Dynamics of NO Release and Linkage Isomer Formation from S-Nitroso-Mercaptoethanol in Aqueous Solutions: Insights from Femtosecond Infrared Spectroscopy

Understanding the photodynamics of S-nitroso-thiol (RSNO), an effective NO transporter in biological systems, is essential for its photochemical applications. S-nitroso-mercaptoethanol (MceSNO), a simple water-soluble RSNO, facilitates high-level quantum calculations. We investigated the photoexcitation dynamics of MceSNO in an aqueous solution, focusing on NO dissociation, recombination, and linkage isomerization using quantum calculations and femtosecond infrared spectroscopy. Upon excitation at 320 nm, MceSNO rapidly dissociates into NO and MceS radicals. Approximately 31 ± 3% of MceS reacts with unexcited MceSNO molecules, forming MceSSMce and releasing additional NO. The remaining MceS undergoes geminate recombination with NO, forming either MceSNO (41 ± 4%) or MceSON (28 ± 3%), the latter being a sulfur-ON linkage isomer observed for the first time in a room-temperature solution. MceSON isomerizes back to MceSNO in 470 ± 30 ps. The formation mechanism of MceSON was verified through a potential energy surface constructed at the CASPT2D(16,11)/cc-pVTZ level. The isomerization barrier was determined to be 3.3 ± 1.2 kcal/mol in water.

Investigating the Photodissociation Dynamics of CF2BrCF2I in CCl4 through Femtosecond Time-Resolved Infrared Spectroscopy

The photodissociation dynamics of CF₂BrCF₂I in CCl₄ at 280 ± 2 K were investigated by probing the C-F stretching mode from 300 fs to 10 μs after excitation at 267 nm using time-resolved infrared spectroscopy. The excitation led to the dissociation of I or Br atoms within 300 fs, producing the CF₂BrCF₂ or CF₂ICF₂ radicals, respectively. All nascent CF₂ICF₂ underwent further dissociation of I, producing CF₂CF₂ with a time constant of 56 ± 5 ns. All nascent g-CF₂BrCF₂ isomerized into the more stable a-CF₂BrCF₂ with a time constant of 47 ± 5 ps. Furthermore, a-CF₂BrCF₂ underwent a bimolecular reaction with either itself (producing CF₂BrCF₂Br and CF₂CF₂) or Br in the CCl₄ solution (producing CF₂BrCF₂Br) at a diffusion-limited rate. The secondary dissociation of Br from a-CF₂BrCF₂ was significantly slow to compete with the bimolecular reactions. Overall, approximately half of the excited CF₂BrCF₂I at 267 nm produced CF₂BrCF₂Br, whereas the other half produced CF₂CF₂. The excess energies in the nascent radicals were thermalized much faster than the secondary dissociation of I from CF₂ICF₂ and the observed bimolecular reactions, implying that the secondary reactions proceeded under thermal conditions. This study further demonstrates that structure-sensitive time-resolved infrared spectroscopy can be used to study various reaction dynamics in solution in real time.

Rotational Isomerization of Carbon-Carbon Single Bonds in Ethyl Radical Derivatives in a Room-Temperature Solution

The rotational isomerization of 1,2-disubstituted ethyl radical derivatives, reaction intermediates often found in the reaction of 1,2-disubstituted ethane derivatives, has never been measured because of their short lifetime and ultrafast rotation. However, the rotational time constant is critical for understanding the detailed reaction mechanism involving these radicals, which determine the stereoisomers of compounds produced via the intermediates. Using time-resolved infrared spectroscopy, we found that the CF₂BrCF₂ radical in a CCl₄ solution rotationally isomerizes with a time constant of 47 ± 5 ps at 280 ± 2 K. From this value and the rotational barrier heights of related compounds, CH₃CH₂ and CH₃CH₂CHCH₃ radicals in CCl₄ were estimated to rotationally isomerize within 1 ps at 298 K, considerably faster than ethane and n-butane, which rotationally isomerize with time constants of 1.8 and 81 ps, respectively. The time constant for the rotational isomerization was similar to that calculated using transition state theory with a transmission coefficient of 0.75.

