Natural Bond Orbital Charge Research Articles

Chemically triggered C–ON bond homolysis in alkoxyamines. Part 7. Remote polar effect

In a recent work (Org. Lett. 2012, 14, 358–361), we showed that the activation by benzylation of alkoxyamine 1 (diethyl (1‐(tert‐butyl(1‐(pyridin‐4‐yl)ethoxy)amino)‐2,2‐dimethylpropyl)phosphonate) afforded a surprisingly large C–ON bond homolysis rate constant kd. Taking advantage of the easy preparation of para‐X‐benzyl‐activated alkoxyamines 2 and of the presence of a shielding methylene group between the two aromatic moieties, we investigated the long range (10 bonds between the X group and the C–ON bond) polar effect for X = H, F, OMe, CN, NO2, NMe2, +NHMe2,Br−. It was observed that the effect was weak (4‐fold) and mainly due to the zwiterionic mesomeric forms generated by the presence of group X on the para position, i.e. kd increased for CN and NO2 and decreased for OMe, NMe2 and +NMe2H,Br−. DFT calculations at the B3LYP/6‐31G(d,p) level were performed to determine orbital interactions (natural bond orbital (NBO) analysis), Mulliken and NBO charges which support the reactivity described. Copyright © 2014 John Wiley & Sons, Ltd.

A Catalyst with Two‐Coordinate Nickel: Theoretical and Catalytic Studies

AbstractThe bisadduct (cAAC)2NiIICl2 [1; cAAC = cyclic (alkyl)(amino)carbene] was directly synthesized by treating cAAC with NiCl2. Compound 1 was reduced to (cAAC)2Ni0 (2) by using lithium diisopropylamide or KC8. Crystals of 2 were stable under an inert gas for several months and decomposed upon heating above 165 °C. On the basis of the calculated natural bond orbital charge values of the nickel atom in 2, the oxidation state of nickel was determined to be between NiI and Ni0 (+0.34). Theoretical calculations suggested a closed‐shell singlet electronic configuration of 2 with little biradical character. Ab initio multiconfigurational C(R)ASSCF/CASPT2 calculations predicted a closed‐shell singlet electronic configuration (Ni0), whereas excited spin states possessed NiI character with unpaired electrons on neighboring carbon atoms. The catalytic activity of complex 2 was investigated for the homocoupling of various unactivated aryl chlorides/fluorides. The biaryls were obtained in good yields at moderate temperature. Theoretical studies showed that an intermediate containing NiIII was more favored than one with NiIV.

Calculations of Electronic Structure and Nonlinear Optical Parameters of 4-Methoxybenzaldehyde-N-Methyl-4-Stilbazolium Tosylate*

The optimized molecular geometry, vibrational frequencies, and gauge including atomic orbital (GIAO) 1H and 13C chemical shifts of 4 -methoxybenzaldehyde-N-methyl-4-stilbazolium tosylate (MBST) were calculated using density functional theory (DFT/B3LYP) and the Hartree–Fock (HF) method with the 6-311++G(d,p) basis set. Nonlinear optical (NLO) parameters [mean polarizability ( ), the anisotropy of the polarizability (Δα), and the mean firstorder hyperpolarizability ( )] of the title compound were investigated theoretically. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies of the MBST compound were calculated with the same methods and basis set. Molecular hardness (η) and electronegativity (χ) were obtained by using the HOMO and LUMO molecular orbital energies. Electronic transitions of the compound were calculated with the time-dependent DFT/B3LYP (TD-DFT/B3LYP) and HF methods. Mulliken, atomic polar tensor (APT), and natural bond orbital (NBO) charges were calculated and compared with each other.

Insight into the acidic behavior of oxazolidin-2-one, its thione and selone analogs through computational techniques

