N-aryl-and N-vinyldiaza-18-crown-6: Synthesis and complexing ability

The nucleophilic aromatic and vinyl substitution using diaza-18-crown-6 as nucleophile afforded a number of its N,N’-diaryl-[aryl = 2,4-(NO₂)₂C₆H₃, 4-C₅F₄N, 4-CF₃C₆F₄] and N,N’-dialkenyl-substituted derivatives [alkenyl = PhC(O)CH=CH, MeOCOCH=CH, (EtO₂C)₂C=C(Ph), etc.]. Arylation of diaza-18-crown-6 with nonactivated aryl bromides, such as 4-Me₂NC₆H₄Br, 4-MeOC₆H₄Br, C₆H₅Br, and 4-CF₃C₆H₄Br, was effected under catalysis by palladium complexes. N,N’-Diaryldiaza-18-crowns-6 having electron-acceptor substituents in the aromatic rings turned out to be incapable of forming complexes with metal cations, while their analogs containing electron-donor para-methoxy and para-dimethylamino groups gave complexes with barium perchlorate.     

Antiferromagnetic complexes with a metal-metal bond: XV. Antiferromagnetic heterometallospirane clusters [(CH3C5H4)2Cr2(μ-SCMe3)(μ3-S)2]2M (M = MnII,FeII): Synthesis and molecular structures

The reaction of (CH₃C₅H₄CrSCMe₃)₂S (Ia) with Cp₂Mn in boiling toluene (containing some THF) has been used to prepare a pentanuclear cluster, [(CH₃C₅H₄)₂Cr₂(SCMe₃)(μ₃-S)₂]Mn (II), which is antiferromagnetic and crystallizes into the monoclinic crystal system: space group Cc, a 26.540(10), b 9.208(3), c 21.595(9) Å; β 135.30(2)°, V = 3712.1 ų, Z = 4. According to X-ray analysis, cluster II contains a metallospirane core, Cr₄Mn, which appears to be strongly distorted, compared to its earlier studied cyclopentadienyl analogue [Cp₂Cr₂SCMe₃(μ₃-S)₂]₂Mn, due to the short intramolecular contacts CH₃…S (2.9–3.1 Å). The angle between the metal triangle planes of Cr₂Mn is 109.60°. Here, the two long CrMn bonds (3.019(3) and 3.104(4) Å) are combined with the shorter Cr Cr bond (2.651(6) Å) in one triangle and, vice versa, the less extended CrMn bonds (2.839(4) and 2.967(3) Å) are combined with a longer CrCr bond (2.726(6) Å) in the other triangle of Cr₂Mn. By the reaction of Ia with [CpFe(CO)₂]₂ (taken in the ratio of $\text{2}\text{1}$) in boiling toluene, the antiferromagnetic cluster [(CH₃C₅H₄)₂Cr₂(SCMe₃)(μ₃-S)₂]₂Fe (III) has been synthesized in which the same distortions as in cluster II are present, as revealed by X-ray analysis. In the metallospirane core of the molecule of III, the Cr₂Fe triangles make an angle of 113.84° with each other. In this cluster, the CrCr distances in the peripheral binuclear fragments (CH₃C₅H₄)₂Cr₂(μ-SCMe₃)(μ₃-S)₂ are practically equal (2.688(3) and 2.661(3) Å), whereas the FeCr bond lengths are markedly different (2.749(2) and 2.827(2) Å in on triangle and 2.910(2) and 2.969(2) Å in the other). The dependence of the geometries of clusters II and III on the steric effects of the methyl substituents in the cyclopentadienyl ligands and on the electronic effect of the central metal atom (Mn^II or Fe^II) is discussed.     

Synthesis and structure of the heterotrinuclear chromium-palladium clusters (RC5H4CrCl)2-(μ3S)(μ-SCMe3)2Pd(PPh3) with ferromagnetic exchange interactions over the CrSCr system

