Catalogo Articoli (Spogli Riviste)

OPAC HELP

Titolo:
Determination of syn/anti isomerism in DCNQI derivatives by 2D exchange spectroscopy: Theoretical underpinning
Autore:
Cossio, FP; de la Cruz, P; de la Hoz, A; Langa, F; Martin, N; Prieto, P; Sanchez, L;
Indirizzi:
Univ Pais Vasco, Fac Quim, San Sebastian 20080, Spain Univ Pais Vasco SanSebastian Spain 20080 im, San Sebastian 20080, Spain Univ Castilla La Mancha, Fac Ciencias Medio Ambiente, Toledo 45001, Spain Univ Castilla La Mancha Toledo Spain 45001 Ambiente, Toledo 45001, Spain Univ Castilla La Mancha, Fac Quim, E-13071 Ciudad Real, Spain Univ Castilla La Mancha Ciudad Real Spain E-13071 071 Ciudad Real, Spain Univ Complutense Madrid, Fac Quim, Dept Quim Organ, E-28040 Madrid, Spain Univ Complutense Madrid Madrid Spain E-28040 rgan, E-28040 Madrid, Spain
Titolo Testata:
EUROPEAN JOURNAL OF ORGANIC CHEMISTRY
fascicolo: 13, , anno: 2000,
pagine: 2407 - 2415
SICI:
1434-193X(200007):13<2407:DOSIID>2.0.ZU;2-3
Fonte:
ISI
Lingua:
ENG
Soggetto:
CHARGE-TRANSFER COMPLEXES; REVERSIBLE REDOX SYSTEMS; N,N'-DICYANOQUINONE DIIMINES DCNQIS; RADICAL-ANION SALTS; ELECTRON-TRANSFER; ORGANIC METALS; NMR; ELECTROCHEMISTRY; ACCEPTORS; 1,4-BENZOQUINONES;
Keywords:
ab initio calculations; NMR spectroscopy; 2D EXSY; cyanoimines;
Tipo documento:
Article
Natura:
Periodico
Settore Disciplinare:
Physical, Chemical & Earth Sciences
Citazioni:
74
Recensione:
Indirizzi per estratti:
Indirizzo: de la Hoz, A Univ Pais Vasco, Fac Quim, Apdo 1072, San Sebastian 20080, Spain Univ Pais Vasco Apdo 1072 San Sebastian Spain 20080 0, Spain
Citazione:
F.P. Cossio et al., "Determination of syn/anti isomerism in DCNQI derivatives by 2D exchange spectroscopy: Theoretical underpinning", EUR J ORG C, (13), 2000, pp. 2407-2415

Abstract

The dynamic syn/anti isomerism resulting from the inversion of the cyano group at the C=N double bond in a series of substituted DCNQIs has been investigated in solution by two-dimensional exchange spectroscopy (2D EXSY). The isomers formed were characterised by H-1 NMR at 223 K to slow down the inversion process of the cyanoimine group as well as by H,H COSY spectra, Whereas compounds 6, 7, 9 and 10 show only one isomer, compounds 8 and 11 showtwo isomers with two inversion processes and compounds 12 and 13 show three isomers and four inversion processes. The energy barrier for the syn/antiisomerization of the NCN groups has been estimated from the rate constantsdetermined from the 2D EXSY spectra, and very close values (13.16-13.93 kcal/mol for a single inversion) were found for all compounds. Isomerizationsinvolving two NCN groups (compounds 12 and 13) exhibited higher activationfree energy values (13.41-14.40 kcal/mol). Theoretical calculations of these free energy barriers are in excellent agreement with the experimental values, especially when solvent effects are taken into account. Theoretical calculations at B3LYP/6-31G* level predict planar geometries for the DCNQI derivatives studied (6 and 8), in particular when solvent effects are considered. This is in perfect agreement with the experimental data. Two-electronstabilizing interactions, as well as solvent effects, are related to the relative energies of the different stereoisomers. The relative equilibrium populations of the different isomers have been calculated using semiempirical energies and Boltzmann's distribution. Although the most stable isomer asdetermined by AM1 method is in qualitative agreement with that deduced from NMR experiments in all DCNQI derivatives studied, better quantitative correlations are obtained at the B3LYP(L1A1)/6-31G* calculation level. Comparison of both semiempirical and ab initio calculations reveal that in these DCNQI systems AM1 is the method of choice for those larger systems whose size prevents energy calculations at higher levels.

ASDD Area Sistemi Dipartimentali e Documentali, Università di Bologna, Catalogo delle riviste ed altri periodici
Documento generato il 04/12/20 alle ore 15:29:13