We report Tip Enhanced Raman Spectroscopy (TERS) mapping and Density Functional (DFT) calculations of aminothiophenol (ATP) grafted on a gold surface. The TERS mapping has demonstrated Raman modes of (ATP) and its dimerised derivative Dimercaptoazobenzene (DMAB). This feature confirms that the plasmon activated chemical reaction of ATP has occurred during TERS measurements. In some specific part of the samples some unidentified Raman modes are observed. We suggest that they could come from intermediate species formed during the conversion of ATP into DMAB. These modes are compared with calculated Raman spectra of some possible intermediate species. These results confirm the high potentiality of TERS measurements for nanochemistry.