Rate Constants for the Gas-Phase Reactions of OH Radicals with Dimethyl Phosphonate over the Temperature Range of 278−351 K and for a Series of Other Organophosphorus Compounds at ∼280 K

Probing the Thermochemistry Properties and Rate Kinetics of Trimethyl Phosphate (TMP): An H-Atom Abstraction (HAA) Reactions Perspective

Trimethyl Phosphate (TMP), an organophosphorus liquid compound, is valued for its versatile qualities and applications in various fields. In modern chemical research and industry, processes involving Trimethyl Phosphate are optimized for minimal negative environmental impact, and scientific advancement is driven by adherence to stringent regulations to provide sustainable solutions and resource preservation. Thermochemical insights enhance our understanding of monomer incorporation, initiation, and propagation energetics. This study comprehensively investigates the thermochemistry and rate kinetics that govern H-atom abstractions in TMP through advanced computational techniques. The theoretical framework encompasses methodologies for conducting conformer searches, exploring transition states, and performing energy calculations. This study calculates rate constants for eight H-atom abstraction reactions involving TMP with stable species, O2 (oxygen), H (hydrogen), and radicals [ȮH (hydroxyl), ĊH3 (methyl), CH3Ȯ (methoxy), HȮ2 (hydroperoxyl), ṄH2 (amino), and ĊN (cyano)], and further analogies are related to barrier heights. Bond dissociation energies are also determined, highlighting TMP's susceptibility to various reaction pathways. The discussion and findings elucidate the need for further experimental validation for practical applications of TMP in chemical synthesis, combustion, flame-retardant technologies, environmental processes, and pharmaceutical research.

Atmospheric Autoxidation of Organophosphate Esters

Organophosphate esters (OPEs), widely used as flame retardants and plasticizers, have frequently been identified in the atmosphere. However, their atmospheric fate and toxicity associated with atmospheric transformations are unclear. Here, we performed quantum chemical calculations and computational toxicology to investigate the reaction mechanism of peroxy radicals of OPEs (OPEs-RO₂^•), key intermediates in determining the atmospheric chemistry of OPEs, and the toxicity of the reaction products. TMP-RO₂^• (R₁) and TCPP-RO₂^• (R₂) derived from trimethyl phosphate and tris(2-chloroisopropyl) phosphate, respectively, are selected as model systems. The results indicate that R₁ and R₂ can follow an H-shift-driven autoxidation mechanism under low NO concentration ([NO]) conditions, clarifying that RO₂^• from esters can follow an autoxidation mechanism. The unexpected autoxidation mechanism can be attributed to the distinct role of the ─(O)₃P(═O) phosphate-ester group in facilitating the H-shift of OPEs-RO₂^• from commonly encountered ─OC(═O)─ and ─ONO₂ ester groups in the atmosphere. Under high [NO] conditions, NO can mediate the autoxidation mechanism to form organonitrates and alkoxy radical-related products. The products from the autoxidation mechanism have low volatility and aquatic toxicity compared to their corresponding parent compounds. The proposed autoxidation mechanism advances our current understanding of the atmospheric RO₂^• chemistry and the environmental risk of OPEs.

Kinetics of the Gas-Phase Reaction of Hydroxyl Radicals with Dimethyl Methylphosphonate (DMMP) over an Extended Temperature Range (273–837 K)

The kinetics of the reaction of hydroxyl radical (OH) with dimethyl methylphosphonate (DMMP, (CH₃O)₂CH₃PO) (reaction 1) OH + DMMP → products (1) was studied at the bath gas (He) pressure of 1 bar over the 295–837 K temperature range. Hydroxyl radicals were produced in the fast reaction of electronically excited oxygen atoms O(¹D) with H₂O. The time-resolved kinetic profiles of hydroxyl radicals were recorded via UV absorption at around 308 nm using a DC discharge H₂O/Ar lamp. The reaction rate constant exhibits a pronounced V-shaped temperature dependence, negative in the low temperature range, 295–530 K (the rate constant decreases with temperature), and positive in the elevated temperature range, 530–837 K (the rate constant increases with temperature), with a turning point at 530 ± 10 K. The rate constant could not be adequately fitted with a standard 3-parameter modified Arrhenius expression. The data were fitted with a 5-parameter expression as: k₁ = 2.19 × 10⁻¹⁴(T/298)^2.43exp(15.02 kJ mol⁻¹/RT) + 1.71 × 10⁻¹⁰exp(−26.51 kJ mol⁻¹/RT) cm³molecule⁻¹s⁻¹ (295–837 K). In addition, a theoretically predicted pressure dependence for such reactions was experimentally observed for the first time.

