Reactions on polymers with amine groups. V. Addition of pyridine and imidazole groups with acetylenecarboxylic acids

Unsaturated macromolecular carboxybetaines were obtained by reaction of poly(4-vinylpyridine) and poly(N-vinylimidazole) with propiolic acid. A kinetic model was presented for 4-methylpyridine. It consists of three coupled reactions: neutralization, addition which involves two molecules of acid and leads to a cation–anion pair structure, where the cation results from the addition of the amine nitrogen to the triple bond of acid, and an equilibrium reaction between the ion-pair structure and the betaine structure. The addition rate was found to be higher for poly(4-vinylpyridine) than for poly(N-vinylimidazole); it was also higher in water than in a water–methanol mixture. The reaction with acetylenedicarboxylic acid was carried out on poly(N-vinylimidazole), but the transformed units showed the structure that results from propiolic acid. The betaine products from 4-methylpyridine did not polymerize by radical initiation. The polymeric products show characteristics of photocrosslinking polymers. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1615–1623, 1998     

Reaction of 2,2′-azinobis (3-ethylbenzothiazoline-6-sulfonic acid (ABTS) derived radicals with hydroperoxides. Kinetics and mechanism

Tert-Butyl hydroperoxide and hydrogen peroxide readily react with the radical cation derived from 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS). The reaction is inhibited by ABTS and protons, and can be interpreted in terms of a mechanism comprising a partially reversible electron transfer ROOH+ABTS^•+↔ ROO · + ABTS + H⁺ (1) followed by the self-reactions of the hydroperoxide derived radicals and reactions between them and another ABTS derived radical. A complete kinetic analysis allows an evaluation of the rate constant for reaction (1). A value of 0.2 M⁻¹ s⁻¹ was obtained for both compounds. The back reaction of process (1) is more relevant when tert-butyl hydroperoxide is employed. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 565–570, 1998     

Kinetics and mechanism of ninhydrin reaction with copper(II) complexes of glycine and α-alanine. Elucidation of the template mechanism

Ninhydrin has been found to react with Cu(glycine)⁺ and Cu(alanine)⁺ in the ratio of 1:1. The kinetic studies of the reaction were carried out at different concentrations of the reactants at 80°C (pH = 5.0). The reaction proceeds through the formation of a ternary labile complex of ninhydrin with Cu(II) complexes of glycine and alanine. The kinetics were found to follow pseudo-first-order reaction path with respect to Cu(II)-complex in presence of excess [Ninhydrin]. Formation of a ternary labile complex indicates a template reaction mechanism based on the reactions with coordinated ligands. The variation of pseudo-first-order rate constants with [ninhydrin] was found to be in good agreement with equation where B₁ and B₂ are the unknown empirical parameters. The [acetate ion] has no significant effect on the rate constants. On the basis of observed data a probable mechanism has been proposed. © 1993 John Wiley & Sons, Inc.     

Kinetic study of the CN radical reaction with 2-methylfuran

The gas phase reaction of the ground state cyano-radical (CN (X²∑⁺)) with 2-methylfuran (2-MF) is investigated in a quasi-static reaction cell at pressures ranging from 2.2 to 7.6 Torr and temperatures ranging from 304 to 440 K. The CN radicals are generated in their ground electronic state by pulsed laser photolysis of gaseous cyanogen iodide (ICN) at 266 nm. Their concentration is monitored as a function of reaction time using laser-induced fluorescence at 387.3 nm on the B²∑⁺ (ν′ = 0) ← X²∑⁺ (ν″ = 0) vibronic band. The reaction rate coefficient is found to be rapid and independent of pressure and temperature. Over the investigated temperature and pressure ranges, the rate coefficient is measured to be 2.83 (± 0.18) × 10⁻¹⁰ cm³ molecules s⁻¹. The enthalpies of the stationary points and transition states on the CN + 2-MF potential energy surface are calculated using the CBS-QB3 computational method. The kinetic results suggest the lack of a prereactive complex on the reaction entrance channel with either a very small or nonexistent entrance energy barrier. In addition, the potential energy surface calculations reveal only submerged barriers along the minimum energy path. Based on comparisons between previous CN reactions with unsaturated hydrocarbons, the most likely reaction pathway is CN addition onto one of the unsaturated carbons followed by either H or methyl elimination. The implications for the transformation of biomass-derived fuels in nitrogen-rich flames is discussed.     

