Isodesmic Reaction Approach Research Articles

A computational study of the conformational stability, vibrational spectra, and thermochemical properties of 2,6-dichlorobenzamide, 2-(trifluoromethyl)benzamide, 2-(trifluoromethyl)benzoic acid, and 3-chloro-5-(trifluoromethyl)pyridine-2-carboxylic acid

Abstract 2,6-Dichlorobenzamide (BAM), 2-(trifluoromethyl)benzamide (TBAM), 2-(trifluoromethyl)benzoic acid (TBA), and 3-chloro-5-(trifluoromethyl)pyridine-2-carboxylic acid (PCA) are degradation by-products of fluopicolide and fluopyram fungicides. In this work, a detailed theoretical study of their different molecular, spectroscopic and thermochemical properties was carried out with different formulations of the density functional theory and high-level model chemistries. The mean values of −146.0 ± 6.3, −763.2 ± 6.3, −949.0 ± 6.3, and −919.4 ± 6.3 kJ mol−1 for the standard enthalpies of formation of BAM, TBAM, TBA and PCA, respectively, were derived for the first time at the G3MP2//DFT and G4MP2//DFT levels of theory (DFT = B3LYP, BMK, and B98). Additionally, a good agreement between formation enthalpies derived from isodesmic reaction approach and from Benson’s group additivity method was obtained.

Quantum chemical calculations of formation enthalpies of cations and anions of ionic liquids

The development of advanced ionic-liquid-based applications requires accurate thermochemical data for both ionic liquids and their constituent ions. Herein, the formation enthalpies of alkylimidazolium cations ([CnMIm+], n = 2, 4, 6, 8, 10) and various anions (trifluoromethanesulfonate ([TfO−]), ethyl sulfate ([ES−]), dicyanamide ([DCA−]), and bis(trifluoromethane)sulfonylimide ([NTf2−])) in the ideal gas state were obtained by quantum chemical calculations using density functional theory (DFT: B3LYP, B98, M06, M06-2X) and composite (G4) thermochemical models in the framework of the isodesmic reaction approach. The enthalpies of heterolytic dissociation of neutral ion pairs (NIPs) for the [CnMIm+][X−] series (n = 2, 4, 6, 8, 10; X− = [TfO−], [ES−], [NTf2−], [DCA−], [BF4−], [PF6−], [Cl−], [Br−], [I−]) were derived using the obtained data and literature formation enthalpies (ΔfH°) of NIPs. Analysis of the thermochemical property trends revealed that the formation enthalpies of [CnMIm+] decrease linearly as the alkyl chain length increases. In addition, the use of the isodesmic reaction approach improved the reliability of the computed data. Furthermore, owing to its internal self-consistency, the G4 method was found to be the most appropriate for describing the thermodynamic quantities of the cations and anions of ionic liquids.

A group additivity methodology for predicting the thermochemistry of oxygen‐containing organosilanes

AbstractA combinatorial approach was applied to devise a set of reference Si–C–O–H species that is used to derive group‐additivity values (GAVs) for this class of molecules. The reference species include 62 stable single‐bonded, 19 cyclic, and nine double‐bonded Si–C–O–H species. The thermochemistry of these reference species, that is, the standard enthalpy of formation, entropy, and heat capacities covering the temperature range from 298 to 2000 K was obtained from quantum chemical calculations using several composite methods, including G4, G4MP2, and CBSQB3, and the isodesmic reaction approach. To calculate the GAVs from the ab initio based thermochemistry of the compounds in the training set, a multivariable linear regression analysis is performed. The sensitivity of GAVs to the different composite methods is discussed, and thermodynamics properties calculated via group additivity are compared with available ab initio calculated values from the literature.

