59th Annual Report on Research 2014

The strengths of different types of hydrogen bonds in hydrogen bond networks (HBNs) were quantified in the gas phase and in solution via photoelectron spectroscopy and pK_a measurements, respectively. The latter results indicated that hydrogen bonds to a charged center and those one solvation shell further away in a simple pentaol provide 5.3 and 2.5 pK_a units per hydrogen bond of stabilization in DMSO. These values are predicted to increase in benzene to 8.4 and 3.9 pK_a units, respectively. In a larger heptaol, hydrogen bonds that are more remote from the charged center were still found to energetically non-trivial.

Anion binding constants of two flexible diols were determined in different solvents and binary mixtures. The association constants (K) in CD₃CN and CDCl₃ are surprisingly similar, and CD₃CN–solvent mixtures led to reduced values of K that are smaller than in either pure solvent. This led to an unexpected U–shaped dependence of K relative to the solvent composition.

Rigid tricyclic 1,3,5-cyclohexanetriol derivatives with 0–3 trifluoromethyl groups were synthesized via multistep processes and the photoelectron spectra of their conjugate bases and chloride anion clusters were reported. The electron binding energies provide an indication of the stabilization resulting from the hydrogen bond network and inductive effects. Taken together, hydrogen bond networks appear to be similar to resonance delocalization in that both appear to provide long range stabilizing pathways.

Work that had previously been carried out on reactive intermediates with partial support from the Petroleum Research Fund was published in Chem. Rev. and J. Org. Chem. The latter study made use of a thermodynamic cycle to obtain the heat of formation of c-C₃H₃^¥ (118.9 ± 4.0 kcal mol^–1) and the allylic C–H bond dissociation energy (BDE) of cyclopropene (104.4 ± 4.0 kcal mol^–1). These experimental values are well reproduced by high level G3 and W1 computations and reveal that the BDE is similar to those for cyclopropane and the vinyl position of cyclopropene. This is unprecedented and reflects the unusual nature of cyclopropene.