A variant is presented of the relative intensity method (RIM) in microwave spectroscopy to determine barriers to internal rotation. In this method torsional energies are determined directly from the slope of the straight line relating the logarithm of the intensity ratio and the reciprocal of the temperature. The advantage of this simple and direct method is that unknown theoretical or experimental weight factors are cancelled out in the determination of torsional energies from the measured intensity ratios. Using a newly developed Stark absorption cell without a mid-electrode, this RIM has been applicated to the molecules CH3SiH3, HCCOCH3 and HCCSCH2D. The results of the height of the barriers to internal rotation in these molecules are in agreement with the values obtained from frequency splitting methods (FSM). Furthermore, the accuracy obtained in the RIM (3–5%) can compete with that of the FSM (2–4%)
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