bluuebunny/crossref_metadata_2025 huggingface dataset:
2‐Cyanoacrylates as reagents in heteroatomic synthesis (a review)
Authors: Gololobov Yuri G. , Krylova Tatiana O.
Published: January 2005

Several types of addition reactions to the CC bond of alkyl 2‐cyanoacrylates, CH2C(CN)COOR (1), are considered. The first examples deal with addition of CH‐Acids (pKa less than 13) and of H2S in the presence of catalytic amounts of strong amines, also of mercaptans, thiocarboxylic, and thiophosphoric acids. P‐Sulfenylchlorides and acidic phosphites add irreversibly at 20°C to form addition products in accordance with the distribution of charges in 1. HCl reversibly adds to 1 and to the acid chloride CH2C(CN)COCl (2). Alcohols and H2O also add reversibly to the acid CH2C(CN)COOH (3) and to esters of 1 to transform 1 and 3 into polymers. Triethylsilane in the presence of CF3COOH (4) reduces the CC bond of 1 and 3 to the corresponding saturated derivatives.

The second set of examples involves reactions of 1 with P‐III compounds in the presence or absence of 4. Ph3P as well as other weak nucleophiles reversibly add to 1 in the absence of 4 to cause instant polymerization. However, 4 protonates an initially formed zwitter‐ion in the reaction of 1 with Ph3P,(EtO)2PCl,Ph2PCl and thiourea to afford stable addition products. IR spectroscopy reveals the formation of H‐complexes of 4 with the CN and COOR groups of 1, which stimulates the addition of the weak nucleophile (o‐C6H4O2)PCl to the CC bond of 1. This reagent does not react with 1 in the absence of 4. Strong nucleophiles, Alk3P, and (Et2N)3P in excess irreversibly add at 20°C to 1 to form zwitterions, which specifically react with PhNCO to give stable products. 1,3,2‐Dioxaphospholes react with 1 either to form spirophosphoranes or 2‐cyano‐3‐phosphoranylpropionates.
Hypervalent tetracoordinate organobismuth compounds (10‐Bi‐4)
Authors: Chen Xiang , Yamamoto Yohsuke , Akiba Kin‐Ya
Published: January 2005

A stable hypervalent 10‐Bi‐4 species, tetraethylammonium bis[α,α‐bis(trifluoromethyl)benzenemethanolato(2‐)‐C2],Obismuthanate(1‐), was prepared by the reaction of bismuth trichloride with 2 equiv of lithium 2‐(2‐lithiophenyl)‐2‐propoxide derivatives. The ate complex was inert toward MeI, instead, the corresponding nonfluorinated analogue, bis[α,α‐bis(dimethyl)benzenemethanolato(2‐)‐C2, O]bismuthanate(1‐), was reactive enough toward MeI to give O‐methylated product. Regioselective methylation at the nonfluorinated methanolate was observed in the reaction of unsymmetrically substituted ate complex, [α,α‐bis(dimethyl)benzenemethanolato(2‐)‐C2, O[]α,α‐bis(trifluoromethyl) benzenemethanolato(2‐)‐C2, O]bis‐muthanate(1‐). Mechanism of isomerization of these ate complexes and related protonated compounds and the synthesis and stability of [α,α‐bis(trifluoromethyl)‐benzenemethanolato (2‐) ‐C2, O]diarylbismuthanate‐(1‐) were also described.
Tricoordinated phosphorus‐containing macrocycles: New synthetic strategies
Authors: Prévoté Delphine , Galliot Christophe , Caminade Anne‐Marie , Majoral Jean Pierre
Published: January 2005

Phosphorus dialdehydes RP (OC6H4CHO)2 (R = Ph, Me2N) react with phosphodihydrazides PhP(Y)‐[N(CH3)NH2]2 (Y = S, O) to give macrocycles 6a–c arising from [2 + 2] cyclocondensation reactions. Treatment of phosphodihydrazone PhP(S) [OC6H4CHN–N(Me)H]2 7 with phenyldichlorophosphine affords macrocycle 8 possessing tri and tetracoordinated phosphorus atoms. Clean desulfurization of thiophosphorus macrocycles 9 and 12 gives rise selectively to new tricoordinated phosphorus containing macrocycles 11 and 13.
Synthesis of onio‐, dionio‐, and trionio‐substituted phosphines; the nucleophilic behavior of DBN and DBU toward main group electrophiles
Authors: Bouhadir Ghenwa , Reed Robert W. , Réau Régis , Bertrand Guy
Published: January 2005

1,5‐Diazabicyclo[4.3.0]non‐5‐ene (DBN) and 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) react with chloro‐ and dichloro‐phosphines leading to onio‐ and dionio‐substituted phosphines. Similarly, onio‐substituted silicon and tin derivatives are prepared; they are used as onio‐substituent transfer reagents in the synthesis of a trionio‐substituted phosphine.
Novel synthesis of 2,6‐benzothiazonine derivatives
Authors: Kim Sung Hoon , Kim Kyongtae
Published: January 2005

The reaction of 1‐alkylamino‐1‐alkylthio‐3‐phenyl‐3‐thioxopropenes with phthaloyl chloride in toluene at 60°C, followed by treatment with triethylamine, afforded 6‐alkyl‐3‐phenyl‐2,6‐benzothiazonine‐1,5,7‐triones in good to excellent yields.
17O and 11B NMR spectroscopic study of substituted benzo[d]‐2,2‐difluoro‐1,3,2‐oxoniaoxaboratins
Authors: Boykin David W. , Kumar Arvind , Chandrasekaran Subramanian
Published: January 2005

17O NMR data are reported for 10 benzo[d]‐2,2‐difluoro‐1,3,2‐oxoniaoxaboratins derived from various ortho‐hydroxyacetophenones and for 2,2‐difluoro‐1,3,2‐oxoniaoxaboratins derived from related hydroxyacetyl naphthalenes and hydroxybenzophenones. The signal for the carbonyl‐like oxygen for these compounds is substantially shielded and appears at 288 ± 22 δ. The single bonded oxygen signal for the 1,3,2‐oxoniaoxaboratins appears at 122 ± 6 δ, except for the naphthalene analogs, whose signal appears at 146 ± 10 δ. The 11B NMR signal for these compounds is insensitive to structural changes and appears at 0.85 ± 0.25 δ. © 1995 John Wiley & Sons, Inc.
Phosphorylated imidazo[1,2‐a]pyridines
Authors: Tolmachev Andrew A. , Yurchenko Alexander A. , Kozlov Ernest S. , Merkulov Anatoliy S. , Semenova Marina G. , Pinchuk Aleksander M.
Published: January 2005

The reaction of phosphorus(III) halides with imidazo[1,2‐a]pyridines in the presence of bases leads to the formation of 3‐phosphorylated imidazo[1,2‐a]pyridines. The reaction proceeds in high yield and requires no catalysts.

