Trimethylene Oxide. II. Structure, Vibration-Rotation Interaction, and Origin of Potential Function for Ring-Puckering Motion

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Abstract
In the previous paper of this series, the microwave spectra of four isotopic species of trimethylene oxide were investigated and the potential function for the puckering of the four‐membered ring was determined. Further analysis of the data has now enabled the molecular structure to be calculated. The structural parameters deduced from the rotational constants are: r(C–C) = 1.549±0.003 A, r(C–O) = 1.449±0.002 A, r(Cα–Hα) = 1.091±0.002 A, r(Cβ–Hβ) = 1.100±0.003 A, ∠Cα–Cβ–Cα = 84°33′±1′, ∠Cα–O–Cα = 91°59′±7′, ∠Cβ–Cα–O = 91°44′±3′, ∠Hα–Cα–Hα = 110°18′±10′, ∠Hβ–Cβ–Hβ = 110°44′±3′. The alphamethylene planes are slightly deflected towards the oxygen atom away from the bisectors of the ∠Cβ–Cα–O's. However, the actual angle of deflection is rather uncertain. Using these structural parameters, models have been constructed to calculate the vibration‐rotation interaction due to the ring‐puckering vibration. The experimentally observed rotational constant variations were found to be very well reproduced if the out‐of‐plane bending motion is assumed to follow a curvilinear path without any stretching of the C–C and C–O bonds. The potential function determined in the previous paper is also interpreted in terms of force fields within the molecule.

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