  The boundary of the neutron $i_{13/2}$ single-particle energy is investigated
with exact shell-model calculations, where random two-body interactions are
adopted to overcome the bias from effective interactions. Excitation energies
of $3^-_1$ state in $^{134}$Te and $^{136}$Xe, as well as those of $13/2^+_1$
states in $^{135}$Te and $^{137}$Xe, are taken as touchstones of our samplings.
A robust upper limit of $\varepsilon_{i13/2}<3.1$ MeV emerges in our
calculations, given the mixing of $i_{13/2}$ single-neutron configuration and
$f_{7/2}\otimes 3^-$ configuration in $13/2^+_1$ states of $N=83$ isotones.