Insight of UV-vis spectra and atmospheric implication for the reaction of ˙OH radical towards glyphosate herbicide and its hydrates

The rate constant of the reactions of ˙OH radicals with glyphosate (GPS) and its hydrates (GPS(H₂O)_n=1–3) were evaluated using the dual method M06-2X/6-311++G(df,p)//6-31+G(df,p) over the temperature range of 200–400 K. The results served to estimate the atmospheric lifetime along with the photochemical ozone creation potential (POCP). The calculations yielded an atmospheric lifetime of 2.34 hours and a POCP of 24.7 for GPS. Upon addition of water molecules, there is an increase of lifetime and decrease of POCP for water monomer and water dimer. The POCP for water trimer is slightly above the gaseous GPS. However, the POCPs of GPS and its hydrates are comparable to that of alkanes. The GPS and its hydrates were found to be a potential reservoir of CO₂. The acidification potential (AP) of GPS was found to be 0.189 and decreases upon addition of water molecules. This shows negligible contribution to rain acidification as the AP is less than that of SO₂. The UV-vis spectra were attained using the M06-L/6-311++G(3df,3pd) method and cover the range 160–260 nm which fits well with experiment.

The rate constant of the reactions of ˙OH radical with glyphosate (GPS) and its hydrates (GPS(H₂O)_n=1–3) were evaluated using the dual method M06-2X/6-311++G(df,p)//6-31+G(df,p) over the temperature range of 200–400 K.

Understanding the Impact of Relative Humidity and Coexisting Soluble Iron on the OH-Initiated Heterogeneous Oxidation of Organophosphate Flame Retardants

The current uncertainties in the reactivity and atmospheric persistence of particle-associated chemicals present a challenge for the prediction of long-range transport and deposition of emerging chemicals such as organophosphate flame retardants, which are ubiquitous in the global environment. Here, the OH-initiated heterogeneous oxidation kinetics of organophosphate flame retardants (OPFRs) coated on inert (NH₄)₂SO₄ and redox-active FeSO₄ particles were systematically determined as a function of relative humidity (RH). The derived reaction rate constants for the heterogeneous loss of tricresyl phosphate (TCP; k_TCP) and tris(2-butoxyethyl) phosphate (TBEP; k_TBEP) were in the range of (2.69-3.57) × 10^-12 and (3.06-5.55) × 10^-12 cm³ molecules^-1 s^-1, respectively, depending on the RH and coexisting Fe(II) content. The k_TCP (coated on (NH₄)₂SO₄) was relatively constant over the investigated RH range while k_TBEP was enhanced by up to 19% with increasing RH. For both OPFRs, the presence of Fe(II) enhanced their k by up to 53% over inert (NH₄)₂SO₄. These enhancement effects (RH and Fe(II)) were attributed to fundamental changes in the organic phase state (higher RH lowered particle viscosity) and Fenton-type chemistry which resulted in the formation of reactive oxygen species, respectively. Such findings serve to emphasize the importance of ambient RH, the phase state of particle-bound organics in general, and the presence of coexisting metallic species for an accurate description of the degradation kinetics and aging of particulate OPFRs in models used to evaluate their atmospheric persistence.

Effects of Atmospheric Water on ·OH-initiated Oxidation of Organophosphate Flame Retardants: A DFT Investigation on TCPP

Tris(2-chloroisopropyl) phosphate (TCPP), a widely used organophosphate flame retardant, has been recognized as an important atmospheric pollutant. It is notable that TCPP has potential for long-range atmospheric transport. However, its atmospheric fate is unknown, restricting its environmental risk assessment. Herein we performed quantum chemical calculations to investigate the atmospheric transformation mechanisms and kinetics of TCPP initiated by ·OH in the presence of O₂/NO/NO₂, and the effects of ubiquitous water on these reactions. Results show the H-abstraction pathways are the most favorable for the titled reaction. The calculated gaseous rate constant and lifetime at 298 K are 1.7 × 10^-10 cm³molecule^-1 s^-1 and 1.7 h, respectively. However, when considering atmospheric water, the corresponding lifetime is about 0.5-20.2 days. This study reveals for the first time that water has a negative role in the ·OH-initiated degradation of TCPP by modifying the stabilities of prereactive complexes and transition states via forming hydrogen bonds, which unveils one underlying mechanism for the observed persistence of TCPP in the atmosphere. Water also influences secondary reaction pathways of selected TCPP radicals formed from the primary H-abstraction. These results demonstrate the importance of water in the evaluation of the atmospheric fate of newly synthesized chemicals and emerging pollutants.

Heterogeneous OH initiated oxidation: a possible explanation for the persistence of organophosphate flame retardants in air

