Three isomers detected for the whisky lactone: 5-butyl-4-methyl tetrahydrofuran-2-one by Fourier transform microwave spectroscopy and quantum chemical calculations

• Published in: Journal of molecular spectroscopy 2017-05, Vol.335, p.27-32
• Main Authors: Kawashima, Yoshiyuki, Katsuragi, Ryusuke, Hirota, Eizi
• Format: Article
• Language: English
• ISSN: 0022-2852; 1096-083X
• DOI: 10.1016/j.jms.2017.02.007

[Display omitted] •We have detected three stereo-isomers of the WL by FTMW combined with quantum chemical calculations: the most stable conformer of the trans-TTT (eq, eq), followed by the cis-TTT (ax, eq) and cis-GTT (ax, eq) forms.•We have also observed and analyzed small splittings due to internal rotation of the terminal methyl in the butyl group for these three forms to derive the potential barriers V3. The ground-state rotational spectra of the whisky lactone (WL): 5-butyl-4-methyl tetrahydrofuran-2-one were observed and analyzed by molecular beam Fourier transform microwave spectroscopy combined with quantum chemical calculations. We have detected three stereo-isomers: the trans-TTT form with a methyl CH3 group attached to C(4) in an equatorial position (eq) and a butyl C4H9 group to C(5) in an eq position, for which 110 b-type and 113 a-type transitions were assigned, the cis-TTT form with a CH3 to C(4) in an axial position (ax) and a C4H9 to C(5) in eq, for which 96 a-type, 101 b-type, and 45 c-type transitions were observed, and the cis-GTT form with a CH3 to C(4) in ax and a C4H9 to C(5) in eq, for which 158 a-type, 52 b-type, and 17 c-type transitions were observed, where TTT and GTT denote the conformations about the C(6)C(5), C(7)C(6), and C(8)C(7) bonds, with T and G designating trans and gauche, respectively. The rotational constants thus derived agree with the predictions made by quantum chemical calculations, MP2/6-311++G(d, p) within 1.2%. The trans-TTT form was calculated to be the most stable. The splittings due to internal rotation of the terminal methyl in the butyl group were observed for all the three stereo-isomers and were analyzed by the XIAM program to determine the threefold potential barrier V3 to be 966.4 (25), 978.8 (11), and 1098.7 (48) in cm−1 for the trans-TTT (eq, eq), the cis-TTT (ax, eq), and the cis-GTT (ax, eq) forms, respectively, to be compared with quantum chemically calculated values: 1055, 1055, and 1053 in cm−1.