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

Off‐nucleus isotropic magnetic shielding (σ (iso)(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 σ (iso)(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 σ (iso)(r) responses along each bond of FD were examined. Also, a σ (iso)(r) 2D‐scan was performed to obtain σ (iso)(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 σ (iso)(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 σ (iso)(r) trend was observed for the C(2)−C(6) bond during FD isomerization. Based on NICS(0‐2 Å) values, the degree of aromaticity follows the order of cis FD<trans FD<furan<TS FD.