Influence of boron atom on the acidity o f alcohol in both ga s phase an d solution phase, A DFT study

In this study, the drastic influence of the boron atom on the acidity of alcohol has been consid ered. The calculated ǻHacidity (۳ ۲۰.۹ - ۳۳۸.۱ kcal/mol) and pKa range of boroncontaining alcoho l (-۰.۱ – ۹.۴) indicate that the bo ration of alcohol lead s to consid erable enhancement of its acidity. For instance, we have obtained the ǻHacidity values ۳۳۸.۱, ۳۳۵.۲ kcal/m ol and the pKa values ۴.۱۲, ۲.۸۱ for BH۲CH۲OH, BF۲C H۲OH alcohols, respectively, which are much smaller than that of CH۳OH (with ǻHacidity = ۳ ۷۴.۹ kcal/m ol and pKa = ۱۵). The in crease in acidity of boronated alco hol can be related to th e stabilizat ion of alkoxy ion due to overlap of unoccupied orbital of bo ron atom with the electron pairs of negative oxygen. All gas phase computations were performed at level MP ۲/۶-۳۱۱++G(d,p)//(B۳L YP/۶-۳۱+G (d). The primary result s indicate th at the pres ence of bor on atom in an alcohol might make it as acidic a s nitric acid. The geom etry optimization of stud ied structures was perfo rmed with DFT computation and optimized structures were used to carry out Nat ural Bond O rbital (NBO) analysis. NBO analy sis reveale d that the increase in acidity of boron-conta ining alcoho ls is due to the charge transfer fro m the negative oxygen (in deprotanated structure) to the em pty orbital of Ȃ ଶ an d Ȃ ଶ. Qu antum theory of atoms in molecules (QTAIM) was also a pplied to determine the nature of bonds formed in the deprotonated stru cture.

Keywords:

Acidity , Boron atom , pKa , D FT , Natural bonding or bital (NBO) , Quantum theory of ato ms in molecules (QTAI M)