Part 1: of 2

The thermodynamic quantities associated with the formation of a hydrogen bond between chloroform and triethylamine are investigated computationally using MOPAC and experimentally via temperature-dependent nuclear magnetic resonance. The two methods provide complementary information about the weak hydrogen bond. Computational results predict a vibrational signature of the hydrogen bond which is observed experimentally.

Temperature effects in equilibrated chemical systems: Hydrogen Bonding

Under conditions of thermodynamic equilibrium, varying temperature will change relative concentrations in a reaction mixture, from which the equilibrium constant, Keq, can be determined. Using the temperature dependence of Keq, the energy changes for the process - H,S, andG - are calculated.

Molecular modeling can predict energy changes in chemical processes and predict spectroscopic signatures of molecular species.

Temperature dependent NMR provides an excellent means to investigate equilibrium in systems which under go rapid exchange. The observed spectra display a weighted average of the species in solution.

Hydrogen bonding is a fundamental attractive intermolecular interaction that exhibits rapid exchange. The specific system investigated in this experiment was chloroform and triethylamine.

Experimental Details

Instrument

200MHz H-1 and C-13 NMR spectrometer with Temperature control

Varian Gemini 2000

Computational Software

CAChe Scientific Molecular editor and visualizer interface to MOPAC.

Computational Hardware

Apple Power Macintosh 7100

Modeling Methodology

Using MOPAC structural optimization, the reaction enthalpy is determined by first calculating formation enthalpy separately for each reactant and product. Using Hess's law, the difference was taken to estimate the reaction enthalpy. With this technique MOPAC was used to estimate the enthalpy of formation for the hydrogen bond between chloroform and triethylamine.

Experimental Analysis

The chemical shift of the proton on chloroform observed in this hydrogen bond experiment, _observed, is an average of the chemical shift of the free chloroform and the bonded chloroform, weighted by the relative concentrations of each species in the equilibrium solution. 

The molar quantities of chloroform, triethylamine, and the hydrogen-bonded species in equilibrium are represented by C, T, and B respectively while the initial molar quantities are Co, and To.

The equilibrium constant Keq, as defined by the law of mass action, uses the mole fraction of each species in solution.

These two equations, with an equation accounting for the reaction stoichiometry and the conservation of mass, can be manipulated to yield the following equation describing the observed data. The experimentally controlled variable is the mole ratio of triethylamine to chloroform, R = To/ Co, which was used to make each solution. The two parameters fit to the equation will produce the equilibrium constant, Keq, and the relative chemical shift of the hydrogen-bonded chloroform proton, which can not be directly observed.

Measured Results

Solutions with seven different concentration ratios, R, were measured in 10° increments between -30°C and +30°C. A concentric arrangement of NMR tubes was used to simultaneously measure the chemical shift of the hydrogen-bonded proton relative to a sample of pure chloroform. The data are plotted with the fit to above equation.

Calculation of enthalpy

Fitting the data at each temperature yields the temperature dependence of Keq. This was analyzed using the van't Hoff equation.

A simple linear least squares fit is used to calculate the reaction enthalpy.

Predicted and Measured IR spectrum of H-Bonded species

From three independent MOPAC force calculations, a computed IR spectrum was generated for chloroform, triethylamine and the H-bonded species. An absorption peak due to the H-bond is predicted at 2600 cm^-1, and is observed experimentally at 2500 cm^-1.

Summary of quantitative results

Property	Experimental	MOPAC Computation	Comment
H	-15.0 0.6 kJ/mol	-18 kJ/mol *	* Depends on Computation Specifics
S (298K)	-41.3 0.3 kJ/mol-K	**	** Statistical Entropy too small for accurate computation
G (298K)	-2.8 0.9 kJ/mol	**
IR	2500 cm-1	2600 cm-1
Bond Length, r	-	1.80Å

The thermodynamic and spectroscopic consequences of hydrogen bonding were computed using MOPAC. The energetics of the bond formation are accessible via the equilibrium changes and NMR spectroscopy at several temperatures. MOPAC predicted a vibrational signature of the bond which was observed experimentally.

Acknowledgments:

Support

CAChe Scientific (Modeling Software )

Meyer Grant (Instrumentation grant: NMR, FTIR )

Dreyfus Foundation (Faculty Grant: Computer )

Pacific Faculty

James O. Currie

Pacific Students

Chris Bock, Shane Carson, Mark Phillips, Kaleo Taft

Presented at the Fourth Annual Murdock Conference on Undergraduate Research
Whitman College, Walla Walla, Washington
November 1995

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Modeling and Measurement of Rotational Barrier

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