This research article published by Science Publishi9ng Group, 2014The interaction between glucosamine molecules has been studied theoretically. The geometrical structures of monomer (A), dimer (AA) and trimer (AAA) molecules of glucosamine were optimized and vibrational spectra were calculated by DFT/B3LYP method using GAMESS software (Firefly version 8.0.0). The theoretical vibrational spectra for the glucosamine dimer and trimer correspond well to the experimental IR spectrum of chitosan. The energies and enthalpies of association of A to form the dimer and trimer have been determined. The enthalpies of dimerization, A + A = AA + H2O, and trimerization, AA + A = AAA + H2O, are 48 and 45 kJ/mol, respectively. The thermodynamic functions of the monomer, dimer, and trimer molecules of glucosamine have been calculated.