Effect of solvation on ion binding to imidazole and methylimidazole

Quantum chemical [MP2(FULL)/6-311++G-(d,p)] calculations are done on the binding of hydrated Li(+), Na(+), K(+), Mg(2+), Cu(+), and Zn(2+) metal ions with biologically relevant heteroaromatics such as imidazole and methylimidazole. The computed interaction energies are found to be in good agreement with the available experimental data. The effect of hydration on hydrogen bonding has been studied in detail and it shows that the hydrogen bond strength between H(2)O···H-N(1) substantially increases in the presence of metal ions. The present study quantifies the cooperativity between M···imidazole (M = Li(+), Na(+), K(+), Mg(2+), Cu(+), and Zn(2+)) and N(1)-H···OH(2) interactions. Topological atoms in molecules (AIM) analysis and charge analysis support the variation in hydrogen-bonding strength and the variation in M···imidazole binding strength. Effect of hydration on N(1)-H stretching frequency is studied, and it shows a clear shift in the stretching frequency after sequential hydration of metal ion as well as the N(1) of imidazole. The present study provides a detailed account on the biologically important M-histidine motif interaction with metal ions, where histidine is modeled by imidazole and methylimidazole.     

Thermochemical properties of molecules and cations of MgNnHm (n=1,2 and m=1–6): enthalpies of formation, proton affinities and ionization energies

Ab initio molecular orbital calculations are reported for small neutral molecules and cations containing magnesium, nitrogen and hydrogen. Structures have been optimized using gradient techniques at B3LYP/6-31+G(d) and at MP2(full)/6-311++G(d,p). Single-point calculations are reported at QCISD(T)(full)/6-311++G(2df,p) and at CCSD(T)(full)/6-311++G(2df,p) levels using geometries optimized at MP2(full)/6-311++G(d,p). Standard enthalpies of formation at 298 K have been calculated at these two higher levels of theory. Other thermochemical properties calculated include ionization energies and proton affinities. The binding enthalpies of ammonia to Mg⁺, MgNH₂⁺ and MgNH₃⁺ are also reported.     

<Emphasis Type="Italic">AB initio</Emphasis> quantum chemical study of the reaction mechanism of ethynide ion formation in the C<Subscript>2</Subscript>H<Subscript>2</Subscript>/MOH/DMSO system (M = Li,Na, K)

The reaction mechanism of the formation of alkali metal ethynides C₂H₂ + MOH → C₂HM + H₂O (M = Li, Na, K) is studied for the gas phase (MP2/6-311++G**//RHF/6-31+G*) and also with regard to the solvent effect of dimethyl sulfoxide (DMSO) included within the continuum model. Among all acetylene complexes with alkali metal hydroxides considered (C₂H₂·MOH (M = Li, Na, K)), only the complex with KOH is thermodynamically stable in DMSO solution. The formation of this structure results in activation of the acetylene molecule towards electrophilic attack. The formation of alkali metal ethynide in solution is also thermodynamically favorable only in the system with potassium hydroxide of a whole series of metals considered. Further, the ethynide ion can interact in KCCK·HOH systems.     

Accurate enthalpies of formation of alkali and alkaline earth metal oxides and hydroxides: assessment of the correlation consistent composite approach (ccCA)

Computing the enthalpies of formation for alkali metal and alkaline earth metal oxides (M(x)O) and hydroxides [M(OH)(n)] using the Gaussian-n (Gn) and Weismann-n (Wn) ab initio model chemistries is difficult due to an improper treatment of core-valence electron correlation effects. Using a new model chemistry called the correlation consistent Composite Approach (ccCA), enthalpies of formation were determined for eight different alkali/alkaline earth metal oxides and hydroxides. Unlike the Gn and Wn model chemistries, which must be modified to properly account for core-valence electron correlation, the standard implementations of the ccCA provide acceptable results, and all enthalpies of formation obtained with the ccCA are within the accepted range of recommended values.     

