Determination of kinetic parameters for atom formation at constant temperature in graphite furnace atomic absorption spectroscopy

A new method for the simultaneous determination of the kinetic order and activation energy for atom release under isothermal condition in a graphite furnace has been developed. Tungsten wire probe atomization was employed to examine the validity of the present method. By means of this model, the kinetic parameters for the atomization of Bi, Ge, Pb and Mn at constant temperatures were successfully determined. The values of the kinetic order and activation energy were found to be 0.67 ± 0.01 and 302 ± 8 kJ mol⁻¹ for Bi, 1.01 ± 0.08 and 109 ± 2 kJ mol⁻¹ for Ge, 0.46 ± 0.01 and 159 ± 2 kJ mol⁻¹ for Pb and 0.97 ± 0.03 and 372 ± 5 kJ mol⁻¹ for Mn, respectively. The atomization mechanism for these four elements from the tungsten probe surface was also discussed.     

Evaluation of propagation rate constants for the free radical polymerization of methacrylonitrile by pulsed laser photolysis

Propagation rate constants for the free radical polymerization of methacrylonitrile (MAN) have been obtained by pulsed laser photolysis (PLP). The temperature dependence of the propagation rate constants indicates a frequency factor of 10^{(6,43 ± 0,26)} L · mol⁻¹ · s⁻¹ and an activation energy of 29,7 ± 1,5 kJ · mol⁻¹. These parameters suggest that the relatively slow rate of propagation in MAN polymerization in relation to other common monomers (methyl methacrylate, styrene) can be attributed to the relative steric bulk and stability of the propagation species.     

Kinetics and Mechanism Study of the Substitution Reactions of the Chloride Ligand in Dichloro(5,10,15,20)‐tetraphenyl‐21H,23H‐porphinato)tin(IV) by Organic Bases in Dimethylformamide Solvent

Substitution reactions of a Cl⁻ ligand in [SnCl₂(tpp)] (tpp=5,10,15,20‐tetraphenyl‐21H,23H‐porphinato(2−)) by five organic bases i.e., butylamine (BuNH₂), sec‐butylamine (^sBuNH₂), tert‐butylamine (^tBuNH₂), dibutylamine (Bu₂NH), and tributylamine (Bu₃N), as entering nucleophile in dimethylformamide at I=0.1M (NaNO₃) and 30–55° were studied. The second‐order rate constants for the substitution of a Cl⁻ ligand were found to be (36.86±1.14)⋅10⁻³, (32.91±0.79)⋅10⁻³, (22.21±0.58)⋅10⁻³, (19.09±0.66)⋅10⁻³, and (1.36±0.08)⋅10⁻³ M ⁻¹s⁻¹ at 40° for BuNH₂, ^tBuNH₂, ^sBuNH₂, Bu₂NH, and Bu₃N, respectively. In a temperature‐dependence study, the activation parameters ΔH^≠ and ΔS^≠ for the reaction of [SnCl₂(tpp)] with the organic bases were determined as 38.61±4.79 kJ mol⁻¹ and −150.40±15.46 J K⁻¹mol⁻¹ for BuNH₂, 40.95±4.79 kJ mol⁻¹ and −143.75±15.46 J K⁻¹mol⁻¹ for ^tBuNH₂, 30.88±2.43 kJ mol⁻¹ and −179.00±7.82 J K⁻¹mol⁻¹ for ^sBuNH₂, 26.56±2.97 kJ mol⁻¹ and −194.05±9.39 J K⁻¹mol⁻¹ for Bu₂NH, and 39.37±2.25 kJ mol⁻¹ and −174.68±7.07 J K⁻¹ mol⁻¹ for Bu₃N. From the linear rate dependence on the concentration of the bases, the span of k₂ values, and the large negative values of the activation entropy, an associative (A) mechanism is deduced for the ligand substitution.     

