Kinetic and mechanistic study on the thermal isomerization of ocimene in the liquid phase

The rate of thermal isomerization of ocimene in the liquid phase has been investigated in the range 90–150 °C. The rate constant for the disappearance of ocimene may be expressed by k = 1. 3 × 10¹⁰e^{– 11994. 2/T}(min^{– 1}), from which we can infer that the activation energy is 99.7 kJ mol^–1 and the pre‐exponential factor is 1.3 × 10¹⁰ min^–1. The half‐life for the disappearance of ocimene may be expressed by t_1/2 = 5. 2 × 10^{– 11}e^{11994. 2/T}(min). The conclusion has been supported by the study results that the ocimene is safe when temperature is below 100 °C. A discussion of the mechanism concerning the conversion is included. Copyright © 2011 John Wiley & Sons, Ltd.     

Kinetics and mechanisms of gas‐phase decarbonylation of α‐methyl‐trans‐cinamaldehyde and E‐2‐methyl‐2‐pentenal under homogeneous catalysis of hydrogen chloride

The kinetics of the gas‐phase elimination of α‐methyl‐trans‐cinamaldehyde catalyzed by HCl in the temperature range of 399.0–438.7 °C, and the pressure range of 38–165 Torr is a homogeneous, molecular, pseudo first‐order process and undergoing a parallel reaction to produce via (A) α‐methylstyrene and CO gas and via (B) β‐methylstyrene and CO gas. The decomposition of substrate E‐2‐methyl‐2‐pentenal was performed in the temperature range of 370.0–410.0 °C and the pressure range of 44–150 Torr also undergoing a molecular, pseudo first‐order reaction gives E‐2‐pentene and CO gas. These reactions were carried out in a static system seasoned reactions vessels and in the presence of toluene free radical inhibitor. The rate coefficients are given by the following Arrhenius expressions:

• Products formation from α‐methyl‐trans‐cinamaldehyde
• α‐methylstyrene :
• β‐methylstyrene :
• Products formation from E‐2‐methyl‐2‐pentenal
• E‐2‐pentene :

The kinetic and thermodynamic parameters for the thermal decomposition of α‐methyl‐trans‐cinamaldehyde suggest that via (A) proceeds through a bicyclic transition state type of mechanism to yield α‐methylstyrene and carbon monoxide, whereas via (B) through a five‐membered cyclic transition state to give β‐methylstyrene and carbon monoxide. However, the elimination of E‐2‐methyl‐2‐pentenal occurs by way of a concerted cyclic five‐membered transition state mechanism producing E‐2‐pentene and carbon monoxide. The present results support that uncatalyzed α‐β‐unsaturated aldehydes decarbonylate through a three‐membered cyclic transition state type of mechanism. Copyright © 2014 John Wiley & Sons, Ltd.     

NMR,IR, and ESR spectroscopic investigation of reaction of α‐silylamines with carbon tetrachloride

This paper reports about high reactivity of α‐silylamines in the reaction with CCl₄. Unlike Et₃N, α‐silylamines rapidly react with CCl₄ upon irradiation with daylight to form α‐silylamine hydrochloride salts in 92–98% yields. The influence of structure of α‐silylamines and solvent on the degree of conversion was displayed. The interaction of α‐silylamines with CCl₄ was studied by NMR, ESR, and IR spectroscopy. C‐centered radicals of α‐silylamines were detected by ESR spectroscopy with spin traps (MNP, ND, and PBN) in reaction mixtures in CH₃CN and C₆H₆ and it show the radical character of this reaction. Both CH₃CN and C₆H₆ serve as solvents as well as reagents for this reaction. A mechanism of an interaction between α‐silylamines and CCl₄ is discussed. Copyright © 2008 John Wiley & Sons, Ltd.     

Relative stabilities and molecular structures of the isomeric enol ethers and carboxylic esters derived from α‐acetyl‐γ‐butyrolactone and α‐acetyl‐δ‐valerolactone

Recently recorded ¹⁷O NMR spectra of compounds studied in a previous work (Taskinen E. Acta Chem. Scand. 1985; B39 : 489–494) dealing with the thermodynamics of isomerization of the enol ethers of α‐acetyl‐γ‐butyrolactone reveal an error in compound identification, caused by an unexpected isomerization reaction during the synthetic procedure. Thus, acid‐catalyzed treatment of the lactone with HC(OR)₃ in the respective alcohol ROH is shown to lead initially to the desired enol ethers which, however, are gradually isomerized to a mixture of the enol ethers and an ester of 2‐methyl‐4,5‐dihydrofuran‐3‐carboxylic acid. As a result, only one of the two isomeric compounds detected in the previous equilibration study was the expected enol ether (the thermodynamically more stable E isomer) of α‐acetyl‐γ‐butyrolactone, while the other, dominating species was the respective carboxylic ester. In the present work, the evidence provided by the ¹⁷O NMR spectra is presented, and the relative stabilities of the isomeric compounds are discussed on the basis of computational enthalpy data. The treatment is also extended to the respective isomeric compounds derived from α‐acetyl‐δ‐valerolactone. Copyright © 2007 John Wiley & Sons, Ltd.     

