The conformational enthalpy (Δ H °) of 3-methyltetrahydropyran (3-MTHP) has been measured by (2+1) resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The measured value (1.60 ± 0.12 kcal/mol) is in good agreement with results from ab initio calculations but somewhat higher than the Δ H ° values predicted by molecular mechanics calculations. Under our supersonic expansion conditions, vibration cooling of the axial and equatorial conformers of 3-MTHP was observed to be faster than interconversion between them up to the highest temperature investigated (383 K). The measurement of Δ G ° between the conformers shows that oscillator strengths for the 3s ← n transition of the conformers are different from each other. A molecular mechanics calculation demonstrates that the interconversion barrier of 11 kcal/mol from the equatorial chair to the axial chair form in 3-MTHP is close to that in tetrahydropyran and cyclohexane.
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Kim et al. (1999) studied this question.
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