Exploration of relative π-electron localization in naphthalene aromatic rings by C-HMIDLINE HORIZONTAL ELLIPSISπ interactions: experimental evidence, computational criteria, and database analysis

Based on the crystal engineering concept, four new esters containing alpha and beta-substituted naphthalene rings were designed, synthesized and fully characterized. The crystal structure analysis of the designed molecules revealed that the C-H-based interactions especially C-HMIDLINE HORIZONTAL ELLIPSIS pi have a significant role in the stabilization of crystal architectures. Regarding the molecular structure of naphthalene rings, one can find that bond lengths are divided into two groups (shorter and longer) so that relative localization is likely to occur in this system. Since pi-electron localization in aromatic rings has been previously introduced as responsible for stronger pi-stacking, this issue was investigated through C-HMIDLINE HORIZONTAL ELLIPSIS pi interaction in this study. Cambridge Structural Database (CSD) analysis was performed by exploration of suitable geometries for C-HMIDLINE HORIZONTAL ELLIPSIS pi(c) and C-HMIDLINE HORIZONTAL ELLIPSIS pi(e) in naphthalene and phenol systems (pi(c) and pi(e) are the centre and edge of the aromatic rings, respectively). The tendency factor (TF) of the C-H moiety toward ring bonds as a new CSD-based criterion was used to detect some of the resonance forms in aromatic rings. The results indicated that C-HMIDLINE HORIZONTAL ELLIPSIS pi(e) is a ubiquitous interaction which can be employed as an experimental probe for the detection of pi-localization in naphthalene rings. Systematic experimental studies, structural evidence and computational results on the title compounds confirmed the relative pi-electron localization, as well.