Rotational isomerism in acetic acid: the first experimental observation of the high-energy conformer

The high-energy conformer of acetic acid (cis-AA) is produced in an Ar matrix by vibrational excitation of the OH stretching overtone of the ground conformational state (trans-AA). IR-absorption spectroscopy provides a clear identification of the reaction product. cis-AA converts back to trans-AA in a time scale of minutes at 8 K by tunneling.     

Time-resolved IR spectroscopy of N-methylthioacetamide: trans-->cis isomerization upon n-pi and pi-pi excitation and cis-->trans photoreaction

Time-resolved infrared spectroscopy was used to study the photoisomerization of N-Methylthioacetamide (NMTAA) in D2O in both the cis-->trans and the trans-->cis direction upon selective excitation of the n-pi (S1) and pi-pi (S2) electronic transitions. While isomerization and the return to the ground state takes place on two distinct time scales (cis isomerization is 30-40%, independent of the electronic state excited, while the cis-->trans isomerization proceeds with a 60-70% quantum efficiency. These results support a mechanism by which isomerization takes place via one common intermediate state independent of electronic excitation energy and initial conformation.     

Large-amplitude dynamics in vinyl radical: the role of quantum tunneling as an isomerization mechanism

We report tunneling splittings associated with the large amplitude 1,2 H-atom migration to the global minima in the vinyl radical. These are obtained using a recent full-dimensional ab initio potential energy surface (PES) [A. R. Sharma, B. J. Braams, S. Carter, B. C. Shepler, and J. M. Bowman, J. Chem. Phys. 130(17), 174301 (2009)] and independently, directly calculated "reaction paths." The PES is a multidimensional fit to coupled cluster single and double and perturbative treatment of triple excitations coupled-cluster single double triple (CCSD(T)) with the augmented correlation consistent triple zeta basis set (aug-cc-pVTZ). The reaction path potentials are obtained from a series of CCSD(T)/aug-cc-pVnTZ calculations extrapolated to the complete basis set limit. Approximate 1D calculations of the tunneling splitting for these 1,2-H atom migrations are obtained using each of these potentials as well as quite different 1D Hamiltonians. The splittings are calculated over a large energy ranges, with results from the two sets of calculations in excellent agreement. Though negligibly slow (>1 s) for the vibrational ground state, this work predicts tunneling-promoted 1,2 hydride shift dynamics in vinyl to exhibit exponential growth with internal vibrational excitation, specifically achieving rates on the sub-μs time scale at energies above E ≈ 7500 cm(-1). Most importantly, these results begin to elucidate the possible role of quantum isomerization through barriers without dissociation, in competition with the more conventional picture of classical roaming permitted over a much narrower window of energies immediately below the bond dissociation limit. Furthermore, when integrated over a Boltzmann distribution of thermal energies, these microcanonical tunneling rates are consistent with sub-μs time scales for 1,2 hydride shift dynamics at T > 1400 K. These results have potential relevance for combustion modeling of low-pressure flames, as well as recent observations of nuclear spin statistical mixing from high-resolution IR/microwave spectroscopy on vinyl radical.     

Effect of hydroacoustic treatment on the rate of hydrolytic degradation of chitosan in acetic acid solutions

Effect of a hydroacoustic treatment in a rotary-pulsatory apparatus on the rate of hydrolytic degradation of chitosan in acetic acid solutions was studied. The influence exerted on the rate of activated hydrolysis of chitosan by its molecular weight and the process temperature was considered.     

Rotational isomerism: XVII—solvent effects on the NMR spectra and rotamer energies of some tetrahaloethanes

