Marjan Senegačnik

15N and 18O kinetic isotope effects in the thermal decomposition of N2O catalyzed by bromine

15N and 18O kinetic isotope effects (KIEs) in the thermal decomposition of N2O catalyzed by bromine were experimentally determined in the temperature range 773–873 K, resulting in KIE (15N)=−2.07+4020/T and KIE (18O)=−0.41+3290/T. For theoretical interpretation, based on the Bigeleisen formalism, the following planar transition states were taken into account: trans (N–N–O–Br)‡, trans (Br–N–N–O)‡, and branched (N–N<OBr)‡. In addition to KIE, activation energy according to the Sanderson bond‐energy–bond‐order relationship and pre‐exponential factor were calculated as subsidiary parameters in selecting an appropriate transition state among all probable configurations. The result reveals that it is meaningless to speculate whether the Br atom approaches the central or the terminal N atom of the N2O molecule, since both transition states could generate acceptable values of the kinetic isotope effects, activation energy, and pre‐exponential factor.

Nitrogen-15 and oxygen-18 kinetic isotope effects in the catalytic decomposition of N2O over MgO

The 15N kinetic isotope effects (KIE) in the catalytic decomposition of nitrous oxide on MgO powder were determined in the temperature range 675–875 K, and the following temperature dependence was found: KIE(15N)=(0.231±0.149)+(1353±114)/T. At initial pressures of N2O between 40 and 60 kPa, the reaction is of the order of +1 in N2O and has an apparent activation energy of 125±4 kJ mol−1. According to the Bigeleisen formalism, the rate-determining and isotope fractionation governing step was well represented by two transition states: a bent product-like NNO and a fork-type NNOO.

Experimental and theoretical studies of the decomposition of N2O catalyzed by chlorine

Kinetic isotope effects (KIEs) for the thermal decomposition of N2O catalyzed by chlorine were experimentally determined in the temperature range 773–923 K, and may be expressed as follows: KIEt(15N)=(4100/T−1.90)±0.15, KIEp(15N)=(3940/T−2.35)±0.10 and KIE(18O)=(6990/T−3.60)±0.25. An Arrhenius fit to the measured rate constants resulted in an activation energy of 136±8 kJ mol−1 and a preexponential factor of 7.7×107±0.1 m3 mol−1 s−1. The KIEs were interpreted according to the Bigeleisen formalism. Furthermore, we calculated the activation energy following the Sanderson bond‐energy–bond‐order relationship, and the preexponential factor from transition state theory and compared them to experimental values. Additionally, ab initio molecular theory was employed to study parts of the potential energy surface of the elementary bimolecular reaction between a N2O molecule with a Cl atom. Equilibrium geometries, energies and harmonic vibrational frequencies were calculated at the HF/6‐31G* and MP2/6‐31G* level for...

Self-association of cholesterol in benzene and toluene solutions

The self-association of cholesterol in dilute benzene and toluene solutions was investigated by conventional infrared spectroscopy. The sample spectra, recorded in the fundamental OH stretching range, were resolved into the bands of functionally different OH groups of associated cholesterol and its predominant species identified by analysis of their absorbances. The formation constants of oligomers were derived from monomer absorbances measured as a function of cholesterol concentration. In addition to the monomers A1, open A2 (ΔH2=−11±1 kJ mol−1) and cyclic dimers Â2 (ΔĤ2=−24±2 kJ mol−1) in benzene and open dimers A2 (ΔH2=−12±1 kJ mol−1) and tetramers A4 (ΔH4=−47±6 kJ mol−1) in toluene were established as the prevailing species.

SELF-ASSOCIATION OF CHOLESTEROL IN SOME CHLORINATED HYDROCARBON SOLVENTS

Abstract The self-association of cholesterol in dilute solutions in 1,2-dichloroethane, trichloromethane and tetrachlorormethane and over the temperature range 298–328 K was investigated using conventional infrared spectroscopy. The information on the size of the predominant cholesterol species was obtained by numerical processing of the solution spectra, recorded in the fundamental OH stretching range. Formation constants were derived from monomer absorbances measured as a function of cholesterol concentration. The following prevailing species and their enthalpies of formation were established: the monomer A1 and monomer-solvent complex A1* (ΔH1*=−9±3 kJ mol−1) in 1,2-dichloroethane; the monomer and open dimer (A2) (ΔH2=−15.0±1.2 kJ mol−1) in trichloromethane; the monomer, open trimer (A3) (ΔH3=−31±3 kJ mol−1) and cyclic hexamer (Â6) (ΔĤ6=−70±10 kJ mol−1) in tetrachloromethane.