X-Ray and X-ray Photoelectron Spectroscopy Studies of the Electronic Structure of Borane Derivatives

This paper reviews the results of electronic structure studies of a number of typical members of borane series by X-ray and X-ray photoelectron spectroscopy using quantum chemical calculations. Fragment analysis of the molecular orbital structure is given. The nature of chemical interaction in boron cluster compounds is studied on models: simple molecules NH₃, BH₃, and BF₃ and their adducts NH₃BH₃ and NH₃BF₃. The electronic structure of B₁₀H₁₂L₂ type compounds with Lewis bases L = NH₃, (CH₃)₂S, (C₆H₅)₃P is analyzed. The complexes are considered in terms of the concept of donor–acceptor interactions between the fragments. The donor–acceptor bond has contributions from both occupied and vacant acceptor orbitals. X-ray photoelectron data on the charged states of atoms in the compounds are overviewed. Electron distribution in complex compounds with transition metals [(1,2-B₉C₂H₁₁)₂M]^–, M = Fe^III, Co^III, Ni^III, and Ni^IV and chain type polycobaltocarborane anions {[(3)-1,2-B₉C₂H₁₁]₂Co _n [(3,6)-1,2-B₈C₂H₁₀]_n-1} ^n– , n = 2-7, is considered.     

Hydrolysis of the Borohydride Anion BH4−: A 11B NMR Study Showing the Formation of Short-Living Reaction Intermediates including BH3OH−

In hydrolysis and electro-oxidation of the borohydride anion BH₄⁻, key reactions in the field of energy, one critical short-living intermediate is BH₃OH⁻. When water was used as both solvent and reactant, only BH₃OH⁻ is detected by ¹¹B NMR. By moving away from such conditions and using DMF as solvent and water as reactant in excess, four ¹¹B NMR quartets were observed. These signals were due to BH₃-based intermediates as suggested by theoretical calculations; they were DMF·BH₃, BH₃OH⁻, and B₂H₇⁻ (i.e., [H₃B−H−BH₃]⁻ or [H₄B−BH₃]⁻). Our results shed light on the importance of BH₃ stemming from BH₄⁻ and on its capacity as Lewis acid to interact with Lewis bases such as DMF, OH⁻, and BH₄⁻. These findings are important for a better understanding at the molecular level of hydrolysis of BH₄⁻ and production of impurities in boranes synthesis.     

Hydrolysis of hydro(pyrrolyl-1)borates

The hydrolysis of hydro(pyrrolyl-l)borates ([BH_n(NC₄H₄)_4-n]⁻, n = 1,2,3) can be treated as a kinetically one-step reaction outside of the mildly acidic region. In strongly acidic medium the hydrolysis takes place in a stepwise manner; the intermediates (boranes and the cationic boron compounds) being hydrolyzed more slowly than the borate anion. In the first step of the hydrolysis of [BH₃(NC₄H₄)]⁻ the B---H bond, while in case of [BH₂(NC₄H₄)₂]⁻ and [BH(NC₄H₄)₃]⁻ the B---N bond is breaking.In neutral and mildly alkaline medium, the hydrolysis is a general acid catalyzed reaction (A---S_E2 mechanism). It becomes to a special H⁺-ion catalyzed reaction (A-1 mechanism) in strongly alkaline region since the protonated intermediate can be reversed to the original borate upon reaction with the OH⁻ ion. The hydrolysis presumably takes place through an intermediate which is protonated on the pyrrolyl nitrogen. Concomitant to the hydrolysis an isotopic exchange reaction was observed on the C_α and C_β atoms of the pyrrolyl group in heavy water. In the hydrolysis of the [BH₃(NC₄H₄)]⁻-anion the N-protonated intermediate is assumed to be able to reverse to the original borate even in acidic or neutral region, at least in part.     

Reaktion von Phosphinboran,Phenylphosphinboran und Phosphoniumjodid mit Natriumtetrahydridoborat

Zusammenfassung Phosphinboran und Phosphoniumjodid reagieren mit NaBH₄ unter H₂-Entwicklung zu H₂P(BH₃)₂–Na⁺(I), Phenylphosphinboran zuPhHP(BH₃)₂–Na⁺ (II). Methylphosphinboran, Phenylphosphin und die Anionen von I und II reagieren nicht mit NaBH₄, da die Acidität des an Phosphor gebundenen Wasserstoffs zu gering ist. Auch (PhHP·BCl₂)₃ reagiert mit NaBH₄ unter Wasserstoffentwicklung.

