Ab Initio Molecular Orbital Theory Research Articles

Ab initio chemical kinetics for the ClOO + NO reaction: Effects of temperature and pressure on product branching formation

The kinetics and mechanism for the reaction of ClOO with NO have been investigated by ab initio molecular orbital theory calculations based on the CCSD(T)/6-311+G(3df)//PW91PW91∕6-311+G(3df) method, employed to evaluate the energetics for the construction of potential energy surfaces and prediction of reaction rate constants. The results show that the reaction can produce two key low energy products ClNO + (3)O(2) via the direct triplet abstraction path and ClO + NO(2) via the association and decomposition mechanism through long-lived singlet pc-ClOONO and ClONO(2) intermediates. The yield of ClNO + O(2) ((1)△) from any of the singlet intermediates was found to be negligible because of their high barriers and tight transition states. As both key reactions initially occur barrierlessly, their rate constants were evaluated with a canonical variational approach in our transition state theory and Rice-Ramspergen-Kassel-Marcus/master equation calculations. The rate constants for ClNO + (3)O(2) and ClO + NO(2) production from ClOO + NO can be given by 2.66 × 10(-16) T(1.91) exp(341/T) (200-700 K) and 1.48 × 10(-24) T(3.99) exp(1711/T) (200-600 K), respectively, independent of pressure below atmospheric pressure. The predicted total rate constant and the yields of ClNO and NO(2) in the temperature range of 200-700 K at 10-760 Torr pressure are in close agreement with available experimental results.

New Insights into the Mechanism of Amine/Nitroxide Cycling during the Hindered Amine Light Stabilizer Inhibited Oxidative Degradation of Polymers

High-level ab initio molecular orbital theory calculations are used to identify the origin of the remarkably high inhibition stoichiometric factors exhibited by dialkylamine-based radical-trapping antioxidants. We have calculated the free energy barriers and reaction energies at 25, 80, and 260 °C in the gas phase and in aqueous solution for a broad range of reactions that might, potentially, be involved in amine/nitroxide cycling, as well as several novel pathways proposed as part of the present work, including that of N-alkyl hindered amine light stabilizer activation. We find that most of the literature nitroxide regeneration cycles should be discarded on either kinetic or thermodynamic grounds; some are even inconsistent with existing experimental observations. We therefore propose a new mechanistic cycle that relies on abstraction of a β-hydrogen atom from an alkoxyamine (R(1)R(2)NOCHR(3)R(4)). Our results suggest that this cycle is energetically feasible for a range of substrates and provides an explanation for previously misinterpreted or unexplained experimental results. We also explore alternative mechanisms for amine/nitroxide cycling for cases where the alkoxyamines do not possess an abstractable β-hydrogen.

Conformations of the Glycine Tripeptide Analog Ac-Gly-Gly-NHMe: A Computational Study Including Aqueous Solvation Effects.

A computational study of the conformational preferences of the glycine tripeptide analog, Ac-Gly-Gly-NHMe, has been carried out. The molecule is considered in isolation as well as with a continuum model of aqueous solvation. In the absence of solvent, several low-energy conformers are found that exhibit turnlike structures including type I and type II β turns. Upon consideration of aqueous solvation, two conformers, corresponding to the type I and II turn structures are found to be significantly lower in energy than all others. Results from ab initio molecular orbital theory calculations at MP2/aug-cc-pVTZ//MP2/6-311+G(d,p) are compared with those from density functional theory with B3LYP, ωB97X-D, B97-D, and M06-2X as well as several empirical force fields.

Two Possible Photoreaction Pathways on the L-Valine Optical Isomerization

The photoreaction mechanism of L-valine optical isomerization is studied by density functional theory (DFT) and ab initio molecular orbital theory. The geometric parameters of reactant, product, intermediates, and transition states, as well as the reaction energy barriers, on the reaction paths in S-0 and T-1 states are optimized at the B3LYP and MP2 levels using the 6-311++G(d, p) basis set. The equilibrium geometries of the S-1 state of valine are optimized using time dependent density functional theory (TD-DFT) at the B3LYP/6-311++G(d, p) level. From the analysis of each geometric stationary point on the reaction path, the photoreaction mechanism of L-valine optical isomerization is proposed. The reaction proceeds via hydrogen transfer with the help of carbonyl O or amino N atom in the excited state. Furthermore, the effect of solvent on the isomer reaction mechanism is discussed based on the polarizable continuum model (PCM) of self-consistent reaction field theory.

