Studies on the structure, stability, and spectral signatures of hydride ion-water clusters

The gas-phase structure, stability, spectra, and electron density topography of H(-)W(n) clusters (where n = 1-8) have been calculated using coupled-cluster CCSD(T) and M?ller-Plesset second-order perturbation (MP2) theory combined with complete basis set (CBS) approaches. The performance of various density functional theory (DFT) based methods such as B3LYP, M05-2X, M06, M06-L, and M06-2X using 6-311++G(d,p), and aug-cc-pVXZ (aVXZ, where X = D, T, and Q) basis sets has also been assessed by considering values calculated using CCSD(T)/CBS limit as reference. The performance of the functionals has been ranked based on the mean signed/unsigned error. The comparison of geometrical parameters elicits that the geometrical parameters predicted by B3LYP/aVTZ method are in good agreement with those values obtained at MP2/aVTZ level of theory. Results show that M05-2X functional outperform other functionals in predicting the energetics when compared to CCSD(T)/CBS value. On the other hand, values predicted by M06-2X, and M06 methods, are closer to those values obtained from MP2/CBS approach. It is evident from the calculations that H(-)W(n) (where n = 5-8) clusters adopt several interesting structural motifs such as pyramidal, prism, book, Clessidra, cubic, cage, and bag. The important role played by ion-water (O-H···H(-)) and water-water (O-H···O) interactions in determining the stability of the clusters has also been observed. Analysis of the results indicates that the most stable cluster is made up of minimum number of O-H···H(-) interaction in conjugation with the maximum number of O-H···O interactions. The Bader theory of atoms in molecules (AIM) and natural bond orbital (NBO) analyses has also been carried out to characterize the nature of interactions between hydride ion and water molecules. It can be observed from the vibrational spectra of H(-)W(n) clusters, the stretching frequencies involving ion-water interaction always exhibit larger redshift and intensities than that of water-water (inter solvent) interactions.     

Complexation of Phenol and Thiophenol by Amine N‐Oxides: Isothermal Titration Calorimetry and ab Initio Calculations

To develop a new solvent‐impregnated resin (SIR) system for removal of phenols from water, the complex formation of dimethyldodecylamine N‐oxide (DMDAO), trioctylamine N‐oxide (TOAO), and tris(2‐ethylhexyl)amine N‐oxide (TEHAO) with phenol (PhOH) and thiophenol (PhSH) is studied. To this end we use isothermal titration calorimetry (ITC) and quantum chemical modeling (on B3LYP/6‐311G(d,p)‐optimized geometries: B3LYP/6‐311+G(d,p), B3LYP/6‐311++G(2d,2p), MP2/6‐311+G(d,p), and spin component scaled (SCS) MP2/6‐311+G(d,p); M06‐2X/6‐311+G(d,p)//M06‐2X/6‐311G(d,p), MP2 with an extrapolation to the complete basis set limit (MP2/CBS), as well as CBS‐Q). The complexes are analyzed in terms of structural (e.g., bond lengths) and electronic elements (e.g., charges). Furthermore, complexation and solvent effects (in benzene, toluene, and mesitylene) are investigated by ITC measurements, yielding binding constants K, enthalpies ΔH⁰, Gibbs fre energies ΔG⁰, and entropies ΔS⁰ of complex formation, and stoichiometry N. The ITC measurements revealed strong 1:1 complex formation between both DMDAO–PhOH and TOAO–PhOH. The binding constant (K=1.7–5.7×10⁴ M ^?1) drops markedly when water‐saturated toluene was used (K=5.8×10³ M ^?1), and π–π interaction with the solvent is shown to be relevant. Quantum mechanical modeling confirms formation of stable 1:1 complexes with linear hydrogen bonds that weaken on attachment of electron‐withdrawing groups to the amine N‐oxide moiety. Modeling also showed that complexes with PhSH are much weaker than those with PhOH, and in fact too weak for ITC determination. CBS‐Q incorrectly predicts equal or even higher binding enthalpies for PhSH than for PhOH, which invalidates it as a benchmark for other calculations. Data from the straightforward SCS‐MP2 method without counterpoise correction show very good agreement with the MP2/CBS values.     