Reactions of (RC₅H₄)₂Cr₂(SCMe₃)₂S(I, R = H; II, R = Me) with (PPh₃)₂PdCl₂ in benzene at 20°C gives trinuclear complexes (RC₅H₄)₂Cr₂Cl₂(μ₃-S)(μ-SCMe₃)₂Pd(PPh₃)(III, R = H; IV, R = Me). The structure of IV as a monobenzene solvate is established by an X-ray analysis (black-green triclinic crystals space group P$\text{1}$ with a = 11.403(4), b = 14.933(5), c = 14.131(5) Å, α = 99.13(3), β = 112.72(3), γ = 95.65(3)°, V = 2201.6 Å, Z = 2; IV·C₆H₆). The structure was solved by direct methods and refined in an anisotropic approximation to R = 0.046, R_w = 0.058 for 7643 reflections with I ? 2σ(I). In the molecule of IV metal atoms are separated by non-bonding distances (Cr … Cr 4.079(I), Cr … Pd 3.230(I) and 3.380(I) Å) but linked by the bridging tridentate sulphur atom (CrS 2.339(2) and 2.329(2), PdS 2.327(2) Å), and two SCMe₃ groups between Pd and Cr (CrS 2.396(2) and 2.403(2), PdS 2.350(2) and 2.381(2) Å, Cr?Pd 85.14(6) and 89.92(6)°). The Cl atoms are transferred from Pd to Cr atoms (CrCl 2.308(2) Å) and being terminally coordinated are in trans-positions to each other (as well as η-CH₃C₅H₄ rings) with respect to the Cr₂Pd plane. Cr atoms in III and IV exhibit ferromagnetic exchange interactions over the Cr?Cr system (+2J = 28 and 11 cm^?1, respectively).     

Étude de l'oxydation de triphénylamines par voltammétrie à variation linéaire de tension en couche mince

The mechanism of the oxidation of substituted triphenylamines R₁R₂R₃N (R₂=R₃=C₆H₅, R₁=p-OCH₃C₆H₄ (1) or p-CH₃C₆H₄ (2); R₁=R₂=R₃=p-OCH₃C₆H₄ (3) or C₆H₅ (4)) has been investigated by using thin layer linear potential sweep voltammetry. The tetraphenylbenzidine which is formed during the oxidation results from a dimerization of two cation radicals. The rate constants are calculated for (1) and (2).     

Structure of arsenic 8-mercaptoquinolinate: Synthesis and crystal structure of arsenic 4-methoxy-8-mercaptoquinolinate As[C<Subscript>9</Subscript>H<Subscript>5</Subscript>(OCH<Subscript>3</Subscript>)NS]<Subscript>3</Subscript>

Arsenic 4-methoxy-8-mercaptoquinolinate As[C₉H₅(4-OCH₃)NS]₃ (I) was synthesized and studied by X-ray diffraction. Crystals are trigonal: space group R3, a = b = 13.9867(4) Å, c = 12.4991(5) Å, γ = 120°, V = 2117.58(12) ų, ρ = 1.519 g/cm³, Z = 3. An arsenic atom in the crystal structure occupies a special position on axis 3. The structural unit of the crystal (neutral complex I) has symmetry C3. 4-Methoxy-8-mercaptoquinoline acts as a bidentate (N,S-) ligand. The coordination polyhedron of the arsenic atom is a symmetric octahedron (3S + 3N) or, with allowance for the lone electron pair, ψ-one-capped octahedron (3S + 3N + E). Bond lengths are as follows: As-S, 2.3179(7)Å; As…N 2.688(3) Å. The geometries of coordination polyhedra of arsenic atoms are compared in the crystal structures of As(C₉H₆NS)₃, As[C₉H₅(2-CH₃)NS]₃, and As[C₉H₅(4-CH₃)NS]₃.     

Site directed synthesis of mono and disubstituted SF5-polyfluoroalkyl benzenes

A novel method for the preparation of o-, m-, p-SF₅CF₂CFYC₆H₄X (Y = Br, F and X = m-Br, p-Br, Cl, CH₃, CF₃, NO₂, o-NO₂, F, CF₃, CH(CH₃)₂) derivatives was devised by a two-step reaction: SF₅Br-addition to o-, m-, p-CF₂CFC₆H₄X followed by reaction of AgBF₄ with the o-, m-, p-SF₅CF₂CFBrC₆H₄X adducts. Additional studies have been carried out with several derivatives and includes the preparation of SF₅CF₂C(O)C₆H₅, p-CF₃CFBrC₆H₄NO₂, SF₅CF₂CF₂C₆H₃(NO₂)₂, SF₅CF₂CF₂C₆H₃(NH₂)₂, and an SF₅CF₂CF₂-containing polyimide and dye. The complete characterization (IR, NMR, and MS) of these compounds is given.     

