As Miklos said, it's not necessary. It depends on the potential energy surface in this particular region: there are instances, where it can be really flat, resulting in small imaginary frequency values for transition states.

From the practical point of view, you should take a look at what kind of transition state are you looking for. For strong (covalent) bond breaking an imaginary frequency of less than 100 cm-1 for a transition state is suspicious, and likely wrong. But for e.g. rotations or inversions it's possibly and actually likely that the transition states have quite low (below 100 cm-1) frequencies.

There is not a single critical value. (You mean the absolute value of the imaginary frequency, right?)

As Miklos said, it's not necessary. It depends on the potential energy surface in this particular region: there are instances, where it can be really flat, resulting in small imaginary frequency values for transition states.

From the practical point of view, you should take a look at what kind of transition state are you looking for. For strong (covalent) bond breaking an imaginary frequency of less than 100 cm-1 for a transition state is suspicious, and likely wrong. But for e.g. rotations or inversions it's possibly and actually likely that the transition states have quite low (below 100 cm-1) frequencies.

Thank you so much Bartosz and Miklos . Actually i am looking for C-H bond dissociation transition state . So in this case it  is possible if i am getting imaginary frequency less than 100 cm-1 .

Thankyou once again

For a covalent C-H bond, which is definitely a strong bond, such imaginary frequency is at least suspicious, if not outright wrong. I could imagine, though, instances where there is a series of steps and a catalyst invlved and the bond breaking may occur in mild conditions and have rather low imaginary freqs.

It is more likely that the

Thank You Allen Clabo...,thankyou all once again.

Hello  Sumi!

i have optimized  TSs with imaginary frequencies -739.3 and 1319.5 cm-1 for

C-H activation of CH4 using dioxiranes and metal-peroxo systems.