Atmospheric Chemistry of N-Methylmethanimine (CH3N═CH2): A Theoretical and Experimental Study.

The OH-initiated photo-oxidation of N-methylmethanimine, CH3N═CH2, was investigated in the 200 m3 EUPHORE atmospheric simulation chamber and in a 240 L stainless steel photochemical reactor employing time-resolved online FTIR and high-resolution PTR-ToF-MS instrumentation and in theoretical calculations based on quantum chemistry results and master equation modeling of the pivotal reaction steps. The quantum chemistry calculations forecast the OH reaction to primarily proceed via H-abstraction from the ═CH2 group and π-system C-addition, whereas H-abstraction from the -CH3 group is a minor route and forecast that N-addition can be disregarded under atmospheric conditions. Theoretical studies of CH3N═CH2 photolysis and the CH3N═CH2 + O3 reaction show that these removal processes are too slow to be important in the troposphere. A detailed mechanism for OH-initiated atmospheric degradation of CH3N═CH2 was obtained as part of the theoretical study. The photo-oxidation experiments, obstructed in part by the CH3N═CH2 monomer-trimer equilibrium, surface reactions, and particle formation, find CH2═NCHO and CH3N═CHOH/CH2═NCH2OH as the major primary products in a ratio 18:82 ± 3 (3σ-limit). Alignment of the theoretical results to the experimental product distribution results in a rate coefficient, showing a minor pressure dependency under tropospheric conditions and that can be parametrized k(T) = 5.70 × 10-14 × (T/298 K)3.18 × exp(1245 K/T) cm3 molecule-1 s-1 with k298 = 3.7 × 10-12 cm3 molecule-1 s-1. The atmospheric fate of CH3N═CH2 is discussed, and it is concluded that, on a global scale, hydrolysis in the atmospheric aqueous phase to give CH3NH2 + CH2O will constitute a dominant loss process. N2O will not be formed in the atmospheric gas phase degradation, and there are no indications of nitrosamines and nitramines formed as primary products.

Atmospheric Chemistry of Methyl Isocyanide-An Experimental and Theoretical Study.

The reaction of CH3NC with OH radicals was studied in smog chamber experiments employing PTR-ToF-MS and long-path FTIR detection. The rate coefficient was determined to be kCH3NC+OH = (7.9 ± 0.6) × 10-11 cm3 molecule-1 s-1 at 298 ± 3 K and 1013 ± 10 hPa; methyl isocyanate was the sole observed product of the reaction. The experimental results are supported by CCSD(T*)-F12a/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ quantum chemistry calculations showing the reaction to proceed primarily via electrophilic addition to the isocyanide carbon atom. On the basis of the quantum chemical data, the kinetics of the OH reaction was simulated using a master equation model revealing the rate coefficient to be nearly independent of pressure at tropospheric conditions and having a negative temperature dependence with kOH = 4.2 × 10-11 cm3 molecule-1 s-1 at 298 K. Additional quantum chemistry calculations on the CH3NC reactions with O3 and NO3 show that these reactions are of little importance under atmospheric conditions. The atmospheric fate of methyl isocyanide is discussed.

Experimental and Theoretical Study of the OH-Initiated Photo-oxidation of Formamide.

The kinetics of OH radical reaction with formamide was studied by the relative rate method employing proton transfer reaction-mass spectrometry detection at the European Photochemical Reactor in Valencia, Spain. The rate coefficient was determined to be (4.5 ± 0.4) × 10(-12) cm(3) molecule(-1) s(-1) at 309 ± 3 K and 1013 ± 1 hPa. Isocyanic acid was observed as the sole product. The experimental results are supported by quantum chemical calculations and kinetic simulations using a master equation model. The calculated rate coefficient is independent of pressure at tropospheric conditions and can be accurately described by an Arrhenius expression having negative activation energy. The reaction is predicted to proceed exclusively via C-H abstraction.

The reactions of N-methylformamide and N,N-dimethylformamide with OH and their photo-oxidation under atmospheric conditions: experimental and theoretical studies.

