Nitrogen-14 NMR relaxation, and reorientation behavior of dissolved dinitrogen

• In recent years, nitrogen- 14 and - 15 NMR spectroscopy has become an important technique in organic chemistry and in biochemistry ( Z-4). A frequent occurrence in the NMR spectra of both nuclei is the presence of a signal at approximately -7 1.5ppm (referred to neat nitromethane; -66 ppm referred to aqueous nitrate). This resonance, which has been observed in water (4) and a range of organic solvents (.5-7), has been the subject of some confusion in the literature. In the case of nitrogen-14 NMR spectra, the signal for the quadrupolar nucleus has a remarkably narrow linewidth (about 25 Hz) compared to those recorded for a wide variety of other substances. This indicates a fairly high degree of electronic symmetry about the nitrogenatom and/or a very short correlation time ( 7,). In a recent report (8)) this signal has been assigned to dissolved dinitrogen on the basis that it could be removed by degassing´the solution; however, no reference was made to the fact that it had a remarkably narrow linewidth compared to other 14N NMR resonances. When detected in the 15NCIDNP spectra of the decomposition products of diazonium ions (5, 6) as well as azo compounds ( 7)) the signal has been assigned either to dinitrogen (6, 7) or to a terminal diazonium nitrogen (5). This work is in general agreement with our own observations over a period of years which indicate that the signal arises from dissolved dinitrogen. We have measured the 14N chemical shift of dissolved N2 in a number of solvents at 25°C and have also determined the T, and T2 relaxation times under a variety of conditions in solvents of different viscosity. This Note is concerned with a discussion of these 14N NMR observations and with the determination of the correlation time of dissolved dinitrogen which permits the determination of both the enthalpy and the entropy of activation via the Eyring equation. These results will be compared with earlier reported data regarding the chemical shift and relaxation of liquid nitrogen obtained under a variety of conditions ( 9-12). All spectra were obtained on a Bruker AM-400 wide-bore NMR spectrometer operating in the FT mode at a frequency of 28.9 MHz for 14N and 40.5 MHz for 15N, using a 10 mm broadband probe. Typical conditions for the acquisition of 14N spectra

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NATURVETENSKAP  -- Kemi -- Fysikalisk kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Physical Chemistry (hsv//eng)

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