Ein einfacher Weg zu α-substituierten (E)-3-Oxo-1-alkenylphosphonsäureestern

Zusammenfassung -Substituierte -Acylvinylphosphonate3 mitE-Konfiguration [R ²CO-CH=C(R ¹)-P(O)(OR)₂], werden in guten Ausbeuten durchWittig-Reaktion von Acylphosphonsäureestern1 [R ¹CO-P(O)(OR)₂,R ¹=Alkyl oder Aryl] mit (2-Oxoalkyliden)triphenylphosphoranen2 [R ²CO-CH=PPh ₃,R ²=Alkyl, O-Alkyl oder CH₂ X (X=Br, OMe, CO₂ Et)] erhalten.

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A convenient route to -substituted dialkyl (E)-3-oxo-1-alkenylphosphonates

-Substituted dialkyl (E)--acylvinylphosphonates [R ²CO-CH=C(R ¹)-P(O)(OR)₂,3], are easily obtained in good yields byWittig-reaction of dialkyl acylphosphonates1 [R ¹CO-P(O)(OR)₂,R ¹=alkyl or aryl) with 2-oxoalkylidene triphenylphosphoranes2 [R ²CO-CH=PPh ₃,R ²=alkyl, O-alkyl and CH₂ X (X=Br, OMe, CO₂ Et)].

     

Crystal structure of the synthetic analog of radtkeite Hg<Subscript>3</Subscript>S<Subscript>2</Subscript>Cl<Subscript>1.00</Subscript>I<Subscript>1.00</Subscript>

The results of structural studies of the synthetic analog of the radtkeite mineral Hg₃S₂Cl_1.00I_1.00 are analyzed. The crystal structure of the compound has been refined; the unit cell parameters are a _m = 16.827(4) , b _m = 9.117(1) , c _m = 13.165(5) , = 130.17(2)°, V = 1543.3(8) ³, space group C2/m, Z = 8, R = 0.0527. A possible transition a ₀ = a _m; b ₀ = a _m + 2c _m; c ₀ = –b _m to the pseudo-orthorhombic F cell previously determined for radtkeite, where one of the angles ( ₀ ) is slightly different from 90° (89.55°), has been found. Each sulfur atom in the structure is bonded to three mercury atoms, forming SHg₃ umbrellas with distances 2.240(6) –2.474(8) and angles HgSHg 94.7(2)°–102.9(2)°. The SHg₃ fragments are linked through Hg vertices to form corrugated [Hg₁₂S₈] layers. The halogen atoms lie inside and between the [Hg₁₂S₈] layers; the distances are Hg-Cl and Hg-I 2.783(7) , 2.961(7) , and 3.083(4) –3.311(3) , respectively.Original Russian Text Copyright © 2004 by N. V. Pervukhina, S. V. Borisov, S. A. Magarill, D. Yu. Naumov, V. I. Vasiliev, and B. G. NenashevTranslated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 4, pp. 755–758, July–August, 2004.This revised version was published online in April 2005 with a corrected cover date.     

Chain entanglement,mechanical properties and drawability of poly(lactide)

The observed brittle fracture behavior of amorphous polylactides seems to be contradicted by the low value ofC =2 determined for poly(L-lactide) by Flory and coworkers. Such very flexible polymer chains deform by shear yielding, and fracture in a ductile manner. In this study,C was estimated in a number of ways, resulting in much higher values ofC =11.7 andC =9.1 for poly(L-lactide) and L- and D-lactide copolymers, respectively. These high values ofC and the low entanglement density account for the brittle fracture behavior of amorphous poly(lactide), as well as for the maximum attainable draw ratios of poly(L-lactide) networks and melt spun fibers. Bulk polymerized poly(L-lactide) networks, where crystallization during polymerization impedes severe entangling, could be hot-drawn most effectively to draw ratios of 8–16, resulting in very strong materials with tensile strengths of 550–805 MPa. By comparison, amorphous, non-crystallizable L/D lactide networks, which do not crystallize during polymerization, could be drawn less, to =7. These materials with strengths up to 460 MPa could, nevertheless, be oriented much more effectively than linear, amorphous L/D lactide copolymers.     