Hydrogen Bonding in Phosphine Oxide/Phosphate–Phenol Complexes

To develop a new solvent‐impregnated resin (SIR) system for the removal of phenols and thiophenols from water, complex formation by hydrogen bonding of phosphine oxides and phosphates is studied using isothermal titration calorimetry (ITC) and quantum chemical modeling. Six different computational methods are used: B3LYP, M06‐2X, MP2, spin component‐scaled (SCS) MP2 [all four with 6‐311+G(d,p) basis set], a complete basis set extrapolation at the MP2 level (MP2/CBS), and the composite CBS‐Q model. This reveals a range of binding enthalpies (ΔH) for phenol–phosphine oxide and phenol–phosphate complexes and their thio analogues. Both structural (bond lengths/angles) and electronic elements (charges, bond orders) are studied. Furthermore, solvent effects are investigated theoretically by the PCM solvent model and experimentally via ITC. From our calculations, a trialkylphosphine oxide is found to be the most promising extractant for phenol in SIRs, yielding ΔH=?14.5 and ?9.8 kcal mol^?1 with phenol and thiophenol, respectively (MP2/CBS), without dimer formation that would hamper the phenol complexation. In ITC measurements, the ΔH of this complex was most negative in the noncoordinating solvent cyclohexane, and slightly less so in π–π interacting solvents such as benzene. The strongest binding is found for the dimethyl phosphate–phenol complex [?15.1 kcal mol^?1 (MP2/CBS)], due to the formation of two H‐bonds (P?O???H‐O‐ and P‐O‐H???O‐H); however, dimer formation of these phosphates competes with complexation of phenol, and would thus hamper their use in industrial extractions. CBS‐Q calculations display erroneous trends for sulfur compounds, and are found to be unsuitable. Computationally relatively cheap SCS‐MP2 and M06‐2X calculations did accurately agree with the much more elaborate MP2/CBS method, with an average deviation of less than 1 kcal mol^?1.     

Molecular and electronic structure, magnetotropicity and absorption spectra of benzene-trinuclear copper(I) and silver(I) trihalide columnar binary stacks

The molecular and electronic structures, stabilities, bonding features, magnetotropicity and absorption spectra of benzene-trinuclear Cu(I) and Ag(I) trihalide columnar binary stacks with the general formula [c-M(3)(μ(2)-X)(3)](n)(C(6)H(6))(m) (M = Cu, Ag; X = halide; n, m ≤ 2) have been investigated by means of electronic structure calculation methods. The interaction of c-M(3)(μ(2)-X)(3) clusters with one and two benzene molecules yields 1:1 and 1:2 binary stacks, while benzene sandwiched 2:1 stacks are formed upon interaction of two c-M(3)(μ(2)-X)(3) clusters with one benzene molecule. In all binary stacks the plane of the alternating c-M(3)(μ(2)-X)(3) and benzene components adopts an almost parallel orientation. The separation distance between the centroids of the benzene and the proximal c-M(3)(μ(2)-X)(3) metallic cluster found in the range 2.97-3.33 ? at the B97D/Def2-TZVP level is indicative of a π···π stacking interaction mode, for the centroid separation distance is very close to the sum of the van der Waals radii of Cu···C (3.10 ?) and Ag···C (3.44 ?). Energy decomposition analysis (EDA) at the SSB-D/TZP level revealed that the dominant term in the c-M(3)(μ(2)-X)(3)···C(6)H(6) interaction arises from dispersion and electrostatic forces while the covalent interactions are predicted to be negligible. On the other hand, charge decomposition analysis (CDA) illustrated very small charge transfer from C(6)H(6) toward the c-M(3)(μ(2)-X)(3) clusters, thus reflecting weak π-base/π-acid interactions which are further corroborated by the respective electrostatic potentials and the fact that the total dipole moment vector points to the center of the metallic ring of the c-M(3)(μ(2)-X)(3) cluster. The absorption spectra of all aromatic columnar binary stacks simulated by means of TD-DFT calculations showed strong absorptions in the UV region. The main features of the simulated absorption spectra are thoroughly analyzed, and assignments of the contributing electronic transitions are given. The magnetotropicity of the binary stacks evaluated by the NICS(zz)-scan curves indicated an enhancement of the diatropicity of the inorganic ring upon interaction with the aromatic benzene molecule. Noteworthy is the slight enhancement of the diatropicity of the benzene ring, particularly in the region between the interacting rings, probably due to the superposition (coupling) of the diamagnetic ring currents of the interacting aromatic ring systems.     

