Gas-phase reactions between thiourea and Ca2+: new evidence for the formation of [Ca(NH3)]2+ and other doubly charged species.

The gas-phase reactions between Ca(2+) and thiourea are investigated by means of electrospray ionization/mass spectrometry experiments. The MS/MS spectra of [Ca(thiourea)](2+) complexes show the appearance of new doubly charged species formed by the loss of NH(3) and HNCS. Other intense peaks at m/z 43, 56, 60, 73, 76 and 98 are also observed, and assigned to monocations produced in different coulomb-explosion processes. The structures and bonding characteristics of the different stationary points of the [Ca(thiourea)](2+) potential energy surface (PES) were theoretically studied by DFT calculations carried out at B3LYP/cc-pWCVTZ level. The analysis of the topology of this PES permits to propose different mechanisms for the loss of ammonia and HNCS, and to identify, the m/z 43, 56, 60, 73, 76 and 98 peaks as H(2)NCNH(+), CaNH(2) (+), H(2)NCS(+), CaSH(+), thiourea(+) and CaNCS(+) ions respectively. There are significant dissimilarities between the reactivity of urea and thiourea, which are related to the lower ionization energy of the latter, and to the fact that thioenols are intrinsically more stable than enols with respect to the corresponding keto forms.     

An experimental and theoretical investigation of gas-phase reactions of Ca2+ with glycine

The gas-phase reactions between Ca(2+) and glycine ([Ca(gly)](2+)) have been investigated through the use of mass spectrometry techniques and B3-LYP/cc-pWCVTZ density functional theory computations. The major peaks observed in the electrospray MS/MS spectrum of [Ca(gly)](2+) correspond to the formation of the [Ca,C,O(2),H](+), NH(2)CH(2) (+), CaOH(+), and NH(2)CH(2)CO(+) fragment ions, which are produced in Coulomb explosion processes. The computed potential energy surface (PES) shows that not only are these species the most stable product ions from a thermodynamic point of view, but they may be produced with barriers lower than for competing processes. Carbon monoxide is a secondary product, derived from the unimolecular decomposition of some of the primary ions formed in the Coulomb explosions. In contrast to what is found for the reactions of Ca(2+) with urea ([Ca(urea)](2+)), minimal unimolecular losses of neutral fragments are observed for the gas-phase fragmentation processes of [Ca(gly)](2+), which is readily explained in terms of the topological differences between their respective PESs.     

Modelling peptide-metal dication interactions: formamide-Ca(2+) reactions in the gas phase

The collision induced dissociation of formamide-Ca(2+) complexes produced in the gas phase through nanoelectrospray ionization yields as main products ions [CaOH](+), [HCNH](+), [Ca(NH(2))](+), HCO(+) and [Ca(NH(3))](2+) and possibly [Ca(H(2)O)](2+) and [C,O,Ca](2+), the latter being rather minor. The mechanisms behind these fragmentation processes have been established by analyzing the topology of the potential energy surface by means of B3LYP calculations carried out with a core-correlated cc-pWCVTZ basis set. The Ca(2+) complexes formed by formamide itself and formimidic acid play a fundamental role. The former undergoes a charge separation reaction yielding [Ca(NH(2))](+) + HCO(+), and the latter undergoes the most favorable Coulomb explosion yielding [Ca-OH](+) + [HCNH](+) and is the origin of a multistep mechanism which accounts for the observed loss of water and HCN. Conversely, the other isomer of formamide, amino(hydroxyl)carbene, does not play any significant role in the unimolecular reactivity of the doubly charged molecular cation.     

Theoretical study of interaction of urate with li(+), na(+), k(+), be(2+), mg(2+), and ca(2+) metal cations

The geometries and energetics of complexes of Li(+), Na(+), K(+), Be(2+), Mg(2+), and Ca(2+)metal cations with different possible uric acid anions (urate) were studied. The complexes were optimized at the B3LYP level and the 6-311++G(d,p) basis set. Complexes of urate with Mg(2+), and Ca(2+)metal cations were also optimized at the MP2/6-31+G(d) level. Single point energy calculations were performed at the MP2/6-311++G(d,p) level. The interactions of the metal cations at different nucleophilic sites of various possible urate were considered. It was revealed that metal cations would interact with urate in a bi-coordinate manner. In the gas phase, the most preferred position for the interaction of Li(+), Na(+), and K(+) cations is between the N(3) and O(2) sites, while all divalent cations Be(2+), Mg(2+), and Ca(2+) prefer binding between the N(7) and O(6) sites of the corresponding urate. The influence of aqueous solvent on the relative stability of different complexes has been examined using the Tomasi's polarized continuum model. The basis set superposition error (BSSE) corrected interaction energy was also computed for complexes. The AIM theory has been applied to analyze the properties of the bond critical points (electron densities and their Laplacians) involved in the coordination between urate and the metal cations. It was revealed that aqueous solvation would have significant effect on the relative stability of complexes obtained by the interaction of urate with Mg(2+) and Ca(2+)cations. Consequently, several complexes were found to exist in the water solution. The effect of metal cations on different NH and CO stretching vibrational modes of uric acid has also been discussed.     

