Studying the Effect of Temperature on the Formation of Hydrogen

Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

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Studying the Effect of Temperature on the Formation of Hydrogen Bond Dimers: A FTIR and Computational Chemistry Lab for Undergraduate Students P. G. Rodríguez Ortega,* M. Montejo, M. S. Valera, and J. J. López González Department of Physical and Analytical Chemistry, Faculty Experimental Sciences, University of Jaén, E-23071 Jaén, Spain

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S Supporting Information *

ABSTRACT: This laboratory experiment allows one to study the effect of temperature in the process of formation of hydrogen bound cyclic dimers of benzoic acid (BA). The implementation of the proposed methodology, which comprises the use of FTIR spectroscopy and quantum chemistry calculations, enables the obtaining of the thermodynamic parameters of the dimerization reaction and their subsequent validation by computational chemistry calculations. Hence, the students are involved in a practical learning scheme focused in the study of a central topic in chemistry, such as hydrogen bonding, with the final purpose being to train chemistry students in applying theoretical-experimental approaches to extract meaningful data and valuable information for explaining experimental observables. KEYWORDS: Upper-Division Undergraduate, Physical Chemistry, Computational Chemistry, Hydrogen-Bonding, IR Spectroscopy

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range 17−61 kJ mol−1,14 which, according to the classification reported by Desiraju,15 would correspond to strong HBs). The experimental structural characterization of a HB can be performed by NMR, XRD, and/or spectroscopic techniques (e.g., UV−visible, IR, MW). Among these, Fourier transform infrared spectroscopy (FTIR) provides the opportunity to get insight into the structural features of the species being studied as it relies on the selective absorption by the sample of IR radiation at certain wavelengths (energies) for particular vibrational motions. The characteristics (intensity, profile, wavenumber) of the bands appearing in the IR spectra give exquisite information about the structure of the sample (identification of specific functional groups and arrangements), its conformational landscape and, also, its environment when the focus is on specific vibrations that are particularly sensitive to inter- or intramolecular interactions. Such is the case of the carbonyl stretching (ν CO) band, for which shape, intensity, and wavenumber change as a consequence of hydrogen bond formation. Thus, FTIR spectroscopy is a powerful and accessible tool for exploring the structure of hydrogen bonded systems and allows the study of the different solution-phase hydrogen bonded states of a molecule.16−20 The above-mentioned importance of HB in such a vast variety of research areas makes of it a relevant concept that should be studied in-depth in any Chemistry curricula. As so,

INTRODUCTION Hydrogen bonding (HB) is a central topic in many different scientific disciplines including chemistry, biology, biophysics and material science.1,2 The importance of hydrogen bonding in these areas is reflected in the many published research works that can be found in the literature devoted to the study of the HB characteristics, energetics and structural implications either by experimental or theoretical methods. This specific type of bonding is the underlying cause of the surprisingly high boiling and melting points of compounds intermolecularly connected through HB compared with analogues where such interactions are not possible, with the water molecule being a paradigmatic example. HB not only determines the properties of water,3 but also the structure of proteins4,5 and the DNA molecule,6 the crystallization outcome of crystalline materials,7−9 the properties of polymeric products2 and the structure and function of supramolecular assemblies.10 They are formed when a hydrogen covalently bonded to an electronegative atom interact (through Coulombic, dispersive and even covalent interactions11) with another electronegative atom in a different molecule (forming an intermolecular HB) or in a different part of the same molecule (forming an intramolecular HB). Typically, this intermolecular cohesive interaction is less than 20−25 kJ mol−112 and provides a high stabilization to the systems. However, due to cooperative effects,13 the associated energies to the HB formation in some systems can be much higher (e.g., in OH and NH containing crystalline solids HBs are within the © XXXX American Chemical Society and Division of Chemical Education, Inc.

Received: March 19, 2019

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DOI: 10.1021/acs.jchemed.9b00237 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

