Harpooning effect

Figure 1. Diagram of the venom duct of Conus. The venom is produced in the venom duct, apparently expelled from the duct into the proboscis by contraction of the venom bulb. Simultaneously, a harpoon-like tooth is transferred from the radula sac to the proboscis. When injection takes place, the venom is pushed through the hollow tooth and flows into the prey through a hole at the tip of the tooth. Typically, fish-hunting cones will strike at a fish only once and grasp the tooth after injection has occurred, effectively harpooning their prey while injecting the paralytic venom. In contrast, snail-hunting cones will usually sting their prey several times before total paralysis occurs. (Reprinted with permission from the Second Revised Edition of Ref. 8. Copyright 1988 Darwin Press, Inc.)...

The possibility of a barrier which inhibits a reaction in spite of the attractive ion-dipole potential suggests that one should make even crude attempts to guess the properties of the potential hypersurface for ion reactions. Even a simple model for the long range behavior of the potential between neutrals (the harpoon model ) appears promising as a means to understand alkali beam reactions (11). The possibility of resonance interaction either to aid or hinder reactions of ions with neutrals has been suggested (8). The effect of possible resonance interaction on cross-sections of ion-molecule reactions has been calculated (25). The resonance interaction would be relatively unimportant for Reaction 2 because the ionization potential for O (13.61 e.v.) is so different from that for N2 (15.56 e.v.). A case in which this resonance interaction should be strong and attractive is Reaction 3 ... 

In summary, preliminary experiments have demonstrated that the efficiency and outcome of electron ionization is influenced by molecular orientation. That is, the magnitude of the electron impact ionization cross section depends on the spatial orientation of the molecule widi respect to the electron projectile. The ionization efficiency is lowest for electron impact on the negative end of the molecular dipole. In addition, the mass spectrum is orientation-dependent for example, in the ionization of CH3CI the ratio CHjCriCHj depends on the molecular orientation. There are both similarities in and differences between the effect of orientation on electron transfer (as an elementary step in the harpoon mechanism) and electron impact ionization, but there is a substantial effect in both cases. It seems likely that other types of particle interactions, for example, free-radical chemistry and ion-molecule chemistry, may also exhibit a dependence on relative spatial orientation. The information emerging from these studies should contribute one more perspective to our view of particle interactions and eventually to a deeper understanding of complex chemical and biological reaction mechanisms. 

The results, which are detailed elsewhere, reveal that the transition state of CT reactions can be studied directly at well-defined impact geometries. The dissociative CT reaction of benzenes with iodine occurs with an elementary harpoon/electron transfer mechanism. The time scales for the CT and for the product (I) formation define the degree of concertedness and, as reported elsewhere, are significant to the recent elegant studies in condensed media by Wiersma and colleagues and by Sension. So far we have studied the electron donors of benzene, toluene, xylene, mesitylene, and cyclohexane and we plan extension to other systems. We have also studied the effect of solvation in clusters and in solutions. 

For harpoon reactions of alkaline metal atoms with iodine molecule I2, the interaction radii, Re, calculated using the formula Re = (ajji) 12 from the experimentally measured cross-sections a, are compared in Table 3 with the distances, Ru, calculated with the help of eqn. (40) and the sums of the gas-kinetic radii i M + i l2 of the reagents. In these calculations, effective radii of alkaline metal atoms have been used as RM, while the radii of the molecule I2, calculated from the data on the viscosity of I2 vapour at T > co and at T = 273 K, have been used as i l2 (the values of RM + i ,2 given in brackets correspond to the latter) [71], It is seen that the values of Re exceed Rm + Rh, i.e. electron transfer occurs at large impact parameters. 

Pure rotational and vibrational Raman spectra of At2 Raman spectroscopic study of kinetics of Ar, formation in a supersonic expansion seeded with Nj Electronic absorption spectrum of HgAr Rotational Zeeman effect in ArHBr t HgCl2 collision complex formed in harpoon reaction of Hg with Clj investigated via excitation of the HgCL van der Waals complex... 

The geometry of molecular anions generally differs from that of the neutral molecule. For instance, N20 is bent whereas the N2O molecule is linear. This is one of the reasons why the vertical electron affinity of N20 is negative. This is also the reason why bending excitation increases the electron affinity [EA) of the molecule. From Magee s equation (see Eq. 3), we know that the increase in EA results in a larger distance Rc for the harpoon. A larger cross-section is therefore expected. This effect has actually been observed semiquantitatively in the chemiluminescent reaction Ba -t- N2O BaO + N2 [125, 126]. The same effect was confirmed later in an experiment where the internal excitation of N2O was carefully prepared [127]. It... 

This effect is best viewed in single harpoon reactions such as those of alkali metal atoms with halogen-containing molecules discussed in Section 2.3.1. A series of studies conducted in a crossed-beam experiment by Lee s group at Berkeley have demonstrated how the electronic excitation of sodium affects the dynamics of these reactions. 

E.E.Nikitin, Charge-exchange indueed reactions, Teor.Eksp.Khim. 4, 593 (1968) E.E.Nikitin, Quantum effects in electron-harpoon reactions, Teor. Eksp. Khim. 4, 751 (1968)... 

The difference may arise from the radical character of the monomers versus the closed-shell electronic structure of the dimer Nj O4. In the former, because of the high electron afhnity, a harpooning process is expected, while in the dimers the interaction is short range with covalent character. It is important to realize that for collision energies of more than 10.5 kcal the cross section for the N2O4 reaction exceeds that of the monomeric process. It is difficult to conclude on the third-body effect in this reaction because of the differences in the electronic structure between the two species NO2 and N2O4. Usually one would like to investigate the cluster s effect where only small perturbations exist. 

R. Brooks [42-44] the process of electron transfer for K to oriented t-butyl bromide is found strongly dependent on the orientation. Systems involving metal atoms are traditional favorites of molecular beam studies, particularly of stereodynamics. In recent experiments [45], with brute force oriented ICl, experimental determination was made of the cone of acceptance for reactivity (steric effect) in a "harpooning" reaction, Sr + ICl leading to electroiucally excited products detected via their chemiluminescence... 

It is of interest not only to perforate vesicle membranes but also to destroy them after they have served their purpose as transport vehicles, in particular for DNA. Natural vesicles, so-called endosomes, contain about 50% cholesterol. The disruption of such cholesterol-containing lipid bilayers by Triton XI00 or sodium deoxycholate, examples of artificial and natural detergents, results in a leaky membrane at low concentration and in a catastrophic rupture process above the cmc of the amphiphiles. Vesicles made of fluid phospholipid bilayers devoid of cholesterol showed only leakiness under the same conditions. Amphiphiles with a carboxylate end group and a very bulky hydrophobic end (e.g., with two tert. butyl groups) disrupt membranes at pH 5 and have no effect above pH 7 (harpoons). For an example, see Figure 6.5.3. 

The harpooning effect fi om p. 308 represents also an example of a vibronic coupling, if the two diabatic states the ionic one R) and the neutral one R) are considered... 

Hernandez-Trujillo and Bader studied the evolution of the electron densities of two separated atoms into an equilibrium molecular distribution, and considered a range of interactions from closed-shell with and without charge transfer, through polar-shared, to equally shared interactions. The harpoon mechanism operative in the formation of LiF was found to exert dramatic effects on the electron density and on the atomic and molecular properties. The virial, the Hellmann-Feynman and the Ehrenfest force theorems provided an imderstanding of the similarities and differences in the bonding. 

Simulating the harpooning effect in the NaCl molecule Polyatomic molecules and the conical intersection ((DO)... 

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