# Is it possible to have a negative overall activation energy for a whole reaction mechanism?

I am trying to find out correlation for ignition delay for alkane fuels. The activation energy part of the correlation gives me a negative value. For the ignition delay formulation, is it possible to have an overall activation energy part being negative?

$$\tau = P^aT^b\phi^c\exp\left(\frac{E_\mathrm{a}}{RT}\right)$$