not only the number of scientific researches concerning this topic is immense, but also the number of educative proposals devoted to its study at undergraduate level using NMR,21 UV− visible22 and/or FTIR23 spectroscopies. Moreover, is worth mentioning that the combined used of FTIR and computational methods is becoming a popular approach in many undergraduate lab procedures.24−28 Furthermore, the use of FTIR spectroscopy instrumentation is common and widespread in the undergraduate chemistry lab since it is relatively easy to handle by the students and can be used in large group activities as the typical measurements require less than 2−3 min. In this experiment, students investigate experimentally the dimerization of benzoic acid (BA, eq 1) in acetonitrile at different temperatures which enables the estimation of the thermodynamic state functions, ΔH°, ΔS°, and ΔG° of the reaction process by probing the carbonyl stretching band at ∼1700 cm−1. Our proposal consists of an experimental handson that comprises the determination of the dimerization constant (KD) at different temperatures, by implementation of a concentration dependent FTIR based methodology reported elsewhere,16 and the subsequent application of the van’t Hoff equation. The experience is completed with the implementation of quantum chemistry calculations. Thus, the analysis of the optimized geometries of BA dimer and monomer allow the students to reach a deeper understanding of the hydrogen bonding formation process through the study of the geometric changes that the monomeric units undergo during dimerization and to rationalize the impact of these changes in the vibrational frequencies of the carbonyl moieties. Finally, in silico estimation of the thermodynamic parameters of the reaction give support to the values determined experimentally. In the following sections, the theoretical background of the experiment will be described, followed by an outlining of the experimental procedure, hazards, and materials. The results obtained by one group of students are presented in the final section, together with the conclusions where we briefly summarize the concepts being worked in this lab procedure as well as the student response.

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Figure 1. Experimental IR spectrum recorded for a 0.5 M solution of BA in CH3CN at 22 °C: Pm = monomer peak; Pd = dimer peak.

absorbance of the vibrational band assigned to the carbonyl stretching of the monomeric form, i.e., Pm. Thus, according to the methodology reported elsewhere,16 the experimental data obtained must fit the equation: 2KD c0 1 = + εmlA m A m2 (εml)2

where c is the analytical molar concentration of the sample (mol L−1), Am is the integrated absorbance of the monomer band, εm is the molar absorptivity of the monomeric species, and l is the path length used during measurements. A related IBL experience integrating the above-mentioned methodology and focused on the dependence of the equilibrium constant with the polarity of the medium has been recently reported.29 After obtaining a set of KD values at different temperatures, the thermodynamic state functions of the dimerization process can be obtained by implementation of the van’t Hoff equation: ΔH ° ΔS ° + (3) RT R which allows, by plotting ln KD values vs 1/T, one to estimate the changes in enthalpy (ΔH°) and the entropy (ΔS°) of the association process. The sign of ΔH° informs about the strength of the hydrogen bond and, together with the ΔS° value, will allow to determine the Gibbs free energy, ΔG°, associated with the formation of dimers of BA in the medium, value that informs about the thermodynamic spontaneity of the process. ln KD = −

THEORETICAL BACKGROUND

When benzoic acid (BA) is dissolved in moderately low polar solvents, the concomitant presence of monomers and hydrogen bound dimers can be deduced from the analysis of the FTIR spectrum of the solutions in the ν CO stretching region that will show two different peaks separated by ca. 20 cm−1 (Figure 1) corresponding to the CO stretching of the monomer (Pm, at higher energy) and the dimer (Pd, at lower energy). The merely subjective analysis (through visual inspection) of the relative intensities of these two bands can be used to roughly estimate the relative fractions of monomers and dimers in the medium. A more thorough analysis of the integrated absorbances of these two bands will allow one to estimate the thermodynamic constant of the equilibrium of the dimerization, according to M + M V D;

KD =

[D ] [M ]2

(2)

0

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EXPERIMENTAL PROCEDURE

Experimental Measurements

Solutions of BA were prepared in acetonitrile at concentrations 0.50, 0.45, 0.40, 0.35, 0.30, and 0.25 M in glass vials. IR spectra were acquired using a Jasco FVS 6000 FTIR/VCD spectrometer, equipped with a ceramic Globar source and an MCT detector, by using its thermostatic sealed cell (50 μm path length, equipped with BaF2 windows) and temperature controller (TC) for the collection of the spectra at different temperatures. Spectra were obtained in absorbance mode, with 50 scans averaged and a resolution of 4 cm−1. All measurements were acquired with a continuous N2 flow to ensure water had been removed from the FTIR and allowing the samples to reach thermic equilibrium prior to scanning (each

(1)

where KD is the equilibrium constant. This constant can be measured at any given temperature by carrying out a concentration-dependent FTIR study that involves the experimental determination of the integrated B




level Chemistry degree; 6−8 students per group), dedicated to both experimental work in the Vibrational Spectroscopy lab and performing the theoretical study through electronic structure calculations. This lab can be completed in a 2 week period with a total in-class and lab time of 8 h (four sessions of 2 h each). Four computers and a FTIR spectrometer were available to the students, which were distributed so that while one group was using the FTIR spectrometer, other students were compiling and analyzing results from computational calculations. Each group needed approximately 2 h to collect all the spectra. A comprehensive guide was facilitated to students to ease the manipulation of the Gaussian output files using the Gaussview program. See the Supporting Information for further suggestions and details to the instructor.

