Molecular enthalpies: 1. Ground-state thermodynamic and computed enthalpies of the norbornadiene cycle

The enthalpies of formation of norbornadiene, norbornene, norbornane, nortricyclane, and quadricyclane, which are experimentally known, have been calculated using the semiempirical programs MNDO, AM1, and PM3 and the empirical force-field method MMP2. MMP2 agrees most closely with experimental results. Of the semiempirical methods, PM3 agrees most closely with experimental results and is very good for the alkenes. Enthalpies of hydrogenation, calculable fromH _f, are inconclusive on the question of ground-state homoaromatic stabilization of norbornadiene.     

Molecular enthalpies 3: Ground-state thermodynamic and computed enthalpies of the valence isomers of benzene

This paper is a study of benzene, Dewar benzene, benzvalene, [3]prismane, and 3,3-bicyclopropenyl by the MM2, MM3, MNDO, AM1, and PM3 calculational methods. Comparisons are made with experimental results and ab initio molecular orbital calculations from the literature.     

The stepwise heats of hydrogenation of barrelene,an ab initio study

Ab initio Hartree-Fock calculations at the 6–31G*//3–21G level of theory are reported for bicyclo[2.2.2]-2,5,7-octatriene (barrelene), 1 , bicyclo[2.2.2]-2,5-octadiene, 2 , bicyclo[2.2.2]-2-octene, 3 , and bicyclo-[2.2.2]octane, 4 . The stepwise heats of hydrogenation of 1 were found to be 38.1, 31.8, and 28.4 kcal/mol, respectively. The unusually large heat of hydrogenation for the first double bond is attributed to the destabilizing electronic effects involving the interaction of the three double bonds of 1 .     

Estimation of the Relative Stabilities of the E,Z-Isomers of α,β-Dialkylsubstituted Methyl Vinyl Ethers by Various Computational Methods

The relative thermodynamic stabilities (relative enthalpies) of the E,Z-isomers of ,-dialkylsubstituted methyl vinyl ethers MeOC(R₁)=CHR₂ have been estimated by various computational methods including molecular mechanics, semiempirical, ab initio, and DFT calculations. The best performance, approaching the accuracy of the experimental method of chemical equilibration, is shown by the DFT calculations. Ab initio methods, provided that electron correlation is taken into account, are also satisfactory, but clearly less successful than the DFT calculations. The reliability of the semiempirical methods AM1 and PM3 is considerably less good, and varies in an unpredictable manner from case to case. The poorest general performance is shown by the MM2 and MM3 calculations, which may overestimate the relative stability of the Z isomer by as much as 18 kJ mol^–1.     

A calorimetric study of the specific heat of cytisine and enthalpies of its dissolution in water and ethanol

The enthalpies of dissolution of an alkaloid cytisine at dilutions [mole of cytisine : moles of water; mole of cytisine : moles of 96% ethanol] equal to 1 : 9000, 1 : 18 000, and 1 : 36 000 were measured by means of calorimetry. The experimental data obtained were used to calculate the standard enthalpies of dissolution of cytisine in infinitely diluted (standard) aqueous and ethanolic solutions. The enthalpies of combustion, melting, and crystallization of cytisine were calculated. The method of dynamic calorimetry in the temperature range 198–298.15 K was applied to study the specific heat of the alkaloid, and an equation describing the dependence C _p ^o f(T) was derived.Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 12, 2004, pp. 1942–1944.Original Russian Text Copyright © 2004 by Abildaeva, Kasenova, Tuleulov, Adekenov, Kasenov.     

An algorithm for estimating the enthalpies of formation of amine molecular ions and immonium fragment ions

A method of estimating the enthalpies of formation of amino molecular ions, ΔH^? ([C_nH_2n+1NH₂]⁺˙) and of immonium ions ΔH([C_nH_2n+1N]⁺) is reported. It is based on the fact that CH₃ is isoelectronic with NH₂, CH₂ with NH and CH with N. Some calculated values of the enthalpies of formation of amine molecular ions and immonium ions are reported and estimates are made of the accuracy of such calculation.     

Ionic enthalpies of transfer from water to water-sulfolane mixtures

NaCl, NaBr, NaI, NaClO₄, KCl, KClO₄, NaBPh₄, and Ph₄PBr solution enthalpies were measured in water-sulfolane mixtures at 30°C. Ionic enthalpies of transfer from water to mixed solvents were calculated on the basis of the assumption H _s ^o (BPh ₄ ^– )=H _s ^o (Ph₄P⁺). The variation of the ionic enthalpies of transfer with solvent composition is discussed in terms of ion-solvent interactions and of the effects caused by sulfolane on the structure of water.     

