bluuebunny/crossref_metadata_2025_split huggingface dataset:
Alkaline earth metal ion catalysis of alcoholysis of crown ether aryl acetates. Effect of the base–solvent system
Authors: Cacciapaglia Roberta , Mandolini Luigi , Reinhoudt David N. , Verboom Willem 
Published: January 2005

AbstractThe effect of strontium and barium ions on the acyl transfer reaction to ethoxide ion from 2‐acetoxy‐1,3‐xylyl‐15‐crown‐4, 2‐acetoxy‐1,3‐xylyl‐18‐crwon‐5, 2‐acetoxy‐1,3‐xylyl‐21‐crown‐6 and the model compound phenyl acetate was investigated in ethanol at 25°C. Significant catalytic effects were observed in all of the studied reactions as a result of stronger metal ion associations with the transition states than with reactants. The rate enhancements observed with the crown ether substrates are much larger than those observed with phenyl acetate, the largest acceleration being displayed by 2‐acetoxy‐1,3‐xylyl‐21‐crown‐6, which reacts with EtOBaBr 5 × 105 times faster than with EtONMe4. These findings provide strong evidence that cooperation of electrostatic binding to the negative charge developing at the carbonyl oxygen and coordinative binding to the polyether chain is highly efficient in the metal‐bound transition state. Comparison of rate data reported here with analogous data obtained in methanol solution clearly shows that both modes of interaction contributing to the stability of the metal‐bound transition state are much more efficient in ethanol than in methanol solution.
Guest dynamics and crystal structure of the tri‐o‐thymotide–acetone clathrate studied by solid‐state NMR, X‐ray diffraction and molecular modelling
Authors: Facey G. A. , Ratcliffe C. I. , Hynes R. , Ripmeester J. A. 
Published: January 2005

AbstractThe structure of the acetone complex of tri‐o‐thymotide was solved by single‐crystal x‐ray diffraction. The acetone molecule is situated on the twofold axis of the cage and shows no sign of unusual disorder. Solid‐state 2H and 13C NMR were used to show that, in addition to fast methyl group rotation, there is a rotation of the acetone molecule about the carbonyl bond. The motion is best described as a twofold flip plus the temperature‐dependent population of a secondary site 63 ± 10° away from the minimum energy position. Molecular modelling calculations were used to confirm the potential shape, which was remarkably sensitive to methyl group orientation. 2H spin–lattice relaxation was used to derive an activation energy of 13 ± 6 ± 0 ± 8 kJ mol−1 for the twofold flips, and the secondary site was shown to be energetically less favourable by 4.4 ± 0.5 kJ mol−1 from the temperature dependence of the line shape. Neither motion is seen in the diffraction experiment.
Conformational analysis of calix[n] arenes with chiral substituents by using circular dichroism
Authors: Ikeda Atsushi , Nagasaki Takeshi , Shinkai Seiji 
Published: January 2005

AbstractCalix [n] arenes (n= 4 and 6) bearing chiral substituents (R*) on the lower rim (1n) or on the upper rim (4n) were synthesized. The isomers derived from 14 were conformationally immobile and showed different circular dichroism (CD) spectra. In the 1H NMR spectra the proton signals in R* shifted to higher magnetic field when the lower rim is sterically crowded and in the CD spectra a strong CD band appeared. Therefore, the change in the CD spectra is rationalized in terms of R*…π interactions. In contrast, compound 44 was not conformationally immobilized and the CD spectra changed in response to the metal binding to the methoxy oxygens arranged on the lower rim. The specific metal interaction enabled the metal binding event to be detected by the CD technique.
Regioselectivity in a benzophenone‐mediated photo‐substitution of some cage‐shaped hydrocarbons
Authors: Chow Tahsin J. , Yan Jeng‐Kuo , Chen Yuh‐Liang 
Published: January 2005

AbstractBenzophenone‐mediated protolyses of five cage‐shaped compounds in benzene solutions result in the formation of phenyl‐substituted products with high regioselectivity. The regioselectivity is rationalized based on the relative bond strengths and structural parameters of each molecule.
Participation and rearrangement in the gas‐phase elimination kinetics of 3‐(o‐methoxyphenyl)propyl‐1‐methanesulphonate and 4‐(p‐methoxyphenyl)butyl‐1‐methanesulphonate
Authors: Martin Ignacio , Chuchani Gabriel , Dominguez Rosa , Rotinov Alexandra 
Published: January 2005

