Calculation of optimum geometries and force fields by the CNDO/force method

CNDO/Force calculations have been performed on a series of molecules, H₂CO, F₂CO, CF₄, CHF₃, CH₂F₂ and CH₃F. The optimum geometries and force fields are reported. It is found that the method can successfully predict the geometries of polyatomic molecules. The bending force constants and interaction force constants are, in general, comparable with experimental values both with respect to sign and magnitude. The stretching force constants have higher values than the experimental force constants. However, the trend in stretching force constants of a series of molecules is comparable with that of the corresponding experimental values.     

Photoionization mass spectrometry studies of deuterated acetaldehydes CH3CDO and CD3CHO

Investigations of the fragmentation processes of acetaldehyde were performed by photoionization mass spectrometry of its deuterium labeled species CH₃CDO and CD₃CHO. Intramolecular exchange of hydrogen atoms (hydrogen scrambling) was observed. Obviously this process is accompanied by predissociation of the parent ion. Results are compared with previous work on acetaldehyde CH₃CHO.     

Reactions of methyl radicals with acetaldehyde and acetaldehyde-d1. II. BEBO calculations of the temperature dependence of the rate constants

The BEBO method was used to calculate the kinetic isotope effect for formyl-hydrogen abstraction from acetaldehyde by methyl radicals. The calculated isotope effect and experimental ratios of the rate constants obtained at 785°K for the reactions of CH₃ with CH₃CHO and CH₃CDO, together with the theoretical temperature dependence of the specific rates (as formulated by the BEBO theory), were used to obtain rate constants for the steps CH₃ + CH₃CHO → CH₄ + CH₃CO (2a), CH₃ + CH₃CHO → CH₄ + CH₂CHO (2b), and CH₃ + CH₃CDO → CH₃D + CH₃CO (1a) between 298 and 1224°K. It was shown that the curvature apparent in the Arrhenius plot of the rate coefficient k₂ reported for the reaction of methyl radicals with acetaldehyde in the temperature range of 298–1224°K is caused both by the simultaneous contribution of steps (2a) and (2b) to methane formation, and by the curvature in the Arrhenius plots of the two elementary rate constants themselves. The predicted curve agrees well with the experimental data, especially if the tunneling correction is applied.     

Study on the method of chemical trapping for formyl intermediates

The method of chemical trapping for formyl intermediates has been studied, with syngas conversion to ethanol over rhodium-based catalysts as the diagnostic reaction concerned, and CH₃I as the trapping reagent. Two species of acetaldehyde, i.e., CH₃CHO and CH₃CDO, were produced in the trapping reaction following CO + 2D₂ reaction. It was shown that the formation of CH₃CHO in the trapping reaction resulted from dehydrogenation of CH₃ from CH₃I to give H, which induced the formation of CH₃CHO in the presence of CO and CH₃ So there may be two pathways for the formation of CH₃CDO in the trapping reaction: one, methylation of DCO adspecies; the other, deuteration of CH₃ CO formed by CO insertion into CH₃ The catalyst surface was purged with Ar following CO + 2D₂ reaction before the trapping reaction was performed. By means of this modified method of chemical trapping for formyl intermediates, CH₃CDO was found to be mainly derived from the methylation of DCO adspecies. Accordingly, it could be concluded that formyl is a C₁ intermediate in the syngas conversion to ethanol over rhodium-based catalysts.     

Towards an Understanding of Halide Interactions with the Carbonyl-Containing Molecule CH3CHO

The anion photoelectron spectra of Cl⁻⋅⋅⋅CD₃CDO, Cl⁻⋅⋅⋅(CD₃CDO)₂, Br⁻⋅⋅⋅CH₃CHO, and I⁻⋅⋅⋅CH₃CHO are presented with electron stabilisation energies of 0.55, 0.93, 0.48, and 0.40 eV, respectively. Optimised geometries of the singly solvated species featured the halide appended to the CH₃CHO molecule in-line with the electropositive portion of the C=O bond and having binding energies between 45 and 52 kJ mol⁻¹. The doubly solvated Cl⁻⋅⋅⋅(CH₃CHO)₂ species features asymmetric solvation upon the addition of a second CH₃CHO molecule. Theoretical detachment energies were found to be in excellent agreement with experiment, with comparisons drawn between other halide complexes with simple carbonyl molecules.     