Aqua(2,2′‐di­amino‐4,4′‐bi‐1,3‐thia­zole‐κ2N,N′)(imino­diacetato‐κ3O,N,O′)­chromium(III) chloride mono­hydrate

Crystals of the title compound, [Cr(C₄H₅NO₄)(C₆H₆N₄S₂)(H₂O)]Cl·H₂O, consist of Cr^III complex cations, Cl⁻ counter‐ions and lattice water mol­ecules. The complex cation assumes an octahedral coordination geometry, formed by a tridentate imino­di­acetate dianion (IDA), a di­amino­bi­thia­zole (DABT) mol­ecule and a water mol­ecule. The planar DABT group chelates the Cr^III ion with normal Cr—N distances [2.0574 (17) and 2.0598 (17) Å], but the DABT mol­ecule is inclined to the coordination plane by a dihedral angle of 17.23 (7)°. In the monodentate carboxylate groups of the IDA ion, the coordinated C—O bonds [1.288 (3) and 1.284 (3) Å] are much longer than the uncoordinated C—O bonds [1.222 (3) and 1.225 (3) Å].     

Chlorobis(polyfluorophenyl)thallium(III) complexes and their reactions with gold(I) and tin(II) compounds

The reaction of TlCl₃ with RLi leads to complexes of the general formula TlR₂Cl (R = C₆F₅, p-C₆F₄H, m-C₆F₄H, 2,4,6-C₆F₃H₂, p-C₆FH₄ or m-CF₃C₆H₄). Some of these undergo oxidative addition reactions with gold(I) complexes to give polyfluorophenyl derivatives of the types AuR₂ClL and Au(C₆F₅)R₂(tht) (tht = tetrahydrothiophen), and with SnCl₂ to give oily materials from which stable solids of the general formula Q[SnR₂Cl₃] can be isolated by addition of QCl (Q = Et₄N or Ph₃BzP).     

Bis[trans‐dibromo(2,2‐di­methylpropane‐1,3‐diamine‐κ2N,N′)chromium(III)] dibromide hydrogen perchlorate hexahydrate

In the title compound, [CrBr₂(C₅H₁₄N₂)₂]₂Br₂·HClO₄·6H₂O, there are two independent Cr^III complex cations which are conformational isomers of each other. The Cr atoms lie respectively on a center of symmetry and on a mirror plane and have octahedral environments, coordinated by the N atoms of two 2,2‐di­methylpropane‐1,3‐diamine ligands and by two Br atoms in trans positions. The Cr—N and Cr—Br bond lengths are in the ranges 2.078 (3)–2.089 (3) and 2.4495 (9)–2.5017 (9) Å, respectively. The crystal structure consists of two Cr^III complex cations, two Br^? anions, a (ClO₄)^? anion and an [H₁₃O₆]⁺ hydrogen‐bonded cluster cation.     

Triruthenium carbonyl complexes containing bidentate pyridine–alkoxide ligands for highly efficient oxidation of primary and secondary alcohols

Reactions of substituted pyridylalkanol 6-CH₃PyCH₂CH(OH)R (R = Ph (L¹H), R = 4-CH₃C₆H₄ (L²H), R = 4-OCH₃C₆H₄ (L³H), R = 4-ClC₆H₄ (L⁴H), R = 4-BrC₆H₄ (L⁵H), R = 4-CF₃C₆H₄ (L⁶H)) with Ru₃(CO)₁₂ in refluxing tetrahydrofuran afforded the corresponding ruthenium carbonyl complexes [6-CH₃PyCH₂CHRO]₂Ru₃(CO)₈ (R = Ph ( 1a ), R = 4-CH₃C₆H₄ ( 1b ), R = 4-OCH₃C₆H₄ ( 1c ), R = 4-ClC₆H₄ ( 1d ), R = 4-BrC₆H₄ ( 1e ), R = 4-CF₃C₆H₄ ( 1f )) in good yields. These ruthenium complexes were well characterized using elemental analysis and Fourier transform infrared and NMR spectroscopies. Furthermore, their crystal structures were determined using single-crystal X-ray diffraction analysis. Complexes 1a – 1f were found to be highly active toward oxidation of a wide range of primary and secondary alcohols to corresponding aldehydes and ketones within 5 minutes in the presence of N-methylmorpholine-N-oxide as oxidant.     