Theoretical approaches to estimating homolytic bond dissociation energies of organocopper and organosilver compounds

Although organocopper and organosilver compounds are known to decompose by homolytic pathways among others, surprisingly little is known about their bond dissociation energies (BDEs). In order to address this deficiency, the performance of the DFT functionals BLYP, B3LYP, BP86, TPSSTPSS, BHandHLYP, M06L, M06, M06-2X, B97D, and PBEPBE, along with the double hybrids, mPW2-PLYP, B2-PLYP, and the ab initio methods, MP2 and CCSD(T), have been benchmarked against the thermochemistry for the M-C homolytic BDEs (D(0)) of Cu-CH(3) and Ag-CH(3), derived from guided ion beam experiments and CBS limit calculations (D(0)(Cu-CH(3)) = 223 kJ·mol(-1); D(0)(Ag-CH(3)) = 169 kJ·mol(-1)). Of the tested methods, in terms of chemical accuracy, error margin, and computational expense, M06 and BLYP were found to perform best for homolytic dissociation of methylcopper and methylsilver, compared with the CBS limit gold standard. Thus the M06 functional was used to evaluate the M-C homolytic bond dissociation energies of Cu-R and Ag-R, R = Et, Pr, iPr, tBu, allyl, CH(2)Ph, and Ph. It was found that D(0)(Ag-R) was always lower (～50 kJ·mol(-1)) than that of D(0)(Cu-R). The trends in BDE when changing the R ligand reflected the H-R bond energy trends for the alkyl ligands, while for R = allyl, CH(2)Ph, and Ph, some differences in bond energy trends arose. These trends in homolytic bond dissociation energy help rationalize the previously reported (Rijs, N. J.; O'Hair, R. A. J. Organometallics2010, 29, 2282-2291) fragmentation pathways of the organometallate anions, [CH(3)MR](-).     

One pot-synthesis of chiral Ni6 clusters involving Ni3 subunits: a combined structural, magnetic and DFT study