Kinetics of polymerization of acrylic acid initiated by Mn3+–isobutyric acid redox system. II

The kinetics of redox -initiated polymerization of acrylic acid (AA) by the systerm Mn³⁺-isobutyric acid (IBA) in sulfuric acid was studied in the temperature range of 35–50°C. The overall rates of polymerization (R_p), disappearance of manganic ion (?R_m), and degree of polymerization (X _n), were measured with variation in [monomer], [Mn³⁺], [IBA], H⁺, μ, [Mn²⁺], and temperature. The polymerization is initiated by the organic free radical that develops from the Mn³⁺-isobutyric acid oxidation reaction. Two types of termination reactions, one by the metal ion (Mn³⁺) and the other by the MN³⁺-isobutyric acid complex are proposed to explain the kinetic results. The various rate parameters were evaluated an discussed.     

Kinetics and mechanisms of the reactions of aluminium(III) with β-diketones in aqueous solution

The kinetics and mechanisms of the reactions of aluminium(III) with pentane-2,4-dione (Hpd), 1,1,1-trifluoro pentane-2,4-dione (Htfpd) and heptane-3,5-dione (Hhptd) have been investigated in aqueous solution at 25°C and ionic strength 0.5 mol dm⁻³ sodium perchlorate. The kinetic data are consistent with a mechanism in which aluminium(III) reacts with the β-diketones by two pathways, one of which is acid independent while the second exhibits a second-order inverse-acid dependence. The acid-independent pathway is ascribed to a mechanism in which [Al(H₂O)₆]³⁺ reacts with the enol tautomers of Hpd, Htfpd, and Hhptd with rate constants of 1.7(±1.3)×10⁻², 0.79(±0.21), and 7.5(±1.6)×10⁻³ dm³ mol⁻¹ s⁻¹, respectively. The inverse acid pathway is consistent with a mechanism in which [Al(H₂O)₅(OH)]²⁺ reacts with the enolate ions of Hpd, Htfpd, and Hhptd with rate constants of 4.32(±0.18)×10⁶, 5.84(±0.24)×10³, and 1.67(±0.05)×10⁷ dm³ mol⁻¹ s⁻¹, respectively. An alternative formulation involves a pathway in which [Al(H₂O)₄(OH)₂]⁺ reacts with the protonated enol tautomers of the ligands. This gives rate constants of 2.79(±0.12)×10⁴, 3.86(±0.16)×10⁵, and 8.98(±0.25)×10³ dm³ mol⁻¹ s⁻¹ for reaction with Hpd, Htfpd, and Hhptd, respectively. Consideration of the kinetic data reported here together with data from the literature, suggest that [Al(H₂O)₅(OH)]²⁺ reacts by an associative or associative-interchange mechanism. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 257–266, 1998.     

Anionic polymerization of 4-methyl-2-oxetanone (β-butyrolactone)

Polymerization of 4-methyl-2-oxetanone ( 1 ) initiated with potassium acetate-dibenzo-18-crown-6 complex ( 2 ) in THF as solvent, was studied. Transfer reactions, leading to both crotonate anions and carboxylic acid formation, have been observed. Two kinetic effects of these reactions, hampering the living polymerization, have been established. The first results from reinitiation with the crotonate anions and thereby lowers the polymer molecular weight. The second is the decrease in the overall polymerization rate due to complexation of the growing carboxylate anions with carboxylic acid moieties. Kinetic scheme of polymerization involves propagation accompanied by transfer followed by slow reinitiation. This scheme, including complexation of the active species has been solved numerically. The apparent rate and equilibrium constants (k_p, k_tr, k_ri, and K_ass and respectively) have been determined. Although these kinetic parameters depend strongly on the polymerization conditions, but the ratio of the rate constants k_p : k_t : k_ri is fairly constant and equal to 10⁻⁴ : 10⁻⁶ : 10⁻⁶, respectively (at 20°C). Conditions of the controlled anionic synthesis of the amorphous poly(4-methyl-2-oxetanone) with $\bar M_n$ as high as 1.7 × 10⁴ and ${{ \le \bar M_n } \mathord{\left/ {\vphantom {{ \le \bar M_n } {\bar M_n }}} \right. \kern-\nulldelimiterspace} {\bar M_n }} \le 1.20$ have also been elaborated.     