Far-infrared spectra and barriers to internal rotation of ethyl nitrate

The far-infrared spectra of gaseous and solid ethyl nitrate, CH₃CH₂ONO₂, have been recorded from 500 to 50 cm⁻¹. The fundamental asymmetric torsion of the trans conformer which has a heavy atom plane has been observed at 112.50 cm⁻¹ with two excited states failing to lower frequencies, and the corresponding fundamental torsion of the gauche conformer was observed at 109.62 cm⁻¹ with two excited states also falling to lower frequencies. The results of a variable temperature Raman study indicate that the trans conformer is more stable than the gauche conformer by 328 ± 96 cm⁻¹ (938 ± 275 cal mol⁻¹). An asymmetric potential function governing the internal rotation about the CH₂O bond is reported which gives a trans to gauche barrier of 894 ± 15 cm⁻¹ (2.56 ± 0.04 kcal mol⁻¹) and a gauche to gauche barrier of 3063 ± 68 cm⁻¹ (8.76 ± 0.20 kcal mol⁻¹) with the trans conformer more stable by 220 ± 148 cm⁻¹ (0.63 ± 0.42 kcal mol⁻¹). Transitions arising from the symmetric CH₃ and NO₂ torsions are observed for both conformers, from which the threefold and twofold periodic barriers to internal rotation have been calculated. For the trans conformer the values are 1002 cm⁻¹ (2.87 kcal mol⁻¹) and 2355 ± 145 cm⁻¹ (6.73 ± 0.42 kcal mol⁻¹) and for the gauche conformer they are 981 cm⁻¹ (2.81 kcal mol⁻¹) and 2736 ± 632 cm⁻¹ (7.82 ± 1.81 kcal mol⁻¹) for the CH₃ and NO₂ rotors, respectively. These results are compared to the corresponding quantities for some similar molecules.     

Mutual isomerization of cyclopentyl and 1-Penten-5-y1 radicals

Unimolecular reactions of mutual isomerization of cyclopentyl and 1-penten-5-yl radicals have been investigated by chemical activation. The radicals were generated by adding energized hydrogen atoms (E_H about 23 kcal mol⁻¹) to the double bond of either cyclopentane or 1,4-pentadiene. Based on the extensive steady-state RRKM calculations employing the experimental data from this work as well as from the literature, the threshold energies for the cyclopentyl ring opening and closure are 32 ± 0.3 and 16.2 ± 0.3 kcal mol⁻¹, respectively. The entropy of activation for the ring opening is close to zero.     

Rate constants and activation energies for ozonolysis of isoprene methacrolein and methyl‐vinyl‐ketone in aqueous solution: Significance to the in‐cloud ozonation of isoprene

Rate constants and activation energies for the reactions of ozone with isoprene, methacrolein, and methyl‐vinyl‐ketone in aqueous solution have been determined at temperatures from 5 to 30°C, using the stopped‐flow‐technique and monitoring ozone decay. The rate constants at 25°C and the activation energies have been found to be 4.1 (±0.2) × 10⁵ M⁻¹ s⁻¹ and 19.9 (±0.5) kJ mol⁻¹ for isoprene, 2.4 (±0.1) × 10⁴ M⁻¹ s⁻¹ and 23.9 (±0.5) kJ mol⁻¹ for methacrolein, and 4.4 (±0.2) × 10⁴ M⁻¹ s⁻¹ and 18.0 (±0.5) kJ mol⁻¹ for methyl‐vinyl‐ketone. A UV spectrum of a transient intermediate with a lifetime of about 15 s formed during the ozonation of isoprene was obtained in the range 220 to 300 nm. It rises steadily toward 220 nm. It is suggested that the spectrum can be attributed to the two unsaturated Criegee‐intermediates (carbonyl oxides), which would conceivably be stabilized by resonance. Lifetime considerations indicate that the oxidation of isoprene and its first‐generation reaction products, methacrolein and methyl‐vinyl‐ketone, by ozone and OH in the aqueous phase of a cloud environment play only a minor role compared to homogeneous gas‐phase processing. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 182–190, 2001     

Kinetic studies of the performic acid epoxidation of natural rubber latex stabilized by cationic surfactant

The kinetics of the epoxidation of natural rubber latex stabilized by hexadecyltrimethyl-ammonium chloride were studied at 3, 15 and 25°C by the sampling technique. The samples were characterized by gravimetric, DSC, i.r. and ¹³C-NMR analysis. The rate determining step of the epoxidation was found to be the formation of performic acid. The activation energy of the reaction was found to be 55 ± 6 KJ mol⁻¹ and the entropy of activation −172 ± 22 J mol⁻¹ K⁻¹.     

