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Temperature-dependent Blueshifting 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) spectroscopic measurements reveal that covalent O-H bonds actually strengthen and therefore shorten when heated. In 1957, Finch and Lippincott proposed a simple 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 modelling 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 inter-chain hydrogen bonds are weakened by thermal ex- citation of chain-separation modes, while intra-chain 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.