In solid ammonia, each `NH_(3)` molecule has six other `NH_(3)` molecules as nearest neighbours. `DeltaH` sublimation of `NH_(3)` at the melting point is `30.8 k J mol^(-1)`, and the estimated `DeltaH` sublimation in the absence of hydrogen bonding is `14.4 kJ mol^(-1)`. the strength of a hydrogen bond is `NH_(3)` is