Synthesis and X-Ray Crystal Structure Study of ]2-Methyl-4-(2-methylbenzoyl)-phenoxy] Acetic Acid Hydrazide

Benzophenone analog 3 has been synthesized and characterized by the X-ray diffraction (XRD) method. The compound crystallizes in a monoclinic space group P2₁/c with cell parameters a = 7.701(8) Å, b = 7.151(5) Å, c= 28.323(3) Å, = 104.639(4)°, Z = 4. The structure exhibits intra- and intermolecular hydrogen bonding of the type N–HO, C–HO, and N–HN. The molecules are interlinked through hydrogen bonds forming an infinite chain. This polymeric-like structure may play an important role in biological activity.     

Strukturelle Abwandlungen an partiell silylierten Kohlenhydraten mittels Triphenylphosphan/Azodicarbonsäurediethylester, 4. Mitt.: Transformationen an Mannose und Galaktose

Reaction of methyl -D-galactopyranoside (1) with two equivalents oft-butyldimethylchlorosilane yields methyl 2,6-bis-O-(tBDMSi)--D-galactopyranoside (1 b), methyl 3,6-bis-O-(tBDMSi)--D-galactopyranoside (1 c) and methyl 4,6-bis-O-(tBDMSi)--D-galactopyranoside (1 d). Likewise methyl -D-mannopyranoside (6) affords methyl 2,6-bis-O-(tBDMSi)--D-mannopyranoside (6 d) and methyl 3,6-bis-O-(tBDMSi)--D-mannopyranoside (6 b), which can be isomerised withTPP/DEAD to methyl 4,6-bis-O-(tBDMSi)--D-mannopyranoside (6 f). Methyl 6-O-(tBDMSi)--D-galactopyranoside (1 a) and methyl 6-O-(tBDMSi)--D-mannopyranoside (6 a) can be prepared from1 or6 with one equivalent oft-butyldimethylchlorosilane.Without an external nucleophile the sugar derivatives1 a and1 b react withTPP/DEAD to form the 3,4-carbonato--D-galactopyranosides1 h and1 i and the 3,4-carbonato-2-O-ethoxycarbonyl--D-galactoside (1 j). In contrast to the formation of the compound1 i by means ofTPP/DEAD the reaction of1 a withTPP and Di-t-butyl-azodicarboxylate (DTBAD) yields the 2,3-anhydro--D-taloside (4 b) and only a small amount of1 i. The epoxide4 b can be cleaved withp-nitrobenzoylchloride/pyridine to the 3-chloro-3-deoxy-2,6-di-O-p-nitrobenzoyl--D-idoside (5). Reaction of1 c and1 d withTPP/DEAD yields the 2,3-anhydro--D-gulopyranoside (2), which can be transformed with NaN₃/NH₄Cl to the 2-azido-2-deoxy--D-idopyranoside (3).Likewise6 a and6 d can be converted to the 3,4-anhydro--D-talosides (7 a and7 b). Reaction of7 b or6 d withTPP/DEAD/NH₃ leads to 3,4-anhydro-2-azido-2-deoxy--D-galactopyranoside (8) and 3-azido-3-deoxy--D-altropyranoside (10), resp.The epoxide7 b is opened with NaN₃/NH₄Cl to the 4-azido-4-deoxymannosides (11 a and11 c) and the 3-azido-3-deoxy--D-idopyranoside (12), while the epoxide8 affords the 2,4-di-azido-2,4-dideoxy--D-glucopyranoside (9).Structures were elucidated by¹H-NMR-analysis of the corresponding acetates.

H. H. Brandstetter undE. Zbiral, Helv., im Druck.     

Synthesis of some 5′-O-amino acid derivatives of uridine as potential inhibitors ofUDP-glucuronosyltransferase

Summary In an attempt to develop potential inhibitors ofUDP-glucuronosyltransferase, some 5-O-amino acid derivatives of uridine were synthesized. N-protectedL-amino acids were coupled at the 5-O-position of 2,3-O-isopropylideneuridine by esterification employing the method of symmetrical anhydrides in presence of 4-dimethylaminopyridine, 5-O-(N-benzyloxycarbonyl-O-tert.butyl-L-threonl)-23-O-isopropylideneuridine (1), 5-O-(N-tert.butyloxycarbonyl-O-benzyl-L-seryl)-2,3-O-isopropylideneuridine and (2), 5-O-(N-tert.butyloxycarbonyl-L-valyl)-2,3-O-isopropylideneuridine (3), and 5-O-(N-tert.butyloxycarbonyl-L-valyl)-2,3-O-isopropylideneuridine (4) were obtained in good yield after column chromatography on silica gel. The treatment of2 withTFA/CH₂Cl₂ (6:1) at room temperature for 30 min led to a selective removal of theBoc group without deblocking of the 2,3-O-isopropylidene group of uridine. Treatment of2 withTFA/H₂O (5:1) at room temperature for 1 h, however, released bothBoc and 2,3-isopropylidene groups. TheZ group of1 was deprotected by catalytic hydrogenolysis over 10% Pd/C/ammonium formate.