Prediction of standard enthalpies of formation of boron nitride nanocones

Prediction of the standard enthalpy of formation (ΔfH0298) of boron nitride nanocone (BNNCs) structures with disclination angles 60°, 120°, and 180°is recommended to be performed by the isodesmic reaction approach. NH3, BH3, and N2H4 were selected as key reference compounds. In order to calculate ΔfH0298 of nanocones, we must first calculate the enthalpies of nanocone rings at their apexes. For this purpose, ΔfH0298 values of 40 different structures, such as boron, nitrogen, and hydrogen compounds, have been calculated by combining Gaussian-4 (G4) theory calculations with the isodesmic and other balanced reactions approach. At each stage of the calculations, the previously estimated enthalpies of formation of nanocones were used as reference points for new molecules in the isodesmic and other balanced reactions. The results of enthalpies of formation of reference compounds were then used as reference values to estimate the enthalpy of formation of rings of BNNCs at their apexes. Finally, enthalpies of formation of BNNCs with disclination angles of 60°, 120°, and 180°and cone heights of 1-5 Å, 3-7 Å, and 4-8 Å were calculated. The results show that the reactions are highly exothermic. An increase in cone height causes the enthalpy of formation of boron nitride nanocone structures to become more negative.

Enthalpy of Formation and O-H Bond Dissociation Enthalpy of Phenol: Inconsistency between Theory and Experiment.

Gas-phase O–H homolytic bond dissociation enthalpy in phenol, DH298°(C6H5O–H), is still disputed, despite a large number of experimental and computational studies. In estimating this value, the experimental enthalpy of formation of phenol, ΔfH298°(C6H5OH, g) = −96.4 ± 0.6 kJ/mol (Cox, J. D. Pure Appl. Chem. 1961, 2, 125−128), is often used assuming high accuracy of the experimental value. In the present work a substantially less negative value of ΔfH298°(C6H5OH, g) = −91.8 ± 2.5 kJ/mol was calculated combining G4 theory with an isodesmic reaction approach. A benchmark quality of this result was achieved by using a large number of reliable reference species in isodesmic reaction calculations. Among these are the most accurate ΔfH298° values currently available from the Active Thermochemical Tables (ATcT) for 36 species (neutral molecules, radicals, and ions), anisole with recently reassessed enthalpy of formation, and 13 substituted phenols. The internal consistency of the calculated ΔfH298°(C6H5OH, g) value with the experimental enthalpies of formation of more than 50 reference species suggests that the reported experimental enthalpy of formation of phenol is in error. Taking into account that the enthalpy of formation of phenol has not been investigated experimentally since 1961, the new measurements would be extremely valuable. Using the accurate enthalpies of formation of C6H5OH and C6H5O• calculated in the present work, we obtained DH298°(C6H5O–H) = 369.6 ± 4.0 kJ/mol. This value is in satisfactory agreement with that determined from the most precise experimental measurement.

Gas-Phase Enthalpies of Formation and Enthalpies of Sublimation of Amino Acids Based on Isodesmic Reaction Calculations

Accurate gas-phase enthalpies of formation (ΔfH298°) of 20 common α-amino acids, seven uncommon amino acids, and three small peptides were calculated by combining G4 theory calculations with an isodesmic reaction approach. The internal consistency over a set of ΔfH298°(g) values was achieved by sequential adjustment of their values through the isodesmic reactions. Four amino acids, alanine, β-alanine, sarcosine, and glycine, with reliable internally self-consistent experimental data, were chosen as the key reference compounds. These amino acids together with about 100 compounds with reliable experimental data (their accuracy was supported by G4 calculations) were used to estimate the enthalpies of formation of remaining amino acids. All of the amino acids with the previously established enthalpies of formation were later used as the reference species in the isodesmic reactions for the other amino acids. A systematic comparison was made of 14 experimentally determined enthalpies of formation with the results of calculations. The experimental enthalpies of formation for 10 amino acids were reproduced with good accuracy, but the experimental and calculated values for 4 compounds differed by 11–21 kJ/mol. For these species, the theoretical ΔfH298°(g) values were suggested as more reliable than the experimental values. On the basis of theoretical results, the recommended values for the gas-phase enthalpies of formation were also provided for amino acids for which the experimental ΔfH298°(g) were not available. The enthalpies of sublimation were evaluated for all compounds by taking into account the literature data on the solid-phase enthalpies of formation and the ΔfH298°(g) values recommended in our work. A special attention was paid to the accurate prediction of enthalpies of formation of amino acids from the atomization reactions. The problems associated with conformational flexibility of these compounds and harmonic treatment of low frequency torsional modes were discussed. The surprisingly good agreement between the ΔfH298°(g) values calculated from the atomization and isodesmic reactions is largely the result of a fortuitous mutual compensation of various corrections used in the atomization reaction procedure.