HeLi^n^+(n=0, 1)簇合物的稳定性和垂直激发态光谱

本文用ab initio研究了簇合物HeLi^n^+(n=0, 1)的几何构型和成键性质。在MP2(FULL)/6-31G**, 水平优化所得LeLi^+的平衡键长为0.2062nm, 与实验值0.205nm十分吻合。比较了HeLi^+(X^1∑^+和a^3∑^+), HeLi(X^2∑^+和a^4II)以及HLi(X^1∑^+)的稳定性, 计算了HeLi^+基态的相关能, 势能曲线和垂直激发态光谱。计算采用了6-31G**, 6-311G**,6-311G(2df, 2pd), 6-311G(3df, 2pd)和6-311+G(3df, 2pd)基组; 采用的方法包括MP2(FULL), MP4, MCSCF, MRSDCI, CCD和ST4CCD。计算表明, 同价HeLi^n^+中激发态的离解能均远比基态的大, 其中HeLi^+(a^3∑^+)的离解能最高(60.49kj/mol),说明激发态是稳定束缚态。HeLi^+基态比等电子体HLi分子基态的稳定性小得多。HeLi^+由A^1∑^+到B^1II的垂直跃迁(3σ→1π)振子强度较大而垂直跃迁能较小。     

Theoretical study of light-metal tetrahydroaluminates

Ab initio calculations of the structural, energetic, vibrational, and magnetic characteristics of the lowest-lying structures for isolated molecules and ions of light-metal tetrahydroaluminates (Li, Na, Be, Mg, and Al) have been performed by the perturbation theory (MP2), quadratic configuration interaction (QCISD(T)), coupled cluster (CCSD(T)), and density functional theory (B3LYP) methods using the 6-31G*, 6-31G**, 6-311+G**, and 6-311++G** basis sets. The trends in the behavior of the molecular characteristics have been analyzed in various related series of these compounds. The results are compared with the data on analogous light-metal tetrahydroborates calculated at the same levels of theory. The differences in structure and stability between analogous hydroborate and alanate complexes are examined. The economical approximation B3LYP/6-311++G**//B3LYP/6-311+G** has been shown to adequately reproduce the results obtained at the higher level of theory CCSD(T)/6-311++G**//MP2/6-31G* even though it requires considerably shorter CPU times and smaller amounts of memory.     

A theoretical study of the H-abstraction reactions from HOI by moist air radiolytic products (H, OH, and O (3P)) and iodine atoms (2P(3/2))

The rate constants of the reactions of HOI molecules with H, OH, O ((3)P), and I ((2)P(3/2)) atoms have been estimated over the temperature range 300-2500 K using four different levels of theory. Geometry optimizations and vibrational frequency calculations are performed using MP2 methods combined with two basis sets (cc-pVTZ and 6-311G(d,p)). Single-point energy calculations are performed with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (pertubatively) electron excitations CCSD(T) using the cc-pVTZ, cc-pVQZ, 6-311+G(3df,2p), and 6-311++G(3df,3pd) basis sets. Reaction enthalpies at 0 K were calculated at the CCSD(T)/cc-pVnZ//MP2/cc-pVTZ (n = T and Q), CCSD(T)/6-311+G(3df,2p)//MP2/6-311G(d,p), and CCSD(T)/6-311++G(3df,3pd)//MP2/6-311G(d,p) levels of theory and compared to the experimental values taken from the literature. Canonical transition-state theory with an Eckart tunneling correction is used to predict the rate constants as a function of temperature. The computational procedure has been used to predict rate constants for H-abstraction elementary reactions because there are actually no literature data to which the calculated rate constants can be directly compared. The final objective is to implement kinetics of gaseous reactions in the ASTEC (accident source term evaluation code) program to improve speciation of fission products, which can be transported along the reactor coolant system (RCS) of a pressurized water reactor (PWR) in the case of a severe accident.     

A quantum mechanical approach to the kinetics of the hydrogen abstraction reaction H2O2 + •OH → HO2 + H2O

