Showing papers in "Journal of Molecular Structure-theochem in 2002"

Segmented contraction scheme for small-core lanthanide pseudopotential basis sets

Abstract: Gaussian (14s13p10d8f6g)/[10s8p5d4f3g] valence basis sets using a segmented contraction scheme have been derived for relativistic energy-consistent small-core lanthanide pseudopotentials of the Stuttgart-Bonn variety. The present basis sets are only slightly larger than previously published (14s13p10d8f6g)/[6s6p5d4f3g] atomic natural orbital basis sets, which use a generalized contraction scheme, and achieve a similar accuracy in atomic and molecular calculations. For calibration purposes multi-configuration self-consistent field and subsequent multi-reference averaged coupled-pair functional calculations are presented for the first to fourth ionization potentials of all lanthanide elements. In addition, results of molecular calibration studies using the coupled-cluster singles, doubles and perturbative triples approach as well as gradient-corrected density functional theory are reported for the monohydrides, monoxides and monofluorides of lanthanum and lutetium.

Non-linear optical properties of novel fluorenyl derivatives—ab initio quantum chemical calculations

Abstract: We report accurate ab initio studies of the first static hyperpolarizabilities ( β ) of fluorenyl derivatives in which electron donating (D) and electron accepting (A) groups were introduced either side of the fluorenyl ring system. Geometries of all molecules were optimized at the Hartree–Fock level in a series of steps, first with the STO-3G minimal basis set, then with the 3-21G split valence basis set and finally with the 6-31G basis set. The first static hyperpolarizabilities of these molecules were calculated using Hartree–Fock level using 6-31G basis set using gaussian 98W. The calculated hyperpolarizabilities of these molecules were compared with biphenyl derivatives and other available data in the literature. To understand this phenomenon in the context of molecular orbital picture, we examined the molecular HOMOs and molecular LUMOs generated via gaussian 98W. The study reveals that the fluorenyl derivatives have large β values hence in general may have potential applications in the development of non-linear optical materials.

Quantum chemical studies on some imidazole derivatives as corrosion inhibitors for iron in acidic medium

Abstract: In this work, computational studies on some imidazole derivatives which behave as corrosion inhibitors at various levels for iron in hydrochloric acid were carried out in gaseous and aqueous phases using semiemprical methods such as MINDO/3, MNDO, PM3 and AM1. Possible correlations between experimentally obtained inhibition efficiencies and calculated highest occupied molecular energy level ( E HOMO ), lowest unoccupied molecular energy level ( E LUMO ) and the differences between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies ( E LUMO − E HOMO ) were studied. Correlation between inhibition efficiencies and total charges ( Z ) of molecules, were also investigated. Regression analysis were performed on inhibition efficiencies ( P ), ( E HOMO ) and ( E LUMO ) energies of the studied molecules.

Density functional calculations of bond dissociation energies for NO2 scission in some nitroaromatic molecules

Abstract: Bond dissociation energies for rupture of the weakest bond in some nitroaromatic molecules are calculated using Density Functional Theory. The weakest bond in the nitroaromatic molecules presented in this work corresponds to that between an NO2 group and the aromatic ring. The relationship between the bond dissociation energy (BDE) and susceptibility to detonation (sensitivity) is examined. The results indicate a correlation between sensitivity and the BDE for removal of an NO2 group.

Quantum chemistry study on the relationship between molecular structure and corrosion inhibition efficiency of amides

Abstract: Quantum chemical calculations were performed on four typical amides compounds e.g. urea, thiourea, thioacetamide and thiosemicarbazide, using the semi-empirical method MINDO/3 within program package HyperChem 6.03. Obvious correlations were found between corrosion inhibition efficiency and some quantum chemical parameters such as highest occupied molecular obital (HOMO), lowest unoccupied molecular orbital (LUMO) energy levels, HOMO–LUMO energy gap and electronic density etc. Calculation results indicated that the great difference of inhibition efficiencies between these amides can be clearly explained in terms of frontier molecular orbital theory. The agreement with the experimental data was also found to be satisfactory.

