Vibrational and theoretical studies of non-steroidal anti-inflammatory drugs Ibuprofen [2-(4-isobutylphenyl)propionic acid]; Naproxen [6-methoxy-α-methyl-2-naphthalene acetic acid] and Tolmetin acids [1-methyl-5-(4-methylbenzoyl)-1H-pyrrole-2-acetic acid]