AbstractThe gas‐phase unimolecular elimination of the methanesulphonates in the temperature range 289.0‐331.4°C and pressure range 18–152.5 Torr follows a first‐order rate law. The rate coefficients for the homogeneous reactions are expressed by the following equations: for 3‐(o‐methoxyphenyl)propyl‐1‐methanesulphonate log[k1(s−1)] = (12.04 ± 0.32) − [(167.8 ± 3.6) kJ mol−1] (2.303RT)−1 and for 4‐(p‐methoxyphenyl)butyl‐1‐methanesulphonate log [k1(s−1)] = (12.82 ± 0.30) − [(175.1 ± 3.4) kJ mol−1] (2.303RT)−1. The oxygen atom of the CH3O substituent in 3‐(o‐methoxyphenyl)propyl‐1‐methanesulphonate appears to participate directly in the C–O bond polarization in order to produce some of the cyclic product dihydrobenzopyran. A parallel reaction occurs with 4‐(p‐methoxyphenyl)butyl‐1‐methanesulphonate where the p‐anisyl substituent participates in the elimination process through a five‐membered spiro intermediate for the formation of the cyclic product 6‐methoxy‐1,2,3,4‐tetrahydronaphthalene. The second pathway of this elimination takes place via normal formation of the corresponding unsaturated aromatic hydrocarbons. These reactions are interpreted in terms of an intimate ion‐pair type of mechanism.
Inclusion compounds of hydroxy hosts. 6. Structures and thermal analysis of 1,2,3,4,5‐pentaphenylcyclopenta‐2,4‐dien‐1‐ol with methanol and ethanol
Authors: Bourne Susan A. , Nassimbeni Luigi R. , Niven Margaret L. 
Published: January 2005

AbstractThe crystal and molecular structures of the methanol and ethanol inclusion compounds of 1,2,3,4,5‐pentaphenylcyclopenta‐2,4‐dien‐1‐ol have been elucidated. (I) C35H26O · CH4O is triclinic, PĪ, a = 10.377, b = 10.778, c = 14.537 Å, α = 102.66(2), β = 90.26(2), γ = 117.33(2)1, V = 1398(15) Å3, Z = 2; (II) (C35H26O)2 · C2H6O is triclinic, P1, a = 10.901(2), b = 13.498(2), c = 19.037(3)Å, α = 101.88(1), β = 93.76(1), γ = 90.84(1), V = 2734(8) Å3, Z = 2. The structures were refined to final residuals of 0.127 for 2207 reflections and 0.073 for 4720 reflections. Thermal analysis of the complexes confirmed the crystallographic results and gave information about their thermal decompositions.
Solvation and metal ion effects on structure and reactivity of phosphoryl compounds. 1. β‐substituted alkylphosphonic esters
Authors: Belciug Marie‐P. , Modro Agnes M. , Modro Tom A. , Wessels Philippus L. 
Published: January 2005

AbstractNMR spectroscopic and conformational analyses were performed for three β‐substituted β‐phenylethylphosphonic esters, PhCHXCH2PO3Me2 (X = OH, OMe, Cl) in five solvents and in acetone containing sodium and magnesium salts. Strong attractive interactions between the phosphoryl group and the oxygen‐containing substituent X were demonstrated, and were shown to involve intramolecular hydrogen bonding, donor‐acceptor O → P effect and the chelation of a metal ion. These effects led to marked selectivity in the population of individual conformations of the phosphonates. The analogous intramolecular effects were found to be much weaker in the corresponding carboxylic ester systems.
Theoretical studies on the gas‐phase rearrangement of deprotonated allyl phenyl ether
Authors: Kim Chang Kon , Lee Bon‐su , Lee Ikchoon 
Published: January 2005

AbstractThe Wittig and anoinic Claisen rearrangements of deprotonated allyl phenyl ether, PhOC̄HCH=CH2, were investigated by MO theory employing the AM1 method. The most favoured reaction pathway for the Wittig rearrangement is the intramolecularSNAr process proceeding by the addition‐elimination mechanism involving a Meisenheimer complex‐type, three‐membered‐ring intermediate. For the anionic Claisen rearrangement a three‐step mechanism is favoured, in which the intramolecular proton transfer occurs first from theorthoposition of the phenyl ring to the anionic carbon centre, which is then followed by a process involving a six‐membered ring intermediate. The two types of rearrangements can compete, but the Wittig type is more facile and favoured than the Claisen process owing to the lower activation enthalpy. The results are in good agreement with gas‐phase experimental results.
Acid‐catalysed depurination of DI‐, TRI‐ and polydeoxyribonucleotides: Effect of molecular environment on the cleavage of adenine residue
Authors: Hakala Harri , Oivanen Mikko , Saloniemi Eija , Gouzaev Andrei , Lönnberg Harri 
Published: January 2005