In the compounds obtained, in contrast to phosphorylated indolizines, the phosphorus–heterocycle bond is stable and not cleaved by dry hydrogen chloride, alcohols, or water.

Imidazo[1,2‐a]pyridines with the phosphinic and phosphinous groups can be alkylated both at the phosphorus and at the nitrogen atom of the heterocycle, the alkylation direction being dependent on the strength of the alkylation reagent used. © 1995 John Wiley & Sons, Inc.
Preparation, crystal and molecular structure, and evaluation of plant growth regulating activity of guanidinoalkanephosphinates and phosphonates
Authors: Mucha Arthur , Tyka Roman , Kafarski Pawel , Glowiak Tadeusz , Goplañska Anna
Published: January 2005

A series of previously unknown α‐guanidinoalkanephosphonous, α‐ and β‐guanidinoalkanephosphonic acids has been prepared in order to study their structures and biological activity. Aminoalkanephosphonous and phosphonic acids have been converted into their guanidino derivatives by means of S‐methylisothiourea hydroiodide or cyanamide amidination. The crystal and molecular structures of three guanidino acids have been determined by single‐crystal X‐ray diffraction. The plant growth regulating activity of all synthesized guanidinoalkanephosphinates and phosphonates has been evaluated on Lepidium sativum. © 1995 John Wiley & Sons, Inc.
C‐Phosphorylated pyrazoles. Reaction of N‐methyl‐ and N‐phenylpyrazoles with phosphorus(III) halides
Authors: Tolmachev Andrew A. , Sviridon Anzhelika I. , Kostyuk Alexander N. , Pinchuk Alexander M.
Published: January 2005

The reactions of N‐methyl‐ and N‐phenylpyrazole derivatives with phosphorus(III) halides have been studied. The preparative method for synthesis of a variety of 4‐phosphorylated pyrazoles, including pyrazolyl substituted halo‐ and dihalophosphines, has been elaborated. Migration of an alkyl group from O to P(III) in 4‐phosphorylated 5‐alkoxypyrazoles was found to give a P‐ylides in a vinylogous manner with respect to the Arbuzov reaction. © 1995 John Wiley & Sons, Inc.
Methoxymethylsilylene: (1 + 6)‐ and (1 + 4)‐cycloadditions to heterotrienes
Authors: Heinicke Joachim W. , Gehrhus Barbara
Published: January 2005

Co‐pyrolytic gas phase reactions of 1,2‐dimethyl‐1,1,2,2‐tetra‐(methoxy)disilane with conjugated 1‐oxatrienes in a flow‐reactor furnish, via intermediate methoxymethylsilylene, mixtures of diastereoisomers of 3‐phenyl‐1‐oxa‐2‐silacyclohepta‐4,6‐dienes and 3‐styryl‐1‐oxa‐2‐silacyclopent‐4‐enes, usually including smaller amounts of the corresponding 1‐oxa‐2‐silacy‐clopent‐3‐ene isomers. In co‐pyrolysis with an analogous N‐isopropylazatriene, five‐membered rings were formed preferentially, and seven‐membered isomers could not be detected by NMR spectroscopy. © 1995 John Wiley & Sons, Inc.
Crystal and molecular structure of novel 10‐membered ring compound: 10‐Chloro‐1,7‐ditosyl‐8‐phenyl‐4,1,7‐benzoxadiazecine
Authors: Bartczak Tadeusz J. , Kajkowski Tomasz , Trzebiñski Piotr , Mikiciuk‐Olasik Elżbieta , Kotełko Barbara
Published: January 2005

10‐Chloro‐1,7‐ditosyl‐8‐phenyl‐4,1,7‐benzoxadiazecine is obtained by reacting the sodium salt of N,N′‐ditosyl‐2‐amino‐5‐chlorobenzhydrylamine with the ditosylate of bis(hydroxyethyl) ether in DMF. The structure of the title compound was established by X‐ray crystal structure analysis. The 10‐membered ring that is the core of the molecule is asymmetric and considerably folded. © 1995 John Wiley & Sons, Inc.
Crystal and molecular structure of cis‐ and trans‐2,4‐dihydroxy‐2,4‐dimethylcyclohexanetrans‐1‐acetic acid γ‐lactone
Authors: Wieczorek Michal W. , Majzner Wieslaw , Sikora Magdalena , Kula Józef
Published: January 2005

During the transformation process of limonene to tetrahydrofuran derivatives, the title compounds (±)‐(4) have been obtained as crystalline products and subjected to X‐ray analysis. The crystals of trans‐(4) are orthorhombic, space group P212121, with the lattice constants a = 7.0445(5) Å, b = 10.0908(4) Å, c = 14.0309(6) Å; the absolute configuration at atoms C1, C2, and C4 is Rc1, Sc2, and Rc4, respectively. The isomeric form cis‐(4) crystallizes in the monoclinic system, space group P21, with the following unit‐cell parameters: a = 10.8275(4) Å, b = 8.6994(5) Å, c = 16.4722(6) Å, β = 106.515(3)°. The asymmetric part of the unit cell of cis‐(4) contains three independent molecules. Each of these three molecules has the identical absolute configuration at all centers of chirality: Sc1, Sc2, and Rc4. © John Wiley & Sons, Inc.
Synthesis of some new spiroindoline derivatives incorporated with pyrazoloheterocycles
Authors: Al‐Thebeiti Marzoog S. , El‐Zohry Maher F.
Published: January 2005

