Exploration for electronic transitions and photodissociation mechanism of hydrogen iodide

The electronic transitions and excited-state fragmentation of hydrogen iodide have been studied within the A-band continuum. The extinction intensity for the excitations from the ground to the low-lying electronic states are derived by performing the wave packet simulations of nuclear dynamics in this study. The quantum yields of the spin-excited I* product at the different photon energies are determined as well. The results suggest that the possibility of intersystem crossing can be neglected. Employing the time-dependent density functional theory (TDDFT), the four highest occupied and the two lowest unoccupied orbitals of hydrogen iodide have been analyzed, and the transition to the state is found to be most probable in the first absorption band.     

Calculated electronic transitions of the water ammonia complex

We have calculated vertical excitation energies and oscillator strengths of the low lying electronic transitions in H2O, NH3, and H2ONH3 using a hierarchy of coupled cluster response functions [coupled cluster singles (CCS), second order approximate coupled cluster singles and doubles (CC2), coupled cluster singles and doubles (CCSD), and third order approximate coupled cluster singles, doubles, and triples (CC3)] and correlation consistent basis functions (n-aug-cc-pVXZ, where n=s,d,t and X=D,T,Q). Our calculations indicate that significant changes in the absorption spectra of the photodissociative states of H2O and NH3 monomers occur upon complexation. In particular, we find that the electronic transitions originating from NH3 are blueshifted, whereas the electronic transitions originating from H2O are redshifted.     

Investigations of electronic transitions and photodissociation of the bromine molecule

The electronic transitions and photodissociation of the bromine molecule were studied in the visible-near UV continuum using dynamic simulation. The molar extinction coefficients in this study were obtained in numerical calculations. The quantum yields of the spin-orbit Br*(² P _1/2) product at different photon frequencies were determined. Time-dependent density functional theory was used to analyze the highest five occupied and lowest five unoccupied Br₂ orbitals. The transition to the ¹Π _u state was found to be most probable in the visible-near UV absorption range.     

Topological implications of Y-conjugation for electronic transitions of cyanine dyes

To understand connections between electronic transitions of dyes having related conjugated systems, topological arguments from graph theory are often helpful. Using the Chebyshev expansion of the characteristic polynomials of cyanines, it is shown that the two possible structures of tribranched cyanines, i.e., a strongly out-of-plane orientation of one of the conjugated branches or a Y-conjugation of the entire unsaturated system, are both consistent with the similarities between visible absorption of these compounds and of simple chains. To choose between these two structures, evidence from other sources should be added.     

Photochemistry of molecules relevant to the atmosphere: photodissociation of nitric acid in the gas phase.

This Minireview gives an account of the photochemical decay of nitric acid HNO3 in the gas phase, which has been well investigated under bulk and molecular-beam conditions. Due to the importance of this molecule in atmospheric chemistry, attention was paid to the irradiation regions around 300 and 200 nm, where solar photolysis of HNO3 is expected to be particularly efficient. While the low-energy region is characterized by the products OH and NO2, the high-energy region gives rise to a variety of photochemical decay pathways, dominated by channels which lead to the products HONO + O in different electronic states.     

The photodissociation of vibrationally excited ozone in the upper atmosphere

The photodissociation of vibrationally excited O₃ in the sunlit mesosphere is investigated. Dissociation rate coefficients for specific vibrationally excited states of O₃ are calculated for the conditions of an overhead sun. Possible vibrational enhancements of the O(¹D) and O₂(a¹Δ_g) production rates are assessed. It is shown that such enhancements should make only minor contributions to the daytime production of these species in the mesosphere and lower thermosphere.     

A surface effect in the photodissociation of CO2 and its significance for the theory of the Martian atmosphere

The pressure and radiation path length dependence of CO₂ photodissociation by 1849 Å, show that it occurs in adsorbed layers, rather than in the gas phase. Previous estimates of dissociation rates in the Martian atmosphere may therefore be much too large.     

