Raman and infrared spectra, novel normal coordinate analysis, vibrational assignments and DFT calculations of hybrid perovskites: trimethyl sulfonium lead halides

Abstract The derivatives of hybrid perovskites, (CH 3 ) 3 SPbI 3-x Br x and (CH 3 ) 3 SPbI 3-x Cl x , where x = 0, 1, 2, and 3 have been synthesized. Aided by Density Functional Theory (DFT) computations employing B3LYP and B3PW91 methods, the molecular geometry was initially optimized for the isolated trimethyl sulphonium ion, [CH 3 ) 3 S] + , up to 6-311++G(d,p) basis. However, combined basis sets employing both 6-31G(d) and 6-311 + G(d) was combined with LANL2DZ, in case of anions that contain(s) “lead and iodine heavy atoms”. The optimized structures at equilibria for [CH 3 ) 3 S] + , [PbCl 3 ] – , [PbBr 3 ] – , [PbI 3 ] – ions were restricted to C 3v point group, while the anions, [PbI 2 Cl] – , [PbI 2 Br] – , [PbICl 2 ] – , [PbIBr 2 ] – obey C s symmetry followed by frequency calculations at the same basis set. Otherwise, combined basis set were used for ions containing Pb, Cl and Br. The already reported Raman (10–3,500 cm −1 ) and infrared (200–4,000 cm −1 ) spectra of perovskite samples in the solid phase were analyzed based on estimated frequencies, normal coordinate analysis (NCA) and potential energy distributions (PEDs). All observed bands were correlated to the appropriate normal modes of vibration. The computed structural parameters (Ps) were in good agreement with those reported from X-ray single crystal. Minor deviations in the X–Pb–X bond angles were observed, indicating that the unit has been isolated as a distorted face-sharing [PbI 6 ] polyhedral chain extending one-dimensionally along the c-axis, rather than as a monomeric molecule. Aided by potential surface scan, methyl barriers to internal rotations of 674–810 cm −1 (1.93–2.32 kcal/mol) and 652–790 cm −1 (1.86–2.26 kcal/mol) were estimated from B3LYP and B3PW91 at 6-311++G(d,p) basis set, respectively. All results presented herein are compared with similar molecules whenever appropriate.