Ni(6) clusters of the general formula [{Ni(3)L(n)(OAc)(OH)}(2)(X)(OAc)(H(2)O)(2)] (n = 1, 2; X = Cl(-) or N(3)(-), (L(n))(3-) = hexadentate tritopic ligands) can be isolated by spontaneous self-assembly, from mixtures of Ni(OAc)(2), H(3)L(n), NMe(4)OH·5H(2)O and NaX in adequate molar ratios. Thus, four new hexanuclear complexes [{Ni(3)L(1)(OAc)(OH)}(2)Cl(OAc)(H(2)O)(2)]·7.5H(2)O (1·7.5H(2)O), [{Ni(3)L(2)(OAc)(OH)}(2)Cl(OAc)(H(2)O)(2)]·2H(2)O·7.5MeOH (2·2H(2)O·7.5MeOH), [{Ni(3)L(1)(OAc)(OH)}(2)(N(3))(OAc)(H(2)O)(2)]·6H(2)O (3·6H(2)O) and [{Ni(3)L(2)(OAc)(OH)}(2)(N(3))(OAc)(H(2)O)(2)]·4H(2)O (4·4H(2)O) were obtained and fully characterised. 1·7.5H(2)O and 2·2H(2)O·7.5MeOH were isolated in the form of single crystals, the latter losing solvate on drying, to yield 2·2H(2)O. Recrystallisation of 3·6H(2)O in MeCN/MeOH also generates single crystals of 3·H(2)O·2MeOH·2MeCN. Their X-ray characterisation shows that these Ni(6) clusters can be considered to be built from two triangular trinuclear [Ni(3)L(n)(OAc)(OH)](+) subunits with different connectors. In addition, these studies demonstrate that the (L(n))(3-) ligands behave as trinucleating, adopting such a conformation that induces chirality in the isolated compounds. In this way, 3·H(2)O·2MeOH·2MeCN appears particularly interesting, since it emerges as homochiral after undergoing spontaneous resolution upon crystallisation. The magnetic characterisation of 1·7.5H(2)O to 3·6H(2)O reveals that the three compounds present an overall antiferromagnetic coupling. The intricate magnetic behaviour of these clusters, mediated by a total of 14 bridges of different kinds, was analysed and satisfactorily interpreted in light of DFT calculations.     

The shape of gaseous n-butylbenzene: assessment of computational methods and comparison with experiments

Conformational landscape of neutral and ionized n-butylbenzene has been examined. Geometries have been optimized at the B3LYP/6-31G(d), B3LYP/6-31+G(d,p), B3LYP-D/6-31+G(d,p), B2PLYP/6-31+G(d,p), B2PLYP-D/6-31+G(d,p), B97-D/6-31+G(d,p), and M06-2X/6-31+G(d,p) levels. This study is complemented by energy computations using 6-311++G(3df,2p) basis set and CBS-QB3 and G3MP2B3 composite methods to obtain accurate relative enthalpies. Five distinguishable conformers have been identified for both the neutral and ionized systems. Comparison with experimentally determined rotational constants shows that the best geometrical parameters are provided by B3LYP-D and M06-2X functionals, which include an explicit treatment of dispersion effects. Composite G3MP2B3 and CBS-QB3 methods, and B2PLYP-D, B3LYP-D, B97-D, and M06-2X functionals, provide comparable relative energies for the two sets of neutral and ionized conformers of butyl benzene. An exception is noted however for conformer V(+) the stability of which being overestimated by the B3LYP-D and B97-D functionals. The better stability of neutral conformers I, III, and IV, and of cation I(+) , demonstrated by our computations, is in perfect agreement with conclusions based on micro wave, fluorescence, and multiphoton ionization experiments.     

Assessment of density functionals with long‐range and/or empirical dispersion corrections for conformational energy calculations of peptides

Density functionals with long‐range and/or empirical dispersion corrections, including LC‐ωPBE, B97‐D, ωB97X‐D, M06‐2X, B2PLYP‐D, and mPW2PLYP‐D functionals, are assessed for their ability to describe the conformational preferences of Ac‐Ala‐NHMe (the alanine dipeptide) and Ac‐Pro‐NHMe (the proline dipeptide) in the gas phase and in water, which have been used as prototypes for amino acid residues of peptides. For both dipeptides, the mean absolute deviation (MAD) is estimated to be 0.22–0.40 kcal/mol in conformational energy and 2.0–3.2° in torsion angles ? and ψ using these functionals with the 6‐311++G(d,p) basis set against the reference values calculated at the MP2/aug‐cc‐pVTZ//MP2/aug‐cc‐pVDZ level of theory in the gas phase. The overall performance is obtained in the order B2PLYP‐D ≈ mPW2PLYP‐D > ωB97X‐D ≈ M06‐2X > MP2 > LC‐ωPBE > B3LYP with the 6–311++G(d,p) basis set. The SMD model at the M06‐2X/6‐31+G(d) level of theory well reproduced experimental hydration free energies of the model compounds for backbone and side chains of peptides with MADs of 0.47 and 4.3 kcal/mol for 20 neutral and 5 charged molecules, respectively. The B2PLYP‐D/6‐311++G(d,p)//SMD M06‐2X/6‐31+G(d) level of theory provides the populations of backbone and/or prolyl peptide bond for the alanine and proline dipeptides in water that are consistent with the observed values. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Adsorption of polycyclic aromatic hydrocarbons and inversion barriers of curved conjugated systems inside the molecular cage ExCage6+

