Determining the Energetics of the Hydrogen Bond through FTIR: A Hands-On Physical Chemistry Lab Experiment Abby C. Guerin, † Kristi Riley, † Kresimir Rupnik, and Daniel G. Kuroda* Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States * S Supporting Information ABSTRACT: Hydrogen bonds are very important chemical structures that are responsible for many unique and important properties of solvents, such as the solvation power of water. These distinctive features are directly related to the stabilization energy conferred by hydrogen bonds to the solvent. Thus, the characterization of hydrogen bond energetics has been vital for many areas of science. We present a laboratory experiment for physical chemistry in which the hydrogen bond energetics between methyl acetate and water is investigated by Fourier transform infrared spectroscopy (FTIR). The experiment consists of measuring the temperature dependent IR spectra of methyl acetate to determine the changes in the enthalpy and entropy of making/breaking hydrogen bonds. This experiment aims at providing the students with hands-on experience in the following topics: solution and sample cell preparation, IR spectra collection and analysis, and data modeling and thermodynamic calculations. The overall objective of this experiment is to familiarize chemistry students with a methodology used to extract meaningful and up-to-date physical chemistry properties from real experimental data. KEYWORDS: Upper-Division Undergraduate, Laboratory Instruction, Physical Chemistry, Hands-On Learning/Manipulatives, Computer-Based Learning, Equilibrium, Hydrogen Bonding, Infrared Spectroscopy, Thermodynamics H ydrogen bonds impact many different areas of science, including, but not limited to, chemistry, biology, and biophysics. 1, 2 In particular, they play a major role in determining the properties of water 3 and defining the structure of proteins 4,5 and DNA, 6 and recently, they have been used for the design of large molecular assemblies in supramolecular 7 and polymer chemistry. 8 Thus, the hydrogen bond is a very important concept that should be present in any college Chemistry curricula. The formation of a hydrogen bond occurs when a hydrogen covalently bonded to an electronegative atom, such as oxygen, interacts with another electronegative atom. This interaction confers a stabilization energy to the molecular system of a few kilojoules per mole, but not greater than 25 kJ/mol. 9 The formation and rupture of a hydrogen bond occurs on an ultrafast time scale, i.e., on the order of a few picoseconds (10 -12 s). 10 The fast interchange dynamics complicates the characterization of the hydrogen bond by most conventional techniques, such as NMR. 9,11,12 Methodologies relying on the absorption of light are not limited to any particular dynamic time scale because they quantify the different species in a sample by measuring the number of photons that they absorb. 13 However, the drawback of light absorption method- ologies is that most of them do not provide structural information about the species being detected. Fourier transform infrared spectroscopy (FTIR) is a spectroscopic methodology that measures the infrared light absorbed by a sample 14 as a consequence of their molecules being promoted from vibrational ground states to vibrational excited states. 15 Some vibrational modes, such as the carbonyl stretch, are well localized modes, allowing one to investigate confined regions of the molecule. In addition, vibrational modes are very good probes of the molecular environment because their associated vibrational frequency is sensitive to the different interactions with the environment. 16 In particular, vibrational modes are very susceptible to the formation of hydrogen bonds because hydrogen bonds significantly alter the electronic structure of the molecular system and, consequently, their associated vibrational transitions. 17 The effect of the hydrogen bond on vibrational transitions is directly observed in the IR spectrum as a shift of the central frequency of the vibrational transition. 17 Thus, IR spectroscopy can be used to measure the different hydrogen bonded states of a molecule. However, it has been very difficult to unequivocally assign the different peaks in the IR spectrum to the different vibrational modes due to the presence of other vibrational transitions, such as those arising from overtones and combinational modes. 18 Recently, the introduction of nonlinear ultrafast IR spectros- copy has allowed the interpretation and assignment of the peaks observed in the IR spectrum to specific vibrational Received: December 17, 2015 Revised: April 8, 2016 Laboratory Experiment pubs.acs.org/jchemeduc © XXXX American Chemical Society and Division of Chemical Education, Inc. A DOI: 10.1021/acs.jchemed.5b01014 J. Chem. Educ. XXXX, XXX, XXX-XXX