Isotopic shift

The isotopic shift (also called isotope shift) is the shift in various forms of spectroscopy that occurs when one nuclear isotope is replaced by another. If atomic spectra also have hyperfine structure the shift refers to the center of gravity of the spectra. There are two effects which contribute to this shift: In NMR spectroscopy, Isotopic effects on chemical shifts are typically small, far less than1 ppm the typical unit for measuring shifts. The 1H NMR signals for 1H2 and 1H2H ('HD') are readily distinguished in terms of their chemical shifts. The asymmetry of the signal for the 'protio' impurity in CD2Cl2 arises from the differing chemical shifts of CDHCl2 and CH2Cl2. Isotopic shifts are best known and most widely used in vibration spectroscopy where the shifts are large, being proportional to the ratio of the square root of the isotopic masses. In the case of hydrogen, the 'H-D shift' is (1/2)1/2 or 1/1.41. Thus, the (totally symmetric) C-H vibration for CH4 and CD4 occur at 2917 cm−1 and 2109 cm−1, respectively. This shift reflects the differing Reduced mass for the affected bonds.