We investigated the hosting of planar and curved π systems by ExCage⁶⁺. The M06‐2X/6‐311G* method and including basis set superposition error (BSSE) corrections showed good agreement with the experimental free energy changes in solution. The mean absolute deviation (MAD) with respect to experiment was 1.3 kcal/mol. The M06‐2X/6‐31G* method exhibited a MAD of 2.1 kcal/mol, so it may be useful to investigate large systems. The good agreement between the M06‐2X/6‐311G*+BSSE results and the M06‐2X/6‐31G* ones was due to a fortuitous error cancelation between basis set incompleteness and BSSE. The interaction energies decreased linearly with the number of CC double bonds present in the PAH, but the shape of the PAH is an important factor in determining the binding strength. Finally, we studied how effective is ExCage⁶⁺ in reducing the inversion barriers of buckybowls. For corannulene, sumanene and dibenzo[a,g]corannulene they are reduced by 2.0, 2.7, and 2.0 kcal/mol, respectively. Although these values indicate an induced fit catalysis by ExCage⁶⁺, they are far from those values calculated inside bilayer graphene. Therefore, much work is necessary to replicate the reduction of inversion barriers observed for graphene.     

Theoretical Studies on the Aminolysis of Ester: Effects of the Catalysis and Substituent to the Reaction of Methyl Indole-3-acetate with Ammonia

A comprehensive exploration of the aminolysis mechanism for methyl indole-3-acetate with ammonia is carried out by employing the B3 LYP/6-311++G(d,p), M06-2 X/6-311++G(d,p) and MP2/6-311++G(d,p)//M06-2 X/6-311++G(d,p) levels. Two alterative reaction channels of the concerted and addition/elimination stepwise processes including the uncatalyzed, base-catalyzed reactions are taken into consideration. Subsequently, the substituent effects and solvent effects in methanol are also evaluated at the M06-2 X/6-311++G(d,p) level. The calculated results indicate that the calculated values of M06-2 X level are quite close to those of MP2, the stepwise pathway has more advantages to the concerted one for all of the reaction processes and the catalyst facilitates the proton migration and decreases the energy barriers as well. It is shown that the most preferred mechanism is the based-catalyzed stepwise process, the substituent of NH2 group slightly accelerates all the aminolysis reaction processes, and the solvent effect does not remarkably change the mechanism of the reaction.     

Redox energetics of hypercloso boron hydrides B(n)H(n) (n = 6-13) and B12X12 (X = F, Cl, OH, and CH3)

