deshielding effect at its edges. Trends in chemical shift are explained based on the degree of shielding or deshielding. Nuclei are found to resonate in a wide range to the left (or more rare to the right) of the internal standard. When a signal is found with a higher chemical shift: the applied effective magnetic field is lower, if the resonance frequency is fixed (as in old traditional CW spectrometers) the frequency is higher, when the applied magnetic field is static (normal case in FT spectrometers) the nucleus is more deshielded the signal or shift is downfield or at low field or paramagnetic Conversely a lower chemical shift is called a diamagnetic shift, and is upfield and more shielded. Diamagnetic shielding In real molecules protons are surrounded by a cloud of charge due to adjacent bonds and atoms. In an applied magnetic field () electrons circulate and produce an induced field () which opposes the applied field. The effective field at the nucleus will be . The nucleus is said to be experiencing a diamagnetic shielding. Factors causing chemical shifts Important factors influencing chemical shift are electron density, electronegativity of neighboring groups and anisotropic induced magnetic field effects. Electron density shields a nucleus from the external field. For example, in proton NMR the electron-poor tropylium ion has its protons downfield at 9.17 ppm, those of the electron-rich cyclooctatetraenyl anion move upfield to 6.75 ppm and its dianion even more upfield to 5.56 ppm. A nucleus in the vicinity of an electronegative atom experiences reduced electron density and the nucleus is therefore deshielded. In proton NMR of methyl halides (CH3X) the chemical shift of the methyl protons increase in the order from 2.16 ppm to 4.26 ppm reflecting this trend. In carbon NMR the chemical shift of the carbon nuclei increase in the same order from around −10 ppm to 70 ppm. Also when the electronegative atom is removed further away the effect diminishes until it can be observed no longer. Anisotropic induced magnetic field effects are the result of a local induced magnetic field experienced by a nucleus resulting from circulating electrons that can either be paramagnetic when it is parallel to the applied field or diamagnetic when it is opposed to it. It is observed in alkenes where the double bond is oriented perpendicular to the external field with pi electrons likewise circulating at right angles. The induced magnetic