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ABSTRACT: Twin polymerization is a novel technique for synthesizing hybrid polymers with domain sizes in the nanometer range. While a broad variety of monomers have been investigated, the mechanistic details and the most important influences on the nanostructure formation are unknown. A scale bridging approach is presented to simulate the twin polymerization of 2,2'-spirobi[4H-1,3,2-benzodioxasiline]. This approach is based on detailed quantum chemical calculations that yield insight into reactivity and structure at the molecular level while the kinetics of the network formation process and the influences that govern structure formation are investigated at the mesoscopic level by a coarse-grained simulation.
Macromolecular Theory and Simulations 11/2012; 21(9):615-628. · 1.71 Impact Factor
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ABSTRACT: Access to elusive alkynyl azides was accomplished with good yields by the key reaction of alkynyliodonium salts with azide salts; this synthesis opens a door not only to isolate and characterize azidoacetylene but also to prepare a variety of products via short-lived azidovinylidene and cyanocarbene intermediates. These two main aspects are reported in back-to-back Communications by K. Banert, A. Auer, et al. (DOI: 10.1002/anie.201203626) and I. F. D. Hyatt and M. P. Croatt (DOI: 10.1002/anie.201203062). The cover picture is by Lauren Nichols.
Angewandte Chemie International Edition 07/2012; · 13.45 Impact Factor
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ABSTRACT: Sleeping Beauty awakes: After 102 years of unsuccessful attempts to synthesize azidoacetylene, spectroscopic evidence for this compound has been shown. This highly explosive compound was synthesized by the treatment of ethynyliodonium salts with azide (QN(3) =n-C(16) H(33) Bu(3) PN(3) ). Azidoacetylene can be trapped by a cycloaddition reaction to yield a stable triazole, otherwise cleavage to generate cyanocarbene dominates.
Angewandte Chemie International Edition 07/2012; 51(30):7515-8. · 13.45 Impact Factor
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ABSTRACT: The first procedures to prepare 4-bromo-4-methylpentanal and 4-azido-4-methylpentanal are reported. The latter compound and also the parent 4-azidobutanal do not lead to 4,5-dihydro-1,2,3,4-oxatriazoles by intramolecular 1,3-dipolar cycloaddition, although it was claimed to be so in the literature. The NMR spectroscopic data of such heterocycles reported previously do not agree with those of similar substances and are incompatible with (13)C NMR spectroscopic chemical shifts calculated by quantum chemical methods in the presented work. These calculations show furthermore that the intramolecular cycloaddition of 4-azidobutanals to give the title compounds is strongly endothermic and thus most probably not possible.
Chemistry 05/2011; 17(20):5539-43. · 5.93 Impact Factor
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ABSTRACT: A new approximation for post-Hartree-Fock (HF) methods is presented applying tensor decomposition techniques in the canonical product tensor format. In this ansatz, multidimensional tensors like integrals or wavefunction parameters are processed as an expansion in one-dimensional representing vectors. This approach has the potential to decrease the computational effort and the storage requirements of conventional algorithms drastically while allowing for rigorous truncation and error estimation. For post-HF ab initio methods, for example, storage is reduced to O(d·R·n) with d being the number of dimensions of the full tensor, R being the expansion length (rank) of the tensor decomposition, and n being the number of entries in each dimension (i.e., the orbital index). If all tensors are expressed in the canonical format, the computational effort for any subsequent tensor contraction can be reduced to O(R(2)·n). We discuss details of the implementation, especially the decomposition of the two-electron integrals, the AO-MO transformation, the Møller-Plesset perturbation theory (MP2) energy expression and the perspective for coupled cluster methods. An algorithm for rank reduction is presented that parallelizes trivially. For a set of representative examples, the scaling of the decomposition rank with system and basis set size is found to be O(N(1.8)) for the AO integrals, O(N(1.4)) for the MO integrals, and O(N(1.2)) for the MP2 t(2)-amplitudes (N denotes a measure of system size) if the upper bound of the error in the l(2)-norm is chosen as ε = 10(-2). This leads to an error in the MP2 energy in the order of mHartree.
The Journal of chemical physics 02/2011; 134(5):054118. · 3.09 Impact Factor
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ABSTRACT: Within this study, the influence of ester groups in mixed monolayers on the surface properties will be discussed. Detailed investigations on the macroscopic and microscopic characteristics on mixed monolayers with different content of ester groups in an alkyl surrounding are done by contact angle measurements and atomic force spectroscopy. Density functional theory (DFT) calculations show a statistical distribution and a directed orientation of the ester molecules. In the experiments an increasing amount of ester groups leads to a fast increasing polarity followed by a nearly constant polarity in the regime of 25% and 40% of ester in the monolayer and a further increase at higher amounts of ester groups, which clearly differ from the behavior expected by Cassie. By DFT calculations it can be shown that water molecules form ring-like structures around the ester group. These solvent shells increase the hydrophilic fraction on the surface explaining the disproportional growth in the polarity of the monolayer. This rise in polarity is maximal for single ester groups (monomers) or dimers of esters. The amount of these monomers and dimers is estimated by Monte Carlo simulation showing clearly that the linear regime at fractions between 0.25 and 0.4 are caused by the transition from mainly monomers to mainly dimers. Thus, we show for the first time that adsorbed water molecules forming a solvent shell around hydrophilic groups in hydrophobic surroundings influence the surface properties of mixed monolayers on a macroscopic and microscopic scale which therefore must be taken into account when preparing, investigating, using and understanding such monolayers.