Study of Mechanism Keto-Enol Tautomerism (isomeric reaction) Structure Cyclohexanone by Using Ab initio Molecular Orbital and Density Functional Theory (DFT) Method with NBO Analysis

Initial quantum mechanic studies and density function theory (DFT) in the level of HF/6-311+G**, B3LYP/3-21+G* and B3LYP/6-311+G** on Keto-Enol Tautomerism (isomeric reaction) cyclohexanone structure, that results shows that activation energy for this reaction equal 64.6143(kcal.mol-1) and transition state has the highest energy level equivalent whit -194354.27(kcal.mol-1) that due to breaking of C-H bond and composing of O-H bond. The results of NBO analysis of showed that the bond π are in resonance condition with lone-pair electrons oxygen of and therefore providing enol state and the transition state in these reactions usually is the structure between ketone state and enol state. Bond order and density of electrons aren’t the same in structures enol state, transition state and ketone state. Also tautomerism cyclohexanone structure is a kind of endothermic isomeric reaction.

Kinetic Study and NBO Analysis of the Dehydrogenation Mechanism of Five‐membered Ring Heterocyclic 2,5‐Dihydro‐[furan, thiophene, selenophene

AbstractThe theoretical study of the dehydrogenation of 2,5‐dihydro‐[furan (1), thiophene (2), and selenophene (3)] was carried out using ab initio molecular orbital (MO) and density functional theory (DFT) methods at the B3LYP/6‐311G**//B3LYP/6‐311G** and MP2/6‐311G**//B3LYP/6‐311G** levels of theory. Among the used methods in this study, the obtained results show that B3LYP/6‐311G** method is in good agreement with the available experimental values. Based on the optimized ground state geometries using B3LYP/6‐311G** method, the natural bond orbital (NBO) analysis of donor‐acceptor (bond‐antibond) interactions revealed that the stabilization energies associated with the electronic delocalization from non‐bonding lone‐pair orbitals [LP(e)X3] to δ*C(1)H(2) antibonding orbital, decrease from compounds 1 to 3. The LP(e)X3→δ*C(1)H(2) resonance energies for compounds 1–3 are 23.37, 16.05 and 12.46 kJ/mol, respectively. Also, the LP(e)X3→δ*C(1)H(2) delocalizations could fairly explain the decrease of occupancies of LP(e)X3 non‐bonding orbitals in ring of compounds 1–3 (3>2>1). The electronic delocalization from LP(e)X3 non‐bonding orbitals to δ*C(1)H(2) antibonding orbital increases the ground state structure stability, Therefore, the decrease of LP(e)X3→δ*C(1)H(2) delocalizations could fairly explain the kinetic of the dehydrogenation reactions of compounds 1–3 (k1>k2>k3). Also, the donor‐acceptor interactions, as obtained from NBO analysis, revealed that the (C(4)C(7)→δ*C(1)H(2) resonance energies decrease from compounds 1 to 3. Further, the results showed that the energy gaps between (C(4)C(7) bonding and δ*C(1)H(2) antibonding orbitals decrease from compounds 1 to 3. The results suggest also that in compounds 1–3, the hydrogen eliminations are controlled by LP(e)→δ* resonance energies. Analysis of bond order, natural bond orbital charges, bond indexes, synchronicity parameters, and IRC calculations indicate that these reactions are occurring through a concerted and synchronous six‐membered cyclic transition state type of mechanism.

Role of CH–π interaction energy in self-assembled gear-shaped amphiphile molecules: correlated ab initio molecular orbital and density functional theory study

Ab initio molecular orbital and density functional theory calculations with inclusion of dispersion interaction effect are employed to reveal the characteristic features of intermolecular interactions for the molecular capsule (16) consisting of six gear-shaped amphiphile molecules (1) discovered by Hiraoka et al. (J Am Chem Soc 130:14368–14369, 2008). The contributions of CH–π and π–π type dispersion energies are found to be indispensable for the formation of hexameric capsule 16 by the analysis of decomposed interaction energies between fragmented-model species in the 1 molecule. We have also calculated the hexameric capsule (26) from demethylated 1 molecule (2). Such subtle structural difference induces the different characters of intermolecular interactions, in which the stabilization energy of hexameric 26 capsule is about 40 kcal/mol smaller than that of the original 16 capsule, due to the lack of three methyl groups for the CH–π interactions in 2 molecules.