Indole‐2,3‐dione (1) was treated with malonic acid (2) in a mixture of ethanol/pyridine to afford 1‐[3‐(2‐oxoindolinylidene)]acetic acid (3). Compound 3 reacted with thionyl chloride to give the corresponding acid chloride (4). The acid chloride 4 reacted with arenes in the presence of AlCl3 to yield 3‐(2‐oxoindolinylidene)acetophenones 5a–c. Compounds 5a–c reacted with 3‐methylpyrazolin‐5‐one derivatives 6a,b to give 3‐aracyl‐3‐[4′‐(3′‐methylpyrazolin‐5‐onyl)]‐indoline‐2‐one derivatives 7a–f. Compounds 7a–f were treated with phosphorus pentoxide in phosphoric acid, with ammonium acetate or methanolic methylamine and with phosphorus pentasulfide to give spiro[indoline‐3,4′‐(pyrazolo[4,5‐b]pyran)]‐2‐ones 8a–f, spiro[indoline‐3,4′‐(pyrazolo[4,5‐b]‐dihydropyridine)]‐2‐ones 9a–f, 10a–f and spiro[indoline‐3,4′‐(pyrazolo[4,5‐b]thiopyran)]‐2‐ones 10a–f, respectively. All of the synthesized spiroheterocycle derivatives were identified by conventional spectroscopic methods (IR, 1H NMR) and elemental analyses. © John Wiley & Sons, Inc.
Reductive allylation of poly(chlorotrifluoroethylene)
Authors: Taylor Richard T. , Allison Steven , Kamolratanayothin Thanaporn
Published: January 2005

Treatment of poly(chlorotrifluoroethylene) with allyltin reagents results in the clean replacement of chlorine by the allyl group under free‐radical–chain conditions (AIBN, allyltributyltin), affording about a 10% degree of functionalization. An improved method using one‐electron reduction of the polymer with cobalt(II) (generated by reduction of chloro(pyridine)bis(dimethylglyoximato)cobalt(III) with Mg) and allyltributyltin gives 50% functionalization. Further modification of the allyl group is realized through hydroboration‐oxidation to the alcohol and subsequent Cr(VI) oxidation to the acid. A mechanistic discussion is provided. © John Wiley & Sons, Inc.
Syntheses and characterization of new (2S)‐2‐hydroxy‐1,4‐oxazin‐3‐ones derived from β‐aminoalcohols
Authors: Mancilla Teresa , De Jesus Rosales Ma. , García‐Baez Efren V. , Carrillo Lourdes
Published: January 2005

The reactions of methyl glyoxylate hemiacetal 1 with 2‐(methylamino)ethanol 2, (1R,2S)‐(–)‐ephedrine 3 and (1S,2S)‐(+)‐pseudoephedrine 4 provide the (2S)‐2‐hydroxy‐1,4‐oxazin‐3‐ones 2a, 3a, 4a in good yield. They were characterized by 1H, 13C, NMR, and infrared and mass spectroscopy. The structures of 2a and 3a were established by X‐ray diffraction. The configuration of C2 (S) is demonstrated by 1H NMR data and confirmed for compounds 2a and 3a by single‐crystal X‐ray diffraction studies. © John Wiley & Sons, Inc.
Acid‐Induced cyclotrimerization of a phosphaalkyne with formal incorporation of water [1]
Authors: Mackewitz Thomas W. , Bergsträßr Uwe , Leininger Stefan , Regitz Manfred
Published: January 2005

tert‐Butylphosphaacetylene 1 can be cyclotrimerized with incorporation of water to furnish the 1,2,4‐triphosphabicyclo[2.1.1]hex‐2‐ene 5 in the presence of trifluoroacetic acid. This novel bicyclic system has been fully characterized by spectral and analytical data. The constitution of 5 has been unambiguously demonstrated by X‐ray crystallography. © John Wiley & Sons, Inc.
Crystal and molecular structure of cyclic sulfoxides: 2‐cyano‐2‐ethoxycarbonyl‐3,6‐dihydro‐4,5‐dimethyl‐2H‐thiapyran 1‐oxide and 2‐phenyl‐2‐methoxycarbonyl‐3,6‐dihydro‐2H‐thiapyran 1‐oxide
Authors: Wieczorek Michał W. , Błaszczyk Jarosław , Dziuba Błażej , Kiełbasiński Piotr , Mikołajczyk Marian , Zwanenburg Binne , Rewinkel Johannes B. M.
Published: January 2005

Crystal and molecular structures of the title compounds 1b and 1c have been determined by the X‐ray method. Crystals of 1b are monoclinic, space group P21/c, and crystals of 1c are orthorhombic, space group P212121. The sulfinyl oxygen atom and the alkoxycarbonyl group in both compounds are trans oriented, and the six‐membered rings adopt a sofa conformation. The differences between conformation, bond lengths, and angles in the compounds investigated are discussed. © John Wiley & Sons, Inc.
Characterization of molecular orbitals by counting nodal regions
Authors: Hatano Yasuyo , Yamamoto Shigeyoshi , Tatewaki Hiroshi
Published: January 2005

The number of nodal regions can be used as an index for characterizing molecular orbitals. A computer program has been developed to count the number of nodal regions, based on the labeling and contraction algorithms. This program is applied to the water molecule, the hydrogen sulfide molecule, the hydrogen atomic orbitals, the Rydberg excited states of ethylene, dissociation of carbon monoxide, and CASSCF calculations of formaldehyde. Because the number of nodal regions is independent of the coordinate system, the method is applicable even when the molecular structure changes drastically as in bond rotation or bond elongation. Changes of nodal regions with bond elongation are investigated for carbon monoxide. A prescription for problems arising with basis set expansion techniques is also given. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 325–333, 2005
The measure of wear in N‐vinyl pyrrolidinone (NVP) modifed glass‐ionomer cements
Authors: Yamazaki Tomoko , Brantley William , Culbertson Bill , Seghi Robert , Schricker Scott
Published: January 2005

Glass‐ionomer cements (GICs) are very useful and important as a dental restorative material because of their unique properties, such as adhesion to moist tooth structure and base metals, long‐term fluoride release that inhibits recurrent caries, thermal expansion coefficient that closely matches tooth structure, biocompatibility and low cytotoxicity. Poor mechanical properties, however, have prevented wider clinical use of these materials. Recent advances have shown that the use of NVP as a co‐monomer in the glass‐ionomer polyacid can improve mechanical properties. This work will focus on the synthesis of four NVP modified polyacids and the evaluation of their mechanical properties and wear properties. There is no significant difference between the wear of a commercial glass‐ionomer (Fuji IX) and the four experimental materials. However, the attrition component of wear appears to correlate well with the measured mechanical properties. Copyright © 2005 John Wiley & Sons, Ltd.
Electrical characterization of organic based transistors: stability issues
Authors: Gomes Henrique L. , Stallinga Peter , Dinelli Franco , Murgia Mauro , Biscarini Fabio , de Leeuw Dago M. , Muccini M. , Müllen K.
Published: January 2005