Deprotonation thermochemistry of Oxazolidin-2-one (OXA), Oxazolidine-2-thione (OXA-S), and Oxazolidine-2-selone (OXA-Se) has been studied in order to find the most acidic site and relative acidities of these heterocyclics at various sites. The deprotonation enthalpies at MP2/6-311++G**//MP2/6-31+G* and B3LYP/6-31+G* levels, while the free energies for deprotonation process and pKa values at B3LYP/6-31+G* level both in gas and aqueous phase (using PCM continuum model) of the anions of the three heterocyclics have been computed at 298 K. Calculated aqueous phase pKa values of OXA vary by ~6–7 units from the experimental aqueous phase pKa values of OXA and its derivatives. The deprotonation at the nitrogen is favored in OXA over the carbon atoms in contrast to the OXA-S and OXA-Se where in the deprotonation at the carbon attached to the nitrogen is most preferred. Deprotonation at this carbon induces an important C–O bond rupture in OXA-S and OXA-Se promoting an energetically favored ring-opening process. The finding offers a rare case when C–H acidity is able to dominate over the N–H acidity. In order to explain the relative stabilities, relative acidities and deprotonation enthalpies various characteristics of these molecules as well as their anions such as molecular electrostatic potential surface (MEP), frontier molecular orbital (FMO) features, chemical hardness, softness have been governed. The three dimensional MEP maps and HOMO–LUMO orbitals encompassing these molecules yield a reliable relative stability and reactivity (in terms of acidity) map displaying the most probable regions for deprotonation. The differential distribution of the electrostatic potential over the neutral and anionic species of OXA, OXA-S, and OXA-Se molecules is authentically reflected by HOMO–LUMO orbitals and NBO charge distribution analysis. The lone pair occupancies, second order delocalization energies for orbital interactions and the distribution of atomic charges over the entire molecular framework as obtained from natural bond orbital (NBO) analysis are found to faithfully replicate the predictions from the MEP maps and HOMO–LUMO band gaps in respect of explaining the relative stabilities and acidities in most of the cases. Good linear correlations have been obtained between HOMO–LUMO gap and pKa values in the aqueous phase for OXA and OXA-S molecules.

Investigation of Carbon Monoxide Adsorption on Cationic Gold- Palladium Clusters

Density functional calculations have been performed for the carbon monoxide molecule adsorption on Au n Pd+ m (n+m ≤ 6) clusters. In the process of CO adsorption, small Au clusters and Pd clusters tend to be an Au atom and three Pd atoms adsorption, respectively. For the mixed Au-Pd clusters, an Au atom, a Pd atom, two atoms consisted of an Au atom and a Pd atom, two Pd atoms, and three Pd atoms adsorption structures are displayed. The highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps and natural bond orbital charge population are calculated. Moreover, CO adsorption energy, CO stretching frequency, and CO bond length (upon adsorption) are also analysed in detail. The results predict that the adsorption strength of Au clusters with CO and the C-O vibration strength is enhanced and reduced after doping of Pd in the Au n Pd m CO+ complexes, respectively

AuSn20 Eutectic Electrodeposition through Alternative Complexing of Pyrophosphoric Acid: Insights from Electrochemical and DFT Methods

Eutectic AuSn20 solder is an important material for electronic packaging technology due to its superior mechanical and thermal conductive properties. In this work, AuSn20 alloy films are prepared via the electrodeposition method for the first time. The electrodeposition is cost-effective with improved control over the alloy content when compared to traditional powdered metallurgy methods. Pyrophosphoric acid was found to be an effective complexing agent to minimize the difference of the deposition potentials between Au and Sn, making the codeposition of AuSn alloys possible. Importantly, electrochemical characterization was combined with density functional theory (DFT) calculations to provide insight into the mechanism of the alloy codeposition when pyrophosphoric acid was used as the complexing agent. In particular, natural bond orbital (NBO) charge distribution and the lowest unoccupied molecular orbital (LUMO) characteristics of [P2O7]4–-Sn(II) and [P2O7]4–-Au(I) complexes are calculated, suggesting th...

A theoretical investigation on the conformation and the interaction of CHF2OCF2CHF2 (desflurane II) with one water molecule

The conformation and the interaction of CHF₂OCF₂CHF₂ (desflurane II) with one water molecule is investigated theoretically using the ab initio MP2/aug-cc-pvdz and DFT-based M062X/6-311++G(d,p) methods. The calculations include the optimized geometries, the harmonic frequencies of relevant vibrational modes along with a natural bond orbital (NBO) analysis including the NBO charges, the hybridization of the C atom and the intra- and intermolecular hyperconjugation energies. In the two most stable conformers, the CH bond of the F2HCO- group occupies the gauche position. The hyperconjugation energies are about the same for both conformers and the conformational preference depends on the interaction between the non-bonded F and H atoms. The deprotonation enthalpies of the CH bonds are about the same for both conformers, the proton affinity of the less stable conformer being 3 kcal mol−1 higher. Both conformers of desflurane II interact with water forming cyclic complexes characterized by CH…O and OH…F hydrogen bonds. The binding energies are moderate, ranging from −2.4 to −3.2 kcal mol−1 at the MP2 level. The origin of the blue shifts of the ν(CH) vibrations is analyzed. In three of the complexes, the water molecule acts as an electron donor. Interestingly, in these cases a charge transfer is also directed to the non bonded OH group of the water molecule. This effect seems to be a property of polyfluorinated ethers.