Theory‐Guided Experiments on the Mechanistic Elucidation of the Reduction of Dinuclear Zinc, Manganese, and Cadmium Complexes

Since the recognition of the first Zn Zn bond in the dinuclear sandwich decamethyldizincocene [(h-C5Me5)Zn Zn(h-C5Me5)], [1] the chemistry of Zn Zn-bonded species has grown so rapidly that many complexes of the type LZn ZnL have been characterized and studied. Regardless of the denticity of the supporting ligands L, they all coordinate to Zn in a terminally chelating mode. However, formation of these dinuclear compounds has not been mechanistically examined. We recently described the characterization of dinuclear Zn Zn-bonded species [{k-Me2Si(NDipp)2}Zn Zn{k-Me2Si(NDipp)2}] 2 (2) (Dipp= 2,6-iPr2C6H3) from KC8 reduction of dinuclear zinc complex [Zn2(m-k -Me2Si(NDipp)2)2] (1), whereby the coordination mode of the diamido ligands dramatically changes from bridging to chelating. We thus became interested in the structural preference and the formation mechanism of Zn Zn-bonded complexes. Elaborate calculations were performed to understand the reduction of 1, and a plausible mechanism was then proposed (Scheme 1). On two-electron reduction of 1, two intermediates, Ia and Ib, are generated, and the energy difference between them is only 0.3 kcalmol . The Zn Zn-bonded mixed-valent intermediate Ia is produced by one-electron reduction of 1, and subsequently undergoes a dramatic structural rearrangement to give Ib, in which one threecoordinate and one one-coordinate Zn atoms are proposed. The exact valence of the Zn atoms in Ib is still not clear. Although the application of quantum chemical methods (ab initio molecular orbital and density functional theory) to elucidate reaction mechanisms has been very successful, most of the time it is difficult to prove the theoretically developed reaction mechanisms by experiments. This is indeed the case for the transformation from 1 to 2. Attempts to probe both intermediates Ia and Ib failed. To this end, we turned our attention from zinc to manganese and cadmium, because they not only show structural similarity in the reported M M-bonded dinuclear complexes [(k-Nacnac)M M(k-Nacnac)] (M=Zn, Mn; Nacnac= HC[C(Me)NDipp]2) and [Ar’M MAr’] (M=Zn, Cd; Ar’= 2,6-(2,6-iPr2C6H3)2C6H3), but also feature an identical M M s-bonding scheme. Herein we report structural transformations on reduction of dinuclear manganese and cadmium complexes [Mn2{k -Me2Si(NDipp)2}2] (3) and [Cd2{mk-Me2Si(NDipp)2}2] (4). Characterization of the products supports the computed mechanism shown in Scheme 1. As shown in Scheme 2, reactions of the dilithiated diamido ligand and 1 equiv of anhydrous MnCl2 and CdCl2 in diethyl ether and THF, respectively, yielded the corresponding dimeric compounds 3 and 4 in good yields. The dinuclear nature of 3 and 4 was deciphered by single-crystal X-ray crystallography, and their molecular structures are provided in Figures S1 and S2 of the Supporting Information. Complex 3 is essentially composed of two MnN2Si fourmembered rings, which are brought together by two Mn N bonds, and consequently exhibit a boat conformation with two manganese atoms at the stern and two Si atoms at the bow. Each Mn atom is embraced by three nitrogen atoms and adopts a distorted T-shaped geometry. The central Mn2N2 Scheme 1. Calculated mechanism of transformation of 1 into 2.

Solvent distributions, solvent orientations and specific hydration regions around 1-adamantyl chloride and adamantane in aqueous solution

Ab initio molecular orbital theory and Monte Carlo method with molecular mechanics are used to calculate the hydration numbers and solvent configurations around 1-adamantyl chloride in 324 water molecules. In the highest probability region for distribution of water molecules, a water molecule forms two hydrogen bonds with Cl and the adamantane skeleton; in the second highest probability region, a water molecule is located at the rear side. The CH⋯O interaction in the rear side is enhanced by the Cl substitution. The water orientation in the rear side of 1-adamantyl chloride is different from that around a hydrophobic adamantane molecule.

