Unstable intermediates. Part XCVI. The effect of hydrogen-bonding on the e.s.r. parameters for NH2, NH3+, and H2CN

Abstract

The isotropic hyperfine coupling to ¹⁴N increases markedly for both NH₂ and H₂CN when these radicals are hydrogen-bonded to the matrix. This is discussed in terms of two models: in one attenuation of lone-pair electrons results in increased spin-polarisation, and in the other hydrogen-bonding changes the structure so that 2s-character is added directly to the nominally 2p-orbital of the unpaired electron.

To do this for NH₂, a zero-point bending is required which must also occur for the conjugate acid, NH₃⁺. In support of this there is a marked fall in A_iso(¹⁴N)(19·3 to 17·3 G) on going from NH₃⁺ to ND₃⁺. For H₂CN the effect can also be understood by analogy with that expected for complete protonation, since the conjugate acid H₂CNH⁺, isoelectronic with H₂CĊH is expected to be non-linear with respect to the C–N–H bonds.