TY - JOUR

T1 - The Scent of Maibowle – π Electron Localization in Coumarin from Its Microwave-Determined Structure

AU - Nguyen, Ha Vinh Lam

AU - Grabow, Jens Uwe

N1 - Funding Information: This work was supported by the Agence Nationale de la Recherche ANR (project ID ANR‐18‐CE29‐0011). J.‐U.G. thanks the Deutsche Forschungsgemeinschaft (DFG) and the Land Niedersachsen for funds.

PY - 2020/6/17

Y1 - 2020/6/17

N2 - The microwave spectra of the natural substance coumarin, a planar aromatic molecule with the specific scent of maibowle, a popular fruit punch served in spring and early summer, were recorded using a molecular jet Fourier transform microwave spectrometer working in the frequency range from 4.0 to 26.5 GHz. The rotational constants and centrifugal distortion constants were determined with high precision, reproducing the spectra to experimental accuracy. The spectra of all singly-substituted 13C and 18O isotopologues were observed in their natural abundances to determine the experimental heavy atom substitution rs and semi-experimental equilibrium reSE structures. The experimental bond lengths and bond angles were compared to those obtained from quantum chemical calculations and those of related molecules reported in the literature with benzene as the prototype. The alternation of the C−C bond lengths to the value of 1.39 Å found for benzene reflects the localization of π electrons in coumarin, where the benzene ring and the lactone-like chain −CH=CH−(C=O)−O− are fused. The large, negative inertial defect of coumarin is consistent with out-of-plane vibrations of the fused rings.

AB - The microwave spectra of the natural substance coumarin, a planar aromatic molecule with the specific scent of maibowle, a popular fruit punch served in spring and early summer, were recorded using a molecular jet Fourier transform microwave spectrometer working in the frequency range from 4.0 to 26.5 GHz. The rotational constants and centrifugal distortion constants were determined with high precision, reproducing the spectra to experimental accuracy. The spectra of all singly-substituted 13C and 18O isotopologues were observed in their natural abundances to determine the experimental heavy atom substitution rs and semi-experimental equilibrium reSE structures. The experimental bond lengths and bond angles were compared to those obtained from quantum chemical calculations and those of related molecules reported in the literature with benzene as the prototype. The alternation of the C−C bond lengths to the value of 1.39 Å found for benzene reflects the localization of π electrons in coumarin, where the benzene ring and the lactone-like chain −CH=CH−(C=O)−O− are fused. The large, negative inertial defect of coumarin is consistent with out-of-plane vibrations of the fused rings.

KW - coumarin

KW - isotopologue

KW - microwave spectroscopy

KW - rotational spectroscopy

KW - structural determination

UR - http://www.scopus.com/inward/record.url?scp=85085580938&partnerID=8YFLogxK

U2 - 10.1002/cphc.202000234

DO - 10.1002/cphc.202000234

M3 - Article

C2 - 32297685

AN - SCOPUS:85085580938

VL - 21

SP - 1243

EP - 1248

JO - CHEMPHYSCHEM

JF - CHEMPHYSCHEM

SN - 1439-4235

IS - 12

ER -