The ab initio study and NBO analysis of the solvent dielectric constant effects and the implicit water molecules on the structural stability of the 1-(6-chloroquinoxalin-2-yl)-2-(4-(trifluoromethyl)-2,6-dinitrophenyl) hydrazine

Ab initio molecular orbital (MO) and density functional theory (DFT) methods were used to analyze the structure and the relative stability of 1-(6-chloroquinoxalin-2-yl)-2-(4-(trifluoromethyl)-2,6-dinitrophenyl) hydrazine in gas phase and the different solvent media. The effects of solvent dielectric constant and the implicit water molecules were investigated on the structural stability and intramolecular interactions. All used methods revealed that by the increase of solvent dielectric constant, the relative stability of the considered compound increase. Hence, the most stable structure is perceived in aqueous solution. Furthermore, natural bond orbital (NBO) analysis demonstrated that in the presence of implicit water molecules, the lone pair electrons of nitrogen have the most contribution in the resonance interactions of the aromatic rings and their stability. These facts may be the probable reasons behind the structural stability of the considered structure in the water solution based on energetic data and NBO analysis at the microscopic level.

Dipole preserving and polarization consistent charges

A method for estimating dipole preserving and polarization consistent (DPPC) charges is described, which reproduces exactly the molecular dipole moment as well as the local, atomic hybridization dipoles determined from the corresponding wave function and can yield accurate molecular polarization. The method is based on a model described by Thole and van Duijnen and a new feature is introduced to treat molecular polarization. Thus, the DPPC method offers a convenient procedure to describe molecular polarization in applications using semiempirical models and ab initio molecular orbital theory with relatively small basis functions such as 6-31+G(d,p) or without inclusion of electron correlation; these methods tend to underestimate molecular polarizability. The trends of the DPPC partial atomic charges are found to be in good accord with those of the CM2 model, a class IV charge analysis method that has been used in a variety of applications. The DPPC method is illustrated to mimic the correct molecular polarizability in a water dimer test case and in water-halide ion complexes using the explicit polarization (X-Pol) potential with the Austin model 1 Hamiltonian.

An Ab Initio Investigation of the Chain-Length Dependence of the Addition–Fragmentation Equilibria in RAFT Polymerization

Ab initio molecular orbital theory has been used to study and explain the effects of chain length on the addition–fragmentation equilibrium constant in reversible addition–fragmentation chain transfer (RAFT) polymerization. New data is presented for azobisisobutyronitrile-initiated t-butyl dithiobenzoate-mediated polymerization of methyl methacrylate, and 2-(((ethylthio)carbonothioyl)thio)propanoic acid-mediated polymerization of acrylamide, and compared with published results for a dithiobenzoate-mediated polymerization of styrene and a trithiocarbonate-mediated polymerization of methyl acrylate. The effects of primary and penultimate substituents on the addition–fragmentation equilibrium constants in RAFT polymerization can be very large (up to eight orders and four orders of magnitude respectively) and should be taken into account in kinetic models. Antepenultimate unit effects are relatively small, implying that, for most systems, chain length effects have largely converged by the dimer stage. However, for sterically bulky monomers capable of undergoing anchimeric interactions such as hydrogen bonding, the onset and convergence of these substituent effects is delayed to slightly longer chain lengths. The magnitude and direction of chain-length effects in the addition–fragmentation equilibrium constants varies considerably with the nature of the RAFT agent, the initiating species, the propagating radical, and the solvent. The observed substituent effects arise primarily in the differing stabilities of the attacking radicals, but are further modified by homoanomeric effects and, where possible, hydrogen-bonding interactions.