Synthesis and alkali metal complexation studies of novel cage-functionalized cryptands

The synthesis of novel cage-functionalized cryptands 1–5 containing adamantane-, 2-oxaadamantane- or noradamantane-moiety [i.e., 1,3-diethyladamantano[2.2.0]cryptand (1), 1,3-diethoxyadamantano[2.2.2]cryptand (2), 1,3-di[(ethyloxy)methyl]adamantano[2.2.2]-cryptand (3), 1,3-di[(ethyloxy)methyl]-2-oxaadamantano[2.2.3]cryptand (4), and 1,2-diethyloxynoradamantano[2.2.2]cryptand (5)] and their alkali metal binding properties are reported. The results obtained by extraction experiments showed that all the cryptands displayed lower extraction capabilities than the parent [2.2.2]cryptand. However, cryptands 1 and 2 showed much higher selectivity toward K⁺ than the reference [2.2.2]cryptand. When the third bridge is enlarged by two additional CH₂-groups as well as by two oxygen atoms, as in cryptands 3 and 4, the complexational abilities for bigger cations (K⁺, Rb⁺ and Cs⁺) are enhanced. Cryptand 5 displayed very good extraction capabilities of all cations, but showed practically no selectivity towards any of the alkali metal cation. The experimental findings are corroborated by calculation studies consisting of force field based conformational search using Monte Carlo method followed by investigation of the stabilities of the complexes of cryptands with Na⁺ and K⁺ metal ions in chloroform by means of quantum chemical calculations at the density functional theory level.     

Synthesis and molecular structure of 1,4-dimethyl-4,5,7,8-tetrahydro-6H-imidazo[4,5-e][1,4]-diazepine-5,8-dithione

1,4-dimethyl-4,5,7,8-tetrahydro-6H-imidazo[4,5-e][1,4]-diazepine-5,8-dithione was synthesized by boiling 1,4-dimethyl-4,5,7,8-tetrahydro-6H-imidazo[4,5-e][1,4]-diazepine-5,8-dione (a cyclic homolog of theobromine) with P₂S₅. Its molecular and crystal structures were determined by X-ray structure analysis, PMR spectroscopy and the calculations using the MM2 program. The crystals are monoclinic, sp. gr. P2 ₁ /n with a=9.305(4), b=9.464(3), c=11.628(3) Å, -90.49(3)^o, Z=4 for C₈H₁₀N₄S₂. M.p. 268–269 °C. The 7-membered heterocycle has a boat conformation in the crystal, while in solution at room temperature it undergoes interconversion. The geometrical parameters of the molecule obtained by X-ray structure analysis, by PMR spectroscopy below the coalescence temperature (290 K), and by MM2 calculations are in good agreement.A. V. Bogatsky Physico-Chemical Institute, Ukrainian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 34, No. 3, pp. 86–90, May–June 1993.Translated by T. Yudanova.     

Experimental and computational bridgehead C-H bond dissociation enthalpies

Bridgehead C-H bond dissociation enthalpies of 105.7 ± 2.0, 102.9 ± 1.7, and 102.4 ± 1.9 kcal mol(-1) for bicyclo[2.2.1]heptane, bicyclo[2.2.2]octane, and adamantane, respectively, were determined in the gas phase by making use of a thermodynamic cycle (i.e., BDE(R-H) = ΔH°(acid)(H-X) - IE(H(·)) + EA(X(·))). These results are in good accord with high-level G3 theory calculations, and the experimental values along with G3 predictions for bicyclo[1.1.1]pentane, bicyclo[2.1.1]hexane, bicyclo[3.1.1]heptane, and bicyclo[4.2.1]nonane were found to correlate with the flexibility of the ring system. Rare examples of alkyl anions in the gas phase are also provided.     

A quantum chemical study on the reaction of 1-aryl-1,2-dihydrophosphinine oxides with dimethyl acetylenedicarboxylate

A β-oxophosphorane/ylide (2a) and an oxaphosphete (3a), the product and the possible intermediate of an inverse Wittig type reaction of 1-(2,4,6-triisopropylphenyl-)1,2-dihydrophosphinine oxide with dimethyl acetylenedicarboxylate were studied by quantum chemical calculations. The reaction of the title reagents following either a traditional [4 + 2] cycloaddition protocol to afford phosphabicyclo[2.2.2]octadiene 5 or a novel route yielding eventually β-oxophosphorane/ylide 2 was evaluated by energy calculations. The mechanism for the formation of intermediate 3a₂ was refined by HF/6-31G* transition state calculations. Analysis of the HOMO-LUMO orbitals of the reagents justified the reactivity experienced.     