Unimolecular reactivity of strong metal-cation complexes in the gas phase: ethylenediamine-Cu(+)

The gas-phase reactions between ethylenediamine (en) and Cu(+) have been investigated by means of mass spectrometry techniques. The MIKE spectrum reveals that the adduct ions [Cu(+)(H(2)NCH(2)CH(2)NH(2))] spontaneously decompose by loosing H(2), NH(3) and HCu, the loss of hydrogen being clearly dominant. The spectra of the fully C-deuterated species show the loss of HD, NH(3) and CuD but no losses of H(2), D(2), NH(2)D, NHD(2), ND(3) or CuH are observed. This clearly excludes hydrogen exchange between the methylene and the amino groups as possible mechanisms for the loss of ammonia. Conversely, methylene hydrogen atoms are clearly involved in the loss of molecular hydrogen. The structures and bonding characteristics of the Cu(+)(en) complexes as well as the different stationary points of the corresponding potential energy surface (PES) have been theoretically studied by DFT calculations carried out at B3LYP/6-311+G(2df,2p)//B3LYP/6-311G(d,p) level. Based on the topology of this PES the most plausible mechanisms for the aforementioned unimolecular fragmentations are proposed. Our theoretical estimates indicate that Cu(+) strongly binds to en, by forming a chelated structure in which Cu(+) is bridging between both amino groups. The binding energy is quite high (84 kcal mol(-1)), but also the products of the unimolecular decomposition of Cu(+)(en) complexes are strongly bound Cu(+)-complexes.     

The significance of the alkene size and the nature of the metal ion in metal-alkene complexes: a theoretical study

Cation interactions with π-systems are a problem of outstanding contemporary interest and the nature of these interactions seems to be quite different for transition and main group metal ions. In this paper, we have systematically analyzed the contrast in the bonding of Cu(+) and main group metal ions. The molecular structures and energetics of the complexes formed by various alkenes (A = C(n)H(2n), n = 2-6; C(n)H(2n- 2), n = 3-8 and C(n)H(2n + 2), n = 5-10) and metal ions (M = Li(+), Na(+), K(+), Ca(2+), Mg(2+), Cu(+) and Zn(2+)) are investigated by employing ab initio post Hartree-Fock (MP2/6-311++G**) calculations and are reported in the current study. The study, which also aims to evaluate the effect of the size of the alkyl portion attached to the π-system on the complexation energy, indicates a linear relationship between the two. The decreasing order of complexation energy with various metal ion-alkene complexes follows the order Zn(2+)-A > Mg(2+)-A > Ca(2+)-A > Cu(+)-A > Li(+)-A > Na(+)-A > K(+)-A. The increased charge transfer and the electron density at (3,-1) intermolecular bond critical point corroborates well with the size of the π-system and the complexation energy. The observed deviation from the linear dependency of the Cu(+)-A complexes is attributed to the dπ→π* back bonding interaction. An energy decomposition analysis via the reduced variational space (RVS) procedure was also carried out to analyze which component among polarization, charge transfer, coulomb and exchange repulsion contributes to the increase in the complexation energy. The RVS results suggest that the polarization component significantly contributes to the increase in the complexation energy when the alkene size increases.     

