Structure, vibrational, electronic, NBO and NMR analyses of 3-methyl-2,6-diphenylpiperidin-4-one (MDPO) by experimental and theoretical approach G. Velraj a,⇑ , S. Soundharam b , C. Sridevi c a Department of Physics, Periyar University, Salem 636011, Tamil Nadu, India b Department of Physics, Selvamm Arts and Science College (Autonomous), Namakkal 637003, Tamil Nadu, India c Department of Physics, Sona College of Technology, Salem 636005, Tamil Nadu, India highlights  The optimized geometrical parameters of MDPO were calculated by DFT method.  Atomic charges, NBO and MEP of MDPO were analysed.  The reactivity of the MDPO was described by global descriptors.  The title compound exhibits good NLO activity.  NMR chemical shifts of MDPO were calculated by GIAO method. article info Article history: Received 28 July 2013 Received in revised form 25 October 2013 Accepted 4 December 2013 Available online 21 December 2013 Keywords: 3-Methyl 2,6-diphenyl piperidin-4-one DFT MEP NBO NLO NMR abstract The optimized molecular struture,vibrational wavenumbers, atomic charges, molecular electrostatic potential, NBO, electronic properties, NLO, 1 H NMR and 13 C NMR chemical shifts of molecule 3- methyl-2,6-diphenylpiperidin-4-one (MDPO) have been studied in the best way in this investigation. Theoretical calculations were performed by density functional theory (DFT)/B3LYP method using 6- 31+G(d,p) basis set. The results show that the vibrational wavenumbers and chemical shifts calculated were obtained to be in a good agreement with the experimental data. The chair conformation of the piperidine ring was deduced by structural parameters and NMR analysis. The stability and charge delo- calization of the molecule were studied by natural bond orbital (NBO) analysis. In addition, a molecular electrostatic potential map (MEP) of the title compound has been analysed for predicting the reactive sites. The reactivity of the compound of interest was described by HOMO–LUMO energies and global descriptors. The electronic properties were also discussed and the transition corresponds to p ? p  . The title compound exhibited good nonlinear optical activity and nine times greater than that of urea. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Piperidine is one of the most recognizable structural entities among heterocyclic molecules [1]. Piperidine has a six-membered ring similar to cyclohexane with one methylene (CH 2 ) replaced by secondary amino (NH) group. Many piperidine natural products with substitution on nitrogen and carbons are frequently encoun- tered among lycine-derived alkaloids. For example homoproline [2], this is related to the essential amino acid proline [3]. Pseud- oconhydrine [4], sedamine [5], b-conhydrine [6] are some of the examples for piperidines with substitution on C2. Dihydropinidine [7], solemopsine [8], deoxoprosopinine [9] are a few examples of piperidine with substitution on 2 and 6 positions. Well-known hal- lucinogen morphine [10] also contains piperidine ring. Piperidine series c-secretase inhibitors have been evaluated for treatment of Alzheimer’s disease (AD) [11]. Some piperidines were found to possess high profile biological activities, including cyto- toxic and anticancer properties [12]. The piperidine ring is a fea- ture of oral anaesthetics and narcotic analgesics [13,14]. Piperidine derivatives are used clinically to prevent post-operative vomiting, to speed up gastric emptying before anaesthesia, to facilitate radiological investigations and to correct variety of dis- turbances of gastrointestinal functions [15]. Several 2,6-disubsti- tuted piperidines were found to be useful, as tranquilizer and some possess hypotensive activity [16] and a combination of stimulant and depressant effects on the central nervous system and herbicidal activities [17]. The 2,6-disubstituted piperidines 0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2013.12.040 ⇑ Corresponding author. Tel.: +91 427 2345766; fax: +91 427 2345124. E-mail address: gvelraj@yahoo.co.uk (G. Velraj). Journal of Molecular Structure 1060 (2014) 156–165 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc