Theoretical Study on the Formation of H- and O-Atoms, HONO, OH, NO, and NO2from the Lowest Lying Singlet and Triplet States inOrtho-Nitrophenol Photolysis

Femtosecond to Microsecond Observation of Photochemical Pathways in Nitroaromatic Phototriggers Using Transient Absorption Spectroscopy

The synthetic accessibility and tolerance to structural modification of phototriggered compounds (PTs) based on the ortho- nitrobenzene (ONB) protecting group have encouraged a myriad of applications including optimization of biological activity, and supramolecular polymerization. Here, a combination of ultrafast transient absorption spectroscopy techniques is used to study the multistep photochemistry of two nitroaromatic phototriggers based on the ONB chromophore, O-(4,5-dimethoxy-2-nitrobenzyl)-l-serine (DMNB-Ser) and O-[(2-nitrophenyl)methyl]-l-tyrosine hydrochloride (NB-Tyr), in DMSO solutions on femtosecond to microsecond time scales following the absorption of UV light. From a common nitro-S1 excited state, the PTs can either undergo excited state intramolecular hydrogen transfer (ESIHT) to an aci-S1 isomer within the singlet state manifold, leading to direct S1 → S0 internal conversion through a conical intersection, or competitive intersystem crossing (ISC) to access the triplet state manifold on time scales of (1.93 ± 0.03) ps and (13.9 ± 1.2) ps for DMNB-Ser and NB-Tyr, respectively. Deprotonation of aci-T1 species to yield triplet anions is proposed to occur in both PTs, with an illustrative time constant of (9.4 ± 0.7) ns for DMNB-Ser. More than 75% of the photoexcited molecules return to their electronic ground states within 8 μs, either by direct S1 → S0 relaxation or anion reprotonation. Hence, upper limits to the quantum yields of photoproduct formation are estimated to be in the range of 13-25%. Mixed DMSO/H2O solvents show the influence of the environment on the observed photochemistry, for example, revealing two nitro-S1 lifetimes for DMNB-Ser in a 10:1 DMSO/H2O mixture of 1.95 ps and (10.1 ± 1.2) ps, which are attributed to different microsolvation environments.

Photo-induced structural dynamics of o-nitrophenol by ultrafast electron diffraction

The photo-induced dynamics of o-nitrophenol, particularly its photolysis, has garnered significant scientific interest as a potential source of nitrous acid in the atmosphere. Although the photolysis products and preceding photo-induced electronic structure dynamics have been investigated extensively, the nuclear dynamics accompanying the non-radiative relaxation of o-nitrophenol on the ultrafast timescale, which include an intramolecular proton transfer step, have not been experimentally resolved. Herein, we present a direct observation of the ultrafast nuclear motions mediating photo-relaxation using ultrafast electron diffraction. This work spatiotemporally resolves the loss of planarity which enables access to a conical intersection between the first excited state and the ground state after the proton transfer step, on the femtosecond timescale and with sub-Angstrom resolution. Our observations, supported by ab initio multiple spawning simulations, provide new insights into the proton transfer mediated relaxation mechanism in o-nitrophenol.

Ultrafast Excited-State Proton Transfer in 4-Nitrocatechol: Implications for the Photochemistry of Nitrophenols

Nitrophenols are a class of environmental contaminants that exhibit strong absorption at atmospherically relevant wavelengths, prompting many studies of their photochemical degradation rates and mechanisms. Despite the importance of photochemical reactions of nitrophenols in the environment, the ultrafast processes in electronically excited nitrophenols are not well understood. Here, we present an experimental study of ultrafast electron dynamics in 4-nitrocatechol (4NC), a common product of biomass burning and fossil fuel combustion. The experiments are accompanied by time-dependent quantum mechanical calculations to help assign the observed transitions in static and transient absorption spectra and to estimate the rates of singlet-to-triplet intersystem crossing. Our results suggest that electronic triplet states are not efficiently populated upon 340 nm excitation, as efficient proton transfer occurs in the excited state on a time scale of a few picoseconds in water and tens of picoseconds in 2-propanol. This suggests that triplet states do not play a significant role in the photochemical reactions of 4NC in the environment and, by extension, in nitrophenols in general. Instead, consideration should be given to the idea that this class of molecules may serve as strong photoacids.