New Synthetic Routes to β-Olefinic Trialkoxyphosphonium Salts and Phosphonates: Organometallic Variants of the Michaelis-Arbuzov Reaction

The low-temperature addition of tertiary phosphites to [(allyl)Fe (CO)₄]⁺X^? complexes proceeds regio- and stereospecifically and produces metal-coordinated β-olefinic trialkoxyphosphonium ions. These can be converted by various routes into the uncomplexed phosphonium salts or phosphonates. Similar reactions of acyclic [(dienyl)Fe(CO)₃]⁺X^? compounds give metal-coordinated (2,4-dien-1-yl)trialkoxyphosphonium salts or dialkyl (2,4-dien-1-yl)phosphonates. The mechanisms and their relationship to the classical Michaelis-Arbuzov reaction are discussed. The new compounds are characterized, if possible, by ¹H-, ¹³C-, and ³¹P-NMR spectra. The new phosphonium salts and phosphonates, potentially useful for Wittig-Horner reactions, are difficult to obtain by conventional routes.     

Kinetics of electron transfer reactions of manganese (III) complexes of trans-cyclohexane-1,2-diamine,-NNN′N′-tetraacetic acid and 2,2′-bipyridyl with ascorbic acid in aqueous acid media

The kinetics of the reactions of manganese(III) complexes of trans-cyclohexane-1,2-diamine-NNN′N′-tetraacetic acid (H₄cydta) and 2,2′-bipyridyl (bpy) have been investigated in the acid ranges [H⁺] = 1.00 ×10⁻⁵ − 3.16 × 10⁻³ M and [H⁺] = 0.10 − 1.00 M, respectively, at different temperatures and at a constant ionic strength. Both the molecular and mono-anionic forms of ascorbic acid have been found to be reactive in the experimental acid ranges. The monoanionic species has been found to be more reactive than the molecular form. Attempts have been made to correlate the kinetic results in terms of the Marcus relationship for outer-sphere electron transfer reaction. Differences between the calculated rates (k_0.8) and experimental rates (k) by approximately four and seven orders of magnitude (k_0.8≪ k) suggest an innersphere pathway for the reaction of Mn(cydta)(H₂O)⁻. Inner-sphere reaction is also believed to be operative for the other reaction. The activation parameters have been evaluated and compared with other reported systems.     

Kinetics and Mechanism of Radical‐Chain Oxidation of 1,2‐Substituted Ethylene and 1,4‐Substituted Butadiene‐1,3

A combination of microvolumetry, the rotating sector method, ESR, ¹H NMR, and IR allowed to establish a detailed mechanism of liquid‐phase oxidation of vinyl compounds X₁CH=CHX₂ and X₁CH=CH–CH=CHX₂ (X₁ and X₂—a polar substitute: С₆Н₅–, CO–, СOO–) initiated by azobisisobutyronitrile. A distinctive feature of the mechanism is the fact that the oxidation chain is carried out by a low‐molecular hydroperoxide radical joining the π‐bond. For nine compounds in the temperature range of 303–353 K, relative chain propagation and termination rate constants were measured (k ₂•k ₃^−0.5). Absolute values of k ₂ were obtained for diphenylethylene (110 L·mol⁻¹·s⁻¹), ethyl ether of trans‐phenyl‐pentadiene acid (13 L·mol⁻¹·s⁻¹), and methyl ether of trans‐phenyl‐pentadiene acid (14.2 L·mol⁻¹·s⁻¹) at T = 323 K. For the same conditions, 10⁻⁸k ₃ were calculated for diphenylethylene (0.87 L·mol⁻¹·s⁻¹) and methyl ether of trans‐phenyl‐pentadiene acid (1.21 L·mol⁻¹·s⁻¹). A cyclic mechanism of the oxidation chain termination on introduced antioxidants (stable nitroxyl radicals of the piperidine series ( > NO^●) and the transition metal compounds (Meⁿ )) was established. The inhibition factor (f ) showing how many reaction chains are terminated by the one particle of the antioxidant is equal to 10². The cyclic chain termination is caused by the following reactions: HO₂^● + > NO^● → NOH + O₂, HO₂● + NOH → >NO^● + H₂O₂ (for >NO^●) and HO₂^● + Meⁿ → Me^{n +1} + HO₂^●−, HO₂^● + Me^{n +1} → Meⁿ + H⁺ + O₂ (for Meⁿ ).     