Effect of vanadium on oxygen diffusivity in niobium

The effect on the diffusivity of oxygen of vanadium additions to niobium was investigated by a diffusion couple technique. The addition of vanadium to niobium results in an increase of the activation energy for oxygen diffusion from 107 kJ mol⁻¹ for oxygen in niobium to 176 ± 9 kJ mol⁻¹ for the Nb-0.5at.%V alloy and to 194 ± 9 kJ mol⁻¹ for the Nb-10at.%V alloy. This increase in the activation energy is attributed to the trapping of oxygen by vanadium atoms. Applying Kirchheim's trapping model and the results of internal friction measurements, trapping energies of about −64 and −49 kJ mol⁻¹ were obtained for the Nb-0.5at.%V and the Nb-10at.%V alloys respectively.     

Kinetics and mechanism of formation of gehlenite,Al–Si spinel and anorthite from the mixture of kaolinite and calcite

The kinetics and mechanism of formation of gehlenite, Al–Si spinel phase, wollastonite and anorthite from the mixture of kaolinite and calcite was investigated by differential thermal analysis under the heating rate from 283 to 293 K min⁻¹ using Kissinger equation. The changes in the phase composition of the sample during the thermal treatment were investigated via simultaneous TG-DTA, in situ high-temperature x-ray diffraction analysis and high-temperature heating-microscopy. The crystallizations of gehlenite and Al–Si spinel phase show apparent activation energy of (411 ± 5) kJ mol⁻¹ and (550 ± 9) kJ mol⁻¹, respectively. The value of kinetic exponent corresponds to the process limited by the decreasing nucleation rate for gehlenite while constant nucleation rate is determined for Al–Si spinel phase. Anorthite crystallizes from the eutectic melt and the process shows the apparent activation energy of (1140 ± 25) kJ mol⁻¹. The process is limited by the constant nucleation rate of a new phase.     

Kinetic study of the ligand substitution on the trimethylplatinum(iv) complexes with unidentate ligands

Ligand substitution kinetics for the reaction [Pt^IVMe₃(X)(NN)]+NaY=[Pt^IVMe₃(Y)(NN)]+NaX, where NN=bipy or phen, X=MeO⁻, CH₃COO⁻, or HCOO⁻, and Y=SCN⁻ or N₃⁻, has been studied in methanol at various temperatures. The kinetic parameters for the reaction are as follows. The reaction of [PtMe₃(OMe)(phen)] with NaSCN: k₁=36.1±10.0 s⁻¹; ΔH₁^≠=65.9±14.2 kJ mol⁻¹; ΔS₁^≠=6±47 J mol⁻¹ K⁻¹; k₋₂=0.0355±0.0034 s⁻¹; ΔH₋₂^≠=63.8±1.1 kJ mol⁻¹; ΔS₋₂^≠=−58.8±3.6 J mol⁻¹ K⁻¹; and k₋₁/k₂=148±19. The reaction of [PtMe₃(OAc)(bipy)] with NaN₃: k₁=26.2±0.1 s⁻¹; ΔH₁^≠=60.5±6.6 kJ mol⁻¹; ΔS₁^≠=−14±22 J mol⁻¹K⁻¹; k₋₂=0.134±0.081 s⁻¹; ΔH₋₂^≠=74.1±24.3 kJ mol⁻¹; ΔS₋₂^≠=−10±82 J mol⁻¹K⁻¹; and k₋₁/k₂=0.479±0.012. The reaction of [PtMe₃(OAc)(bipy)] with NaSCN: k₁=26.4±0.3 s⁻¹; ΔH₁^≠=59.6±6.7 kJ mol⁻¹; ΔS₁^≠=−17±23 J mol⁻¹K⁻¹; k₋₂=0.174±0.200 s⁻¹; ΔH₋₂^≠=62.7±10.3 kJ mol⁻¹; ΔS₋₂^≠=−48±35 J mol⁻¹K⁻¹; and k₋₁/k₂=1.01±0.08. The reaction of [PtMe₃(OOCH)(bipy)] with NaN₃: k₁=36.8±0.3 s⁻¹; ΔH₁^≠=66.4±4.7 kJ mol⁻¹; ΔS₁^≠=7±16 J mol⁻¹K⁻¹; k₋₂=0.164±0.076 s⁻¹; ΔH₋₂^≠=47.0±18.1 kJ mol⁻¹; ΔS₋₂^≠=−101±61 J mol⁻¹ K⁻¹; and k₋₁/k₂=5.90±0.18. The reaction of [PtMe₃(OOCH)(bipy)] with NaSCN: k₁ =33.5±0.2 s⁻¹; ΔH₁^≠=58.0±0.4 kJ mol⁻¹; ΔS₁^≠=−20.5±1.6 J mol⁻¹ K⁻¹; k₋₂=0.222±0.083 s⁻¹; ΔH₋₂^≠=54.9±6.3 kJ mol⁻¹; ΔS₋₂^≠=−73.0±21.3 J mol⁻¹ K⁻¹; and k₋₁/k₂=12.0±0.3. Conditional pseudo-first-order rate constant k₀ increased linearly with the concentration of NaY, while it decreased drastically with the concentration of NaX. Some plausible mechanisms were examined, and the following mechanism was proposed. [Note to reader: Please see article pdf to view this scheme.] © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 523–532, 1998     