An investigation into the stability of metal insulator semiconductor (MIS) transistors based on α‐sexithiophene is reported. In particular the kinetics of the threshold voltage shift upon application of a gate bias has been determined. The kinetics follow a stretched‐hyperbola type behavior, in agreement with the formalism developed to explain metastability in amorphous–silicon thin film transistors. Using this model, quantification of device stability is possible. Temperature‐dependent measurements show that there are two processes involved in the threshold voltage shift, one occurring at T≈220 K and the other at T≈300 K. The latter process is found to be sample dependent. This suggests a relation between device stability and α‐sexithiophene deposition parameters. Copyright © 2005 John Wiley & Sons, Ltd.
Nanocomposite systems based on unsaturated polyester and organo‐clay
Authors: Mironi‐Harpaz I. , Narkis M. , Siegmann A.
Published: January 2005

Unsaturated polyester (UP) systems give rise to numerous possible approaches in synthesizing nanocomposites. A simultaneous mixing method was used to synthesize UP‐resin/organo‐clay nanocomposites. The effects of various mixing processes, using several organically‐modified clay types, were investigated. The incorporation of these organo‐clays resulted in an intercalated structure, the extent of which depended mainly on the type of the clay organic treatment. Organo‐clays that exhibited the highest intercalation levels were further studied using a sequential mixing method. The UP‐alkyd (without styrene) was mixed with different organo‐clays. Processing parameters such as mixing modes, applied shearing levels, clay contents, and mixing‐temperatures were investigated. Prolonged high shear levels promoted the intercalation and exfoliation of the silicate layers, resulting in a better dispersion of clay particles. The high shear levels effects were achieved by vigorous mechanical mixing and were intensified by using large amounts of clay and optimized matrix viscosity. Rheological studies of the nanocomposites were found complementary and in correlation with morphological and thermal characterization. This methodological approach provides a basis for understanding the structuring processes involving the formation of the UP/clay nanocomposites and establishing materials‐processing‐structure interrelations. Polym. Eng. Sci. 45:174–186, 2005. © 2005 Society of Plastics Engineers.
Diamine coupling of maleic‐anhydride‐ modified polyethylene
Authors: Colbeaux Aimeline , Fenouillot Françoise , Gerard Jean‐François , Taha Mohamed , Wautier Henri
Published: January 2005

The efficiency of the diamines 1,12‐diaminododecane (C12N2) and (4,4′‐methylene)bis(2,6‐diethylaniline) (MDEA) used as coupling agents for maleic‐anhydride‐grafted polyethylene (PEgMA) was compared. The effect of the miscibility of the diamines on the course of the reaction with PEgMA was investigated. The reaction in the molten state with the functional PE was run at 150 °C in an internal mixer and the resulting modified PE was characterized. The coupling agents reacted very fast, especially the aliphatic diamine. The MDEA was slower. Moreover, the reaction of the latter with the anhydride functions was not total owing to its immiscibility with PEgMA. With C12N2, the NH2‐to‐anhydride‐function stoichiometry leading to the higher degree of coupling was 2, indicating that the imide ring was not formed at this temperature. In this case, the coupling of the PEgMA chains resulted in a highly branched structure characterized by a high viscosity of the molten polymer and a substantial insoluble PE‐rich fraction. Copyright © 2005 Society of Chemical Industry
Structure of radical intermediates appearing in the grignard reaction of monoketones
Authors: Maruyama Kazuhiro , Katagiri Toshimasa
Published: January 2005

The structure of radical intermediates appearing in the reactions of MeMgBr with aromatic monoketones was investigated by using ESR and visible spectroscopy. Stable radical intermediates in the reacting solutions were assigned to the dimeric radical ion pairs.
Effect of cosolvent additives on relative rates of photooxidation on semiconductor surfaces
Authors: Sackett Debra D. , Fox Marye Anne
Published: January 2005

The effect of solvent additives on the course of TiO2 photocatalyzed oxygenation of α‐methylstyrene has been studied. While the addition of small amounts of nonhalogenated alcohols to TiO2 powders suspended in acetonitrile was found to decrease the rate of photooxygenation, added halogenated alcohols increase the rate of reaction. In addition, the solvent additives affect the observed product distribution.
The effect of hydrophobic‐lipophilic interactions on chemical reactivity. 11. The dependence of the wrapping‐up capability of sodium carboxymethylamylose on its degree of substitution
Authors: Jiang Xi‐Kui , Li Xing‐Ya , Huang Bang‐Zhou
Published: January 2005

The dependency of the wrapping‐up capability of sodium carboxymethylamylose (NaCMA) on its degree of substitution (D. S. = 0·00, 0·12, 0·18, 0·24, 0·29, 0·35 and 0·41) have been studied, using two guest species, iodine and cetyl‐trimethylammonium bromide (CTAB). The λmax values of NaCMA‐iodine helical inclusion complexes decrease with increasing D. S. values and the amounts of encapsulated iodine by NaCMA as measured by amperometric titrations also decrease with increasing D. S. values. With CTAB as the substrate, the largest number of binding sites, n, and the dissociation constants Kd have been determined by the method of surface tension versus the CTAB concentration plots. The results show that n decreases while Kd increases with increasing D. S. values. All these observations point to the fact that the wrapping‐up capability of NaCMA decreases with increasing degrees of substitution. The results are discussed in terms of host‐guest and host‐solvent hydrophobic‐lipophilic interactions as well as intramolecular hydrogen‐bonding.
The Effect of hydrophobic‐lipophilic interactions on chemical reactivity. 12. Mechanistic investigations of NaCMA‐catalyzed hydrolysis of esters
Authors: Jiang Xi‐Kui , Li Xing‐Ya , Huang Bang‐Zhou
Published: January 2005

