  We propose a simple, general, and accurate formula for analyzing the
tunneling between classical configurations of a non-planar molecule in a gas
medium, as a function of the thermodynamic parameters of the gas. We apply it
to two interesting cases: i) The shift to zero frequency of the inversion line
of ammonia, upon increase of the pressure of the gas; ii) The destruction of
the coherent tunneling of D2S2 molecules in a gas of He. In both cases, we
compare our analysis with previous theoretical and experimental results.