Modeling the interactions between peptide functions and Sr2+: formamide-Sr2+ reactions in the gas phase

The interactions between formamide, which can be considered a prototype of a peptide function, and Sr(2+) have been investigated by combining nanoelectrospray ionization/mass spectrometry techniques and G96LYP DFT calculations. For Sr an extended LANL2DZ basis set was employed, together with a 6-311+G(3df,2p) basis set expansion for the remaining atoms of the system. The observed reactivity seems to be dominated by the Coulomb explosion process yielding [SrOH](+) + [HNCH](+), which are the most intense peaks in the MS/MS spectra. Nevertheless, additional peaks corresponding to the loss of HNC and CO indicate that the association of Sr(2+) to water or to ammonia leads to long-lived doubly charged species detectable in the timescale of these experimental techniques. The topology of the calculated potential energy surface permits us to establish the mechanisms behind these processes. Although the interaction between the neutral base and Sr(2+) is essentially electrostatic, the polarization triggered by the doubly charged metal ion results in the activation of several bonds, and favors different proton transfer mechanisms required for the formation of the [SrOH](+), [SrOH(2)](2+) and [SrNH(3)](2+) products.     

Collision induced dissociation of doubly-charged ions: Coulomb explosion vs. neutral loss in [Ca(urea)](2+) gas phase unimolecular reactivity via chemical dynamics simulations

In this paper we report different theoretical approaches to study the gas-phase unimolecular dissociation of the doubly-charged cation [Ca(urea)](2+), in order to rationalize recent experimental findings. Quantum mechanical plus molecular mechanical (QM/MM) direct chemical dynamics simulations were used to investigate collision induced dissociation (CID) and rotational-vibrational energy transfer for Ar + [Ca(urea)](2+) collisions. For the picosecond time-domain of the simulations, both neutral loss and Coulomb explosion reactions were found and the differences in their mechanisms elucidated. The loss of neutral urea subsequent to collision with Ar occurs via a shattering mechanism, while the formation of two singly-charged cations follows statistical (or almost statistical) dynamics. Vibrational-rotational energy transfer efficiencies obtained for trajectories that do not dissociate during the trajectory integration were used in conjunction with RRKM rate constants to approximate dissociation pathways assuming complete intramolecular vibrational energy redistribution (IVR) and statistical dynamics. This statistical limit predicts, as expected, that at long time the most stable species on the potential energy surface (PES) dominate. These results, coupled with experimental CID from which both neutral loss and Coulomb explosion products were obtained, show that the gas phase dissociation of this ion occurs by multiple mechanisms leading to different products and that reactivity on the complicated PES is dynamically complex.     

Theoretical study of the reaction mechanism of HCN+ and CH4 of relevance to Titan's ion chemistry

Titan is the largest satellite of Saturn. In its atmosphere, CH4 is the most abundant neutral after nitrogen. In this paper, the complex doublet potential-energy surface related to the reaction between HCN+ and CH4 is investigated at the B3LYP/6-311G(d,p), CCSD(T)/6-311G++(3df,2pd)(single-point), and QCISD/6-311G(d,p) computational levels. A total of seven products are located on the PES. The initial association of HCN+ with CH4 is found to be a prereaction complex 1 (HCNHCH3(+)) without barrier. Starting from 1, the most feasible pathway is the direct H-abstraction process (the internal C-H bond dissociation) leading to the product P1 (HCNH++CH3). By C-C addition, prereaction complex 1 can form intermediate 2 (HNCHCH3(+)) and then lead to the product P2 (CH3CNH++H). The rate-controlling step of this process is only 25.6 kcal/mol. It makes the Path P2 (1) R --> 1 --> TS1/2 --> 2 --> TS2/P2 --> P2 another possible way for the reaction. P3 (HCNCH3(+) + H), P5 (cNCHCH2(+) + H2), and P6 (NCCH3(+) + H2) are exothermic products, but they have higher barriers (more than 40.0 kcal/mol); P4 (H + HCN + CH3(+)) and P7 (H + H2 + HCCNH+) are endothermic products. They should be discovered under different experimental or interstellar conditions. The present study may be helpful for investigating the analogous ion-molecule reaction in Titan's atmosphere.     

Comprehensive density functional theory study on serine and related ions in gas phase: conformations, gas phase basicities, and acidities

