Electronic [UV–Visible] and vibrational [FT-IR, FT-Raman] investigation and NMR–mass spectroscopic analysis of terephthalic acid using quantum Gaussian calculations N. Karthikeyan a , J. Joseph Prince b , S. Ramalingam c,⇑ , S. Periandy d a Department of Physics, Indra Ganesan College of Engineering, Tiruchirappalli, Tamilnadu, India b Department of Physics, Anna University, BIT Campus, Tiruchirappalli, Tamilnadu, India c Department of Physics, A.V.C. College, Mayiladuthurai, Tamilnadu, India d Department of Physics, Tagore Arts College, Puducherry, India highlights  The calculations were carried out at HF, B3LYP and B3PW91 methods.  The NLO, optical properties, ESP, ED and the MEP were analyzed.  The vibrational spectra and UV– Visible showed the change of electronic configuration.  The calculated data are compared with experimental data. graphical abstract The molecular mass spectral data related to base molecule and substitutional group of the compound was analyzed. The modification of the chemical property by the reaction mechanism of the injection of dicar- boxylic group in the benzene ring was investigated and it is displayed in the following figure. The orbital energies of different levels of HOMO and LUMO were calculated and the molecular orbital lobe overlap- ping showed the inter charge transformation between the base molecule and ligand group. The interac- tion over the frontier molecular orbitals the possibility of electrophilic and nucleophilic hit is also presented in the figure. The present molecule was fragmented with respect to atomic mass and the mass variation depends on the substitutions have also been studied. article info Article history: Received 16 September 2014 Received in revised form 19 November 2014 Accepted 25 November 2014 Available online 24 December 2014 Keywords: Terephthalic acid Dicarboxylic group Optical properties Hybrid Gaussian NLO activity FMO abstract In this research work, the vibrational IR, polarization Raman, NMR and mass spectra of terephthalic acid (TA) were recorded. The observed fundamental peaks (IR, Raman) were assigned according to their dis- tinctiveness region. The hybrid computational calculations were carried out for calculating geometrical and vibrational parameters by DFT (B3LYP and B3PW91) methods with 6-31++G(d,p) and 6- 311++G(d,p) basis sets and the corresponding results were tabulated. The molecular mass spectral data related to base molecule and substitutional group of the compound was analyzed. The modification of the chemical property by the reaction mechanism of the injection of dicarboxylic group in the base mol- ecule was investigated. The 13 C and 1 H NMR spectra were simulated by using the gauge independent atomic orbital (GIAO) method and the absolute chemical shifts related to TMS were compared with experimental spectra. The study on the electronic and optical properties; absorption wavelengths, excitation energy, dipole moment and frontier molecular orbital energies, were performed by hybrid Gaussian calculation methods. The orbital energies of different levels of HOMO and LUMO were http://dx.doi.org/10.1016/j.saa.2014.11.112 1386-1425/Crown Copyright Ó 2014 Published by Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel./fax: +91 04364 225367. E-mail address: ramalingam.physics@gmail.com (S. Ramalingam). Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 229–242 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa