Absolute Nitrogen Shieldings

From a theoretical point of view, it is interesting to gain a good knowledge of the absolute nitrogen shielding. Unfortunately, direct experimental determination is not actually available and the total screening 

---

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. 

In the stratosphere, the region of the atmosphere from 10 up to 50 km, ozone is synthesized from dioxygen in a complex cycle of reactions. Thanks to its strong absorption of ultraviolet radiation in the range between 232 and 290 nm, ozone provides a protective shield for the plant and animal life on the earth s surface. Because no other atmospheric species can absorb the intense radiation in this range, the ozone layer is absolutely critical to the earth s inhabitants and has been the focus of numerous investigations. A major concern is the possible depletion of the ozone layer by nitrogen oxides from automobile exhausts and by Freons from aerosal sprays. 

Nitrogen shifts can be referred to an absolute scale based on spin-rotation measurements of NH3 values for other compounds are obtained by subtraction of the chemical shift (Table 1). Absolute shieldings are of interest for comparison with the values given by quantum-mechanical calculations, or with estimated effects of substituents, as well as in comparisons involving different nuclei, as with C shieldings in Section 4. 

---