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For non axially symmetric systems, the energy levels deviate further and the degeneracy of the m ≠ 0 levels is split. The energy levels are then expressed in terms of an asymmetry parameter η = (qxx - qyy)/qzz. In the NQR Spectrometer, the sample is placed in an inductance, L, which is tuned to the absorption frequency by varying the capacitance of the capacitors C, connected to L. The voltage output which is affected by the absorption is observed in an oscilloscope.
NQR can not be used as widely as NMR because of fewer I ≥1 nuclei and the requirement of the solid state sample. However for compounds containing N and Cl, good use of NQR has been made to generate the molecular data for e2qQ and η in different environments. In metal cyano complexes, the migration of the electrons of the central metal to the empty π* orbitals of the cyano groups affects the field gradient values at 14N. In the case of group III trihalides, the terminal halides in dimerised AlX3 have different frequencies than the bridging halides. |
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Figure 15.5. Structure of AlBr3 dimer. The terminal bromines absorb at 113.79 and 115.45 MHz, while bridging bromines absorb at 97.945 MHz, Chlorides of I, Au and Ga absorb at lower frequencies.
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The data on group frequencies of C-Cl can be used as an analytical tool. Values of C-Cl absorption frequencies in MHz for some groups are –COCl (29-34), O-C-Cl (19.7 to 32.7), C-Cl (31-43), CCl2 (34.5 – 42.5), CCl3 (38-42), S-C-Cl (33 to 34.6) and = C-Cl (36.5 to 39.5). Intermolecular hydrogen bonding reduces the field gradients at nuclei such as 35Cl and 14N by about 20% and this can be detected in NQR. |