Strain,orbital interaction,and conformation of propella[34]prismane and its precursor diolefin

Factors that influence the intramolecular [2+2] photochemical ring closure reaction of multiply bridgedsyn-tricyclo[4.2.0.0^2.5]octa-3,7-diene (2) leading to propella[3₄]prismane (1) are analyzed. The unfavorable, large increase in strain (89 kcal mol^–1) calculated for this reaction is overcome by the favorable ordering of frontier molecular orbitals, which is caused primarily by bridging of the juxtaposed double bonds with trimethylene chains in2. This conclusion is based on detailed analysis of the frontier molecular orbitals (FMO) of cycloocta-1,5-diene (6), cyclodeca-1,6-diene (7), and tricyclo-[4.2.0.0^2.5]octa-3, 7-diene (3), bridged with alkylene groups of various lengths, in terms of Paddon-Row's theory of through-bond interaction by the use of AM1 semiempirical MO method. In addition to the -orbital interaction through an even number of C-C bonds, the double-bond-double-bond distance is recognized to be an important factor for the ordering of FMOs. It is suggested that reduction of this distance in6 by only a few tenths of an Ångstrom will change the FMO ordering and allow the intramolecular [2+2] cycloaddition to proceed. Potential energy calculations by MM2 indicate that the trimethylene bridges in1 and2 are so flexible that they appear flat on the NMR time scale.     

Diazabicycloalkanes with nitrogen atoms in the nodal positions. 7. Synthesis and configuration of 2,3-diphenyl-1,4-diazabicyclo[2.2.2]-octane

The reaction of ethylene oxide with the meso and d,l diasteromers of 1,2-diphenyl-ethylenediamine gave N,N'-bis(-hydroxyethyl)-1,2-diphenylethylenediamines, which upon refluxing with SOCl₂ form N,N'-bis (-chloroethyl)-1,2-ethylenediamines. The latter upon heating in dimethylformamide (DMF) at 150 ° C undergo cyclization to give, respectivley, cis- and trans-2,3-diphenyl-1,4-diazabicyclo[2.2.2]octane. Evidence for the spatial orientation of the phenyl substituents in the diazabicyclooctanes obtained was obtained by means of an analysis of the multiplicity of the methylene protons in the PMR spectra. The vicinal spin-spin coupling constants for the benzyl protons of the isomeric 2,3-diphenyl-1,4-diazabicyclo[2.2.2]octanes in the ¹³C-H satellites were measured. The values obtained were compared with the literature data and with the dihedral angles calculated from mechanical models of the molecules.See [1] for Communication 6.Translated from Khimiya Geterotsiklicheskikh Soedinenii. No. 1. pp. 95–100. January, 1982.The authors thank V. I. Mamatyuk and Yu. V. Gatilov for their assistance in interpreting the PMR spectra and performing the mathematical calculations.     

Correlation between MO eigenvectors and enthalpies of formation for alkanes

A new model for the calculation of enthalpies of formation of alkanes (up to C₈) is presented. An additive bond energy scheme, using the experimental methane and diamond values for the CH and CC bond energies, respectively, is supplemented by correction for the CC π antibonding character of the highest occupied molecular orbitals (HOMOs), effectively adjusting the CC bond energies. The effect is calculated by the summation of products of appropriate eigenvectors from semiempirical PM3 or HF/STO-3G calculations, after orthogonal transformation. The enthalpy of formation can then be expressed in terms of only one adjustable parameter. With HF/STO-3G eigenvectors, the mean discrepancy between experimental and calculated enthalpies of formation, after a one-parameter correction for 1,4 steric interactions, is 2.2 kJ mol⁻¹, comparable with more highly parameterized models. The results using PM3 eigenvectors are less satisfactory, probably on account of the neglect of overlap in the semiempirical scheme.     

Tetrakis(Isothiocyanato)zinc(II) 4,7,13,16,21,24-Hexaoxa-1,10-diazoniabicyclo[8.8.8]hexacosane: Synthesis and Crystal Structure

The salt tetrakis(isothiocyanato)zinc(II) 4,7,13,16,21,24-hexaoxa-1,10-diazoniabicyclo[8.8.8]hexacosane, [H₂(Crypt-222)]²⁺ · [Zn(NCS)₄]^2– (I), was synthesized and its structure was determined using X-ray diffraction analysis: space group P2₁/n, a = 13.734 Å, b = 11.627 Å, c = 20.816 Å, = 91.51°, Z = 4. The structure was solved by direct methods and anisotropically refined by the full-matrix least-squares method to R =0.093 for 4920 independent reflections (CAD4 autodiffractometer, CuK radiation). The structural units of crystal I are 2.2.2-cryptand dications (with two protonated nitrogen atoms) and complex anions [Zn(NCS)₄]^2–. The coordination polyhedron of the Zn²⁺ cation is a distorted tetrahedron. The 2.2.2-cryptand dication contains trifurcate N–H(...O)₃ hydrogen bonds.     

