Temperature-Dependent Blue Shifting of O–H Stretching Frequencies in Crystalline Cellulose Explained

Increasing the temperature of a chemical system generally
causes covalent bonds to lengthen and weaken, often the first step
in initiating chemical reactions. However, for some hydrogen-bonded
systems, infrared (IR) spectroscopy measurements reveal that covalent
O–H bonds actually strengthen and therefore shorten when heated.
In 1957, Finch and Lippincott proposed a simple one-dimensional (1D)
model to explain this effect, in which thermal excitation of intermolecular
stretching modes leads to lengthening and weakening of intermolecular
O–H···O hydrogen bonds, thereby indirectly strengthening
the associated covalent O–H bonds. Taking cellulose (an infinitely
repeating polymer of d-glucose) as an example, we use molecular
dynamics modeling to show that the same mechanism is responsible for
temperature-dependent blue shifting of O–H stretching bands
in IR spectra of carbohydrate biopolymers, except that interchain
hydrogen bonds are weakened by thermal excitation of chain-separation
modes, while intrachain hydrogen bonds are weakened by thermally induced
changes in ring puckering and orientation of ring substituents but
not reorientation of glucose units relative to one another or overall
twisting of the cellulose chains.