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Synthese von 5-O-Aminosäurederivaten des Uridins als potentielle Inhibitoren derUDP-Glukuronosyl-Transferase

Zusammenfassung In einem Versuch, potentielle Inhibitoren derUDP-Glukuronosyl-Transferase zu entwickeln, wurden einige 5-O-Aminosäurederivate des Uridins synthetisiert. N-GeschützteL-Aminosäuren wurden durch Veresterung mit der 5-O-Position des 2,3-isopropylidenuridins gekuppelt (Methode der symmetrischen Anhydride in der Gegenwart von 5-Dimethylaminopyridin). Solcherweise wurden 5-O-(N-Benzyloxycarbonyl-O-tert.butyl-L-threonly)-2,3-O-isopropylidenuridin (1), 5-O-(N-tert.Butyloxycarbonyl-O-benzyl-L-seryl)-2,3-O-isopropylidenuridin (2), 5-O-(N-tert.Butyloxycarbonyl-L-leucyl)-2,3-O-isopropylidenuridin (3) und 5-O-(N-tert.Butyloxycarbonyl-L-valyl)-2,3-O-isopropylidenuridine (4) nach Säulenchromatographie (Kieselgel) in guter Ausbeute hergestellt. Die Behandlung von2 mitTFA/CH₂Cl₂ (6:1) bei Zimmertemperatur (30 min) führte zu einer selektiven Abspaltung derBoc-Gruppe ohne Deblockierung der 2,3-O-Isopropylidengruppe des Uridins. Eine Behandlung von2 mitTFA/H₂O (5:1) bei Zimmertemperatur für 1 Stunde führte hingegen zur Abspaltung sowohl derBoc als auch der 2,3-O-Isopropylidengruppe. DieZ-Gruppe von1 wurde durch katalytische Hydrogenolyse auf 10% Pd/C/Ammoniumformiat abgespalten.

     

2,2′-Diphenyl-Δ3,3′-bi-3H-indole-1,1′-dioxide: Molecular interactions and crystal structure

The Dinitrone 2,2-diphenyl-^3,3-bi-3H-indole-1,1-dioxide acts as a demethylating and dehydrogenating agent. The mechanism of interaction of the dinitrone with donors and acceptors does not involve intermediate charge-transfer complexes probably due to a self association between dinitrone molecules (as supported by X-ray determinations). The crystal structure of the dinitrone was obtained by direct methods;a=9.967 (2),b=19.817 (3),c=10.875 (2) Å, =111.2 (2)°, space group P2₁/n. The finalR andR _w were 0.089 and 0.063 for all measured reflexes.

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2,2-Diphenyl-^3,3-bi-3H-indol-1,1-dioxid: Molekulare Wechselwirkungen und Kristallstruktur

Zusammenfassung Das Dinitron 2,2-Diphenyl-^3,3-bi-3H-indol-1,1-dioxid wirkt als Demethylierungs- und Oxydationsmittel. Die Wechselwirkung des Dinitrons mit Elektronen-Acceptoren und Elektronen-Donatoren geht wegen der Selbstassoziation zwischen den Dinitron-Molekülen ohne die dazwischenliegende Bildung eines Charge-Transfer-Komplexes vor sich; das wird auch von Röntgenstrukturuntersuchungen gestützt. Die Kristallstruktur wurde mit direkten Methoden ermittelt:a=9.967 (2),b=19.817 (3),c=10.875 (2) Å; =111.2 (2)°. P2₁/n. Die endgültigen WerteR undR _w waren 0.089 und 0.063 für alle gemessenen Reflexe.

