Question

Which technique is more sensitive to isotopic shifts: infrared or microwave spectroscopy? Explain your answer.

Answer 1

Infrared spectroscopy (Vibrational spectroscopy) is more sensitive to isotopic shifts. Because rotational energies are smaller than vibrational energies, then rotational isotopic shifts also smaller than vibrational isotopic shifts.

If shifts are very smaller then peak corresponding to molecule with isotope may be merged with peak corresponding to original molecule. So it is difficult identify the peaks corresponding isotopic substituted molecule and it is difficult calculate the difference in rotational constant values of normal molecule and isotopic substituted molecule.