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.

Atomic spectra

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:

• The mass difference. This difference induces a change in the reduced electronic mass. This purely kinematical effect, studied theoretically by Hughes and Eckart^[1] is important for light elements.

• The volume difference. This difference induces a change in the electric charge distribution of the nucleus. This effect is important in heavy elements and its first theory was formulated by Pauli and Peierls.^[2][3][4]

NMR spectroscopy

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.

Vibrational spectra

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 and 2109 cm-1, respectively.^[5] This shift reflects the differing Zero-point energy for the affected bonds.

References