Enhancement of second-order nonlinear optical response in boron nitride nanocone: Li-doped effect

The unusual properties of Li-doped boron nitride nanomaterials have been paid further attention due to their wide applications in many promising fields. Here, density functional theory (DFT) calculations have been carried out to investigate the second-order nonlinear optical (NLO) properties of boron nitride nanocone (BNNC) and its Li-doped BNNC derivatives. The natural bond orbital charge, electron location function, localized orbital locator and frontier molecular orbital analysis offer further insights into the electron density of the Li-doped BNNC derivatives. The electron density is effectively bounded by the Li atom and its neighboring B atoms. The Li-doped BNNC molecules exhibit large static first hyperpolarizabilities (βtot) up to 1.19×103a.u. for Li@2N-BNNC, 5.05×103a.u. for Li@2B-BNNC, and 1.08×103a.u. for Li@BN-BNNC, which are significantly larger than that of the non-doped BNNC (1.07×102a.u.). The further investigations show that there are clearly dependencies of the first hyperpolarizabilities on the transition energies and oscillator strengths. Moreover, time-dependent DFT results show that the charge transfer from BNNC to Li atom becomes more pronounced as doping the Li atom to BNNC. It is also found that the frequency-dependent effect on the first hyperpolarizabilities is weak, which may be beneficial to experimentalists for designing Li-doped BNNC molecules with large NLO responses.

Si‐Doped B12N12 Nanocage as an Adsorbent for Dissociation of N2O to N2 Molecule

ABSTRACTAdsorption of N2O molecule by using density functional theory calculations at the B3LYP/6–31G* level onto pristine and Si‐doped B12N12 nanocage in terms of energetic, geometric, and electronic properties was investigated. The results of calculations showed that the N2O molecule is physically adsorbed on the pristine and Si‐doped B12N12 (SiN) models, releasing energies in the range of –1.13 to –2.02 kcal mol−1. It was found that the electronic properties of the models have not changed significantly upon the N2O adsorption. On the other hand, the adsorption energy of N2O on the Si‐doped B12N12 (SiB model) was about –67.20 kcal mol−1and the natural bond orbital charge of 0.58|e| is transferred from the nanocage to the N2O molecule. In the configuration, the O atom of N2O molecule is bonded to the Si atom of the nanocage, so that an N2 molecule escapes from the wall of the nanocage. The results showed that the SiB model can be an adsorbent for dissociation of the N2O molecule.

Isomerisation Reactions of α-Methyl Allyl [Acetate, Trifluoroacetate]: Theoretical Study