Quantum mechanical study of sulfuric acid hydration: atmospheric implications

The role of the binary nucleation of sulfuric acid in aerosol formation and its implications for global warming is one of the fundamental unsettled questions in atmospheric chemistry. We have investigated the thermodynamics of sulfuric acid hydration using ab initio quantum mechanical methods. For H(2)SO(4)(H(2)O)(n) where n = 1-6, we used a scheme combining molecular dynamics configurational sampling with high-level ab initio calculations to locate the global and many low lying local minima for each cluster size. For each isomer, we extrapolated the M?ller-Plesset perturbation theory (MP2) energies to their complete basis set (CBS) limit and added finite temperature corrections within the rigid-rotor-harmonic-oscillator (RRHO) model using scaled harmonic vibrational frequencies. We found that ionic pair (HSO(4)(-)·H(3)O(+))(H(2)O)(n-1) clusters are competitive with the neutral (H(2)SO(4))(H(2)O)(n) clusters for n ≥ 3 and are more stable than neutral clusters for n ≥ 4 depending on the temperature. The Boltzmann averaged Gibbs free energies for the formation of H(2)SO(4)(H(2)O)(n) clusters are favorable in colder regions of the troposphere (T = 216.65-273.15 K) for n = 1-6, but the formation of clusters with n ≥ 5 is not favorable at higher (T > 273.15 K) temperatures. Our results suggest the critical cluster of a binary H(2)SO(4)-H(2)O system must contain more than one H(2)SO(4) and are in concert with recent findings (1) that the role of binary nucleation is small at ambient conditions, but significant at colder regions of the troposphere. Overall, the results support the idea that binary nucleation of sulfuric acid and water cannot account for nucleation of sulfuric acid in the lower troposphere.     

Calculated band profiles of the OH-stretching transitions in water dimer

We have calculated the band profiles of the OH-stretching fundamental and overtone transitions in the proton donor unit of the water dimer complex. We have used a local mode Hamiltonian that includes both OH-stretching and OO-stretching motion but separates these adiabatically. The variation of OH-stretching frequency and anharmonicity with OO displacement from equilibrium contributes to the effective OO-stretching potentials for each OH-stretching state. The resulting OO-stretching energy levels and wave functions are used to simulate the vibrational profile of each OH-stretching transition. The coupled cluster with singles, doubles, and perturbative triples ab initio method with an augmented triple-zeta correlation consistent basis set has been used to obtain the necessary parameters, potentials, and dipole moment functions. We find that the OO-stretching transitions associated with a given hydrogen bonded OH-stretching transition are spread significantly and this spread increases with overtone. The spread is minor for the free OH-stretching transition. The inclusion of the OO-stretching mode has a limited effect on the overall OH-stretching band intensity.     

Hydrolysis mechanisms for the acetylpyridinephenylhydrazone ligand in sulfuric acid

The excess acidity method was applied to rate data obtained for 2-acetylpyridinephenylhydrazone hydrolysis in strong acid media using various aqueous/organic solvents, and it was observed that the reaction rate decreases with increasing permittivity of the medium. Two hydrolysis mechanisms are indicated. Below 0.6 M H(2)SO(4), no hydrolysis was observed; between 0.6 and 6.0 M H(2)SO(4), the substrate hydrolyzes by an A-S(E)2 mechanism and switches to an A-2 mechanism at higher acidity. This change of mechanism was justified on the basis of the syn and anti rotational conformers of the diene -N((1))=C-C=N((2))- group; the greater stability of the former can be explained by the formation of hydrogen bonds between the proton and the N((1)) and N((2)) nitrogen atoms, giving rise to a very stable five-membered ring. If the syn conformer is predominant, then no hydrolysis is observed; above 0.6 M, the attack of a second proton gives rise to a balance between the syn and anti forms, the latter being responsible for the hydrolysis of the hydrazone group.     

Heterogeneous chemistry of butanol and decanol with sulfuric acid: implications for secondary organic aerosol formation

Recent environmental chamber studies suggest that acid-catalyzed reactions between alcohols and aldehydes in the condensed phase lead to the formation of hemiacetals and acetals, enhancing secondary organic aerosol (SOA) growth. We report measurements of heterogeneous uptake of butanol and decanol on liquid H2SO4 in the range of 62-84 wt % and between 273 and 296 K. Both alcohols exhibit two distinct types of uptake behaviors (partially irreversible vs totally irreversible uptake), depending on the acid concentration and temperature. For the partially irreversible uptake, a fraction of the alcohol was physically absorbed while the other fraction underwent irreversible reaction. For the totally irreversible uptake, the alcohols were completely lost onto the sulfuric acid. The Henry's law solubility constant (H*) was determined from the time-dependent uptake, while the reactive uptake coefficients were calculated from the time-independent irreversible loss. Coexistence of butanol or decanol with octanal or decanal did not show enhanced uptake of the aldehydes in the sulfuric acid. Protonation and dissolution likely account for the reversible uptake, while formation of alkyl sulfate or dialkyl sulfate explains irreversible uptake of the alcohols. The results suggest that heterogeneous uptake of larger alcohols is unlikely of significant importance in the lower atmosphere except in the case of freshly nucleated aerosols that may have high acid concentrations.     