Gas phase model of an ionic liquid: semi-empirical and ab initio bonding and molecular structure

Abstract: Semi-empirical (AM1 and PM3) and ab initio structures of the gas phase ion pair, bmim+(1-butyl-3-methyl imidazolium)–PF6−, have been calculated. The ab initio calculations include fully optimized structures at the RHF/3-21G(∗), RHF/6-31G∗, RHF/6-31G∗∗, MP2/6-31G∗, B3LYP/6-31G∗ and B3LYP/6-31G∗∗ levels. Semi-empirical structures (AM1 and PM3) are similar to those obtained from ab initio methods. The non-bonded interactions in these ion pairs are of the C–H⋯X type (C–H⋯F hydrogen bonds) and exist between F's on PF6− and (a) the imidazolium ring N–H, (b) the methyl group H adjacent to the ring and (c) H's on the butyl (bmim+) side chain. The ab initio ion pair structures contain hydrogen bonds that shorten with increasing basis set complexity. The movement of the bmim+ imidazolium ring C2–H along the proton transfer coordinate towards PF6− produces similar anharmonic functions for the RHF/6-31G∗//6-31G∗∗, MP2/6-31G∗//6-31G∗∗, RHF/6-31G∗∗/6-31+G(d,p)//6-31G∗∗, MP2/6-31G∗∗/6-31+G(d,p)//6-31G∗∗, B3LYP/6-31G∗//6-31G∗∗ and B3LYP/6-31G∗∗/6-31+G(d,p)//6-31G∗∗ levels. The existence of anharmonic functions that describe the proton transfer coordinate is consistent with the Del Bene model of hydrogen bonding.

Predicting the antioxidant activity of curcumin and curcuminoids

Abstract: The curcuminoids, which include curcumin and related molecules, are effective antioxidants with demonstrated medicinal effects. The curcumin structure contains a variety of functional groups including the β-diketo group, carbon–carbon double bonds and phenyl rings containing varying amounts of hydroxyl and methoxy substituents. The literature on the site of activity and the reaction mechanism(s) responsible for the antioxidant effects is controversial, with some authors claiming that the activity is due to the hydroxyl moiety while others invoke carbon-centered radicals or involvement of the carbonyl groups. We present a systematic theoretical study of these molecules, aimed at clarifying the active sites, and discuss the validity of proposed mechanisms based on our calculations. We also discuss more general aspects of free radical chemistry, especially the selectivity arising from choice of the free radical/antioxidant system.

Vibrational spectroscopic calculations on pyrogallol and gallic acid

Abstract: Infrared and Raman spectra of pyrogallol and gallic acid were recorded. Optimized geometries, vibrational frequencies and infrared intensities were calculated utilizing the post-HF DFT method with the Becke3P86 functional and the 6-311G(d,p) basis set for one of the possible conformers of both molecules. Normal coordinate analysis was carried out. The results of the calculations were applied to simulate the infrared spectra.

Calculation of excitation energies of organic chromophores: a critical evaluation

Abstract: A series of organic compounds with first-row elements was calculated to determine the performance of TD-DFRT (time-dependent density functional response theory) and to compare with the performance of approximate or semiempirical methods (TD-DFRT-TB, ZINDO/S, PM3/S). The mean absolute deviation between theoretical and experimental excitation energies is lowest for TD-DFRT. Because of poor results in some cases the error of TD-DFT was distinctly larger in the calculations of sulfur-free than in the calculation of sulfur-containing compounds. General spectral absorption features are reasonably well reproduced by the approximate TD-DFRT in tight-binding approximation (TD-DFRT-TB) and by semiempirical MO-CI methods based on the NDO (ZINDO/S) or the NDDO approach (PM3/S). If the limitations of ZINDO/S are considered the method is useful in calculating n→π∗- and π→π∗-type transitions.

Is the intramolecular hydrogen bond energy valuable from internal rotation barriers

Abstract: A method for evaluating the energy of intramolecular hydrogen bridges utilizing the rotation barriers of the donor and/or of the acceptor groups is proposed. The procedure has been tested successfully on various molecules containing O–H⋯O, O–H⋯Halogen, O–H⋯N, N–H⋯O, N–H⋯N and O–H⋯S bridges. Comparison between the obtained results and those coming from the classic method (ΔE between the open and chelate conformations) shows good agreement and reliability. Failure has been noted only for the S–H⋯O and S–H⋯S hydrogen bridges.

