Nitrogen shielding constants

Most small Hquid helium containers are unpressurized heat leak slowly bods away the Hquid, and the vapor is vented to the atmosphere. To prevent plugging of the vent lines with solidified air, check valves of some sort are included in the vent system. Containers used for air transportation are equipped with automatic venting valves that maintain a constant absolute pressure with the helium container in order to prevent Hquid flash losses at the lower pressures of flight altitudes and to prevent the inhalation of air as the pressure increases during the aircraft s descent. Improved super insulation has removed the need for Hquid nitrogen shielding from almost all small containers. 

Large positive solvent shifts have been calculated for the shielding constants of sulfur and nitrogen nuclei in H2S and HCN, while the shielding constants for carbon in HCN has been found to decrease as the polarity of the medium is increased [42],... 

Model Nitrogen species Calculated N shielding constant o-iso (ppm)... 

The dashed line shows the pressure-time relationship for a vessel containing only saturated vapor. In this case, the temperature rise accompanying the pressure increase is significant and the assumption of constant heat input may not be valid. A temperature equal to the shield temperature (77°K) will be reached at 387 psig. Further temperature rise would be prevented by cooling of the helium by the liquid-nitrogen shield. 

Difficulties have arisen because of the desire of some workers to minimize negative signs, as achieved for H and with TMS. A major source of nitrogen NMR information uses shielding constants (low frequency positive) instead of chemical shifts (and some workers defy the lUPAC sign convention). Another... 

Because of the sensitivity of nitrogen shielding (and coupling constants) to the presence of a lone pair on nitrogen, and to 77-delocalization, useful criteria of structure can be formulated ... 

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