AMMRLers,
An interesting scientific discussion I had with a student last week prompts
me to write today.
My understanding of the hydrogen bond is that the proton on the heteroatom
(X) has its electronic environment perturbed by the Lewis base (electron
pair donor) nearby, giving a chemical shift that cannot be explained using a
simplistic electronegativity argument.
For example, a hydroxyl proton is, when attached to an aliphatic carbon,
typically found in the 2-5 ppm range. Based on the Pauli electronegativity
of oxygen compared with that of carbon, we might expect the chemical shift
of the proton to be nearer to 5 ppm than to 2 ppm, and yet a lone electron
pair from the solvent or another solute molecule (or even from a different
part of the same solute molecule) apparently provides some electron density
to the hydroxyl proton, thus shielding it additionally and pushing its
chemical shift upfield (to the right, to lower ppm values). The extent to
which this additional shielding is provided by electron pair donors
therefore accounts for the variability in the observed chemical shifts of
hydroxyl protons.
Amide protons, on the other hand, are observed to experience a downfield
chemical shift in the presence of electron pair donors (they are observed to
move from about 7 ppm to about 9 ppm in the presence of hydrogen bonding).
This shift can also be explained by supposing that the N-H bond is
lengthened by the competing electron pair donor, and thus the proton finds
itself far from any significant concentration of electron density - i.e.,
far from the heteroatom to which it is formally bound. So the proton, due
to a shallowness of the energy potential well as the N-H bond length is
increased, finds itself far from the shielding effect of any higher-Z atoms,
and thus experiences the applied field more strongly, which gives a
downfield chemical shift compared to the case in which the electron pair
donor is not elongating the N-H bond (in the absence of the hydrogen bond).
Therefore, we have two opposite trends. Presumably a short distance between
the heteroatom bearing the proton and the heteroatom donating the lone pair
will result in increased electron density and a concomitant upfield shift,
while with a long inter-heteroatom distance (but still with a strong
hydrogen bond) the proton, being far from the shielding provided by
electron-density-rich-regions, experiences a downfield shift.
Infrared spectroscopy can also be used to examine the nature of the hydrogen
bond, because the O-H and N-H stretching frequency will move up or down as
the force constant changes. When two heteroatoms are competing for a
proton, I would expect the force constant (and ir band frequency) to
increase, but when the potential well is flattened because of greater
stabilization by the electron pair donor, I would expect the force constant
(and ir frequency) to decrease.
If anybody has any additional insight to add (or can tell me where I am
going wrong in my viewing of this matter), or even a few experimental data
points to offer, I would appreciate it. Given sufficient interest, I will
post a summary to the group.
Thanks for reading,
Jeff
Jeff Simpson, Director
Department of Chemistry Instrumentation Facility
Massachusetts Institute of Technology
77 Massachusetts Avenue, 18-0090
Cambridge, MA, 02139
617-253-2016 (1806 lab)
Received on Fri May 14 2010 - 04:40:52 MST