IR spectrum were recorded 5 min after the TC checked the established temperature). Spectra were collected using the Jasco Spectra Measurement software package (version 2.14.06) over the range 900−2000 cm−1. Computational Calculations

Geometries optimizations and frequencies calculations were performed on the BA monomer and dimer (Figure 2) using

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HAZARDS Students are required to work exclusively in the hood and to wear personal protective equipment: protective clothing, protective gloves, and safety goggles. Benzoic acid: corrosion (GHS05) and health (GHS08) hazards; causes skin irritation (H315), causes serious eye damage (H318), and causes damage to organs through prolonged or repeated exposure (H372). Acetonitrile: flammable (GHS02) and GHS08 hazards; highly flammable liquid and vapor (H225), H302, H319. In case of swallowing, inhaling, and/or contact with eyes, immediately call a poison center, doctor, or physician, remove contact lenses (if applicable), and rinse cautiously with water for several minutes.

Figure 2. Optimized structures (DFT/B3LYP/aug-cc-pVDZ) of the (a) benzoic acid monomer and (b) benzoic acid dimer.

Gaussian16,30 and the results were visualized with Gaussview6. Other computational programs are freely available to calculate these properties and so this part of the experiment can be adapted to the resources available. A density functional theory (DFT)31 based method, B3LYP,32 was chosen for computational calculations because of its good accuracy/computation cost (time) relation and because of its known reliability in predicting vibrational frequencies. It is important to note that the correct modeling of the hydrogen bonded motif requires the use of polarization and diffuse functions; thus, the basis set selected was Dunning’s correlation consistent basis set called aug-cc-pVDZ.33 The monomer and dimer structures were first modeled using Gaussview 6.0 and subjected to geometrical optimization at the above-mentioned level of theory. The subsequent calculation of their vibrational frequencies (that yielded no imaginary values) allowed the confirmation of each structure as true minima on their respective potential energy surfaces (PES).34 The optimized structures were further subjected to geometry optimization and frequency calculation in a dielectric continuum medium (IEF-PCM)35 using the dielectric constant of acetonitrile (ε = 37.5) to simulate the solvent environment employed in the experiment. Considering the computation times required for the required levels of calculations and in order to optimize resources, it could be convenient to the instructor to carry out the calculations beforehand and simply provide the students with the files containing the optimized molecular geometries and the harmonic vibrational frequencies calculations for the monomer and dimer, ready to be inspected in Gaussview or any other software (see the Supporting Information). From the calculations performed, students were asked to compile structural information (mainly carboxylic bond lengths), thermodynamic magnitudes, and vibrational frequencies associated with the carboxylic group.

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MATERIALS AND EQUIPMENT Benzoic acid (BA, CAS # 65-85-0, ACS reagent, ≥99.5%) and acetonitrile (CAS # 2206-26-0) were purchased from SigmaAldrich and used without further purification. The measurements were carried out on a Jasco FVS 6000 spectrometer equipped with a ceramic Globar source and MCT detector and using its thermostatic cell (50 μm equipped with BaF2 windows) and temperature controller for the collection of the spectra at different temperatures. Data processing was carried out by using OPUS software v. 7.2. Full details of material and equipment needed to implement the activity can be found in the Supporting Information. Electronic structure calculations were performed using Gaussian16.30

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RESULTS AND DISCUSSION The experimental measurement of the enthalpy (ΔH°) and entropy (ΔS°) of the BA dimer formation reaction was performed after determining the dimerization constants (KD) in a range of increasing temperatures (17, 22, 27, 32, 37, and 42 °C). To this end, the IR spectra of a set of BA solutions in CH3CN with concentrations ranging 0.25−0.5 M were recorded using a thermostatic cell. The IR spectra of the samples showed two well differentiated peaks which relative intensities got balanced as the BA concentration increased. The theoretical IR spectra calculated were used to confirm the vibrational assignments of those observed bands to the monomer and the hydrogen bound cyclic dimer of the species. Besides, the calculated geometries for the monomer and dimer permit explaining the shift observed in the ν C = O stretching band for each species (see Table S1 in the Supporting Information) on the basis of the modification that the formation of a hydrogen bond induces in

