Natural linear-scaled coupled-cluster theory with local transferable triple excitations: applications to peptides

The natural linear-scaled coupled-cluster (NLSCC) method ( Flocke, N.; Bartlett, R. J. J. Chem. Phys. 2004, 121, 10935 ) is extended to include approximate triple excitations via a coupled-cluster with single, double, and triple excitation method (CCSDT-3). The triples contribution can potentially be embedded in a larger singles and doubles region. NLSCC exploits the extensivity of the CC wave function to represent it in terms of transferable natural localized molecular orbitals (NLMOs) or functional groups thereof that are obtained from small quantum mechanical (QM) regions. Both occupied and virtual NLMOs are local because they derive from the single-particle density matrix. Noncanonical triples amplitudes are avoided by applying the unitary localization matrix to the canonical CC wave function for a QM region. A generalized NLMO code interfaced to the ACES II quantum chemistry software package provides NLMOs for the relevant number of atoms in a given functional group. Applications include linear polyglycine and the pentapeptide met-enkephalin, which was chosen as a more realistic three-dimensional system with nontrivial side chains. The results show that the triples contributions are quite large for aromatic bonds suggesting an interesting active space method for triples in which different bonds require different excitation levels. The NLSCC approach recovers a very large percentage (>99%) of the CCSD or CCSDT-3 correlation energy.     

A coupled cluster study of the electronic spectroscopy and photochemistry of Cr(CO)6

The transition energies to the low-lying singlet and triplet excited states of Cr(CO)(6) are computed by equation-of-motion coupled cluster singles and doubles (EOM-CCSD) and similarity transformed equation-of-motion coupled cluster singles and doubles (STEOM-CCSD) methods with all-electrons basis sets. Both experimental and optimized geometries are used for the calculations. Calculations with various basis sets, among them one of the largest calculations performed at the EOM-CCSD level, based on atomic natural orbitals with 627 functions, were used to evaluate the basis set influence on computed transition energies. The presence of a shoulder at 3.9 eV in the experimental absorption spectrum, assigned to the (1)A(1g)-->(1)T(2u) transition, which was not reproduced by recent density functional theory (DFT) or multi-state complete active space perturbation theory (MS-CASPT2) is supported by the present STEOM-CCSD calculations with a theoretical value of 3.92 eV. In addition to this weak (1)A(1g)--> a (1)T(2u) absorption, we observe two strong absorptions corresponding to (1)A(1g)--> a (1)T(1u) at 4.37 eV (vs. an experimental value of 4.46 eV) and (1)A(1g)--> b (1)T(1u) at 5.20 eV (vs. an experimental value of 5.53 eV). Both are characterized as metal-to-ligand charge-transfer (MLCT) allowed transitions. The first metal-centered (MC) absorption at 4.37 eV in our best calculation is degenerate with the lowest MLCT absorbing state. The one-dimensional potential energy curves associated to the low-lying singlet MLCT and MC states as a function of the chromium axial carbonyl bond distance q(a) = [Cr-CO(axial)] show that an avoided crossing exists between the a (1)T(1g) (MC) and a (1)T(1u) (MLCT) states near 1.92 A, which is very close to the equilibrium Cr-CO distance. Moreover, the MC state seems to be dissociative for the CO loss. These two important features could explain the ultra-fast dissociation of CO (100 fs) observed in recent low intensity laser probed gas phase experiments.     

Octahedral and trigonal prismatic structure preferences in a bicyclic hexaamine cage for zinc(II), cadmium(II) and mercury(II) ions

The synthesis and characterisation of complexes of the hexaamine cage ligand facial-1,5,9,13,20-pentamethyl-3,7,11,15,18,22-hexaazabicyclo[7.7.7]tricosane (fac-(Me)(5)-D(3 h)tricosaneN(6)) with Zn(II), Cd(II) and Hg(II) is reported. Single crystal X-ray structural analyses of the Cd(II) and Hg(II) complexes reveal that the coordination spheres of both cations have an unusual trigonal prismatic stereochemistry organised by the ligand substituents and cavity size. This is unprecedented for hexaamine complexes of these metal ions, and in stark contrast to the distorted octahedral stereochemistry found previously for the analogous Zn(II) complex. An X-ray structural analysis of single crystals of the diprotonated ligand [fac-(Me)(5)-D(3h)tricosaneN(6) - 2H](CF(3)SO(3))(2) shows that it also prefers to adopt a trigonal prismatic structure. The (13)C NMR spectra of the metal complexes indicate that their structures are preserved at 20 degrees C in solution. However, heating the Zn(II) complex to approximately 130 degrees C appears to convert it to the trigonal prismatic form. In contrast cooling the trigonal prismatic Hg(II) complex to -80 degrees C does not convert it to the octahedral structure. The results are also compared to the structures of various other transition metal ion complexes of the same or similar ligands. This comparison yields overall an appreciation of the factors that determine the final structures of complexes formed with such tricosaneN(6) ligands.     

Synthesis and characterization of ruthenium bis(beta-diketonato) pyridine-imidazole complexes for hydrogen atom transfer

Ruthenium bis(beta-diketonato) complexes have been prepared at both the RuII and RuIII oxidation levels and with protonated and deprotonated pyridine-imidazole ligands. RuII(acac)2(py-imH) (1), [RuIII(acac)2(py-imH)]OTf (2), RuIII(acac)2(py-im) (3), RuII(hfac)2(py-imH) (4), and [DBU-H][RuII(hfac)2(py-im)] (5) have been fully characterized, including X-ray crystal structures (acac = 2,4-pentanedionato, hfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionato, py-imH = 2-(2'-pyridyl)imidazole, DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene). For the acac-imidazole complexes 1 and 2, cyclic voltammetry in MeCN shows the RuIII/II reduction potential (E1/2) to be -0.64 V versus Cp2Fe+/0. E1/2 for the deprotonated imidazolate complex 3 (-1.00 V) is 0.36 V more negative. The RuII bis-hfac analogues 4 and 5 show the same DeltaE1/2 = 0.36 V but are 0.93 V harder to oxidize than the acac derivatives (0.29 and -0.07 V). The difference in acidity between the acac and hfac derivatives is much smaller, with pKa values of 22.1 and 19.3 in MeCN for 1 and 4, respectively. From the E1/2 and pKa values, the bond dissociation free energies (BDFEs) of the N-H bonds in 1 and 4 are calculated to be 62.0 and 79.6 kcal mol(-1) in MeCN - a remarkable difference of 17.6 kcal mol(-1) for such structurally similar compounds. Consistent with these values, there is a facile net hydrogen atom transfer from 1 to TEMPO* (2,2,6,6-tetramethylpiperidine-1-oxyl radical) to give 3 and TEMPO-H. The DeltaG degrees for this reaction is -4.5 kcal mol(-1). 4 is not oxidized by TEMPO* (DeltaG degrees = +13.1 kcal mol(-1)), but in the reverse direction TEMPO-H readily reduces in situ generated RuIII(hfac)2(py-im) (6). A RuII-imidazoline analogue of 1, RuII(acac)2(py-imnH) (7), reacts with 3 equiv of TEMPO* to give the imidazolate 3 and TEMPO-H, with dehydrogenation of the imidazoline ring.     

Graded annihilators of modules over the Frobenius skew polynomial ring, and tight closure

This paper is concerned with the tight closure of an ideal in a commutative Noetherian local ring of prime characteristic . Several authors, including R. Fedder, K-i. Watanabe, K. E. Smith, N. Hara and F. Enescu, have used the natural Frobenius action on the top local cohomology module of such an to good effect in the study of tight closure, and this paper uses that device. The main part of the paper develops a theory of what are here called `special annihilator submodules' of a left module over the Frobenius skew polynomial ring associated to ; this theory is then applied in the later sections of the paper to the top local cohomology module of and used to show that, if is Cohen-Macaulay, then it must have a weak parameter test element, even if it is not excellent.

     

Analysis of long-range effects in many-body correlation approaches for one-dimensional periodic systems

We analyze the divergent contributions to the Hamiltonian for extended, nonmetallic systems in one dimension, to both the ground-state correlation energy and to the correlated band structure. It is shown that the contribution from the long-range divergent part of the Hamiltonian tends to zero as 1/M⁻², where M is the extent of the troublesome lattice summations. Therefore, it is well justified to neglect such contributions in higher order many-body perturbation theory or coupled cluster treatments of the electronic structure for polymers. © 1997 John Wiley & Sons, Inc.     

赛灵思与微软的智能车载信息系统

根据美国交通部的一项研究,全世界人们每周在汽车上度过的交通时间超过5亿小时.既然花在汽车上的时间如此之多,人们希望能够利用这些时间来享受娱乐,同心爱的人说说话,甚至完成一些通常需要在工作场所才能完成的任务.     

Polymer Brushes via Controlled,Surface‐Initiated Atom Transfer Radical Polymerization (ATRP) from Graphene Oxide

A method for growing polymers directly from the surface of graphene oxide is demonstrated. The technique involves the covalent attachment of an initiator followed by the polymerization of styrene, methyl methacrylate, or butyl acrylate using atom transfer radical polymerization (ATRP). The resulting materials were characterized using a range of techniques and were found to significantly improve the solubility properties of graphene oxide. The surface‐grown polymers were saponified from the surface and also characterized. Based on these results, the ATRP reactions were determined to proceed in a controlled manner and were found to leave the structure of the graphene oxide largely intact.

     

Spongy Graphene as a Highly Efficient and Recyclable Sorbent for Oils and Organic Solvents

In this work, spongy graphene (SG), a shape‐mouldable and nanoporous material with a high specific surface area used as a versatile and recyclable sorbent material, is proposed and studied. SG shows highly efficient absorption of not only petroleum products and fats, but also toxic solvents such as toluene and chloroform (up to 86 times of its own weight), requiring no further pretreatment, which is tens of times higher than that of conventional absorbers. Moreover, SG can be regenerated (>10 times) by heat treatment, yielding the full release of adsorbates (>99%). The present work suggests SG a widespread potential for applications in industry as well as topics regarding environmental protection.     

Synthesis of cyclic N‐nitrourethanes by the simultaneous oxidative desulfurization and nitration of cyclic thiourethanes

The reaction of five‐ and six‐membered cyclic thiourethanes with acetyl nitrate results in a vigorous reaction that generates copious amounts of red–brown nitrogen oxide fumes and produces the corresponding cyclic N‐nitrourethanes in high yields (>95%). The overall yield of the cyclic N‐nitrourethanes starting from an aminoalcohol using the “thiourethane” route is superior to the conventional route going through the cyclic urethane. J. Heterocyclic Chem., (2011).