¹H/²H NMR Studies of Geometric H/D Isotope Effects on the Coupled Hydrogen Bonds in Porphycene Derivatives

Abstract

The 1H and 2H NMR spectra of porphycene (1), 2,3,6,7,12,13,16,17-octaethylporphycene (2), 2,7,12,17-tetra-n-propylporphycene (3), and 2,7,12,17-tetra-(tert-butyl)-3,6-13,16-dibenzo[cde;mno]porphycene (4) partially deuterated in the mobile proton sites are reported. These compounds exhibit two intramolecular NHN hydrogen bonds of increasing strength representing models of the concerted HH transfer in the parent compound, porphycene. The 1H chemical shifts of the mobile protons are correlated with the difference of the energies of the amino- and imino-N1s orbitals reported by Ghosh A.; Moulder J.; Bröring M.; Vogel E. Angew. Chem., Int. Ed. 2001, 113, 445-448. The chemical shifts of 4 indicate a reduced contribution of the aromatic ring current as compared to the other compounds which is associated to the nonplanarity of this molecule. The primary H/D isotope effects on the chemical shifts give information about the primary, secondary, and vicinal geometric isotope effects of the two inner hydrogen bonds of porphycenes. The vicinal effects indicate a cooperative coupling of the two hydrogen bonds which may favor a concerted double proton-transfer mechanism.

Excited-State Proton Transfer via Hydrogen-Bonded Acetic Acid (AcOH) Wire for 6-Hydroxyquinoline

“…However, the results of ab initio calculations and the two-color resonant two-photon ionization (2C-R2PI) spectrum of 7HQ-(NH 3 ) 3 suggest that a preliminary step of fast hydroxyl depro- tonation in excited 7HQ producing the 7HQ anion species occurs before imine protonation to generate the excited 7HQ-keto tautomer. Notice that the order of proton transfer is sensitive to the environment pH for hydroxyquinoline.…”

A questionable excited-state double-proton transfer mechanism for 3-hydroxyisoquinoline

“…[12][13][14][15][16][17][18][19][20] Proton transfer (PT) is one of the most important reactions in chemical and biological acid-base dynamics resulting from site-specific interactions through hydrogen bonding. [21][22][23][24] The study of excited-state intramolecular and intermolecular proton transfer (ESIPT) processes has been an active area of research since the first experimental observation of the phenomenon by Weller et al in the middle of the last century. 25 PT occurs in molecules containing both acidic and basic groups in close proximity that may rearrange in the excited electronic state via a proton or hydrogen atom transfer.…”

Mode‐Selective Promotion and Isotope Effects of Concerted Double‐Hydrogen Tunneling in Porphycene Embedded in Superfluid Helium Nanodroplets

“…Up to now, experimental data did not provide conclusive evidence for the stepwise or concerted mechanisms in Pc. [2,3] …”