Ab Initio Chemical Kinetics for SiH3 Reactions with SixH2x+2 (x = 1−4)

Gas-phase kinetics and mechanisms of SiH(3) reactions with SiH(4), Si(2)H(6), Si(3)H(8), and Si(4)H(10), processes of relevance to a-Si thin-film deposition, have been investigated by ab initio molecular orbital and transition-state theory (TST) calculations. Geometric parameters of all the species involved in the title reactions were optimized by density functional theory at the B3LYP and BH&HLYP levels with the 6-311++G(3df,2p) basis set. The potential energy surface of each reaction was refined at the CCSD(T)/6-311++G(3df,2p) level of theory. The results show that the most favorable low energy pathways in the SiH(3) reactions with these silanes occur by H abstraction, leading to the formation of SiH(4) + Si(x)H(2x+1) (silanyl) radicals. For both Si(3)H(8) and n-Si(4)H(10) reactions, the lowest energy barrier channels take place by secondary Si-H abstraction, yielding SiH(4) + s-Si(3)H(7) and SiH(4) + s-Si(4)H(9), respectively. In the i-Si(4)H(10) reaction, tertiary Si-H abstraction has the lowest barrier producing SiH(4) + t-Si(4)H(9). In addition, direct SiH(3)-for-X substitution reactions forming Si(2)H(6) + X (X = H or silanyls) can also occur, but with significantly higher reaction barriers. A comparison of the SiH(3) reactions with the analogous CH(3) reactions with alkanes has been made. The rate constants for low-energy product channels have been calculated for the temperature range 300-2500 K by TST with Eckart tunneling corrections. These results, together with predicted heats of formation of various silanyl radicals and Si(4)H(10) isomers, have been tabulated for modeling of a-Si:H film growth by chemical vapor deposition.

X-ray Crystal Structure, Raman Spectroscopy, and Ab Initio Density Functional Theory Calculations on 1,1,3,3-Tetramethylguanidinium Bromide

The salt 1,1,3,3-tetramethylguanidinium bromide, [((CH(3))(2)N)(2)C═NH(2)](+)Br(-) or [tmgH]Br, was found to melt at 135(5) °C, forming what may be referred to as a moderate temperature ionic liquid. The chemistry was studied and compared with the corresponding chloride compound. We present X-ray diffraction and Raman evidence to show that also the bromide salt contains dimeric ion pair "molecules" in the crystalline state and probably also in the liquid state. The structure of [tmgH]Br determined at 120(2) K was found to be monoclinic, space group P2(1)/n, with a = 7.2072(14), b = 13.335(3), c = 9.378(2) Å, β =104.31(3)°, Z = 2, based on 11769 reflections, measured from θ = 2.71-28.00° on a small colorless needle crystal. Raman and IR spectra are presented and assigned. When heated, both the chloride and the bromide salts form vapor phases. The Raman spectra of the vapors are surprisingly alike, showing, for example, a characteristic strong band at 2229 cm(-1). This band was interpreted by some of us to show that the [tmgH]Cl gas phase should consist of monomeric ion pair "molecules" held together by a single N-H(+)···Cl(-) hydrogen bond, the stretching vibration of which should be causing the band, based on ab initio molecular orbital density functional theory type calculations. It is not likely that both the bromide and chloride should have identical spectra. As explanation, the formation of 1,1-dimethylcyanamide gas is proposed, by decomposition of [tmgH]X leaving dimethylammonium halogenide (X = Cl, Br). The Raman spectra of all gas phases were quite identical and fitted the calculated spectrum of dimethylcyanamide. It is concluded that monomeric ion pair "molecules" held together by single N-H(+)···X(-) hydrogen bonds probably do not exist in the vapor phase over the solids at about 200-230 °C.

Thermochemistry of Lewis Adducts of BH3 and Nucleophilic Substitution of Triethylamine on NH3BH3 in Tetrahydrofuran