A computational study of the thermolysis of β‐hydroxy ketones in gas phase and in m‐xylene solution

Theoretical calculations at the M05‐2X/6‐31+G(d) level of theory have been carried out in order to explore the nature of the mechanism of the thermal decomposition reactions of the β‐hydroxy ketones, 4‐hydroxy‐2‐butanone, 4‐hydroxy‐2‐pentanone, and 4‐hydroxy‐2‐methyl‐2‐pentanone in gas phase and in m‐xylene solution. The mechanism proposed is a one‐step process proceeding through a six‐membered cyclic transition state. A reasonable agreement between experimental and calculated activation parameters and rate constants has been obtained, the tertiary : secondary : primary alcohol rate constant ratio being calculated, at T = 503.15 K, as 5.9:4.7:1.0 in m‐xylene solution and 44.1:5.0:1.0 in the gas phase, compared with the experimental values, 3.7:1.3:1.0 and 13.5:3.2:1.0, respectively. The progress of the thermal decomposition reactions of β‐hydroxy ketones has been followed by means of the Wiberg bond indices. The lengthening of the O₁–C₂ bond with the initial migration of the H₆ atom from O₅ to O₁ can be seen as the driving force for the studied reactions. Calculated synchronicity values indicate that the mechanisms correspond to concerted and highly synchronous processes. The transition states are “advanced”, nearer to the products than to the reactants. Copyright © 2012 John Wiley & Sons, Ltd.     

β‐Elimination of an aziridine to an allylic amine: a mechanistic study

The base‐induced rearrangement of aziridines has been examined using a combination of calculations and experiment. The calculations show that the substituent on nitrogen is a critical feature that greatly affects the favorability of both α‐deprotonation, and β‐elimination to form an allylic amine. Experiments were carried out to determine whether E2‐like rearrangement to the allylic amine with lithium diisopropyl amide (LDA) is possible. N‐tosyl aziridines were found to deprotonate on the tosyl group, preventing further reaction. A variety of N‐benzenesulfonyl aziridines having both α‐ and β‐protons decomposed when treated with LDA in either tetrahydrofuran or hexamethylphosphoramide. However, when α‐protons were not present, allylic amine was formed, presumably via β‐elimination. Copyright © 2011 John Wiley & Sons, Ltd.     

Catalysis by hydrogen chloride in the gas‐phase elimination kinetics of 2‐phenyl‐2‐propanol and 3‐methyl‐1‐buten‐3‐ol

A homogeneous, molecular, gas‐phase elimination kinetics of 2‐phenyl‐2‐propanol and 3‐methyl‐1‐ buten‐3‐ol catalyzed by hydrogen chloride in the temperature range 325–386 °C and pressure range 34–149 torr are described. The rate coefficients are given by the following Arrhenius equations: for 2‐phenyl‐2‐propanol log k₁ (s^?1) = (11.01 ± 0.31) ? (109.5 ± 2.8) kJ mol^?1 (2.303 RT)^?1 and for 3‐methyl‐1‐buten‐3‐ol log k₁ (s^?1) = (11.50 ± 0.18) ? (116.5 ± 1.4) kJ mol^?1 (2.303 RT)^?1. Electron delocalization of the CH₂?CH and C₆H₅ appears to be an important effect in the rate enhancement of acid catalyzed tertiary alcohols in the gas phase. A concerted six‐member cyclic transition state type of mechanism appears to be, as described before, a rational interpretation for the dehydration process of these substrates. Copyright © 2007 John Wiley & Sons, Ltd.     

Exploring the reaction of iodine with α‐diazo esters

In this paper we describe the unprecedented reaction between α‐diazo esters 1 and iodine. The reaction, carried out in the presence of aqueous NaHCO₃, afforded the Z‐isomer of the corresponding unsaturated‐2‐iodo ester 8 . The configuration of compounds 8 was determined using the ³J_{C? H} coupling between carbonyl carbon atom and alkene proton. Mechanistic considerations accounting for the observed phenomena and including quantum chemical calculations are proposed. Copyright © 2008 John Wiley & Sons, Ltd.     