AbstractFirst‐order rate constants for the acid‐catalysed cleavage of adenine base from 2′‐deoxyadenosine, its 3′ ‐ and 5′‐ monophosphates, various dinucleoside monophosphates, trinucleoside diphosphates and hexameric 2′‐deoxyadenylic acid were measured by the method of initial velocity. The results obtained were compared with the effect that hetroassociation with caffeine has on the pre‐equilibrium protonation and rate‐limiting heterolysis of 2′ ‐deoxyadenosine and its isosteric analogue, 1‐(2‐deoxy‐β‐D‐erythro‐pentofuranosyl) benzimidazole. The kinetics of the depurination of poly (dA‐T), poly(dA) and a mixture of poly(dA) and poly(T) were determined, and the rate variations observed are discussed on the basis of the known structures of these polymers.
Generation of vinyl cations by nitrosation of N‐silylated imines and enamines
Authors: Alvarez Roberto martínez , Hanack Michael , Subramanian L. R. 
Published: January 2005

AbstractThe reaction of monosilylated imines and a disilylated enamine with nitrosyl salts in dichloromethane at −78°C is shown to proceed via intermediate vinyl diazonium salts and vinyl cations.
Precipitation of strong uncharged organic acids with various crown ethers or with acyclic polyethers from aqueous medium. Characterization of adducts in the solid state
Authors: Kolthoff Izaak M. , Chantooni M. K. 
Published: January 2005

AbstractIt was found that substituted picric acids form slightly soluble 1 : 2 : 2 adducts with several simple crown ethers or with tri‐, tetra‐, or pentaethylene glycol dimethyl ether (glymes 4, 5 and 6) in water, whereas 2,4,6‐trinitrobenzenesulfonic acid forms a 1 : 1 : 1 adduct with dicyclohexano‐18‐crown‐6 (cis‐syn‐cis). The ability of a crown ether to precipitate a given acid follows the order dicyclohexano‐18‐crown‐6 (cis‐syn‐cis) > benzo‐15‐crown‐5 > dicyclohexano‐24‐crown‐8 > 18‐crown‐6 > 21‐crown‐7, 15‐crown‐5 ≫ 12‐crown‐4, roughly in the order of its base strength. Precipitation can occur even when the concentration(s) of one or more of the components are < 10−4 M. The solubility products of the 18‐crown‐6 complexes with dichloro‐ or dimethylpicric acids were estimated. In the solid state the 1 : 2 : 2 dicyclohexano‐18‐crown‐6 (cis‐anti‐cis)–dichloropicric acid–water complex, which reportedly has two pseudohydronium ions lacking C3v symmetry of H3O+, exhibits an IR spectrum in the OH stretching region characteristic of an aqua complex.
Elimination kinetics and mechanism of primary, secondary and tertiary α‐hydroxycarboxylic acids in the gas phase
Authors: Chuchani Gabriel , Martin Ignacio , Rotinov Alexandra , Dominguez Rosa M. 
Published: January 2005

AbstractThe rates of elimination of primary, secondary and tertiary α‐hydroxycarboxylic acids were determined in a seasoned, static reaction vessel over the temperature range 280–390°C and the pressure range 30–201 Torr. The reactions, in the presence of a free radical inhibitor, are homogeneous, unimolecular and follow a first‐order rate law. The rate coefficients are given by the following equations: for glycolic acid, log k1 (s−1) = (14·03 ± 0·24) − (209·3 ± 1·5) kJ mol−1 (2·303RT)−1; for lactic acid, log k1 (s−1) = (12·24 ± 0·11) − (182·8 ± 1·3) kJ mol−1 (2·303RT)−1; and for 2‐hydroxyisobutyric acid, log k1 (s−1) = (12·91 ± 0·13) − (174·7 ± 1·5) kJ mol−1 (2·303RT)−1. The basicity and the ease with which the hydroxy group is removed from primary to tertiary α‐hydroxycarboxylic acids are reflected in rate enhancement. The mechanism of these eliminations appears to proceed through a semi‐polar five‐membered cyclic transition state.
α‐Versus β‐carbon nucleophilic attack in vinylic substitution
Authors: Shainyan B. A. 
Published: January 2005