The thermochemistry of the formation of Lewis base adducts of BH(3) in tetrahydrofuran (THF) solution and the gas phase and the kinetics of substitution on ammonia borane by triethylamine are reported. The dative bond energy of Lewis adducts were predicted using density functional theory at the B3LYP/DZVP2 and B3LYP/6-311+G** levels and correlated ab initio molecular orbital theories, including MP2, G3(MP2), and G3(MP2)B3LYP, and compared with available experimental data and accurate CCSD(T)/CBS theory results. The analysis showed that the G3 methods using either the MP2 or the B3LYP geometries reproduce the benchmark results usually to within ~1 kcal/mol. Energies calculated at the MP2/aug-cc-pVTZ level for geometries optimized at the B3LYP/DZVP2 or B3LYP/6-311+G** levels give dative bond energies 2-4 kcal/mol larger than benchmark values. The enthalpies for forming adducts in THF were determined by calorimetry and compared with the calculated energies for the gas phase reaction: THFBH(3) + L → LBH(3) + THF. The formation of NH(3)BH(3) in THF was observed to yield significantly more heat than gas phase dative bond energies predict, consistent with strong solvation of NH(3)BH(3). Substitution of NEt(3) on NH(3)BH(3) is an equilibrium process in THF solution (K ≈ 0.2 at 25 °C). The reaction obeys a reversible bimolecular kinetic rate law with the Arrhenius parameters: log A = 14.7 ± 1.1 and E(a) = 28.1 ± 1.5 kcal/mol. Simulation of the mechanism using the SM8 continuum solvation model shows the reaction most likely proceeds primarily by a classical S(N)2 mechanism.

ChemInform Abstract: The Energy of N2H2 and Related Compounds

Ab initio molecular orbital theory at the G2 level has been used to study the energy of N2H2 and related compounds. Overall, the agreement between theory and experiment is good. The G2 enthalpy of formation ΔH0f0(N2H2) of 49.6 kcal/mol supports the experimental estimate of ≥46.6 kcal/mol derived by Ruscic and Berkowitz [J. Chem. Phys. 95, xxx4 (1991)] in a recent photoionization study. Predicted dissociation energies are D0(HN■NH)=122.8 kcal/mol, D0(HNNH–H)=43.6 kcal/mol, and D0(H2N■NH–H)=82.1 kcal/mol. The G2 value for the proton affinity (PA) of N2 at 298 K is PA298=118.1 kcal/mol. The G2 ionization potential of N2H3 of 7.54 eV is in agreement with the new value of ≤7.61 eV reported by Ruscic and Berkowitz. The G2 results for the ionization potential of N2H4 and the appearance potential of N+2 from N2H2 are in disagreement with experimental results suggesting that detection of the origins in these cases are thwarted by large geometry changes and significant Franck–Condon effects.

ChemInform Abstract: In Pursuit of Cyclopropanethione: Cyclopropanethione S-Oxide and S,S- Dioxide.

The geometries of cyclopropanethione (la), methylenethiuane (2a), cyclopropanethione S-oxide (lb), methylenethiirane S-oxide (Zb), cyclopropanethione S,S-dioxide (IC), methylenethiirane S,S-dioxide (2c), thioformaldehyde S$-dioxide (sulfene), and the tetramethyl derivatives of lb and 2b, tetramethylcyclopropanethione S-oxide (Id) and 3,3-dimethyl-2-isopropylidenethiirane S-oxide (td), respectively, were optimized at the SCF level by using ab initio molecular orbital theory with a polarized double zeta basis set. The difference in energy for each pair of isomers is as follows: 2a more stable than la by 6.4 kcal/mol; lb more stable than 2b by 8.2 kcal/mol; 2c more stable than IC by 0.2 kcal/mol; and Id more stable than 2d by 0.3 kcal/mol. Fluorodesilylation of 1-(trimethy1silyl)cyclopropanesulfonyl chloride (3) in the presence of 1 -(N,N-diethylamino) 1 -propyne affords 4-(N,N-diethylamino)-3-methyl-2-thiaspiro( 3.21 hex-3-ene 2,2-dioxide (19) in 56% yield by way of IC. Treatment of cyclopropanesulfinyl chloride (7) with triethylamine gives triethylamine hydrochloride in 90% yield along with S- 1 -chlorocyclopropyl cyclopropanethiosulfonate (18), S-cyclopropyl cyclopropanethiosulfonate (9), and cyclopropanesulfonyl chloride (8). The formation of these latter four products is consistent with the intermediacy of lb. Attempts to fluorodesilylate 1-(trimethylsi1yl)cyclopropyl aryl disulfides generating cyclopropanethione la itself led instead to products derived from rearrangement of aryldithio a-cyclopropyl carbanions. A cyclopropanethione, dispiro(2.0.2.l)heptane-7-thione (20), which is calculated to be stable relative to its methylenethiirane tautomer, is proposed.

