Fourier transform pure nuclear quadrupole resonance by pulsed field cycling

We report the observation of Fourier transform pure NQR by pulsed field cycling. For deuterium, well resolved spectra are obtained with high sensitivity showing the low frequency nu0 lines and allowing assignments of quadrupole couplings and asymmetry parameters to inequivalent deuterons. The technique is ideally applicable to nuclei with low quadrupolar frequencies (e.g., 2D, 7Li, 11B, 27Al, 23Na, 14N) and makes possible high resolution structure determination in polycrystalline or disordered materials

Mechanism of CO₃^2- substitution in carbonate-fluorapatite: Evidence from FTIR spectroscopy, ¹³C NMR and quantum mechanical calculations

The substitution of CO32- in carbonate-fluorapatite (“francolite”) has a destabilizing effect on the apatite structure that results in increased solubility. A commonly proposed substitution mechanism involves the replacement of a PO43- ion by CO3F3-, in which C is at the center of a tetrahedron and is surrounded by three O atoms and one F atom at the corners. However, no spectroscopic evidence exists for this hypothetical anion.We have investigated the microenvironment of CO32- in 13C-enriched type B synthetic and natural sedimentary carbonate-fluorapatite samples by 13C static and MAS NMR and FTIR spectroscopies. (True CO32- substitution in the apatite structure was ascertained by X-ray diffraction and FTIR.) The CO32- ion itself consists of an sp2-hybridized planar arrangement of three O atoms about the central C atom. If the CO32- geometry were changed to an sp3 hybrid with F- ate one corner, the NMR chemical isotropic shift would change considerably, owing to the altered environment about the central C atom. Also, there would be an internuclear dipolar interaction between 13C and 19F.Simulated NMR spectra for 13C-19F distances of 1.38 and 2.00 Å based on the sp3 model are very different from those obtained on our apatite samples, which are instead characteristic of C in a planar sp2 arrangement and no 13C-19F dipolar coupling. No evidence from direct bonding between CO32- and F was found. This result is confirmed by ab-initio quantum mechanical calculations, which show that the hypothetical CO3F3- ion is unstable in an apatitic environment. Ab-initio results, as well as FTIR data, favor the existence of an interstitial position for F- if nonstoichiometric amounts of this element are present in apatite

2,3,5,6-Tetrakis(Methylene)-1,4-Cyclohexanediyl (1,2,4,5-Tetramethylenebenzene), a Disjoint Non-Kekule Singlet Hydrocarbon Biradical

The title compound 4a, a purple transient whose main UV-vis absorption band occurs near 490 nm (epsilon = 5000 M-1 cm-1), has been generated by irradiation of 2,3,5,6-tetramethylene-7-oxonorbornane (5a). It has been observed directly by immobilization in frozen matrices or in polymer films and by time-resolved spectroscopy following nanosecond later flash photolysis of the ketone precursor 5a. Although the matrix-immobilized irradiated samples containing biradical 4a show a triplet electron spin resonance (ESR) spectrum at low temperature, some other species is the carrier, since the ESR spectrum persists even after complete photobleaching of the UV-vis spectrum characteristic of 4a. The singlet spin state for biradical 4a, predicted by theory, is confirmed not only by the absence of an ESR spectrum assignable to it but also by the appearance of a C-13 cross-polarization magic angle spinning NMR spectrum (delta = 113 ppm) observed when glassy preparations of 5a-2,3-di-(CH2)-C-13 are irradiated. The rates of dimerization of 4a in fluid media, (1.9-3) x 10(10) M-1 s-1, and the rate of capture by O2, 1.86 x 10(7) M-1 s-1, are also consistent with the behavior expected of a singlet biradical