Density functional theory (DFT) calculations have been performed to investigate the gas-phase conformations of serine and its three related ions (serineH(+), serine(-), and serine(2-)). The full ensemble of possible conformations, 324 conformations for serine, 108 for serineH(+), 162 for serine(-) and 54 for serine(2-), were first surveyed at B3LYP/6-31G level, and then the obtained unique conformations were further refined at B3LYP/6-311+G level. From full optimizations, 74 unique conformations for seine, 14 for serineH(+), 11 for serine(-), and 4 for serine(2-) were located, and their relative energies were also determined at B3LYP/6-311+G level. Atoms in molecules (AIM) analysis was carried out to establish rigorous definition of hydrogen bonds. Six types of intramolecular H-bonds in conformers of serine, six types in serineH(+), three types in serine(-), and two types in serine(2-) were identified within the framework of AIM theory and their relative strengths were determined based on topological properties at bond critical points (BCPs) of H-bonds. The intramolecular H-bonds were demonstrated to play an important role in deciding the relative stability of conformations of amino acids and the related ions. The enthalpies and Gibbs free energies of protonation and deprotonation reactions of serine and its related ions were calculated at B3LYP/6-311+G//B3LYP/6-31G, and B3LYP/6-311+G//B3LYP/6-311+G level. The calculated results are both in excellent agreement with the experimental data. We demonstrate in this study that B3LYP is an efficient and accurate method to predict the thermochemical and structural parameters of amino acids and the related ions.     

Formation of C7H7(+) from benzyl chloride and chlorotoluene molecular ions: a theoretical study

The potential energy surface (PES) for the formation of C7H7(+) from benzyl chloride and chlorotoluene ions was obtained by quantum chemical calculations at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) level. On the basis of the PES, the RRKM model calculations were carried out to predict the rate constants of the dissociations of the molecular ions of o-, m-, and p-chlorotoluene, all of which agreed well with previous experimental results. The kinetic analysis showed that the benzylium ion was the predominant product in the dissociations of the four isomeric molecular ions, below the thresholds of the formation of tolylium ions.     

Energetics and mechanisms of C-H bond activation by a doubly charged metal ion: guided ion beam and theoretical studies of Ta2+ + CH4

A guided ion beam tandem mass spectrometer is used to study the kinetic-energy dependence of doubly charged atomic tantalum cations (Ta(2+)) reacting with CH4 and CD4. As for the analogous singly charged system, the dehydrogenation reaction to form TaCH2(2+) + H2 is exothermic. The charge-transfer reaction to form Ta(+) + CH4(+) and the charge-separation reaction to form TaH(+) + CH3(+) are also observed at low energies in exothermic processes, as is a secondary reaction of TaCH2(2+) to form TaCH3(+) + CH3(+). At higher energies, other doubly charged products, TaC(2+) and TaCH3(2+), are observed, although no formation of TaH(2+) was observed. Modeling of the endothermic cross sections provides 0 K bond dissociation energies (in electronvolts) of D0(Ta(2+)-C) = 5.42 +/- 0.19 and D0(Ta(2+)-CH3) = 3.40 +/- 0.16. These experimental bond energies are in poor agreement with density functional calculations at the B3LYP/HW+/6-311++G(3df,3p) level of theory. However, the Ta(2+)-C bond energy is in good agreement with calculations at the QCISD(T) level of theory, and the Ta(2+)-CH3 bond energy is in good agreement with density functional calculations at the BHLYP level of theory. Theoretical calculations reveal the geometric and electronic structures of all product ions and are used to map the potential energy surface, which describes the mechanism of the reaction and key intermediates. Both experimental and theoretical results suggest that TaH(+), TaCH2(2+), and TaCH3(2+) are formed through a H-Ta(2+)-CH3 intermediate.     

Infrared multiple photon dissociation spectroscopy of cationized histidine: effects of metal cation size on gas-phase conformation

The gas phase structures of cationized histidine (His), including complexes with Li(+), Na(+), K(+), Rb(+), and Cs(+), are examined by infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light generated by a free electron laser, in conjunction with quantum chemical calculations. To identify the structures present in the experimental studies, measured IRMPD spectra are compared to spectra calculated at B3LYP/6-311+G(d,p) (Li(+), Na(+), and K(+) complexes) and B3LYP/HW*/6-311+G(d,p) (Rb(+) and Cs(+) complexes) levels of theory, where HW* indicates that the Hay-Wadt effective core potential with additional polarization functions was used on the metals. Single point energy calculations were carried out at the B3LYP, B3P86, and MP2(full) levels using the 6-311+G(2d,2p) basis set. On the basis of these experiments and calculations, the only conformation that reproduces the IRMPD action spectra for the complexes of the smaller alkali metal cations, Li(+)(His) and Na(+)(His), is a charge-solvated, tridentate structure where the metal cation binds to the backbone carbonyl oxygen, backbone amino nitrogen, and nitrogen atom of the imidazole side chain, [CO,N(α),N(1)], in agreement with the predicted ground states of these complexes. Spectra of the larger alkali metal cation complexes, K(+)(His), Rb(+)(His), and Cs(+)(His), have very similar spectral features that are considerably more complex than the IRMPD spectra of Li(+)(His) and Na(+)(His). For these complexes, the bidentate [CO,N(1)] conformer in which the metal cation binds to the backbone carbonyl oxygen and nitrogen atom of the imidazole side chain is a dominant contributor, although features associated with the tridentate [CO,N(α),N(1)] conformer remain, and those for the [COOH] conformer are also clearly present. Theoretical results for Rb(+)(His) and Cs(+)(His) indicate that both [CO,N(1)] and [COOH] conformers are low-energy structures, with different levels of theory predicting different ground conformers.     

