Experimental and theoretical study for substituent effects of three 2-bromothiophene derivatives

The electronic structure and substituent effects (SEs) in 2-bromo-5-chlorothiophene, 2-bromo-5-methylthiophene, and 2-bromo-5-nitrothiophene have been investigated by HeI photoelectron spectroscopy (PES). The observed PES bands were analyzed by combining empirical arguments and theoretical methods. The outermost electrons of the three compounds all reside mainly in thiophene ring. The analysis of electronic effects of the donor or acceptor substituent groups is essential for the reliable assignment of the observed photoelectron spectra. The investigation of π- and n-orbital ionization potentials has enabled us to describe the substituent effects and the relative reactivities. Furthermore, the natural bond orbital (NBO) analysis was applied for better understanding the nature of the intermolecular interaction.