Heterogeneous reactions between OH radicals and emerging flame retardant compounds coated on inert particles have been investigated. Organophosphate esters (OPEs) including triphenyl phosphate (TPhP), tris-2-ethylhexyl phosphate (TEHP), and tris-1,3-dichloro-2-propyl phosphate (TDCPP) were coated on (NH4)2SO4 particles and exposed to OH radicals in a photochemical flow tube at 298 K and (38.0 ± 2.0) % RH. The degradation of these particle-bound OPEs was observed as a result of OH exposure, as measured using a Time-of-Flight Aerosol Mass Spectrometer. The derived second-order rate constants for the heterogeneous loss of TPhP, TEHP, and TDCPP were (2.1 ± 0.19) × 10(-12), (2.7 ± 0.63) × 10(-12), and (9.2 ± 0.92) × 10(-13) cm(3) molecule(-1) s(-1), respectively, from which approximate atmospheric lifetimes are estimated to be 5.6 (5.2-6.0), 4.3 (3.5-5.6), and 13 (11-14) days. Additional coating of the OPE coated particles with an OH radical active species further increased the lifetimes of these OPEs. These results represent the first reported estimates of heterogeneous reaction rate constants for these species. The results demonstrate that particle bound OPEs are highly persistent in the atmosphere with regard to OH radical oxidation, consistent with the assumption that OPEs can undergo medium or long-range transport, as previously proposed on the basis of field measurements. Finally, these results indicate that future risk assessment and transport modeling of emerging priority chemicals with semi- to low-volatility must consider particle phase heterogeneous loss processes when evaluating environmental persistence.

Atmospheric chemistry of methyl and ethyl N,N,N',N'-tetramethylphosphorodiamidate and O,S-dimethyl methylphosphonothioate

Rate constants for the reactions of OH radicals with methyl N,N,N',N'-tetramethylphosphorodiamidate [CH3OP(O)[N(CH3)2]2; MTMPDA], ethyl N,N,N',N'-tetramethylphosphorodiamidate [C2H5OP(O)[N(CH3)2]2; ETMPDA], and O,S-dimethyl methylphosphonothioate [CH3OP(O)(CH3)SCH3; OSDMMP] have been measured over the temperature range 281-349 K at atmospheric pressure of air using a relative rate method. The rate expressions obtained were 4.96 × 10(-12) e((1058±71)/T) cm(3) molecule(-1) s(-1) (1.73 × 10(-10) cm(3) molecule(-1) s(-1) at 298 K) for OH + MTMPDA, 4.46 × 10(-12) e((1144±95)/T) cm(3) molecule(-1) s(-1) (2.07 × 10(-10) cm(3) molecule(-1) s(-1) at 298 K) for OH + ETMPDA, and 1.31 × 10(-13) e((1370±229)/T) cm(3) molecule(-1) s(-1) (1.30 × 10(-11) cm(3) molecule(-1) s(-1) at 298 K) for OH + OSDMMP. The rate constant for OH + OSDMMP was independent of O2 content over the range 2.1-71% O2 at 296 ± 2 K. In addition, rate constants for the reactions of NO3 radicals and O3 with MTMPDA, of (1.4 ± 0.1) × 10(-12) cm(3) molecule(-1) s(-1) and <3.5 × 10(-19) cm(3) molecule(-1) s(-1), respectively, were measured at 297 ± 2 K. Products of the OH radical- and, for MTMPDA, NO3 radical-initiated reactions were investigated using gas chromatography and in situ atmospheric pressure ionization mass spectrometry. A product of molecular weight 180 was observed from the OH and NO3 radical-initiated reactions of MTMPDA, and this is attributed to CH3OP(O)[N(CH3)2]N(CH3)CHO. Similarly, a product of molecular weight 194 was observed from the OH + ETMPDA reaction and attributed to C2H5OP(O)[N(CH3)2]N(CH3)CHO. Possible reaction mechanisms are discussed.

Atmospheric chemistry of dichlorvos

Dichlorvos [2,2-dichlorovinyl dimethyl phosphate, (CH(3)O)(2)P(O)OCH═CCl(2)] is a relatively volatile in-use insecticide. Rate constants for its reaction with OH radicals have been measured over the temperature range 296-348 K and atmospheric pressure of air using a relative rate method. The rate expression obtained was 3.53 × 10(-13) e((1367±239)/T) cm(3) molecule(-1) s(-1), with a 298 K rate constant of (3.5 ± 0.7) × 10(-11) cm(3) molecule(-1) s(-1), where the error in the 298 K rate constant is the estimated overall uncertainty. In addition, rate constants for the reactions of NO(3) radicals and O(3) with dichlorvos, of (2.5 ± 0.5) × 10(-13) cm(3) molecule(-1) s(-1) and (1.7 ± 1.0) × 10(-19) cm(3) molecule(-1) s(-1), respectively, were measured at 296 ± 2 K. Products of the OH and NO(3) radical-initiated reactions were investigated using in situ atmospheric pressure ionization mass spectrometry (API-MS) and (OH radical reaction only) in situ Fourier transform infrared (FT-IR) spectroscopy. For the OH radical reaction, the major initial products were CO, phosgene [C(O)Cl(2)] and dimethyl phosphate [(CH(3)O)(2)P(O)OH], with equal (to within ±10%) formation yields of CO and C(O)Cl(2). The API-MS analyses were consistent with formation of (CH(3)O)(2)P(O)OH from both the OH and NO(3) radical-initiated reactions. In the atmosphere, the dominant chemical loss processes for dichlorvos will be daytime reaction with OH radicals and nighttime reaction with NO(3) radicals, with an estimated lifetime of a few hours.