La Spectroscopie de résonance magnétique nucléaire (RMN)

La spectroscopie de résonance magnétique nucléaire trouve un grand emploi pour la détermination des structures flavoniques ^[56]. C’est une méthode précise et efficace, mais nécessite une grande quantité du produit ce qui limite son utilisation par rapport aux autres méthodes d’analyses.

 Spectre RMN 1

H: Elle informe sur l’environnement des différents protons flavoniques qui résonnent généralement entre 6 et 8 ppm ^[56-60], elle permet de connaître :

 La position et le nombre de divers protons porté par le flavonoïde  Le nombre de substituants méthoxyles porté par le squelette flavonique  Le nombre et la nature des sucres liés à l’aglycone.

 Spectre RMN 13

C: Il permet de déterminer le nombre d’atomes de carbone et de connaître leurs déplacements chimiques et leurs natures (C quaternaire, CH, CH2, ou CH3). Il donne aussi des informations utiles et parfois nécessaires pour mieux identifier la molécule telles que ^{[40], [60]} :

- Le nombre total d’atomes de carbone du composé flavonique ainsi que leur environnement.

- la connaissance de type des liaisons –C et / ou –O sucres.  Spectre COSY (2D 1

H-¹H) : Il permet de déterminer les couplages ¹H-¹H de protons géminés ou vicinaux. On observe des taches de corrélation entre deux protons voisins, distants de 2 liaisons (2J) et 3 liaisons (3J).

 Spectre HSQC (2D 1

H-¹³C) : Il permet d’établir les corrélations directes ¹H-¹³C. On observe sur le spectre des taches de corrélation entre les atomes de carbones et les protons directement liés.

 Spectre HMBC (2D 1

H-¹³C) : Il permet d’établir les corrélations entre protons

1

H-¹³C (Fig II.14) à longue distance à travers les liaisons. On observe sur le spectre des taches de corrélation entre les atomes de carbone et les protons distants de 2 ou 3 liaisons.

Figure II.13: Principes des techniques de RMN à deux dimensions  Spectre NOESY (2D 1

H-¹H):Il indique les couplages ¹H-¹H à travers l’espace.  Spectre ROESY (2D1

H-¹H):Ce type d’expérience permet de visualiser les corrélations dipolaires proton-proton entre deux voisins proches. Les corrélations peuvent être d’origine intra- ou extra-résidu.

Les spectres NOESY et ROESY sont spécialement intéressants pour la détermination de distances interatomiques

 L’élucidation structurale nécessite l’analyse conjuguée de ces différents spectres. Les corrélations COSY permettent dans un premier temps de déterminer l’enchaînement des CH, CH2, et CH3 et d’élaborer des motifs de structure. Ceux-ci seront ensuite connectés grâce aux corrélations HMBC mettant en évidence les carbones quaternaires. Enfin, les corrélations NOESY permettent de déterminer la disposition spatiale des protons et d’établir la configuration relative de la molécule.

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CHAPITRE III. ETUDE CHIMIQUE DE DEUX ESPECES DE GENISTA