Spatial Contributions to 1H NMR Chemical Shifts of Free-Base Porphyrinoids

Abstract

A recently developed methodology for calculating, analyzing, and visualizing nuclear magnetic shielding densities is used for studying spatial contributions including ring-current contributions to 1H nuclear magnetic resonance (NMR) chemical shifts of aromatic and anti-aromatic free-base porphyrinoids. Our approach allows a visual inspection of the spatial origin of the positive (shielding) and negative (deshielding) contributions to the nuclear magnetic shielding constants. Diatropic and paratropic current-density fluxes yield both shielding and deshielding contributions implying that not merely the tropicity of the current density determines whether the contribution has a shielding or deshielding character. Instead the shielding or deshielding contribution is determined by the direction of the current-density flux with respect to the studied nucleus.

Current-density pathways in figure-eight-shaped octaphyrins

“…multiplied with the first derivative of the vector potential of the nuclear magnetic moment with respect to the strength of the magnetic moment in the limit of vanishing magnetic moment [46][47][48][49][50][51][52][53] The Biot-Savart expression for calculating the shielding tensor elements then reads…”

“…The sign change leads to the alternating shielding and deshielding contributions as shown in Figure 6. 52,53 Table 3 The average 1 H NMR shielding constant (in ppm) of the inner hydrogen atoms and of the outer β hydrogen atoms. For N4 and the N6 dication with 26 occupied conjugated orbitals the averaged 1 H NMR shielding constants are 21.3 ppm and 22.5 ppm, respectively.…”

“…. [46][47][48][49][50][51][52][53] The Biot-Savart expression for calculating the shielding tensor elements then reads…”

“…6. 52,53 Since the inner protons of the pyrrole rings are sensitive to the magnetic shielding or deshielding due to the ring-current, these signals are useful for estimating the aromatic character of porphyrinoids. The averaged 1 H NMR shielding constants of the inner hydrogen atoms shift are 18.3 ppm for o-N2, 16.9 ppm for p-N2 and 16.7 ppm for the N4 dication with 25 occupied conjugated orbitals.…”

Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday

“…Other papers present theoretical and methodological advances in the calculation of the magnetic response: a topological analysis of the magnetically induced current density in strong magnetic fields [13], the partitioning of Hückel-London currents into cycle contributions [14], and decomposition in spatial contributions to 1H NMR chemical shifts [15].…”