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Reaction of phosphine borane, phenylphosphine borane and phosphonium iodide with sodium tetrahydridoborate

The reaction of phosphine borane and phosphonium iodide with NaBH₄ yields H₂P(BH₃)₂–Na⁺ (I), of phenylphosphine boranePhHP(BH₃)₂–Na⁺ (II) hydrogen being evolved in both reactions. Methylphosphine borane, phenylphosphine and the anions of I and II do not react with NaBH₄ on account of the reduced acidity of the hydrogen atoms bound to phosphorus. Likewise hydrogen is evolved if (PhHP·BCl₂)₃ reacts with NaBH₄.

     

Minimization of the energy of a molecule with respect to nonlinear parameters of the basis set

We study characteristic features of minimization of the Hartree-Fock-Roothaan energy with respect to nonlinear parameters of the Gaussian basis set. We describe and apply regularization of the discrete Newton-Raphson method based on the analysis of eigenvalues of the Hessian matrix. We discuss results of groundstate energy calculations for the molecules LiH, CH⁺, CH^–, He₃ ²⁺, BH₂ ⁺, and H₂O in optimal ls-Gaussian basis sets. We find that, for molecules with four to six electrons, good accuracy is obtained with small basis sets consisting of ls-functions only.Translated from Teoreticheskaya i Éxperimental'naya Khimiya, Vol. 24, No. 2, pp. 215–218, March–April, 1988.     

The interaction of water with sulfonium ions and the effects of hydration on the energetics of methyl group transfer: An ab initio molecular orbital study of the hydration of (CH3)3S+ and (CH3)2S+CH2CO2 −

The interactions of the sulfonium ions (CH₃)₃S⁺, (CH₃)₂S⁺CH₂CO₂ ^–, and (CH₃)₂S⁺-CH₂CH₂CO₂ ^– with up to four water molecules have been studied by ab initio molecular orbital methods. Complexes of (CH₃)₃S⁺ with one to three water molecules involve strong electrostatic sulfur-oxygen interactions; in contrast, the sulfide (CH₃)₂S interacts with water molecules via weak S-H hydrogen bonds, suggesting that methyl-group transfer from (CH₃)₃S⁺ in aqueous solution involves a significant alteration of the hydration pattern around the sulfur atom. Two conformers of (CH₃)₂S⁺CH₂CO₂ ^– were found that display sulfur-oxygen distances which are approximately 0.3 å less than the sum of the sulfur and oxygen van der Waals radii, indicating a strong intramolecular electrostatic interaction. For the complexes (CH₃)₂S⁺CH₂CO₂ ^–·nH₂O(n =1–4), water interacts primarily with the carboxylate group via hydrogen bonds, rather than electrostatically with the sulfur atom, although in complexes with the three- and four-water complexes, the proximity of the positively charged sulfur atom to the carboxylate group significantly alters the hydration pattern compared to that in the corresponding of complexes CH₃SCH₂CO₂ ^–· Thus, methyl transfer from (CH₃)₂S⁺CH₂CO₂ ^– to an acceptor in aqueous solution also involves substantial changes in the hydration pattern around the carboxylate group.     

Synthese und Struktur von assoziiertem μ-Phosphinodiboran und von phosphorsubstituierten Derivaten

Associated -phosphinodiborane, (-H₂PB₂H₅) _n , is formed in the reaction of H₂P(BH₃)₂Na with HCl in diethyl ether solution at –96°C. The formation of B–H–B bridges is demonstrated by IR and¹¹B-NMR spectra. (-H₂PB₂H₅) _n decomposes thermally to diborane and polymeric phosphinoborane analogous to -H₂NB₂H₅. Other phosphorus substituted -phosphinodiboranes associated via B–H–B bridges are formed in the reaction of the salts (CH₃)PH(BH₃)₂Li, (CH₃)₂P(BH₃)₂Li, andPhPH(BH₃)₂Li with HCl.

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Mit 6 Abbildungen

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Herrn Professor Dr.E. Hayek zum 70. Geburtstag gewidmet.     

