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Solid-State 25Mg NMR Spectroscopic and Computational Studies of Organic Compounds. Square-Pyramidal Magnesium(II) Ions in Aqua(magnesium) Phthalocyanine and Chlorophyll a

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posted on 24.08.2006, 00:00 by Alan Wong, Ramsey Ida, Xin Mo, Zhehong Gan, Jennifer Poh, Gang Wu
We report a solid-state 25Mg NMR spectroscopic study of two magnesium-containing organic compounds:  monopyridinated aqua(magnesium) phthalocyanine (MgPc·H2O·Py) and chlorophyll a (Chla). Each of these compounds contains a Mg(II) ion coordinating to four nitrogen atoms and a water molecule in a square-pyramidal geometry. Solid-state 25Mg NMR spectra for MgPc·H2O·Py were obtained at 11.7 T (500 MHz for 1H) for a 25Mg-enriched sample (99.1% 25Mg atom) using both Hahn-echo and quadrupole Carr−Purcell Meiboom−Gill (QCPMG) pulse sequences. Solid-state 25Mg NMR spectra for Chla were recorded at 25Mg natural abundance (10.1%) at 19.6 T (830 MHz for 1H). The 25Mg quadrupole parameters were determined from spectral analyses:  MgPc·H2O·Py, CQ = 13.0 ± 0.1 MHz and ηQ = 0.00 ± 0.05; Chla, CQ = 12.9 ± 0.1 MHz and ηQ = 1.00 ± 0.05. This work represents the first time that Mg(II) ions in a square-pyramidal geometry have been characterized by solid-state 25Mg NMR spectroscopy. Extensive quantum mechanical calculations for electric-field-gradient (EFG) and chemical shielding tensors were performed at restricted Hartee−Fock (RHF), density functional theory (DFT), and second-order Møller−Plesset perturbation theory (MP2) levels for both compounds. Computed 25Mg nuclear quadrupole coupling constants at the RHF and MP2 levels show a reasonable basis-set convergence at the cc-pV5Z basis set (within 7% of the experimental value); however, B3LYP results display a drastic divergence beyond the cc-pVTZ basis set. A new crystal structure for MgPc·H2O·Py is also reported.