The [3,3]-sigmatropic rearrangement reactions of α-methyl allyl acetate (1), and α-methyl allyl trifluoroacetate (2) have been studied using ab initio molecular orbital (MO) methods at the HF, B3LYP, B3PW91, and BHandHLYP levels with a 6-31++G** basis set. Among the methods used in this study, the values for activation parameters obtained with the BHandHLYP/6-31++G** method are in good agreement with experimental values. The calculated data demonstrate that in the rearrangement reactions of the compounds studied, the polarisation of the O3–C4 bond is rate determining. Optimised transition states at the BHandHLYP/6-31++G** level were used for calculating the nucleus-independent chemical shift (NICS) and also a natural bond orbital (NBO) analysis was carried out at the same level. Based on the optimised ground state geometries using the BHandHLYP/6-31++G** method, the NBO analysis of donor–acceptor (bonding–antibonding) interactions revealed that the stabilisation energies associated with electronic delocalisation from the σO(3)–C(4) bonding orbital to the π*C(5)=C(6) antibonding orbital, increase from compounds 1 to 2. The σO(3)–C(4) →π*C(5)=C(6) resonance energies for compounds 1 and 2 are 1.59 and 1.62 kcal mol−1, respectively. Also, analysis of the donor–acceptor interactions σO(3)–C(4) →π*C(5)=C(6) suggests that in these rearrangements, the TS structure in compound 2 has more aromatic character than for compound 1. The NBO results revealed that in these rearrangements, activation energies are not controlled by the σO(3)–C(4) →π*C(5)=C(6) resonance energy. NICS results indicate that the aromaticities of the transition states are controlled by the out-of-plane component. The predicted high-pressure-limit rate constants for the rearrangement reactions of compounds 1 and 2 are represented as 6.12 × 1012 exp(–21604/T), and 1.04 × 1013 exp(–17846/T) s−1, respectively. The fall-off pressures for the rearrangement reactions 1 and 2 are found to be 9.56 × 10−5 and 1.09 × 10−3 mmHg, respectively. The fall-off pressures of compounds 1 and 2 in both reactions are in the following order: P1/2(2) > P1/2(1) and the rates follow as: rate(2) > rate(1). Analysis of bond order, NBO charges, bond indices and synchronicity parameters suggest the Claisen rearrangement reactions of 1 and 2 occur through a concerted and slightly asynchronous six-membered cyclic transition state type of mechanism.

Theoretical study of interaction of formamide with kaolinite

The interactions between small molecules and clay minerals have attracted more interest of scientists recently due to the wide use of them in the catalysts and environmental absorbents. In the clays, kaolinite as an important layered aluminosilicate presents a variety of physicochemical properties. In this paper, the clusters of Si6O18H12, Al6O24H30 and Si6Al6O42H42 were constructed for tetrahedral, octahedral layers and isolated kaolinite to investigate the microscopic properties of formamide (FA) on kaolinite layers by Density Function Theory (DFT) methods. All computations were performed at the B3LYP (Becke, three-parameter, Lee–Yang–Parr exchange–correlation functional) level using the 6-31G(d) basis set. The properties, such as the optimal structures, structural parameters, interaction energies, NBO (natural bond orbital) charge distributions, vibrational frequencies, electrostatic potential, were extracted from them. The results show that hydrogen bonds between FA and the surface hydroxyl groups of the octahedral layer or the basal oxygen atoms of the tetrahedral layer in kaolinite are formed. The absolute values of calculated interaction energies on the tetrahedral layer are smaller than those on the octahedral layer and the interaction energies of adsorbed systems are smaller than intercalated one, which indicate that the adsorption between FA and octahedral layer of kaolinite is stronger than that of FA on the tetrahedral layer of kaolinite, and the intercalation is more stable than the adsorption.

The symmetric and asymmetric thiophene-fused benzocarborane: structures and first hyperpolarizabilities

The unusual properties of thiophene-fused benzocarborane have attracted a lot of interest in recent years due to their wide applications in photonics and optoelectronics. In the present work, nine molecules [M, N] (M, N are labeled as the number of thiophene rings on the left and right part, respectively) on the basis of thiophene-fused benzocarborane were considered. The first hyperpolarizability (β tot) values of three synthesized symmetric molecules [1, 1], [2, 2], [3, 3] (M=N, Chem. Eur. J 2012. 18, 11251-11257) and six asymmetric molecules [1, 2], [1, 3], [1, 4], [1, 5], [2, 3], [2, 4] (M≠N) were investigated, β tot values of symmetric molecules show the order: 39 of [1, 1]< 800 of [2, 2]< 903 au of [3, 3], which indicate that β tot value increases with increasing the number of thiophene ring for symmetric molecules. The other order of β tot values can be observed: 39 of [1, 1]< 800 of [1, 2]< 3553 of [1, 3]< 7998 of [1, 4]< 13049 au of [1, 5] and 66 of [2, 2]< 3240 of [2, 3]< 8029 au of [2, 4]. Interestingly, when sum of M and N is constant, larger difference between M and N is, larger β tot value is: 800 au of [2, 2]< 3553 au of [1, 3]; 3240 au of [2, 3] < 7998 au of [1, 4]; 903 au of [3, 3]< 8029 au of [2, 4]< 13049 au of [1, 5]. Significantly, [1, 5] with six thiophene rings has the largest β tot value (13049 au) which is greatly larger than 903 au of [3, 3] with six thiophene rings. Furthermore, the natural bond orbital (NBO) charge populations, the nucleus-independent chemical shift (NICS), the bond length alternation (BLA) of the nine molecules and crucial transition were studied in our work. We hope that the present work will be beneficial for future theoretical and experimental studies on the electro-optical properties of thiophene-fused benzocarborane molecules.

