Magnetic inequivalence

In an Hg22+ asymmetrically complexed by two crown-ethers of different ring size, an extremely high coupling constant /( "Hg, 9Hg) of 284 kHz between the magnetically inequivalent Hg nuclei has been observed.4 3... 

One of the most promising techniques for studying transition metal ions involves the use of zeolite single crystals. Such crystals offer a unique opportunity to carry out single crystal measurements on a large surface area material. Suitable crystals of the natural large pore zeolites are available, and fairly small crystals of the synthetic zeolites can be obtained. The spectra in the faujasite-type crystals will not be simple because of the magnetically inequivalent sites however, the lines should be sharp and symmetric. Work on Mn2+ in hydrated chabazite has indicated that there is only one symmetry axis in that material 173), and a current study in the author s laboratory on Cu2+ in partially dehydrated chabazite tends to confirm this observation. 

NMR time scale (ca. 10 s), the lithium spins see an average manganese oxidation state of 3.5 (i.e., Mn 5+ ions), and thus there is only one magnetically inequivalent lithium site (the 8a site). The NMR spectra are clearly sensitive to this hopping mechanism, and NMR spectroscopy may be used to follow the partial charge-ordering process that occurs just below room temperature in the stoichiometric material. 

Si—Si bond length, the Sil—Si2 being slightly shorter than Si2—Si3 (2.226 vs 2.263 A), and explains the magnetic inequivalence of Sil and Si3 noted above. 

IrCl(CO)(H)2(PPh3)2, with two chemically equivalent but magnetically inequivalent hydrides [203, 211, 212],... 

Some examples to illustrate the use of this spin system notation to distinguish between first- and second-order systems and to explain the concept of magnetic inequivalence will now be discussed. Because is the only spin-1/2 nucleus with 100% natural abundance that forms a wide variety of inorganic ring systems, most of the examples are taken from phosphorus chemistry (for other examples, see Chapter 11). 

The cyclophosphinophosphonium ions Me(PR) ( = 3, 4, 5) (see Section 11.3.1.2 and Scheme 11.2) provide an informative example of magnetic inequivalence. Whereas the P H NMR spectra for the three-membered ring 3.12a and the four-membered ring 3.12b are essentially first order (A2X and A2MX, respectively), the five-membered architecture of 3.12c results in magnetic inequivalence of the two pairs of phosphorus atoms that, in solution, are related by a C2 axis. Consequently, complex second-order spectra resulting from an AA BB X spin system are observed. 

The material YCaA104 is of interest for solid state lasers. The orientational dependence of the HFEPR spectra at 250 GHz was measured32 for an iron-doped crystal using a goniometer in a quasi-optical spectrometer at 253 K The spectra of Fe3+, S — 5/2, revealed the existence of two magnetically inequivalent sites of roughly equal concentration. Only the site with its z-axis along the c-axis of the crystal was studied in detail and was found to have a 0-factor close to 1.99 and a ZFS of about 29 GHz. 

The 55Mn2+ ion, S = 5/2,1 = 5/2, was studied in a-ZnV207. The orientation dependence of the spectra at 250 GHz was measured32 using a goniometer in a quasi-optical spectrometer at 253 K. Two magnetically inequivalent, but physically equivalent, sites were detected that could be described by the same set of spin Hamiltonian parameters. The (/-factor was found to be isotropic with a value close to 2 and the ZFS was 6 GHz. 

The EPR of a Mn2+ impurity in mixed crystal N ClajIm has been observed38 between X-band and 250 GHz. Spectra from three magnetically inequivalent, but physically equivalent, Mn2+ ions were observed. The temperature dependence of the linewidth revealed phase transitions at 228 K and 242 K. The analysis of all the spectra was consistent with a model of an axially symmetric crystal field... 

Point 2 is the more obvious and can be demonstrated for a complex such as III. Clearly the two magnetically inequivalent tin atoms... 

Ammonium nitrate has two magnetically inequivalent nitrogens. The ammonium nitrogen frequencies have not been observed by NQR from NMR measurements, the quadrupole... 

Ans. The material is 9.55. The substrate has two enantiofaces and each en-antioface can have either up or down orientation with respect to NiL, which has Cl symmetry. This gives four diastereomers with two P atoms, each being magnetically inequivalent. At higher temperatures the diastereomers interconvert. 9.54 to 9.55 cannot be the enantioselection step (see A. L. Casalnuovo et al., J. Am. Chem. Soc. 116, 9869-82, 1994). 

A single crystal XH NMR study of potassium hydrogen maleate has established the chemical shift tensors of all magnetically inequivalent XH nuclei in the unit cell [131]. 

The crystal structure of benzoic acid is monoclinic and contains four molecules per unit cell in the form of two magnetically inequivalent dimers with equal values of the chemical shift tensors but with their principal axis systems oriented... 

The H NMR spectra also indicate that the methylene protons in the isobu-toxy group are equivalent in the carbenium ion [11], in contrast to their magnetic inequivalence in the covalent precursor. That is, addition of Lewis acid to the alkyl chloride leads to rapid ionization as shown by one broad signal at 3.6 ppm for the methylene group. This resolves into a doublet if ionization is sufficiently fast. Two AB doublets are observed if exchange is slow. Note that the methylene carbon atom and carbocationic center are connected through an oxygen atom in the Newman projection shown in Eq. (5). 

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