The microwave spectrum,structure, and dipole moment of amino acetonitrile

The microwave spectra of three isotopic species of amino acetonitrile (NH₂CH₂CN, NHDCH₂CN, and ND₂CH₂CN) have been investigated to learn something about the structure and bonding in this and similar compounds. The only rotamer observed is the form in which both NH bonds are gauche to the CN group and the structure is quite rigid. From the available data only the bond angles are well determined. The amino NCC angle is 114.5(3)°, the HCH angle is 103(2)°, the HNC angle is 109.6(4)°, and the HNH angle is 107(1)°. The dipole moment components are μ_a = 2.577(7) D and μ_b = 0.5754(10) D; these agree quite well in magnitude and direction with the sum of the acetonitrile and methyl amine dipoles. The rigidity of the molecule and its preference for the form in which the amino protons are closest to the triple bond confirms a suggestion based on earlier studies on similar molecules that there is a strong hydrogen bonding interaction between the amino group and the nitrile group, although in this molecule dipole-dipole forces also probably play a significant role in determining the structure and its rigidity.