The dependence of the catalytic efficiencies of sodium carboxymethylamylose (NaCMA) for the hydrolysis of N‐lauryl‐3‐acetoxypyridinium iodide (1) and p‐nitrophenyl dodecanoate (3) on its degree of substitution (D. S. = 0·00, 0·12, 0·18, 0·24, 0·29 and 0·35) and on the pH values of the solutions (pH = 7·32, 7·80, 8·10 and 9·30) have been studied. At fixed D. S. values, the observed hydrolysis rates of 1 and 3 increase with increasing concentrations of NaCMA and follow saturation kinetics. At fixed concentration of NaCMA, the rates increase with decreasing D. S. values until they reach maxima at D. S. = 0·00. Furthermore, at any D. S. value the catalytic efficiency increases with increasing pH values of the solutions. All these results indicate that the hydroxyl groups are actually the principal catalyzing groups.
Reaction branching as a mechanistic alternative to tunneling in explaining anomalous temperature‐dependencies of kinetic isotope effects
Authors: Thibblin Alf
Published: January 2005

The temperature‐dependence of the kinetic isotope effects for branched reactions proceeding via a common intermediate has been simulated by calculations. It is shown that, under certain conditions, anomalously small isotope effects on the Arrhenius preexponential factors, as well as unusually large observed isotope effects, may originate from the branching.
Participation of beta carbon‐silicon bonds in the development of positive charge in five‐membered rings
Authors: Lambert Joseph B. , Wang Gen‐Tai
Published: January 2005

The development of positive charge β to silicon in a saturated five‐membered ring has been studied in the cis‐ and trans‐2‐(trimethylsilyl)cyclopentyl trifluoroacetates. The cis substrate solvolyzes in 97% trifluoroethanol at 25° C about 5 times faster than the analogous six‐membered ring, after correction for differences in ring strain. The trans substrate solvolyzes about 360 times more slowly than the analogous six‐membered ring. These changes are in agreement with a hyperconjugative mechanism for interaction between the silyl group and the developing positive charge. The expected cosine‐squared dependence of hyperconjugation on the SiCCX dihedral angle suggests that the cis dihedral angle is reduced somewhat from the 60° in the six‐membered ring, and the trans dihedral angle is reduced substantially from the 180° in the six‐membered ring.
Ab initio Mo calculations on the rearrangements of 7‐oxa‐2‐bicyclo[2.2.1]heptyl cations. The facile migration of acyl group in wagner‐meerwein rearrangements
Authors: Carrupt Pierre‐Alain , Vogel Pierre
Published: January 2005

Ab initio STO‐3G and 6‐31G minimized geometries of 5‐oxo‐ (9), 6‐oxo‐7‐oxabicyclo[2.2.1]hept‐2‐yl cation (12) and 7‐oxabicyclo[2.2.1]hept‐2‐yl cation (15) were obtained. The energy barriers for their Wagner‐Meerwein rearrangements to the more stable 5‐oxo‐ (11), 6‐oxo‐3‐oxabicyclo[2.2.1]hept‐2‐yl (13) and 3‐oxabicyclo[2.2.1]hept‐2‐yl (17) cations, respectively, have been evaluated and compared with those calculated for the rearrangement of the 5‐oxo (18) and 6‐oxobicyclo[2.2.1]hept‐2‐yl cations (20). In agreement with experimental data, the ‘true migratory aptitude’ of an acyl group is higher than that of β‐oxoalkyl group in competitive Wagner‐Meerwein rearrangments that are ‘energetically unbiased’. The ease of the acyl group 1,2‐shift toward an electron‐deficient center is related to the electron‐donating ability of the carbonyl group due to favorable n(CO) ⟷ σ σ ⟷ p(C+) hyperconjugative interaction.
Ion pairs in solvolysis reactions. Kinetic deuterium isotope effects for deprotonation of carbocation intermediates in aqueous solvent
Authors: Thibblin Alf
Published: January 2005

Solvolysis of 2‐X‐2‐phenylpropane (1‐X) in 25 vol% acetonitrile in water at 25°C produces 2‐hydroxy‐2‐phenylpropane (1‐OH) and 2‐phenylpropene (3). The carbocationic intermediate discriminates between different nucleophiles; azide anion, acetate anion, and methanol are more efficient nucleophiles than water, k/k = 42 kOAc/k = 3, and kMeOH/k = 2·9 (ratio of second‐order rate constants). The fraction of the elimination product 3 increases with increasing basicity of the leaving group X as well as by addition of general bases. The Brønsted parameter for this catalysis is small, β = 0·13, with substituted acetate anions. The kinetic deuterium isotope effect for the dehydronation of the intermediate has been measured (assuming the reaction from intermediate to alcohol is insensitive to isotopic substitution) employing the hexadeuterated substrate d6‐1‐X as k/k = 3·5 ± 0·2 for the chloride 1‐Cl with acetate anion, and, without added base, 3·1 ± 0·2 for the acetate 1‐OAc, and 3·1 ± 0·2 for the p‐nitrobnzoate 1‐PNB, respectively, and ∼5 for the protonated methyl ether 1‐OMeH+. The variation in isotope effect with change in leaving group is discussed in terms of elimination from contact ion pairs and ‘free’ carbocation. The overall kinetic isotope effect for the solvolysis was found to be k/k = 1·31 (1‐OMeH+), 1·38 (1‐OAc), 1·40 (1‐PNB), and 5·7 (1‐OH). These isotope effects consist of the isotope effect k/k for the formation of the substitution product 1‐OH and k/k for production of the olefin 3. It is concluded that the latter isotope effect is enlarged owing to a branched mechanism in which the deprotonation of the carbocationic intermediate competes with formation of the substitution product. As large an isotope effect as k/k ∼6·5 has been measured for 1‐OMeH+.
The use of IR spectroscopy to monitor the conversion of matrix isolated phenylacetyl chloride to phenylacetyl cation, without decarbonylation to benzyl cation
Authors: Jarret Ronald M. , Sin Ny , Ramsey Timothy , Saunders Martin
Published: January 2005

IR spectroscopy is presented as a convenient means to monitor the formation of acyl (and alkyl) cations from corresponding acid chlorides, with Lewis acids at low temperature in the solid state. Phenylacetyl chloride is co‐deposited (under vacuum at −173°C with antimony pentafluoride on a KBr window), as in the ‘molecular beam’ method of cation generation. The initial IR spectrum of the deposition shows (in addition to starting materials) that a small amount of phenylacetyl cation (2274 cm−1) has been formed. Warming the solid matrix to −123°C Promotes the smooth conversion of acid chloride to acyl cation. Ultraviolet irradiation (through quartz windows) at −123°C to −73°C facilitates this conversion but does not result in the loss of carbon monoxide from the acyl cation to form benzyl cation. When benzyl chloride is used in place of phenylacetyl chloride, there is no noticeable difference between IR spectra taken before and after warming and/or photolysis of the solid deposition.
Twisted internal charge transfer emission in B,B‐bis(mesityl) pyrroloboranes
Authors: Brittelli David R. , Eaton David F.
Published: January 2005