Estimation of the 2.05 helix type i→i hydrogen bond energy at Aib∗-Oxa motif: an isodesmic approach

Bending at the valence angle N–Cα–C′ (τ) is a known control feature for attenuating the stability of the rare intramolecular i→i hydrogen bonded pseudo five-membered ring C5 structures, the so called 2.05 helices, at Aib. The competitive 310-helical structures still predominate over the C5 structures at Aib for most values of τ. However at Aib∗, a mimic of Aib where the carbonyl O of Aib is replaced with an imidate N (in 5,6-dihydro-4H-1,3-oxazine=Oxa), in the peptidomimic Piv-Pro-Aib∗-Oxa (1), the C5i structure is persistent in both crystals and in solution. Here we show that the i→i hydrogen bond energy is a more determinant control for the relative stability of the C5 structure and estimate its value to be 18.5±0.7kJ/mol at Aib∗ in 1, through the computational isodesmic reaction approach, using two independent sets of theoretical isodesmic reactions.

Is the Isodesmic Reaction Approach a Better Model for Accurate Calculation of pK a of Organic Superbases? A Computational Study

The acid-base dissociation constant (p<i>K</i> _a) can be related to the solubility and binding of drugs. However, measuring accurate p<i>K</i> _a values is a challenging task. In this study, we have examined the p<i>K</i> _a of various organic superbases: naphthalenes, cyclic guanidines, vinamidines, and acyclic guanidines computationally. We have calculated the p<i>K</i> _a of such superbases by employing two methods: a conventional thermodynamic cycle and a second method based on an isodesmic reaction. The thermodynamic cycle involves computation of solvation free energy by using gas-phase free energy and the difference in solvation free energies (∆G_solv) between products and reactants. Calculations performed with the isodesmic reaction approach do not use the free energy of solvation; hence, the accuracy of the approach is less sensitive to solvent molecules and global charges of the calculated species. The root-mean-square errors (RMSE) predict that the p<i>K</i> _a of the studied organic superbases are more accurate when calculated with the isodesmic reaction approach.

Accurate Prediction of Enthalpies of Formation of Organic Azides by Combining G4 Theory Calculations with an Isodesmic Reaction Scheme

Accurate gas-phase enthalpies of formation (ΔfH298°) of 29 azides are recommended by combining G4 theory calculations with an isodesmic reaction approach. The internal consistency over a set of ΔfH298° values was achieved by sequential adjustment of their values through the isodesmic reactions. The HN₃ was chosen as a key reference compound. Of the experimental data available for 16 compounds, our predictive values agree well with 9 of them, while the deviations from 25 to 55 kJ/mol are observed for 7 compounds; possible systematic errors in the experimental data for phenyl azide, 2-azidoethanol, azidocyclopentane, azidocyclohexane, 3-azido-3-ethylpentane, 2-azido-2-phenylpropane, and 1-azidoadamantane are discussed. The recommended enthalpies of formation of organic azides were used as reference values to estimate the enthalpy of formation of four nitrogen-rich carbon nitrides. The calculations do not support the high value of the solid-state enthalpy of formation of TAAT (4,4',6,6'-tetra(azido)azo-1,3,5-triazine); its value is estimated to be 300-400 kJ/mol lower than that measured experimentally.