     

A spectrophotometric determination of the formation constants of the inclusion complexes ofα- andβ-cyclodextrins with the azonium and ammonium tautomers of methyl orange and methyl yellow

The color fading caused by the addition of-cyclodextrin or-cyclodextrin to an aqueous solution of a tautomeric mixture of methyl orange or methyl yellow is studied spectrophotometrically at pH 1.1 and 25.0°C. A model involving 1 : 1 stoichiometry has been used to analyze the spectrophotometric data. The addition of a cyclodextrin shifts the tautomeric mixture towards the side of the ammonium tautomer. An expression allowing the calculation of the tautomeric equilibrium constant of the inclusion complexes is derived. The formation constants of the inclusion complexes of the individual tautomers are determined. Both- and-cyclodextrins bind the ammonium tautomer stronger than the azonium tautomer. The inclusion complexes of-cyclodextrin are more stable than the corresponding ones of-cyclodextrin.     

Electrochemical performance of Zn-substituted Ni(OH)2 for alkaline rechargeable batteries

Zn-substituted Ni(OH)₂ for alkaline rechargeable batteries was prepared by a chemical coprecipitation method. The structures were characterized by X-ray diffraction and scanning electron microscopy, and the electrochemical performance, including charge–discharge behavior, the proton diffusion coefficient (D_H+), and the cycle life, was investigated in detail. The results showed that the charge–discharge potentials of Zn-substituted -Ni(OH)₂ are much higher than those of Zn-substituted -Ni(OH)₂. For a single (for 30.5–48.4% Zn content) or a single (from 0 to 9.3% Zn content) phase in the sample, the discharge potentials increase with the increase of Zn content. However, when there is an and phase mixture in the sample, the discharge potential decreases with an increase of Zn content. The D_H+ values of Zn 0% and Zn 38.1% samples measured by the current-pulse relaxation method are much lower than those of Zn 9.3% and Zn 19.6% samples. D_H+ of all the samples decreases with an increase of the depth of discharge. The effects of different Zn contents on the charge–discharge potentials of the nickel electrodes can be attributed to the differences of the electrochemical and diffusion polarization.     

Triterpene glycosides ofScheffleropsis angkae. II. Structure of glycosides L-E1, L-E2, L-K1, and L-K2

The structures of four triterpene glycosides from leaves ofScheffleropsis angkae (Araliaceae) are established using chemical and NMR methods. The structures 3-O--D-glucopyranosyl-(1-3)-O--L-arabinopyranosides of oleanic and ursolic acids and their 28-O--L-rhamnopyranosyl-(1-4)-O--gentiobiosyl ethers are proposed for L-E₁, L-E₂, L-K₁, and L-K₂, respectively. L-K₁ and L-K₂ are new triterpene glycosides.Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 239–241, May–June, 2000.     

Diatomic interactions in momentum space. The 2pπ u and 3dπ g states of H 2 + system

The recently proposed method of momentum electron density for interatomic interactions is applied to the two states of the H ₂ ⁺ system. The processes of the attractive 2P _u and repulsive 3d _g interactions are analysed based on the behaviour of the momentum density and Compton profile. The results are compared with the previous ones for the 1 _{S g} and 2p _u states. The guiding principle of contraction and expansion for the energy-density relation in momentum space is shown to be common to both the and states.     

Ein einfacher Zugang zu (R)-γ-Amino-β-hydroxybuttersäure (GABOB) aus natürlichem (2S, 4R)-4-Hydroxyprolin

The oxidative decarboxylation sequence (1a 2a 3a 4a 5a) affording -aminobutanoic acid (5a) is adapted to the synthesis of its hydroxy derivative5b. A facile high yield conversion of (2S, 4R)-4-hydroxyproline-methylester-hydrochloride (7) to (R)-GABOB (5b) on a preparative scale is reported with the hydroxypyrrolidone8 as the intermediate.