The reduction potentials (E°(Red) versus SHE) of hypercloso boron hydrides B(n)H(n) (n = 6-13) and B(12)X(12) (X = F, Cl, OH, and CH(3)) in water have been computed using the Conductor-like Polarizable Continuum Model (CPCM) and the Solvation Model Density (SMD) method for solvation modeling. The B3LYP/aug-cc-pvtz and M06-2X/aug-cc-pvtz as well as G4 level of theory were applied to determine the free energies of the first and second electron attachment (ΔG(E.A.)) to boron clusters. The solvation free energies (ΔG(solv)) greatly depend on the choice of the cavity set (UAKS, Pauling, or SMD) while the dependence on the choice of exchange/correlation functional is modest. The SMD cavity set gives the largest ΔΔG(solv) for B(n)H(n)(0/-) and B(n)H(n)(-/2-) while the UAKS cavity set gives the smallest ΔΔG(solv) value. The E°(Red) of B(n)H(n)(-/2-) (n = 6-12) with the G4/M06-2X(Pauling) (energy/solvation(cavity)) combination agrees within 0.2 V of experimental values. The experimental oxidative stability (E(1/2)) of B(n)X(n)(2-) (X = F, Cl, OH, and CH(3)) is usually located between the values predicted using the B3LYP and M06-2X functionals. The disproportionation free energies (ΔG(dpro)) of 2B(n)H(n)(-) → B(n)H(n) + B(n)H(n)(2-) reveal that the stabilities of B(n)H(n)(-) (n = 6-13) to disproportionation decrease in the order B(8)H(8)(-) > B(9)H(9)(-) > B(11)H(11)(-) > B(10)H(10)(-). The spin densities in B(12)X(12)(-) (X = F, Cl, OH, and CH(3)) tend to delocalize on the boron atoms rather than on the exterior functional groups. The partitioning of ΔG(solv)(B(n)H(n)(2-)) over spheres allows a rationalization of the nonlinear correlation between ΔG(E.A.) and E°(Red) for B(6)H(6)(-/2-), B(11)H(11)(-/2-), and B(13)H(13)(-/2-).     

Assessment of ab initio MP2 and density functionals for characterizing the potential energy profiles of the SN2 reactions at N center

The potential energy profiles of five selected bimolecular nucleophilic substitution (S_N2) reactions at nitrogen (N) center have been reinvestigated with the CCSD(T), G3[MP2,CCSD(T)], MP2, and some density functional methods. The basis sets of 6‐31+G(d,p) and 6‐311+G(3d,2p) are used for the MP2 and density functional calculations. Taking the relative energies at the CCSD(T)/CBS level of theory as benchmarks, we recommend the MP2, B97‐K, B2K‐PLYP, BMK, ωB97X‐D, M06‐2X, M05‐2X, CAM‐B3LYP, M08‐SO, and ωB97X methods to generally characterize the potential energy profiles for the S_N2 reactions at N center. Furthermore, these recommended methods with the relatively small 6‐31+G(d,p) basis set may also be used to perform direct classical trajectory simulations to uncover the dynamic behaviors of the S_N2 reactions at N center. © 2012 Wiley Periodicals, Inc.     

Syntheses, structures, and magnetic properties of a family of tetra-, hexa-, and nonanuclear Mn/Ni heterometallic clusters