Avoiding gas-phase calculations in theoretical pK a predictions

CBS-QB3, two simplified and less computationally demanding versions of CBS-QB3, DFT-B3LYP, and HF quantum chemistry methods have been used in conjunction with the CPCM continuum solvent model to calculate the free energies of proton exchange reactions in water solution following an isodesmic reaction approach. According to our results, the precision of the predicted pK a values when compared to experiment is equivalent to that of the thermodynamic cycles that combine gas-phase and solution-phase calculations. However, in the aqueous isodesmic reaction schema, the accuracy of the results is less sensitive to the presence of explicit water molecules and to the global charges of the involved species since the free energies of solvation are not required. In addition, this procedure makes easier the prediction of pK a values for molecules that undergo large conformational changes in solvation process and makes possible the pK a prediction of unstable species in gas-phase such as some zwitterionic tautomers. The successive pK a values of few amino acids corresponding to the ionization of the α-carboxylic acid and α-amine groups, which is one of the problematic cases for thermodynamic cycles, were successfully calculated by employing the aqueous isodesmic reaction yielding mean absolute deviations of 0.22 and 0.19 pK a units for the first and second ionization processes, respectively.

Theoretical studies on nitrogen rich energetic azoles

Different nitro azole isomers based on five membered heterocyclics were designed and investigated using computational techniques in order to find out the comprehensive relationships between structure and performances of these high nitrogen compounds. Electronic structure of the molecules have been calculated using density functional theory (DFT) and the heat of formation has been calculated using the isodesmic reaction approach at B3LYP/6-31G* level. All designed compounds show high positive heat of formation due to the high nitrogen content and energetic nitro groups. The crystal densities of these energetic azoles have been predicted with different force fields. All the energetic azoles show densities higher than 1.87 g/cm(3). Detonation properties of energetic azoles are evaluated by using Kamlet-Jacobs equation based on the calculated densities and heat of formations. It is found that energetic azoles show detonation velocity about 9.0 km/s, and detonation pressure of 40GPa. Stability of the designed compounds has been predicted by evaluating the bond dissociation energy of the weakest C-NO(2) bond. The aromaticity using nucleus independent chemical shift (NICS) is also explored to predict the stability via delocalization of the π-electrons. Charge on the nitro group is used to assess the impact sensitivity in the present study. Overall, the study implies that all energetic azoles are found to be stable and expected to be the novel candidates of high energy density materials (HEDMs).

Molecular electrostatic potentials and electron densities in nitrotriprismanes

Molecular electrostatic potential and Electron density topography in a series of nitrotriprismanes (C 6H 6− α (NO 2) α , α=1–6) have been analyzed using the Hartree–Fock and hybrid density functional methods. Weak, electrostatic C–H⋯O interactions, which render more stability to the isomers of nitroprismane, manifest in the shallow minima of the nitro groups along a series. As revealed from the molecular electron density topography the critical points of the X–N (X=C or N) bonds correlate well with the heat of formation derived from the isodesmic reaction approach.

Molecular electrostatic potentials and electron densities in azatriprismanes and nitroazatriprismanes

Triprismane, a C6H6 isomer of benzene, belongs to a class of strained hydrocarbons. In this work, the energy and charge distributions in a series of aza (C6N α H6− α ) and nitroaza (CNO2)6− α N α , (with α=1...6) derivatives of triprismane have been analyzed using the ab initio Hartree–Fock (HF)-derived molecular electrostatic potentials and molecular electron densities. Electrostatic potential investigations have shown that the electron-rich regions around nitrogen along a series of azatriprismanes and those near oxygens of nitro group in nitroazatriprismanes become smaller on encompassing from the hexanitroaza to nitroazatriprismane. As revealed from the molecular electrostatic potential (MESP) topography for the series of azatriprismanes the MESP minimum near nitrogen become shallow with increasing nitrogen content. Heats of formation obtained from the isodesmic reaction approach in the C6N6H6− α and (CNO2)6− α N α series correlate well with the electron density at the bond-critical point of the X— N (X = C or N) bonds of the triprismane framework.

