  We use Instant Preheating as a mechanism to reheat the universe when its
evolution is modeled by a non-oscillating background. Once we obtain the
reheating temperature, we calculate the number of e-folds using two different
methods, which allows us to establish a relationship between the reheating
temperature and the spectral index of scalar perturbations. We explore this
connection to constrain the spectral index for different Quintessential
Inflation models.