A Comparative 19F NMR study of electronic effects in some fluoroaryl compounds of antimony and bismuth and in their carbon and nitrogen analogues

A number of m- and p-fluorophenyl compounds of type Ar₂QC₆H₄F (Q = Sb, Bi, N and CH; Ar = C₆H₅, C₆H₄F) have been prepared. The ^l0F chemical shifts relative to fluorobenzene have been measured in cyclohexane, chloroform and pyridine for all these compounds. On the basis of the data obtained, the electronic nature of the Ar₂Sb and Ar₂Bi substituents has been studied and compared with that of the corresponding (C₆H₅)₂N and (C₆H₅)₂CH groups. The electronic effect of the Ar₂Sb and Ar₂Bi substituents has been shown to be mainly inductive, its solvent susceptibility being in most cases close to zero relative to both electron- and proton- donating solvents.     

Syntheses, structures and reactivities of diindenyl-coordinated diiron bridging carbene complexes

Compound [Fe₂(μ-CO)₂(CO)₂(η⁵-C₉H₇)₂] (1) reacts with aryllithium reagents, ArLi (Ar = C₆H₅, p-CH₃C₆H₄, p-CF₃C₆H₄) followed by alkylation with Et₃OBF₄ to give the diindenyl-coordinated diiron bridging alkoxycarbene complexes [Fe₂{μ-C(OC₂H₅)Ar}(μ-CO)(CO)₂(η⁵-C₉H₇)₂] (2, Ar = C₆H₅; 3, Ar = p-CH₃C₆H₄, 4, Ar = p-CF₃C₆H₄). Complex 4 reacts with HBF₄ · Et₂O at low temperature to yield cationic bridging carbyne complex [Fe₂(μ-CC₆H₄CF₃-p)(μ-CO)(CO)₂(η⁵-C₉H₇)₂]BF₄ (5). Cationic 5 reacts with NaBH₄ in THF at low temperature to afford diiron bridging arylcarbene complex [Fe₂{μ-C(H)C₆H₄CF₃-p}(μ-CO)(CO)₂(η⁵-C₉H₇)₂] (6). The reaction of 5 with NaSC₆H₄CH₃-p under the similar conditions gave the bridging arylthiocarbene complex [Fe₂{μ-C(C₆H₄CF₃-p)SC₆H₄CH₃-p}(μ-CO)(CO)₂(η⁵-C₉H₇)₂] (7). Complex 5 can also react with carbonylmetal anionic compounds Na[M(CO)₅(CN)] (M = Cr, Mo, W) to produce the diiron bridging aryl(penta-carbonylcyanometal)carbene complexes [Fe₂{μ-C(C₆H₄CF₃-p)NCM(CO)₅}(μ-CO)(CO)₂(η⁵-C₉H₇)₂] (8, M = Cr; 9, M = Mo; 10, M = W). The structures of complexes 4, 6, 7, and 10 have been established by X-ray diffraction studies.     

Preparation of cobalt-containing bulky monodentate phosphines with electron-withdrawing/donating substituents on bridged arylethynyl and their applications in Suzuki coupling reactions

Several cobalt-containing bulky monodentate phosphines (μ-PPh₂CH₂PPh₂)Co₂(CO)₄(μ,η-(^tBu)₂PCC(C₆H₄R)) (4a: R=H; 4b: R=p-F; 4cp: R=p-CF₃; 4cm: R=m-CF₃; 4d: R=p-OMe) were prepared from the reactions of (^tBu)₂PCC(R-C₆H₄) (2a: R=H; 2b: R=p-F; 2cp: R=p-CF₃; 2cm: R=m-CF₃; 2d: R=p-OMe) with Co₂(CO)₆(μ-PPh₂CH₂PPh₂) 3. Further reactions of 4a, 4b, 4cp, 4cm, and 4d with Pd(OAc)₂ yielded unique palladium complexes (μ-PPh₂CH₂PPh₂)Co₂(CO)₄(μ,η-(^tBu)₂PCC(C₆H₃R)-κC¹)Pd(μ-OAc) (5a: R=H; 5b: R=p-F; 5cp: R=p-CF₃; 5cm: R=m-CF₃; 5d: R=p-OMe, respectively). The strong electron-withdrawing substituents, -F and -CF₃, assist the ortho-metalation process during the formation of 5b, 5cp, and 5cm. The more positively charged palladium center in 5b (or 5cp, 5cm) enhances the probability for PhB(OH)₃⁻ to attack the metal center and the rate of reduction thereafter. DFT studies on the charges of these palladium centers support this assumption. The enhancement of the reaction rates of the Suzuki-Miyaura cross-coupling reactions using 5b, 5cp, and 5cm is thereby attributed to this effect.     

