Adsorption kinetics and electrode processes

Prof. H. A. Laitinen (Urbana):We have used double layer capacity measurements to measure adsorption as a function of time during the life of slowly forming mercury drops. For many substances that are not extremely surface active, so that the solution concentration is 1 to 10 mM, the double layer capacity per unit area is essentially constant during the time interval of 5 to 15 sec of drop life. It appears, therefore, that adsorption equilibrium has essentially been attained during the later portions of drop life.Prof. P. Delahay:We have also used a similar method of following the differential capacity during drop life. I would agree that one can achieve conditions for which adsorption equilibrium with respect to bulk concentration is practically reached (high enough concentration and long drop time). But this is quite often not the case (drop time of 3–4 sec in polarography).Mr. G.C. Whitnack (China Lake, U.S.A.);In determination of nitrate esters in presence of phthalate esters we have observed considerable dropping of current height of nitrate ester wave before start of phthalate ester wave. Is this due to an adsorption process occurring at the DME?Prof. P. Delahay:It might well be, but I would hesitate to comment any further on this point because more information would be required. You probably have quite a complicated situation. Perhaps we can discuss the matter later?Prof. Dr. H. Fischer (Ettingen):Ich möchte hinzufügen, class es Fälle geben kann, in denen mcht allem der Blockierungseffekt don Grenzstrom verringert, sondern eine Veränderung des Diffusionsfilmes durch Sekundärreaktion des Inhibitors. Dies ist z.B. bei der Abscheidung von Wasserstoff an einer festen Elektrode (Fe) der Fall, wenn der Inhibitor sich spaltet in eine schwerlösliche Verbindung und ein Proton (RNH⁺ → [RN] + H⁺). Dies beobachtet man z.B, bei den Kationen von heterozyklischen Aminen (Acridin). Offentlich bildet sich eine diffusionshindernde Barriere aus.Prof. P. Delahay:The case of the hydrogen electrode is complicated, and I shall attempt to answer your question only in the case of an ideally smooth electrode. Then, diffusion toward the electrode (partially covered with an adsorbed substance) is little affected by adsorption because the size of “the holes” in the film is very small in comparison with the diffusion layer thickness. Of course, there is no diffusion where there is complete blocking.Prof. B. Breyer (Sydney):The importance of the chemical nature of the film adsorbed at the interface, which has been mentioned by Prof. delahay, seems to me to play a major part in the type of processes discussed. Thus it must be kept in mind that complex formation between the diachargeable ion and the adsorbed film might occur (cf. e.g. heyrovsky? and matyas, 1941). The fact that T1+ ion is little influenced by the presence of an adsorbed film at the electrode solution interface could then be partly explained by the notoriously low co-ordination tendency of that ion.Prof. P. Delahay:The difficulty involved in the “blocking” of the limiting current for thallium is due, I think, to the small size of this ion (large diffusion coefficient of Tl+ in comparison with other ions). Of course, complex ions can be relatively very bulky and this enhances “blocking”.Prof. E. Lange (Erlangen):I agree with prof. delahay that it is very important to investigate the connections between the adsorption and electrode reaction.This is easy for a steady state, e.g., each heterogeneous reaction between two phases that is accompanied by a transfer of ions or electrons, i.e. of electrical charges, from one phase to the other. In such a case, the Galvani tension does not change and the transfer between the two phases must be compensated by a corresponding transfer of charges within the phases.But in the non-steady state, also, an adsorption process may behave as an “electrode reaction” for instance, even an adsorption of a dipole molecule may cause a “transfer of charge” accompanied by a corresponding change of the Galvani tension. In this manner, it seems to me that for the non-steady state it is necessary to define precisely what one means by “electrode reaction”.Prof. P. Delahay:I entirely agree with Prof. i.ange about the necessity of clear definitions. I think that the fact that a steady state with respect to diffusion of the reductible or oxidizable substance has not been reached is not too serious because this scarcely affects the Galvani potential (large excess of supporting electrolyte).Variations in the amount of adsorbed organic substances indeed affect the Galvani potential (dipole orientation), but this effect is included in the dependence of the rate constant ks (at the standard potential) on the electrode coverage.Prof. N. Tanaka (Sendai, Japan).I am very grateful to Prof. delahay for his beautiful work on adsorption kinetics. I should like to make one comment in connection with the rotated dropping mercury electrode. The relation between log i and log t on the current — time curve was found to be 0.5 only in the absence of the surface-active substance. In the presence of surface-active substance, the slope of log i vs. log t changed at a certain point of the current — time curve. This can be explained when the slow adsorption of the surface-active substance on the surface of the electrode is taken into consideration.Prof. P. Delahay:Even for stirred solutions, adsorption equilibrium is not reached very rapidly. A simple calculation based on a model of the Nernst diffusion layer shows that perhaps 1–5 sec are required. Your conclusion is, therefore, quite correct.Prof. W. Kemula (Warsaw):We have recently published that, in several cases, the addition of extremely small concentrations of organic surface-active substances provokes at first a rise of the diffusion current, this current then being suppressed by additional quantities of the substance.     

