  The electronic and magnetic properties of K$_{2}$CuP$_{2}$O$_{7}$ were
investigated by means of susceptibility, specific heat and $^{31}$P nuclear
magnetic resonance (NMR) measurements and by LDA band structure calculations.
The temperature dependence of the NMR shift $K(T)$ is well described by the
$S=1/2$ Heisenberg antiferromagnetic chain (HAF) model with nearest neighbor
exchange $J_{1}$ $\simeq $ $(141\pm 5)$ K. The corresponding mapping of an
LDA-derived tight-binding model leads to $J_{1}^{LDA}$ $\simeq$ 196 K. The spin
lattice relaxation rate $1/T_{1}$ decreases with temperature below 300 K but
becomes nearly temperature independent between 30 K and 2 K as theoretically
expected for an $S = {1/2}$ HAF chain. None of the investigated properties give
any evidence for long range magnetic order above 2K, in agreement with the
results of the band structure calculation, which yield extremely weak exchange
to the next nearest neighbor (NNN) and a very small and frustrated inter-chain
exchange. Thus, K$_{2}$CuP$_{2}$O$_{7}$ seems to be a better realization of a
nearest neighbor $S = {1/2}$ HAF chain than the compounds reported so far.