     

Hypothetical transformation of Ca(OH)<Subscript>2</Subscript> into CaCO<Subscript>3</Subscript> in solid-state reactions of portland cement

Summary Previous study of the hydration and ageing products of two cement pastes created the basis for the postulate of the course of solid-state reactions between the portlandite Ca(OH)₂ and the CO₂ from air in the hydrated and air dry cement. XRD basal spacing d(001) of portlandite exceeded the nominal value and increased with ageing, with the wetting and drying procedure and with carbonate content of the paste, indicating that a part of OH^- ions was gradually substituted by CO₃^2- ions, which are about twice bigger. IR spectroscopy showed a considerable content of portlandite, of CO₃^2- of water and silicates. Also HCO₃^- H₂O and CO₂ in cavities between hexagonal rings and hexagonal hydrates were indicated. By MS (mass spectrometry) in vacuum the evaporation of sorbed water was detected at 100-120°C, of gel water at 350°C of portlandite water at 400°C and of high temperature water between 500 and 700°C, simultaneously with CO₂ escape. Slightly higher peak temperatures were found by the TG test either in air or in argon. From these results and from geometric considerations it is postulated that the solid-state reactions take place on ageing of the cement paste and on its heating: hexagonal portlanditecalcium carbonate hydroxy hydratecalcium carbonate hydratehexagonal vaterite and/or orthorhombic aragoniterhombohedral calcite The analysis of the standard files of the calcium carbonate hydroxy hydrates supports this postulate and indicates a gradual transformation.     

Ab initio SCF and CI study of the electronic spectrum of pyridine N-oxide

Configuration interaction (CI) studies of ground, n *, * * electronically excited states are reported for pyridine N-oxide. The transition energy to the lowest * excited ¹ B ₂ state is calculated at 4.35 eV, compared to the experimental spectrum range of 3.67–4.0 eV. This state lies below the lowest n * excited ¹ A ₂ state calculated at 4.81 eV above the ground state. The only experimentally reported triplet state at 2.92 eV above the ground state is predicted to be the ³ A ₁ (*) state. The calculated energy lies at 3.27 eV. Numerous other high-lying singlet states as well as the triplet states have also been calculated. The intramolecular charge transfer character of the ground and the excited states have been studied in terms of the calculated dipole moment and other physical properties.     

Stereochemistry of planarchiral compounds,part XIII: Crystal structure of Meso-10,10′-dibromo-2,2′-bi-(1,6-methano-[10]-annulenyl)

Summary Crystallization of the mesoform of the title compound1 from benzene-pentane in the presence of 5% of (+)(R) _p (S) _a (R) _p -1 afforded crystals, the structure of which was determined in the achiral spacegroup P2₁/a-C _2h ⁵ . The configuration (R) _p (S) _p was confirmed, the torsional angle around the 2,2-bond is 68.6° and 72.1° (C3-C2-C2-C3 and C1-C2-C2-C1, resp.). The twist of the best planes through the perimeter carbonatoms 3, 4, 5 and 7, 8, 9 (3, 4, etc., resp.) is 26.0° and 19.8°. The colours of the crystals of the stereoisomers of1 depend on the torsional angles and thereby on the conjugation of the -systems. Meso, monoclinic (70.3°): light yellow; racemate, opt. inactive (56.2°): yellow; enantiomer of the racemate (34.4°): orange (average -values).

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Stereochemie planarchiraler Verbindungen, 13. Mitt.: Die Kristallstruktur von Meso-10,10-dibrom-2,2-bi-(1,6-methano-[10]-annulenyl)

Zusammenfassung Kristallisation der Meso-Form der Titelverbindung1 aus Benzol-Pentan in Gegenwart von 5% (+)(R) _p (S) _a (R) _p -1 lieferte Kristalle, deren Struktur in der achiralen Raumgruppe P2₁/a-C _2h ⁵ bestimmt werden konnte: Die Konfiguration (R) _p (S) _p wurde bestätigt, der Torsionswinkel um die 2,2-Bindung beträgt 68.6 bzw. 72.1° (C3-C2-C2-C3 bzw. C1-C2-C2-C1). Die Verkippung der besten Ebenen, definiert durch die Perimeter-C-Atome 3, 4, 5 und 7, 8, 9 (bzw. 3, 4, etc.) beträgt 26.0 bzw. 19.8°. Die Farben der Kristalle der Stereoisomere von1 hängen von den Torsionswinkeln und damit von der Konjugation der -Systeme ab: Meso, monoklin (70.3°): hellgelb; Racemat, opt. inaktiv (56.2°): gelb: Enantiomer des Racemates (34.4°): orange (gemittelte Werte).