The kinetics of the hydrogen abstraction from H₂O₂ by ^?OH has been modeled with MP2/6‐31G*//MP2/6‐31G*, MP2‐SAC//MP2/6‐31G*, MP2/6‐31+G**//MP2/6‐31+G**, MP2‐SAC// MP2/6‐31+G**, MP4(SDTQ)/6‐311G**//MP2/6‐31G*, CCSD(T)/6‐31G*//CCSD(T)/6‐31G*, CCSD(T)/6‐31G**//CCSD(T)/6‐31G**, CCSD(T)/6‐311++G**//MP2/6‐31G* in the gas phase. MD simulations have been used to generate initial geometries for the stationary points along the potential energy surface for hydrogen abstraction from H₂O₂. The effective fragment potential (EFP) has been used to optimize the relevant structures in solution. Furthermore, the IEFPCM model has been used for the supermolecules generated via MD calculations. IEFPCM/MP2/6‐31G* and IEFPCM/CCSD(T)/6‐31G* calculations have also been performed for structures without explicit water molecules. Experimentally, the rate constant for hydrogen abstraction by ^?OH drops from 1.75 × 10^?12 cm³ molecule^?1 s^?1 in the gas phase to 4.48 × 10^?14 cm³ molecule^?1 s^?1 in solution. The same trend has been reproduced best with MP4 (SDTQ)/6‐311G**//MP2/6‐31G* in the gas phase (0.415 × 10^?12 cm³ molecule^?1 s^?1) and with EFP (UHF/6‐31G*) in solution (3.23 × 10^?14 cm³ molecule^?1 s^?1). © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 502–514, 2005     

Ab Initio Calculations of Structure and Energetics of the Hypermetalated Hydrogen Oxide Molecules: Li2NaOH and LiNa2OH

We made ab initio electronic calculations of the structure and energetics of mixed hypermetalated hydrogen oxides, Li₂NaOH and LiNa₂OH. There exist five equilibrium geometries for each complex. In all levels of calculation the global minimum structure for Li₂NaOH has C_2v symmetry and a large distance between sodium and oxygen, 4.24 Å (MP2/6-31G*). The dissociation energies to all possible products were also calculated. Li₂NaOH → Na + Li₂OH δH = +25.33 kcal/mol (at MP4/6-311++G**//6-31G* + ZPE scaled by 0.9). All other dissociation processes are highly endothermic. Similar procedures were applied to LiNa₂OH. The global minimum structure for LiNa₂OH belongs to point group C_s. It is also endothermic to all possible dissociation paths. LiNa₂OH →Na + LiNaOH δH = +12.72 kcal/mol (at MP4/6-311++G*//6-31G* + ZPE scaled by 0.9). The nuclear repulsion energy is crucial in energetics of the structures. The distribution of electron density and bonding properties for these equilibrium structures were analyzed.     

Influence of H2 on the gas-phase decomposition of formic acid: a theoretical study

Gas-phase decomposition of formic acid results in final products CO + H2O and CO2 + H2. Experimentally, the CO/CO2 ratio tends to be large, in contradiction with mechanism studies, which show almost equal activation energies for dehydration and decarboxylation. In this work, the influence of H2 on the decomposition mechanism of HCOOH was explored using ab initio calculations at the CCSD(T)/6-311++G**//MP2/6-311++G** level. It was found that, in the presence of H2, the reaction channels leading to CO + H2O are more than those leading to CO2 + H2. With competitive energy, H2 addition to HCOOH can reduce the latter into HCHO, which then dissociates into CO + H2 catalyzed by H2O. Compared to trans-HCOOH, cis-HCOOH and cis-C(OH)2, conformers required for decarboxylation, are less populated due to interactions with H2.     

Sulfur and selenium ylide bond enthalpies

The bond dissociation enthalpies (BDEs) of sulfur and selenium ylides have been estimated by applying MP2/6-311++G(3df,2p)//MP2/6-31G(d,p), G3, and other computational methods. Computed sulfoxide bond enthalpies were compared to experimental results to ensure the reliability of the computational methods before extending to related compounds. The examined ylides include the following: sulfoxides, sulfilimines, S,C-sulfonium ylides, and selenoxides. Selenoxides have BDEs about 10 kcal/mol smaller than the corresponding sulfoxides. N-H sulfilimines and CH2-S,C-sulfonium ylides have low BDEs, unless the sulfilimine or S,C-sulfonium ylide is stabilized by an electronegative substituent on N or C, respectively. Incorporation of the S or Se into a thiophene or selenophene-type ring lowers the BDE for the ylide.     

