Aromaticities of azines relative to benzene; a theoretical approach through the dimethyldihydropyrene probe Maria a , Riffat U. Nisa a , Muhammad Hanif a , Adeem Mahmood a† and Khurshid Ayub a,b * The aromaticities of azines relative to benzene have been estimated by fusion with 15,16-dimethyldihydropyrene. Chemical shift data for the azine-fused dihydropyrenes (calculated at GIAO HF/6-31G*//B3LYP/6-31 + G*) were used to estimate the reduction in the dihydropyrene nucleus aromaticity. Choice of the saturated reference model was quite crucial in reliable estimation of aromaticity. Reference models with partial unsaturation at azine (21,23,25–32) gave better estimate of aroma- ticity than the parent dimethyldihydropyrene. Aromaticities of azines through chemical shift data and geometric parameter analysis were found to be 90–100% to that of benzene, highly consistent with the aromaticity estimation by nucleus indepen- dent chemical shift (0)πzz calculations. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: Aromaticity quantification; Azines; Density functional theory; Dimethyldihydropyrene probe; Magnetic and geometric criteria INTRODUCTION Aromaticity is a general, commonly used, but quite controversial concept in organic chemistry. Qualitative description of a compound as aromatic, non-aromatic or anti aromatic is gener- ally less contentious; however, quantitative estimation of aroma- ticity is not trivial and generally leads to controversies, primarily due to the quantification methods applied. [1] The aromaticity of a compound may vary considerably depending on the method used for quantitative analysis. Three major categories to quantify aromaticity are energetic, structural and magnetic, essentially all theorectical Dewar resonance energy, [2–5] Huckel resonance energy, [6–8] Hess-Schaad resonance energy, [9–14] Schleyer isomerization sta- bilization energies [15] and topological resonace energies [16–18] are a few important energetic criteria. The Harmonic Oscillator Model of Aromaticity (HOMA), [19–21] Julg aromaticity index, [22] Bird’ s aromaticity index [23–27] and Fringuelli structural index [28,29] are the most important structure-based methods for the quantification of aromaticity. The most common magnetic criteria include magnetic suscepti- bility exaltation, [30–41] nuclear magnetic resonance (NMR) [36,42–48] and nucleus independent chemical shifts (NICS). [49] NMR-based methods are generally more diverse and include chemical shift analysis of 3 He and 7 Li nuclei placed above the aromatic nucleus, [36,42–48] 1 H chemical shift [50,51] analysis of probe protons usually in the center of the nucleus under consideration and coupling constants [52,53] analysis in H-NMR (Gunther Q-values). However, NMR-based methods generally require that a suitable model or probe is chosen. NMR-based methods may even provide experimental scale of aromaticity for theoretical NICS values. [54] A probe of high accuracy based on 1 H NMR chemical shift is 15,16-dimethyldihydropyrene 1. The internal methyl protons in 15,16-dimethyldihydropyrene 1 appear at δ 4.25 and its comparison with the non-conjugated model 2 δ 0.97 indicates large shielding of ~5.2 ppm due to a strong ring current. [50] When an aromatic ring is [a]- or [e]- fused to the dimethyldihydropyrene, the ring current in the latter is re- duced. The internal protons of [e]- fused benzodihydropyrene [50] 3 appear at 1.85 ppm which means that the internal methyl protons in 3 are shielded by 2.82 ppm, and this leads to an experimental estimate of the aromaticity for 3 relative to 1 to be 52%. The greater the aromaticity of the fused ring, the greater is the reduction in the ring current of the dihydropyrene (DHP), and this concept can be used to compare the relative aromatic- ities of any two molecules provided the following two conditions are met: (i) the effect of fusion on the geometry of the probe * Correspondence to: K. Ayub, Department of Chemistry COMSATS Institute of Information Technology Abbottabad 22060, Pakistan. Email: kayub@kfu.edu.sa; khurshid@ciit.net.pk † Current Address: Chemistry Department, King Saud University, Riyadh 11451, Saudi Arabia a Maria, R. U. Nisa, M. Hanif, A. Mahmood, K. Ayub Department of Chemistry, COMSATS Institute of Information Technology, Abbottabad 22060, Pakistan b K. Ayub Department of Chemistry, College of Science, King Faisal University, Al Ahsa 31982, Kingdom of Saudi Arabia Research Article Received: 23 March 2014, Revised: 27 July 2014, Accepted: 6 August 2014, Published online in Wiley Online Library: 17 September 2014 (wileyonlinelibrary.com) DOI: 10.1002/poc.3348 J. Phys. Org. Chem. 2014, 27 860–866 Copyright © 2014 John Wiley & Sons, Ltd. 860