Production of levulinic acid from wood raw material in the presence of sulfuric acid and its salts

The catalytic activities of sulfuric acid and of cobalt(II), iron(III), and aluminum sulfates in the thermal splitting of the cellulose and wood of various species in the presence of superheated steam at 250–350°C under flow conditions and at 150–250°C under autoclave conditions have been studied. The yield of levulinic acid from cellulose reaches 35 wt-% and from wood 16.0–18.0 wt-%.Spruce, according to Table 3 and the experimental part-Translator.Institute of the Chemistry of Natural Organic Raw Material, Siberian Division, Russian Academy of Sciences, Krasnoyarsk, fax (3912) 43 93 42. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 226–230, March–April, 1998.     

Solubility of acetic acid and trifluoroacetic acid in low-temperature (207-245 k) sulfuric acid solutions: implications for the upper troposphere and lower stratosphere

The solubility of gas-phase acetic acid (CH(3)COOH, HAc) and trifluoroacetic acid (CF(3)COOH, TFA) in aqueous sulfuric acid solutions was measured in a Knudsen cell reactor over ranges of temperature (207-245 K) and acid composition (40-75 wt %, H(2)SO(4)). For both HAc and TFA, the effective Henry's law coefficient, H*, is inversely dependent on temperature. Measured values of H* for TFA range from 1.7 × 10(3) M atm(-1) in 75.0 wt % H(2)SO(4) at 242.5 K to 3.6 × 10(8) M atm(-1) in 40.7 wt % H(2)SO(4) at 207.8 K. Measured values of H* for HAc range from 2.2 × 10(5) M atm(-1) in 57.8 wt % H(2)SO(4) at 245.0 K to 3.8 × 10(8) M atm(-1) in 74.4 wt % H(2)SO(4) at 219.6 K. The solubility of HAc increases with increasing H(2)SO(4) concentration and is higher in strong sulfuric acid than in water. In contrast, the solubility of TFA decreases with increasing sulfuric acid concentration. The equilibrium concentration of HAc in UT/LS aerosol particles is estimated from our measurements and is found to be up to several orders of magnitude higher than those determined for common alcohols and small carbonyl compounds. On the basis of our measured solubility, we determine that HAc in the upper troposphere undergoes aerosol partitioning, though the role of H(2)SO(4) aerosol particles as a sink for HAc in the upper troposphere and lower stratosphere will only be discernible under high atmospheric sulfate perturbations.     

Ultrafast formation of the benzoic acid triplet upon ultraviolet photolysis and its sequential photodissociation in solution

Time-resolved infrared (TR-IR) absorption spectroscopy in both the femtosecond and nanosecond time domain has been applied to examine the photolysis of benzoic acid in acetonitrile solution following either 267 nm or 193 nm excitation. By combining the ultrafast and nanosecond TR-IR measurements, both the excited states and the photofragments have been detected and key mechanistic insights were obtained. We show that the solvent interaction modifies the excited state relaxation pathways and thus the population dynamics, leading to different photolysis behavior in solution from that observed in the gas phase. Vibrational energy transfer to solvents dissipates excitation energy efficiently, suppressing the photodissociation and depopulating the excited S(2) or S(3) state molecules to the lowest T(1) state with a rate of ～2.5 ps after a delayed onset of ～3.7 ps. Photolysis of benzoic acid using 267 nm excitation is dominated by the formation of the T(1) excited state and no photofragments could be detected. The results from TR-IR experiments using higher energy of 193 nm indicate that photodissociation proceeds more rapidly than the vibrational energy transfer to solvents and C-C bond fission becomes the dominant relaxation pathway in these experiments as featured by the prominent observation of the COOH photofragments and negligible yield of the T(1) excited state. The measured ultrafast formation of T(1) excited state supports the existence of the surface intersections of S(2)/S(1), S(2)/T(2), and S(1)/T(1)/T(2), and the large T(1) quantum yield of ～0.65 indicates the importance of the excited state depopulation to triplet manifold as the key factor affecting the photophysical and photochemical behavior of the monomeric benzoic acid.     

