Unusual Raman spectra of para-nitroaniline by sequential Fermi resonances

In this communication, we report the unusual Raman spectra of para-nitroaniline (PNA) by sequential Fermi resonances. The combinational mode 1292 cm-1 in the experimental Raman spectrum indirectly gains the initial spectral weight at 1392 cm-1 by three sequential Fermi resonances. These Fermi resonances result in the strong interaction between the donor group of NH2 and the acceptor group of NO2. Our theoretical calculations provide reasonable interpretation for the abnormal Raman spectra of PNA. Experimental surface enhanced Raman scattering (SERS) spectrum of PNA further confirmed our conclusion, where the strongest Raman peak at 1292 cm-1 is very weak, while the Raman peak at 1392 cm-1 becoming the strongest Raman peak, which is consistent with the theoretical simulations.