Two isomorphous azastilbene derivatives: 1‐(2‐chlorobenzyl)‐4‐[(E)‐2‐(4‐hydroxyphenyl)ethenyl]pyridinium chloride and 1‐(2‐bromobenzyl)‐4‐[(E)‐2‐(4‐hydroxyphenyl)ethenyl]pyridinium bromide

The crystal structures of the two title (E)‐stilbazolium halogenates, C₂₀H₁₇ClNO⁺·Cl⁻ and C₂₀H₁₇BrNO⁺·Br⁻, are isomorphous, with an isostructurality index of 0.985. The azastyryl fragments are almost planar, with dihedral angles between the benzene and pyridine rings of ca 4.5°. The rings of the benzyl groups are, in turn, almost perpendicular to the azastyryl planes, with dihedral angles larger than 80°. The cations and anions are connected by O—H...X⁻ (X = halogen) hydrogen bonds. The halide anions are `sandwiched' between the charged pyridinium rings of neighbouring molecules, and weak C—H...O hydrogen bonds and C—H...X and C—H...π interactions also contribute to the crystal structures.     

Secondary interactions in the iso­morphous compounds 2,6‐bis­(chloro­methyl)­pyridinium chloride and 2,6‐bis­(bromo­methyl)­pyridinium bromide

The title compounds, C₇H₈Cl₂N⁺·Cl⁻ and C₇H₈Br₂N⁺·Br⁻, are isomorphous. In the crystal packing, layers parallel to the ac plane are formed by a classical N⁺—H⋯X⁻ hydrogen bond (X = halogen) and two X⋯X contacts. A third X⋯X contact links the layers, and a fourth, which is however very long, completes a ladder‐like motif of halogen atoms. Hydro­gen bonds of the form C—H⋯X play at best a subordinate role in the packing.     

A convenient synthesis of α,α′‐ homo‐ and α,α′‐hetero‐bifunctionalized poly(ε‐caprolactone)s by ring opening polymerization: The potentially valuable precursors for miktoarm star copolymers

Two new ring opening polymerization (ROP) initiators, namely, (3‐allyl‐2‐(allyloxy)phenyl)methanol and (3‐allyl‐2‐(prop‐2‐yn‐1‐yloxy)phenyl)methanol each containing two reactive functionalities viz. allyl, allyloxy and allyl, propargyloxy, respectively, were synthesized from 3‐allylsalicyaldehyde as a starting material. Well defined α‐allyl, α′‐allyloxy and α‐allyl, α′‐propargyloxy bifunctionalized poly(ε‐caprolactone)s with molecular weights in the range 4200–9500 and 3600–10,900 g/mol and molecular weight distributions in the range 1.16–1.18 and 1.15–1.16, respectively, were synthesized by ROP of ε‐caprolactone employing these initiators. The presence of α‐allyl, α′‐allyloxy and α‐allyl, α′‐propargyloxy functionalities on poly(ε‐caprolactone)s was confirmed by FT‐IR, ¹H, ¹³C NMR spectroscopy, and MALDI‐TOF analysis. The kinetic study of ROP of ε‐caprolactone with both the initiators revealed the pseudo first order kinetics with respect to ε‐caprolactone consumption and controlled behavior of polymerization reactions. The usefulness of α‐allyl, α′‐allyloxy functionalities on poly(ε‐caprolactone) was demonstrated by performing the thiol‐ene reaction with poly(ethylene glycol) thiol to obtain (mPEG)₂‐PCL miktoarm star copolymer. α‐Allyl, α′‐propargyloxy functionalities on poly(ε‐caprolactone) were utilized in orthogonal reactions i.e copper catalyzed alkyne‐azide click (CuAAC) with azido functionalized poly(N‐isopropylacrylamide) followed by thiol‐ene reaction with poly(ethylene glycol) thiol to synthesize PCL‐PNIPAAm‐mPEG miktoarm star terpolymer. The preliminary characterization of A₂B and ABC miktoarm star copolymers was carried out by ¹H NMR spectroscopy and gel permeation chromatography (GPC). © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 844–860     

The effect of amino acid backbone length on molecular packing: crystalline tartrates of glycine, β‐alanine, γ‐aminobutyric acid (GABA) and dl‐α‐aminobutyric acid (AABA)