Assignment of the electronic transitions in benzene

The problem of the assignment of π-electronic transitions in benzene is discussed using all criteria presently available. It is shown that based on this information a few different assignments are impossible to exclude. The correct assignment ³B_1u < ³E_1u < ¹B_2u < ³B_2u < ¹B_1u < ³E⁺_2g < ¹E_1u < ¹E^?_2g < ³E^?_2g has been selected as result of theoretical considerations based on a new approach to the semi-empirical π-electron theory. The results confirm the adequacy of the π-electron model for energy level calculations, and emphasise the fundamental importance of multi-excited configurations.     

Fluorosulfonic acid and chlorosulfonic acid: possible candidates for OH-stretching overtone-induced photodissociation

We have calculated the stationary points and internal reaction coordinate pathway for the dissociation of fluorosulfonic acid (FSO3H) and chlorosulfonic acid (ClSO3H). These sulfonic acids dissociate to sulfur trioxide and hydrogen fluoride and chloride, respectively. We have calculated the frequencies and intensities of the OH-stretching transitions of FSO3H and ClSO3H with an anharmonic oscillator local mode model. We find that excitation of the fourth and third OH-stretching overtones provide adequate energy for photodissociation of FSO3H and ClSO3H, respectively. We propose that experimental detection of the products of OH-stretching overtone-induced photodissociation of FSO3H and ClSO3H would be easier than the sulfuric acid (H2SO4) equivalent. The photodissociation of H2SO4 is thought to be important in the stratosphere. The FSO3H and ClSO3H experiment could be used in proxy to support the recently proposed OH-stretching overtone-induced photodissociation mechanism of H2SO4.     

蛋氨酸衍生物的合成及其缓蚀性能研究

本文合成了一种新型蛋氨酸衍生物酸洗缓蚀剂,运用红外光谱及核磁共振氢谱对其结构进行了鉴定。采用失重法和电化学法研究了在0. 5mol·L~(-1)硫酸介质中其对碳钢试片的缓蚀性能,并通过吸附等温模型对缓蚀机理进行初步的探讨。结果表明,蛋氨酸衍生物的缓蚀效率约为90%,整体用量适中,是一种有望得到良好应用的绿色缓蚀剂。电化学分析表明,蛋氨酸衍生物为混合型缓蚀剂,其通过增大金属表面的电荷转移电阻而降低电化学腐蚀速率。     

Conformational memory in photodissociation of formic acid

The trans and cis forms of formic acid (HCOOH) in solid argon favor essentially different photodissociation (193 nm) products, H2O + CO and H2 + CO2, respectively. The branching ratio of these channels differs between the two conformers by a factor of >10. The observed selective photodissociation features conformational memory when the transition state of a molecule is reached before torsional randomization. These data demonstrate that the photodissociation products can be efficiently steered with selective narrow-band infrared radiation promoting rotational isomerism, which makes a strong case of optically controlled chemical reactions     

Methane activation and oxidation in sulfuric acid

The H/D exchange observed when methane is contacted with D(2)SO(4) at 270-330 degrees C shows that the alkane behaves as a sigma base and undergoes rapid and reversible protonation at this temperature. DFT studies of the hydrogen exchange between a monomer and a dimer of sulfuric acid and methane show that the transition states involved in the exchange are bifunctional, that is one hydrogen atom is transferred from a hydroxy group in sulfuric acid to methane, while one hydrogen atom is abstracted from methane by a non-hydroxy oxygen atom in sulfuric acid. All the transition states include a CH(5) moiety, which shows similarities to the methanium ion CH(5) (+). The calculated potential activation energy of the hydrogen exchange for the monomer is 174 kJ mol(-1), which is close to the experimental value (176 kJ mol(-1)). Solvation of the monomer and the transition state of the monomer with an extra sulfuric acid molecule, decrease the potential activation energy by 6 kJ mol(-1). The acid-base process is in competition, however, with an oxidative process involving methane and sulfuric acid which leads to CO(2), SO(2), and water, and thus to a decrease of acidity and loss of reactivity of the medium.