Accurate scaling of the vibrational spectra of aniline and several derivatives

Abstract: The structure of aniline was studied by quantum chemical methods. Complete geometry optimization of the minimum energy structure and of the transition states for internal rotation and inversion of the amino group was carried out using several levels. The performance of the different methods in calculating and describing the vibrational wavenumbers of aniline was determined. The normal modes were characterized by the magnitude and direction of the displacement vectors. Three procedures were used to obtain the scaled wavenumbers, two of them new, using specific scale factors and scaling equations from benzene molecule. The errors obtained were compared with those calculated through other standard procedures. A reassignment of several bands was made. A comparison of the cost/effective method and procedure of scaling was carried out. Specific scale factors and scale equations were determined for the amino group to be used in large aromatic amines.

Electric field induced transitions in water clusters

Abstract: The influence of an external uniform electrostatic field on the internal energy and polarization of a medium-sized water cluster, consisting of 40 molecules is studied at four temperatures: 150, 170, 200, and 240 K, by means of the Monte Carlo method. The external field is slowly varied in the 0.5×10 7 –7×10 7 V/cm range. The system shows an abrupt change of its properties at some critical value of the field, E tr , where a transition from a normal (solid-like or liquid-like) to a superpolarized cluster state is observed. This process has the character of a first-order phase transition and is accompanied by the absorption of heat. For intermediate field strengths, polarization has been found to increase with increasing temperature, an effect, which through a simple analytical model has been shown to be the result of the interplay between the strength of the attractive water–water interactions and the strength of the external field. Also E tr has been found to decrease with increasing temperature, in contrast to the behavior dictated by the Clausius–Clayperon equation for field-induced transitions between equilibrium states.

Proper basis set for quantum mechanical studies of potential energy surfaces of carbohydrates

Abstract: In the current paper we address the optimal selection of basis set for carbohydrates. The following basis sets are used with B3LYP method: 6-31G(d), 6-31G(d,p) 5d, 6-31+G(d,p), 6-311+G(d,p), 6-311++G(d,p), 6-311+G(2d,2p), and 6-311++G(2d,2p). The following molecules were used for the comparison: water, conformers of α- l -fucose, and β- d -glucose. The 6-31G(d) or 6-31G(d,p) basis set in combination with B3LYP functional provide unacceptably poor results for carbohydrates. (Although the HF/6-31G(d) results are quite good.) The introduction of the diffuse functions on heavy atoms is necessary for good results if B3LYP functional is used (suggested methods are B3LYP/6-31+G(d,p) or 6-311+G(d,p), the latter is closer to basis set limit). The introduction of the diffuse functions (++) on hydrogen atoms is not necessary if B3LYP or other density functional method is used for carbohydrates

Molecular structures of (trifluoromethyl)iodine dihalides CF3IX2 (X = F, Cl): Ab initio and DFT calculations

Abstract: Ab initio and density functional theory calculations have been performed to investigate the equilibrium molecular structures, rotational barriers, vibrational spectra of (trifluoromethyl)iodine dihalides, CF 3 IX 2 (X=F, Cl), with the several basis sets. For each molecule the eclipsed form is found to be more stable than the staggered form. The staggered conformer corresponds to the transition state structure of which energy barrier is less than ∼0.5 kcal/mol. The natural bond orbital analysis at the restricted Hartree–Fock level shows that the energy preference of an eclipsed form over a staggered form is primarily due to the σ IX →σ CF ∗ interactions.

Theoretical study of tautomeric structures and fluorescence spectra of Hoechst 33258

Abstract: Various tautomers of Hoechst 33258 have been studied in this work, and their UV absorption and fluorescence spectra calculated theoretically These have been compared with the experimental spectra and the likely species in the ground and excited states have been discussed The results indicate that proton transfer takes place in the excited state, yielding a highly conjugated species that fluoresces at long wavelengths The findings explain the two kinds of fluorescence observed in DNA-bound Hoechst 33258 in terms of two binding modes involving different tautomers

The hydration effect on the uracil frequencies: an experimental and quantum chemical study

Abstract: This work describes the performance of different quantum chemical theoretical methods in calculating the vibrational frequencies of uracil and some derivatives, and the effect of hydration on the uracil frequencies. The Raman spectra of polycristalline uracil with different water contents are discussed. To correct the deficiency of the theoretical quantum chemical methods, several procedures are described in detail. Two of them are new. For these new procedures, scaling factors and scaling equations were determined at different levels. With them, a significant reduction in the error of the predicted frequencies was obtained over the one-factor scaling standard procedure. A comparison of the cost/effective method and procedure of scaling was carried out on uracil molecule. Scale factors transferred from uracil to related molecules provided an a priori prediction of fundamental frequencies and intensities, permitting several corrections to be proposed for earlier assignments.