Ab initio studies of the vibrational spectra of some hydrogen-bonded complexes of fluoroacetylene

Ab initio molecular orbital theory has been used to compute the properties of a number of hydrogen-bonded complexes between fluoroacetylene as proton donor and ammonia, water, hydrogen fluoride, phosphine, hydrogen sulfide, and hydrogen chloride as proton acceptors. The properties considered were the vibrational spectra, the molecular structures, the hydrogen-bond energies, and the electron densities, and one of the aims of the study was to ascertain whether there was any evidence of blue-shifting hydrogen-bond character in the complexes formed. The adducts with NH3, H2O, PH3, and H2S were of the conventional CH···X kind (X = N, O, P, S), with hydrogen-bond energies decreasing in the order NH3 > H2O > PH3≈ H2S. Those formed with HF and HCl showed the presence of three alternative structures; in addition to the CH···F(Cl) complexes, adducts of the F(Cl)H···F and F(Cl)H···π type were also found to be stationary points on the potential energy surfaces, with stabilities in the order F(Cl)H···π > CH···F(Cl) > F(Cl)H···F. The hydrogen-bond energies of the CH···X series correlated with the gas-phase basicities of the proton acceptors; moreover, the CH bond-length changes, the wavenumber shifts, the complex–monomer infrared intensity ratios of the CH stretching modes, and the amounts of charge transferred on complex formation were all found to track with the hydrogen-bond energies. All those properties considered here are consistent with the formation of red-shifting hydrogen bonds, to the exclusion of the blue-shifting alternatives.

Benchmark Calculations of Absolute Reduction Potential of Ferricinium/Ferrocene Couple in Nonaqueous Solutions.

High-level ab initio molecular orbital theory is used to obtain benchmark values for the ferricenium/ferrocene (Fc(+)/Fc) couple, the IUPAC recommended reference electrode for nonaqueous solution. The gas-phase ionization energy of ferrocene is calculated using the high-level composite method, G3(MP2)-RAD, and two higher-level variants of this method. These latter methods incorporate corrections for core correlation and, in the case of the highest level considered, use (RO)CCSD(T)/6-311+G(d,p) in place of (RO)CCSD(T)/6-31G(d) as the base level of theory. All methods provide good agreement with one another and the corresponding experimental values. Solvation energies have been calculated using PCM, CPCM, SMD, and COSMO-RS. Using G3(MP2)-RAD-Full-TZ gas-phase energies and COSMO-RS solvation energies, the absolute redox potentials of the Fc(+)/Fc couple have been calculated as 4.988, 4.927, and 5.043 V in acetonitrile, 1,2-dichloroethane, and dimethylsulfoxide solutions, respectively.

A Quantum‐Chemical Study on Understanding the Dehydrogenation Mechanisms of Metal (Na, K, or Mg) Cation Substitution in Lithium Amide Nanoclusters

AbstractThe hydrogen‐releasing activity of (LiNH2)6–LiH nanoclusters and metal (Na, K, or Mg)‐cation substituted nanoclusters (denoted as (NaNH2)(LiNH2)5, (KNH2)(LiNH2)5, and (MgNH)(LiNH2)5) are studied using ab initio molecular orbital theory. Kinetics results show that the rate‐determining step for the dehydrogenation of the (LiNH2)6–LiH nanocluster is the ammonia liberation from the amide with a high activation energy of 167.0 kJ mol−1 (at B3LYP/6‐31 + G(d,p) level). However, metal (Na, K, Mg)‐cation substitution in amide–hydride nanosystems reduces the activation energies for the rate‐determining step to 156.8, 149.6, and 144.1 kJ mol−1 (at B3LYP/6‐31 + G(d,p) level) for (NaNH2)(LiNH2)5, (KNH2)(LiNH2)5, and (MgNH)(LiNH2)5, respectively. Furthermore, only the −NH2 group bound to the Na/K cation is destabilized after Na/K cation substitution, indicating that the improving effect from Na/K‐cation substitution is due to a short‐range interaction. On the other hand, Mg‐cation substitution affects all –NH2 groups in the nanocluster, resulting in weakened N–H covalent bonding together with stronger ionic interactions between Li and the –NH2 group. The present results shed light on the dehydrogenation mechanisms of metal‐cation substitution in lithium amide–hydride nanoclusters and the application of (MgNH)(LiNH2)5 nanoclusters as promising hydrogen‐storage media.