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Bond Length Alternation Observed Experimentally: The Case of 1 H ‐Indazole
Author(s) -
Uriarte Iciar,
Reviriego Felipe,
Calabrese Camilla,
Elguero José,
Kisiel Zbigniew,
Alkorta Ibon,
Cocinero Emilio J.
Publication year - 2019
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201901666
Subject(s) - indazole , isotopologue , bond length , chemistry , alternation (linguistics) , crystallography , conformational isomerism , molecule , rotational spectroscopy , spectroscopy , computational chemistry , stereochemistry , crystal structure , physics , organic chemistry , linguistics , philosophy , quantum mechanics
Bond length alternation is a chemical phenomenon in benzene rings fused to other rings, which has been mainly predicted theoretically. Its physical origin is still not clear and has generated discussion. Here, by using a strategy that combines microwave spectroscopy, custom‐made synthesis and high‐level ab initio calculations, we demonstrate that this phenomenon is clearly observed in the prototype indazole molecule isolated in the gas phase. The 1 H ‐indazole conformer was detected by rotational spectroscopy, and its 17 isotopologues resulting from single and double heavy atom substitution ( 13 C and 15 N) were also unambiguously observed. Several experimental structures were determined and, in particular, the most useful semi‐experimental equilibrium structure ( r e SE ), allowed determination of the heavy atom bond lengths to milli‐Ångstrom precision. The experimentally determined bond length alternation is estimated to correspond to 60:40 contributions from the two resonant forms of 1 H ‐indazole.