Molecular structure and decomposition mechanism of peracetic acid esters AcOOR (R = Me, But)

The molecular structure and conformational mobility of methyl and tert-butyl esters of peracetic acid AcOOR (R = Me (1), But (2)) were studied by the ab initio MP4(SDQ)//MP2(FC)/6-31G(d,p) method and density functional B3LYP/6-31G(d,p) approach. The B3LYP calculated equilibrium conformations of the molecules are characterized by the C-O-O-C torsion angles of 93.6° (1) and 117.0° (2). Structural features of the molecules under study and a distortion of tetrahedral bond configuration at the Cα atom were explained using the natural bonding orbital approach. The standard enthalpies of formation of AcOOMe (−328.5 kJ mol−1) and AcOOBut (−440.4 kJ mol−1) were determined using the G2 and G2(MP2) computational schemes and the isodesmic reaction approach. The transition state of AcOOMe decomposition into AcOOH and formaldehyde was calculated (Ea = 122.8 kJ mol−1). The thermal effects of homolytic decomposition of the peroxy esters following a concerted mechanism (Me· + CO2 + ·OR) and simple homolysis of the peroxide bond (AcO· + ·OR) were found to be 97.5±0.3 and 155.1±0.3 kJ mol−1, respectively. At temperatures below 400 K, the most probable decomposition mechanism of peroxy esters 1 and 2 involves simple homolysis of the O-O bond.

Molecular electrostatic potentials and electron densities in nitrocubanes C 8H 8− α(NO 2) α ( α=1–8): ab initio and density functional study

Charge distributions in nitro derivatives of cubane, C 8H 8− α(NO 2) α ( α=1–8) have been derived employing the ab initio Hartree-Fock and hybrid density functional calculations using the topography of the molecular electrostatic potential (MESP) and the electron density as a tool. The electron-rich regions around the oxygen atoms of the nitro group emerged from the electrostatic potential get smaller with increasing number of nitro groups in the series. Within different isomers of nitrocubane, the lowest energy isomer engender large electron-rich regions relative to the other isomers, which has been supported by the shallow MESP minima near oxygen atoms of nitro group. The electrostatic potential at the center of the cube (cage critical point) on the other hand increases on encompassing from mono- to octa-nitrocubane. Thus it has been inferred that the successive replacement of cubyl hydrogens by NO 2 groups pushes the added electrons along a cube with an increase of curvature of the C–C bonds. It has been found that the higher value of the dipole moment for the most destabilized isomer stems from the large electron-rich regions in the electrostatic potential. Further the lowest frequency vibration and energy gap of the highest occupied and the lowest unoccupied molecular orbital in the series of octa-nitrocubane suggest it to be most labile in the series. The electron density at the bond critical point and the curvature of the C–C bonds in a cubanoid increases from mono- to octa-nitrocubane. Heats of formation derived from the isodesmic reaction approach increase with increased destabilization in the nitrocubane isomers.

Single-site mutation and secondary structure stability: an isodesmic reaction approach. The case of unnatural amino acid mutagenesis Ala-->Lac.