The axial N-base has minor influence on Co-C bond cleavage in cobalamins

Abstract: We have investigated the properties of cobalamin complexes with imidazolate using the density functional B3LYP method, In particular, we have compared imidazolate (Imm) with imidazole and 5,6-dimethylbenzimidazole (DMB), and studied how constraints in the axial Co-N bond length may affect the strength of the Co-C bond. The results show that the optimum Co-N-Imm bond is similar to0.2 Angstrom shorter than that of imidazole. There is no indication from crystal structures that the histidine ligand would be deprotonated in the enzymes. However, it is likely that it attains some imidazolate character through its hydrogen bond to a conserved aspartate residue. The Co-N bond with imidazolate is three times more rigid than that with imidazole or DMB, but twice as flexible as the Co-C bond. Constraints in the Co-N-Imm bond length give rise to a larger change in the corrin conformation than imidazole, but smaller than for DMB. The resulting effect for the Co-C bond dissociation energy is larger for imidazolate than for imidazole or DMB. However, even the largest reasonable distortion can only enhance catalysis by 15 kJ mol(-1). Therefore, we conclude that, irrespective of the nature of the N-base, constraints in the axial Co-N bond lengths cannot be the main reason for the catalytic power of cobalamin enzymes. (Less)

Representing Born -Oppenheimer breakdown radial correction functions for diatomic molecules

Abstract: The empirical determination of Born–Oppenheimer breakdown (BOB) correction functions from the analysis of high resolution diatomic molecule spectra is becoming increasingly common. However, in virtually all applications to date, the analytic expressions used to represent those functions have unphysical limiting behaviour which makes the resulting overall potential energy functions unusable at long-range. This paper delineates the limiting physical constraints, which should be imposed on the form of such BOB correction functions, and presents flexible new expressions incorporating those constraints. The utility of these new forms is illustrated by a re-analysis of recent high resolution IR data for the ground electronic state of AgH, which yields an updated potential energy function and more meaningful adiabatic (potential) and non-adiabatic (centrifugal) BOB correction functions for this system. Further examination of this AgH system also illustrates the problem of model dependence associated with efforts to determine physically unique potential energy and centrifugal BOB correction functions.

Ab initio studies on geometry and vibrational spectra of N-methyl formamide and N-methylacetamide

Abstract: The molecular conformation, ground state molecular vibrations and force field of N-methylformamide (NMF) and N-methylacetamide (NMA) have been studied by the ab initio method at the Hartree–Fock level using the $6-31+g^*$ basis set. The potential energy surface of NMA is investigated by the ab initio method with full geometry optimization. For both the molecules, trans conformer was found to be the most stable. For trans-NMF, methyl group in the staggered conformation and for trans-NMA, methyl groups in cis–trans position with respect to –CONH– group represent global minima. The vibrational spectral analysis has been carried out for both NMF and NMA. Comparisons with the previous assignments for amide bands have been made. The present results are compared with the previous results of structure and vibrational spectra and discussed.

Studies of solvent effects on conformers of glycine molecule

Abstract: Conformational stability and solvent effects on selected conformers of glycine under different environment, such as polar and apolar solvents have been studied using ab initio and density functional theory (DFT) methods. The molecular geometries have been optimized using HF/6-31+G ∗ method of ab initio and B3LYP/6-31+G ∗ and B3PW91/6-31+G ∗ hybrid DFT methods. The effects of solvent on the geometrical parameters, relative stability and physical properties, such as dipole moment, etc. have been studied for the conformers of glycine.

Theoretical study of interactions of dickite and kaolinite with small organic molecules

Abstract: Intercalates and adsorbates of two minerals of the kaolin group (kaolinite and dickite) with formamide (FA), N -methylformamide (MFA) and dimethylsulfoxide (DMSO) were investigated using the B3LYP/3-21G ∗ method of representative cluster models. Geometry optimisation of interacting molecules as well as of outer –OH groups of the octahedral sheet of the dickite and kaolinite layer was performed. Positions of molecules with respect to mineral layers were found. Formations and orientations of hydrogen bonds between intercalated or adsorbed molecules and layers were indicated. Interaction energies of intercalates and adsorbates were calculated and corrected to the basis set superposition error (BSSE). Intercalation energies are significantly larger than adsorption ones, thus showing the importance of additional stabilisation of the intercalated molecule in the interlayer space. The order of the stabilisation of intercalates is dickite–FA>dickite–MFA>kaolinite–DMSO. The same order is also observed for adsorbed systems.