AbstractThe mechanism of substitution of the bromine atom in (Z)‐α‐bromo‐β‐arylthiovinyl phenyl sulphones, PhSO2CBr = CHSAr, with arylthiolates, Ar′S− was studied. The same mixture of the four possible products, PhSO2C(SAr′) = CHSAr (Ar, Ar′ = p‐Tol, p‐CIC6H4), was formed in both cross‐experiments (Ar ≠ Ar′). A mechanism involving 1,2‐intramolecular migration of the arylthio group is suggested.
Phosphonic systems. Part 8. Solution and solid‐state conformation of diastereomeric adducts of diethyl prop‐2‐enylphosphonate to p‐nitrobenzaldehyde
Authors: Müller Elmar L. , Roos H. Marita , Modro Tomasz A. 
Published: January 2005

AbstractA mixture of siastereomeric adducts of lithiated diethyl prop‐2‐enylphosphonate to p‐nitrobenzaldehyde was separated and the solution and the solid‐state structures of the individual stereoisomers were studied. Conformational analysis demonstrated that in solution the most populated rotamer involves in both cases trans orientation of the PO3Et2 and O2NC6H4 groups and a gauche relationship of the phosphoryl and hydroxyl groups. In the solid state the crystal structure determination demonstrated that the molecules exist in the same conformation as in solution, although the PO ⃛HO hydrogen bonding changed from intramolecular to intermolecular (dimeric) interactions.
Nucleophilic solvent intervention in the solvolysis of 2‐aryl‐2‐Chloropropanes. An example to demonstrate the advantage of using the YBnCl scale in correlation analysis
Authors: Liu Kwang‐Ting , Chen Pang‐Shao , Hu Chia‐Ruei , Sheu Hung‐Chan 
Published: January 2005

AbstractThe advantage of using the YBnCl scale over the YCl scale in Grunwald–Winstein‐type correlation analysis was demonstrated by the kinetic evidence of a significant nucleophilic solvent intervention in the solvolysis of 2‐aryl‐2‐chloropropanes. A depression of log k values measured in ethanol–trifluoroethanol mixtures and a positive azide salt effect were observed.
Versatile packing modes of 9‐substituted 9‐fluorenols. X‐ray crystal structures of 9‐methyl‐, 9‐phenyl‐, 9‐(1‐naphthyl)‐ and 9‐(2‐biphenylyl)fluoren‐9‐ol
Authors: Csöregh Ingeborg , Czugler Mátyás , Weber Edwin 
Published: January 2005

AbstractThe four studied semi‐rigid fluorenol derivatives differ only in the size of the 9‐substituents. All four compounds crystallize with two molecules per asymmetric unit and with relatively low crystallographic symmetry [P1for the 9‐methyl derivative (1) andP21/nfor the 9‐(1‐naphthyl) (3) and 9‐(2‐biphenylyl) (4) derivatives]. Crystal data: 1,a= 8·779(1),b= 11·570(1),c= 11·747(1), α = 91·197(5), β = 104·345(4), γ = 105·848(6)° 2,a= 8·425(1),b= 13·819(1),c= 23·902(2)Å, β = 95·87(1)° 3,a= 15·679(4),b= 8·567(2);c= 23·884(17)Å, β = 92·60(6)° 4,a= 7·770(6),b= 33·766(10),c= 13·649(4)Å, β = 95·30(1)°. Nevertheless, the packing modes are different. The 9‐methylfluoren‐9‐ol molecules, forming H‐bonded tetramers with full H‐bond saturation, give rise to the lowest packing coefficient among the four compounds. The crystals of the 9‐phenyl derivative are built up of H‐bonded dimers, whereas those of the bulky 9‐naphthyl and 9‐diphenylyl derivatives consist of single molecules only. The latter two compounds, however, have a denser packing than the two that contain H‐bonded aggregates. The crystal structures are held together mainly by ordinary Van der Waals forces. In the case of 9‐biphenylyfluoren‐9‐ol, the structure seems to be stabilized also by weak H‐bond‐type interactions from the alcoholic hydroxyl group to the π‐electron cloud of an aromatic ring in the neighbouring molecule the O…centroid distances are 3·182(3) and 3·248(4)Å, and in the 9‐naphthyl derivative the OH groups are involved in some short (˜3·3Å) intermolecular approach, possibly indicating an electrostatically favourable packing for the compound.
Relative stability of crowded isomeric enols: 2‐Mesityl‐2‐phenylethenols and their methyl ethers
Authors: Nadler Ella B. , RÖCk Maik , Schmittel Michael , Rappoport Zvi 
Published: January 2005