Transition state geometry in radical abstraction reactions: comparison of interatomic distances in the intersecting parabolas and Morse curves models with quantum-chemical calculations

Interatomic distances in the transition state were estimated for the reactions of radical abstraction: H^· + H₂, H^· + HCl, H^· + CH₄, N^·H₂ + NH₃, HO^· + H₂O, HO₂ ^· + HOOH, and C^·H₃ + SiH₄. The calculation was performed by the quantum-chemical density functional method or coupled clusters method (QCH), as well as by the methods of intersecting parabolas (IPM) and Morse curves (IMM), using experimental data (activation energies and reaction enthalpies). The results of the latter two methods are close to the quantum-chemical calculation and differ only by the increment a: r(IPM or IMM) = a + r(QCH), where a = –4.5·10^–12 m for IPM and a = +1.9·10^–12 m for IMM.     

A spectrophotometric and thermodynamic study of the sitting-atop complex formation from reaction between free base meso-tetraarylporphyrins and zirconyl nitrate in chloroform solution

The sitting-atop complexation of meso-tetraarylporphyrins and its para-substituted derivatives (H₂t(4-X)pp, X:H, Br, Cl, CH(CH₃)₂, OCH₃, CH₃), as electron donors, with zirconyl, as an electron acceptor, have been investigated spectrophotometrically in chloroform. The mole ratio studies based on physicochemical techniques were employed clearly and revealed the formation of 1:1 sitting-atop complexes which was confirmed by UV–vis, ¹H NMR and IR spectroscopic data. The value of the formation constant was estimated for each complex using a nonlinear optimization of the complex absorbance vs. mole ratio data by package KINFIT. The results showed that the stability of these complexes decreases with the temperature enhancement. Thermodynamic parameters, ΔG°, ΔH° and ΔS°, of the SAT complexes have been determined from the temperature dependence of formation constants by Van’t Hoff equation. Also, the influence of the substituents of the aryl rings in H₂t(4-X)pp on the stability of the SAT complexes is discussed.     

η-Alkynyl and vinylidene transition metal complexes: 10. Reaction of the metal-acetylide [(η-C5H5)(CO)(NO)W---CC---C(CH3)3]Li with 1,2-diiodoethane as electrophile and with various nucleophiles

Treatment of the η¹-acetylide complex [(η⁵-C₅H₅)(CO)(NO)W---CC---C(CH₃)₃]Li (4) with 1,2-diiodoethane in THF at −78 °C, followed by the addition of Li---CC---R [R=C(CH₃)₃, C₆H₅, Si(CH₃)₃, 6a–6c] or n-C₄H₉Li and protonation with H₂O, afforded the corresponding oxametallacyclopentadienyl complexes (η⁵-C₅H₅)W(I)(NO)[η²-O=C(CC---R)CH=CC(CH₃)₃] (7a–7c), 8c and (η⁵-C₅H₅)W(I)(NO)[η²-O=C(n-C₄H₉)CH=CC(CH₃)₃] (9). The formation of these metallafuran derivatives is rationalized by the electrophilic attack of 1,2-diiodoethane onto the metal center of 4 to form first the neutral complex [(η⁵-C₅H₅)(I)(CO)(NO)W---CC---C(CH₃)₃] (5). Subsequent nucleophilic addition of Li---CC---R 6a–6c or n-C₄H₉Li and a reductive elimination step followed by protonation leads to the products 7a–7c and 9. One reaction intermediate could be trapped with CF₃SO₃CH₃ and characterized by a crystal structure analysis. The identity of another intermediate was established by infrared spectroscopic data. The oxametallacyclopentadienyl complex 10 forms in the presence of excess 1,2-diiodoethane through an alternative pathway and crystallizes as a clathrate containing iodine.     

Theoretical study of nonrigid rotation of the tetrahedral cation and anion in NH4BH4

Segments of the potential energy surface of NH₄BH₄ corresponding to nonrigid rotations of the NH₄ ⁺ cation and BH₄ ^– anion relative to each other were calculated using the SCF/3–21G approximation with complete geometrical optimization and MP3/6–31G^* approximation for special points. The barriers for these motions are only several kcal/mole and the molecule is very nonrigid relative to several types of intramolecular rotations (rotation about the B-N bond, rotations with change in the dentation of the cation and anion, and rotations of the cation and anion facilitating the formation of some H₃N-N..H-BH₃ bonds and the cleavage of other such bonds). The strong geometric deformation of the NH₄ ⁺ and BH₄ ^– ions, the polarization of the electron density in these ions, and the H-H interaction between the hydrogen atoms belonging to different ions were discussed. The structural nonrigidity of NH₄BH₄ is closely related to its stability relative to decomposition.Institute of New Chemical Products, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 5, pp. 27–34, September–October, 1989.     

