FTIR, FT-Raman, UV–Visible spectra and quantum chemical calculations of allantoin molecule and its hydrogen bonded dimers Mohammad Jane Alam, Shabbir Ahmad ⇑ Department of Physics, Aligarh Muslim University, Aligarh 202002, India highlights  Anharmonic force field calculations using PT2, VSCF and CC-VSCF.  Anharmonic mode–mode coupling strengths based on 2MR-QFF potential.  FTIR, FT-Raman and UV–Vis spectra.  DFT, TD-DFT and MP2 calculations. graphical abstract article info Article history: Received 23 July 2014 Received in revised form 22 September 2014 Accepted 25 September 2014 Available online 5 October 2014 Keywords: Allantoin VSCF CC-VSCF PT2 FTIR FT-Raman abstract FTIR, FT-Raman and electronic spectra of allantoin molecule are recorded and investigated using DFT and MP2 methods with 6-311++G(d,p) basis set. The molecular structure, anharmonic vibrational spectra, nat- ural atomic charges, non-linear optical properties, etc. have been computed for the ground state of allan- toin. The anharmonic vibrational frequencies are calculated using PT2 algorithm (Barone method) as well as VSCF and CC-VSCF methods. These methods yield results that are in remarkable agreement with the experiment. The coupling strengths between pairs of modes are also calculated using coupling integral based on 2MR-QFF approximation. The simulations on allantoin dimers have been also performed at B3LYP/6-311++G(d,p) level of theory to investigate the effect of the intermolecular interactions on the molecular structure and vibrational frequencies of the monomer. Vibrational assignments are made with the great accuracy using PED calculations and animated modes. The combination and overtone bands have been also identified in the FTIR spectrum with the help of anharmonic computations. The electronic spec- tra are simulated in gas and solution at TD-B3LYP/6-311++G(d,p) level of theory. The important global quantities such as electro-negativity, electronic chemical potential, electrophilicity index, chemical hard- ness and softness based on HOMO, LUMO energy eigenvalues are also computed. NBO analysis has been performed for monomer and dimers of allantoin at B3LYP/6-311++G(d,p) level of theory. Ó 2014 Elsevier B.V. All rights reserved. Introduction Allantoin molecule (5-ureidohydantoin) is a product of purine metabolism and it is present in many plants and bacteria. In most of the mammals, allantoin is formed from uric acid by the action of uricase [1–4]. It has a wide range of applications in cosmetic, anti- perspirant and pharmaceutical products for skin softening, rapid generation of skin cells and treatment of skin diseases [5–10]. Due to its numerous applications, this molecule draws attention for the spectroscopic and quantum chemical investigations. Mootz [11] determined crystal structure of DL-allantoin by using XRD http://dx.doi.org/10.1016/j.saa.2014.09.119 1386-1425/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +91 9412501430. E-mail address: ahmad.shabbir@rediffmail.com (S. Ahmad). Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 961–978 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa