First author: Boulet, P, Photochemistry of the CpNiNO complex. A theoretical study using density functional theory,INORGANIC CHEMISTRY, 40, 7032, (2001)
Abstract: The photochemistry of the CpNiNO complex has been investigated using density functional theory. The whole potential energy curve along the NiNO angle coordinate is presented for the first time with both ground and metastable states, and transition states connecting the minima. The excited states of the GS, MSI, and MSII species have been calculated using time-dependent density functional theory. Furthermore, the structure of the excited states pertaining to the photochemistry of CpNiNO has been optimized. From these results it is shown that the backward GS <– MSII <– MSI reaction is more efficient than the forward GS –> MSII –> MSI scheme.
First author: Michalak, A, DFT studies on the copolymerization of alpha-olefins with polar monomers: Ethylene-methyl acrylate copolymerization catalyzed by a Pd-based diimine catalyst, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 123, 12266, (2001)
Abstract: Gradient-corrected density functional theory has been used to study the elementary reactions for the copolymerization of ethylene with methyl acrylate catalyzed by Pd-based diimine catalysts, modeled by the generic complex N boolean ANDN-Pd(n-C3H7)(+), with N boolean ANDN = -NHCHCHNH-. The steric effects in the real systems are discussed on the basis of the calculations for the catalyst with NAN = -NArCRCRNAr-, R = CH3, and Ar = C6H3(i-Pr-2) and the previous calculations on ethylene/propylene polymerization. Considerations have been given to the different possible acrylate complexes, as well as the transition states and the products (agostic complexes and the alternative chelates) for two acrylate insertion paths (1,2 and 2,1). The chelate-opening reactions have also been studied. The results revealed a strong electronic preference for the 2, 1-insertion paths, with a barrier that is 4.5 kcal/mol lower than any other studied insertion pathway. In the real systems the 2,1-insertion of acrylate is preferred by 0.5 kcal/mol. The 2.1-insertion barrier calculated for the real system of 12.4 kcal/mol is in very good agreement with the experimental value of 12.1 kcal/mol. The six-member chelate is the most stable insertion product with an energy that is 21 kcal/mol lower than the kinetic insertion product. The reactions of the chelate opening by ethylene that start from the lowest energy complexes have the lowest barrier for the four-member ring (23 kcal/mol) and the highest for the six-member structure (30.4 kcal/mol). The high barrier for the opening of the six-member chelate suggests the possibility of a two-step chelate-opening mechanism. The internal barriers for the chelate-opening reactions starting from the higher energy complexes are lower then the one-step reaction that starts from the preferred complex and comparable to those of the ethylene insertion into the Pd-alkyl bond. While the chelate opening by a subsequent acrylate insertion seems to be facile for the generic catalyst, steric effects in the real catalyst are likely to decrease the acrylate pi -complexation energies and increase the insertion barriers to the extent where such a reaction becomes unfeasible.
First author: Grozema, FC, Excited state polarizabilities of conjugated molecules calculated using time dependent density functional theory, JOURNAL OF CHEMICAL PHYSICS, 115, 10014, (2001)
Abstract: In this paper, time-dependent density functional theory (TDDFT) calculations of excited state polarizabilities of conjugated molecules are presented. The increase in polarizability upon excitation was obtained by evaluating the dependence of the excitation energy on an applied static electric field. The excitation energy was found to vary quadratically with the field strength. The excess polarizabilities obtained for singlet excited states are in reasonable agreement with the experimental results for the shorter oligomers, particularly if the experimental uncertainties are considered. For longer oligomers the excess polarizability is considerably overestimated, similar to DFT calculations of ground state polarizabilities. Excess polarizabilities of triplet states were found to be smaller than those for the corresponding singlet state, which agrees with experimental results that are available for triplet polarizabilities. Negative polarizabilities are obtained for the lowest singlet A(g) states of longer oligomers. The polarizability of the lowest B-u and A(g) excited states of the conjugated molecules studied here are determined mainly by the interaction between these two states. Upon application of a static electric field a quadratic Stark effect is observed in which the lower B-u state has a positive excess polarizability and the upper A(g) state exhibits a decrease in polarizability upon excitation. All results are explained in terms of a sum-over-states description for the polarizability.
First author: Bridgeman, AJ, Molecular and electronic structures of six-coordinate W complexes and polyanions containing tri-oxo groups, POLYHEDRON, 20, 3101, (2001)
Abstract: The molecular and electronic structures of [WO3(OL)(3)] (L: CN, H-2, H) complexes and of the [W2O9](6-) and [W4O16](8-) polyanions have been investigated using density-functional methods. Structural and bonding analyses have been performed employing various molecular-orbital and population approaches. Correlations of the trans-influence type between bonds to unshared and to high-coordinate oxygen atoms have been observed for geometrical and electronic parameters, such as W-O distances and orders. The (trans) bridging bonds have been found to be (separately) rather weak, as suggested by their (long) lengths, but the valency and orbital-interaction results have indicated that the overall bonding capacity and strength are similar for all oxygen atoms in each individual polyanion.
First author: Maroulis, G, Dipole, dipole-quadrupole, and dipole-octopole polarizability of adamantane, C10H16, from refractive index measurements, depolarized collision-induced light scattering, conventional ab initio and density functional theory calculations, JOURNAL OF CHEMICAL PHYSICS, 115, 7957, (2001)
Abstract: Refractive index (RI) measurements, depolarized collision-induced light (CILS) scattering and ab initio quantum chemical calculations are used to determine the dipole (alpha), dipole-quadrupole (A), and dipole-octopole polarizability (E) of adamantane, C10H16. For this molecule of symmetry group T-d the three polarizabilities can be represented by a single scalar quantity. From experiment we obtain for the static dipole polarizability at T approximate to 400-500 K 107.5 +/-1.1 e(2)a(0)(2)E(h)(-1), and for the higher polarizabilities \A \ =102.0 +/-7.8 e(2)a(0)(3)E(h)(-1), and \E oe =720 +/- 80 e(2)a(0)(4)E(h)(-1). We have performed conventional ab initio and density functional theory calculations with specifically designed basis sets. A very large [4s3p3d1f/3s2p1d] basis set consisting of 574 basis functions is thought to provide near-Hartree-Fock values for alpha ,A and E: alpha =101.72 e(2)a(0)(2)E(h)(-1), A=-6.5 e(2)a(0)(3)E(h)(-1), and E=-71.0 e(2)a(0)(4)E(h)(-1). Our final theoretical estimates for these properties are alpha =107.5 +/-1.0 e(2)a(0)(2)E(h)(-1), A=-8.0 +/-1.5 e(2)a(0)(3)E(h)(-1), and E=-76.5 +/-5.5 e(2)a(0)(4)E(h)(-1). Very strong electron correlation effects are found for both the first (beta) and second (gamma) hyperpolarizability. Our estimate for beta equivalent to beta (xyz) and the mean <()over bar> are 41.4 +/-5.6 e(3)a(0)(3)E(h)(-2) and (25 +/-2)x10(3) e(4)a(0)(4)E(h)(-3), respectively. For the octopole and hexadecapole moments we propose Omega=-4.4 +/-0.2 ea(0)(3) and Phi=-100.8 +/-5.5 ea(0)(4). The basis sets constructed in this work should provide reliable computational tools for the study of intermolecular interactions of adamantane.
First author: Bickelhaupt, FM, Base-induced 1,4-elimination: Insights from theory and mass spectrometry, MASS SPECTROMETRY REVIEWS, 20, 347, (2001)
Abstract: Experimental and theoretical studies on gas-phase base-induced 1,4-elimination reactions are summarized and discussed. The emphasis is on the synergy that is achieved by combining the complementary data from mass spectrometry and theoretical chemistry. The scope and applications of 1,4-eliminations are discussed and compared with other elementary reactions; e,g., 1,2-elimination and aliphatic (S(N)2) and allylic (S(N)2′) nucleophilic substitution. Furthermore, the syn versus anti stereochemistry of 1,4-elimination reactions and the effect of E versus Z stereochemistry of the substrate are examined. Particular attention is paid to the mechanistic nature of 1,4-elimination, i.e., E2 or E1cb, as well as special features Such as the single-well E2 and E1cb mechanism. Also, new results from density functional theory computations (BP86/TZ2P) are presented.
First author: Deubel, DV, [2+2] versus [3+2] addition of metal oxides across C=C double bonds: Toward an understanding of the surprising chemo- and periselectivity of transition-metal-oxide additions to ketene, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 123, 10085, (2001)
Abstract: The peri-, chemo-, stereo-, and regio selectivity of the addition of the transition-metal oxides OSO4 and LReO3 (L = O-, H3PN, Me, Cp) to ketene were systematically investigated using density-functional methods. While metal-oxide additions to ethylene have recently been reported to follow a [3+2] mechanism only, the calculations reveal a strong influence of the metal on the periselectivity of the ketene addition: OSO4 again prefers a [3+2] pathway across the C=C moiety whereas, for the rhenium oxides LReO3, the [2+2] barriers are lowest. Furthermore, a divergent chemoselectivity arising from the ligand L was found: ReO4- and (H3PN)ReO3 add across the C=O bond while MeReO3 and CpReO3 favor the addition across the C=C moiety. The calculated energy profile for the MeReO3 additions differs from the CpReO3 energy profile by up to 45 kcal/mol due to the stereoelectronic flexibility of the Cp ligand adopting eta (5), eta (3), eta (1) bonding modes. The selectivity of the cycloadditions was rationalized by the analysis of donor-acceptor interactions in the transition states. In contrast, metal-oxide additions to diphenylketene probably follow a different mechanism: We give theoretical evidence for a zwitterionic intermediate that is formed by nucleophilic attack at the carbonyl moiety and undergoes a subsequent cyclization yielding the thermodynamically favored product. This two-step pathway is in agreement with the results of recent experimental work.
First author: Frenking, G, Understanding the nature of the bonding in transition metal complexes: from Dewar’s molecular orbital model to an energy partitioning analysis of the metal-ligand bond, JOURNAL OF ORGANOMETALLIC CHEMISTRY, 635, 9, (2001)
Abstract: The results of an energy partitioning analysis of three classes of transition metal complexes are discussed. They are (i) neutral and charged isoelectronic hexacarbonyls TM(CO)(6)(q) (TMq = Hf2-, Ta-, W, Re+, Os2+, Ir3+); (ii) Group-13 diyl complexes (CO)(4)Fe-ER (E = B, Al, Ga, In, Tl; R = Cp, Ph), Fe(ECH3)(5) and Ni(ECH3)(4); (iii) complexes with cyclic pi -donor ligands Fe(Cp)(2) and Fe(eta (5) -N-5)(2). The results show that Dewar’s molecular orbital model can be recovered and that the orbital interactions can become quantitatively expressed by accurate quantum chemical calculations. However, the energy analysis goes beyond the MO model and gives a much deeper insight into the nature of the metal-ligand bonding. It addresses also the question of ionic versus covalent bonding as well as the relative importance of sigma and pi bonding contributions.
First author: Lein, M, Iron bispentazole Fe(eta(5)-N-5)(2), a theoretically predicted high-energy compound: Structure, bonding analysis, metal-ligand bond strength and a comparison with the isoelectronic ferrocene, CHEMISTRY-A EUROPEAN JOURNAL, 7, 4155, (2001)
Abstract: Quantum-chemical calculations with gradient-corrected (B3LYP) density functional theory have been carried out for iron bispentazole and ferrocene. The calculations predict that Fe(eta (5)- N-5)(2) is a strongly bonded complex which has DM symmetry. The theoretically predicted total bond energy that yields Fe in the D-5 ground state and two pentazole ligands is D-o = 109.0 kcal mol(-1), which is only 29 kcal mol(-1) less than the calculated bond energy of ferrocene (D-o = 138.0 kcal mol(-1) experimental: 158 +/- 2 kcal mol(-1)). The compound Fe(eta (5)-N-5)(2) is 260.5 kcal mol(-1) higher in energy than the experimentally known isomer Fe(N-2)(5), but the bond energy of the latter (D-o = 33.7 kcal mol(-1)) is much less. The energy decomposition analyses of Fe(eta (5)-N-5)(2) and ferrocene show that the two compounds have similar bonding situations. The metal – ligand bonds are roughly half ionic and half covalent. The covalent bonding comes mainly from (e(1g)) eta (5)-N-5(-) –> Fe2+ alpha -donation. The previously suggested MO correlation diagram for ferrocene is nicely recovered by the Kohn-Sham orbitals. The calculated vibrational frequencies and IR intensities are reported.
First author: Spassov, VZ, pK(a) calculations suggest storage of an excess proton in a hydrogen-bonded water network in bacteriorhodopsin, JOURNAL OF MOLECULAR BIOLOGY, 312, 203, (2001)
Abstract: Calculations of protonation states and pK(a) values for the ionizable groups in the resting state of bacteriorhodopsin have been carried out using the recently available 1.55 Angstrom resolution X-ray crystallographic structure. The calculations are in reasonable agreement with the available experimental data for groups on or near the ion transport chain (the retinal Schiff base; Asp85, 96, 115, 212, and Arg82). In contrast to earlier studies using lower-resolution structural data, this agreement is achieved without manipulations of the crystallographically determined heavy-atom positions or ad hoc adjustments of the intrinsic pK(a) of the Schiff base. Thus, the theoretical methods used provide increased reliability as the input structural data are improved. Only minor effects on the agreement with experiment are found with respect to methodological variations, such as single versus multi-conformational treatment of hydrogen atom placements, or retaining the crystallographically determined internal water molecules versus treating them as high-dielectric cavities. The long-standing question of the identity of the group that releases a proton to the extracellular side of the membrane during the L-to-M transition of the photocycle is addressed by including as pH-titratable sites not only Glu204 and Glu194, residues near the extracellular side that have been proposed as the release group, but also an H5O2+ molecule in a nearby cavity. The latter represents the recently proposed storage of the release proton in an hydrogen-bonded water network. In all calculations where this possibility is included, the proton is stored in the H5O2+ rather than on either of the glutamic acids, thus establishing the plausibility on theoretical grounds of the storage of the release proton in bacteriorhodopsin in a hydrogen-bonded water network. The methods used here may also be applicable to other proteins that may store a proton in this way, such as the photosynthetic reaction center and cytochrome c oxidase.
First author: Diefenbach, A, Oxidative addition of Pd to C-H, C-C and C-Cl bonds: Importance of relativistic effects in DFT calculations, JOURNAL OF CHEMICAL PHYSICS, 115, 4030, (2001)
Abstract: To assess the importance of relativistic effects for the quantum chemical description of oxidative addition reactions of palladium to C-H, C-C and C-CI bonds., we have carried out a systematic study of the corresponding reactions of CH4, C2H6 and CH3Cl with Pd-d(10) using nonrelativistic (NR), quasirelativistic (QR), and zeroth-order regularly approximated (ZORA) relativistic density functional theory (DFT) at the BP86/TZ(2)P level. Relativistic effects are important according to both QR and ZORA, the former yielding similar but somewhat more pronounced effects than the latter, more reliable method: activation barriers are reduced by 6-14 kcal/mol and reaction enthalpies become 15-20 kcal/mol more exothermic if one goes from NR to ZORA. This yields, for example, 298 K activation enthalpies DeltaH(298)(not equal) of -5.0 (C-H), 9.6 (C-C) and -6.0 kcal/mol (C-Cl) relative to the separate reactants at ZORA-BP86/TZ(2)P. In accordance with gas-phase experiments on reactions of Pd with alkanes, we find reaction profiles with pronounced potential wells for reactant complexes (collisionally stabilized and observed in experiments for alkanes larger than CH4) at -11.4 (CH4), -11.6 (C2H6) and -15.6 kcal/mol (CH3Cl) relative to separated reactants [ZORA-BP86/TZ(2)P]. Furthermore, we analyze the height of and the relativistic effects on the activation energies DeltaE(not equal) in terms of the activation strain DeltaE(strain)(not equal) in of and the transition-state interaction DeltaE(int)(not equal) between the reactants in the activated complex, with DeltaE(not equal) = DeltaE(strain)(not equal) + DeltaE(int)(not equal).