Spectral characteristics of several simple substituted B,B‐bis(mesityl)pyrroloboranes are reported which support a theoretical treatment by Bonacic‐Koutecky and Michl (J. Am. Chem. Soc. 107, 1765 (1985)) describing the excited states of simple aminoboranes as an example of twisted internal charge transfer. In the aminoboranes the pyrrolo moiety functions as the electron donor group and the empty p‐orbital of the boron atom as the acceptor.
Gas phase reactivities of isomeric ions: Methylenediphenylcyclopropane and diphenylmethylenecyclopropane
Authors: Pineda C.‐J. , Roth H. D. , Mujsce A. M. , Schilling M. L. , Reents W. D.
Published: January 2005

Gas phase ion/molecule reactions have been used to probe the structure of ions obtained by electron impact upon 1‐(diphenylmethylene)cyclopropane and 2,2‐diphenyl‐1‐methylenecyclopropane. The resulting ions of molecular composition C16H14 (m/z 206+) were reacted with charge transfer reagents (6·9 < IP < 8·8eV), giving evidence for the presence of isomeric ions with different reactivities. The less reactive ion is identified as a trimethylenemethane species (TMM+) in which one cyclopropane bond is broken; the more reactive ions are assigned as vertical ions in which the cyclopropane ring is unaffected. The vertical ions have recombination energies of 8·44 ± 0·05eV, whereas TMM+ has one of 7·41±0·05eV. The TMM+ fraction is not constant; it increases with increasing IP of the reagent. This is attributed to a reagent‐catalyzed isomerization of the vertical ion to TMM+. In addition, the reagent ions are observed to undergo a unique reaction with the neutral methylenecyclopropane derivatives: electron transfer and ring opening to yield TMM+. These findings limit the application of the customary equilibrium measurements as a method to determine the substrate IP.
Theoretical studies of the singlet and triplet potential energy surfaces of cyclobutanediyl
Authors: Pranata Julianto , Dougherty Dennis A.
Published: January 2005

Cyclobutanediyl (2) has been studied in both its singlet and triplet states by ab initio electronic structure theory. The triplet, which is the ground state of the molecule, exists in both C2h and C2v forms which interconvert via a Cs transition state. For the singlet, only a C2h form is found. It passes, via a Cs transition state, onto the C2v surface on which bicyclobutane (3) is the only minimum. The ring‐flipping (inversion) process in 3 includes the singlet biradical as an intermediate, and involves a novel, non‐least motion path similar to one previously proposed by Gassman. Semiclassical periodic orbit theory indicates that the various minima on both the singlet and triplet surfaces can interconvert via quantum mechanical tunneling.
Reassignment of the fundamental vibrations of azetidine from ab initio calculations
Authors: Nielsen Per Halfdan , Gajhede Michael
Published: January 2005

The harmonic vibrational frequencies were calculated analytically at the 6‐31** level for azetidine using the GAUSSIAN 82 program. The results strongly indicate the presence of several errors in a recent assignment of the fundamentals of azetidine based on a normal coordinate analysis and a revised assignment is suggested. It is concluded that reliable vibrational data for azetidine in the gas phase are needed in order to resolve the remaining ambiguities in the interpretation of the spectra.
Mechanism of the Grignard reaction
Authors: Maruyama Kazuhiro , Katagiri Toshimasa
Published: January 2005

Mechanism of the Grignard reactions of aromatic ketones in THF was studied by spectroscopic and kinetic methods. The stable radical intermediates generated in the initial electron transfer from Grignard reagent to ketones are in a state of aggregated dimer of corresponding ion–radical pairs; in which two ketone anion radicals are bridged by a dimer di–cation of Grignard reagent. Subsequent alkyl radical transfer from dimeric Grignard reagent cation moiety to ketone anion radical aggregated each other are promoted by a participation of another neutral Grignard reagent. Proposed mechanism by present authors is able to explain well addition products/reduction products ratios in the Grignard reactions.
Alkylating properties of phosphate esters. 3. Reactivity of dialkyl 2‐(dimethylamino) ethyl phosphates
Authors: Modro T. A. , Moorhoff C. M.
Published: January 2005

Reactivity of dialkyl 2‐(dimethylamino)ethyl phosphates, (RO)2P(O)OCH2CH2NMe2 (R = Me, PhCH2) was studied in aqueous solutions. Depending on the initial substrate's concentration, reaction can involve the unimolecular fragmentation to N,N‐dimethylaziridinium dialkylphosphate, or the bimolecular isomerization to the zwitterionic derivative. The latter reaction proceeds via two consecutive SN2 steps and involves the formation of two ionic intermediates which were synthesized independently and allowed to react to give the zwitterionic product. Rate constants for the isomerization of the dimethyl ester (R = Me), as well as rate constant for the reaction between the corresponding intermediates have been determined, and the reactivity of the dimethyl ester has been compared with that of the dibenzyl derivative.
Crystal and molecular structures of dimesitylketene and bis(3,5‐dibromo‐2,4,6‐trimethylphenyl)ketene. Intrinsic torsional angles and buttressing effects in 1,1‐dimesitylvinyl propellers
Authors: Biali Silvio E. , Gozin Michael , Rappoport Zvi
Published: January 2005

The crystal and molecular structures of dimesitylketene (1) and bis(3,5‐dibromo‐2,4,6‐trimethylphenyl)ketene (2), the first free ketenes to be studied by X‐ray diffraction, have been determined. The molecules of 1 and 2 have crystallographic C1 and C2 symmetry, respectively, and exist in a propeller conformation. Molecular mechanics calculations of 1 and 2 using a modification of Allinger's new allene parameters reproduce satisfactorily the structural parameters of the ketenes. The torsional angles of the aryl rings in 1 and 2 are similar (ca. 50°) and are regarded as ‘intrinsic’ values for a 1,1‐dimesitylvinyl propeller. From the similarity in aryl torsional angles in 1 and 2 it is concluded that the buttressing effects in 2 (if any) are not markedly manifested in the structural parameters of 2.
A theoretical and experimental investigation of acid catalyzed rearrangements of small [n]cyclophanes
Authors: Kostermans Gerardus B. M. , Kwakman Pieter J. , Pouwels Petra J. W. , Somsen Gus , De Wolf Willem H. , Bicklehaupt Friedrich
Published: January 2005

