theory of molecular structure and its applications. Pauli repulsions exist only in the eye of the beholder.

Balaban, A. Is aromaticity outmoded? Pure Appl. Becke, A. Density-functional thermochemistry. The role of exact exchange. Behrens, S. Bickelhaupt, F. Lipkowitz and D. The case for steric repulsion causing the staggered conformation of ethane.

Biermann, D. Diels-alder reactivity of polycyclic aromatic hydrocarbons. Acenes and benzologs. Boschi, R. Photoelectron spectra of polynuclear aromatics. The effect of nonplanarity in sterically overcrowded aromatic hydrocarbons.

Bultinck, P. Multicenter bond indices as a new measure of aromaticity in polycyclic aromatic hydrocarbons. Cheeseman, J. A comparison of models for calculating nuclear magnetic resonance shielding tensors. Chen, Z. Clar, E. Polycyclic Hydrocarbons. London: Academic Press. The Aromatic Sextet. New York, NY: Wiley. Cyranski, M. Dabestani, R. A compilation of physical, spectroscopic and photophysical properties of polycyclic aromatic hydrocarbons.

Google Scholar. Danovich, D. Theory Comput. Dewar, M. Ground states of conjugated molecules. Improved treatment of hydrocarbons. Dominikowska, J. Does the concept of Clar's aromatic sextet work for dicationic forms of polycyclic aromatic hydrocarbons?

Dihydrogen contacts in alkanes are subtle but not faint. Nature Chem. El-Hamdi, M. Organometallics 32, — Eskandari, K. Hydrogen—hydrogen interaction in planar biphenyl: a theoretical study based on the interacting quantum atoms and Hirshfeld atomic energy partitioning methods. Feixas, F. A test to evaluate the performance of aromaticity descriptors in all-metal and semimetal clusters.

An appraisal of electronic and magnetic indicators of aromaticity. CrossRef Full Text. On the performance of some aromaticity indices: a critical assessment using a test set. Quantifying aromaticity with electron delocalisation measures. The activation strain model and molecular orbital theory: understanding and designing chemical reactions.

Fliegl, H. Gauge-origin independent calculations of the anisotropy of the magnetically induced current densities. Fradera, X. The Lewis model and beyond. Electron-pairing analysis from localization and delocalization indices in the framework of the atoms-in-molecules theory.

Frisch, M. Self-consistent molecular orbital methods Supplementary functions for Gaussian basis sets. Gaussian 09, Revision C. Wallingford, CT, Gaussian, Inc. Fukui, K. Role of frontier orbitals in chemical reactions. Science , — The role of frontier orbitals in chemical reactions Nobel Lecture. Geuenich, D. Giambiagi, M. Multicenter bond indices as a measure of aromaticity. Grimme, S. When do interacting atoms form a chemical bond? The most common heterocyclic compounds contain carbon along with nitrogen, oxygen, or sulfur.

Because some heterocyclic compounds are aromatic, it is important to discuss how the inclusion of non-carbon atoms affects the determination of aromaticity. The reactivity and general physical properties of aromatic nitrogen heterocycles will be discussed in greater detail in Section Pyridine is an example of a six-membered aromatic heterocycle and has an electronic structure similar to benzene. In the bonding picture of pyridine the five carbons and single nitrogen are all sp 2 hybridized.

All six of these atoms have a p orbital perpendicular to the plane of the ring and each contains one pi electron which allows the ring to be fully conjugated.

This makes the pi electron count for pyridine 6 pi electrons. The lone pair electrons on pyridine's nitrogen are contained in a sp 2 orbital that lies in the same plane as the ring and does not overlap with the p orbitals of the ring. The lone pair electrons are not part of the aromatic system and do not affect the p electron count. Pyrimidine is an another six-membered aromatic heterocycle that is analogous to pyridine. Pyrimidine has four carbons atoms and two nitrogen atoms that are sp 2 hybridized.

Each of these atoms contributes a p orbital and a pi election allowing pyrimidine to be fully conjugated and aromatic. Both of the nitrogens in pyrimidine have lone pair electrons contained in a sp 2 orbitals and are not involved in the aromatic system. An electrostatic potential map of pyrimidine shows that neither set of lone pair electrons is distributed around the ring.

Pyrrole is a five-membered heterocyclic ring which has 5 p orbitals and six pi electrons contributing to its aromaticity. Each carbon in pyrrole contributes one p orbital and pi electron. Aromaticity 14 — Homocyclic aromatic systems. However, pyrrole is electrically neutral. The presence of nitrogen leads to loss of radial symmetry in the molecule.

Also, the resonance structures of pyrrole are not equivalent as in cyclopentadienyl ion, due to the presence of the heteroatom. The first resonance structure has no charge. The next structures have a positive charge on N and the negative charge and double bonds alternate with each other as shown below-.

Similar resonating structures can be written for furan and thiophene. The only difference is that the N-H will be replaced by O and S respectively.

Also, the electronegativity of S and O is more than N. Unlike pyridine, pyrrole is an extremely weak base as the lone pair of electrons are required to maintain the aromatic sextet. Whereas in other molecules, the heteroatoms being more electronegative than carbon, they pull the electron cloud towards themselves.

Thus, there is an uneven charge distribution. More electronegativity of the hetero atom less — is the aromaticity of the compound. Oxygen is the most electronegative and so it is the least aromatic. Sulphur is less electronegative. Also the 3s and 3p electrons are the valence electrons in sulphur atom as opposed to 2s and 2p in oxygen and nitrogen.