Knowledge Bank

The microwave spectra of 2-methyl-1-buten-3-yne, trans-1,3-pentadiene and cis-1,3-pentadiene were all assigned and the methyl group barriers to internal rotation were measured. The results for 2-methyl-1-buten-3-yne are: $A=9359.58\pm 0.02MHz,B\pm 4013.52\pm 0.01MHz,C=2854.80\pm 0.01MHz,V3=2011\pm 70cal/mole$. For trans-1,3-pentadiene $A=28.5GHz,B=2161.6\pm 0.2MHz,C=2033.2\pm 0.2MHz,V3=1813\pm 60$ cal/mole. For cis-1,3-pentadiene: $A=15.6GHz,B=2659.2\pm 0.1MHz,C=2306.1\pm 0.1MHz,Va=750\pm 50$ cal/mole. The molecular electric dipole moments were also measured in the three molecules. The methyl group barriers to internal rotation will be interpreted in light of present theories.