Implementation of the Laboratory

This laboratory was facilitated in four separated sessions of the course entitled “Applied Vibrational Spectroscopy” (upperC




the electronic structure of the carbonyl group and, hence, in the C = O bond distance. The set of spectra registered for the sample in the 1800− 1600 cm−1 is reported in Figure S1 in the Supporting Information. The comparison of the IR region of interest recorded at different temperatures for a given BA concentration allowed a preliminary analysis of the temperature effect on the dimer formation process. Thus, as shown in Figure 3,

Table 1. Dimerization Constant (KD) of Benzoic Acid in CH3CN at Different Temperaturesa T (K)

KD

290.15 295.15 300.15 305.15 310.15 315.15

0.619 0.532 0.510 0.436 0.383 0.327

Spectral fitting corresponding to the determination of each KD are available as in the Supporting Information (Figure S3).

a

Figure 3. IR spectra of a series of 0.3 M solutions of BA in CH3CN recorded at T = 290.15 K (black), T = 295.15 K (green), T = 300.15 K (red), T = 305.15 K (violet), T = 310.15 K (cyan), and T = 315.15 K (orange).

Figure 4. Theoretical (B3LYP/aug-cc-pVDZ/IEF-PCM) IR spectra of monomeric and dimeric species in a 0.5 M BA solution in CH3CN in the 1800−1600 cm−1 region. Lorentzian band-shapes, FMWH = 8 cm−1. The spectra are calculated considering the monomeric (84%) and dimeric (16%) fractions of BA according to its KD at 22 °C.

while the shapes and wavenumbers of the carbonyl stretching bands of monomer and dimer are not significantly altered, a steady decrease on the absorbance of the dimer’s peak is observed as the temperature increases that is accompanied by an increment in the absorption of the monomer band. These observations are congruent with a change in the conditions of the dimerization equilibrium of BA: as the sample is heated the number of H-bound dimers decreases while the concentration of free BA units increases. To quantify the observed effect, the region was Lorentzian fitted and deconvoluted using Opus v. 7.2 which allowed to measure the integrated absorbances of the monomeric band (Am) at each concentration. Table S2 collects Am values obtained for different concentrations of BA in CH3CN at each temperature. These data were used to plot c0/Am2 against 1/Am according to eq 2, and the dimerization constants (KD) were obtained from the slope of the plot (inner graphs in Figure S1). Table 1 collects the calculated values for KD at different temperatures. Data reported in Table 1 give further evidence of the assumption made by the subjective analysis of the evolution of the monomeric and dimeric peaks as a function of temperature: the magnitude of the dimerization constant decreases at higher temperatures. The value of KD also permits the estimation of the monomeric (f m) and dimeric (fd) fractions present in solution at a given temperature. Table S3 reports f m and fd values at 22 °C for BA in acetonitrile. These data can be used to theoretically simulate the IR spectra of BA as shown in Figure 4. This theoretical spectrum nicely reproduce the experimental IR profile recorded for a 0.5 M solution in CD3CN at 22 °C

(Figure 1), hence confirming the accuracy of the estimated monomeric and dimeric fractions through KD. Finally, as shown in Figure 5, a linear relation between the ln KD and 1/T (K−1) values was found that, according to eq 3, permitted estimating the change in enthalpy associated with

Figure 5. Relationship between ln KD and 1/T. Red dots represent the experimental data, and the solid line represents the linear fit. D




the dimer formation reaction, ΔH° = −18.80 kJ mol−1, from the slope of the least-squares fitting of the data (b = 2262.508 K). This value lies within the expected range values for hydrogen bound dimers heat of formation, that according to the scientific literature is ca. 15 kJ mol−1 for other species in organic solvents.36 This analysis also provides the change in entropy associated with the dimerization process, which is obtained from the value of the line intercept on the vertical axis: ΔS° = −68.70 J K−1 mol−1. The change in the Gibbs free energy for the dimer formation reaction of BA in CH3CN at 25 °C thus yields a value of 1.67 kJ mol−1. At this point a comparative theoretical-experimental analysis of the thermodynamics properties for the dimerization of BA in acetonitrile was carried out. Data reported in Table 2 show that theoretical results reproduce the experimental data reasonably.

Finally, since the sign and value of ΔG° can be interpreted as a measurement of the thermodynamic spontaneity of the dimer formation reaction, the obtained value reinforces the observation of the tendency of the monomeric forms of BA to persist isolated in acetonitrile solution that can also be inferred from the values obtained for KD.

