Rapid aqueous-phase dark reaction of phenols with nitrosonium ions: Novel mechanism for atmospheric nitrosation and nitration at low pH

Abstract Dark aqueous-phase reactions involving the nitrosation and nitration of aromatic organic compounds play a significant role in the production of light-absorbing organic carbon in the atmosphere. This process constitutes a crucial aspect of tropospheric chemistry and has attracted growing research interest, particularly in understanding the mechanisms governing nighttime reactions between phenols and nitrogen oxides. In this study, we present new findings concerning the rapid dark reactions between phenols containing electron-donating groups and inorganic nitrite in acidic aqueous solutions with pH levels <3.5. This reaction generates a substantial amount of nitroso- and nitro-substituted phenolic compounds, known for their light-absorbing properties and toxicity. In experiments utilizing various substituted phenols, we demonstrate that their reaction rates with nitrite depend on the electron cloud density of the benzene ring, indicative of an electrophilic substitution reaction mechanism. Control experiments and theoretical calculations indicate that the nitrosonium ion (NO+) is the reactive nitrogen species responsible for undergoing electrophilic reactions with phenolate anions, leading to the formation of nitroso-substituted phenolic compounds. These compounds then undergo partial oxidation to form nitro-substituted phenols through reactions with nitrous acid (HONO) or other oxidants like oxygen. Our findings unveil a novel mechanism for swift atmospheric nitrosation and nitration reactions that occur within acidic cloud droplets or aerosol water, providing valuable insights into the rapid nocturnal formation of nitrogen-containing organic compounds with significant implications for climate dynamics and human health.


Figures S1 to S18
Tables S1 to S2 SI References

S2. Kinetic analyses.
Pseudo-First-Order analyses.According to Atkins' Physical Chemistry, 11 th Edition, the reactions were identified as being pseudo first order in GUA when the initial concentration of NaNO2 is much greater than the initial concentration of GUA ( NaNO ≫ GUA ), could be described with a pseudo-first-order rate constant: where GUA is the concentration of GUA at reaction time t.GUA is the initial concentration of GUA.k is the pseudo-first-order rate constant.t is the half-life of GUA.
Pseudo-Second-Order analyses.The reactions were identified as pseudo second order with respect to GUA when the initial concentrations of NaNO2 and GUA were similar.( NaNO GUA ), could be described with a pseudo-second-order rate constant: where GUA is the concentration of GUA at reaction time t.GUA is the initial concentration of GUA.k is the pseudo-second-order rate constant.t is the half-life of GUA.When NaNO GUA where GUA is the concentration of GUA at reaction time t.NaNO is the concentration of NaNO2 at reaction time t.GUA is the initial concentration of GUA.

NaNO
is the initial concentration of NaNO2.k is the pseudo-second-order rate constant.S2): R is the ideal-gas constant 8.314 J K -1 mol -1 .
We calculate the distribution factor fphenols and the aqueous phase Xaq using: R is the ideal-gas constant 0.08205 atm L mol -1 K -1 .T is the temperature 278 K. L is the cloud/fog liquid water content in g m -3 .
We calculated the degradation rate of phenols with HONO and NO3 radical in the aqueous phases at pH 3.0 and temperature 5 o C. The degradation rate of phenols with oxidants calculates using:

𝑅 𝑘 𝑝ℎ𝑒𝑛𝑜𝑙𝑠 𝑂𝑥
In the aqueous-phase reaction between HONO and phenols, the active reactive species are phenols -and NO + .Previous studies have reported the total reaction rate constants for phenols and NO + .Therefore, the degradation rate of phenols is calculated using the total reaction rate constant as follows: ℎ   is 2.2×10 9 M -1 s -1 for phenol (9).Our experiments show that with the increase of electron cloud density on the aromatic ring, the degradation rate will increase by 3-6 times.Therefore, we estimate that the second-order reaction rate constant  of guaiacol and NO + is 1.32×10 10 M -1 s -1 , and the  is 7.9×10 The degradation rate of phenols with NO3 radical calculates using:   ℎ   is 5×10 7 M -1 s -1 for guaiacol.There is no available  value for syringol, we assume it has the same value as guaiacol (12). is 1×10 -12 M, represents a typical nighttime cloud droplet concentration (13).
Therefore, the contribution of phenolic compounds to the degradation rate at nighttime aqueous phase can be calculated using:

Figure S4 .
Figure S4.The dependence of the pseudo-first-order rate constant for the GUA + NaNO2 reaction on the concentration of NaNO2.Experimental conditions: [GUA] = 0.1 mM, pH = 3.0 ± 0.1, without light, zero air bubbling, room temperature.