Cyclopalladation of mono-, di- and tribenzylamine by palladium(II) acetate: Influence of bulkiness around the nitrogen atom of benzylamine upon internal metallation

Cyclopalladation of mono-, di- and tribenzylamine has been investigated by reacting the corresponding amines with an equimolar amount of palladium(II) acetate (reaction i), or by heating the corresponding bis-amine complexes [Pd(O₂CMe)₂{(PhCH₂)_nNH_3−n}₂] (n=1, 2) (reaction ii). By the reaction i, all the three amines undergo cyclopalladation. However, in the case of the reaction ii, only the dibenzylamine complex [Pd(O₂CMe)₂{(PhCH₂)₂NH}₂] has been converted into a cyclopalladated complex. The reactivity of the three benzylamines towards cyclopalladation has been discussed in terms of the co-ordinating ability influenced by the bulkiness around the nitrogen atom. Temperature-dependent ¹H-NMR spectra are observed for mononuclear cyclopalladated complexes [Pd(O₂CMe){C₆H₄CH₂N(CH₂Ph)₂–C¹, N}L] (L=PPh₃, AsPh₃) and are attributed to the dissociation of the nitrogen atom in the cyclopalladated chelate ring. A heteroleptic bis-cyclopalladated complex [Pd[C₆H₄CH₂N(CH₂Ph)₂–C¹, N](C₆H₄CH₂NMe₂–C¹, N)] has also been prepared. X-ray crystallographic studies on [{Pd(O₂CMe)[C₆H₄CH₂N(CH₂Ph)₂–C¹, N]}₂] and [Pd[C₆H₄CH₂N(CH₂Ph)₂–C¹, N](C₆H₄CH₂NMe₂–C¹, N)] have been reported.     

Die Schwingungsspektren von Trimethylsilyl-alkyl-bzw.-phenyl-aminen

Zusammenfassung Die Infrarot- und die Raman-Spektren der Silylamine (CH₃)₃Si–NH–R (R=CH₃, C₂H₅ und C₆H₅) sowie der analogen N-deuterierten Verbindungen werden mitgeteilt und analysiert. Starke Kopplungen führen zu einer Mischung vonv SiN bei etwa 700 cm^–1 mit anderen Schwingungen des C₃Si–NHR-Skelettes.

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The Infrared and Raman spectra of the silylamino compounds (CH₃)₃Si–NH–R (R=CH₃, C₂H₅, and C₆H₅) and the analogous N-deuterated species are reported and assigned. The SiN stretching mode at about 700 cm^–1 is strongly coupled with other vibrations of the molecules.

     

Preparation of new dinuclear half-titanocene complexes with ortho- and meta-xylene linkages and investigation of styrene polymerization

Half-titanocene is well-known as an excellent catalyst for the preparation of SPS (syndiotactic polystyrene) when activated with methylaluminoxane (MAO). Dinuclear half-sandwich complexes of titanium bearing a xylene bridge, (TiCl₂L)₂{(μ-η⁵, η⁵-C₅H₄-ortho-(CH₂–C₆H₄–CH₂)C₅H₄}, (4 (L = Cl), 7 (L = O-2,6-iPr₂C₆H₃)) and (TiCl₂L)₂{(μ-η⁵, η⁵-C₅H₄-meta-(CH₂–C₆H₄–CH₂)C₅H₄} (5 (L = Cl), 8(L = O-2,6-iPr₂C₆H₃)), have been successfully synthesized and introduced for styrene polymerization. The catalysts were characterized by ¹H- and ¹³C NMR, and elemental analysis. These catalysts were found to be effective in forming SPS in combination with MAO. The activities of the catalysts with rigid ortho- and meta-xylene bridges were higher than those of catalysts with flexible pentamethylene bridges. The catalytic activity of four dinuclear half-titanocenes increased in the order of 4 < 5 < 7 < 8. This result displays that the meta-xylene bridged catalyst is more active than the ortho-xylene bridged and that the aryloxo group at the titanium center is more effective at promoting catalyst activity compared to the chloride group at the titanium center. Temperature and ratio of [Al]:[Ti] had significant effects on catalytic activity. Polymerizations were conducted at three different temperatures (25, 40, and 70 °C) with variation in the [Al]:[Ti] ratio from 2000 to 4000. It was observed that activity of the catalysts increased with increasing temperature, as well as higher [Al]:[Ti]. Different xylene linkage patterns (ortho and meta) were recognized to be a principal factor leading to the characteristics of the dinuclear catalyst due to its different spatial arrangement, causing dissimilar intramolecular interactions between the two active sites.     

