Reaction dynamics of CN radicals with tetrahydrofuran in liquid solutions

Abstract

Transient, broadband infra-red absorption spectroscopy with picosecond time resolution has been used to study the dynamics of reactions of CN radicals with tetrahydrofuran (THF) and d₈-THF in liquid solutions ranging from neat THF to 0.5 M THF in chlorinated solvents (CDCl₃ and CD₂Cl₂). HCN and DCN products were monitored via their v₁ (CN stretching) and v₃ (C−H(D) stretching) vibrational absorption bands. Transient spectral features indicate formation of vibrationally excited HCN and DCN, and the onsets of absorption via the fundamental bands of HCN and DCN show short (5–15 ps) delays consistent with vibrational relaxation within the nascent reaction products. This interpretation is confirmed by non-equilibrium molecular dynamics simulations employing a newly derived analytic potential energy surface for the reaction in explicit THF solvent. The rate coefficient for reactive formation of HCN (as determined from measurements on both the 1¹₀ and 3¹₀ fundamental bands) decreases with increasing dilution of the THF in CDCl₃ or CD₂Cl₂, showing pseudo-first order kinetic behaviour for THF concentrations in the range 0.5–4.5 M, and a bimolecular rate coefficient of (1.57 ± 0.12) × 10¹⁰ M⁻¹ s⁻¹ is derived. Simultaneous analysis of time-dependent HCN 1¹₀ and 3¹₀ band intensities following reaction of CN with THF (3.0 M) in CD₂Cl₂ suggests that C−H stretching mode excitation is favoured, and this deduction is supported by the computer simulations. The results extend our recent demonstration of nascent vibrational excitation of the products of bimolecular reactions in liquid solution to a different, and more strongly interacting class of organic solvents. They serve to reinforce the finding that dynamics (and thus the topology of the reactive potential energy surface) play an important role in determining the nascent product state distributions in condensed phase reactions.