Influence of the water molecules (n?=?1–6) on the interaction between Li+, Na+, K+ cations and indole molecule as tryptophan amino acid residue

Influence of the addition of water molecules (n = 1–6) on the interaction energy between Li⁺, Na⁺, K⁺ cations and indole molecule as tryptophan amino acid residue is considered at MP2(FULL)/6-311++G(d,p)//B3LYP/6-311++G(d,p) levels of theory. The calculations suggest that the size of cation and the number of water molecules are two important factors that affect the interaction energy between the hydrated metal cation and indole molecule. The strength of cation–π interactions get substantially reduced when the metal ion is solvated or the size of metal cation increases. Quantum theory of atoms in molecules analysis of cation–π interaction indicates that there is a correlation between the electron density (ρ(r)) in the cage critical points generated upon complexation and the distance between metal cation and centroid of phenyl ring in indole molecule.     

Exploring the size dependence of cyclic and acyclic pi-systems on cation-pi binding

MP2(FULL)/6-311++G** calculations are performed on the cation-pi complexes of Li+ and Mg2+ with the pi-face of linear (ethylene, butadiene, hexatriene, and octatetraene) and cyclic (benzene, naphthalene, anthracene, phenanthrene and naphthacene) unsaturated hydrocarbons. The interaction energy is found to increase systematically as the size of the pi-system increases. The higher interaction energy is in good correlation with the extent of charge transfer. The increase in the interaction energy is more dramatic in the case of acyclic systems. The computations reveal that larger pi-systems tend to have higher complexation energy with the metal ions, which will have important implications in our understanding of the structural and functional aspects of metal binding.     

Theoretical studies on the reaction of pentafulvenone with water in gas phase and aqueous solvent

In gas phase, the hydrations of pentafulvenone to generate three types of cyclopentadienyl carboxylic acids are studied theoretically at the MP2/6-311+G**//B3LYP/6-311+G** level. A water molecule attacking the C=O double bond of pentafulvenone can yield cyclopentadienyl carboxylic acids via the formation of fulvenediols, and attacking the C=C double bond of pentafulvenone can directly yield cyclopentadienyl carboxylic acid. The barriers of rate-determining transition states are 42.2 and 30.4 kcal mol⁻¹, respectively. The barriers of rate-determining transition states for two water molecules system are 20.2 and 19.6 kcal mol⁻¹, respectively. The products can isomerize to each other. In aqueous solvent, the hydrations of pentafulvenone are investigated using PCM-UAHF model at the MP2 (PCM)/6-311+G**// B3LYP (PCM)/6-311+G** and MP2 (PCM)/6-311+G**// B3LYP/6-311+G** levels. The barriers of all rate-determining transition states are decreased. The added water molecule acts as catalyst in both gas phase and aqueous solvent. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

A theoretical study of the interactions of NF(3) with neutral ambidentate electron donor and acceptor molecules

A theoretical study of the complexes (dimers and trimers) formed between nitrogen trifluoride (NF(3)) and the ambidentate electron donor/acceptor systems HF, FCl, HCN, and HNC has been carried out using DFT [M05-2x/6-311++G(d,p)] and ab initio methods [(MP2/6-311++G(d,p) and MP2/aug-cc-pVTZ)]. Due to its structure, the NF(3) molecule can interact with both electron acceptors and electron donors through its N and F atoms. Thus, five minimum energy structures have been located for the dimers and four minima structures have been studied for the trimer complexes. New σ-hole bonding complexes have been located.     

An ab initio study of intermolecular interactions of nitromethane dimer and nitromethane trimer

Different geometries of nitromethane dimer and nitromethane trimer have been fully optimized employing the density functional theory B3LYP method and the 6-31++G** basis set. Three-body interaction energy has been obtained with the ab initio supermolecular approach at the levels of MP2/6-31++G**//B3LYP/6-31++G** and MP2/aug-cc-pVDZ//B3LYP/6-31++G**. The internal rotation of methyl group induced by intermolecular interaction has been observed theoretically. For the optimized structures of nitromethane dimer, the strength of C--H...O--N H-bond ranges from -9.0 to -12.4 kJ mol(-1) at the MP2/aug-cc-pVDZ//B3LYP/6-31++G** level, and the B3LYP method underestimates the interaction strength compared with the MP2 method, while MP2/6-31++G**//B3LYP/6-31++G** calculated DeltaE(C) is within 2.5 kJ mol(-1) of the corresponding value at the MP4(SDTQ)/6-31G**//B3LYP/6-31++G** level. The analytic atom-atom intermolecular potential has been successfully regressed by using the MP2/6-31++G**//B3LYP/6-31++G** calculated interaction energies of nitromethane dimer. For the optimized structures of nitromethane trimer the three-body interaction energies occupy small percentage of corresponding total binding energies, but become important for the compressed nitromethane explosive. In addition, it has been discovered that the three-body interaction energy in the cyclic nitromethane trimer is more and more negative as intermolecular distances decrease from 2.2 to 1.7 A.     

