Ij. Shannon et al., THEORETICAL PREDICTION OF THE GUEST PERIODICITY OF ALKANE UREA INCLUSION-COMPOUNDS, Journal of the Chemical Society. Faraday transactions, 89(12), 1993, pp. 2023-2029
Citations number
19
Categorie Soggetti
Chemistry Physical","Physics, Atomic, Molecular & Chemical
A mathematical model of one-dimensional inclusion compounds, developed
previously, is applied here to predict and rationalize structural pro
perties of the alkane/urea family of inclusion compounds. Within this
model, the one-dimensional inclusion compound is considered as a linea
r infinite host tunnel (with periodic repeat distance c(h)) containing
a finite periodic arrangement (with periodic repeat distance c(g)) of
identical guest molecules. This mathematical model provides a formali
sm that allows potential-energy functions (describing host-guest inter
action, guest-guest interaction and intramolecular potential energies)
computed for any one-dimensional inclusion compound to be used to pre
dict and rationalize structural properties of the inclusion compound.
Using this approach, the optimum c(g) has been determined for the seri
es of urea inclusion compounds containing the alkane guest molecules C
H3(CH2)(r)CH3, with r = 2-18, and the question of whether these inclus
ion compounds exhibit commensurate or incommensurate behaviour has bee
n assessed. It is predicted and demonstrated that the heptadecane/urea
inclusion compound is commensurate (6c(g) = 13c(h)). The butane/urea
inclusion compound is also predicted to be commensurate, whereas all o
f the other alkane/urea inclusion compounds investigated are predicted
to exhibit incommensurate behaviour. The wider issue of assessing the
commensurate vs. incommensurate nature of such inclusion compounds by
this theoretical approach is discussed. The alkane/urea inclusion com
pounds with guest molecules CH3(CH2)rCH3 (r = 7-15, 18) have been stud
ied by single-crystal X-ray diffraction, and from these data the value
of c(g) at room temperature has been determined. The values of c(g) p
redicted theoretically for these inclusion compounds are in good agree
ment with the values of c(g) determined experimentally. For all the ur
ea inclusion compounds studied, the optimum c(g) is ca. 0.5 angstrom s
horter than the value of c(g) corresponding to the minimum guest-guest
interaction energy; this fact substantiates the claim that, at the op
timum guest periodicity in alkane/urea inclusion compounds, the intera
ction between adjacent guest molecules is repulsive.