Effets de micelles cationiques sur l'hydrolyse alcaline d'acetanilides a groupe acyle active par des substituants electro-attracteurs

Micellar effects of CTAB upon the alkaline hydrolysis of CF₃-CO-N(CH₃)C₆H₅, CHCl₂-CO-N(CH₃)C₆H₄X and CH₂Cl-CO-N(CH₃)C₆H₄X, (X=p-OCH₃H,p-Cl) are reported. Variations of k_obs, and of kinetic order of the reaction with respect to HO^? ion, are interpreted as an acceleration of HO^?-catalyzed steps, and a decrease of catalysis by water for decomposition of tetrahedral intermediates; these two effects oppose each other in HO^? and H₂O catalyzed steps. Differences between micellar and DMSO effects suggest a very small local concentration of HO^? ions in micelles.     

Reactions of palladium(II) cyclometallated benzylideneaniline Schiff's bases. Some relative rates for the synthesis of ortho-substituted carbomethoxy derivatives via CO insertion

The synthesis and characterisation of the complexes [(p-CH₃C₆H₄NCH($\text{C}{}_6\text{H}{}_3\text{Y))P}$d(OAc)]₂ (II) are reported. These complexes react at very different rates with carbon monoxide in methanol to give the ortho-substituted esters, p-CH₃C₆H₄NCHC₆H₃Y - 2R, R = CO₂CH₃, with electron withdrawing Y substituents slowing the reaction. The ¹³C{¹H} data for II show a linear correlation of δ(C(2)) in the 5′-complexes (Y trans to PdC) with δ(C(4)) of monosubstituted benzene compounds. For Y = 5′-NO₂, 4′-NO₂ and 4′-Cl, the bis complex [{p-CH₃C₆H₄NCH(

is formed in a secondary reaction.     

Crystal structure of 1-cyclohexylpiperazine-1,4-diium dichromate(VI)

Single crystals of 1-cyclohexylpiperazine-1,4-diium dichromate(VI), (C₁₀H₂₂N₂)[Cr₂O₇], were obtained by slow evaporation at room temperature from an aqueous solution of potassium dichromate, hydrochloric acid and 1-cyclohexylpiperazine. (C₁₀H₂₂N₂)[Cr₂O₇] is triclinic (P\(\bar 1\)) with a = 10.351(2) Å, b = 12.766(3) Å, c = 6.111(1) Å, α = 91.50(2)°, β = 104.26(3)°, γ = 94.91(2)°, V = 778.8(3) ų, and Z = 2. The structure determination performed from single crystal X-ray diffraction data leads to R₁/wR₂ reliability factors of 0.032/0.078. The asymmetric unit of the title salt C₁₀H₂₂N₂²⁺·Cr₂O₇^2?, consists of one 1-cyclohexylpiperazine-1,4-diium dication and one dichromate dianion. These entities are linked together by N–H···O hydrogen bonds to form {(C₁₀H₂₂N₂)[Cr₂O₇]}_n infinite chains lying parallel to the (100) plane and running along the c axis. The intermolecular N–H···O hydrogen bonds link these chains into a two-dimensional network structure consolidated through C–H···O weak interactions.     

Crystal structure,luminescence, and thermochromic properties of bis(tetraethylammonium) hexabromotellurate(IV)

The atomic structure of ((C₂H₅)₄N)₂TeBr₆ crystals (a = 17.930(8) Å, b = 11.133(5) Å, c = 15.022(7) Å, β = 109.28(9)°, space group C2/c, Z = 4, ρ_calcd = 2.036 g/cm³) has been studied by X-ray diffraction. The ((C₂H₅)₄N)₂TeBr₆ crystal structure consists of isolated [TeBr₆]^2? anions and ((C₂H₅)₄N)⁺ cations. The electronic and geometric aspects that influence the luminescence and thermochromic properties of the complex have been considered.     