Calculation of enthalpies of hydrogenation of hydrocarbons

Sets of hydrogen molecule equivalents have been developed which permit the calculation of hydrogenation of different types of carbon-carbon bonds from ab initio total energies (3-21G and 6-31G* basis sets, and, to a more limited extent, for MP2/6-31G* data) of reactants and products. The calculated enthalpies of hydrogenation are in good agreement with experiment for unstrained molecules, with average errors on the order of 2 kcal/mol. The 6-31G* equivalents allow the enthalpies for strained molecules to be calculated accurately, but the 3-21G equivalents do not. The equivalents for both basis sets have been tested by calculating the enthalpies of hydrogenation of carbon-carbon bonds in nitrogen- and oxygen-containing organic molecules, free radicals, and classical carbocations. The results are in good agreement with experiment in most cases.     

A theoretical study on NH bond dissociation enthalpies of oxo, thio and seleno carbamates and their N-protonated and N-deprotonated species

The effect of N-protonation and N-deprotonation on structure, NH bond dissociation enthalpies (BDEs) and stabilities of radicals formed on H-abstraction from nitrogen atom of carbamates and their thio- and seleno-analogs have been investigated. For those molecules where experimental results are available for comparison, the ROB3LYP/6-311++G(d,p)//B3LYP/6-31+G* theoretical level is in agreement within the estimated experimental uncertainty. The NH BDE of carbamates H₂NC(=X)YCH₃ [X = O; Y = O, S, Se] are higher but lower when X = S, Se and Y = O, S, Se in comparison to NH BDE of NH₃. DFT calculations indicate that the NH bond dissociation enthalpies are decreased by protonation and deprotonation at nitrogen atom; but the effect of deprotonation is rather smaller than the protonation. The variations are analyzed in terms of stabilities of molecules, their protonated and deprotonated species along with their respective radicals. The electron delocalization from nitrogen, X and Y atoms, electrostatic interactions, conjugative interactions and spin delocalization are the important factors affecting the stability. The spin delocalization and shift of radical center to chalcogen X (X = S, Se) are the main determinants for radical stability.     

Kinetics of the diels–alder addition of ethene to cyclohexa-1,3-diene and its reverse reaction in the gas phase

The addition of ethene to cyclohexa-1,3-diene has been studied between 466 and 591 K at pressures ranging from 27 to 119 torr for ethene and 10 to 74 torr for cyclohexa-1,3-diene. The reaction is of the “Diels–Alder” type and leads to the formation of bicyclo[2.2.2]oct-2-ene. It is homogeneous and first order with respect to each reagent. The rate constant (in l./mol sec) is given by The retron-Diels–Alder pyrolysis of bicyclo[2.2.2]oct-2-ene has also been studied. In the ranges of 548–632 K and 4–21 torr the reaction is first order, and its rate constant (in sec^?1) is given by The reaction mechanism is discussed. The heat of formation and the entropy of bicyclo[2.2.2]oct-2-ene are estimated.     

Purification of 3-ethoxycarbonylquinuclidine and its conversion to 3-quinuclidinecarboxylic acid and 3-quinuclidinylmethanol

3-Ethoxycarbonylquinuclidine obtained by the Grob method is a mixture of 3-ethoxycarbonyl-1-azabicylo-[2.2.2]- and, according to ¹³C NMR data, 5-ethoxycarbonyl-5-azatricyclo[3.2.1.0^2,7]octanes (1621). 3-Ethoxy-carbonylquinuclidine was purified by recrystallization of the hydrochloride, hydrolyzed by water to 3-quinuclidinecarboxylic acid, and reduced by LiAlH₄ to 3-quinuclidinylmethanol.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, 1509–1512, November, 1992.     

Synthesis and Crystal Structure of Diaqua(2.2.2-Cryptand)strontium Dichloride Trihydrate

Absract—Diaqua(2.2.2-Cryptand)strontium dichloride trihydrate [Sr(2.2.2-Crypt)(H₂O)₂]²⁺ · 2Cl^– · 3H₂O (I) was prepared and studied by X-ray diffraction. The triclinic structure of I (space group P , a = 9.152 Å, b = 10.140 Å, c = 15.219 Å, = 88.84°, = 88.19°, = 87.62°, Z = 2) was solved by the direct method and refined by full-matrix least-squares calculations in the anisotropic approximation to R = 0.050 for 4188 independent reflections (CAD4 automated diffractometer, CuK radiation). The structure contains the [Sr(2.2.2-Crypt)(H₂O)₂]²⁺ host–guest cation. The Sr²⁺ cation resides in the 2.2.2-cryptand cavity and is coordinated by all eight heteroatoms (6O + 2N) of the cryptand ligand and by two O atoms of water molecules. The Sr²⁺ coordination polyhedron (C.N. 10) is a highly distorted dibase-centered two-cap trigonal prism. The crystal structure of I contains a branched system of ion–ion (intermolecular) hydrogen bonds O(w)–H···Cl^–, which connect the complex cations, the Cl^– anions, and the crystal water molecules to form infinite thick layers parallel to the yz plane.