Indones. J. Chem., 2018, 18 (1), 173 - 178 Sitti Rahmawati et al. 173 DOI: 10.22146/ijc.25170 * Corresponding author. Email address : ama_annisa@students.itb.ac.id Density Functional Theory (DFT) and Natural Bond Orbital (NBO) Analysis of Intermolecular Hydrogen Bond Interaction in "Phosphorylated Nata de Coco - Water" Sitti Rahmawati * , Cynthia Linaya Radiman, and Muhamad Abdulkadir Martoprawiro Inorganic and Physical Chemistry Division, Faculty of Mathematics and Natural Sciences, Bandung Institute of Technology, Jl. Ganesha 10, Bandung 40132, Indonesia Received May 25, 2017; Accepted July 8, 2017 ABSTRACT This study aims to study the conformation, the hydrogen bond network, and the stabilities of all the possible intermolecular interactions in phosphorylated nata de coco membrane with water (NDCF-(H2O)n, n = 1-5). Analysis of Natural Bond Orbital (NBO) was performed to measure the relative strength of the hydrogen bonding interactions, charge transfer, particularly the interactions of n-σ * O-H and to take into account the effect on the stabilities of the molecular structure. All calculation were performed using Density Functional Theory (DFT) method, at B3LYP functional level of theory and 6-311 G** basis set. The charge transfer between the lone pair of a proton acceptor to the anti-bonding orbital of the proton donor provides the substantial to the stabilization of the hydrogen bonds. Interaction between NDCF and (H2O)5 was strongest with the stabilization energy of 37.73 kcal/mol, that indicate the ease of donating lone pair electrons. The contributions of each hydrogen bond to the stability of the complex have been analyzed. Keywords: proton transfer; hydrogen bonds; phosphorylated nata de coco; NBO ABSTRAK Penelitian ini bertujuan untuk mempelajari konformasi, jaringan ikatan hidrogen, dan stabilitas dari semua interaksi molekul yang mungkin, antara membran nata de coco terfosfatasi dengan air (NDCF-(H2O)n, n = 1-5). Perhitungan menggunakan Fungsional B3LYP dengan basis set 6-311 G**. Analisis menggunakan Teori ikatan orbital alami (Natural Bond Orbital, NBO) dilakukan untuk mengukur kekuatan relatif dari interaksi ikatan hidrogen, transfer muatan (khususnya interaksi n-σ* O-H) dan untuk memperhitungkan efeknya pada stabilitas struktur molekul. Transfer muatan antara pasangan elektron bebas (lone pair) dari akseptor proton ke orbital anti ikatan dari donor proton memberikan stabilisasi besar terhadap ikatan hidrogen. Interaksi antara NDCF dengan (H2O)5 ditemukan paling kuat dengan energi stabilisasi 37,73 kkal/mol, menandakan kemudahan dalam mendonorkan lone pair elektron. Kontribusi dari masing-masing ikatan hidrogen untuk stabilitas kompleks NDCF-(H2O)n telah dianalisis. Kata Kunci: transfer proton; ikatan hidrogen; nata de coco terfosfatasi; NBO INTRODUCTION The human needs, especially for energy, are increasing over the time in line with the increasing number of population and their life development. The energy needed is no longer met by the natural resources available. Since the fulfillment of the energy needed still relies on natural resources originated from fossil. The limitation of fossil resources availability in nature and its problem of renewability insist them to find other alternative energy sources. A new kind of energy which has a great potential to substitute fossil fuel is fuel cell. Fuel cell technology has been developed worldwide in recent years since it has excellences such as to be highly efficient and environmentally-friendly energy source (low emission). Fuel cell is a kind of alternative energy sources working on the principles of electrochemical cell, which can directly change the chemical energy contained in the fuel like hydrogen to be electrical energy [1-3]. Many kinds of fuel cells have been developed. The most very promising fuel cell which is very efficient, environmentally friendly, and using polymer membrane as its electrolyte is PEMFC (Proton Exchange Membrane Fuel Cell) [4-7]. The membrane is functioning to transport cations are hydrogen ions (protons) in an electrolyte membrane from anodes to cathodes. But the membrane cannot transport any electron [8-9]. Nowadays, Nafion is an electronic membrane which is widely used for PEMFC. Unfortunately, Nafion has some limitations such as, among others, spending expensive cost (600–1000