Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 25, Issue 43, Pages 10172-10178Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.201901666
Keywords
ab initio calculations; bond alternation; conformational analysis; rotational spectroscopy; structure elucidation
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Funding
- MINECO [CTQ2017-89150-R, PGC2018-094644-B-C22]
- Basque Government [PIBA 2018-11]
- Comunidad Autnoma de Madrid [P2018/EMT-4329 AIRTEC-CM]
- UPV/EHU [PPG17/10]
- CSIC [PICS 2018FR0036]
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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 1H-indazole conformer was detected by rotational spectroscopy, and its 17 isotopologues resulting from single and double heavy atom substitution (C-13 and N-15) 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-angstrom ngstrom precision. The experimentally determined bond length alternation is estimated to correspond to 60:40 contributions from the two resonant forms of 1H-indazole.
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