The NMR spectra of 1,1-dichloro-2,2-difluoroethane ( 1 ), 1,1-dibromo-2,2-difluoroethane ( 2 ), meso and dl 1,2-dichloro-1,2-difluoroethane ( 3 ) and 1,1,2,2-tetrachloroethane ( 4 ) have been analysed in a number of solvents. The ¹⁹F spectrum of 3 in L-bornyl acetate at 56·4 and 94·1 MHz allows an unambiguous identification of the meso and dl isomers. The spectra of the d and l isomers consist of two AA′XX′ spectra with a small chemical shift difference between the d and l forms, whilst that of the meso form is an apparent AA′XX′ spectrum at 56·4 MHz but an ABXX′ spectrum at 94·1 MHz, the ¹⁹F nuclei in this isomer being anisochronous in this solvent. The observed solvent and temperature dependence of the couplings of 1, 2 , meso 3 and 4 when combined with the calculated solvation energies, allow the determination of the rotamer energies and couplings in these molecules. The rotamer energy differences (E_g – E_t) in the liquid and vapour states are 0·6 and ?0·2 kcal/mol ( 1 ); 0·4 and ?0·5 kcal/mol ( 2 ); 0·9 and 0·2 kcal/mol meso ( 3 ) and ?0·1 and ?0·8 kcal/mol ( 4 ). The ³J(HH), ³J(HF) and ³J(FF) couplings for the distinct rotamers are considered together with those of similarly constituted molecules. The general agreement demonstrates that the solvation theory may be applied to multisubstituted ethanes without any basic modifications. The trans oriented HH couplings show a linear substituent electronegativity dependence, which differs appreciably from that obtained for disubstituted ethanes, however. The gauche couplings show the influence of dihedral angle variations as well as substituent electronegativity. The rotamer ³J(FF) couplings in meso 3 are ?38·2 Hz (J_t) and ?17·4 Hz (J_g).     

Sensitivity of femtosecond quantum dynamics and control with respect to non-adiabatic couplings: Model simulations for the cis–trans isomerization of the dideuterated methaniminium cation

We investigate the non-radiative decay in the photo-isomerization of the dideuterated methaniminium cation HDN⁺CDH by means of quantum dynamical simulations of the laser driven torsion. For the present model, the torsional dynamics is nearly adiabatic, but this is shown to be extremely sensitive to the large non-adiabatic coupling elements close to the conical intersection as well as to the apparently small values in the other domains. Neglecting those small couplings artificially changes the nearly adiabatic dynamics into diabatic dynamics. The non-adiabatic coupling terms are calculated ab initio with a test of accuracy which is based on the strict adiabaticity of pure rotational motions. Moreover, we present a novel mechanism for the enhancement of the laser-induced isomerization which may be interpreted as coherent control of competing pump–dump and two interfering non-radiative decay processes.     

H-bonds in acetic acid crystals: Influence of temperature and of isotopic dilution on OH(O-D) stretching BANDS

We describe the evolution of the ν_s bands of CH₃COOH, CD₃COOH, CH₃COOD and CD₃COOD crystals when varying temperature from 10 K to 270 K, and also under isotopic dilutions. We discuss the results obtained and we show how we can estimate from these experiments the values of the anharmonic constant coupling ν_s with ν_σ (parameter b) and also how we can evaluate the energy of resonance interactions of ν_s with other modes. The origin of the shift of the centre of gravity of the ν_s, bands with temperature is discussed and we give arguments against an anharmonic one-dimesional potential for the ν_σ mode being at the origin of such a shift in acetic acid crystals. We suggest that this shift might rather be due to a modulation of ν_s or ν_σ by lower frequency modes of the H-band. In the conclusion we summarize our results on acetic acid crystals and stress the interest of performing such quantitative studies.     

Rotational isomerism—XVI: The AA′BB′X NMR spectrum and rotational isomerism of 2-fluoroethyl trichloroacetate

The complete AA′BB′X analysis of the ¹H and ¹⁹F NMR spectrum of CH₂F.CH₂OCO·CCl₃ ( 1 ) as a neat liquid and in various solvents is given and considered in detail. The differentiation of the two conjugate solutions is obtained both from the X spectrum and by analysis at two field strengths. The coupling constants obtained show that the compound exists almost entirely in the gauche rotamer in all solvents.     