We report a novel 1:1 cocrystal of β‐alanine with dl ‐tartaric acid, C₃H₇NO₂·C₄H₆O₆, (II), and three new molecular salts of dl ‐tartaric acid with β‐alanine {3‐azaniumylpropanoic acid–3‐azaniumylpropanoate dl ‐tartaric acid–dl ‐tartrate, [H(C₃H₇NO₂)₂]⁺·[H(C₄H₅O₆)₂]⁻, (III)}, γ‐aminobutyric acid [3‐carboxypropanaminium dl ‐tartrate, C₄H₁₀NO₂⁺·C₄H₅O₆⁻, (IV)] and dl ‐α‐aminobutyric acid {dl ‐2‐azaniumylbutanoic acid–dl ‐2‐azaniumylbutanoate dl ‐tartaric acid–dl ‐tartrate, [H(C₄H₉NO₂)₂]⁺·[H(C₄H₅O₆)₂]⁻, (V)}. The crystal structures of binary crystals of dl ‐tartaric acid with glycine, (I), β‐alanine, (II) and (III), GABA, (IV), and dl ‐AABA, (V), have similar molecular packing and crystallographic motifs. The shortest amino acid (i.e. glycine) forms a cocrystal, (I), with dl ‐tartaric acid, whereas the larger amino acids form molecular salts, viz. (IV) and (V). β‐Alanine is the only amino acid capable of forming both a cocrystal [i.e. (II)] and a molecular salt [i.e. (III)] with dl ‐tartaric acid. The cocrystals of glycine and β‐alanine with dl ‐tartaric acid, i.e. (I) and (II), respectively, contain chains of amino acid zwitterions, similar to the structure of pure glycine. In the structures of the molecular salts of amino acids, the amino acid cations form isolated dimers [of β‐alanine in (III), GABA in (IV) and dl ‐AABA in (V)], which are linked by strong O—H…O hydrogen bonds. Moreover, the three crystal structures comprise different types of dimeric cations, i.e. (A…A)⁺ in (III) and (V), and A⁺…A⁺ in (IV). Molecular salts (IV) and (V) are the first examples of molecular salts of GABA and dl ‐AABA that contain dimers of amino acid cations. The geometry of each investigated amino acid (except dl ‐AABA) correlates with the melting point of its mixed crystal.     

Kinetics and mechanism of autocatalyzed reaction between Phenyl Hydrazine and Toluidine blue in aqueous solution

The kinetics and mechanism of reduction of aqueous toluidine blue (TB⁺) by phenyl hydrazine (Pz), which exhibits nonlinear behavior, is studied spectrophotometrically at 630 nm. Typical kinetic curves exhibited autocatalytic characteristics. The role of H⁺ as an autocatalyst is established. Rate constants for the uncatalyzed and acid catalyzed reactions are determined. The forward rate constants for the uncatalyzed and acid catalyzed reactions were 1.4 × 10⁻² M⁻¹ s⁻¹ and 60 M⁻¹ s⁻¹. Reaction products are toluidine white, phenol, and an azo dye. From the stoichiometric ratios, the major reaction is Pz + 2 TB⁺ + H₂O = PhOH + 2 TBH + 2 H⁺ + N₂. The rate expression and a detailed 12‐step reaction mechanism supported by simulations are proposed. ©1999 John Wiley & Sons, Inc. Int J Chem Kinet: 31: 83–88, 1999     

Study on the Extraction Kinetics of Ketoconazole with Hydroxypropyl‐β‐Cyclodextrin as a Selector

This paper reports on the kinetics of extraction of ketoconazole enantiomers in a Lewis cell, which accompanies by an inclusion reaction. The mass transfer model of the extraction process is established based on the homogeneous reaction mechanism and two‐film theory. Factors such as stirring speed, phase contact area, and ketoconazole concentration are separately investigated. The obtained data show the inclusion reaction is a fast reaction. The reactions have been found to be first order for ketoconazole enantiomers and second order for hydroxypropyl‐β‐cyclodextrin with forward rate constants of 1.716 × 10⁻³ m⁶/(mol²·s) for (−)‐ketoconazole and 2.067 × 10⁻³ m⁶/(mol²·s) for (+)‐ketoconazole.     

β‐LiMoO2(AsO4)

A new non‐centrosymmetrical form of lithium molybdyl arsenate has been synthesized and grown as a single crystal. The structure of β‐LiMoO₂(AsO₄) is built up of corner‐sharing AsO₄ tetrahedra and MoO₆ octahedra which form a three‐dimensional framework containing tunnels running along the a axis, wherein the Li⁺ cations are located. This novel structure is compared with the compound LiMoO₂(AsO₄) of the same formula, and with those of AMO₂(XO₄) (A is Na, K, Rb or Pb, M is Mo or V, and X is P or As) and B(MoO₂)₂(XO₄)₂ (B is Ba, Pb or Sr).     