Kinetics and product identification of water-dissolved nitroguaiacol photolysis under artificial sunlight

Nitroguaiacols are typical constituents of biomass-burning emissions, including absorbing aerosols which contribute to climate change. Although they are also harmful to humans and plants, their atmospheric fate and lifetimes are still very speculative. Therefore, in this work, the photolysis kinetics of aqueous-phase 4-nitroguaiacol (4NG) and 5-nitroguaiacol (5NG), and the resulting photo-formed products were investigated under artificial sunlight, observing also the effect of sunlight on the absorption properties of the solutions. We found the photolysis of 5NG slower than that of 4NG, whereas the absorbance in the visible range prevailed in the 5NG solutions at the end of experiments. Although we identified dinitroguaiacol as one of the 4NG photolysis products, which increased light absorption of 4NG-containing solutions, considerably more chromophores formed in the 5NG photolyzed solutions, implying its stronger potential for secondary BrC formation in the atmosphere. In general, denitration, carbon loss, hydroxylation, nitration, and carbon gain were characteristic of 4NG phototransformation, while carbon loss, hydroxylation, and carbon gain were observed in the case of 5NG. The photolysis kinetics was found of the first order at low precursor concentrations (<0.45 mM), resulting in their lifetimes in the order of days (125 and 167 h illumination for 4NG and 5NG, respectively), which suggests long-range transport of the investigated compounds in the atmosphere and proposes their use as biomass-burning aerosol tracer compounds.

Photolysis frequency of nitrophenols derived from ambient measurements

Nitrophenols, a class of important intermediate products from the oxidation of aromatics, can participate in photochemistry and influence the atmospheric oxidative capacity. However, the reported photolysis frequencies of nitrophenols show considerable discrepancies. Here, measurements of nitrophenol, and methyl nitrophenol using a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) at both urban and regional sites in southern China are used to constrain photolysis frequencies of nitrophenols. Considerable concentrations with a campaign average of 58 ± 32 ppt for nitrophenol and 97 ± 59 ppt for methyl nitrophenol were observed at the regional site. Based on the in-situ measurement dataset, a steady-state calculation was performed along with a zero-dimensional box model to analyze the budgets of nitrophenols. The result indicates that both primary emission and photolysis have significant impacts on nitrophenols. Primary emission contributes up to 88 % of the total nitrophenols production while photolysis accounts for up to 98 % of the total removal rate. The dominant sink of nitrophenols is photolysis with a rate of about 3.5 % ± 1.3 % of j_NO2 for nitrophenol and 2.4 % ± 1.0 % of j_NO2 for methyl nitrophenol. The results of this study suggest that using advanced mass spectrometry to accurately measure ambient nitrophenols, supplemented by an observation-based box model for budget analysis, provides an important indication for determining photolysis rate constants of nitrophenols.

Excited-state dynamics of o-nitrophenol studied with UV pump-VUV probe time-resolved photoelectron and photoion spectroscopy

Time-resolved photoionization measurements were performed on o-nitrophenol pumped with UV laser pulses at a central wavelength of 255 nm (4.9 eV) and probed with vacuum ultraviolet (VUV) pulses at 153 nm (8.1 eV). The photoelectron spectrum and time of flight mass spectrum for ions were recorded at each pump-probe delay. The measurements are interpreted with the aid of electronic structure calculations for both the neutral and ionic states. Evidence is found for the formation of a bicyclic intermediate followed by NO dissociation through a process of internal conversion and intersystem crossing. The combination of photoelectron and photoion spectroscopy, together with computational results, provides strong evidence of intersystem crossing that is difficult to establish with only a single technique.

Modulating the Roaming Dynamics for the NO Release in ortho-Nitrobenzenes

The dynamics of NO release upon photodissociation of nitroaromatic compounds is dependent on the nature of the interaction between the NO₂ group and substituent in the ortho position. A bimodal (slow and fast) translational energy distribution of the NO photofragment indicates the presence of two distinct NO elimination channels. The slow-to-fast branching ratio for the NO release is regulated by the hydrogen bonding ability of the ortho substituent and follows the order [OH > NH₂ > CH₃ > OCH₃], indicating that the intramolecular hydrogen bonding plays a pivotal role in NO release dynamics. Further, the topology of the triplet state potential energy surface acts as a doorway to the dissociation pathway switching between the roaming and nonroaming mechanisms, with hydrogen bonding substituents (OH and NH₂) favoring the roaming mechanism.

Ultrafast Spectroscopies of Nitrophenols and Nitrophenolates in Solution: From Electronic Dynamics and Vibrational Structures to Photochemical and Environmental Implications