     

Glykosylazide als Ausgangsbasis zur Gewinnung von Nucleosidanalogen, 3. Mitt. Synthese von Alkylaminotetrazol- und Uretidinonnucleosiden

The -ribofuranosylazide1 is transformed after usual derivatization by suitable protecting groups into the P–N-ylid2, which gives the corresponding N-Glykosyl-N-alkylcarbodiimides4 and a small amount of the glykosylisocyanatde-rivative3 by reaction with alkylisocyanates. The carbodiimides4 were reacted with hydrazoic acid to give the alkylaminotetrazolnucleosides5 and finally the free nucleosidanalogs6. In the case of5 c the 5-aziridinyltetrazolnucleosid5 h was formed by an usual neighbouring group reaction. In addition the compound1 is transformed into the 3,5-diprotected anchor derivative7 by reaction withTIPSCl₂. The latter could be transformed by usual steps into the alkylaminotetra-zolnucleosides8 with a free 2-OH group. In the next step the 2-p-tolylthiocarbo-nates9 were prepared followed by transformation to the 2-desoxynucleosides10 by means of tributyltinhydride. Finally the free 2-desoxynucleosides11 were prepared. By reacting the carbodiimides4 with phenylisocyanate a mixture of the two possible regiouretidinonnucleosidderivatives12 and13 are formed. In the case of the N-glykosyl-N-allylcarbodiimide4 d only the one isomer13 d arises.

Herrn Prof. Dr.A. Neckel mit den besten Wünschen zum 60. Geburtstag gewidmet.     

2,2′-Diphenyl-Δ3,3′-bi-3H-indole-1,1′-dioxide: Competitive demethylation and redox reactions

Competitive demethylation and redox reactions induced by 2,2-diphenyl-^3,3-bi-3H-indole-1,1-dioxide,1 (dinitrone) on several nitrogen bearing compounds (pyridines, amides, indoles, hydrazones and amines) are reported.Es wird über kompetitive Demethylierungen und Redoxreaktionen an stickstoffhaltigen Verbindungen (Pyridine, Amide, Indole, Hydrazone und Amine) berichtet, die durch das Dinitron 2,2-Diphenyl-3,3-bi-3H-indol-1,1-dioxid induziert werden.

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2,2-Diphenyl-3,3t-bi-3H-indol-1,1t-dioxid: Kompetitive Demethylierung und Redoxreaktionen

     

Hexafluoroacetone as Protecting and Activating Reagent. Glycosylated Malic, Citramalic, and Thiomalic Acid Derivatives, New Glycosylated Building Blocks for Drug Design [1]

Summary. Glycosylated -hydroxy and -mercapto acids have been synthesized starting from malic/citramalic/thiomalic acid and Ac₄--D-Glc-NH₂/Bzl₄--D-Glc-NH₂ using hexafluoroacetone as protecting and activating reagent.Dedicated to Prof. Dr. Horst Wilde on the occasion of his 65^th birthday     

Local-density-functional approximation for exchange-correlation potential

Local exchange-correlation potential has been derived starting from the free electron gas model.Ab initio way of calculating the parameter of the X method is presented. Self-consistent and statistical exchange-correlation parameters have been determined. The self-consistent parameters have been used to calculate the electron binding energies of Neon, Argon and Krypton. We suggest using statistical exchange-correlation parameter in molecular calculations. The statistical exchange-correlation parameter has been applied to study the electron binding energies of the molecules H₂O and HF. It is shown that the electron binding energies calculated with the self-consistent and the statistical parameters show agreement with the experimental values.Dedicated to Professor J. Koutecký on the occasion of his 65th birthday     

Theoretical investigation of structure and stability of oligomers of LiH,NaH, LiF,and NaF

The molecules Li_nH_n, Na_nH_n, Li_nF_n, n=1,..., 4, and NaF and Na₂F₂ are investigated by means of extended basis set SCF and CEPA-PNO computations. In analogy to the D _2h structure of dimers, it is found that trimers have a planar cyclic D _3h equilibrium geometry. For the tetramer of LiH and NaH, the D _4h structure has about the same energy as the 3-dimensional T _d structure, whereas the latter is definitely favoured for Li₄F₄. Correlation effects are investigated for the oligomerization of LiH and the dimerization of LiF. The effect of electron correlation on corresponding E turns out to be small (<4 kJ/mol), except for the case that the T _d tetramer is involved which has a rather large correlation energy.