Hetero-Diels-Alder Cycloadditions of α,β-Unsaturated Acyl Cyanides,Part 4 , Substituent Effects in Reactions with p-Substituted Styrenes

Cycloadditions of α,β-unsaturated acyl cyanides (=2-oxonitriles) 1 – 6 to styrene and its p-substituted derivatives 7a – f , h are of inverse electron demand and provide, under mild conditions, regio- and stereoselectively 2-aryl-3,4-dihydro-2H-pyran-6-carbonitriles 8 – 13 , generally in good yield. Rates for the cycloaddition of acryloyl cyanide 1 to p-substituted styrenes, determined in competition reactions of substrate pairs relative to that of styrene, increase in the order of electron-donating ability NO₂<Cl<H<AcO<Me<AcNH<MeO of the p-substituent. Linear correlation of log (k_X/k_H), and σ_p⁺ substituent constants (a Hammett-type plot), gives a reaction constant ρ_p⁺ of −1.47±0.17, supporting a concerted mechanism.     

Crystallization of Poly(ε‐caprolactone) under Nanoparticle Confinement

Solid‐state ¹³C‐NMR spectroscopy has been used to characterize the conformation of the hydrophobic poly(ε‐caprolactone) core of a nanoparticle having a cross‐linked hydrophilic poly(acrylic acid)/polyacrylamide shell. The amphiphilic nanoparticles were synthesized from the diblock copolymer, poly(ε‐caprolactone)₁₂₁‐b‐poly(acrylic acid)₁₆₅ by self‐assembly into polymer micelles, followed by cross‐linking via condensation reactions between the carboxylic acid groups of the hydrophilic shell and the amine groups of 2,2′‐(ethylenedioxy)bis(ethylamine). NMR Experiments performed at −30° on nanoparticles rapidly quenched from 60° show that the core is largely noncrystalline and locally disordered. Heating to 25° results in some crystallization, although far less than that observed for bulk poly(ε‐caprolactone) homopolymer. Storage at −30° results in further crystallization and conversion of most rubbery, mobile regions into more rigid, locally ordered amorphous domains. The absence of dipolar coupling between natural‐abundance ¹³C in the poly(ε‐caprolactone) core of the nanoparticle, and ¹⁵N labels dispersed throughout the cross‐linked shell show that the interface between core and shell is sharp. The dipolar coupling measurements were accomplished by ¹³C{¹⁵N} rotational‐echo double resonance.     

Theoretical study of the π→π* excited states of linear polyenes: The energy gap between 11Bu+ and 21Ag− states and their character

Multireference perturbation theory with complete active space self-consistent field (CASSCF) reference functions is applied to the study of the valence π→π* excited states of 1,3-butadiene, 1,3,5-hexatriene, 1,3,5,7-octatetraene, and 1,3,5,7,9-decapentaene. Our focus was put on determining the nature of the two lowest-lying singlet excited states, 1¹B_u⁺ and 2¹A_g⁻, and their ordering. The 1¹B_u⁺ state is a singly excited state with an ionic nature originating from the HOMO→LUMO one-electron transition while the covalent 2¹A_g⁻ state is the doubly excited state which comes mainly from the (HOMO)²→(LUMO)² transition. The active-space and basis-set effects are taken into account to estimate the excitation energies of larger polyenes. For butadiene, the 1¹B_u⁺ state is calculated to be slightly lower by 0.1 eV than the doubly excited 2¹A_g⁻ state at the ground-state equilibrium geometry. For hexatriene, our calculations predict the two states to be virtually degenerate. Octatetraene is the first polyene for which we predict that the 2¹A_g⁻ state is the lowest excited singlet state at the ground-state geometry. The present theory also indicates that the 2¹A_g⁻ state lies clearly below the 1¹B_u⁺ state in decapentaene with the energy gap of 0.4 eV. The 0–0 transition and the emission energies are also calculated using the planar C_2h relaxed excited-state geometries. The covalent 2¹A_g⁻ state is much more sensitive to the geometry variation than is the ionic 1¹B_u⁺ state, which places the 2¹A_g⁻ state significantly below the 1¹B_u⁺ state at the relaxed geometry. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 157–175, 1998