| dc.description.abstract | The investigations report the ability of several donor
macrocyclic polyethers, 15?Crown?5, 18?Crown?6, benzo?15?Crown?5,
dibenzo?18?Crown?6, dicyclohexyl?18?Crown?6 and dibenzo?24?
Crown?8, to form molecular complexes with diverse organic ??acceptors,
dichlorodicyanobenzoquinone (DDQ), trinitro, tetranitro,
9?dicyanomethylene fluorenones (TEF, TEHP, CNTF),
cyanomethylene indanedione (CNIND), tetracyano pyrazine (TCNPY)
and hexafluorobenzene (HFB). Optical absorption, ¹H, ¹?F and
¹³C NMR and IR spectral methods were employed to study
binding constants (K), thermodynamic parameters (?H and ?S)
and charge transfer transition energies (h?_CT).
It is found that the stability of the complexes increases
with the number of oxygen atoms in the donor ethers. The ground
state stabilization in these electron?donor?acceptor complexes
is ascribed to both ?–? and n–? interactions. The n–? interaction
forms 10% of the total contribution to the stabilisation.
The electron affinity (EA) of the ??acceptors correlates
well with the observed binding constants of the complexes and
also with the h?_CT values indicating that charge transfer
forces are dominant. In all the cases, the ionization energy
of donor ethers (I_D) is evaluated. The appearance of multiple
charge transfer absorptions in these complexes is explained on
the basis of I_D values (energies of the highest occupied
molecular orbitals) and available acceptor molecular
orbitals (energies of the lowest unoccupied molecular
orbitals). The ¹H and NMR spectra of the complexes
provided information as to the geometry of the complexes
in solution. That there exists no specific site binding
in these complexes is revealed by the IR spectra of the
complexes. The thermodynamic parameters (?H and ?S)
are discussed in terms of energetics of packing and
orientation effects present in these molecular complexing
systems. These values are low compared to systems where
ion?dipole interactions dominate. Based on these studies
it is proposed that crown ethers form weak complexes with
??acceptors wherein both ?–? and n–? interactions form
the major source of ground state stabilization. | |