The cis—trans isomerization of nitrostilbenes XV: mixed singlet and triplet mechanism for trans → cis photoisomerization of 4-nitro-4′-dialkylaminostilbenes in non-polar solvents

The quantum yields of direct cis trans photoisomerization (φ_c → _t and φ_{t → c}) and of fluorescence of the trans isomers (φ_f) of three 4-nitro-4′-R-stilbenes (R amino (1), dimethylamino (2) and diethylamino (3)) were measured in several saturated hydrocarbons. Formation and decay of the lowest triplet state was observed by nanosecond laser flash photolysis. The triplet yield (φ_T), the triplet lifetime (τ_T), φ_{t → c} and φ_f were measured as a function of temperature and of the concentration of the quenchers ferrocene, azulene (Q) and oxygen. Twisting in the triplet, involving a ³t* ³p* equilibrium, analogous to that in other 4-nitrostilbenes, is suggested on the basis of the effects of temperature and quenchers on φ_T and τ_T. The trans → cis photoisomerization of 1 follows the triplet route almost completely. The existence of a singlet pathway (20% – 30% contribution) for 2 and 3 in non-polar solvents at room temperature is concluded from the non-linear dependence of the φ⁰_{t → c}/φ_{t → c} ratio on the concentration of Q. For these two nitrostilbenes a mixed singlet—triplet mechanism for the trans → cis photoisomerization is suggested.     

Oxidation of L‐amino acids by metal ion (Mn3+) in sulfuric acid medium: Effect of nucleophilicity and hydrophobicity on reaction rate

The kinetics of oxidation of L ‐amino acids (AAs) glycine( 1a ), alanine( 1b ), valine( 1c ), isoleucine( 1d ), leucine( 1e ), proline( 1f ), and phenylalanine( 1g ) by a transition metal ion (Mn³⁺) was studied in the presence of sulfuric acid medium at 26°C by a spectrophotometrical (λ_max = 500 nm) method. In all cases, the kinetics of reactions was of first‐order with respect to each [AA] and [Mn³⁺]. Increased [H⁺], [Mn²⁺] (the reduction product of Mn³⁺), sulfate, and chloride had no effect on the reaction rate. However, the reaction rate increased with increased dielectric constant of the medium. Oxidation rate increased for 1a–g and an apparent correlation was observed between the rate of oxidation and nucleophilicty of AAs except for 1a and 1g . The reaction rate also linearly depended on the hydrophobicity of AAs except for 1f and 1g . The thermodynamic parameters for AA‐metal ion complex formation and activation parameters for the rate‐limiting steps have been evaluated. Analysis of the oxidation products indicated that the AAs underwent oxidative deamination and decarboxylation to form corresponding aldehydes. Based on these data, plausible mechanisms involved in the oxidation of AAs are proposed. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 599–607, 2011     

Effect of quantum interference on tunneling photoionization rates of N2 and O2 molecules

In this work, we reexamine the photoionization rates of N(2) and O(2) molecules using the previously published photoionization rate theory which is based on the original atomic Keldysh theory [K. Mishima et al., Phys. Rev. A 66, 033401 (2002); ibid.66, 053408 (2002)]. We have found that the constructive quantum interference takes place for N(2) molecule while the destructive quantum interference plays an important role for O(2) molecule. This is consistent with the experimental and theoretical results reported in the literature. The formulas derived in this paper clearly show that this is due to the different symmetries of the valence orbitals of N(2) and O(2) molecules.     

Kinetics of the oxidation of some polycyclic aromatic hydrocarbons by lead tetraacetate in aqueous acetic acid and perchloric acid medium

The state of metal cations in CuCr₂O₄/-Al₂O₃ catalysts under its operation in catalytic heat generators has been examined. IR spectra of adsorbed CO indicate copper ion reduction in the catalyst to Cu⁺, whose surface concentration was determined. Diffuse reflectance spectra have revealed that the initial catalyst contains Cr(VI), disappearing after reaction.

---

CuCr₂O₄/-Al₂O₃ (). - CO , Cu⁺. Cu⁺. , Cr(VI), .