Fake molecular-orbital calculations

The FAKE method of approximate molecular-orbital calculations is presented and illustrated by application to a number of molecules. The method is of the extended Huckel type but uses accurately computed kinetic-energy matrix elements and avoids scale factors of the Wolfsberg—Helmholtz type. It also includes neighbor-atom charge effects and single-center off-diagonal matrix elements. These features permit FAKE occupied-orbital energies and charge distributions to come into close agreement with corresponding ab initio quantities.     

On the origin of dynamic instability of molecular systems

The vibronic origin of dynamic instability of molecular systems considered earlier, is here given a more complete and rigorous treatment. It is shown that the nonvibronic contribution to the curvature of the adiabatic potential arising due to nuclear displacements under fixed electronic density distribution, is always positive, and hence the only reason for dynamic instability is the pseudo Jahn-Teller effect. For some examples of special interest (planar equilateral NH₃, planar square CH₄ and linear H ₃ ⁺ ) the molecular excited states, responsible for the instability of the ground state, are revealed by means of ab initio calculations.     

Silylation as a protective method in acetylene chemistry. : Syntheses in the polyeneyne series,R3Si(CC)n(CHCH)4(CC)nSiR3 and H(CC)n(CHCH)4(CC)nH (n = 1, 2)

The Grignard reagents R₃Si(CC)_nMgBr (R = Me, n = 1; R = Et, n = 1,2) couple with cyclooctatetraene dibromide 1 in THF to give, as major products, the silyl-stabilised E, Z, Z, E-polyeneynes, Me₃SiCC(CHCH)₄CCSiMe₃3a, Et₃SiCC(CHCH)₄CCSiEt₃4a and Et₃Si(CC)₂(CHCH)₄(CC)₂SiEt₃6a together with minor proportions of configurational isomers Z, E, Z, Z 3c, all -E 3b, 4b, 6b and compounds in which a bicyclo-octadiene structure 2, 5 and 7 is retained. Irradiation converts the cis(Z)-rich isomers e.g. 3c into the all-trans(E) products. Treatment of the bissilyl compounds 3, 4 and 6 with aqueous base liberates the respective parent polyeneynes, H(CC)_n(CHCH)₄(CC)_nH, in each case.     

Symmetrical, unsymmetrical and bridged calix[4]arene derivatives as neutral carrier ionophores in poly (vinyl chloride) membrane sodium selective electrodes

The complexing ability of a range of 19 symmetrical, unsymmetrical and bridged calix[4]arene derivatives having ester, ketone, amide, amine and thioether functionalities were determined by the picrate extraction method. On incorporating these calix[4]arene derivatives as neutral carrier ionophores in sodium-selective poly (vinyl chloride) membrane electrodes the performance was assessed on the basis of the sensitivity and selectivity over the alkali, alkaline earth metals and hydrogen and ammonium ions. The temperature dependence, response times and lifetimes were also determined. Four ionophores in particular gave excellent sensitivity and selectivity and lifetimes of > 200 days. These electrodes were then tested without additional lipophilic additives and one ionophore was incorporated into poly (vinyl chloride) membrane electrodes with plasticizing solvents of varying polarity.     

Reductive Amination of Aldehydes and Ketones with Sodium Triacetoxyborohydride. Studies on Direct and Indirect Reductive Amination Procedures(1)

Sodium triacetoxyborohydride is presented as a general reducing agent for the reductive amination of aldehydes and ketones. Procedures for using this mild and selective reagent have been developed for a wide variety of substrates. The scope of the reaction includes aliphatic acyclic and cyclic ketones, aliphatic and aromatic aldehydes, and primary and secondary amines including a variety of weakly basic and nonbasic amines. Limitations include reactions with aromatic and unsaturated ketones and some sterically hindered ketones and amines. 1,2-Dichloroethane (DCE) is the preferred reaction solvent, but reactions can also be carried out in tetrahydrofuran (THF) and occasionally in acetonitrile. Acetic acid may be used as catalyst with ketone reactions, but it is generally not needed with aldehydes. The procedure is carried out effectively in the presence of acid sensitive functional groups such as acetals and ketals; it can also be carried out in the presence of reducible functional groups such as C-C multiple bonds and cyano and nitro groups. Reactions are generally faster in DCE than in THF, and in both solvents, reactions are faster in the presence of AcOH. In comparison with other reductive amination procedures such as NaBH(3)CN/MeOH, borane-pyridine, and catalytic hydrogenation, NaBH(OAc)(3) gave consistently higher yields and fewer side products. In the reductive amination of some aldehydes with primary amines where dialkylation is a problem we adopted a stepwise procedure involving imine formation in MeOH followed by reduction with NaBH(4).