Dependence of positive binding energies on side chains--a theoretical prediction on the origin of regular ordering for the amino acid residues in the selectivity filter

The side-chain effects of metalated and protonated dipeptides, including GGH(+)M(+), GAH(+)M(+), AGH(+)M(+), AAH(+)M(+), GWH(+)M(+), GSH(+)M(+), GTH(+)M(+), GFH(+)M(+), GYH(+)M(+), and GVH(+)M(+) (G = glycine, A = alanine, W = tryptophan, S = serine, T = threonine, F = phenylalanine, and V = valine; M = Li, Na, and K), are theoretically explored in this paper on their positive binding energies (PBEs), which are derived from interactions of M+ with the carboxyl oxygen(s). The B3LYP/6-311++G(**)// B3LYP/6-31G(*) calculations suggest that the PBEs of dipeptides with side chain(s) are much smaller than those with no side chain (GGH(+)M(+)). Generally, larger side chains and smaller M(+) radii would lead to fewer PBEs for the M(+) involved systems. On the basis of the direct dependence of PBE on the electrostatic repulsion between two kinds of cations (H(+) and M(+)) in these dipeptide models, it could be reasonably expected that the side-chain effect on the electrostatic repulsion and consequently on the PBEs could offer one good insight, on a chemical-physical basis, into the origin of regular ordering of the amino acids when they form a filter in the K(+) channel protein (MacKinnon, et al. Science 1998, 280, 106).     

Interaction of molecular nitrogen and oxygen with extraframework cations in zeolites with double six-membered rings of oxygen-bridged silicon and aluminum atoms: a DFT study

The interaction of N(2) and O(2) with extraframework cations of zeolite frameworks was studied by DFT, using the B3LYP method. The extraframework cation sites located in the vicinity of the double six-member rings (D6R) of FAU zeolites (SI, SI', SIII') were considered and clusters with composition (M(n)(+))(2/)(n)()H(12)Si(10)Al(2)O(18), M = Li(+), Na(+), K(+), Ca(2+), were selected to represent the adsorption centers. The cation sites SII in the center of single six-membered rings (S6R) were modeled by [M(I)H(12)Si(4)Al(2)O(6)](-) and M(II)H(12)Si(4)Al(2)O(6) clusters. The adsorption energy of N(2) and O(2) is the highest for Li(+) cations at the SIII' cation sites, while for the SI' and SII sites the adsorption energies decrease in the order Ca(2+) > Na(+) > Li(+). The calculated small N(2) adsorption energy for Li(+) cations at SII sites suggests that these sites do not take part in the sorption process in agreement with results of NMR studies and Monte Carlo simulations. The N(2) adsorption complexes with the extraframework cations are linear, while those of O(2) are bent regardless of the extraframework cation location. The SIII' cation sites are the most favorable ones with respect to N(2) adsorption capacity and N(2)/O(2) selectivity; the SII sites are less selective and the SI sites are not accessible.     

Salt effects on the conformational behavior of 5-carboxy- and 5-hydroxy-1,3-dioxane

The varied and essential involvement of metal ions and inorganic salts in biological and chemical processes motivated the present study where 5-carboxy- and 5-hydroxy-1,3-dioxanes are used as model frameworks for the evaluation of the conformational behavior of oxygen-containing receptors in the presence of Li(+), Na(+), K(+), Ag(+), Mg(2+), Ca(2+), Ba(2+), and Zn(2+). Thus, the position of equilibria, established by means of BF(3), between diastereomeric cis- and trans-5-substituted-2-phenyl-1,3-dioxanes, in solvent THF and in the presence of 0, 1, and 5 equiv of salt, has been determined. The observed Delta G(o) degrees values for the conformational equilibria of 5-carboxy-1,3-dioxane show that Ag(+), Li(+), and Ca(2+) complexation leads to increased stability of the axial isomer. In the case of the 5-hydroxy-1,3-dioxane, Mg(2+), Ag(+), and Zn(2+) are the metal ions that stabilize the axial conformer of the heterocycle upon association. Interpretation of the experimental observations was based on DFT molecular modeling studies at the Becke3LYP/6-31G* and Becke3LYP/6-31+G** levels of theory. Although gas-phase calculations give Delta E values that are too large when modeling equilibria involving ionic species in polar solution, the computational results confirm the structural and energetic consequences of metal cation coordination to the oxygen atom in carbonyls or ethers. The results derived from the present study contribute to our understanding of the chemical processes involved in molecular recognition and physiological events.     