Computational study of the proton affinity and basicity of structurally diverse α1‐adrenoceptor ligands

The α₁‐adrenoceptor is a target for the treatment of several conditions from hypertension to benign prostatic hyperplasia. In this paper, we describe a new analysis approach to explore the conformational space of several ligands of the α₁‐adrenoceptor and we also present the calculation of their proton affinity and basicity. For each compound a conformational search followed by a semi‐empirical optimisation was performed and a selection of conformations for each ligand was subjected to further optimisation using density functional theory methods. Different positions were explored to determine the favoured site of protonation, and then, the proton affinity (in the gas phase) and basicity (using the polarisable continuum model for the aqueous solution) were calculated for each of them. In addition, an alternative method using one explicit water molecule in combination with the polarisable continuum model for aqueous solvent was explored. Moreover, the acid dissociation constant (pK_a) in water of these 26 compounds was calculated because this is an important parameter for a ligand when binding to its receptor. The experimental pK_a values of six of these ligands and those of two compounds with a very low and a very large pK_a were used to validate the theoretical methodology. Copyright © 2011 John Wiley & Sons, Ltd.     

Vibrational spectroscopy and DFT calculations of di‐amino acid peptides: α‐ and β‐N‐acetyl‐L‐Asp‐L‐Glu in the solid state

Experimental vibrational spectroscopic studies and density functional theory (DFT) calculations of the di‐amino acid peptide derivatives α‐ and β‐N‐acetyl‐L‐Asp‐L‐Glu have been undertaken. Raman and infrared spectra have been recorded for samples in the solid state. DFT simulations were conducted using the B3‐LYP correlation functional and the cc‐pVDZ basis set to determine energy minimized/geometry optimized structures (based on a single isolated molecule in the gaseous state). Normal coordinate calculations have provided vibrational assignments for fundamental modes, including their potential energy distributions. Significant differences are observed between α‐ and β‐N‐acetyl‐L‐Asp‐L‐Glu both in the computed structures and in the vibrational spectra. The combination of experimental and calculated spectra provide an insight into the structural and vibrational spectroscopic properties of di‐amino acid peptide derivatives. Copyright © 2009 John Wiley & Sons, Ltd.     

Comparative studies on reaction of bis(p‐nitrophenyl) phosphate and α‐nucleophiles in cationic micellar media

We studied the cleave of bis(p‐nitrophenyl) phosphate (BNPP) over a pH range of 7.0–12.0 in the presence of cationic micelles of cetyldiethylethanolammonium bromide, cetyldimethylethanolammonium bromide, cetylpyridinium bromide, cetyltrimethylammonium bromide, and cetylpyridinium chloride by using different α‐nucleophiles, viz acetohydroxamate, benzohydroxamate, salicylhydroxamate, butane‐2,3‐dione monooximate, and α‐benzoin oximate ions. With the use of α‐nucleophiles in cationic micellar media, the hydrolytic cleavage of BNPP was found to be approximately 10⁵‐fold faster than its spontaneous hydrolysis. All reactions followed pseudo‐first‐order kinetics. The effect of various concentrations of cationic micelles for the reaction of BNPP and α‐nucleophiles has been studied. The variation of k_obs values of the reactions depends on the micellar structure, that is, head groups, hydrophobic tail length, and counter ion. Copyright © 2012 John Wiley & Sons, Ltd.     

Acid catalyzed intramolecular attack of β‐phenylthioureido group on amide function. Parallel formation of thiodihydrouracil and 4‐iminothiodihydrouracil. Different pathways in the Edman degradation reaction in the formation of six‐ versus five‐membered cyclic intermediates

