Imaginary frequency and rate constant?

      As it is expected, MOPAC calculates one and only one negative
 vibrational frequency at TS. The value lies (-300 - -150 cm-1). Now, is it
 correct to use this frequency as pre-exponent factor (frequency factor) in
 Arrhenius rate constant equation? The reaction I study is C-C single bond
 forming (or splitting) reaction.
 If there will be interest, I'll give the summary to list.
 Dr. Ivar Martin
 Department of Bioorganic Chemistry           tel: +372 2 526510
 Institute of Chemistry                       fax: +372 2 536371
 Akadeemia tee 15 EE0026                   e-mail: ivarm()at()
 Tallinn, ESTONIA