  The high mass star forming clump W43-MM1 has been mapped in N$_{2}$H$^{+}(4
\rightarrow 3)$, C$^{18}$O$(3 \rightarrow 2)$, SiO$(8 \rightarrow 7)$, and in a
single pointing in DCO$^{+}(5 \rightarrow 4)$ towards the center of the clump.
Column densities from these observations as well as previous HCO$^{+}(4
\rightarrow 3)$, H$^{13}$CO$^{+}(4 \rightarrow 3)$, HCN$(4 \rightarrow 3)$,
H$^{13}$CN$(4 \rightarrow 3)$, and CS$(7 \rightarrow 6)$ data, have been
derived using the RADEX code, results later used to derived chemical abundances
at selected points in the MM1 main axis. Comparing with chemical models, we
estimate an evolutionary age of $10^{4}$ years for a remarkable warm hot core
inside MM1. We also proposed that the dust temperature derived from SED fitting
in MM1 is not representative of the gas temperature deep inside the clump as
dust emission may have become optically thick. By deriving a deuterium
fractionation of $1.2 \times 10^{3}$, we estimate an electron fraction of
$X(e)= 6.5 \times 10^{-8}$. Thus, the coupling between the neutral gas and the
magnetic field is estimated by computing the ambipolar diffusion Reynolds
number $R_{m}=18$ and the wave coupling number W=110. Considering that the
infalling speed is slightly supersonic (M=1.1) but sub-alfvenic, we conclude
that the MM1 clump has recently or is in the process of decoupling the field
from the neutral fluid. Thus, the MM1 clump appears to be in an intermediate
stage of evolution in which a hot core has developed while the envelope is
still infalling and not fully decoupled from the ambient magnetic field.
