- a valence bond (VB) picture where there is a quantum superposition of D-H and H-A bonds (which are partially covalent and partially ionic)
- a molecular orbital picture of 4 electrons in 3 orbitals
- a donor-acceptor natural orbital picture
But is there any definitive experimental evidence for covalent character? Two are cited:
For a physicist and non-expert on NMR it is hard to follow the latter. However, I suspect what is going on is the following.
Please correct me if I am wrong.
There is a hyperfine coupling between the D nucleus (A nucleus) and an electron which is largely localised on the D atom (A atom). But this D electron forms a "bond" (i.e. its spin is partly entangled with) with another electron localised on the H atom. But this electron in the H atom orbital also forms a bond with the electron in the A orbital. The net effect is there is some entanglement (covalency) between the electrons in the D and A orbitals. This in turn leads to entanglement (i.e. a spin-spin interaction) between the D and A nuclear spins.
In contrast, if everything was purely classical and electrostatic there would be no quantum mechanical phase coherence between electron spins associated with the D and A atoms.