Did you use some solvation model (e.g. PCM) to account for solvent effects? This is very important in the case of pKa calculations, values calculated in vacuo are completely different.

Yes. The thermodynamic cycle implies on the calculation of (delta)G of: RNH2 (g), RNH2 (solvent), RNH3+ (g) and RNH3+ (solvent).

In this case I used the keyword SCRF with SMD model.

If the results, not the method, is your goal, then you may want to try our S+pKa predictions:

http://www.simulations-plus.com/Products.aspx?ADMET%20Predictor&grpID=1&cID=10&pID=13

Amines are sensitive to the solvation models. You can try to use this method. It works for many different types of amines.Article Quantum Chemical Calculation of pKas of Environmentally Rele...