This assignment is further supported by the calculated isotope shifts. The predicted shift due to substituting Cr with Cr for a simple diatomic oscillator with a mode of 340 cm -1 was estimated to be ∼13 cm -1 . The experimentally observed shifts are 10.1, 9.4, and 5.6 cm -1 for peaks a, b, and c, respectively. CASPT2 computed values are in excellent agreement at 9.3, 10.3, and 12.2 cm -1 , with the largest deviation for ν(Cr 2 ) where the effect of an elongated Cr-Cr bond distance and anharmonicity of the mode is expected to be greatest. The aggregate Cr isotope shift from the three bands was 25.1 cm -1 in experiment and is 31.9 cm -1 with CASPT2.