Electrochemical and Theoretical Studies of Adsorption and Corrosion Inhibition of N,N′-Bis(2-hydroxyethoxyacetophenone)-2,2-dimethyl-1,2-propanediimine on Low Carbon Steel (API 5L Grade B) in Acidic Solution

Schiff base N,N′-bis(2-hydroxyethoxyacetophenone)-2,2-dimethyl-1,2-propanediimine (H2(hppnptn)) as corrosion inhibitor for API 5L grade B low carbon steel in hydrochloric acid solutions has been studied using electrochemical techniques and surface techniques. Results showed that the inhibition occurs through adsorption of the inhibitor molecules on the metal surface. Thermodynamic parameters for adsorption and activation processes were determined. Polarization data indicated that this compound acts as mixed-type inhibitors, and the adsorption isotherm basically obeys the Langmuir adsorption isotherm. The calculations of reactivity indices of H2(hppnptn) such as the localization of frontier molecular orbitals, EHOMO, ELUMO, energy gap (ΔE), dipole moment (D), hardness (η), softness (σ), the fractions of electrons transferred (ΔN), electrophilicity index (ω), total energy change (ΔET), and natural bond orbital (NBO) charge distributions together with local reactivity by means of Fukui indices were used to explain the electron transfer mechanism between the H2(hppnptn) molecules and the steel surface.

Strong Hyperconjugative Interactions in Isolated and Water Complexes of Desflurane: A Theoretical Investigation

Ab initio MP2/aug-cc-pvDZ and density functional B3LYP calculations with the 6-311++G(d,p) basis set are performed to investigate the conformation of desflurane (CHF2OCHFCF3), its acidity/basicity and its interaction with one water molecule. The calculations include the optimized geometries, the harmonic frequencies of relevant vibrational modes, the binding energies with water, and a detailed natural bond orbital (NBO) analysis Iincluding the NBO charges, the hybridization of the C atoms and the intra- and intermolecular hyperconjugations. The relative energies of the two most stable conformers are discussed as a function of the total hyperconjugative energies resulting from the interaction of lone pairs of the O and F atoms to the different antibonding orbitals of desflurane. The proton affinity is the same for both conformers but the acidity of the CH bond is larger for the less stable conformer. The binding energies of the complexes of two desflurane conformers with one water molecule range from -2.75 to -3.23 kcal mol(-1). Depending on the structure of the complexes, the CH bonds involved in the interaction are contracted or elongated. The σ*(CH) occupation predominates over the hybridization effect in determining the CH bond length. There is an unexpected charge transfer to the external OH bond of the water molecule. This effect is in good agreement with theoretical data on the complexes between fluorinated dimethyl ethers and water and seems to depend on the number of F atoms implanted on the ether molecule.

Comparative studies on melt processable polyimides derived from 2,3,3′,4′-oxydiphthalic anhydride and 2,3,3′,4′-thioetherdiphthalic anhydride

Two thermoplastic polyimides were controllably synthesized with similar molecular weight from 2,3,3′,4′-oxydiphthalic anhydride (3,4′-ODPA) or 2,3,3′,4′-thioetherdiphthalic anhydride (3,4′-TDPA) with 4,4′-diaminodiphenyl ether (ODA) by a one-step method of m-cresol. The solubility and mechanical properties of these polymers were investigated for comparison. The thermal and melt processable properties of these polymers were examined carefully by differential scanning calorimetry, thermogravimetric analysis (TGA), rheometer and melt index, and so on. It was found that the glass transition temperature ( Tg) of 3,4′-TDPA/ODA was about 15°C lower than that of 3,4′-ODPA/ODA, the melt indices of 3,4′-TDPA/ODA at different temperatures were higher than those of 3,4′-ODPA/ODA, while the minimum melt viscosity of 3,4′-TDPA/ODA was lower than those of 3,4′-ODPA/ODA. Furthermore, TGA results showed that 3,4′-TDPA/ODA had different degradation mechanism in air compared with 3,4′-ODPA/ODA. The long-term thermooxidative stability results showed that 3,4′-TDPA/ODA had better thermal oxidative stability than 3,4′-ODPA/ODA at 410°C for 6 h in air, which could be associated with the results of apparent activation energy ( Ea) of polymers calculated by TGA dynamics. The natural bond orbital charge of ethero atoms (S or O), and energy gap of the corresponding model compounds were also obtained by computational simulation to correlate with polymer properties.