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CONCLUSIONS Benzoic acid (BA) is an affordable reagent that readily forms hydrogen bonded dimers in solution. In this laboratory, the BA monomer and dimer were used as a model to study the thermodynamic properties of hydrogen bonding as a function of temperature for this species in acetonitrile solution using a combined theoretical-experimental approach. The experimental study was accomplished through a FTIR temperaturedependent analysis of the carbonyl stretching band for the species using a thermostatic cell. The theoretical analysis implied performing geometry optimizations for the monomeric and dimeric forms of BA in CH3CN. Some of the concepts covered by this laboratory are Ef fect of hydrogen bond in the IR spectrum. This experiment shows the variations induced by H-bonding in the IR spectrum of a sample prone to form H-bonded dimers, with monomeric and dimeric peaks easily detectable in the carbonyl stretching region. Ef fect of hydrogen bond in the vibrational f requency of the carbonyl group and its connection with the electronic structure of the dimeric and monomeric species. The theoretical-experimental study of the carbonyl stretching bands for the monomer and dimer of BA shows that the formation of an intermolecular Hbond produces an elongation of the CO bond length which is reflected in a red-shift of its stretching motion. This elongation relates to electron density withdrawing from the carbonyl oxygen by the hydrogen atom that depolarizes the CO σ bond and, in compensation, polarizes the CO π bond making the carbonyl bond longer and weaker. Ef fect of temperature in the equilibrium constant. This experiment shows that monomeric and dimeric fractions at a given concentration vary with temperature. This is reflected by the value of the corresponding dimerization constants that has been calculated in a series of increasing temperatures as well as by the evolution of the relative absorbances for the dimeric band as T increases. Thermodynamic properties of the dimer formation reaction of BA in acetonitrile solution. The experiment shows that the dimerization process of BA in CH3CN is exothermic and ΔG° for the process is positive. This is consistent with the KD determined experimentally as well as with IR profiles registered for the sample at different temperatures and agrees with the theoretical calculations performed for the sample in solution. The computational component of this laboratory favored a higher engagement of the students as compared with other purely experimental laboratories and served them as a fundamental aid to rationalize their experimental observations, showing the great advantage that combining computational protocols with experimental activities suppose for undergraduate students.

Table 2. Experimental and Theoretical (B3LYP/aug-ccpVDZ) Values for the Changes in Enthalpy (ΔH° in kJ mol−1), Entropy (ΔS° in J K−1 mol−1), and Gibbs Free Energy (ΔG° in kJ mol−1) Associated with the Dimerization Process of BA in CH3CN solution ΔH° ΔS° ΔG°

exptl

theor

−18.80 −68.70 1.67

−45.40 −156.30 1.41

Regarding the quantitative theoretical-experimental deviations in the thermodynamic parameters they can find explanation in the limitations of the theoretical approach. A first error source that must be addressed for a more accurate prediction is the Basis Set Superposition Error (BSSE),37 which would cause an overestimation of the stability of the dimeric species (hence, ΔH° values are calculated larger than expected). The so-called Counterpoise38 correction may compensate this error. Besides, the solvation energies for the corresponding monomeric and dimeric species should be considered for a more accurate theoretical prediction of ΔG°.39 In any case, for the purpose of this laboratory experiment, we consider that a qualitative theoretical validation of the experimental data can be enough. It is important to note that the sign of the changes in the thermodynamic state functions of the dimerization process derived from the experiment (and validated by theoretical calculations) are consistent with the underlying chemistry in the hydrogen bound BA dimer formation process and the observed evolution of the dimeric and monomeric peaks as a function of temperature. Thus, the negative sign of ΔH° is consistent with the experimental observations: (i) the absorbance associated with the carbonyl stretching in the dimer species drops steadily as the temperature increases; (ii) the calculated values for the dimerization constants of BA in CH3CN also decreases with the increment in temperature. In summary, the increase in the temperature has caused a reduction in the relative proportion of products (dimers), which is the expected in an exothermic reaction. Moreover, the formation of a dimeric structure is normally associated with an increment in its local order. Thus, the negative value in ΔS° implies that the system increased its order when the hydrogen bond is formed.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.9b00237. E



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Material and equipment, instructor’s notes, student guidelines and handout, tables and figures cited in the text (PDF and DOCX)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

P. G. Rodríguez Ortega: 0000-0002-9705-4528 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.S.V. acknowledges funding from the Spanish Andalusian Government for a contract supporting an internship in the Physical and Analytical Chemistry Department of the University of Jaén.

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