A method is described to evaluate backbone interactions in proteins via computational unnatural amino acid mutagenesis. Several N-acetyl polyalanyl amides (AcA(n)NH(2)) were optimized in the representative helical (3(10)-, 4(13)-, and a "hybrid" kappa-helix, n = 7, 9, 10, 14) and hairpin (two- and three-stranded antiparallel beta-sheets with type I turns betaalphaalphaepsilon, n = 6, 9, 10) conformations, and extended conformers of N-acetyl polyalanyl methylamides (n = 2, 3) were used to derive multistranded beta-sheet fragments. Subsequently, each residue of every model structure was substituted, one at a time, with l-lactic acid. The resulting mutant structures were again optimized, and group-transfer energies DeltaE(GT) were obtained as heats of the isodesmic reactions: AcA(n)NHR + AcOMe --> AcA(x)LacA(y)NHR + AcNHMe (R = H, CH(3)). These group-transfer energies correlate with the degree of charge polarization of the substituted peptide linkages as measured by the difference Deltae in H and O Mulliken populations in HN-C=O and with the H-bond distances in the "wild-type" structures. A good correlation obtains for the HF/3-21G and B3LYP/6-31G* group-transfer energies. The destabilization effects are interpreted in terms of loss of interstrand and intrastrand H-bonds, decrease in Lewis basicity of the C=O group, and O...O repulsion. On the basis of several comparisons of Ala --> Lac DeltaE(GT)'s with heats of the NH --> CH(2) substitutions, the latter contribution is estimated (B3LYP/6-31G*) to range between 1.5 and 2.4 kcal mol(-1), a figure close to the recent experimental DeltaDeltaG(o) value of 2.6 kcal mol(-1) (McComas, C. C.; Crowley, B. M.; Boger, D. L. J. Am.Chem. Soc. 2003, 125, 9314). The partitioning yields the following maximum values of the electronic association energy of H-bonds in the examined sample of model structures (B3LYP/6-31G* estimates): 3(10)-helix D(e) = -1.7 kcal mol(-1), alpha-helix D(e) = -3.8 kcal mol(-1), beta-sheet D(e) = -6.1 kcal mol(-1). The premise of experimental evaluations of the backbone-backbone H-bonding that Ala --> Lac substitution in proteins is isosteric (e.g., Koh, J. T.; Cornish, V. W.; Schultz, P. G. Biochemistry 1997, 36, 11314) is often but not always corroborated. Examination of the integrity of H-bonding pattern and phi(i), psi(i) distribution identified several mutants with significant distortions of the "wild-type" structure resulting inter alia from the transitions between i, i + 3 and i, i + 4 H-bonding in helices, observed previously in the crystallographic studies of depsipeptides (Ohyama, T.; Oku, H.; Hiroki, A.; Maekawa, Y.; Yoshida, M.; Katakai, R. Biopolymers 2000, 54, 375; Karle, I. L.; Das, C.; Balaram, P. Biopolymers 2001, 59, 276). Thus, the isodesmic reaction approach provides a simple way to gauge how conformation of the polypeptide chain and dimensions of the H-bonding network affect the strength of backbone-backbone C=O...HN bonds. The results indicate that the stabilization provided by such interactions increases on going from 3(10)-helix to alpha-helix to beta-sheet.

Theoretical studies on the molecular electron densities and electrostatic potentials in azacubanes

Energetics and the charge distributions in azacubanes (C8NαH8−α) have been obtained using the ab initio Hartree–Fock, second-order Mo oller–Plesset perturbation theory and hybrid density functional methods. For diazacubane to hexaazacubane the lowest-energy conformers have nitrogen atoms occupying the face opposite corners of a cube. The topography of the molecular electrostatic potential and the electron density of azacubane conformers have been investigated. The electrostatic potential studies have shown that successive substitution of nitrogen instead of CH groups of cubane engenders smaller and more localized electron-rich regions around the nitrogens of a cube. Further the bond ellipticity and the electron density at the bond critical point of the X–N bonds (X=C or N) in a cubanoid increase from azacubane to octaazacubane. The heats of formation of azacubanes calculated by the isodesmic reaction approach using different levels of theory correlate well with the electron density at the bond critical point of X–N (X=C or N) bonds in a cubanoid.

Molecular electrostatic potentials and electron densities in nitroazacubanes.