AbstractThe structure of 2‐mesityl‐2‐phenylethenol (7) obtained by reduction of mesityl phenyl ketene with LiAlH4 and by acid‐catalysed rearrangement of 1‐mesityl‐2‐phenylethylene glycol was determined by x‐ray crystallography to be Z [(Z)‐7]. In contrast with a literature report, the reduction of 2‐acetoxy‐2‐mesityl‐2‐phenylacetaldehyde did not provide the E isomer [(E)‐7], but a mixture of (Z)‐7 and 2‐mesityl‐2‐phenylethenol. An (E)‐7–(Z)‐7 mixture of 1:5 was obtained starting from pure (Z)‐7 at 80°C in dimethyl sulphoxide. The lower stability of (E)‐7 was ascribed to higher steric effects due to a smaller Ph–C=C compared with Mes–C=C torsional angle and a preferred intramolecular π(Mes)–OH in (Z)‐7 over π(Ph)–OH hydrogen bonding. In order to dissect the effects, the corresponding 2‐mesityl‐2‐phenylvinyl methyl ethers (E)‐15 and (Z)‐15, where hydrogen bonding is absent, were prepared and equilibrated in chlorobenzene. The (Z)‐15: (E)‐15 ratio of ca 3:1 between 58° and 132° (ΔG=0·8 kcal mol−1) gives ΔH ≈ 0·6 kcal mol−1 and ΔS ≈ 0·5 e.u. It was concluded that steric effects contribute ca 1 kcal mol−1 and hydrogen bonding ca 1·5 kcal mol−1 to the higher stability of (Z)‐7 over (E)‐7. The unknown mesitylphenylacetaldehyde 16 was obtained from (Z)‐7 at 135°C in 31% yield.
Calorimetric study of human serum albumin in water–dioxane mixtures
Authors: Borisover Mikhail D. , Sirotkin Vladimir A. , Solomonov Boris N. 
Published: January 2005

AbstractThe calorimetric isotherm of water sorption by solid human serum albumin (HSA) was measured in water–dioxane mixtures. This isotherm has been successfully described at low water contents in the solvent within the framework of the presented model. The adsorption equilibrium constant and the monolayer formation energy were estimated from this model. The interaction enthalpies of solid HSA with the solvent in water‐rich mixtures do not differ essentially from the solution enthalpy of HSA in water. This means that the solvent water content may be sufficient for the intermolecular interactions of HSA suspended in a water–organic mixture to be similar to those in aqueous protein solution.
Acid‐catalysed solvolysis of optically active 1‐Phenyl‐1‐methoxythane. The role of ion– molecule pair intermediates
Authors: Thibblin Alf 
Published: January 2005

AbstractThe acid‐catalysed solvolysis of (R)‐1‐phenyl‐1‐methoxyethane was studied in mixtures of dilute aqueous perchloric acid and acetonitrile at 50°C. The rate of loss of optical activity (kα) was found to be the same as the rate of solvolysis (ksolv) at both low and high water contents of the solvent. These results are not in accord with literature data on racemization and oxygen exchange of 1‐phenylethanol in water, which has been discussed in terms of interconversion of chiral ion–dipole pair intermediates. The mechanistic implications of the results are discussed.
Steric deuterium isotope effect in the solvolysis of (Z)‐[Methyl‐D3]‐2‐ethylidene‐1‐adamantyl iodide accelerated by F‐strain
Authors: Ohga Yasushi , Takeuchi Ken'Ichi 
Published: January 2005