Figure S5 .
Figure S5.The dependence of the pseudo-first-order rate constant for the reaction of GUA with NaNO2 on the concentration of GUA.Experimental conditions: [NaNO2] = 1.0 mM, pH = 3.0 ± 0.1, without light, zero air bubbling, room temperature.GUA concentration

Figure S7 .
Figure S7.Two chemical mechanisms for the reactions between N(Ⅲ) inorganic species and phenols.(A) Addition reaction pathway in solutions with a pH above 5.5(6).(B) Radical reaction pathway in solutions with a pH below 5.5(7).

Figure S8 .
Figure S8.The ion speciation of N(Ⅲ) inorganic compounds according to the pKa.In theory, when the pH level of the solution surpasses 3.5, the primary constituents are HNO2 and NO2 -.However, when the pH falls below 1.0, the primary constituents are H2ONO + and HNO2.The black, red and blue lines represent H2ONO + , HNO2, and NO2 -.

Figure S9 .
Figure S9.The modredundant calculation abbreviates the distance between the N atoms of NO+ and the C atoms of the benzene ring.(A) and (B) To form 4-nitrosoguaiacol from guaiacol and guaiacol -. (C) and (D) To form 6-nitrosoguaiacol from guaiacol and guaiacol -.The gray, white, red and blue balls represent C, H, O and N atoms, respectively.

Figure S10 .
Figure S10.The modredundant calculation abbreviates the distance between the N atoms of NO + and the C atoms of the benzene ring.(A) and (B) for the formation of 6nitrosocreosol from creosol and creosol -. (C) and (D) for the formation of 6-nitrosovanillin from vanillin and vanillin -.The gray, white, red and blue balls symbolize C, H, O, and N atoms, respectively.

Figure S11 .
Figure S11.The molecular structures along the reaction coordinates cited (Å) in Figure 3A and 3B.The gray, white, red and blue balls symbolize C, H, O, and N atoms, respectively.

Figure S12 .
Figure S12.The modredundant calculation abbreviates the distance between the N atoms of NO + and the C atoms of the benzene ring.(A) and (B) To form 4-nitrosocatechol from catechol and catechol -. (C) and (D) To form 6-nitrosocatechol from catechol and catechol -.The gray, white, red and blue balls symbolize C, H, O, and N atoms, respectively.

Figure S13 .
Figure S13.Gibbs free-energy (in kcal/mol at 298.15 K) profiles for the reaction of catechol and NO + at the DLPNO-CCSD(T)/aug-cc-pVTZ/SMD(water)//B3LYP-D3(BJ)/aug-cc-pVTZ/SMD(water) level of theory with the Zero Point Energy (ZPE) correction applied.The black line represents the reaction mechanisms for forming 4-nitrosocatechol and the red line represents the reaction mechanisms for forming 6-nitrosocatechol.

Figure S14 .Figure S15 .
Figure S14.The molecular structures along the reaction coordinates cited (Å) in Figure S13.The gray, white, red and blue balls symbolize C, H, O, and N atoms, respectively.

Figure S17 .
Figure S17.The molecular structures along the reaction coordinates cited (Å) in Figure S16.The gray, white, red, blue and green balls symbolize C, H, O, N and Cl atoms, respectively.

Table S1 .
The experiments conducted in this work.aThepseudo-first-order rate constant (min -1 ).b The pseudo-second-order rate constant (mM -1 • min - 1 ).c Bubble using zero air with 84 ppb O3.We calculated the concentrations of phenols in the gas and aqueous phases as a function of liquid water content at 5 o C. The Henry's law constants (KH, 278K) of guaiacol, catechol, syringol, m-benzenediol, and p-benzenediol at 278 k were calculated from measured KH,289 K and the enthalpy of dissolution (ΔHsol) (8) (see Table