Photoacid-induced aqueous acid-base reactions probed by femtosecond infrared spectroscopy

Acid-base reactions involving an excited photoacid have typically been investigated at high base concentrations, but the mechanisms at low base concentrations require clarification. Herein, the dynamics of acid-base reactions induced by an excited photoacid, pyranine (DA), were investigated in the presence of azide ion (N₃^-) in D₂O solution using femtosecond infrared spectroscopy. Specifically, the spectral characteristics of four species (DA, electronically excited DA (DA*), the conjugate base of DA* (A*^-), and the conjugate base of DA (A^-)) were probed in the spectral region of 1400-1670 cm^-1 in the time range of 1 ps-1 μs. This broad timescale encompassed all the acid-base reactions initiated by photoexcitation at 400 nm; thus, reactions related to both DA* and A^- could be probed. Furthermore, changes in the populations of N₃^- and DN₃ were monitored using the absorption bands at 2042 and 2133 cm^-1, respectively. Following excitation, approximately half of DA* relaxed to DA with a time constant of 0.44 ± 0.04 ns. The remainder underwent an acid-base reaction to produce A*^-, which relaxed to A^- with a time constant of 3.9 ± 0.3 ns. The acid-base reaction proceeded via two paths, namely, proton exchange with the added base or simple deuteron release to D₂O (protolysis). Notably, all the acid-base reactions were well described by the rate constant at the steady-state limit. Thus, although the acid-base reactions at low base concentrations (< 0.1 M) were diffusion controlled, they could be described using a simple rate equation.

Photoexcitation dynamics of bromodiphenyl ethers in acetonitrile-d3 studied by femtosecond time-resolved infrared spectroscopy

The efficient decomposition of polybrominated diphenyl ethers (PBDEs), onetime prevalent flame retardants, is central to the reduction of their harmful effects on human health. PBDE photodecomposition is a promising method, but its mechanism and products are not well understood. The photoexcitation dynamics of 3- and 4-bromodiphenyl ethers (BDE-2 and BDE-3) in CD₃CN were studied from 0.3 ps to 10 μs using time-resolved infrared spectroscopy. An excitation at 267 nm dissociated the Br atom from BDE-2 and BDE-3 within 0.3 ps and 14 ± 3 ps, respectively, producing a radical compound (R) and a Br atom. About 85% of R formed an intermediate (IM) that weakly interacted with the Br atom and the surrounding CD₃CN solvent in 7-12 ps. The remaining R separated from the dissociated Br and underwent slow geminate rebinding (GR) with Br within 35 to 54 ns. The IM competitively engaged in GR with the interacting Br in 40-60 ps or formed CD₃CN-bound radical compounds (RS) in 100-130 ps. The RS further degraded via either the dissociation of CD₃-producing a cyano-bound diphenyl ether (DE) in 150 or 550 ns-or the deuterium abstraction of CD₃CN in 180 or 430 ns-producing a deuterated DE. Overall, 33 ± 3 (22 ± 3)% of the photoexcited BDE-2 (BDE-3) decomposed in CD₃CN under 267 nm excitation. Efficient binding of the CD₃CN solvent to R deterred the yield-diminishing GR and slowed the rate of product formation. The observed photoexcitation dynamics of BDE suggest methods for the efficient decomposition of PBDE.

Photodissociation Dynamics of Nitric Oxide from N-Acetylcysteine- or N-Acetylpenicillamine-bound Roussin's Red Ester

The photochemical release of nitric oxide (NO) from a NO precursor is advantageous in terms of spatial, temporal, and dosage control of NO delivery to target sites. To realize full control of the quantitative NO administration from photoactivated NO precursors, it is necessary to have detailed dynamical information on the photodissociation of NO from NO precursors. We synthesized two new water-soluble Roussin's red esters (RREs), [Fe₂(μ-N-acetylcysteine)₂(NO)₄] and [Fe₂(μ-N-acetylpenicillamine)₂(NO)₄], which have five times longer lifetime than the well-known [Fe₂(μ-cysteine)₂(NO)₄]. The photodissociation dynamics of NO from these RREs in water were investigated over a broad time range from 0.3 ps to 10 μs after excitation at 310 and 400 nm using femtosecond time-resolved infrared (IR) spectroscopy. When these RREs are excited, they either release one NO, producing a radical species deficient in one NO (R), [Fe₂(μ-RS)₂(NO)₃], or relax into the ground state without photodeligation via an electronically excited intermediate state (M). R appears immediately after photoexcitation, suggesting that one NO is photodissociated faster than 0.3 ps. A certain fraction of R undergoes geminate recombination (GR) with NO with a time constant of 7-9 ps, while the remaining R competitively binds to the solvent. Solvent-bound R eventually bimolecularly recombines with NO with a rate constant of (1.3-1.6) × 10⁸ M^-1 s^-1. For a given RRE molecule, the fractional yield of M (0.62-0.76) depends on the excitation wavelength (λ_ex); however, the relaxation time of M (6 ± 1 ns) is independent of λ_ex. Although the primary quantum yield of NO photodissociation (Φ₁) was found to be 0.24-0.38, the final yield of NO suitable for other reactions (Φ₂) was reduced to 0.14-0.29 due to the picosecond GR of the dissociated NO with R. Detailed photoexcitation dynamics of RRE can be utilized in the quantitative control of NO administration at a specific site and time, promoting pin-point usage of NO in chemistry and biology. We demonstrate that femtosecond IR spectroscopy combined with quantum chemical calculations is a powerful method for obtaining detailed dynamic information on photoactivated NO precursors such as Φ₁ and Φ₂, the GR yield, and secondary reactions of the nascent photoproducts, which are essential information for the design of efficient photoactivated NO precursors and their quantitative utilization.