Density functional study of technetium and rhenium compounds

Abstract: Density functional calculations employing the B3LYP method and the LANL2DZ basis set have been performed on several Re dioxo complexes of simple polyamines. An extended test of the ability of the methodology to offer a good description of these complexes was performed, calculating the MO 2 , MO 2 + and MO 4 − metal oxides (both for Tc and Re) and the complex [MO 2 (NH 3 ) 4 ] + . It is shown that the results agree well with the experimental information available. The effect of the ligands is to stabilize the linear geometry of the dioxo metal core, much higher in energy than the bent geometry for the isolated species. The ligand-stabilized core exhibits a singlet electronic state, while the triplet is more stable for the isolated core. The consequence of these facts is that the metal–oxygen distance in the complexes is larger than that in any of the metal oxides. The calculated structures of the Re and Tc complexes with the amines exhibit a reasonable, although not perfect, agreement with the experimental data available. The B3LYP/LANL2DZ method seems able to describe with semiquantitative precision, the structure of Re and Tc complexes of the type, studied.

Ab initio potential energy surface and second virial coefficient for He–H2O complex

Abstract: Symmetry-adapted perturbation theory has been applied to compute the helium–water potential with the water molecule assumed rigid. The potential has a single minimum of −34.9 cm −1 at the center of mass separation of 5.92 bohr with the helium atom in the plane of water and making 75° angle with water's symmetry axis (on the oxygen side). The computed points were fitted to an analytic 3D potential surface with correct asymptotic behavior. This potential has been used to calculate the interaction second virial coefficient for He–H 2 O which agreed well with experimental data and is likely more accurate than the latter.

Gauging the applicability of ONIOM (MO/MO) methods to weak chemical interactions in large systems: hydrogen bonding in alcohol dimers

Abstract: The applicability of two-layer ONIOM methods to hydrogen bonding has been systematically investigated. The structures of 12 hydrogen bonded dimers composed of water, methanol, ethanol and n -propanol have been optimized using second-order Moller–Plesset perturbation theory (MP2) in conjunction with a double- ζ basis set with polarization and diffuse functions on all atoms, denoted DZP++. The dissociation energies (both counterpoise corrected and uncorrected) of these dimers were computed at the coupled-cluster level that includes all single and double excitations as well as a perturbative estimate of connected triple excitations [CCSD(T)] with the DZP++ basis set. These target values were used to gauge the performance of various ONIOM schemes. The methods, basis sets, and partitioning schemes were all systematically varied to determine which combinations, if any, produces substituent values ( S values) for the dissociation energy that mimic those at the target level (i.e. CCSD(T) with the DZP++ basis). All possible combinations of four popular methods [SCF, B3LYP, MP2, CCSD(T)] and seven common basis sets (3-21G, 6-31G, 6-31++G, 6-31+G( d ), 6-31++G, 6-31G( d , p ), 6-31++G( d , p ), DZP++) were investigated. Only the MP2 and DZP++ combination gives satisfactory agreement for all 47 model systems. The proton acceptor is very sensitive to the size of the model system. As such, the minimal model recommended for these primary, aliphatic alcohols should include zero C atoms on the donor and one on the acceptor (Model-0,1). With larger models (Model-1,1 or Model-1,2) less expensive methods and smaller basis sets begin to reproduce the target S values.

Theoretical study of an intermediate, a factor determining the quantum yield in photochromism of diarylethene derivatives

Abstract: Experimental and theoretical joint study on the photochemical reaction mechanism of photochromic diarylethenes is presented. An intermediate, suggesting an important role for the determination of the quantum yield, is proposed by ab initio multiconfiguration self-consistent (MCSCF) calculation.

QSAR modeling of toxicity on optimization of correlation weights of Morgan extended connectivity

Abstract: Optimal descriptors calculated in the presence of correlation weights in molecular graph of different kinds of vertices (i.e. chemical elements, e.g. H, C, O, etc.) and different Morgan extended connectivity of third-order values are reasonably better models of the acute aquatic toxicity (−log[LC 50 ]) of benzene derivatives. Statistical characteristics of the model are the following: on training set n =44, r 2 =0.8982, s =0.251, F =371; on test set n =25, r 2 =0.9181, s =0.234, F =258.

