Time resolved infrared absorption studies of geminate recombination and vibrational relaxation in OClO photochemistry

Abstract

Ultrafast time-resolved infrared absorption studies of aqueous chlorine dioxide (OClO) photochemistry are reported. Following photoexcitation at 401 nm, the evolution in optical density at frequencies between 1000 to 1100 cm(-1) is monitored to investigate vibrational energy deposition and relaxation along the asymmetric-stretch coordinate following the reformation of ground-state OClO via geminate recombination of the primary photofragments. The measured kinetics are compared to two proposed models for the vibrational-relaxation dynamics along the asymmetric-stretch coordinate. This comparison demonstrates that the perturbation model derived from molecular dynamics studies is capable of qualitatively reproducing the observed kinetics, where the collisional model employed in previous UV-pump, visible probe experiments demonstrates poor agreement with experiment. The ability of the perturbation model to reproduce the optical-density evolution observed in these studies demonstrates that for aqueous OClO, frequency dependence of the solvent-solute coupling is important in defining the level-dependent vibrational relaxation rates along the asymmetric-stretch coordinate. The absence of optical-density evolution corresponding to the population of higher vibrational levels (n>8) along the asymmetric-stretch coordinate suggests that following geminate recombination, energy is initially deposited into a local Cl-O stretch, with the relaxation of vibrational energy from this coordinate providing for delayed vibrational excitation of the asymmetric- and symmetric-stretch coordinates relative to geminate recombination, as previously observed.

Vibrational dynamics of deoxyguanosine 5′-monophosphate following UV excitation

“…In some cases, however, vibrational energy relaxation proceeds directly from the non-thermal distribution of high frequency modes without redistribution of the vibrational energy. This was observed in CS 2 (aq) 21 and OClO(aq) 22,23 where the energy predominantly relaxes through the asymmetric stretch vibration populated in geminate recombination. Wang et al 25 studied liquid alcohols and demonstrated vibrational energy relaxation in part due to anharmonic coupling to low frequency modes and in part due to a vibrational cascade involving other high frequency modes.…”

“…It is instructive to look at vibrational energy relaxation in two limiting cases: a thermal model, sometimes referred to as an anharmonic coupling model, 4,[16][17][18][19][20] and a non-thermal model. [21][22][23] In the thermal model the excess electronic energy after internal conversion is converted into vibrational motion on the potential surface of the electronic ground state. This vibrational energy is assumed to be in thermal equilibrium at a temperature dictated by the heat capacity of the dGMP molecule, and through the Boltzmann distribution most of the vibrational population thus resides in the low frequency modes of the dGMP molecule.…”

Photochemistry of Nucleic Acid Bases and Their Thio- and Aza-Analogues in Solution

“…In the second model, which is known as the non-thermal relaxation model [187][188][189], the vibrational Scheme 2 The structures of adenosine monophosphate, guanosine monophosphate, and a series of methylated xanthines. Hydrogen bond donor sites are highlighted to emphasize their inverse relationship with the molecule's vibrational cooling lifetime (τ vc ), i.e.…”

Mode-specific vibrational relaxation of photoexcited guanosine 5′-monophosphate and its acid form: a femtosecond broadband mid-IR transient absorption and theoretical study

“…[62][63][64][65][66] It is distinguished from the so-called non-thermal model in which the populations of a small number of high-frequency modes are non-statistical for times long enough to be measured. [67][68][69] In the latter case, the frequency shifts experienced by the excited vibrational modes mainly arise from diagonal anharmonicity.…”