Determination of ammonia, creatinine and inorganic cations in urine using CE with contactless conductivity detection

CE with capacitively coupled contactless conductivity detection (C(4)D) was used to determine waste products of the nitrogen metabolism (ammonia and creatinine) and of biogenic inorganic cations in samples of human urine. The CE separation was performed in two BGEs, consisting of 2 M acetic acid + 1.5 mM crown ether 18-crown-6 (BGE I) and 2 M acetic acid + 2% w/v PEG (BGE II). Only BGE II permitted complete separation of all the analytes in a model sample and in real urine samples. The LOD values for the optimized procedure ranged from 0.8 microM for Ca(2+) and Mg(2+) to 2.9 microM for NH(4)(+) (in terms of mass concentration units, from 7 microg/L for Li(+) to 102 microg/L for creatinine). These values are adequate for determination of NH(4)(+), creatinine, Na(+), K(+), Ca(2+) and Mg(2+) in real urine samples.     

Structures of water octamers (H2O)8: exploration on ab initio potential energy surfaces by the scaled hypersphere search method

The potential energy surface (PES) of water octamers has been explored by the scaled hypersphere search method. Among 164 minima on the PES (based on MP2/6-311++G(3df,2p)//B3LYP/6-311+G(d,p) calculations), the cubic structure with D2d symmetry has been confirmed to be the global minimum. In a thermodynamic simulation using these 164 structures, the cubic structure with S4 symmetry has the highest population at low temperature, though double rings can become dominant as temperature going up, in good accord with a recent Monte Carlo simulation using an empirical potential. A transition temperature from cubic to noncubic has significantly been underestimated when potential energy data of B3LYP/6-311+G(d,p) calculations are employed in the simulation. This serious discrepancy between the MP2 and the B3LYP results suggests an importance of dispersion interactions for discussions on thermodynamics of water octamers.     

Structure of the boronic acid dimer and the relative stabilities of its conformers

Despite the widespread use of boronic acids in materials science and as pharmaceutical agents, many aspects of their structure and reactivity are not well understood. In this research the boronic acid dimer, [HB(OH)(2)](2), was studied by second-order M?ller-Plesset (MP2) perturbation theory and coupled cluster methodology with single and double excitations (CCSD). Pople split-valence 6-31+G*, 6-311G**, and 6-311++G** and Dunning-Woon correlation-consistent cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, and aug-cc-pVTZ basis sets were employed for the calculations. A doubly hydrogen-bonded conformer (1) of the dimer was consistently found to be lowest in energy; the structure of 1 was planar (C(2h)) at most computational levels employed but was significantly nonplanar (C(2)) at the MP2/6-311++G** and CCSD/6-311++G** levels, the result of an intrinsic problem with Pople-type sp-diffuse basis functions on heavy atoms. The dimerization energy, enthalpy, and free energy for the formation of (1) from the exo-endo conformer of the monomer were -10.8, -9.2, and +1.2 kcal/mol, respectively, at the MP2/aug-cc-pVTZ level. Several other hydrogen-bonded conformers of the dimer were local minima on the potential energy surface (PES) and ranged from 2 to 5 kcal/mol higher in energy than 1. Nine doubly OH-bridged conformers, in which the boron atoms were tetracoordinated, were also local minima on the PES, but they were all greater than 13 kcal/mol higher in energy than 1; doubly H-bridged structures proved to be transition states. MP2 and CCSD results were compared to those from the BLYP, B3LYP, OLYP, O3LYP, PBE1PBE, and TPSS functionals with the 6-311++G** and aug-cc-pVTZ basis sets; the PBE1PBE functional performed best relative to the MP2 and CCSD results. Self-consistent reaction field (SCRF) calculations predict that boronic acid dimerization is less favorable in solution than in vacuo.