field lines are parallel to the external field at the location of the alkene protons which therefore shift downfield to a 4.5 ppm to 7.5 ppm range. The three-dimensional space where a diamagnetic shift is called the shielding zone with a cone-like shape aligned with the external field. The protons in aromatic compounds are shifted downfield even further with a signal for benzene at 7.73 ppm as a consequence of a diamagnetic ring current. Alkyne protons by contrast resonate at high field in a 2–3 ppm range. For alkynes the most effective orientation is the external field in parallel with electrons circulation around the triple bond. In this way the acetylenic protons are located in the cone-shaped shielding zone hence the upfield shift. Magnetic properties of most common nuclei 1H and 13C are not the only nuclei susceptible to NMR experiments. A number of different nuclei can also be detected, although the use of such techniques is generally rare due to small relative sensitivities in NMR experiments (compared to 1H) of the nuclei in question, the other factor for rare use being their slender representation in nature and organic compounds. 1H, 13C, 15N, 19F and 31P are the five nuclei that have the greatest importance in NMR experiments: 1H because of high sensitivity and vast occurrence in organic compounds 13C because of being the key component of all organic compounds despite occurring at a low abundance (1.1%) compared to the major isotope of carbon 12C, which has a spin of 0 and therefore is NMR-inactive. 15N because of being a key component of important biomolecules such as proteins and DNA 19F because of high relative sensitivity 31P because of frequent occurrence in organic compounds and moderate relative sensitivity Chemical shift manipulation In general, the associated increased signal-to-noise and resolution has driven a move towards increasingly high field strengths. In limited cases, however, lower fields are preferred; examples are for systems in chemical exchange, where the speed of the exchange relative to the NMR experiment can cause additional and confounding linewidth broadening. Similarly, while avoidance of second order coupling is generally preferred, this information can be useful for elucidation of chemical structures. Using refocussing pulses placed between recording of successive points of the free induction decay, in an analogous fashion to the spin echo technique in MRI, the chemical shift evolution can be scaled to provide apparent low-field spectra on a