Selectivity in resonant vibrational excitation by e− impact (0.3–4.5 eV) on formaldehyde,acetaldehyde and acetone

Vibrational excitation by e⁻ impact via low-energy resonances has been investigated in acetaldehyde and acetone and compared with similar results in formaldehyde. Despite the large number of vibrational modes involved, the three systems exhibit a selective excitation of only several modes. Besides an important excitation of the CO stretch modes (dominant in H₂CO), excitation of CH stretch, CH₃ stretch and CH₃ deformation are also observed in CH₃CHO and (CH₃)₂CO. Interpretation of the energy loss spectra is given in terms of recently developed symmetry considerations together with the character of the LUMO occupied by the extra electron to form the transitory negative ion (resonance). Differential cross sections versus electron energy are presented for elastic and several inelastic processes. Weak oscillations (of the “boomerang” type) are observed on the inelastic cross sections for acetaldehyde, whereas no structure appears for acetone. This is in contrast with the pronounced oscillations observed for H₂CO, and reveals a shorter lifetime for the CH₃CHO⁻ and (CH₃)₂CO⁻ resonant states, compared to H₂CO⁻.     

Force constants in molecular crystals of methyl-and perdeuteriomethylmercury(II) halides

Force constants for the internal vibrations involving the metal and for the lattice vibrations of Hg(CH₃)X and Hg(CD₃)X (X = Cl, Br or I) are calculated on the basis of a D_4h⁷ layer structure. The internal HgX stretching force constants are much lower than for these molecules in solution, but HgC stretching force constants are slightly higher. The HgX and longitudinal translatory force constants within the lattice layer are close in value to the strong and weak HgX bond stretching force constants respectively in the unsymmetrical [Hg(CH₃)X₂]^? complex ions.     

Shock‐tube and modeling study of acetaldehyde pyrolysis and oxidation

Pyrolysis and oxidation of acetaldehyde were studied behind reflected shock waves in the temperature range 1000–1700 K at total pressures between 1.2 and 2.8 atm. The study was carried out using the following methods, (1) time‐resolved IR‐laser absorption at 3.39 μm for acetaldehyde decay and CH‐compound formation rates, (2) time‐resolved UV absorption at 200 nm for CH₂CO and C₂H₄ product formation rates, (3) time‐resolved UV absorption at 216 nm for CH₃ formation rates, (4) time‐resolved UV absorption at 306.7 nm for OH radical formation rate, (5) time‐resolved IR emission at 4.24 μm for the CO₂ formation rate, (6) time‐resolved IR emission at 4.68 μm for the CO and CH₂CO formation rate, and (7) a single‐pulse technique for product yields. From a computer‐simulation study, a 178‐reaction mechanism that could satisfactorily model all of our data was constructed using new reactions, CH₃CHO (+M) → CH₄ + CO (+M), CH₃CHO (+M) → CH₂CO + H₂(+M), H + CH₃CHO → CH₂CHO + H₂, CH₃ + CH₃CHO → CH₂CHO + CH₄, O₂ + CH₃CHO → CH₂CHO + HO₂, O + CH₃CHO → CH₂CHO + OH, OH + CH₃CHO → CH₂CHO + H₂O, HO₂ + CH₃CHO → CH₂CHO + H₂O₂, having assumed or evaluated rate constants. The submechanisms of methane, ethylene, ethane, formaldehyde, and ketene were found to play an important role in acetaldehyde oxidation. © 2007 Wiley Periodicals, Inc. 40: 73–102, 2008     

Vibrational spectra and force field of dimethylphosphines

Vibrational spectra in the range 200–3000 cm^?1 are reported and assigned for the species (CH₃)₂PH, (CH₃)₂PD, (CD₃)₂PH, (CD₃)₂PD, CH₃CD₃PH and CH₃CD₃PD. The spectra in the range 1020–500 cm^?1 are complicated due to the coupling between δPH, ?Me and the skeletal modes of the molecule. Interpretation is only possible through a force field which is markedly different from an earlier one of dimethyl sulphide. This force field predicts uncoupled δPH frequencies of 835 (a) and 909 cm^?1 (a), couples PH bending largely to out-of-skeletal plane methyl rocking (?_i) and includes a low p¦¦(a) bending constant, a high skeletal bending constant and unusual signs for two interaction constants. In the crystalline phase at 78 K, the two methyl groups are non-equivalent.     

Molecular orbital studies of the acidity of substituted methanes

CNDO/2 MO studies have been carried out on CH₄, C₂H₆, CHCL₃ CH₃CN, CH₃NO₂, CH₃CHO, CH₃COCH₃, and their corresponding anions, both in the gas phase and in “aqueous solution” The results closely parallel related experimental studies.     

