Gastone Gilli and Paola Gilli
- Published in print:
- 2009
- Published Online:
- September 2009
- ISBN:
- 9780199558964
- eISBN:
- 9780191720949
- Item type:
- chapter
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780199558964.003.0005
- Subject:
- Physics, Crystallography: Physics
Acid-base parameters of donors (D-H) and acceptors (:A) are crucial in any D-H···:A bond in view of the hypothesis that the difference ΔPA = PA(D-) - PA(:A) or ΔpKa = pKa(D-H) - pKa(A-H+) drive ...
More
Acid-base parameters of donors (D-H) and acceptors (:A) are crucial in any D-H···:A bond in view of the hypothesis that the difference ΔPA = PA(D-) - PA(:A) or ΔpKa = pKa(D-H) - pKa(A-H+) drive H-bond strengths, the bond becoming really strong only when this difference tend to zero. This criterion, known as PA/pKa equalization principle, was suggested many years ago but never verified in its generality. This chapter performs a full thermodynamic analysis of the problem showing that it admits general solution by systematic comparisons of PA or pKa values with H-bond solid-state geometries or gas-phase enthalpies, though the combination pKa-geometry has proved to be of more general use. This last method is exploited by compiling extended tables of donor-acceptor pKa values (-14 = pKa = 53), arranging them in an unique bar-chart (the pKa slide rule), and comparing the ΔpKa values computed with the geometries of more than 10,000 crystal structures of X-H···X and N-H···O/O-H···N bonds.Less
Acid-base parameters of donors (D-H) and acceptors (:A) are crucial in any D-H···:A bond in view of the hypothesis that the difference ΔPA = PA(D-) - PA(:A) or ΔpKa = pKa(D-H) - pKa(A-H+) drive H-bond strengths, the bond becoming really strong only when this difference tend to zero. This criterion, known as PA/pKa equalization principle, was suggested many years ago but never verified in its generality. This chapter performs a full thermodynamic analysis of the problem showing that it admits general solution by systematic comparisons of PA or pKa values with H-bond solid-state geometries or gas-phase enthalpies, though the combination pKa-geometry has proved to be of more general use. This last method is exploited by compiling extended tables of donor-acceptor pKa values (-14 = pKa = 53), arranging them in an unique bar-chart (the pKa slide rule), and comparing the ΔpKa values computed with the geometries of more than 10,000 crystal structures of X-H···X and N-H···O/O-H···N bonds.
Gastone Gilli and Paola Gilli
- Published in print:
- 2009
- Published Online:
- September 2009
- ISBN:
- 9780199558964
- eISBN:
- 9780191720949
- Item type:
- chapter
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780199558964.003.0006
- Subject:
- Physics, Crystallography: Physics
The analysis performed in Chapters 3 and 4 leads to formulate a number of rules which can be considered the empirical laws governing the H-bond. The first is that H-bonds are not an undifferentiated ...
More
The analysis performed in Chapters 3 and 4 leads to formulate a number of rules which can be considered the empirical laws governing the H-bond. The first is that H-bonds are not an undifferentiated group but, conversely, need to be divided in classes according to their strength, mechanism of action, and modes of proton-exchange, acid-base association, and PA/pKa matching, the most important subdivision remaining that in six chemical leitmotifs (CLs), four of which collect all strong H-bonds known in nature. It is eventually concluded that the H-bond behaves as an interaction having a twofold nature (electrostatic and covalent) and a dual logic (two bonds formed by a same proton with two lone-pair donors) which, for these very reasons, is fully interpreted and rationalized in terms of VB theory (the electrostatic-covalent H-bond model) or general acid-base theory (the PA/pKa equalization principle).Less
The analysis performed in Chapters 3 and 4 leads to formulate a number of rules which can be considered the empirical laws governing the H-bond. The first is that H-bonds are not an undifferentiated group but, conversely, need to be divided in classes according to their strength, mechanism of action, and modes of proton-exchange, acid-base association, and PA/pKa matching, the most important subdivision remaining that in six chemical leitmotifs (CLs), four of which collect all strong H-bonds known in nature. It is eventually concluded that the H-bond behaves as an interaction having a twofold nature (electrostatic and covalent) and a dual logic (two bonds formed by a same proton with two lone-pair donors) which, for these very reasons, is fully interpreted and rationalized in terms of VB theory (the electrostatic-covalent H-bond model) or general acid-base theory (the PA/pKa equalization principle).