The difference in reactivity of small [n]cyclophanes towards CF3CO2H is discussed in terms of charge densities, strain energies and proton affinities. These data are calculated with MNDO and MINDO/3 for para‐, meta‐ and ortho‐cyclophanes and for their ipso‐protonation products; an attempt is made to transform gas phase ΔH values into liquid phase ΔH0f values. Experimental evidence is presented that the acid catalyzed rearrangement of [5]paracyclophane to its ortho‐isomer proceeds via two consecutive 1,2‐carbon shifts without deprotonation; intermediate adducts were identified by NMR‐spectroscopy. Thus, a gradual shift in reaction pattern in the series [4]‐, [5]‐ and [6]paracyclophane is observed experimentally, in line with the calculational results.
Oxygen‐17 NMR study on substituted α, α, α‐trifluoroacetophenones
Authors: Liu Kwang‐Ting , Wu Tseng‐Rong , Lin Ying‐Chih
Published: January 2005

An excellent linear correlation of oxygen‐17 substituent chemical shifts (SCS) for twelve α, α, α‐trifluoroacetophenones with single σ+ constants, δ = 24·6σ+ + 556·3 (correlation coefficient 0·998) has been observed. However, from this plot, an SCS of 34·3 ppm for 1,1,1‐trifluoroacetone will give a corresponding σ+ (γ+) value of 1·44 for a methyl group, which is not in agreement with 0·63–0·79 obtained from solvolytic rate data.
Free‐radical reactions of cyclic ethers and sulfides with bromotrichloromethane
Authors: Hallen Richard T. , Gleicher Gerald Jay , Mahiou Belaid , Clapp Gary E.
Published: January 2005

The relative rates of hydrogen atom abstraction from a series of twelve saturated cyclic ethers and sulfides were determined at 70°C. The abstracting radical could be generated from bromotrichloromethane both photolytically or by the thermal decomposition of AIBN. The reaction rates did not show a dependence upon method of radical generation. Reaction occurred only at the position adjacent to the heteroatom. The reactivity of the cyclic ethers was in the order C4H8O > C6H12O > C3H6O > C5H10O. This trend would indicate appreciable influence by ring strain, however, the slightly greater reactivity of tetrahydrofuran relative to oxepane suggests a contribution by stereoelectronic factors as well. The reactivity of the cyclic sulfides, which reacted faster than the corresponding ethers, was in the order C4H8S > C5H10S > C6H12S. This would imply little influence of ring strain. The major structural effect would be that of variable electron donating ability of the sulfur atom. The rate of reaction of thietane was also determined. It was found to preferentially undergo SH2 attack at the sulfur atom followed by ring opening rather than hydrogen abstraction. The reactivities of both series of compounds were decreased by the inductive effect of a second heteroatom beta to the reaciton site.
Photochemistry of matrix‐isolated norbornadiene
Authors: Gȩbicki Jerzy , Kuberski Sławomir , Kamiński Rafał
Published: January 2005

Norbornadiene, isolated in argon, xenon and nitrogen matrices at 20 K, was irradiated with UV light. Characterized by UV–VIS–IR absorption spectroscopy the photoproduct, representing a single species, was assigned to quadricyclane. This result is compared with the literature data on photochemistry of norbornadiene in the gas and liquid phase. The role of the rigid matrix environment on selectivity of the photochemical reaction is discussed.
Bifunctionally catalyzed 1,3‐proton transfer of a propene by a sec‐amidine studied by deuterium isotope effects. A stepwise two‐proton transfer mechanism
Authors: Ahlberg Per , Janné Kjell , Löfås Stefan , Nettelblad Folke , Swahn Lena
Published: January 2005

A two‐hydron transfer mechanism involving hydron transfers from carbon to nitrogen and from nitrogen to carbon was studied. The rearrangement of 1,3,3‐triphenylpropene (1) into 1,1,3‐triphenylpropene (2) catalyzed by 2,10‐diazabicyclo[4.4.0]dec‐1‐ene (3) in benzene at 25·00°C was studied by 2H‐labeling experiments and kinetic 2H‐isotope effects. The synthesis and purification of [6,10‐2H2]‐2,10‐diazabicyclo[4.4.0]dec‐1‐ene ([6,10‐2H2]‐3), [3‐2H]‐1,3,3‐triphenylpropene ([3‐2H]‐1), [3‐2H]‐1,1,3‐triphenylpropene ([3‐2H]‐2) and [3,3‐2H2]‐1,1,3‐triphenylpropene ([3,3‐2H2]‐2) together with their precursors are reported. Partial reaction of [3‐2H]‐1 with [6,10‐1H2]‐3 gave 42% conversion into product 2, which was shown by 1H NMR to be composed of 88% [3‐1H]‐2 and 12% [3‐2H]‐2. Partial reaction of [3‐1H]‐1 with [6,10‐2H2]‐3 gave 43% of 2, composed of 73% [3‐1H]‐2 and 27% [3‐2H]‐2.

These results clearly show that a substantial fraction of the reaction takes place in a bifunctional manner but isotope exchange and/or monofunctionally catalyzed reactions interfere. The following kinetic deuterium isotope effects on the rearrangement 1 → 2 were measured: kHH/kDH = 6·56; kHH/kHD = 1·19; kHH/kDD = 7·08; kHD/kDD = 5·94; and kDH/kDD = 1·08.

On the basis of these results, a concerted two‐hydron transfer mechanism is excluded. Instead, a stepwise mechanism is favored, in which at first the 3‐hydron of 1 is abstracted by 3 yielding an ion pair(s), the carbanion of which in a separate step is then hydronated to yield the product 2.