Theoretical study of light-metal tetrahydroborates

Ab initio calculations of the structural, energetic, vibrational, and magnetic characteristics of the lowest-lying structures for isolated molecules and ions of light-metal tetrahydroborates (Li, Na, Be, Mg, and Al) have been performed by the perturbation theory (MP2), quadratic configuration interaction (QCISD(T)), coupled cluster (CCSD(T)), and density functional theory (B3LYP) methods using the 6-31G*, 6-31G**, 6-311+G**, and 6-311++G** basis sets. The trends in the behavior of the structural parameters, the energies of different decomposition pathways, barriers to internal rotation of BH₄ groups, normal mode frequencies, magnetic shielding constants, and spin density distribution (in radicals) have been analyzed in various related series of these compounds. The results obtained by ab initio methods and at the DFT level are compared. The economical approximation B3LYP/6-311++G**//B3LYP/6-311+G** adequately reproduces the results obtained at the higher level of theory CCSD(T)/6-311++G*s*//MP2/6-31G* even though it requires considerably shorter CPU times and smaller amounts of memory.     

An ab initio quantum chemical study of reaction mechanisms in the C2H2/CH3OH/KOH/DMSO system

An ab initio quantum chemical study (MP2/6-311++G**//B3LYP/6-31+G*) of a number of possible interactions is performed for the gas phase system of acetylene—potassium hydroxide-dimethylsulfoxide(DMSO)—methanol and with regard to the solvent effect within the continuum model. Key structures in the vinylation reaction are shown to be methoxide ion complexes with the alkali metal hydroxide and acetylene molecules. The formation of these complexes results in the activation of the acetylene molecule and an increase in the nucleophilicity of the methoxide ion. In the C₂H₂/CH₃OH/KOH/DMSO reaction system, a proton exchange between the acetylene molecule and the anionic nucleophile ([OH]^- and [CH₃O]^-) is freely performed with the formation of systems with ethynideions, whereas the thermodynamically preferable formation of vinyl alcohol or methyl vinyl ether is determined by a barrier of 20 kcal/mol.     

A computational study on pi and sigma modes of metal ion binding to heteroaromatics (CH)(5-m)X(m) and (CH)(6-m)X(m) (X = N and P): contrasting preferences between nitrogen- and phosphorous-substituted rings

The pi and sigma complexation energy of various heteroaromatic systems which include mono-, di-, and trisubstituted azoles, phospholes, azines and phosphinines with various metal ions, viz. Li(+), Na(+), K(+), Mg(2+), and Ca(2+), was calculated at the post Hartree-Fock MP2 level, MP2(FULL)/6-311+G(2d,2p)//MP2/6-31G. The azoles and azines were found to form stronger sigma complexes than the corresponding pi complexes, whereas the phospholes and phosphinines had higher pi complexation energy with Li(+), Mg(2+), and Ca(2+) while their pi and sigma complexation energies were very comparable with Na(+) and K(+). The strongest pi complex among the five-membered heteroaromatic system was that of pyrrole with all the metals except with Mg(2+), while benzene formed the strongest pi complex among the six-membered heterocyclic systems. The nitrogen heterocyclic system 4H-[1,2,4] triazole and pyridazine formed the strongest sigma complex among the five- and six-membered heteroaromatic systems considered. The complexation energy of the pi and sigma complexes of the azoles and azines was found to decrease with the increase in the heteroatom substitution in the ring, while that of phospholes and phosphinines did not vary significantly. The azoles and azines preferred to form sigma complexes wherein the metal had bidentate linkage, while the phospholes and phosphinines did not show binding mode preference. In the sigma complexes of both azoles and phospholes, the metal binds away form the electron-deficient nitrogen or phosphorus center.