Contrary to the cleavage of α‐phenylthioureido peptides 1 proceeding through intermediate 2‐anilinothiazolinone 2 , the b‐analog cis‐2‐(3‐phenylthioureido)cyclopentane‐carboxamide 5 forms transiently 4‐imino‐2‐thioxopyrimidine 6 . Monitoring amide cyclization and hydrolysis of iminopyrimidine 6 in acid by UV showed that an equilibrium between 5 and 6 was reached followed by slower conversion of both compounds into 2‐oxo‐4‐thioxopyrimidine 7 . Both processes were characterized by isosbestic points, the first due to parallel conversion of 5 into 6 and 7 (or 6 into 5 and 7 ) at a constant ratio while the second identical for both reactants – to conversion of equilibrated 5 and 6 into 7 . The special isosbestic points allowed the determination of the individual constants of Scheme 2. Further confirmation was obtained from NMR product analysis and following the cyclization of amide 5 in DMSO:DCl. Product 2‐oxo‐4‐ thioxopyrimidine 7 hydrolyzed reversibly to thioureido acid 8 . The cyclization rate of 8 allowed the participation of 6‐oxothiazine 10 formed by sulfur attack to be excluded. The absence of sulfur attack in the six‐membered case is explained by the longer C? S bond bringing about greater bond angle strain at the tetrahedral ring atoms due to the geometrical characteristics of five‐ and six‐membered rings with planar segments. The cyclizations of amide 5 to iminopyrimidine 6 and to thiodihydrouracil 7 are first order in [H⁺], while the reactions of protonated imine 6 H⁺ are zero order to amide and ?1 to thiodihydrouracil. The reaction orders can be reconciled by assuming a rate determining proton transfer from the tetrahedral intermediate in amide cyclization. Copyright © 2007 John Wiley & Sons, Ltd.     

Raman spectroscopic study of serum albumins: an effect of proton‐ and γ‐irradiation

Raman spectroscopy was applied to analyse structural changes in serum albumins (bovine serum albumin, BSA; human serum albumin, HSA) following proton and γ‐irradiation (0.5, 5 and 50 Gy). Characteristic Raman bands of both polypeptide backbone and amino acid residues were sensitive to irradiation. Significant damage of HSA/BSA was observed only at the highest dose (50 Gy). Raman spectra confirmed radiation‐induced denaturation, destruction of helical structures and aggregation of serum albumins. The differences in the dose‐dependent effects of proton and γ‐radiation on studied proteins are discussed. Copyright © 2007 John Wiley & Sons, Ltd.     

Solid phase synthesis of novel α/β‐tetrapeptides,electrospray ionization mass spectrometric evaluation of their metal cation complexation behavior,and conformational analysis using density functional theory (DFT)

Thirty‐four novel α/β‐tetrapeptides ( 1–34 ) have been prepared employing solid‐phase and in‐parallel synthetic protocols. α/β ‐Tetrapeptides 1 – 34 were prepared by a combination of three α‐amino acid residues (alanine (Ala), phenylalanine (Phe), and isoleucine (Ile)) with one β‐amino acid residue (β³‐homophenylglycine). The corresponding complexes of several selected α/β‐tetrapeptides with alkali, alkaline earth, and transition metals, [tP + M⁺], were evaluated using ion electrospray‐ionization mass spectrometry (ESI‐MS). According to the results from analysis of mixtures, we can conclude that the position of the β‐amino acid is determinant in the affinity toward different metal cations. Computational modeling (DFT, B3LYP 6‐311++G) provided useful information regarding the most likely coordination sites of the metal ions on the receptor α/β‐tetrapeptide 12 , HO₂C‐α‐Phe‐α‐Phe‐α‐Ile‐β³‐hPhg‐NH₂, as well as the conformational changes induced by the metal upon [tP + M⁺] complex formation. Copyright © 2008 John Wiley & Sons, Ltd.     

Molecular structure,experimental and theoretical 1H and 13C NMR chemical shift assignment of cyclic and acyclic α,β‐unsaturated esters

The experimental ¹H and ¹³C NMR spectra of 13 phenyl cinnamates and four 4‐methylcoumarins were investigated and their chemical shifts assigned on the basis of the two‐dimensional spectra. For the unsubstituted cinnamic acid phenyl ester, optimized molecular structures were calculated at a B3LYP/6‐311++G(d,p) level of theory. ¹H and ¹³C NMR chemical shifts were also calculated with the GIAO method at the B3LYP/6‐311 + G(2d,p) level of theory. The comparison between experimental and calculated NMR chemical shift suggests that the experimental spectra are formed from the superposition spectra of the two lowest energy conformers of the compound in solution. The most stable s‐cis configuration found in our studies is also the conformation adopted for a related phenyl cinnamate in solid state. The experimental results were analyzed in terms of the substituent effects. Copyright © 2013 John Wiley & Sons, Ltd.     