The abstraction of the 3‐hydron from 1 might be hydrogen bond assisted. The two hydron transfer transition states are together rate limiting, although they limit the rate to different extents. A detailed mechanistic analysis is presented together with the results of an investigation of the nature of the catalyst. The dimerization constant for 3 was determined by 1H NMR to be 1·67 l mol−1 at 25·0°C. Isotopomer composition was measured by 1H NMR and GLC was used for the separation of the substrate and products. Computer‐assisted capillary GLC was used for the kinetics.
Dynamic isotope dilution kinetic study of the thermal conversions of bicyclo[3.2.0]hept‐2‐ene to bicyclo[2.2.1]hept‐2‐ene and to cyclopentadiene and ethene
Authors: Baldwin John E. , Belfield Kevin D.
Published: January 2005

Mixtures of bicyclo[3.2.0]hept‐2‐ene and bicyclo[2.2.1]hept‐2‐ene, with one isomer labeled with two deuterium atoms, were isomerized in the gas phase at 276 °C. By following the concentration ratios of d0 and d2 versions of bicyclo[2.2.1]hept‐2‐ene as functions of time one finds that the partitioning of bicyclo[3.2.0]hept‐2‐ene between the primary products bicyclo[2.2.1]hept‐2‐ene and cyclopentadiene plus ethene is 2:1. This and earlier stereochemical results permit the calculation of the partitioning of the [3.2.0] olefin between the orbital symmetry allowed [1,3] carbon shift‐with‐inversion product and several forbidden or non‐concerted reaction channels (approximately 1:1) and the stereochemistry of the ethene d2 formed from exo,exo‐[6,7‐2H2]bicyclo[3.2.0]hept‐2‐ene directly (1:1 E: Z).
Aromatic substitution by free benzyl cations from the nuclear decay of multi‐tritiated toluene. Competitive benzylation of benzene and toluene in the gas phase and in solution
Authors: Giacomello Pierluigi , Angelini Giancarlo , Sparapani Cinzia , Speranza Maurizio
Published: January 2005

Benzyl cations, free of counter ions, have been generated from the nuclear decay of tritium atoms contained exclusively in the side‐chain of multi‐labelled toluene and allowed to react in competition experiments with benzene and toluene. The aromatic substitution was studied in both the gaseous and liquid phase, employing pure aromatics or aromatics dissolved in n‐hexane, carbon tetrachloride and nitromethane as the reaction medium. In the gaseous systems, at 20 Torr total pressure, High meta substitution of toluene indicates extensive isomerization of the excited intermediates, while the increase of the kT/kB value in the Presence of NH3 suggests that selective transalkylation competes with proton transfer to ammonia from the benzyl cation adducts with benzene, but not with toluene. High positional and low substrate selectivities were measured in solution, the decrease in kT/kB (from 2·8 to 1·7) in the different solvents being accompanied by an increase in the o/2p ratio. Competition experiments between benzene and methanol indicate that the reaction is not diffusion controlled. The influence of the reaction medium on the substrate selectivity is explained by postulating the formation of a cation–solvent adduct, where the solvent molecule undergoes displacement by the substrate. Positional selectivity would be controlled in a subsequent step.
Influence of aromatic substituents on the configuration and conformation of calix[4]areneoctols
Authors: Mann G. , Weinelt F. , Hauptmann S.
Published: January 2005

Acid‐catalysed condensation of resorcinol with aromatic aldehydes results in 2,8,14,20‐tetrasubstitued calix[4] areneoctols. Sixteen compounds of this type containing different aromatic substituents were synthesized. The ring closure step, under the conditions of the reaction, is a reversible process. Four configurations are possible for every constitution, viz. cccc, cctt, ccct and ctct, but only cccc and cctt were formed in detectable amounts. In seven cases the thermodynamically more stable cccc isomers (1a–5a, 7a, 8a) and also the kinetically controlled cctt isomers (1b–5b, 7b, 8b) could be isolated. The configurations of the compounds were assigned by temperature‐dependent 1H NMR analysis. The conformational motions of the macrocyclic ring permit only the cccc isomers to show coalescence for the signals of the aromatic Hb protons. The coalescence temperature was determined for isomers 1a (ΔG = 83·5 kJmol−1) and 3a (ΔG = 83·7 kJ mol−1). For steric reasons the ‘chair–chair’ conformers B and the ‘quasi‐boat–chair’ conformers F are favoured. 1H and 13C NMR shifts show that compounds 1a–5a and 1b–5b have a quasi‐axial arrangement of the aromatic substituents, whereas 6–9 have a quasi‐equatorial arrangement of the substituents.
Decarboxylation of 6‐nitrobenzisoxazole‐3‐carboxylate ion in cationic micelles: Effect of head group size
Authors: Germani Raimondo , Ponti Pier Paolo , Romeo Tiziana , Savelli Gianfranco , Spreti Nicoletta , Cerichelli Giorgio , Luchetti Luciana , Mancini Giovanna , Bunton Clifford A.
Published: January 2005

The rate of decarboxylation of 6‐nitrobenzisoxazole‐3‐carboxylate ion increases sharply with increasing head group size in a series of cetyltrialkylammonium bromides (C16H33NR3Br: R = Me, CTABr; R = Et, CTEABr; R = n – Pr, CTPABr; R = n – Bu, CTBABr) with rate enhancements of 102 (CTABr) and 2·8 × 103 (CTBABr). Micellized tetradecylquinuclidinium bromide and hexadecyl‐N‐methylmorpholinium bromide are slightly better catalysts than CTABr, as is 1,3‐bis(N‐cetyl‐N,N‐dimethylamino)propane dibromide, but p‐octyloxybenzyltrialkylammonium bromides (alkyl = Me, n – Bu) are less effective than the corresponding CTA+ surfactants. These differences in catalytic efficiency depend on the head group structure and the extent to which the cationic head groups become less accessible to water rather than the overall micellar structure.
Anionic nucleophile–cation radical combination reactions. The super‐electrophilic properties of cation radicals in solution
Authors: Parker Vernon D. , Reitstöen Björn , Tilset Mats
Published: January 2005

Rate constants were determined for the combination reactions of a series of cation radicals derived from substituted anthracenes with acetate, p‐nitrobenzoate, trifluoroacetate, nitrate and perchlorate ions. Rate constants, depending on the identities of the cation radicals and the nucleophiles, ranging from about 200 to 2 × 1010 1 mol−1 s−1 were observed in acetonitrile at 293 K. The key steps in the reaction are (1) reversible complex formation between the anion and the cation radical followed by (2) irreversible bond formation

.

The preliminary results show that cation radical–anionic nucleophile reactions can be very facile. The cation radicals of 9‐nitro‐ and 9‐cyanoanthracene are particularly reactive, giving rise to rate constants close to the diffusion‐controlled limit with all anionic nucleophile studied and even react moderately rapidly with perchlorate ion. The reaction with perchlorate ion can be compared to the behavior of stable carbenium ions that coexist with the anion in solution and in crystalline salts.