Polyhedral water clusters, II: correlations of connectivity parameters with electronic energy and hydrogen bond lengths

Abstract: Polyhedral water clusters (PWCs) are (H 2 O) n clusters in which every oxygen is three-coordinated. Four polyhedral geometries on which water clusters can be based are the cube ( n =8), pentagonal prism ( n =10), hexagonal prism ( n =12), and 4 4 5 4 octahedron ( n =12). For each of these geometries, a database was obtained of PWCs optimized at the B3LYP/6-311++G ∗∗ level. The total database contained 1311 hydrogen bonds in 82 optimized PWCs. Using linear regression, correlations were sought linking aspects of the clusters' H-bonding pattern with their electronic energies and with the lengths of their H-bonds. Excluding six very high-energy clusters whose H-bonding pattern included a motif called a cyclic component, 98.9% or more of the variance in clusters' electronic energy could be accounted for by just three connectivity parameters, for each of the geometries. Five families of H-bonds were distinguished based on the presence or absence of a pendent H on the acceptor and donor O and on the layout of adjacent H's. Within each family, the presence or absence of pendent H's on first and second neighbor O's accounted for 86–95% of the variance in H-bond length, and could be used to predict H-bond length with an RMS error≤2.4 pm.

Cation-π interactions between ammonium ion and aromatic rings: An energy decomposition study

Abstract: The nature of the cation–π interaction between ammonium ion and a series of aromatic groups has been revisited through standard supermolecular energy decomposition schemes such as the Kitaura–Morokuma and the reduced variational space self-consistent field approaches. The emerged picture describes the ammonium–aromatics interaction as basically governed by electrostatics with significant contributions from the polarization, dispersion, and at a minor extent from, even from the aromatic–ammonium charge transfer. The role of molecular properties of the aromatic partners such as their mutipole moments, first and second order polarizability as well as the electronic properties of the frontier orbitals have been investigated in the light of the decomposed components. A final comment on the implications of our results on the reliability of the force fields currently employed for classical simulations and drug design investigations, has also been addressed.

Electronic structures and spectroscopic properties of dimers Cu2, Ag2, and Au2 calculated by density functional theory

Abstract: Electronic structures and spectroscopic properties of dimers Cu2, Ag2, and Au2 have been studied by density functional theory (DFT). The relativistic effects increase with the increment in electronic number. Zero order regular approximation (ZORA) relativistic correlation gives reliable parameters of electronic structures. By applying B3LYP as the exchange–correlation functional, a same sequence of orbital levels is obtained for Cu2, Ag2, and Au2. The fully filled d-electrons are not completely inert. The contributions of d-electrons to the frontier molecular orbitals are different in these three molecules. A strong interaction between d- and s-electrons causes an up-shift of d-type and a down-shift of s-type molecular orbitals. For Cu2 and Au2, the separation in d- and s-type molecular orbitals is close and small, which account for the much similar characteristics of the excitation states. For Ag2, a big separation in d- and s-type molecular orbitals is shown owing to the extremely small contribution of d electrons to the frontier molecular orbitals. By TDDFT and DSCF, the spectroscopic terms are assigned and explained in terms of the contributions of s- and d-electrons on the excitations. The prediction based on the present calculation matches well with the experimental data.

Ab initio calculation of proton barrier and binding energy of the (H2O)OH− complex

Abstract: The very low barrier (of the order of 1 kJ mol−1) to proton transfer in the (H2O)OH− complex is accurately computed by the coupled-cluster method including single and double excitations plus perturbative corrections for triple excitations [CCSD(T)]. By virtue of the coupled-cluster R12 method [CCSD(T)-R12], which employs explicitly correlated wave functions, results are obtained close to the limit of a complete basis set of atomic orbitals (AOs). The basis-set superposition error is avoided by applying the full function counterpoise approach to the three fragments OH−, H+, and OH−. The most accurate computed value for the proton barrier amounts to δ=0.9±0.3 kJ mol −1 , which is 4–5 times this value below the lowest vibrational energy level. The three lowest vibrational energy levels are calculated from a one-dimensional potential energy curve describing the position of the bonding proton between the two OH− fragments. The electronic binding energy with respect to dissociation into H2O and OH− is estimated to D e =109±10 kJ mol −1 at the level of explicitly correlated CCSD(T)-R12 theory.