Showing papers by "Dante Macciantelli published in 1984"

Absolute rate constants for hydrogen abstraction from aldehydes and conformational studies of the corresponding aromatic acyl radicals

Abstract: On mesure par des techniques de photolyse eclair laser les constantes de vitesse absolue des reactions de radicaux t-butoxyles avec des aldehydes organiques. Parametres d'Arrhenius. Les effets polaires sont tres importants pour determiner la reactivite des aldehydes vis-a-vis des radicaux t-butoxyles

Conformational studies by dynamic nuclear magnetic resonance spectroscopy. Part 27. Kinetics and mechanism of annular tautomerism in isomeric triazoles

Abstract: The rates for the prototropic shift of the NH hydrogen in the two isomeric triazoles [(1)= 1,2,4- and (2)= 1,2,3-triazole] have been measured at low temperature by 1H n.m.r. (100 and 300 MHz). Only the less symmetric (cs) of the two possible annular tautomers was found to be populated in the case of (1), whereas both (cs and c2v) were detected in (2). The relative proportions in the latter compound are dramatically dependent on temperature, concentration, and solvent. In either triazoles the kinetic process appears to be intramolecular and the mechanism corresponds to a 1,2 prototropic shift. The parameters of activation yielded similar ΔH‡(5.8 and 6.5 kcal mol–1) and quite negative ΔS‡(–29 and –19 cal mol–1 K–1) values, thus indicating a relatively low probability of attaining the ordered (three-membered) cyclic transition state.

Conformational studies by dynamic nuclear magnetic resonance. Part 26. Interconversion barriers between syn- and anti-conformers of isomeric thiophenecarbaldehydes

Abstract: The low-temperature 13C n.m.r. spectra (25.16 MHz) of thiophene-2- and -3-carbaldehyde [(1) and (2)] display line-broadening effects due to the exchange between their S,O-syn- and S,O-anti-conformers. In both cases the free energy of activation for rotation of the –CHO moiety was measured, the values being 10.15 and 8.5 kcal mol–1 for (1) and (2), respectively. In the case of thiophene-3-carbaldehyde (2) the signals of both conformers were detected, the ratios being 97:3 at –140 and 93:7 at –100 °C in favour of the S,O-anti-conformer. In thiophene-2-carbaldehyde (1) the minor conformer (S,O-anti) was present in too small an amount to be unambiguously detected by a conventional spectrometer, but spectra taken with a superconducting instrument (75.48 MHz) revealed ca. 1.5% of this conformer at –100 °C.