Ion-molecule complexes

The reactants and products are at the two ends of the curve. The transition structure for the reaction connects two minima. These minima are two ion-molecule complexes, intermediate species through which the reaction proceeds. 

B. Thermochemistry of Ion-Molecule Complexes and Specifically Ion-Hydrates Obtained from Equilibrium Determinations... 

The nucleophilic attack of azide on 76b was also modeled at the pBP/DN //HF/ 6-31G level (Table 10, Fig. 23).44 In the gas phase the transition state is preceded by an ion-molecule complex (Fig. 23a) which is at a minimum on the reaction coordinate and in which the azide anion is 3.7 A from the amide nitrogen, and approaching the axis of the formyloxy-nitrogen (Nl-09) bond but in which the azide possesses all the anionic charge. 

B3LYP predicts a negative overall barrier if X = Y = Cl (i.e. a barrier between the entry and exit ion-molecule complexes that lies below the entrance channel). Adamo and Barone [79] demonstrated that their new mPWlPW91 (modified Perdew-Wang) functional at least yields the correct sign for this problem. 

In general, electrostatic theory is not able to reproduce correct energy surfaces as a whole since the compensation of errors mentioned above holds only around the energy minima (see p. 25). Furthermore, the various spectral data, especially IR, RAMAN and NMR results, which became accessible during the last ten years cannot be interpreted at all by electrostatic models. Many theoreticians feel, therefore, that there is a need for more accurate calculations on ion-molecule complexes based upon quantum mechanics. 

In semiempirical calculations variational principle itself is manipulated, or to give another interpretation, the molecular Hamiltonian is modified in order to save computation time. Two semiempirical methods differing in the concept and the degree of sophistication have been applied frequently to ion-molecule complexes and therefore will be mentioned briefly. 

In order to see whether this kind of intermolecular energy partitioning also works well in ion-molecule complexes, and to study basis set effects on the results, we performed a number of calculations on Li+... OH2 91h The numerical values obtained are shown in Table 3. To facilitate comparison the geometry of the water molecule was frozen at the experimental values and the intermolecular distance was fixed to i ou, = 3.4 a. 1. u. 1.8 A, which is very near to the equilibrium value. 

Typical examples of density- and density difference maps for ion-molecule complexes are shown in Chapter V. For our purpose we do not need the detailed spatial information which is rather confusing for a discussion of charge transfer. We proposed therefore 113> to choose an axis of the complex (2-axis), which is characteristic for the type of interaction that we are interested in. In Li+... OH2 or Li+... OCH2 this is evidently the twofold symmetry axis C2. In other cases like e.g. F-... HOH or (H20)2, the connection line of the three atoms forming the hydrogen bond will be appropriate ). After defining a 2-axis in this way we can calculate a density difference curve Aq(z) by simple integration (35) ... 

The arenium ion/(R)-(— )-2-chlorobutane adducts. A crucial question concerns the chemical identity and the relative spatial arrangement of the components of a microsolvated system, two features of paramount importance to assess the kinetic and the mechanistic role of the corresponding ion-dipole pairs in solution. In the example reported in this section, Cacace and coworkers consider the ion-molecule complexes involved in the classical Friedel-Crafts alkylation of arenes." " At 300 K and under FT-ICR conditions, the benzenium ion CeH reacts with 2-chlorobutane C4H9CI to give the CloHj5 ion with a rate constant of 5 X 10 cm molecule corresponding to a collision efficiency of 2.5% (equations (33) or (34)). ... 

Acid-induced wagner-meerwein rearrangements in chiral alcohols. In view of the considerable interest on ion-molecule complexes involved in gas-phase analogues of solvolytic reactions," ° " a sustained research effort has been directed to the study of Wagner-Meerwein rearrangements in cationized )8-arylaIkyl systems, under conditions excluding nucleophilic assistance by the solvent which in these systems normally interferes with anchimeric assistance of groups adjacent to the... 

Brief discussions of some topics of this chapter may also be found in physical chemistry textbooks however, much better introductions are given in the specialized literature. [4-11] Detailed compound-specific fragmentation mechanisms, ion-molecule complexes, and more are dealt with later (Chap. 6.). 

Hammerum, S. Derrick, P.J. Thermodynamics of Intermediate Ion-Molecule Complexes or Kinetics of Competing Reactions The Reactions of Low-Energy Isobutylamine and Neopentylamine Molecular Ions. J. Chem. Soc.. Perkin Trans. 2 1986, 1577-1580. 

Morton, T.H. Ion-Molecule Complexes in Unimolecular Fragmentations of Gaseous Cations. Alkyl Phenyl Ether Molecular... 

Contrary to expectations, the reductive process leading to the ion-pair 18 is found to be more favorable than the oxidatve path to the ion-molecule complex 17 for all Group II and most Group XII metals. Furthermore, Eq. (3) predicts the energies for the reductive process well for the Group I, II and XII metals if the constant k is adjusted for each group. These results are a little unexpected... 

Density functional theory has also been recently applied to several dissociative pathways of HO—ONO and its anion (ONO—0 ) in aqueous solution. For example, the Gibbs free energy for the homolysis of peroxynitrous acid (HO—ONO HO -h ONO) is calculated to be AG (aq.) = 11.1 kcalmol" , a value in good agreement with experiment (13.6 kcalmoD ). For peroxynitrite homolysis (ONO—O NO2 -b O ) the calculated AG (aq.) = 13.0 kcalmoD was obtained for ion-molecule complexes with watef ... 

The procedure adopted here is to make use again of RRKM theory to calculate k2/k l as a function of the relative barrier height. In this case, the transition state for the, reaction is taken as the loose ion-molecule complex at the Langevin capture distance. The transition state for the reaction k2 is taken as the tetrahedral intermediate RCOYX ". By a suitable choice of the vibrational frequencies and moments of inertia, this type of calculation shows that E 0-E0 for Cl- + CH3COCl should be around — 7 kcal mol 1 in order to reproduce the experimental efficiency. This amounts to an E 0 of 4 kcal mol-1. 

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