Why does the absorption spectrum of aqueous [Ti(OH2)6]3+ exhibit a broad band with a shoulder?
The spectrum you observe is the transition from the ground state 2T2g to the excited state 2Eg of the octahedral aquo ion. It is allowed with respect to the spin rule (DeltaS = 0). The shoulder on the low-energy side is due to Jahn-Teller distortion of the excited 2Eg state. Jahn-Teller (JT) distortions occur when a non-linear molecule is in a degenerate electronic state. The molecule then distorts, its symmetry is lowered and the energy minimized. For Ti(III), d1, the JT distortion of the ground state is small, but the JT of the excited state is sizeable; the 2Eg state splits into 2A1g and 2B1g levels. The splitting is on the order of 2000 cm-1. The band at around 500 nm (20,300 cm-1) corresponds to the transition to 2A1g, the should at around 550 nm to the transition to 2B1g.
The electronic spin-allowed dd-transition plus Jan-Teller effect (e.g. Burns 1993).
Md Abdur Rahman is right, in addition to the splitting I have described the width of the bands is broadened because of vibronic coupling
This may due to the effect of Jahn-Teller distortion. Since Ti+3 is in d 1 configuration and present in the dxy of T2g orbital. There will be possibility of axial bond distortion.
Hope it Helps x
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