Role of Ti-based catalysts in the dehydrogenation mechanism of magnesium borohydride: A cluster approach

The role of Ti and Ti-based catalysts such as TiCl3 and TiF3 in the dehydrogenation of Mg(BH4)2 has been studied using a cluster approach and density functional theory. The optimized geometry of Mg(BH4)2 clusters mimics the structure it has in the crystalline form, but undergoes significant geometric distortion as well as changes in the natural bond orbital (NBO) charge after the addition of the catalysts. While all the catalysts lower the hydrogen desorption energy, elemental Ti appears to be the best catalyst followed by TiCl3 and TiF3. The lowering of the hydrogen desorption is shown to be due to the weakening of the BH bond which is caused by the interaction of Ti with H and Mg.

Chemically triggered C–ON bond homolysis in alkoxyamines: regioselectivity and chemoselectivity

Recently, we examplified the activation of the C-ON bond homolysis by protonation, alkylation, benzylation, acylation, oxidation and complexation with a Lewis acid of the nitrogen atom of the 1-(pyridin-4-yl)ethyl fragment (Chem. Commun., 2011, 4291 and Org. Lett., 2012, 358) and of the 1-(pyridin-2-yl)ethyl fragment (J. Org. Chem. ASAP Doi:10.1021/jo401674v) of (N-(2-methylpropyl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-N-oxyl) SG1-based alkoxyamines. The quaternization of the 1-(pyridin-3-yl)ethyl fragment by the aforementioned reactions was investigated for the corresponding SG1-based alkoxyamines. In sharp contrast to the quaternization at ortho and para positions of the pyridyl moiety, the effect of the quaternization at the meta position was weak. The effects of quaternization at ortho, meta and para positions were investigated through natural bond orbital and Mulliken charges, HOMO-LUMO interactions in the starting materials and the radical stabilization energy of the released 1-puridylmethyl radicals using DFT calculations with the B3LYP/6-31G(d) and UBMK/6-311+G(3df,2p)//R(O)B3LYP/6-31G(d) methods, respectively.

Looking at the Green Fluorescent Protein (GFP) chromophore from a different perspective: A computational insight

In the present contribution Density Functional Theory (DFT) has been applied to explore molecular dipole moment, frontier molecular orbital (FMO) features, chemical hardness, and the molecular electrostatic potential surface (MEPS) characteristics for optimized molecular geometry of the Green Fluorescent Protein (GFP) chromophore p-hydroxybenzylideneimidazolinone (HBDI) both in its protonated (neutral) and deprotonated (anion) forms. The distribution of atomic charges over the entire molecular framework as obtained from Natural Bond Orbital (NBO) analysis is found to faithfully replicate the predictions from the MEP map in respect of reactivity map of HBDI (neutral and anion) and possible sites for hydrogen bonding interactions etc. The three dimensional MEP map encompassing the entire molecule yields a reliable reactivity map of HBDI molecule also displaying the most probable regions for non-covalent interactions. The differential distribution of the electrostatic potential over the neutral and anionic species of HBDI is authentically reflected on MEP map and NBO charge distribution analysis. Thermodynamic properties such as heat capacity, thermal energy, enthalpy, entropy have been calculated and the correlation of the various thermodynamic functions with temperature has been established for neutral molecule. More importantly, however, the computational approach has been employed to unveil the nonlinear optical (NLO) properties of protonated (neutral) and deprotonated (anion) HBDI. Also in an endeavor to achieve a fuller understanding on this aspect the effect of basis set on the NLO properties of the title molecule has been investigated. Our computations delineate the discernible differences in NLO properties between the neutral and anionic species of HBDI whereby indicating the possibility of development of photoswitchable NLO device.

