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Calixarenes are used as molecular scaffolds for the preparation of novel receptors for cations, anions, and neutral molecules. Although widely studied, the nature of the host−guest interactions within these molecules remains obscure. We present a combined density-functional theory and second-order perturbation theory study of the complexation of the alkali cations Na+ and Cs+ with tetramethoxycalix[4]arene. The results show that the host−guest interactions of this simple system are much more complex than previously thought and suggest new approaches for the design of selective cation receptors.

We present an ab initio, quantum mechanical study of 18-crown-6 (18c6) and its interaction with the alkaline earth dications Mg2+, Ca2+, Sr2+, Ba2+, and Ru2+. Geometries, binding energies, and binding enthalpies are evaluated at the restricted Hartree−Fock (RHF) and second-order Moller−Plesset perturbation (MP2) levels of theory using the 6-31+G* basis set and relativistic effective core potentials. The affinity of 18c6 for the dications is considerable, ranging from 150−300 kcal mol-1. The cation−18c6 interaction arises principally from the electrostatic interaction of the dication with the nucleophilic ether backbone and from the polarization of the crown ether by the electric field of the dication. Whereas Ba2+ selectivity is observed for 18c6 in aqueous environments, our calculations clearly show that the crown ether in fact binds Mg2+ most strongly in gas phase. Thus, solvation effects appear to strongly influence cation selectivity. Indeed, Ba2+ selectivity is recovered when we consider the competit...

We present an ab inito, quantum mechanical study of 18-crown-6 (18c6) and its interaction with the alkali metal cations Li[sup +], Na[sup +], K[sup +], Rb[sup +], and Cs[sup +]. Geometries, binding energies, and binding enthalpies are evaluated at the restricted Hartree-Fock (RHF) level using standard basis sets (3-21G and 6-31 + G*) and relativistic effective core potentials. Electron correlation effects are determined at the MP2 level, and wave function analysis is performed by the natural bond orbital (NBO) and associated methods. The affinity of 18c6 for the alkali metal cations is quite strong (50-100 kcal mol[sup [minus]1], depending on cation type), arising largely from the electrostatic (ionic) interaction of the cation with the nucleophilic ether backbone. Charge transfer (covalent bonding) contributions are somewhat less important, only 20-50% as strong as the electrostatic interaction. Agreement of the calculated binding enthalpies and experimentally determined quantities is rather poor. For example, the binding energy for K[sup +]/18c6 (-71.5 kcal mol[sup [minus]1]) is about 30 kcal mol[sup [minus]1] stronger than that determined by experiment, and it is not clear how to reconcile this difference. Our calculations clearly show that solvation effects strongly influence cation selectivity. 48 refs., 12 figs., 5 tabs.

Ab initio molecular orbital theory has been used to calculate accurate enthalpies of formation and adiabatic electron affinities or ionization potentials for N3, N3-, N5+, and N5- from total atomization energies. The calculated heats of formation of the gas-phase molecules/ions at 0 K are DeltaHf(N3(2Pi)) = 109.2, DeltaHf(N3-(1sigma+)) = 47.4, DeltaHf(N5-(1A1')) = 62.3, and DeltaHf(N5+(1A1)) = 353.3 kcal/mol with an estimated error bar of +/-1 kcal/mol. For comparison purposes, the error in the calculated bond energy for N2 is 0.72 kcal/mol. Born-Haber cycle calculations, using estimated lattice energies and the adiabatic ionization potentials of the anions and electron affinities of the cations, enable reliable stability predictions for the hypothetical N5(+)N3(-) and N5(+)N5(-) salts. The calculations show that neither salt can be stabilized and that both should decompose spontaneously into N3 radicals and N2. This conclusion was experimentally confirmed for the N5(+)N3(-) salt by low-temperature metathetical reactions between N5SbF6 and alkali metal azides in different solvents, resulting in violent reactions with spontaneous nitrogen evolution. It is emphasized that one needs to use adiabatic ionization potentials and electron affinities instead of vertical potentials and affinities for salt stability predictions when the formed radicals are not vibrationally stable. This is the case for the N5 radicals where the energy difference between vertical and adiabatic potentials amounts to about 100 kcal/mol per N5.

