Question

why are the dihedral angles of the hydrogens at C1 in gauche butane not exactly 60 degrees or 180 degrees

Answer 1

a rotation of 60 or greater will move the methyl groups further apart (assuming a view thru the C2-C3 carbons). as the methyl groups get further apart, the energy decreases...

~ 11 kJ at 0 vs ~ 3.8 or 0 kJ at 60 or, say 180

this is due to the torsional strength, as well as the steric strength of the e- trying to get as far away from eachother as possible.

Answer 2

found that on yahoo