Synthesis, reactions and X-ray crystal structures of metallacrown ethers with unsymmetrical bis(phosphinite) and bis(phosphite) ligands derived from 2-hydroxy-2′-(1,4-bisoxo-6-hexanol)-1,1′-biphenyl

Chlorodiphenylphosphine and 2,2′-biphenylylenephosphorochloridite react with 2-hydroxy-2′-(1,4-bisoxo-6-hexanol)-1,1′-biphenyl to yield the new α,ω-bis(phosphorus-donor)-polyether ligands, 2-Ph₂PO(CH₂CH₂O)₂–C₁₂H₈-2′-OPPh₂ (1) and 2-(2,2′-O₂C₁₂H₈)P(CH₂CH₂O)₂–C₁₂H₈-2′-P(2,2′-O₂C₁₂H₈) (2). These ligands react with Mo(CO)₄(nbd) to form the monomeric metallacrown ethers, cis-Mo(CO)₄{2-Ph₂PO(CH₂CH₂O)₂–C₁₂H₈-2′-OPPh₂} (cis-3) and cis-Mo(CO)₄{2-(2,2′-O₂C₁₂H₈)P(CH₂CH₂O)₂–C₁₂H₈-2′-P(2,2′-O₂C₁₂H₈)} (cis-4), in good yields. The X-ray crystal structures of cis-3 and cis-4 are significantly different, especially in the conformation of the metal center and the adjacent ethylene group. The very different ¹³C-NMR coordination chemical shifts of this ethylene group in cis-3 and cis-4 suggest that the solution conformations of these metallacrown ethers are also quite different. Both metallacrown ethers undergo cis–trans isomerization in the presence of HgCl₂. Although the cis–trans equilibrium constants for the isomerization reactions are nearly identical, the isomerization of cis-3 is more rapid. Phenyl lithium reacts with cis-3 to form the corresponding benzoyl complexes but does not react with either trans-3 or cis-4. Both the slower rate of cis–trans isomerization of cis-4 and its lack of reaction with PhLi are consistent with weaker interactions between the hard metal cations and the carbonyl oxygens in both trans-3 and cis-4.     

Reaktionshemmender Einflu? von Schwefel auf die Aufkohlung von Nickel in Methan-Wasserstoff-Gemischen

Summary The carburization of nickel in CH₄-H₂ and CH₄-H₂S-H₂ gas mixtures was studied in a flow apparatus at 1060–1280 K applying the resistance-relaxation technique. The reaction of nickel with CH₄ is found to be thermally activated with an activation energy of 128 kJ/mol. The ratecontrolling step is the separation of the first hydrogen atom from the CH₄-molecules impinging on the surface. Small H₂S-amounts, resulting in PH₂S/PH₂-ratios between 8 × 10^–9 and 9 × 10^–5 decrease the initial carburization rate by a factor of up to 100. The kinetic data are interpreted in terms of poisoning of active surface sites by adsorbed sulfur. From this analysis adsorption isotherms for sulfur are derived.     

Liquid chromatography with and of platinum complexes

Summary Reversed-phase, high-performance liquid chromatography (HPLC) on chemically bonded C₁₈-phases with acetonitrile-water mobile phases, contianing platinum complexes like Zeise's salt C₂H₄PtCl₃, the amine derivative C₂H₄–PtCl₂–NH₂–CH(CH₃)–C₆H₅ or the amino acid compound C₂H₄–PtCl–OOC–CH(N(CH₃)₂)–C₆H₅ by analogy with argentation chromatography, was used to increase selectivity for the separation of various types of olefins, amines and heterocyclic compounds. On the other hand, normal-phase adsorption chromatography on silica with n-heptane, dichloromethane and n-propanol mobile phases proves to be an ideal tool for the analytical and preparative separation of diastereomeric platinum complexes of olefins, introduced by Gil-Av, that can be easily preparedin vitro, by the reaction of C₂H₄–PtCl–OOC–CH(N(CH₃)₂)–C₆H₅ with optically active olefins in CH₂Cl₂. The preparation of the intitial complex as well as its application to the separation of several interesting types of enantiomeric olefins is described and discussed. The number and amount of separable diastereomers formed by the above reaction is strongly influenced by sterical effects. By comparison of the chromatographic pattern of either racemic or partly racemic mixtures, it is possible to decide, which peaks belong to one or the other enantiomeric form of the olefin.     