Kinetic Determination of the Gas‐Phase Decarbonylation of Butyraldehyde in the Presence of HCl Catalyst

The gas‐phase kinetics and mechanism of the homogeneous elimination of CO from butyraldehyde in the presence of HCl has been experimentally studied. The reaction is homogeneous and follows the second‐order kinetics with the following rate expression: log k ₁ (s⁻¹ L mol⁻¹) = (13.27 ± 0.36) – (173.2 ± 4.4) kJ mol⁻¹(2.303RT )⁻¹. Experimental data suggested a concerted four‐membered cyclic transition state type of mechanism. The first and rate‐determining step occurs through a four‐membered cyclic transition state to produce propane and formyl chloride. The formyl chloride intermediate rapidly decomposes to CO and HCl gases.     

DSC determination of the sublimation enthalpy of bis(2,4-pentanedionato)oxovanadium(IV) and tetrakis(2,4-pentanedionato)zirconium(IV)

The sublimation enthalpies of bis(2,4-pentanedionato)oxovanadium(IV) and tetrakis(2,4-pentanedionato)zirconium(IV) have been determined by differential scanning calorimetry as 140.7 ± 4.0 and 132.0 ± 6.8 kJ mol⁻¹, respectively. The fusion enthalpy of the latter complex has also been determined as 33.68 ± 2.5 kJ mol⁻¹. A summary of “selected” sublimation enthalpy data for first-row transition metal acetylacetonate complexes is included.     

Far infrared and low frequency gas phase Raman spectra,vibrational assignment,and conformational stability of 1-chloro-2-methylpropane and 1-bromo-2-methylpropane

The Raman (3200—10cm⁻¹) and infrared (3200—50 cm⁻¹) spectra of gaseous and solid 1-chloro-2-methylpropane and 1-bromo-methylpropane, as well as the Raman spectra of the liquids, have been recorded and assigned. The gauche asymmetric torsion of the 1-chloro-2-methylpropane molecules has been observed at 110 cm⁻¹ in the Raman spectrum of the gas. For the 1-bromo-2-methylpropane molecule, both the trans and gauche asymmetric torsions have been observed at 106.70 and 103.94 cm⁻¹, respectively, along with three additional transitions for the gauche conformer. From these data, the asymmetric potential function for the bromide molecules to V₁ = —493 ±16, V₂ = 595 ± 18, and V₃ = 2006 ± 6 cm⁻¹ with the trans conformer being more stable than the gauche conformer by 44 ± 20 cm⁻¹. The trans form is found experimentally to be more stable in the liquid phase by 30 ± 14 cm⁻¹ (83 ± 40 cal mol⁻¹). From the relative intensities, in the Raman spectra, of the CCl stretches measured as a function of temperature, the gauche conformer of the chloride molecules to be 167 ± 71 cm⁻¹ (479 ± 203 cal mol⁻¹) more stable than the trans conformer in the gas phase, and 73 ± 10 cm⁻¹ (208 ± 29 cal mol⁻¹) more stable in the liquid phase. The methyl torsions for the gauche and trans conformers of both molecules are tentatively assigned in the gas phase and the barriers have been calculated. The results of this study are compared with previous studies on these molecules.     