Successive introduction of nitrogen atoms in the cubyl corners instead of C-NO2 groups of octanitrocubane (CNO2)8, the most powerful explosives known to date, leads to a class of energy-rich compounds known as nitroazacubanes. In present work the ab initio Hartree-Fock and hybrid density functional calculations have been carried out on the possible conformers of (CNO2)(8-alpha)Nalpha (with alpha=0-8), nitroazacubanes. The charge distributions in these systems have been derived using the topography of the molecular electrostatic potential and electron density. Molecular electrostatic potential investigations reveal that of different nitroazacubane conformers, the electron-rich regions around nitro oxygens of the lowest energy conformer having face opposite nitrogen atoms within a cube are more delocalized. These conformers are predicted to have the largest difference of the energies of the highest occupied molecular orbital and lowest unoccupied molecular orbital relative to the other conformers. The dipole moments of nitroazacubanes are dependent on the nitrogen sites within a cube, caused by the resultant of C-N bond moments and nearly insensitive to position of the NO2 groups. The lowest frequency vibration (522 cm(-1)) suggests octa-azacubane having robust structure in the nitroazacubane series. Substitution of nitrogen atom instead of C-NO2 group leads to increase in electron density at the bond critical point of the X-N (X=C or N) bonds in a cube. The heats of formation of different nitroazacubanes were calculated by using the isodesmic reaction approach. The present calculation has shown that for the di- though hexanitroazacubanes the most destabilized conformer possess largest dipole moment and the heat of formation as well. A linear correlation of the electron density at the bond critical point of X-N bonds and the heat of formation has been obtained.

1‐amino‐4,5‐8‐naphthalenetricarboxylic acid‐1,8‐lactam‐4,5‐imide‐containing macrocycles: synthesis, molecular modelling and polymerization

AbstractNovel macrocycles containing 1‐amino‐4,5‐8‐naphthalenetricarboxylic acid‐1,8‐lactam‐4,5‐imide and 1,4,5‐8‐naphthalenetetracarboxylic bisimide fragments were synthesized by the high‐temperature pseudo‐high‐dilution acylation of the corresponding diols with isophthaloyl chloride, 4,4′‐ and 2,2′‐dichlorocarbonyl biphenyls with up to 60% yield. An important side‐reaction that impedes cyclization was found to be the reaction of diol OH groups with HCl during the acetylation. The ring strain in synthesized macrocyles and model cycles was estimated using the isodesmic reaction approach at the B3LYP/6–311 + G(d,p)//HF/3–21G level of theory. Lactamimide‐containing macrocycles were found to be more strained than bisimide‐containing macrocycles. The ring‐opening polymerization (ROP) of synthesized macrocycles in the molten state shows that the driving force of this process is the strain release on ring‐opening. The ROP of lactamimide‐containing macrocycles was found to be an efficient way to obtain lactamimide‐containing polymers, which are otherwise difficult to synthesize. Copyright © 2003 Society of Chemical Industry

Structure, thermochemistry, and conformational analysis of peroxides ROOR and hydroperoxides ROOH (R = Me, But, CF3)

The equilibrium structures, total energies, and harmonic frequencies of peroxides ROOR and ROOH (R = Me, But, CF3) were calculated using the perturbation theory (MP4//MP2 method) and density functional approach (B3LYP) in the 6-31G(d,p) basis set. The conformational flexibility of peroxides under rotation about the O-O bond was investigated. It was found that the stable conformation of a peroxide molecule is determined by superposition of the destabilizing effects (repulsion between the lone electron pairs, steric hindrances) and the interaction of the nonbonding orbitals of oxygen atoms with the antibonding orbitals of the adjacent polar bonds. The latter effect stabilizes the nonplanar structure of the peroxide molecule. The role of orbital interactions in manifestation of the d-effect (distortion of the tetrahedral configuration of the X3CO fragment of peroxide molecule) was revealed. The vibrational spectra of peroxides were calculated and compared with the experimental data. The potential energy distribution over normal vibrations was analyzed. The enthalpies of formation and the bond strengths in the molecules of compounds examined were calculated in the framework of the isodesmic reaction approach.