Structural Dynamics of C2F4I2 in Cyclohexane Studied via Time-Resolved X-ray Liquidography

The halogen elimination of 1,2-diiodoethane (C2H4I2) and 1,2-diiodotetrafluoroethane (C2F4I2) serves as a model reaction for investigating the influence of fluorination on reaction dynamics and solute-solvent interactions in solution-phase reactions. While the kinetics and reaction pathways of the halogen elimination reaction of C2H4I2 were reported to vary substantially depending on the solvent, the solvent effects on the photodissociation of C2F4I2 remain to be explored, as its reaction dynamics have only been studied in methanol. Here, to investigate the solvent dependence, we conducted a time-resolved X-ray liquidography (TRXL) experiment on C2F4I2 in cyclohexane. The data revealed that (ⅰ) the solvent dependence of the photoreaction of C2F4I2 is not as strong as that observed for C2H4I2, and (ⅱ) the nongeminate recombination leading to the formation of I2 is slower in cyclohexane than in methanol. We also show that the molecular structures of the relevant species determined from the structural analysis of TRXL data provide an excellent benchmark for DFT calculations, especially for investigating the relevance of exchange-correlation functionals used for the structural optimization of haloalkanes. This study demonstrates that TRXL is a powerful technique to study solvent dependence in the solution phase.

Dynamics of photodissociation of nitric oxide from S-nitrosylated cysteine and N-acetylated cysteine derivatives in water

Cysteine and N-acetylated cysteine derivatives are ubiquitous in biological systems; they have thiol groups that bind NO to form S-nitrosothiols (RSNOs) such as S-nitrosocysteine (CySNO), S-nitroso-N-acetylcysteine (NacSNO), and S-nitroso-N-acetylpenicillamine (NapSNO). Although they have been utilised as thermally or catalytically decomposing NO donors, their photochemical applications are yet to be fully explored owing to the lack of photodissociation dynamics. To this end, the photoexcitation dynamics of these RSNOs in water at 330 nm were investigated using femtosecond time-resolved infrared (TRIR) spectroscopy over a broad time range encompassing the entire reaction, which includes the primary reaction, secondary reactions of the reaction intermediates, and product formation. We discovered that the acetate and amide groups in these RSNOs have strong vibrational bands sensitive to the bondage of NO and the electronic state of the compound, which facilitates the identification of reaction intermediates involved in photoexcitation. The simplest thiol available with the acetate group-thioglycolic acid-was nitrosylated; it produced S-nitrosothioglycolic acid (TgSNO) and was comparatively investigated. Transient absorption bands in the TRIR spectra of the RSNOs were assigned using quantum chemical calculations. Photoexcited cysteine-related RSNOs either decompose into RS and NO within 0.3 ps after excitation at 330 nm with a primary quantum yield (Φ1) of 0.46-1 or relax into an electronically excited intermediate state lying at 42 ± 3 kcal mol-1 above the ground state, which relaxes into the ground state with a time constant of 460-520 ps. A majority (62-80%) of the RS radical geminately rebinds with NO at a time constant of 3-7 ps. The remaining RS reacts with the neighbouring RSNO, which produces additional NO and RSSR with a (nearly) diffusion-limited rate constant that doubles the amount of NO produced; further, it remarkably extends the time window for the dissociated NO to react with the target compound. The final fraction of NO produced from these RSNOs at 330 nm was 0.32-0.58, and it depends on the geminate rebinding yield and Φ1. The detailed dynamics of the photoexcited RSNO can be utilised in the quantitative application of these RSNOs in practical use and in the synthesis of more efficient photoactivated NO precursors.