Excess molar heat capacities and excess molar volumes of some mixtures of propylene carbonate with aromatic hydrocarbons

Excess molar volumes V ^E and excess molar heat capacities C _P ^E at constant pressure have been measured, at 25°C, as a function of composition for the four binary liquid mixtures propylene carbonate (4-methyl-1,3-dioxolan-2-one, C₄H₆O₃; PC) + benzene (C₆H₆;B), + toluene (C₆H₅CH₃;T), + ethylbenzene (C₆H₅C₂H₅;EB), and + p-xylene (p-C₆H₄(CH₃)₂;p-X) using a vibrating-tube densimeter and a Picker flow microcalorimeter, respectively. All the excess volumes are negative and noticeably skewed towards the hydrocarbon side: V ^E (cm³-mol^–1) at the minimum ranges from about –0.31 at x₁=0.43 for {x₁C₄H₆O₃+x₂p-C₆H₄(CH₃)₂}, to –0.45 at x₁=0.40 for {x₁C₄H₆O₃+x₂C₆H₅CH₃}. For the systems (PC+T), (PC+EB) and (PC+p-X) the C _P ^E s are all positive and even more skewed. For instance, for (PC+T) the maximum is at x _1,max =0.31 with C _P,max ^E =1.91 J-K^–1-mol^–1. Most interestingly, C _P ^E of {x₁C₄H₆O₃+x₂C₆H₆} exhibits two maxima near the ends of the composition range and a minimum at x _1,min =0.71 with C _P,min ^E =–0.23 J-K^–1-mol^–1. For this type of mixture, it is the first reported case of an M-shaped composition dependence of the excess molar heat capacity at constant pressure.Communicated at the Festsymposium celebrating Dr. Henry V. Kehiaian's 60^th birthday, Clermont-Ferrand, France, 17–18 May 1990.     

Synthesis,characterization and antimicrobial activity of cobalt(II), nickel(II)and copper(II) complexes with new asymmetrical Schiff base ligands derived from 7-formyanil-substituted diamine-sulphoxine and acetylacetone

Asymmetric 7-formyanil-substituted-imino-4-(4-methyl-2-butanone)-8-hydroxyquinoline-5-sulphonic acid (Schiff bases), react with Co^II, Ni^II and Cu^II ions to give 1:2, 1:1 and 2:1 complexes as established by conductometric titrations in 1:1 DMF:H₂O. The complexes were investigated by elemental analyses, molecular weight determinations, molar conductance, magnetic moments, thermal analysis, i.r., u.v.–vis. and e.s.r. spectra. The complexes have an octahedral crystal structure and general formula [ML·(OH₂)₂], where M^II = Co, Ni and Cu, and L = Na[7—X—HL], (—X— = (CH₂)₂, (CH₂)₃, p-C₆H₄, o-C₆H₄). Antimicrobial activity of these new ligands and their transition metal complexes has been screened in vitro on common fungi and bacteria.     

Hydration of the carboxylate group: Anab initio molecular orbital study of acetate-water complexes

The hydration of the carboxylate group in the acetate anion has been investigated by performingab initio molecular orbital calculations on selected conformers of complexes with the form CH₃CO₂ ^– ·nH₂O·mH₂O, wheren andm denote the number of water molecules in the first and second hydration spheres around the carboxylate group, andn + m 7. The results of RHF/6–31G^* optimizations for all the complexes and MP2/6–31+G^** optimizations for several one-water complexes are reported. The primary consequence of hydration on the structure of the acetate anion is a decrease in the length of the C-C bond. Enthalpy and free energy changes calculated at the MP2/6–31+G^** and MP2/6–311+ +G^** levels are reported for the reactions CH₃CO₂ ^– + [H₂O] _P CH₃CO₂ ^– ·nH₂O ·mH₂O where [H₂O] _P is a water cluster containingp water molecules andp=n+m 7. The calculations show that conformers with the lowest enthalpy change on complex formation are often not those with the lowest free energy change, due to a greater entropic loss in complexes with tighter and more favorable enthalpic interactions. Hydrogen bonding of six water molecules directly to the carboxylate group in CH₃CO₂ ^– is found to account for approximately 40% of the enthalpy change and 37% of the free energy change associated with bulk solvation.