Rate constants for the gas-phase reactions of O3 with a series of carbonyls at 296 K

Rate constants for the gas-phase reactions of O₃ with the carbonyls acrolein, crotonaldehyde, methacrolein, methylvinylketone, 3-penten-2-one, 2-cyclohexen-1-one, acetaldehyde, and methylglyoxal have been determined at 296 ± 2 K. The rate constants ranged from <6 × 10^?21 cm³ molecule^?1 s^?1 for acetaldehyde to 2.13 × 10^?17 cm³ molecule^?1 s^?1 for 3-penten-2-one. The substituent effects of ? CHO and CH₃CO? groups on the rate constants are assessed and discussed, as are implications for the atmospheric chemistry of the natural hydrocarbon isoprene.     

Photoinduced reactions of organic compounds with transition metal complexes: II. Reaction of PtCl62− with acetone to give a σ-acetonyl complex of platinum(IV). Detection of platinum(III) compounds by ESR

Light irradiation of a PtCl₆^2? solution in acetone affords a σ-acetonyl complex of platinum(IV), [CH₃COCH₂PtCl₅]^2?. Prolonged irradiation yields CH₃COCH₂Cl and PtCl₄^2?. The signals of a platinum(III) complex and CH₃COCH₂^. are detected in the ESR spectrum of the frozen solution of PtCl₆^2? in acetone irradiated at 77 K. The proposed mechanism of the reaction involves electron transfer from the enol tautomer of acetone to PtCl₆^2?.     

Theoretical study of the force field and vibrational assignments of acetamide and deuterated analogues

The complete harmonic force constants of acetamide have been evaluated by ab initio calculations at the Hartree-Fock level with the 4–31G(d) basis set. The force field was scaled to compensate for the systematic overestimations of the Hartree-Fock-level force constants by empirical factors using the matrix isolation IR spectra of CH₃CONH₂ and CD₃CONH₂. A normal coordinate treatment has been carried out with the scaled force field to analyze the vibrational spectra of CH₃CONH₂. A normal coordinate treatment has been carried out with the scaled force field to analyze the vibrational spectra of CH₃COND₂, cis-CH₃CONHD and trans-CH₃CONHD. The effect of cis/trans isomerism of CH₃CONHD on the fundamental bands was well reproduced by the calculations. The fundamental vibrations were also predicted for CD₃COND₂, cis-CD₃CONHD and trans-CD₃CONHD.     

Gas-phase ion equilibria studies of protons and chloride ions in propanol and acetone

The gas-phase clustering reactions of proton in propanol and acetone, and chloride ions in acetone were investigated. The −ΔH_n−1,n values obtained for clustering reactions (n−1,n) were as follows: H⁺ (C₃H₇OH)_n−1 + C₃H₇OH ⇄ H⁺ (C₃H₇OH)_n, (2, 3) 18.9 kcal mol⁻¹, (3, 4) 14.2 kcal mol⁻¹, (4, 5) 11.7 kcal mol⁻¹; H⁺ (CH₃COCH₃)₂ + CH₃COCH₃ ⇄ H⁺ (CH₃COCH₃)₃, 14.2 kcal mol⁻¹; and Cl⁻ + CH₃COCH₃ ⇄ Cl⁻ (CH₃COCH₃), 12.4 kcal mol.⁻¹. For clustering reactions, Cl⁻ (CH₃COCH_3n−1 + CH₃COCH₃ ⇄ Cl⁻ (CH₃COCH₃)_n where n ≥ 2, the equilibria could not be established; probably due to the isomerization of ligand acetone molecules from the keto to enol form.     

Gas-phase reactions of pd with acetone: A theoretical investigation using density functional theory

The gas-phase reaction of palladium atom with acetone is investigated using density functional theory. Geometries and energies of the reactants, intermediates, and products involved are calculated. Both ground and excited state potential energy surfaces are investigated in detail. The present results show that the title reaction start with the formation of an ??²-CH₃COCH₃-metal complex, followed by C-O, C-H, and C-C activation. These reactions can lead to four different products (PdO + C₃H₆, PdCH₂COCH₃ + H, PdCH₂ + CH₃CHO, and PdCOCH₂ + CH₄). The present results may be helpful in understanding the mechanism of the title reaction and further experimental investigation of the reaction.     

Decomposition of Acetaldehyde in Atmospheric Pressure Filamentary Nitrogen Plasma

The removal of 500?ppm acetaldehyde in nitrogen at 1?bar is characterized in a pulse dielectric barrier discharge generating a spatial random distribution of plasma filaments. The identification and the quantification of numerous by-products are performed. At 20?°C, CH₃CHO is efficiently dissociated, probably owing to quenching of N₂ metastable states. The most abundant by-products are CO, H₂, and CH₄, in consistency with the three important exit channels for the quenching of the N₂(A³?? _u ⁺ ) state by CH₃CHO proposed by Faider et al. (2011). In order of importance, other products are HCN, C₂H₆, CH₃CN, HNCO, CO₂, CH₃COCH₃, C₂H₄, C₂H₅CN, NH₃, C₂H₂, and a group of nitriles and of ketones. An increase of the temperature from 20?°C up to 300?°C induces a strong decrease of the removal characteristic energy, but the by-products types remain unchanged. Probably the reaction of H with CH₃CHO plays a role in the removal of the molecule at 300?°C.     