First author: Lim, BS, Nickel dithiolenes revisited: Structures and electron distribution from density functional theory for the three-member electron-transfer series [Ni(S(2)C(2)Me2)(2)](0,1-,2-), INORGANIC CHEMISTRY, 40, 4257, (2001)
Abstract: The complexes [Ni(S2C2Me2)(2)](z) (z = 0, 1-, 2-) have been isolated for the purpose of investigating their electronic structures in a reversible three-member electron-transfer series. Members are interrelated by reversible redox reactions with E-1/2(0/1-) = -0.15 V and, E-1/2(1-/2-) = -1.05 V versus SCE in acetonitrile. The three,complexes have nearly planar structures of idealized D-2h symmetry. As the series is traversed in the reducing direction, Ni-S and C-S bond lengths increase; the chelate ring C-C bond length decreases from the neutral complex to the monoanion and does not change significantly in the dianion. Structural trends are compared with previous results for [Ni(S2C2R2)(2)](1-,2-). Following the geometrical changes, values of v(Ni-S) and v(C-S) decrease, while the value of v(C-C) increases with increased reduction. Geometry optimizations at the density functional theory (DFT) level were performed for all members of -the series. Geometrical parameters obtained from the calculations are in. good agreement with the experimental findings. The 5b(2g) orbital was identified as the LUMO in [Ni(S2C2Me2)(2)], the SOMO in [Ni(S2C2Me2)(2)](1-), and the HOMO in [Ni(S2C2Me2)(2)](2-). Unlike in the situation in the [M(CO)(2)-(S2C2Me2)(2)](z) series (M, = Mo, W; z = 0, 1-, 2-), the apparent contribution from the metal d orbital in the electroactive orbital is not constant. In the present series, the d(xz) contribution increases from 13 to 20 to 39% upon passing from the, neutral to the monoanionic to the dianionic complex. Accurate calculation of EPR g-values Of [Ni(S2C2Me2)(2)](1-) by DFT serves as a test for the reliability of the electronic structure calculations.
First author: Bridgeman, AJ, Molecular and electronic structures of six-coordinate chloro-oxo-metalate complexes of V, Nb, Ta, Mo, and W, JOURNAL OF THE CHEMICAL SOCIETY-DALTON TRANSACTIONS, 40, 3556, (2001)
Abstract: The molecular and electronic structures of anionic [MOnCl6-n] (n = 1-3) complexes of V, Nb, Ta, Mo and W have been investigated using density-functional methods. Calculated structural parameters, including geometries and vibrational frequencies, are in good agreement with reported experimental data. A detailed bonding analysis is presented, and the properties of relevance to modelling polyoxoanion chemistry are explored and discussed. Structural trans-influence phenomena have been observed in all species studied. A combined analysis based on Mulliken-Mayer population methods and geometrical and bonding-energy data, supports the idea that both electronic and electrostatic factors are involved in the trans influence. Valuable insight into the redox properties of mono-oxo and cis di-oxo complexes, and the connection with polyoxometalates, has been gained by probing the nature of frontier orbitals. In general, bonding interactions possess metal-d and ligand-p character, and are strongly covalent in M-O bonds, and from moderately to largely ionic in M-Cl bonds.