  The linear spin-1/2 Heisenberg antiferromagnet with exchanges $J_1$, $J_2$
between first and second neighbors has a bond-order wave (BOW) phase that
starts at the fluid-dimer transition at $J_2/J_1 = 0.2411$ and is particularly
simple at $J_2/J_1 = 1/2$. The BOW phase has a doubly degenerate singlet ground
state, broken inversion symmetry and a finite energy gap $E_m$ to the lowest
triplet state.
  The interval $0.4<J_2/J_1<1.0$ has large $E_m$ and small finite size
corrections. Exact solutions are presented up to $N=28$ spins with either
periodic or open boundary conditions and for thermodynamics up to $N=18$. The
elementary excitations of the BOW phase with large $E_m$ are topological
spin-1/2 solitons that separate BOWs with opposite phase in a regular array of
spins. The molar spin susceptibility $\chi_M(T)$ is exponentially small for $T
\ll E_m$ and increases nearly linearly with $T$ to a broad maximum. $J_1$,
$J_2$ spin chains approximate the magnetic properties of the BOW phase of
Hubbard-type models and provide a starting point for modeling alkali-TCNQ
salts.
