Bonding, Aromaticity and Isomerization of Furfuraldehyde through Off-Nucleus Isotropic Magnetic Shielding

Off-nucleus isotropic magnetic shielding (σiso (r)) and multi-points nucleus independent chemical shift (NICS(0-2 Å)) index were utilized to find the impacts of the isomerization of gas-phase furfuraldehyde (FD) on bonding and aromaticity of FD. Multidimensional (1D to 3D) grids of ghost atoms (bqs) were used as local magnetic probes to evaluate σiso (r) through gauge-including atomic orbitals (GIAO) at density functional theory (DFT) and B3LYP functional/6-311+G(d,p) basis set level of theory. 1D σiso (r) responses along each bond of FD were examined. Also, a σiso (r) 2D-scan was performed to obtain σiso (r) behavior at vertical heights of 0-1 Å above the FD plane in its cis, transition state (TS) and trans forms. New techniques for evaluating 2D σiso (r) cross-sections are also included. Our findings showed that bonds in cyclic and acyclic parts of FD are dissimilar. Unlike the C-O bond of furanyl, the C=O bond of the formyl group is magnetically different. C-C and C-H bonds in furanyl are found similar to those in aromatic rings. A unique σiso (r) trend was observed for the C2 -C6 bond during FD isomerization. Based on NICS(0-2 Å) values, the degree of aromaticity follows the order of cis FD Collapse

Key Words

• aromaticity
• chemical bonding
• furfuraldehyde
• nucleus-independent chemical shift NICS
• off-nucleus isotropic shielding

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MESH Headings Collapse

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Affiliation(s)

Muntadar A. H. Al‐Yassiri Department of Chemistry – College of ScienceUniversity of BaghdadAl-JadiryaBaghdadIraq

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Magnetic shielding paints an accurate and easy-to-visualize portrait of aromaticity

Chemists are trained to recognize aromaticity semi-intuitively, using pictures of resonance structures and Frost-Musulin diagrams, or simple electron-counting rules such as Hückel's 4n + 2/4n rule. To quantify aromaticity one can use various aromaticity indices, each of which is a number reflecting some experimentally measured or calculated molecular property, or some feature of the molecular wavefunction, which often has no visual interpretation or may not have direct chemical relevance. We show that computed isotropic magnetic shielding isosurfaces and contour plots provide a feature-rich picture of aromaticity and chemical bonding which is both quantitative and easy-to-visualize and interpret. These isosurfaces and contour plots make good chemical sense as at atomic positions they are pinned to the nuclear shieldings which are experimentally measurable through chemical shifts. As examples we discuss the archetypal aromatic and antiaromatic molecules of benzene and square cyclobutadiene, followed by modern visual interpretations of Clar's aromatic sextet theory, the aromaticity of corannulene and heteroaromaticity.

Ortho-Quinone Methide Driven Synthesis of New O,N- or N,N-Heterocycles

To synthesize functionalized Mannich bases that can serve two different types of ortho-quinone methide (o-QM) intermediates, 2-naphthol and 6-hydroxyquinoline were reacted with salicylic aldehyde in the presence of morpholine. The Mannich bases that can form o-QM and aza-o-QM were also synthesized by mixing 2-naphthol, 2-nitrobenzaldehyde, and morpholine followed by reduction of the nitro group. The highly functionalized aminonaphthol derivatives were then tested in [4+2] cycloaddition with different cyclic imines. The reaction proved to be both regio- and diastereoselective. In all cases, only one reaction product was obtained. Detailed structural analyses of the new polyheterocycles as well as conformational studies including DFT modelling were performed. The relative stability of o-QMs/aza-o-QM were also calculated, and the regioselectivity of the reactions could be explained only when the cycloaddition started from aminodiol 4. It was summarized that starting from diaminonaphthol 25, the regioselectivity of the reaction is driven by the higher nucleophilicity of the amino group compared with the hydroxy group. 12H-benzo[a]xanthen-12-one (11), formed via o-QM formation, was isolated as a side product. The proton NMR spectrum of 11 proved to be very unique from NMR point of view. The reason for the extreme low-field position of proton H-1 could be accounted for by theoretical calculation of structure and spatial magnetic properties of the compound in combination of ring current effects of the aromatic moieties and steric compression within the heavily hindered H(1)-C(1)-C(12b)-C(12a)-C(12)=O structural fragment.

Magnetic Shielding, Aromaticity, Antiaromaticity, and Bonding in the Low-Lying Electronic States of Benzene and Cyclobutadiene

Aromaticity, antiaromaticity, and their effects on chemical bonding in the ground states (S₀), lowest triplet states (T₁), and the first and second singlet excited states (S₁ and S₂) of benzene (C₆H₆) and square cyclobutadiene (C₄H₄) are investigated by analyzing the variations in isotropic magnetic shielding around these molecules in each electronic state. All shieldings are calculated using state-optimized π-space complete-active-space self-consistent field (CASSCF) wave functions constructed from gauge-including atomic orbitals (GIAOs), in the 6-311++G(2d,2p) basis. It is shown that the profoundly different shielding distributions in the S₀ states of C₆H₆ and C₄H₄ represent aromaticity and antiaromaticity "fingerprints" which are reproduced in other electronic states of the two molecules and allow classification of these states as aromatic (S₀ and S₂ for C₆H₆, T₁ and S₁ for C₄H₄) or antiaromatic (S₀ and S₂ for C₄H₄, T₁ and S₁ for C₆H₆). S₂ C₆H₆ is predicted to be even more aromatic than S₀ C₆H₆. As isotropic shielding isosurfaces and contour plots show very clearly the effects of aromaticity and antiaromaticity on chemical bonding, these can be viewed, arguably, as the most succinct visual definitions of the two phenomena currently available.