n-Alkanes are the textbook examples of the odd-even effect: The difference in the periodic packing of odd- and even-numbered n-alkane solids results in odd-even variation of their melting points. However, in the liquid state, in which this packing difference is not obvious, it seems natural to assume that the odd-even effect does not exist, as supported by the monotonic dependence of the boiling points of n-alkanes on the chain length. Herein, we report a surprising odd-even effect in the translational diffusional dynamic properties of n-alkanes in their liquid states. To measure the dynamics of the molecules, we performed quasi-elastic neutron scattering measurements near their melting points. We found that odd-numbered n-alkanes exhibit up to 30 times slower dynamics than even-numbered n-alkanes near their respective melting points. Our results suggest that, although n-alkanes are the simplest hydrocarbons, their dynamic properties are extremely sensitive to the number of carbon atoms.
Porous materials provide a plethora of technologically important applications that encompass molecular separations, catalysis, and adsorption. The majority of research in this field involves network solids constructed from multitopic constituents that, when assembled either covalently or ionically, afford macromolecular arrangements with micro- or meso-porous apertures. Recently, porous solids fabricated from discrete organic cages have garnered much interest due to their ease of handling and solution processability. Although this class of materials is a promising alternative to network solids, fundamental studies are still required to elucidate critical structure-function relationships that govern microporosity. Here, we report a systematic investigation of the effects of building block shape-persistence on the porosity of molecular cages. Alkyne metathesis and edge-specific postsynthetic modifications afforded three organic cages with alkynyl, alkenyl, and alkyl edges, respectively. Nitrogen adsorption experiments conducted on rapidly crystallized and slowly crystallized solids illustrated a general trend in porosity: alkynyl > alkenyl > alkyl. To understand the molecular-scale origin of this trend, we investigated the short and long time scale molecular motions of the molecular cages using ab initio molecular dynamics (AIMD) and classical molecular dynamics (MD) simulations. Our combined experimental and computational results demonstrate that the microporosity of molecular cages directly correlates with shape persistence. These findings discern fundamental molecular requirements for rationally designing porous molecular solids.
n-Alkanes are the textbook examples of the oddeven effect:The difference in the periodic packingofodd-and even-numbered n-alkane solids results in odd-even variation of their melting points.H owever,i nt he liquid state,i nw hich this packing difference is not obvious,i ts eems natural to assume that the odd-even effect does not exist, as supported by the monotonic dependence of the boiling points of n-alkanes on the chain length. Herein, we report as urprising odd-even effect in the translational diffusional dynamic properties of nalkanes in their liquid states.T om easure the dynamics of the molecules,w ep erformed quasi-elastic neutron scattering measurements near their melting points.W ef ound that oddnumbered n-alkanes exhibit up to 30 times slower dynamics than even-numbered n-alkanes near their respective melting points.O ur results suggest that, although n-alkanes are the simplest hydrocarbons,their dynamic properties are extremely sensitive to the number of carbon atoms.Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under http://dx.
As a compelling case of sensitive
structure–property relationship,
an odd–even effect refers to the alternating trend of physical
or chemical properties on odd/even number of repeating structural
units. In crystalline or semicrystalline materials, such odd–even
effects emerge as manifestations of differences in the periodic packing
patterns of molecules. Therefore, due to the lack of long-range order,
such an odd–even phenomenon is not expected for dynamic properties
in amorphous state. Herein, we report the discovery of a remarkable
odd–even effect of dynamical properties in the liquid phase.
In a class of glass-forming diammonium citrate ionic liquids, using
incoherent quasi-elastic neutron scattering measurements, we measured
the dynamical properties including diffusion coefficient and rotational
relaxation time. These directly measured molecular dynamics showed
pronounced alternating trends with increased number of methylene (−CH2−) groups in the backbone. Meanwhile, the structure
factor S(Q) showed no long-range
periodic packing of molecules, while the pair distribution function G(r) revealed subtle differences in the
local molecular morphology. The observed dynamical odd–even
phenomenon in liquids showed that profound dynamical changes originate
from subtle local structural differences.
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