Synthesis, structural, hirshfeld surface, spectroscopic studies and quantum chemical calculation of the proton transfer complex between 2-amino-4-hydroxy-6-methylpyrimidine and salicylic acid Ziya Afroz a , Mohd Faizan b , Mohammad Jane Alam b , Vítor Hugo Nunes Rodrigues c , Shabbir Ahmad b , Afaq Ahmad a, * a Department of Chemistry, Aligarh Muslim University, Aligarh, 202002, UP, India b Department of Physics, Aligarh Muslim University, Aligarh, 202002, UP, India c CFisUC, Department of Physics, Universidade de Coimbra, Rua Larga, 3004-516, Coimbra, Portugal article info Article history: Received 13 March 2018 Received in revised form 5 June 2018 Accepted 5 June 2018 Keywords: PT complex SCXRD DFT calculation 2-Amino-4-hydroxy-6-methylpyridine and salicylic acid abstract The proton transfer (PT) complex of 2-amino-4-hydroxy-6-methylpyrimidine (AHMP) with salicylic acid (SA) has been synthesized and their crystal has been grown by slow evaporation technique. Further, the crystal has been investigated by single crystal X-ray diffraction (SCXRD). The complex crystallizes in the orthorhombic centrosymmetric space group Pbca. The vibrational spectra of PT complex and its con- stituents have been analysed in the solid phase. UVeVis and 1 HNMR spectra of PT complex and its constituents (SA and AHMP have been studied in solution phase. Thermal stability of PT complex has been examined by thermal gravimetric and differential thermal analysis (TG/DTA). The intermolecular interactions and their contribution to the crystal formation have been analysed with the help of 3D Hirshfeld surface and 2D fingerprint plot. The theoretical calculations of the PT complex have been carried out using DFT/TD-DFT methods to support the experimental results as well as to explore various molecular properties such as frontier molecular orbitals, natural atomic charges, chemical reactivity and molecular electrostatic potential mapping (MEP). © 2018 Elsevier B.V. All rights reserved. 1. Introduction PT complexes are gaining considerable interest in the research from last decade, due to their wide potential applications in the field of pharmaceutical science [1], material science [2], bio- electrochemical energy transfer process [3], biological science [4], optoelectronic, optical communication [5,6], organic semi- conductor [7], and DNA binding, antibacterial and antifungal ac- tivity [8]. Pyrimidine and its derivatives are known for their biological and pharmaceuticals importance. These are nitrogen- containing heterocyclic compound and belong to nucleic acid family [9, 10]. Pyrimidine derivatives have exhibited antibacterial, antifungal and anti-HIV activity [11 , 12]. While, SA is widely used in organic synthesis, plant growth regulator, as a preservative in food products, antiseptic, anti-fungal agents [13, 14] and in the many skin care product for acne treatment, psoriasis, calluses, corns, keratosis pilaris and warts [15, 16]. These wide range importances of AHMP and SA have motivated to synthesize their PT complex. Their single crystal has been grown by slow evaporation technique and it was investigated by SCXRD analysis. The PT complex was further investigated by quantum chemical calculations (DFT/TD-DFT) and various experimental techniques (FTIR, UVeVis, 1 H NMR, TG/DTA). The characteristic absorption bands appeared in the FTIR of PT complex have been assigned with great accuracy using animated modes and simulated IR spectrum. SCXRD technique and FTIR, 1 H NMR and UVeVis spectra are used to confirm the present PT complex formation. Hirshfeld surface analysis is utilized to study the contribution of the intermolecular interactions and void field space in the crystal. Thermal behaviour of the studied crystal is analysed by TG/DTA. Moreover, frontier molecular orbital energies, electrostatic potential, natural atomic charge and reactivity parameter have been discussed. * Corresponding author. E-mail addresses: afaqahmad3@gmail.com, zia19a@gmail.com (A. Ahmad). Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc https://doi.org/10.1016/j.molstruc.2018.06.020 0022-2860/© 2018 Elsevier B.V. All rights reserved. Journal of Molecular Structure 1171 (2018) 438e448