Destabilization in the isomeric nitrobenzonitriles: an experimental thermochemical study

The enthalpies of combustion and of sublimation, respectively, of the three isomeric nitrobenzonitriles have been measured: o-, {(−3456.3±2.9), (88.1±1.4)} kJ·mol⁻¹; m-, {(−3442.8±3.3), (92.8±0.3)} kJ·mol⁻¹; p-, {(−3448.2±3.6), (91.1±1.3)} kJ·mol⁻¹. In turn, from these values, the standard molar enthalpies of formation for the condensed and gaseous state, respectively, have been derived: o-, {(130.1±3.1), (218.2±3.4)} kJ·mol⁻¹; m-, {(116.5±3.5), (209.3±3.5)} kJ·mol⁻¹; p-, {(122.0±3.8), (213.1±4.0)} kJ·mol⁻¹. Destabilization energies associated with the presence of the two electron-withdrawing groups have been determined, for o-, m-, and p-nitrobenzonitrile, {(17.6±4.1), (8.7±4.2), and (12.5±4.6)} kJ·mol⁻¹, respectively, and are consistent with those obtained for the corresponding sets of isomeric methyl benzenedicarboxylates, dicyanobenzenes, dinitrobenzenes, and (neutral and ionized) nitrobenzoic acids.     

Theoretical and experimental studies of the diketene system: Product branching decomposition rate constants and energetics of isomers

The kinetics and mechanism for the thermal decomposition of diketene have been studied in the temperature range 510–603 K using highly diluted mixtures with Ar as a diluent. The concentrations of diketene, ketene, and CO₂ were measured by FTIR spectrometry using calibrated standard mixtures. Two reaction channels were identified. The rate constants for the formation of ketene (k₁) and CO₂ (k₂) have been determined and compared with the values predicted by the Rice–Ramsperger–Kassel–Marcus (RRKM) theory for the branching reaction. The first-order rate constants, k₁ (s⁻¹) = 10^{15.74 ± 0.72} exp(−49.29 (kcal mol⁻¹) (±1.84)/RT) and k₂ (s⁻¹) = 10^{14.65 ± 0.87} exp(−49.01 (kcal mol⁻¹) (±2.22)/RT); the bulk of experimental data agree well with predicted results. The heats of formation of ketene, diketene, cyclobuta-1,3-dione, and cyclobuta-1,2-dione at 298 K computed from the G2M scheme are −11.1, −45.3, −43.6, and −40.3 kcal mol⁻¹, respectively. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 580–590, 2007     

Analysis of torsional spectra of molecules with two internal C3ν, rotors—XXVI. A far infrared study of 1,1-dimethylhydrazine

The infrared spectra of 1,1-dimethylhydrazine, (CH₃)₂NNH₂, and two isotopomers, (CD₃)₂NNH₂ and (CH₃)₂NND₂, have been recorded in the region between 600 and 100 cm⁻¹. Very rich and complex spectra were obtained and analysis of the data has been carried out. The interpretation of the spectra arising from the two methyl torsional modes of the −d₀ compound was carried out using a semi-rigid model, and the resulting potential function obtained is V₃₀ = 1685 ± 12 cm⁻¹ (4.82 ± 0.04 kcal mol⁻¹); V₀₃ = 1827 ± 16 cm⁻¹ (5.22 ± 0.05 kcal mol⁻¹); V₆₀ = −92±5cm⁻¹ (−0.26 ± 0.02 kcal mol⁻¹); V₀₆ = −41 ± 6cm⁻¹ (−0.12 ± 0.02 kcal mol⁻¹) and V^′₃₃ = −51 ± 5 cm⁻¹ (−0.15 ± 0.01 kcal mol⁻¹). Ab initio gradient calculations were carried out employing the 3–21G and 6–31G* basis sets, as well as the 6–31G* basis set with electron correlation at the MP2 level. The structural parameters, conformational stability, and three-fold barriers to internal rotation have been determined and the gauche conformer is calculated to be more stable than the trans form by 783 cm⁻¹ (2.24 kcal mol⁻¹) with the MP2/6–31G* basis set. These calculations were also used to re-evaluate the previously reported assignment of the fundamental modes, and to obtain a potential function for the asymmetric torsion. All of these results are discussed and compared with corresponding quantities for some similar compounds.     

Thermodynamic properties of potassium metasilicate (K2SiO3)

The standard enthalpy of formation of potassium metasilicate (K₂SiO₃), determined by hydrofluoric acid solution calorimetry, was found to be ΔH^o_f,298 = −363.866±0.421 kcal mol⁻¹ (−1522.415±1.762 kj mol⁻¹). The standard enthalpy of formation from the oxides was found to beΔH^o₂₉₈ = −64.786±0.559 kcal mol⁻¹ (−271.065±2.339 kJ mol⁻¹).These experimentally determined data were combined with data from the literature to calculate the Gibbs energies of formation and equilibrium constants of formation over the temperature range of the literature data. The standard enthalpies of formation and Gibbs energies of formation are given as functions of temperature. The standard Gibbs energy of formation is ΔG_f,298.15⁰ = −341.705 kcal mol⁻¹ (−1429.694 kJ mol⁻¹).     

Conformational enthalpy and volume changes in bromocyclohoexane determined by Raman spectroscopy

Raman spectra of bromocyclohexane have been recorded as a function of temperature between 22 and 135°C and as a function of pressure from 1 bar to 7.0 kbar. The integrated intensities of the bands at 657 and 687 cm⁻¹ have been used to monitor the relative populations of the axialand equatorial forms respectively. From these data, enthalpy and volume differences of 1.1 ± 0.3 kJ mol⁻¹ and −2.2 ± 0.3 cm³ mol⁻¹ respectively were determined for the equatorial—axial equilibrium.     

Kinetics of D,L–Lactide Polymerization Initiated with Zirconium Acetylacetonate

The kinetic of D,L-lactide polymerization in presence of biocompatible zirconium acetylacetonate initiator was studied by differential scanning calorimetry in isothermal mode at various temperatures and initiator concentrations. The enthalpy of D,L-lactide polymerization measured directly in DSC cell was found to be ΔH=−17.8±1.4 kJ mol⁻¹. Kinetic curves of D,L-lactide polymerization and propagation rate constants were determined for polymerization with zirconium acetylacetonate at concentrations of 250–1000 ppm and temperature of 160–220 °C. Using model or reversible polymerization the following kinetic and thermodynamic parameters were calculated: activation energy E_a=44.51±5.35 kJ mol⁻¹, preexponential constant lnA=15.47±1.38, entropy of polymerization ΔS=−25.14 J mol⁻¹ K⁻¹. The effect of reaction conditions on the molecular weight of poly(D,L-lactide) was shown.     

Enantiomerization of an inherently chiral resorcarene derivative: determination of the interconversion barrier by computer simulation of the dynamic HPLC experiment

The inherently chiral tetrabenzoxazine resorcarene derivative 1 shows characteristic plateau-formation during enantioselective HPLC separation on the chiral stationary phase Chiralpak AD. By computer assisted peak form analysis of the elution profiles, obtained from temperature dependent dynamic HPLC (DHPLC) experiments, with ChromWin, the enantiomerization barrier ΔG^#(298 K)=92±2 kJ mol⁻¹ and the activation parameters ΔH^#=53.0±1.8 kJ mol⁻¹ and ΔS^#=−131±14 J (K mol)⁻¹ were determined.