Concentration‐Dependent Variation of ¹H‐NMR Chemical Shifts of Aromatic Protons in Sampangine Derivatives

Abstract

A concentration dependence of the ¹H‐NMR chemical shifts of the aromatic protons in sampangine derivatives with a fused imidazole ring is observed. This variation is probably ascribable to self‐association of the molecules through an intermolecular π‐stacking interaction of the aromatic rings. The quantitative variation is correlated with the calculated electrostatic potential for these derivatives. The concentration variation appears to be independent of the nature of the substitution in the imidazole ring.

Keywords:

• NMR
• ¹H
• Shift variations
• Concentration dependence
• Molecular modeling
• Intermolecular interaction
• Sampangine derivatives

Acknowledgments

The authors thank Mr. Jianwei Ren for the help with the synthesis of sampangine derivatives (3), Mr. Vernon Rayford and Ms. Sharee Smith for the preparation of sampangine (1). We are also grateful to Dr. David Lankin (GD Searle, Chicago), and Dr. Marcey L. Waters (University of North Carolina) for useful suggestions and discussions.

Notes

^aElectrostatic potentials (kcal/mol) for 3a on H‐3, ‐4, and ‐5 are 31.6, 35.2, and 37.7, while that on H‐9 and ‐10 are 20.3 and 19.4, respectively.

^bElectrostatic potentials (kcal/mol) for 3b on H‐3, ‐4, and ‐5 are 31.5, 38.0, and 43.3, while that on H‐9 and ‐10 are 20.2 and 21.0, respectively.

^cThe derivative 3c was prepared by using the same method described in Ref. [27].