The reactions of OH radicals with CH3NHCHO (N-methylformamide, MF) and (CH3)2NCHO (N,N-dimethylformamide, DMF) have been studied by experimental and computational methods. Rate coefficients were determined as a function of temperature (T = 260-295 K) and pressure (P = 30-600 mbar) by the flash photolysis/laser-induced fluorescence technique. OH radicals were produced by laser flash photolysis of 2,4-pentanedione or tert-butyl hydroperoxide under pseudo-first order conditions in an excess of the corresponding amide. The rate coefficients obtained show negative temperature dependences that can be parameterized as follows: kOH+MF = (1.3 ± 0.4) × 10(-12) exp(3.7 kJ mol(-1)/(RT)) cm(3) s(-1) and kOH+DMF = (5.5 ± 1.7) × 10(-13) exp(6.6 kJ mol(-1)/(RT)) cm(3) s(-1). The rate coefficient kOH+MF shows very weak positive pressure dependence whereas kOH+DMF was found to be independent of pressure. The Arrhenius equations given, within their uncertainty, are valid for the entire pressure range of our experiments. Furthermore, MF and DMF smog-chamber photo-oxidation experiments were monitored by proton-transfer-reaction time-of-flight mass spectrometry. Atmospheric MF photo-oxidation results in 65% CH3NCO (methylisocyanate), 16% (CHO)2NH, and NOx-dependent amounts of CH2[double bond, length as m-dash]NH and CH3NHNO2 as primary products, while DMF photo-oxidation results in around 35% CH3N(CHO)2 as primary product and 65% meta-stable (CH3)2NC(O)OONO2 degrading to NOx-dependent amounts of CH3N[double bond, length as m-dash]CH2 (N-methylmethanimine), (CH3)2NNO (N-nitroso dimethylamine) and (CH3)2NNO2 (N-nitro dimethylamine). The potential for nitramine formation in MF photo-oxidation is comparable to that of methylamine whereas the potential to form nitrosamine and nitramine in DMF photo-oxidation is larger than for dimethylamine. Quantum chemistry supported atmospheric degradation mechanisms for MF and DMF are presented. Rate coefficients and initial branching ratios calculated with statistical rate theory based on molecular data from quantum chemical calculations at the CCSD(T*)-F12a/aug-cc-pVTZ//MP2/aug-cc-pVTZ level of theory show satisfactory agreement with the experimental results. It turned out that adjustment of calculated threshold energies by 0.2 to 8.8 kJ mol(-1) lead to agreement between experimental and predicted results.

Atmospheric gas phase chemistry of CH2═NH and HNC. A first-principles approach.

Quantum chemical methods were used to investigate the OH initiated atmospheric degradation of methanimine, CH2═NH, the major primary product in the atmospheric photo-oxidation of methylamine, CH3NH2. Energies of stationary points on potential energy surfaces of reaction were calculated using multireference perturbation theory and coupled cluster theory. The results show that hydrogen abstraction dominates over the addition route in the CH2═NH + OH reaction, and that the major primary product is HCN, while HNC and CHONH2 are minor primary products. HNC is found to react with OH exclusively via addition to the carbon atom followed by O-H scission leading to HNCO; N2O is not a product in the atmospheric photo-oxidation of HNC. Additional G4 calculations of the CH2═NH + O3 reaction show that this is too slow to be of importance at atmospheric conditions. Rate coefficients for the CH2═NH + OH and HNC + OH reactions were calculated as a function of temperature and pressure using a master equation model based on the coupled cluster theory results. The rate coefficients for OH reaction with CH2═NH and HNC at 1000 mbar and room temperature are calculated to be 3.0 × 10(-12) and 1.3 × 10(-11) cm(3) molecule(-1) s(-1), respectively. The atmospheric fate of CH2═NH is discussed and a gas phase photo-oxidation mechanism is presented.

Synthesis and microwave spectrum of vinyl isoselenocyanate (H2C═CHNCSe), a compound with a quasilinear CNCSe chain.

The first α,ß-unsaturated isoselenocyanate, vinyl isoselenocyanate (H(2)C═CHNCSe), has been synthesized, and its microwave spectrum has been investigated in the 11.5-77.0 GHz spectral range. The microwave work was augmented by quantum chemical calculations using four different methods, namely, CCSD(T), CCSD, B3LYP, and M062X, with the cc-pVTZ basis set. It is generally assumed that two rotamers having the isoselenocyanide group in an antiperiplanar or a synperiplanar position can exist for this compound. However, these four methods all predict that there is only one rotameric form of the molecule, namely, the antiperiplanar form. The CNC angle of the antiperiplanar rotamer is calculated to vary from 151° to 170° depending on the quantum chemical methodology. CCSD(T) and B3LYP potential functions of the in-plane CNC bending vibrations were calculated. These functions have one shallow minimum corresponding to the antiperiplanar form. The spectra of the ground and one vibrationally excited state of this rotamer were assigned. Spectral searches for the synperiplanar form were performed but were not successful, so this form must have a relatively high energy, if it exists at all. The vibrationally excited state is presumably the lowest in-plane bending vibration of the CNC angle. Relative intensity measurements yielded a very low frequency of 18(25) cm(-1) for this vibration. The large-amplitude vibration of this mode suggests that this compound should rather be regarded as having a quasilinear CNCSe link of atoms than a rigid, bent antiperiplanar form.

Energetics and mechanisms for the unimolecular dissociation of protonated trioses and relationship to proton-mediated formaldehyde polymerization to carbohydrates in interstellar environments.

We report the unimolecular decomposition of protonated glyceraldehyde, [HOCH(2)CH(OH)CHO]H(+), and protonated dihydroxyacetone, [HOCH(2)C(O)CH(2)OH]H(+). On the basis of mass spectrometric experiments and computational quantum chemistry, we have found that these isomeric ions interconvert freely at energies below that required for their unimolecular decompositions. The losses of formaldehyde and water (the latter also followed by CO loss) are the dominating processes, with formaldehyde loss having the lower energetic threshold. The reverse of the formaldehyde loss, namely, the addition of formaldehyde to protonated glycolaldehyde, appears to be an inefficient reaction at low temperature and pressure in the gas phase, leading to dissociation products. The relevance of these findings to interstellar chemistry and prebiotic chemistry is discussed, and it is concluded that the suggestion made in the literature that successive addition of formaldehyde by proton-assisted reactions should account for interstellar carbohydrates most likely is incorrect.