Theoretical studies on the role of π-electron delocalization in determining the conformation of N-benzylideneaniline with three types of LMO basis sets

Enhancing magnetic coupling through protonation of benzylideneaniline-bridged diradicals and comparison with stilbene- and azobenzene-based diradicals

Taking nitroxide radicals as spin sources, we explore the intramolecular magnetic coupling interactions of the trans- and cis-forms of benzylideneaniline (BA)-bridged diradicals, in which the central -CH[double bond, length as m-dash]N- unit can undergo single protonation to convert to its protonated counterpart or vice versa. The calculated results for these two pairs of diradicals (protonated versus unprotonated trans and cis forms) verify that the signs of their magnetic coupling constants J do not change, but the magnitudes significantly increase after protonation. In the structure, the better conjugation of the protonated trans diradical and two reduced CCNC and CCCN torsion angles of the protonated cis one make for a more efficient spin transport, promoting the spin polarization, thus leading to larger spin couplings. In terms of mechanism, the proton-induced magnetic enhancement should be attributed to strong participation of the coupler BA through its lowest unoccupied molecular orbital (LUMO) with a lower energy level after protonation, and the small HOMO-LUMO (HOMO: highest occupied molecular orbital) gap of the coupler BA through protonation is crucial in explaining such remarkable spin-coupling enhancement. Furthermore, different linking modes of the radical groups to the couplers are also considered to confirm our conclusions. In addition, we also make a comparison of the magnetic coupling strengths among their isoelectronic analogues of BA-, AB- and stilbene-bridged nitroxide diradicals before or after protonation, and find a linear correlation among them. It should be noted that the magnetic behaviors of all these diradicals obey the spin alternation rule and singly occupied molecular orbital (SOMO) effect. This work provides helpful information for the rational design of promising magnetic molecular switches.

New procedure to evaluate aromaticity at the density functional theory, Hartree-Fock, and post-self-consistent field levels

The spatial exchange interaction, arising from the exchange-type two-electron integrals (i(p)j(q)/i'(p)j'(q)) between two different groups P and Q, is another driving force for the delocalization of π-electrons besides orbital charge-transfer and exchange interactions. We have developed a new combination program for restricted geometry optimization, in which all of the orbital and spatial interactions among isolated groups were excluded from the localized geometry of a conjugated molecule. This was achieved by deleting particular Fock elements and the 15 types of exchange-type two-electron integrals, ensuring that the corresponding π-electrons are completely localized within their respective groups and the π-orbitals are fully localized. The extra stabilization energy (ESE) of benzene is -36.3 kcal/mol (B3LYP/6-31G*), and the level of density functional theory, Hartree-Fock, and post-self-consistent field (Møller-Plesset 2, configuration interaction singles and doubles, and singles and doubles coupled-cluster) and the basis sets have slight effect on the ESE. Based on the comparisons between our procedure, Morokuma's energy decomposition analysis and the block-localized wave function method, it was confirmed that our program calculates reliable results. The nonaromaticity of acyclic polyenes and antiaromaticity of cyclobutadiene and planar cyclooctatetraene were also estimated. Comparison of the CC single bond lengths in the ground state with its π-localized geometries showed that shortening of the single bonds in acyclic polyenes and butadiyne should be attributed to different hybridization, demonstrating that the effect of π-delocalization on single bonds is so small as to be negligible.

Restricted Geometry Optimization: A Different Way To Estimate Stabilization Energies for Aromatic Molecules of Various Types

At RHF, MPn, and DFT levels, a procedure of geometry optimization under the restrictions of pi-orbital interactions (GOR) was developed, thus providing a conjugated molecule with the following two types of localized reference geometries: a "GL" geometry where all double bonds are localized, and n different "GE-n" geometries, in each of which only two double bonds were permitted to conjugate. Interestingly, the molecular energy differences between the corresponding pairs of GE-n and GL geometries were found to be additive in each of the acyclic polyenes, and these were not additive for benzene. As a result, an extra stabilization energy (ESE) value of -39.0 kcal/mol was found in benzene. Afterward, GOR was applied to benzene- and furan-like species, strained aromatic molecules, and substituted benzenes, and the calculated ESEs for these molecules were found to be in reasonable ranges. The GOR can isolate a specific group from other groups, and it has several special functions. First, with regard to the substituent effect, the ESE difference between substituted benzene and benzene can be partitioned into conjugative and inductive parts. Second, the behavior of strained aromatic molecules can be ascertained from the roles of their resonance interactions, strained-induced bond localization (SIBL), and inductive effects, indicating that it is resonance interactions, rather than SIBL, which are responsible for localizing double bonds. Emphatically, it is the GL and GE-n geometries of aromatic molecules, rather than nonaromatic compounds, which can be used as the reference structures for calculating ESE. Particularly, these localized geometries are no longer arbitrary.