فهرست نشریات علمی دسترسی آزاد ایران

Reliable kinetic modeling of bimolecular reactions from multivariate spectroscopic data often relies on the pseudo-first-order (PFO) approximation. In this approximation, one reactant is assumed to be in constant excess. While this simplification facilitates exponential fitting, its quantitative validity under realistic experimental conditions is not well understood. In this study, we evaluated the accuracy of the PFO approximation compared to the true second-order kinetic model using simulated spectroscopic data generated from a bilinear Beer–Lambert law. The simulations covered factorial combinations of four concentration ratios ([B]0/[A]0 = 1, 10, 50, and 100), four second-order rate constants (k = 0.04–0.60 M⁻¹ s⁻¹), and three noise levels (σ = 0.0005–0.01 AU). We employed global nonlinear least-squares fitting with a Newton–Gauss/Levenberg–Marquardt algorithm to simultaneously recover the kinetic parameters (k or kobs) and pure component spectra.The results showed that the PFO model systematically underestimates the true second-order rate constant when the excess reactant is depleted by more than a few percent. Under near-stoichiometric conditions ([B]0/[A]0 ≈ 1), recovery errors ranged from 15 to 45 percent and decreased monotonically with increasing excess. Recovery errors fell below 1 percent only when [B]0/[A]0 was greater than or equal to 50. Noise had a negligible influence on the mean parameter estimates, but it markedly increased their uncertainties. Therefore, for multivariate spectroscopic kinetic data, the full second-order model should be the default fitting strategy, and the PFO approximation should be reserved for extreme cases of reactant excess where its assumptions are valid.