Structure of the Boronic Acid Dimer and the Relative Stabilities of Its Conformers

Despite the widespread use of boronic acids in materials science and as pharmaceutical agents, many aspects
of their structure and reactivity are not well understood. In this research the boronic acid dimer, [HB(OH)2]2,
was studied by second-order Møller−Plesset (MP2) perturbation theory and coupled cluster methodology
with single and double excitations (CCSD). Pople split-valence 6-31+G*, 6-311G**, and 6-311++G** and
Dunning−Woon correlation-consistent cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, and aug-cc-pVTZ basis sets were
employed for the calculations. A doubly hydrogen-bonded conformer (1) of the dimer was consistently found
to be lowest in energy; the structure of 1 was planar (C2h) at most computational levels employed but was
significantly nonplanar (C2) at the MP2/6-311++G** and CCSD/6-311++G** levels, the result of an intrinsic
problem with Pople-type sp-diffuse basis functions on heavy atoms. The dimerization energy, enthalpy, and
free energy for the formation of (1) from the exo−endo conformer of the monomer were −10.8, −9.2, and
+1.2 kcal/mol, respectively, at the MP2/aug-cc-pVTZ level. Several other hydrogen-bonded conformers of
the dimer were local minima on the potential energy surface (PES) and ranged from 2 to 5 kcal/mol higher
in energy than 1. Nine doubly OH-bridged conformers, in which the boron atoms were tetracoordinated,
were also local minima on the PES, but they were all greater than 13 kcal/mol higher in energy than 1;
doubly H-bridged structures proved to be transition states. MP2 and CCSD results were compared to those
from the BLYP, B3LYP, OLYP, O3LYP, PBE1PBE, and TPSS functionals with the 6-311++G** and aug-cc-pVTZ basis sets; the PBE1PBE functional performed best relative to the MP2 and CCSD results. Self-consistent reaction field (SCRF) calculations predict that boronic acid dimerization is less favorable in solution
than in vacuo.