A simple method exists (Angew. Chemie Int. Ed., submitted) for finding an approximate relationship between the molecular weights of small molecules (< 1000 g/mol) containing light atoms (sulfur or lighter) and their diffusion coefficients D in common solvents. The method requires only the molecular weight of the solute and the viscosity of the solvent to yield an estimate of D with an rms error of about 15%. A simple Excel spreadsheet is available here
DvsMW.xls (right click and chose 'save ... as')
that allows an estimate of D to be obtained from the MW of the solute and the identity of the solvent, or conversely an estimate of MW to be found from D (note that the uncertainty in MW is about three times the uncertainty in D). Instructions maybe found by hovering the mouse over the red triangles in the spreadsheet.