Experimental and Computational Studies of the Reaction of Carbon Dioxide with Pincer-Supported Nickel and Palladium Hydrides

A series of Ni(II) and Pd(II) hydrides supported by PNP and PCP ligands, including iPr2PNP(CH3)PdH (iPr2PNP(CH3) = N(2-PiPr2-4-MeC6H3)2), iPr2PNP(CH3)NiH, iPr2PNP(F)PdH (iPr2PNP(F) = N(2-PiPr2-4-C6H3F)2), CyPhPNPPdH (CyPhPNP = N(2-P(Cy)(Ph)-4-MeC6H3)2), tBu2PCPPdH (tBu2PCP = 2,6-C6H3(CH2PtBu2)2), tBu2PCPNiH, Cy2PCPPdH (Cy2PCP = 2,6-C6H3(CH2PCy2)2), and Cy2PCPNiH, were prepared using literature methods. In addition, the new Ni and Pd hydrides Cy2PSiPMH (M = Ni, Pd; Cy2PSiP = Si(Me)(2-PCy2-C6H4)2) supported by PSiP ligands were synthesized. The analogous metal hydride complexes supported by the Ph2PSiP ligand (Ph2PSiP = Si(Me)(2-PPh2-C6H4)2) could not be prepared. Instead, the Ni(0) and Pd(0) η2-silane complexes Ph2PSiHPM(PPh3) (M = Ni, Pd; Ph2PSiHP = (H)Si(Me)(2-PPh2-C6H4)2), which have been proposed to be in equilibrium with Ph2PSiPMH (M = Ni, Pd) and PPh3, were prepared. Facile carbon dioxide insertion into the metal–hydride bond to form the metal formate complexes tBu2PCPM-OC(O)H (M = Ni, Pd) or Cy2PCPM-OC(O)H (M = Ni, Pd) was observed for PCP-supported species, and a similar reaction was observed for Cy2PSiP-supported hydrides to form Cy2PSiPM-OC(O)H (M = Ni, Pd). No reaction with carbon dioxide was observed for any complexes supported by PNP ligands. The η2-silane complex Ph2PSiHPPd(PPh3) reacted rapidly with carbon dioxide to give Ph2PSiPPd-OC(O)H and PPh3, while the corresponding Ni complex Ph2PSiHPNi(PPh3) did not react with carbon dioxide. DFT calculations indicate that carbon dioxide insertion is thermodynamically favorable for PSiP- and PCP-supported hydrides because the strong trans influence of the anionic carbon donor destabilizes the metal–hydride bond. In contrast, carbon dioxide insertion is thermodynamically unfavorable for the PNP-supported species. In the case of the η2-silane complexes, carbon dioxide insertion is thermodynamically favorable for Pd and unfavorable for Ni. This is because the equilibrium between the metal hydride and PPh3 and the η2-silane complex more strongly favors the metal hydride for Pd than for Ni. In the cases of metal hydrides, the thermodynamic favorability of carbon dioxide insertion can be predicted from the natural bond orbital charge on the hydride. The pathway for carbon dioxide insertion into the metal hydride is concerted and features a four-centered transition state. The energy of the transition state for carbon dioxide insertion decreases as the trans influence of the anionic donor of the pincer ligand increases.

A first-principles study of hydrogen interaction and saturation on ScAl3

Complex metal hydrides are promising hydrogen storage material for mobile applications. The presence of scandium catalysts in complex metal hydrides is found to improve significantly the kinetics of H2 adsorption and desorption. The exact role of these additives is not completely understood. NMR studies suggests the presence of distorted ScAl3 phase which is attributed to the enhancement of reversible uptake and release rates of H2. In this study, we have investigated the hydrogen interaction and saturation on stable ScAl3 cluster at various sites using the density functional theory by employing unrestricted B3LYP/6-311++G(d,p) and the GGA-PW91/DNP methods. From natural bond orbital charge analysis and electrostatic potential superimposed on electron density plots, ScAl3 is found to be more polarized and Sc atom in ScAl3 cluster has higher negative charge and electron density over Al atoms. H2 undergoes dissociative chemisorption on the Sc atop site in ScAl3 resulting in Sc–H–Al bridge structure. The hydrogen saturation is studied by sequential H2 adsorption on ScAl3 cluster. In the Sc site, the first H2 molecule undergoes dissociative chemisorption and further addition results in H2 physisorption with the maximum loading of 6 H2 molecules resulting in stable ScAl3H12 structure. To obtain mechanistic insights into the hydrogenation process, the transition state and activation energy of H2 dissociation on the ScAl3 system has been determined.