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Structure and Dynamics of Hydrogen Chelates. Part 1. NMR‐spectroscopic studies in the quinazoline‐2‐acetonitrile series
Author(s) -
Gehring Holger,
Daltrozzo Ewald
Publication year - 1998
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19980810206
Subject(s) - chemistry , conformational isomerism , acetonitrile , tautomer , quinazoline , chelation , hydrogen bond , nuclear magnetic resonance spectroscopy , proton nmr , computational chemistry , pyrimidine , stereochemistry , molecule , organic chemistry
To get informations on both the structure and dynamics of hydrogen chelates 1 of heteroaromatic systems, a great variety of quinazoline‐2‐acetonitrile chelates were synthesized (see 2–4 ). Similarly to the situation of the corresponding H‐chelates in the pyrimidine‐2‐acetonitrile series, the investigation of these new derivatives 2–4 by NMR spectroscopic methods (DNMR, COSY, NOESY, ROESY, EXSY, HMQC, HMBC) confirms the presence of an equilibrium of the two possible H‐chelate structures (two ‘rotamers’ I and II , i.e. , ( E )/( Z ) isomers; see Scheme ). The corresponding equilibria I ⇌ II were determined by complete 1 H‐NMR signal assignment at low temperatures (after freezing the rotational processes). In addition, the tautomer equilibria A ⇌ B (relative energies of the two minima of the nonsymmetrical double‐well potential) for both ‘rotamers’ are ascertained by H,H and C,H couplings. The results are an important basis for the interpretation of both the UV/VIS absorptions and the dependence of fluorescence and fluorescence quantum yields on temperature.