Introduction
The hydrogen bond occurs when a hydrogen attached to an electronegative atom (usually fluorine or oxygen, but occasionally nitrogen) is attracted to a neighboring electronegative atom in its vicinity. The neighboring atom may be on a different molecule (intermolecular hydrogen bond) or on the same molecule (intramolecular hydrogen bond). The enthalpy of a hydrogen bond is typically in the 4- 40 kJ mol-1 range, compared to the much higher values expected for covalent bonds (100-600 kJ mol-1). Experiments 9-12 utilize spectroscopic methods (NMR, IR, UV- visible, fluorescence) to explore the nature of the hydrogen bond.
NMR study of the Hydrogen Bond
The proton NMR spectrum is a very sensitive measure of the environment experienced by a hydrogen. Increased hydrogen bonding reduces the electron density at a hydrogen, resulting in a deshielding of the proton undergoing hydrogen bonding. In this experiment, you will measure the chemical shift of the hydroxyl proton in ethanol in various solvents as a function of concentration. From this, the association equilibrium constant may be obtained for ethanol in each solvent (1):
(d d/dx)(x=0) = 2 K (dD - dM)
where x is the apparent ethanol mole fraction, d the observed chemical shift, dM and dD are the monomer and dimer chemical shifts, respectively, and K is the association equilibrium constant.
Procedure
Each group will choose one solvent (benzene, chloroform, cyclohexane, or nitromethane) and acquire spectra at ethanol mole fractions ranging from 0.10 to 1.00 in increments of 0.10. Record the chemical shift of the ethanol hydroxyl proton for each solution.
Calculations
Plot the chemical shift of the hydroxyl proton versus mole fraction. You may assume that the limiting (extrapolated x = 0.0 and observed x=1.0) values of the chemical shift correspond to pure monomer and dimer shifts, respectively. Determine the value of K from the slope of the curve fitting the data at the low concentration limit, using the equation above.
(1). Huggins, C. M.; Shoolery, J. N. J. Phys.
Chem. 1956 60, 1311.