Enthalpies of formation of radicals and the mass spectra of the products of tetrafluoroethylene polymerization in acetone

The mass spectra of the dissociative electron-impact ionization products of telomers formed upon the radiation-chemical telomerization of tetrafluoroethylene in acetone were measured over the range of m/z from 1 to 204. The most intense bands at m/z = 43, 51, and 57 were attributed to the CH₃CO⁺, CF₂H⁺ and CH₃COCH₂⁺ cations—the main dissociation products of the H(C₂F₄) _n CH₂COCH₃ telomers. The telomer composition was consistent with a radical telomerization mechanism, in which chain growth and chain transfer are due to the formation of the CH₃COCH₂^· radical. Based on published data supplemented with quantum-chemical calculations, the enthalpies of formation of the radicals R(CF₂) _n (n = 2–8; R = H, CH₃, CH₃CO, and CH₃COCH₂) were tabulated. The formation of telomers with the same terminal groups is consistent with thermodynamic data and a polymerization mechanism in which the chain growth reaction is diffusion-limited and the chain transfer reaction is activated hydrogen-atom transfer.     

Relative rate constants for the reactions of trichloropropenyl radical. Application of the two-parameter Taft equation

The relative rate constants for the hydrogen atom abstraction by CCl₃CH?CH· radical from CH₂Cl₂, CHCl₃, CH₃COCH₃, CH₃CN, C₆H₅CH₃, C₆H₅OCH₃, CH₃CHO, and CH₃OH in the liquid phase at 20°C have been measured. It was shown that these reaction rate constants are correlated by the two-parameter Taft equation with ρ* = 0.726 ± 0.096, r* = 1.22 ± 0.16. A relationship between r* and bond dissociation energy D(R? H) has been found for the abstraction reactions of different free radicals.     

High-temperature pyrolysis of acetaldehyde

High-temperature (>1000°K) pyrolysis of acetaldehyde (～1% in an atmosphere of pure nitrogen) was examined in a turbulent flow reactor which permits accurate determination of the spatial distribution of the stable species. Results show that the products in order of decreasing importance are CO, CH₄, H₂, C₂H₆, and C₂H₄. Rates of formation were consistent with the Rice–Herzfeld mechanism by including reactions to explain C₂H₄ formation and the possible presence of ketene. A steady-state treatment of the complete mechanism indicates that the overall reaction order decreases from \documentclass{article}\pagestyle{empty}\begin{document}$ \frac{3}{2} $\end{document} to 1, which is supported by the new experimental data. Using earlier low-temperature results, the rate constant for the reaction CH₃CHO → CH₃ + CHO (1) was found as k₁=10^15.85±0.21 exp (?81,775±1000/RT) sec^?1. Also, data for the ratio of rate constants for reactions CH₃CHO + CH₃ → CH₄ + CH₃CO (4) and 2CH₃ → C₂H₆(6) were fitted to the empirical expression k₄/k₆^1/2=10^?13.89±0.03T^6.1 exp(?1720±70/RT) (cm³/mole·sec)^1/2 and causes for the curvature are discussed. The noncatalytic effect of oxygen on acetaldehyde pyrolysis at high temperature is explained.     

Reactivity of acetonyl anion with nitroaromatics: an atmospheric pressure chemical ionization study

Nucleophilic attack by the acetonyl anion (CH₃COCH) on highly electron‐deficient nitroaromatics such as trinitrotoluene (TNT) results in the carbon‐bonded anionic σ‐complexes (Meisenheimer complexes). The complexes are generated at atmospheric pressure with relatively low internal energy and do not dissociate under atmospheric pressure chemical ionization conditions, but can be dissociated upon collisional activation to produce characteristic nitrobenzyl‐substituted acetonyl anions via elimination of HNO₂. Further fragmentation produces deprotonated nitrotoluene anions through the loss of CH₂CO, while loss of CH₂COCH₃ and CH₃COCH₃ yields nitroaromatic molecular anions and their H‐loss counterparts. Hydrogen/deuterium (H/D) scrambling is observed in the fragmentation products of the [TNT · CD₃COCD₂]^? complex to extents which vary for different fragmentation pathways. Selective Meisenheimer complex formation, together with its distinctive fragmentation pattern, supplies a highly discriminatory method for detection of TNT and related compounds. Copyright © 2005 John Wiley & Sons, Ltd.