The concerted dihydrogen exchange reaction between eclipsed ethane and ethylene yielding ethylene and ethane is ''symmetry allowed''. Nonetheless, on the basis of SCF and full optimized reaction space (FORS) MCSCF calculations, a reaction barrier of about 71 kcal/mol is predicted. The electronic rearrangements and the origin of the barrier are analyzed.

MCSCF calculations with an STO-3G basis set have been carried out to obtain the optimal geometries and associated energies for a grid of points on the lowest singlet potential surface connecting methylenecyclopropane (MCP) and trimethylenemethane (TMM). Two methylene rotation angles were used to generate a two-dimensional projection of the 24-dimensional hypersurface, and a double Fourier fit to the computed energies gave a closed form expression for the energy on the two-dimensional surface. Three minima, connected by four transition states, were located on the surface. The effect of basis set expansion and inclusion of CI on the relative energies was determined. In accord with Hammond's postulate, the energies of transition states leading to orthogonal and planar TMM were found to be in the same order as the energies of these two intermediates. The conrotary transition state leading to planar TMM was found to be slightly lower in energy than the disrotary one. From a Fourier fit to the triplet energies, calculated at the optimal singlet geometries on the twedimensional surface, the line along which the singlet and triplet surfaces intersect has been found. The lowest point along this line was computed to be higher in energy than orthogonal TMM. The implications of this finding are discussed. The thermal rearrangements of methylenecyclopropane (MCP) derivatives have been the subject of numerous experimental investigations.] These studies suggest that the major pathway involves the orthogonal geometry (1) in which one methylene group H. 'C / H . The preference for 1 over 2 in the lowest singlet state contrasts with the geometry of the lowest triplet state. Both the null value of the zero field splitting parameter E" and the hyperfine coupling patternI2 suggest a triplet geometry of at least C, symmetry. Ab initio calculations predict the triplet geometry to be planar &,.I3 The difference between the preferred geometry of the singlet and triplet has been rationalized in terms of electron repulsion effects.14 A large number of calculations have been carried out to determine the relative energies of the planar and orthogonal singlet states, and all predict that the orthogonal singlet is 1 0 w e r . ' ~ ~ ~ H \ / H H \ / l H \ , / F. *, / I I1 H\C/:+C/H H\, /C\ , /H

The /sup 11/B/sub 2/ molecule has been trapped in neon and argon matrices at 5 +/- 1 K for ESR (electron spin resonance) investigations. Laser sputtering from solid boron was used to generate the B/sub 2/ molecule whose ground electronic state has been established experimentally for the first time to be /sup 3/..sigma../sub g//sup -/. The ESR nuclear hyperfine parameters for B/sub 2/ and B atoms have been compared with the results of ab initio CI theoretical calculations. Good agreement between theory and experiment is found for the dipolar component of the A tensor but not for the small and difficult to calculate isotropic hfs. Reasons that make the Fermi contact term especially difficult to calculate in this particular case are presented. The argon matrix ESR results yield the following magnetic parameters for /sup 11/B/sub 2/ in its ground electronic state: g/sub parallel/ = g/sub perpendicular/ = 2.0015 (4); A/sub parallel/ = +/- 11 (1) MHz and A/sub perpendicular/ = +/- 27.4 (3) MHz for /sup 11/B and the zero field parameter; and the D value is 3633 (3) MHz. As discussed, the most reasonable signs for the A values are A/sub parallel/ 0.more » The neon matrix value of D was found to be 3840 (3) MHz and A/sub perpendicular/ in neon is +/- 26.6 (3) MHz.« less

Emploi de la theorie Jahn-Teller au 2nd ordre pour montrer que les differences d'electronegativite et la conjugaison cyclique peuvent chacune minimiser l'importance de la resonance allylique dans les redicaux libres, bien que la repulsion electronique provoque une resonance importante dans les diradicaux. Confirmation de ces previsions par des calculs MC-SCF et CI sur O(CH 2 ) 2 + et O(CH 2 ) 2