Microwave Spectra, Barriers to Internal Rotation of the Methyl Group, and Molecular Electrical Dipole Moments in trans- and cis-1,3-Pentadiene

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Abstract
The microwave spectra of trans‐ and cis‐1,3‐pentadiene have been assigned. The barriers to internal rotation of the methyl group and the molecular electric dipole moments are also measured. The barriers to internal rotation are discussed in light of recent theories and the experimental work on other substituted propenes. The specific results obtained here are listed below. trans‐1,3‐pentadiene: The rotation constants are A = 28.2 ± 0.7 GHz , B = 2160.61 ± 0.01 MHz , and C = 2033.21 ± 0.01 MHz . The barrier to internal rotation of the methyl group is V3 = 1805 ± 60 cal/mole . The electric dipole moments in the molecular plane are | μa | = (0.561 ± 0.002)D , | μb | = (0.16 ± 0.03)D , and | μ | = (0.585 ± 0.010)D . The total dipole moment is at an angle of 16.5° ± 3.0° from the a axis (see Fig. 1). cis‐1,3‐pentadiene: The rotational constants are A = 15.7 ± 0.1 GHz , B = 2659.17 ± 0.02 MHz , and C = 2306.12 ± 0.01 MHz . The barrier to internal rotation is 741 ± 50 cal/mole. The electric dipole moments in the molecular plane are | μa | = (0.465 ± 0.010)D , | μb | = (0.185 ± 0.015) D , and | μ | = (0.500 ± 0.015) D . The total dipole moment is at an angle of 21.6° ± 1.5° from the a axis.