conc=(1./sqrt(4*pi*D*t)*exp.(-1*((x-v*t).^2./4*D)));
% ^
This is interpreted as "exp" beeing a struct and "(-1*((x-v*t).^2./4*D)))" is the dynamic fieldname.
Such typos are more obvious if you use spaces:
conc = 1 ./ sqrt(4 * pi * D * t) * exp(-D * (x - v * t) .^ 2 ./ 4);
The spaces avoid another ambiguity also:
a.^2./x
% Does this mean:
a .^ 2.0 / x
% or:
a .^ 2 ./ x
Although the Matlab's interpreters have strict and unique rules for these cases, a human reader can be confused.