Synthesis and spectroscopic studies of homo- and heteroleptic N-arylsalicylaldiminates of titanium(IV), zirconium(IV) and chromium(III)

Homo- and heteroleptic N-arylsalicylaldiminate derivatives of Ti^IV and Zr^IV of the type, MX_4–x (OC₆H₄CH=NAr) _x (X = OPrⁱ, x = 2,3; X = Cl, x = 1,2,3,4; Ar = C₆H₃Me₂-2,6, C₆H₃Et₂-2,6) have been prepared by reactions in the desired molar ratios of: (i) Ti(OPrⁱ)₄/Zr(OPrⁱ)₄·PrⁱOH with N-arylsalicylaldimines in benzene, and (ii) MCl₄ (M = Ti, Zr) with Me₃SiOC₆H₄CH=NAr or HOC₆H₄CH=NAr in the presence of Et₃N as a base or the potassium salt of N-arylsalicylaldimines in benzene. The three homoleptic derivatives of Cr^III, Cr(OC₆H₄CH=NAr)₃ (Ar = C₆H₂Me₃-2,4,6, C₆H₃Et₂-2,6, C₆H₃Prⁱ ₂-2,6) have also been prepared by salt-elimination. All of these new derivatives have been characterized by elemental analyses, spectroscopic [i.r., ¹H and ¹³C-n.m.r. (Ti and Zr complexes), and electronic (for Cr complexes)] studies, as well as molecular weight measurements.     

β-Diketiminato Rare-Earth Metal Complexes: The Influence of Monoatomic Substituents in the N-aryl Moieties on Structures and Properties

Treatment of β-diketiminate ligands bearing different N-aryl monoatomic substituents [HL^H = (C₆H₅)N = C(Me)CH=C(Me)NH(C₆H₅), HL^F = (2,6-F₂C₆H₃)N=C(Me)CH=C(Me)NH(2,6-F₂C₆H₃), and HL^Cl = (2,6-Cl₂C₆H₃)N=C(Me)CH=C(Me)NH(2,6-Cl₂C₆H₃)] with Ln(CH₂SiMe₃)₃(THF)₂ (Ln = Y and Lu) afforded a variety of β-diketiminato rare-earth metal complexes depending on substituents, namely, phenyl ring C–H bond activated complexes (L')(L^H)Lu(THF) ( 1b , L' = (C₆H₄)N = C(Me)CH=C(Me)N(C₆H₅)), six-coordinate homoleptic complexes (L^H)₃Ln [Ln = Y ( 1aa ), Lu ( 1bb )], five-coordinate monoalkyl complexes (L^F)₂Ln(CH₂SiMe₃) [Ln = Y ( 2a ), Lu ( 2b )], and four-coordinate dialkyl complexes (L^Cl)Ln(CH₂SiMe₃)₂ [Ln = Y ( 3a ), Lu ( 3b )]. All these complexes were characterized with NMR spectroscopy, and lutetium complexes 1b , 1bb and 3b were structurally validated by single-crystal X-ray diffraction analysis. Moreover, dialkyl complexes 3 promoted the polymerization of 2-vinylpyridine (2-VP) to produce atactic poly(2-vinylpyridine) (P2VP) with quantitative yield. On activation with an equimolar amount of [Ph₃C][B(C₆F₅)₄], complexes 3 afforded highly isotactic P2VP with an mm value up to 94 %. Both ¹H NMR spectrum and MALDI-TOF mass analysis of an oligomer indicate that the polymerization was initiated by coordination insertion of 2-VP into the Y-CH₂SiMe₃ bond.     

Kinetics and mechanism of dimerization of arylmercurials

The kinetics of dimerization of arylmercurials XC₆H₄HgCl (X ? H, p-CH₃, m-CH₃, p-Cl, m-OCH₃, m-CO₂C₂H₅, and o-OCH₃) in the presence of [ClRh(CO)₂]₂ was studied in hexamethylphosphoramide (HMPA). The experimental rate law obtained is ?d[ArHgCl]/dt=k[ArHgCl]². The kinetic parameters of these reactions have been reported, and the variation, therein, has been explained on the basis of steric effect of substituents. The apparent activation energy E is linearly proportional to pre-exponential factor lnA. A most plausible mechanism has been proposed on the basis of experimental results. © 1993 John Wiley & Sons, Inc.