Nitrophenols are a group of small organic molecules with significant environmental implications from the atmosphere to waterways. In this work, we investigate a series of nitrophenols and nitrophenolates, with the contrasting ortho-, meta-, and para-substituted nitro group to the phenolic hydroxy or phenolate oxygen site (2/3/4NP or NP^-), implementing a suite of steady-state and time-resolved spectroscopic techniques that include UV/Visible spectroscopy, femtosecond transient absorption (fs-TA) spectroscopy with probe-dependent and global analysis, and femtosecond stimulated Raman spectroscopy (FSRS), aided by quantum calculations. The excitation-dependent (400 and 267 nm) electronic dynamics in water and methanol, for six protonated or deprotonated nitrophenol molecules (three regioisomers in each set), enable a systematic investigation of the excited-state dynamics of these functional "nanomachines" that can undergo nitro-group twisting (as a rotor), excited-state intramolecular or intermolecular proton transfer (donor-acceptor, ESIPT, or ESPT), solvation, and cooling (chromophore) events on molecular timescales. In particular, the meta-substituted compound 3NP or 3NP^- exhibits the strongest charge-transfer character with FSRS signatures (e.g., C-N peak frequency), and thus, does not favor nitroaromatic twist in the excited state, while the ortho-substituted compound 2NP can undergo ESIPT in water and likely generate nitrous acid (HONO) after 267 nm excitation. The delineated mechanistic insights into the nitro-substituent-location-, protonation-, solvent-, and excitation-wavelength-dependent effects on nitrophenols, in conjunction with the ultraviolet-light-induced degradation of 2NP in water, substantiates an appealing discovery loop to characterize and engineer functional molecules for environmental applications.

Unimolecular Decomposition Reactions of Picric Acid and Its Methylated Derivatives─A DFT Study

To handle energetic materials safely, it is important to have knowledge about their sensitivity. Density functional theory (DFT) has proven a valuable tool in the study of energetic materials, and in the current work, DFT is employed to study the thermal unimolecular decomposition of 2,4,6-trinitrophenol (picric acid, PA), 3-methyl-2,4,6-trinitrophenol (methyl picric acid, mPA), and 3,5-dimethyl-2,4,6-trinitrophenol (dimethyl picric acid, dmPA). These compounds have similar molecular structures, but according to the literature, mPA is far less sensitive to impact than the other two compounds. Three pathways believed important for the initiation reactions are investigated at 0 and 298.15 K. We compare the computed energetics of the reaction pathways with the objective of rationalizing the unexpected sensitivity behavior. Our results reveal a few if any significant differences in the energetics of the three molecules, and thus do not reflect the sensitivity deviations observed in experiments. These findings point toward the potential importance of crystal structure, crystal morphology, bimolecular reactions, or combinations thereof on the impact sensitivity of nitroaromatics.

A Comprehensive Model Test of the HONO Sources Constrained to Field Measurements at Rural North China Plain

As nitrous acid (HONO) photolysis is an important source of hydroxyl radical (OH), apportionment of the ambient HONO sources is necessary to better understand atmospheric oxidation. Based on the data HONO-related species and various parameters measured during the one-month campaign at Wangdu (a rural site in North China plain) in summer 2014, a box model was adopted with input of current literature parametrizations for various HONO sources (nitrogen dioxide heterogeneous conversion, photoenhanced conversion, photolysis of adsorbed nitric acid and particulate nitrate, acid displacement, and soil emission) to reveal the relative importance of each source at the rural site. The simulation results reproduced the observed HONO production rates during noontime in general but with large uncertainty from both the production and destruction terms. NO₂ photoenhanced conversion and photolysis of particulate nitrate were found to be the two major mechanisms with large potential of HONO formation but the associated uncertainty may reduce their importance to be nearly negligible. Soil nitrite was found to be an important HONO source during fertilization periods, accounted for (80 ± 6)% of simulation HONO during noontime. For some episodes of the biomass burning, only the NO₂ heterogeneous conversion to HONO was promoted significantly. In summary, the study of the HONO budget is still far from closed, which would require a significant effort on both the accurate measurement of HONO and the determination of related kinetic parameters for its production pathways.

Ultrafast stimulated emission of nitrophenolates in organic and aqueous solutions

Early-time dynamics of nitroaromatics and its coressponding bases can give valuable insights into photo-induced reactions relevant to atmospheric and environmental processes. In this work, femtosecond broadband absorption spectroscopy between 350 and 700 nm has been applied to explore the ultrafast dynamics of o-, p- and m-nitrophenol anions (NP-) in basic organic and aqueous solution. Excitation at 400 nm promotes these compounds into the first bright electronic singlet state, which is a charge-transfer state. A surprising finding for all nitrophenolates was a characteristic, spectrally broad stimulated emission (SE) from the electronically excited state into the ground state. The corresponding lifetime was on the order of a few hundred femtoseconds for o- and p-NP- while it was roughly ten times larger for m-NP-. In line with earlier observations, the SE is governed by an out-of-plane torsional motion of the nitro group, leading to a close energetic approach of the relevant electronically excited singlet and ground states. Subsequent dynamics can be assigned to excited state absorption and ground state relaxation due to energy dissipation of the vibrational modes to the solvent that occur for up to several tens of picoseconds. No longer-lasting transient absorption (TA) was found; instead, a complete recovery of the ground state bleaching was observed indicating that triplet state relaxation is either not significantly involved in this spectral part or shifted to other regions. In the aqueous system, time constants for all processes are much smaller than in organic solution, a fact that can be explained by the larger dipole moment of the solvent and the correspondingly stronger intermolecular coupling between NP- and the aqueous solvent.