  We present new high-resolution HI observations of the disk of the collisional
infrared luminous (L$_{\rm IR}=2.2\times10^{11}$ L$_{\sun}$) galaxy NGC 1144,
which reveal an apparent large-scale azimuthal and kinematic segregation of
neutral hydrogen relative to the molecular gas distribution. Even among
violently collisional galaxies, the CO/HI asymmetry in NGC 1144 is unusual,
both in the inner regions, and in the outer disk. We suggest that we are
observing Arp 118 at a special moment, shortly after a high-speed collision
between NGC 1144 and its elliptical companion NGC 1143. HI emission with an
average molecular fraction f$_{mol}$ $<$ 0.5 is observed on one side (NW) of
the rotating disk of NGC 1144, while the other side (SE) is dominated by dense
molecular complexes in which f$_{mol}$ is almost unity. The interface region
between the warm-- and cool--cloud dominated regions, lies on a deep
spiral-like dust-lane which we identify as a shock-wave responsible for the
relative shift in the dominance of HI and H$_2$ gas. A strong shock being fed
by diffuse HI clouds with unusually large ($>$ 400 km s$^{-1}$) rotational
velocities can explain: 1) the CO/HI asymmetries, 2) a large velocity jump (185
km s$^{-1}$) across the arm as measured by HI absorption against a radio bright
continuum source which straddles the arm, and 3) the asymmetric distribution of
star formation and off-nuclear molecular gas resulting from likely streaming
motions associated with the strong-shock. The new results provide for the first
time a coherent picture of Arp 118's many peculiarities, and underlines the
potentially complex changes in the gas-phase that can accompany large
gravitational perturbations of gas-rich galaxies.
