Thermodynamics in Solution (solvation) from quantum chemistry
The best way is to take all the structures into account and use Maxwell-Boltzmann distribution to weigh them accordingly. Here is what I would do
For each chemical structures/isomers (here I used F to denote Gibbs free energy),
$ F_{liq} =  F_{gas} +  F_{solvation}$
Now, $ F_{gas} =  E_{scf} - k_BTlnQ$
Where, $Q=q_{translational} \times q_{vibrational} \times q_{rotational} $
Using appropriate method (implicit solvation or QM/MM) you can find $F_{solvation}$. Now you have $F_{liq}$ for each isomers. Then use the following method to weigh them according to Maxwell-Boltzmann distribution.  

In the above image, I use $\epsilon_i$ to denote $F_i$.