A family of Mn(III)/Ni(II) heterometallic clusters, [Mn(III)(4)Ni(II)(5)(OH)(4)(hmcH)(4)(pao)(8)Cl(2)]·5DMF (1·5DMF), [Mn(III)(3)Ni(II)(6)(N(3))(2)(pao)(10)(hmcH)(2)(OH)(4)]Br·2MeOH·9H(2)O (2·2MeOH·9H(2)O), [Mn(III)Ni(II)(5)(N(3))(4)(pao)(6)(paoH)(2)(OH)(2)](ClO(4))·MeOH·3H(2)O (3·MeOH·3H(2)O), and [Mn(III)(2)Ni(II)(2)(hmcH)(2)(pao)(4)(OMe)(2)(MeOH)(2)]·2H(2)O·6MeOH (4·2H(2)O·6MeOH) [paoH = pyridine-2-aldoxime, hmcH(3) = 2, 6-Bis(hydroxymethyl)-p-cresol], has been prepared by reactions of Mn(II) salts with [Ni(paoH)(2)Cl(2)], hmcH(3), and NEt(3) in the presence or absence of NaN(3) and characterized. Complex 1 has a Mn(III)(4)Ni(II)(5) topology which can be described as two corner-sharing [Mn(2)Ni(2)O(2)] butterfly units bridged to an outer Mn atom and a Ni atom through alkoxide groups. Complex 2 has a Mn(III)(3)Ni(II)(6) topology that is similar to that of 1 but with two corner-sharing [Mn(2)Ni(2)O(2)] units of 1 replaced with [Mn(3)NiO(2)] and [MnNi(3)O(2)] units as well as the outer Mn atom of 1 substituted by a Ni atom. 1 and 2 represent the largest 3d heterometal/oxime clusters and the biggest Mn(III)Ni(II) clusters discovered to date. Complex 3 possesses a [MnNi(5)(μ-N(3))(2)(μ-OH)(2)](9+) core, whose topology is observed for the first time in a discrete molecule. Careful examination of the structures of 1-3 indicates that the Mn/Ni ratios of the complexes are likely associated with the presence of the different coligands hmcH(2-) and/or N(3)(-). Complex 4 has a Mn(III)(2)Ni(II)(2) defective double-cubane topology. Variable-temperature, solid-state dc and ac magnetization studies were carried out on complexes 1-4. Fitting of the obtained M/(Nμ(B)) vs H/T data gave S = 5, g = 1.94, and D = -0.38 cm(-1) for 1 and S = 3, g = 2.05, and D = -0.86 cm(-1) for 3. The ground state for 2 was determined from ac data, which indicated an S = 5 ground state. For 4, the pairwise exchange interactions were determined by fitting the susceptibility data vs T based on a 3-J model. Complex 1 exhibits out-of-phase ac susceptibility signals, indicating it may be a SMM.     

Ab initio study of hydrogen-bond formation between aliphatic and phenolic hydroxy groups and selected amino acid side chains

Hydrogen bonding was studied in 24 pairs of isopropyl alcohol and phenol as one partner, and water and amino-acid mimics (methanol, acetamide, neutral and protonated imidazole, protonated methylalamine, methyl-guanidium cation, and acetate anion) as the other partner. MP2/6-31+G* and MP2/aug-cc-pvtz calculations were conducted in the gas phase and in a model continuum dielectric environment with dielectric constant of 15.0. Structures were optimized in the gas phase with both basis sets, and zero-point energies were calculated at the MP2/6-31+G* level. At the MP2/aug-cc-pvtz level, the BSSE values from the Boys-Bernardi counterpoise calculations amount to 10-20 and 5-10% of the uncorrected binding energies of the neutral and ionic complexes, respectively. The geometry distortion energy upon hydrogen-bond formation is up to 2 kcal/mol, with the exception of the most strongly bound complexes. The BSSE-corrected MP2/aug-cc-pvtz binding energy of -27.56 kcal/mol for the gas-phase acetate...phenol system has been classified as a short and strong hydrogen bond (SSHB). The CH3NH3+...isopropyl alcohol complex with binding energy of -22.54 kcal/mol approaches this classification. The complete basis set limit (CBS) for the binding energy was calculated for twelve and six complexes on the basis of standard and counterpoise-corrected geometry optimizations, respectively. The X...Y distances of the X-H...Y bridges differ by up to 0.03 A as calculated by the two methods, whereas the corresponding CBS energy values differ by up to 0.03 kcal/mol. Uncorrected MP2/aug-cc-pvtz hydrogen-bonding energies are more negative by up to 0.35 kcal/mol than the MP2/CBS values, and overestimate the CCSD(T)/CBS binding energies generally by up to 5% for the eight studied complexes in the gas phase. The uncorrected MP2/aug-cc-pvtz binding energies decreased (in absolute value) by 11-18 kcal/mol for the ionic species and by up to 5 kcal/mol for the neutral complexes when the electrostatic effect of a polarizable model environment was considered. The DeltaECCSD(T) - DeltaEMP2 corrections still remained close to their gas-phase values for four complexes with 0, +/-1 net charges. Good correlations (R2 = 0.918-0.958) for the in-environment MP2/aug-cc-pvtz and MP2/6-31+G* hydrogen-bonding energies facilitate the high-level prediction of these energies on the basis of relatively simple MP2/6-31+G* calculations.     