     

Photopolymerization of 2‐hydroxyethyl methacrylate: Effect of the medium properties on the polymerization rate

The effect of solvent properties on the polymerization rate of 2‐hydroxyethyl methacrylate (HEMA) was examined with the photoreaction of 4,4′‐azobis(2‐amidinopropane) and photoinduced electron transfer of a thioxanthone derivative and triethanolamine as the radical source. The polymerization rate of HEMA was markedly affected by pH and the medium polarity. The rate increased over a pH range of 6–8. The dependence of the polymerization rate on the amine concentration photoinitiated by the bimolecular system was different in water and acetonitrile as solvents. In aqueous medium, pH 9.5, the rate increased with the amine concentration reaching a constant value at 0.025 M amine; further amine addition inhibited the polymerization. In organic media the inhibition effect was not observed. Triethanolamine addition did not change the polymerization rate photoinitiated by the azo compound. Photochemical studies of the thioxanthone were carried out under the polymerization conditions. These studies allowed us to simulate the dependence of the polymerization rate on the amine concentration. The results are explained in terms of the interaction of the ketone excited states with the amine in the different media. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2368–2373, 2003     

预裂化理论研究:基质表面酸性位类型及不同类型酸性位接触顺序对裂化过程小分子烯烃收率的影响

在区分氢负离子转移反应与氢转移反应、非选择性氢转移反应与总的氢转移反应的情况下,通过合成物性相近但酸性不同的氧化铝,用以作为裂化催化剂基质材料,在固定床反应器上考察了催化裂化过程,基质酸性位类型及基质表面Lewis及Brönsted酸性位接触顺序对小分子烯烃(丙烯、丁烯)收率的影响。结果表明,催化裂化生成小分子烯烃过程中,分子筛与基质所呈现出的反应特点存在较大的区别,前者活性虽高,但总的氢转移反应活性过强。基质材料裂化活性虽低但其表面以氢负离子转移反应为主,反应路径角度更有利于小分子烯烃收率的提高。另外,基质表面存在Brönsted酸性位,或原料油首先与基质表面Lewis酸性位相接触再与Brönsted酸性位反应的预裂化过程,会在促进裂化反应发生的同时抑制总的氢转移反应,更有利于小分子烯烃收率的提高。     

Thermal evolution of acetic acid nanodeposits over 123-180 K on noncrystalline ice and polycrystalline ice studied by FTIR reflection-absorption spectroscopy: hydrogen-bonding interactions in acetic acid and between acetic acid and ice

Acetic acid vapor-deposited on ultrathin noncrystalline ice (NCI) and polycrystalline ice (PCI) films (less than 6 nm thick) under ultrahigh vacuum conditions has been investigated by using Fourier Transform Infrared Reflection-Absorption Spectroscopy. Pristine acetic acid deposited at 123 K (on a copper support) appears as an amorphous solid, which undergoes an irreversible phase transformation to a more structurally ordered (polycrystalline) form upon annealing to 153 K. Acetic acid is found to adsorb on NCI and PCI films initially through hydrogen bonding between C=O and dangling OH (of ice), followed by the formation of multilayers at 123 K. Thermal evolution studies of a low exposure of acetic acid on the ultrathin NCI and PCI films show that acetic acid undergoes coevaporation with water likely as an acetic acid hydrate at 155 K, which continues until the entire ice film has been exhausted at 165 K. Above 165 K, the remaining acetic acid solid appears to evaporate without undergoing the phase transformation, in contrast to the case of a high acetic acid exposure. Coevaporation of acetic acid with water is also found to proceed at a faster rate than the subsequent evaporation of acetic acid, which is consistent with the weaker interactions observed in the H-bonded acetic acid hydrate than that in acetic acid solid.     

Infrared spectroscopic and theoretical studies on the reactions of copper atoms with carbon monoxide and nitric oxide molecules in rare-gas matrices

Reactions of laser-ablated Cu atoms with CO and NO mixtures in solid argon and neon have been investigated using matrix-isolation infrared spectroscopy. Copper carbonyls and copper nitrosyls have been observed, whereas copper carbonyl nitrosyl complexes are absent from the present experiments. New products, (CuCO)2, [NO]Cu[NO], Cu2(mu2-NO), and Cu(NO)2Cu, have been formed in the copper experiments and characterized using infrared spectroscopy on the basis of the results of the isotopic shifts, mixed isotopic splitting patterns, stepwise annealing, the change of reagent concentration and laser energy, and comparison with theoretical predictions. Density functional theory calculations have been performed on these copper carbonyls and copper nitrosyls, which support the identification of these products from the matrix infrared spectrum. A plausible reaction mechanism has been proposed to account for the formation of copper carbonyls and copper nitrosyls. Similar matrix experiments with Ag and Au produce no new species.