AbstractThe deuterium isotope effect in the solvolysis of (Z)‐ and (E)‐[methyl‐d3]‐2‐ethylidene‐1‐adamantyl mesylates [(Z)‐ and (E)‐1d‐OMs] and iodides [(Z)‐ and (E)‐1d‐I] was studied in 2,2,2‐trifluoroethanol at 25·0°C for mesylates and at 50·0°C for iodides. For the mesylates, which show a relatively small F‐strain effect, the (Z/E)H rate ratio (117 ± 1) is essentially identical with the (Z/E)D rate ratio (116 ± 1) at 25·0°C. On the other hand, for the iodides, which show a larger F‐strain effect, the (Z/E)H rate ratio (5413 ± 57) is greater than the (Z/E)D rate ratio (5040 ± 58) at 50·0°C. This indicates that (Z)‐1h‐I has greater F‐strain than (Z)‐1d‐I in the ground state. These results again confirm that the F‐strain effect in the (Z)‐2‐ethylidene‐1‐adamantyl derivatives exists between the (Z)‐methyl group and the leaving group atom directly attached to the reaction centre.
Nucleophilic substitution reactions of isopropyl benzenesulphonates with anilines and benzylamines
Authors: Oh Hyuck Keun , Kwon Young Bong , Lee Ikchoon 
Published: January 2005

AbstractThe nucleophilic substitution reactions of isopropyl arenesulphonates with anilines and benzylamines in acetonitrile at 65·0°C were investigated. The cross‐interaction constants ρxz (and βxz) between substituents in the nucleophile (X) and leaving group (Z) are positive and relatively small. The transition state (TS) variation is consistent with that predicted by the More O'Ferrall–Jencks diagram, as expected from the positive ρxz. The small magnitude of ρxz correctly reflects a relatively loose TS structure, in contrast to the tight TS for ethyl (or methyl) derivatives under similar reaction conditions.
Magnetic field effects upon photochemistry of bifuctional chain molecules
Authors: Nakagaki Ryoichi , Tanimoto Yoshifumi , Mutai Kiyoshi 
Published: January 2005

AbstractExternal magnetic field effects are discussed for three typical examples of photoreactions involving biradical intermediates: photo‐induced intramolecular electron transfer in chain molecules D(CH2)nA consisting of an electron donor (D) and an acceptor (A), intramolecular hydrogen abstraction in bifunctional species :X(CH2)nYH containing a hydrogen donor (YH) and an acceptor (:X in the excited triplet state) and photochemical homolytic ring opening of a cyclic ketone which yields a biradical consisting of an acyl and an alkyl group. There has been progress in the elucidation of reaction mechanisms through the analysis of magnetic field effects on lifetimes of reaction intermediates and product yields in radical reactions involving biradicals. Extremely small interactions such as the Zeeman energy and hyperfine interaction may give rise to a large change in the distribution of photo‐products derived from bifunctional chain molecules.
Base catalysed photochemically induced hydrogen isotope exchange in o‐methylbenzophenone
Authors: Michalak Jacek , Wolszczak Marian , Gȩbicki Jerzy 
Published: January 2005

AbstractPhotochemically induced hydrogen–deuterium and hydrogen–tritium exchange between o‐methylbenzophenone and labelled methanol is shown to be catalysed by sodium carbonate. Both the (E) and (Z)‐photoenols participate in the exchange reaction whereas in absence of the catalyst only the (E)‐photoenol is reactive. Analysis of the measured quantum yields for hydrogen isotope exchange reaction allowed the determination of the relative population of transient photoenols and to its comparison with that obtained from the flash photolysis experiments. Both hydrogen kinetic and solvent isotope effects are discussed.
Protonation of 2‐(2‐thienyl)pyrrole and 2‐(2‐thienyl)‐1‐vinylpyrroles
Authors: Sigalov Mark V. , Trofimov Alexander B. , Shmidt Elena Yu. , Trofimov Boris A. 
Published: January 2005

Abstract2‐(2‐Thienyl)pyrrole and its 3‐alkyl‐1‐vinyl derivatives have been shown (1H NMR) to be protonated by HSO3F, CF3COOH, HCl and HBr at the pyrrole C‐5 atom. Reaction with the superacid system HSO3FSbF5–SO2 results in the equilibrium between pyrrolium and thiophenium ions. The heats of formation (ΔH), charges and HOMO partial electron densities for 2‐(2‐thienyl)pyrrole and its protonated forms have been calculated by the MNDO method. The calculated ΔH values of thienylpyrrolium and pyrrolythiophenium ions are in agreement with the experimentally observed ratio.