Mario Barzaghi

Iterative computer analysis of complex dynamic EPR bandshapes. Fast motional region

Abstract A general iterative least-squares procedure for the lineshape fitting of dynamic EPR spectra in the fast motional region is outlined. The method makes use of the full information content of a spectrum, and allows the choice of different minimization algorithms in order to ensure convergence to the correct solution and speed in the least-squares fitting. Basic equations are derived, which explicitly include the effect of relatively large modulation amplitude and/or microwave power on lineshape. The algorithm adopted produces both in-phase and out-of-phase spectra, detected at any harmonic of the modulation field. These models and procedures are incorporated in the computer program EPR80, which is able to handle any problem, whose size (i.e., the number of allowed transitions and parameters) is limited only by the available main storage and the available CPU time of the computer. A few examples, relative to the EPR spectra of p -nitrobenzoate dianion radical and 2-nitro-3-aminopyridine anion radical, illustrate the application of the program.

Cyclopropane ring closure in 11,11-disubstituted 1,6-methano [10] annulenes

Abstract The effect of substituents at the bridging carbon atom of 11,11-disubstituted 1,6-methano [10]annulenes is investigated by means of the topological theory of molecular structure 3-21G ab initio calculations of the potential energy hypersurface of the parent hydrocarbon, within C2v symmetry constraints, indicate three distinct stationary points, which correspond to the norcaradienic structure (1), the annulenic structure (2) and the transition state (TS) for their interconversion. Topological analysis of the charge density of the three stationary points shows that 1 is built by two 6MRs and one 3MR condensed on the C1C6 bond, while 2 is formed by two 7MRs which share the C1C11 and C6C11 bonds. In TS the 3MR and C1C6 bond critical points merge to yield a singularity in ϱ and the system goes through a catastrophe configuration of the bifurcative type. The bond equalization in 2 is far from being complete and, beside the fundamental 10π-electron aromatic system, a conjugative coupling of the bridging bonds to the [10]annulenic framework is of a clear-cut relevance.

The conformational structure of 1,2-dimethoxy-ethane in the gas phase

The conformational structure of 1,2-dimethoxyethane in the gas phase has been studied by Hartree-Fock and Moller-Plesset perturbation theory. Full geometry optimizations within C2 symmetry constraints for the aga structure and within C2h symmetry constraints for the aaa structure were performed using both 3-21G and 6-31G* basis sets. Our best energy estimate favors the aaa structure by 0.613 kcal mol−1, which reduces to 0.518 kcal mol−1 when the (unscaled) 3-21G zero-point energy contribution is included. The transition states for the conversion of the aaa structure into the aga structure and for the degenerate rearrangement of the ag+a structure into the ag−a structure have been determined.

Substituent effect on the planarization energy and the relative stability of Winstein and Möbius structures of the homotropylium cation

Abstract The substituent (OH, OC 2 H 5 , Cl, z.sbnd;CHCHCHCH) effects on the relative stabilities of Winstein and Mobius structures of the homotropylium cation, 1 , have been investigated by the MNDO model. Electron donor substituent at C 1 , C 3 , C 5 , and C 7 favor Mobius structures in the order C 1 ,C 7 >C 3 ,C 5 , whereas Winstein structures are stabilized by electron donor substituents at position C 2 , C 4 , and C 6 , and in the order C 2 ,C 6 > C 4 . Electron withdrawing substituents at C 8 enhance stability of the homoaromatic structures, in the order OH>Cl. In any case, the substituent effect is found to be nearly additive. The reaction path for the three-carbon membered ring opening the eight-carbon membered ring inversion has been derived for all the compounds investigated. The equilibrium populations of Winstein Mobius structures were then obtained from the calculated probability distribution functions along the reaction path. The unsubstituted compound 1 was found to be a non-homoaromatic cation in the gas phase, as it adopts a Mobius conformation with a large C 1 -C 7 , internuclear distance.

Thermal rearrangements of the homotropylium cation: An MO investigation of some relevant stationary points on the potential energy surface

Abstract Semiempirical (MINDO and MNDO) and ab initio molecular orbital calculations (using 3-21G, 6-31G, and 6-31G* basis sets and including electron correlation through second-order Moller-Plesset calculations) have been applied in a study of the thermal rearrangements of the homotropylium cation (circumambulation, conformational ring inversion, and transannular hydride transfer). On the basis of our calculations and estimates, we conclude that the conformational ring inversion is the most facile process, with an activation energy of 28.7 kcal/mol, which is 2.2 kcal/mol lower than that for the transannular hydride transfer. The walk rearrangement is a symmetry-forbidden one-step reaction, which occurs with inversion at the migrating center and has an activation energy of 52.4 kcal/mol. The quantum theory of atoms in molecules has been applied to the chemical characterization of the transient species, through a topological analysis of the corresponding 6-31G* charge density functions. MINDO and MNDO estimates have been made of the influence that dimethyl substitution at the migrating carbon has on the activation energies. It is found that substitution makes the walk reaction energetically comparable to the ring inversion.

Dynamic EPR and ENDOR spectroscopies of organic radicals

Abstract Applications of density matrix theory to the analysis of dynamic EPR and ENDOR spectra of organic radicals in solution are reviewed. Using some significant examples it is shown that lineshape analysis often uniquely provides direct kinetic information on reorganizing chemical systems at thermodynamic equilibrium. Temperature-dependent hyperfine coupling constants are shown to be useful for extracting information on the dynamical processes. The potency and drawbacks of dynamic EPR and ENDOR techniques are compared and discussed.

An electron spin resonance investigation of the intermediates and products in the basic reduction of cis- and trans-p-nitro-β-bromostyrenes

The mechanism of chemical and electrolytic reduction of cis- and trans-p-nitro-β-bromostyrenes has been studied by e.s.r. spectroscopy. Both isomers undergo dehydrobromination by electrolytic reduction, whereas chemical reduction by potassium t-butoxide also induces debromination. The anion radicals of cis- and trans-p-nitro-β-bromostyrenes, p-nitrostyrene, p-nitrophenylacetylene, and p-nitrobenzoic acid were identified, through the corresponding e.s.r. spectra, as intermediates or products of the reactions investigated. p-Nitrophenylacetylene and p-nitrobenzoic acid were also isolated from the reaction mixture. The effects of the solvent and the counterion on the yield of electrolytic conversion of cis-p-nitro-β-bromostyrene to p-nitrophenylacetylene are also reported.

An electron spin resonance study of side reactions in the equilibria of formation of triple ions

Reduction of aromatic compounds A (A = nitrobenzene,1,2-dinitrobenzene, 1,3-dinitrobenzene, 1,4-dinitrobenzene, 3-nitropyridine, 3,5-dinitropyridine, 4-nitrobenzophenone, 4,4′-dinitrobenzophenone, benzonitrile, 1,2-dicyanobenzene, 1,3-dicyanobenzene, 1,4-dicyanobenzene, and 4,4′-dicyanobenzophenone) by alkali metals in ethereal solvents containing sodium tetraphenylborate, yields ion pairs A–˙Na+, which are in equilibrium with the corresponding triple ions Na+A–˙Na+. In a few cases side reactions in the formation equilibria of triple ions are observed by e.s.r. spectroscopy. The decomposition of sodium tetraphenylborate is ascertained and the formation of biphenyl and p-benzosemiquinone is explained as dependent on the number and the position of the substituents in the radical anion A–˙.