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Luis Sánchez – One of the best experts on this subject based on the ideXlab platform.

  • determination of syn anti isomerism in dcnqi derivatives by 2D Exchange Spectroscopy theoretical underpinning
    European Journal of Organic Chemistry, 2000
    Co-Authors: Fernando P. Cossío, Pilar De La Cruz, Antonio De La Hoz, Fernando Langa, Nazario Martín, Pilar Prieto, Luis Sánchez

    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 1H 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 show two isomers with two inversion processes and compounds 12 and 13 show three isomers and four inversion processes. The energy barrier for the syn/anti isomerization of the NCN groups has been estimated from the rate constants determined from the 2D EXSY spectra, and very close values (13.16−13.93 kcal/mol for a single inversion) were found for all compounds. Isomerizations involving two NCN groups (compounds 12 and 13) exhibited higher activation free 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-electron stabilizing 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 as determined 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.

  • Determination of syn/anti Isomerism in DCNQI Derivatives by 2D Exchange Spectroscopy: Theoretical Underpinning
    European Journal of Organic Chemistry, 2000
    Co-Authors: Fernando P. Cossío, Pilar De La Cruz, Antonio De La Hoz, Fernando Langa, Nazario Martín, Pilar Prieto, Luis Sánchez

    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 1H 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 show two isomers with two inversion processes and compounds 12 and 13 show three isomers and four inversion processes. The energy barrier for the syn/anti isomerization of the NCN groups has been estimated from the rate constants determined from the 2D EXSY spectra, and very close values (13.16−13.93 kcal/mol for a single inversion) were found for all compounds. Isomerizations involving two NCN groups (compounds 12 and 13) exhibited higher activation free 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-electron stabilizing 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 as determined 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.

Antonio De La Hoz – One of the best experts on this subject based on the ideXlab platform.

  • Microwave‐Assisted Synthesis and Dynamic Behaviour of N2,N4,N6‐Tris(1H‐pyrazolyl)‐1,3,5‐triazine‐2,4,6‐triamines
    QSAR & Combinatorial Science, 2005
    Co-Authors: Angel Díaz-ortiz, Antonio De La Hoz, José Elguero, Andrés Moreno, Sergio Moreno, Agustín Jiménez, Ana Sánchez-migallón

    Abstract:

    A series of N 2 ,N 4 ,N 6 -tris(1H-pyrazolyl)-1,3,5-triazine-2,4,6-triamines has been synthesised under microwave irradiation in solvent-free conditions. By reaction of pyrazolylamines with cyanuric chloride and 2-chloro-4,6-diamino-1,3,5-triazines under microwave irradiation, l,3,5-triazine-2,4,6-triamies with symmetrical and asymmetrical substitution, respectively, can be obtained. In the latter case, the procedure can be easily adapted by addition of a small amount of Dimethyl Sulfoxide (DMSO) for the preparation of polymer-supported triazines, with application in supramolecular combinatorial synthesis. At low temperature, the presence of two or four conformers has been detected for symmetrically and asymmetrically substituted derivatives respectively. ID- and 2DExchange Spectroscopy studies in various solvents and at different temperatures have been used to determine the equilibrium constants and the activation free energies of the restricted rotation about the amino – triazine bond. A plot of the activation free energy versus temperature shows a good linear correlation and confirms that the same process is present in all of the compounds under investigation.

  • Synthesis, structural determination and dynamic behavior of 2-chloro-4,6-bis(pyrazolylamino)-1,3,5-triazines
    Organic & biomolecular chemistry, 2003
    Co-Authors: Angel Díaz-ortiz, Antonio De La Hoz, José Elguero, Concepción Foces-foces, Andrés Moreno, Sergio Moreno, Ana Sánchez-migallón, Gema Valiente

    Abstract:

    A series of 2-chloro-4,6-bis(pyrazolylamino)-1,3,5-triazines with applications in crystal engineering have been prepared. At low temperature, the presence of two or three isomers has been detected and these assigned to 4,6-diamino-1,3,5-triazine structures on the basis of comparison with model compounds. 2DExchange Spectroscopy studies in various solvents and at different temperatures have been used to determine the equilibrium constants and the activation free energies of the restricted rotation about the amino–triazine bond. A plot of the activation free energy versus temperature showed a good linear correlation and confirmed that the same process is present in all of the compounds under investigation. Comparison with model compounds also confirmed both the occurrence of the restricted rotation and the 4,6-diamino-1,3,5-triazine tautomerism for triazines 1–4 in solution. The structure of compound 1 has been determined in the solid state by X-ray crystallography and consists of a 4,6-diamino-1,3,5-triazine structure stabilized by intra and intermolecular hydrogen bonds.

  • determination of syn anti isomerism in dcnqi derivatives by 2D Exchange Spectroscopy theoretical underpinning
    European Journal of Organic Chemistry, 2000
    Co-Authors: Fernando P. Cossío, Pilar De La Cruz, Antonio De La Hoz, Fernando Langa, Nazario Martín, Pilar Prieto, Luis Sánchez

    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 1H 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 show two isomers with two inversion processes and compounds 12 and 13 show three isomers and four inversion processes. The energy barrier for the syn/anti isomerization of the NCN groups has been estimated from the rate constants determined from the 2D EXSY spectra, and very close values (13.16−13.93 kcal/mol for a single inversion) were found for all compounds. Isomerizations involving two NCN groups (compounds 12 and 13) exhibited higher activation free 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-electron stabilizing 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 as determined 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.

Fernando P. Cossío – One of the best experts on this subject based on the ideXlab platform.

  • determination of syn anti isomerism in dcnqi derivatives by 2D Exchange Spectroscopy theoretical underpinning
    European Journal of Organic Chemistry, 2000
    Co-Authors: Fernando P. Cossío, Pilar De La Cruz, Antonio De La Hoz, Fernando Langa, Nazario Martín, Pilar Prieto, Luis Sánchez

    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 1H 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 show two isomers with two inversion processes and compounds 12 and 13 show three isomers and four inversion processes. The energy barrier for the syn/anti isomerization of the NCN groups has been estimated from the rate constants determined from the 2D EXSY spectra, and very close values (13.16−13.93 kcal/mol for a single inversion) were found for all compounds. Isomerizations involving two NCN groups (compounds 12 and 13) exhibited higher activation free 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-electron stabilizing 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 as determined 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.

  • Determination of syn/anti Isomerism in DCNQI Derivatives by 2D Exchange Spectroscopy: Theoretical Underpinning
    European Journal of Organic Chemistry, 2000
    Co-Authors: Fernando P. Cossío, Pilar De La Cruz, Antonio De La Hoz, Fernando Langa, Nazario Martín, Pilar Prieto, Luis Sánchez

    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 1H 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 show two isomers with two inversion processes and compounds 12 and 13 show three isomers and four inversion processes. The energy barrier for the syn/anti isomerization of the NCN groups has been estimated from the rate constants determined from the 2D EXSY spectra, and very close values (13.16−13.93 kcal/mol for a single inversion) were found for all compounds. Isomerizations involving two NCN groups (compounds 12 and 13) exhibited higher activation free 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-electron stabilizing 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 as determined 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.