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Polymorphism vs. Desmotropy: The Cases of 3‐Phenyl‐ and 5‐Phenyl‐ 1 H ‐pyrazoles and 3‐Phenyl‐1 H ‐indazole
Author(s) -
Angeles García María,
López Concepción,
Claramunt Rosa M.,
Kenz Ahmed,
Pierrot Marcel,
Elguero José
Publication year - 2002
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 - 10.1002/1522-2675(200209)85:9<2763::aid-hlca2763>3.0.co;2-r
Subject(s) - chemistry , tautomer , indazole , crystallography , molecule , pyrazole , melting point , density functional theory , carbon 13 nmr , crystal structure , group (periodic table) , solid state , stereochemistry , computational chemistry , organic chemistry
Two desmotropes, 3‐phenyl‐1 H ‐pyrazole ( 1a ) and 5‐phenyl‐1 H ‐pyrazole ( 1b ) have been isolated and the conditions for their interconversion established. The X‐ray structure of 1b has been determined ( a =10.862(1), b =5.7620(5), c =12.927(2) Å, β =111.435(2)°, space group P 2 1 / c ), and both tautomers 1a and 1b were characterized by NMR in the solid state ( 13 C‐ and 15 N‐CPMAS). In the case of 3‐phenyl‐1 H ‐indazole ( 2a ), two concomitant polymorphs have been analyzed by X‐ray crystallography, and their NMR spectral properties were determined. The low‐melting‐point polymorph, at 106.7°, contains three molecules in the asymmetric unit ( a =41.086(1), b =7.3860(2), c =23.391(1) Å, β =117.697(1)°, space group C 2/ c ) and the high‐melting‐point one, 115.3°, six molecules ( a =13.7818(4), b =13.7976(5), c =18.9445(5) Å, α =94.300(3), β =95.131(3), γ =119.428(3)°, space group P ‐1). Here, too, it has been experimentally determined how to transform one form into the other. Density‐functional‐theory calculations at the B3LYP/6‐31G** level have been performed in both examples to rationalize the stability of the different tautomers.