NICS Difference Determination of Relative Aromaticity

RM 208

Aromatic compounds commonly occur in pharmaceuticals, fuel additives, polymers, etc. Key characteristics that make up an aromatic structure are the delocalization of pi, particular stability of the structure, and the distinct electronic field generated, that can be detected running an NMR analysis. NICS (short for Nucleus Independent Chemical Shifts) is the computational method for predicting the NMR spectrum. In this Project, NICS values are obtained using four programs: (1) Spartan to build the molecules, optimized structures (using the DFT B3LYP 6-311+G** method), and record geometry measurements such as bond lengths and angles; (2) Avogadro to insert probe/ ghost atoms which are where the calculations for NICS are determined; (3) a text editor to set up proper input page for the final step, and (4) Submission to GAMESSQ (General Atomic and Molecular Electronic Structure System) to calculate shielding values for NICS. NICS has taken some criticism for not being able determine the relative aromaticity of rings of different sizes. Our novel method, NICS difference scans uses two NICS scans one for the structure in question and its reference structure. This project demonstrates, the ability of our novel NICS difference method to further enhance NICS scans in predicting aromaticity, as can be examined by modelling benzene and distorted benzene derivatives