O₂(aΔ_g) Absorption and O₂(b¹Σ_g⁺) Emission in Solution:  Quantifying the a−b Stokes Shift

Abstract

In a nanosecond time-resolved infrared spectroscopic study of dissolved oxygen, O 2 (a 1 ∆ g ) absorption, i.e., a 1 ∆ g f b 1 Σ g + , and O 2 (b 1 Σ g + ) emission, i.e., b 1 Σ g + f a 1 ∆ g , were monitored at ∼5200 cm -1 in a number of solvents. The maxima of the respective spectra depend significantly on the solvent, indicating that the O 2 (a 1 ∆ g ) and O 2 (b 1 Σ g + ) energy levels likewise depend significantly on the solvent. The corresponding Stokes shifts, however, are small. The latter, recorded as the difference between the absorption and emission maxima, do not exceed the uncertainty limits that derive from the step-scan Fourier transform spectroscopic measurements (∼ (3 cm -1 ). Nevertheless, the data clearly indicate that the difference between the equilibrium and nonequilibrium solvation energies for the O 2 (a 1 ∆ g ) and O 2 (b 1 Σ g + ) states is not large. Within the error limits, it is not possible to ascertain if the Stokes shifts are solvent dependent. Ab initio computational methods were used to model the data, and the results indicate that both long-and short-range interactions between oxygen and the perturbing solvent must be considered to adequately describe spectroscopic transitions in dissolved oxygen. The computational results indicate that a 1:1 complex between oxygen and the perturbing molecule embedded in a dielectric continuum appears to provide a sufficiently accurate model that can be used to probe subtle solvent-oxygen interactions.

Overview of Theoretical and Computational Methods Applied to the Oxygen‐Organic Molecule Photosystem

“…By default, the community invariably assumes a 1:1 complex between M and O 2 . Although it has been demonstrated computationally that such a 1:1 interaction is often sufficient to model certain phenomena (66,67), one should not exclude the likely possibility that multiple M molecules simultaneously interact with O 2 , certainly in condensed phase systems. Of course, modeling interactions with more than one M increases the complexity of the computation.…”

“…Although this so‐called dielectric continuum approach is easy to implement, and arguably accounts reasonably well for long‐range interactions between O 2 and the surrounding medium, it suffers from the obvious limitation that the all‐important short range interactions with their unique molecule‐dependent specificity are not properly implemented (66,67). This latter problem can be addressed by incorporating the concept of the M‐O 2 collision complex.…”

“…This latter problem can be addressed by incorporating the concept of the M‐O 2 collision complex. In the so‐called semicontinuum solvation approach, where the M‐O 2 complex is immersed into the corresponding dielectric continuum that reflects an ensemble of M molecules, solution‐phase behavior can be modeled reasonably well (66,67).…”

On the Mechanism of Polyamide Degradation in Chlorinated Water

“…Polymer samples were mounted on the stage of a microscope (Bruker IRscopeII) attached to the FT-IR spectrometer, and the spectra were recorded from only a small spatial domain to avoid artifacts from heterogeneities (e.g., cracks, blisters) that appeared during the course of the degradation reaction. A complete description of our FT-IR spectrometer and attached microscope is provided elsewhere [25] [26]. For quantitative studies of the change in IR absorbance (e.g., Fig.…”

Effect of Solvent on the O2(a1Δg) → O2(b1Σg+) Absorption Coefficient

“…9 Approximately 20 years ago, we constructed a step-scan FTIR spectrometer that could record the time-resolved O 2 (a 1 Δ g ) → O 2 (b 1 Σ g + ) absorption spectrum, which occurs at ∼5200 cm −1 (i.e., ∼1925 nm) in liquid solvents. 16,17 We subsequently used this instrument to quantify solvent-dependent spectral changes in both the O 2 (a 1 Δ g ) → O 2 (b 1 Σ g + ) and 18,19 In proof-of-principle experiments, we also demonstrated that this instrument can be used to quantify k ba in a way that is useful to the study of Minaev's model for the perturbation of oxygen by liquid solvents. 20 In the present study, we modified our FTIR spectrometer to obtain a better signal-to-noise ratio and then set out to quantify O 2 (a 1 Δ g ) → O 2 (b 1 Σ g + ) absorption coefficients in a wide range of common organic solvents.…”