Theoretical search for alternative nine-electron ligands suitable for superhalogen anions

The calculations performed at the OVGF/6-311++G(3df,3pd)//MP2/6-311++G(d,p) level for the representative NaX(2)(-) and AlX(4)(-) anions matching the MX(k+1)(-) superhalogen formula and utilizing 9-electron systems (i.e., consisting of various possible combinations of atoms containing nine electrons when brought together) revealed that the OH, Li(2)H(3), and NH(2) groups might be considered as alternative ligands X due to their thermodynamic stability and large values of electron binding energy (approaching or even exceeding 6 eV in some cases). All aluminum-containing AlX(4)(-) anions (excluding Al(HBLi)(4)(-)) were predicted to be thermodynamically stable, whereas the NaX(2)(-) anions for X = CH(3), HBLi, CLi, BeB, and H(2)BeLi were found to be susceptible to the fragmentations leading to Na(-) loss. Among the MX(k+1)(-) (M = Na, Al; X = Li(2)H(3), OH, H(2)BeLi, BeB, NH(2), HBLi, CH(3), Be(2)H, CLi) anions utilizing systems containing 9 electrons (and thus isoelectronic with the F atom) the largest vertical electron detachment energy of 6.38 eV was obtained for Al(OH)(4)(-).     

Assessment of multicoefficient correlation methods, second-order Møller-Plesset perturbation theory, and density functional theory for H3O(+)(H2O)n (n = 1-5) and OH(-)(H2O)n (n = 1-4)

We have assessed the ability of 52 methods including 15 multicoefficient correlation methods (MCCMs), two complete basis set (CBS) methods, second-order M?ller-Plesset perturbation theory (MP2) with 5 basis sets, the popular B3LYP hybrid functional with 6 basis sets, and 24 combinations of local density functional and basis set to accurately reproduce reaction energies obtained at the Weizmann-1 level of theory for hydronium, hydroxide, and pure water clusters. The three best methods overall are BMC-CCSD, G3SX(MP3), and M06-L/aug-cc-pVTZ. If only microsolvated ion data is included, M06-L/aug-cc-pVTZ is the best method; it has errors only half as large as the other density functionals. The deviations between the three best performing methods are larger for the larger hydronium- and hydroxide-containing clusters, despite a decrease in the average reaction energy, making it impossible to determine which of the three methods is overall the best, so they might be ranked in order of increasing cost, with BMC-CCSD least expensive, followed by M06-L/aug-cc-pVTZ. However, the cost for M06-L will increase more slowly as cluster size increases. This study shows that the M06-L functional is very promising for condensed-phase simulations of the transport of hydronium and hydroxide ions in aqueous solution.     

The electronic spectra of the sandwich stacked PFBT: a theoretical study

Stacked models that include 9,9'-bis(6'-N,N,N-trimethylammonium)hexyl]fluorene-co-alt-4,7-(2,1,3-benzothiadiazole)dibromide (F(BT)F) monomer sandwiched between two stacked 2,1,3-benzothiadiazole (BT) units were explored using theoretical approaches. Molecular structures and the optical characteristics of the investigated species were investigated at the M06-2X/6-311G(d,p)//TD-M06-2X/6-311G(d,p) level of theory. In all models, the electronic excitation to the lowest singlet ππ* excited state (S1(ππ*)) is governed by the highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO → LUMO) transitions. The obtained results suggest that stacking interaction might have only minor effects on the transition energy for both absorption and emission processes. Instead, the reduction in the excitation energy of the stacked complexes should be attributed to the dipole-dipole interaction. The larger the interaction energy of the stacked models, the bigger the observed differences between absorption-emission energies. The presence of the solvation medium with small dielectric constant may increase the absorption-emission energy differences. It is expected that the largest absorption-emission shift can be observed in the benzene solution.