Kinetics and mechanism of gas‐phase pyrolysis of ylides. Part 3.1 Thermal reactivity of α‐carbonyl‐ and thiocarbonyl‐stabilized methylenetriphenylphosphoranes

Fourteen ketone/thione‐stabilized triphenylphosphonium methylides were subjected to conventional gas‐phase and flash vacuum pyrolysis (FVP). The kinetics of the first‐order thermal gas‐phase reactions of all these compounds were investigated over 360–653 K temperature range. The values of the Arrhenius log A and energy of activation of these ylides averaged 11.52 ± 0.34 s^?1 and 133.20 ± 3.14 kJ mol^?1, respectively. The products of sealed‐tube (static) and FVP were analyzed and compared. A mechanism is proposed to account for the products of reaction. The rate constants [k (s^?1)] of the substrates at 500 K were calculated and used to substantiate the proposed mechanism of pyrolysis, and to rationalize the thermal gas‐phase reactivities of the ylides under study. Copyright © 2010 John Wiley & Sons, Ltd.     

Stimulated Raman scattering influenced by concentration,fluorescence profile and bandwidth of β‐carotene in liquid‐core optical fiber

We demonstrated stimulated Raman scattering (SRS) of carbon disulfide (CS₂) influenced by β‐carotene in a liquid‐core optical fiber (LCOF). Owing to the double fluorescence characteristics and large third‐order optical nonlinearity of β‐carotene, the high‐order Stokes lines, such as the seventh‐order Stokes line of CS₂, can be observed at a relatively low input‐laser power. The thresholds of Stokes lines lowered with the addition of the carotenoid when the concentration of solution was within 10⁻¹² and 10⁻⁷ mol/L; the threshold increments and intensities of Stokes lines were correlative with the fluorescence profile of β‐carotene: when the fluorescence intensity of the wavenumber region on the spectrum was high, the Stokes line intensity was also high and its threshold increment was small, and vice versa. These results are expected to be worthy of the applications on the tunable laser and the seeding laser. Copyright © 2009 John Wiley & Sons, Ltd.     

Mechano‐Chemical Changes of Nano Sized α‐Al2O3 During Wet Dispersion in Stirred Ball Mills

The mechano‐chemical changes in synthetic α‐Al₂O₃ during wet dispersion in a stirred ball mill, are studied. It is shown that a second phase is generated which can be identified as an aluminum hydroxide (Bayerite) by use of dynamic scanning calorimetry (DSC), X‐ray diffraction (XRD), thermogravimetry (TG), Fourier transformed infrared (FTIR) spectroscopy and transmission electron microscopy (TEM). The amount of the Bayerite phase produced increases with milling time. A comparison of the evaluated amounts from TG‐ and DSC‐measurements are in reasonable agreement. Furthermore, the measured BET surface area shows a strong dependence on the temperature of sample preparation. Depending on the temperature, the bayerite phase is seen to convert to the α‐phase again and also forms meso‐ and possibly micro‐pores, leading to a high BET surface area. In this paper, we highlight the necessity for using different methods to characterize the dispersion process, and for a correct interpretation of the measurement results.     

Surface‐enhanced Raman scattering of 4‐mercaptopyridine on sub‐monolayers of α‐Fe2O3 nanocrystals (sphere,spindle, cube)

We report surface‐enhanced Raman scattering (SERS) spectra from 4‐mercaptopyridine (4‐Mpy) adsorbed on sub‐monolayers of α‐Fe₂O₃ nanocrystals (sphere, spindle, cube). The maximum enhancement factor has been estimated to be about 10⁴ compared to that of 4‐Mpy in solution. A possible mechanism has been proposed that the charge transfer between the α‐Fe₂O₃ nanocrystals and the 4‐Mpy molecules is most likely responsible for the observed enhancement of Raman intensity of adsorbed 4‐Mpy molecules as surface plasmon resonances have not occurred. Copyright © 2009 John Wiley & Sons, Ltd.     

Raman spectroscopic study of calf thymus DNA: an effect of proton‐ and γ‐irradiation

Raman spectroscopy was applied to analyse structural changes in calf thymus double‐stranded deoxyribonucleic acid (dsDNA) after proton‐ and γ‐irradiation (0.5, 5 and 50 Gy). Characteristic Raman bands of phosphodiester linkages, nucleic bases and deoxyribose moieties were sensitive to irradiation. A significant damage of the macromolecules was observed only at the highest dose (50 Gy) of both types of radiations. Spectral changes confirmed a radiation‐induced alteration of the native structure of dsDNA. Nucleic bases, especially pyrimidines, were the most sensitive to radiation, while some alterations in the sugar–phosphate backbone were also detected. The differences in the dose‐dependent effects of proton vs γ‐irradiation on studied biomolecule are discussed. Copyright © 2007 John Wiley & Sons, Ltd.