1637 Pure Appl. Chem., Vol. 83, No. 8, pp. 1637–1641, 2011. doi:10.1351/PAC-REC-10-01-02 © 2011 IUPAC, Publication date (Web): 8 July 2011 Definition of the hydrogen bond (IUPAC Recommendations 2011)* Elangannan Arunan 1,‡ , Gautam R. Desiraju 2 , Roger A. Klein 3 , Joanna Sadlej 4 , Steve Scheiner 5 , Ibon Alkorta 6 , David C. Clary 7 , Robert H. Crabtree 8 , Joseph J. Dannenberg 9 , Pavel Hobza 10 , Henrik G. Kjaergaard 11 , Anthony C. Legon 12 , Benedetta Mennucci 13 , and David J. Nesbitt 14 1 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India; 2 Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India; 3 30 Kimberley Road, Chesterton, Cambridge, CB4 1HH, UK; 4 Department of Chemistry, Laboratory of Intermolecular Interactions, University of Warsaw, Warsaw, PL-02093, Poland; 5 Department of Chemistry and Biochemistry, 0300 Old Main Hall, Utah State University, Logan, UT 84322, USA; 6 Medicinal Chemistry Institute, Juan de la Cierva 3, Madrid E-28006, Spain; 7 Department of Physical and Theoretical Chemistry, Oxford University, South Parks Road, Oxford OX1 3QZ, UK; 8 Department of Chemistry, 225 Prospect Street, Yale University, New Haven, CT 06511-8499 USA; 9 Department of Chemistry and Biochemistry, City University of New York, Hunter College, 695 Park Avenue, New York, NY 10065, USA; 10 Institute of Organic and Biochemistry, Academy of Sciences of Czech Republic, Flemingovo nam 2, Praha CZ 16610, Czech Republic; 11 Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark; 12 School of Chemistry, University of Bristol, Bristol BS8 1TS, UK; 13 Department of Chemistry, University of Pisa, Via Risorgimento 35 Pisa 1-56125, Italy; 14 Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA Abstract: A novel definition for the hydrogen bond is recommended here. It takes into account the theoretical and experimental knowledge acquired over the past century. This def- inition insists on some evidence. Six criteria are listed that could be used as evidence for the presence of a hydrogen bond. Keywords: bonding; charge transfer; hydrogen bond; IUPAC Physical and Biophysical Chemistry Division; molecular interactions; noncovalent interactions. 1. PREAMBLE The task group recommends the definition given here for the hydrogen bond. The short definition is fol- lowed by a list of experimental and/or theoretical criteria, which can be used as evidence for the pres- ence of the hydrogen bond. Finally, some characteristics that are typical of hydrogen-bonded systems *Sponsoring body: IUPAC Physical and Biophysical Chemistry Division: see more details on p. 1640. ‡ Corresponding author: E-mail: arunan@ipc.iisc.ernet.in