Langmuir 06/2010; 26(11):8301-8. · 4.19 Impact Factor
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ABSTRACT: High-level ab initio benchmark calculations of the (15)N and (31)P NMR chemical shielding constants for a representative set of molecules are presented. The computations have been carried out at the Hartree-Fock self-consistent field (HF-SCF), density functional theory (DFT) (B-P86 and B3-LYP), second-order Moller-Plesset perturbation theory (MP2), coupled cluster singles and doubles (CCSD), and CCSD augmented by a perturbative treatment of triple excitations [CCSD(T)] level of theory using basis sets of triple zeta quality or better. The influence of the geometry, the treatment of electron correlation, as well as basis set and zero-point vibrational effects on the shielding constants are discussed and the results are compared to gas-phase experimental shifts. As for the first time a study using high-level post-HF methods is carried out for a second-row element, we also propose a family of basis sets suitable for the computation of (31)P shielding constants. The mean deviations observed for (15)N and (31)P are 0.9 [CCSD(T)/13s9p4d3f] and -3.3 ppm [CCSD(T)/15s12p4d3f2g], respectively, when corrected for zero-point vibrational effects. Results obtained at the DFT level of theory are of comparable accuracy to MP2 for (15)N and of comparable accuracy to HF-SCF for (31)P. However, they are not improved by inclusion of zero-point vibrational effects. The PN molecule is an especially interesting case with exceptionally large electron correlation effects on shielding constants beyond MP2 which, therefore, represents an excellent example for further benchmark studies.
The Journal of chemical physics 02/2010; 132(6):064109. · 3.09 Impact Factor
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Stefan Spange,
Patrick Kempe,
Andreas Seifert, Alexander A Auer,
Petra Ecorchard,
Heinrich Lang,
Meiken Falke,
Michael Hietschold,
Andreas Pohlers,
Walter Hoyer,
Gerhard Cox,
Emanuel Kockrick,
Stefan Kaskel
Angewandte Chemie International Edition 09/2009; 48(44):8254-8. · 13.45 Impact Factor
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Alexander A Auer
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ABSTRACT: Benchmark calculations of (17)O NMR chemical shifts for a series of 19 molecules with 22 chemical shifts are presented. This includes calculations at the HF-SCF, DFT (BP86 and B3-LYP), MP2, CCSD(T), and for a special case full CCSDT level of theory using basis sets of quadruple zeta quality and better. The effects of the quality of the geometry, electron correlation, basis set, and the inclusion of zero-point vibrational and temperature corrections are discussed in detail and the results are compared to gas-phase experimental values. Mean and standard deviations are 6 and 24 ppm for HF-SCF, -20 and 14 ppm for BP86, -20 and 13 ppm for B3-LYP, and 26 and 12 ppm for MP2. Results at the CCSD(T)/pz3d2f level of theory using geometries optimized at the CCSD(T)/cc-pVTZ level of theory exhibit a mean deviation of 16 ppm and a standard deviation of 6 ppm. A mean deviation of 6 ppm and a standard deviation of 4 ppm are obtained if these values are corrected for zero-point vibrational and temperature effects.
The Journal of chemical physics 08/2009; 131(2):024116. · 3.09 Impact Factor
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ABSTRACT: The electrophilic substituent constant sigma(+) was derived from the relative rate constants of the solvolysis of substituted alpha-cumyl chlorides in an acetone/water solvent mixture in the original work by Brown and Okamoto (J. Am. Chem. Soc. 1958, 80, 4979). As an extension of this procedure, we were looking for methods to determine sigma(+) values in nonpolar media. We have calculated the exocyclic charges q(R) for alpha-cumyl cations (R-C(6)H(4)CMe(2)(+)) as a measure of the electron-donating capacity of meta and para substituents. The UV-vis absorption nu(max) of the analogously substituted nitrobenzene derivatives has been used for the experimental measurement of the electron donating capacity of substituents R in nonpolar media. A linear relationship between nu(max) and Deltaq(R) (= q(R-C(6)H(4)CMe(2)(+)) - q(C(6)H(5)CMe(2)(+))) was obtained. A description of the electron-donating capacity in nonpolar media can be achieved both for substituents investigated already and for those for which the experimental determination of sigma(+) has previously been difficult. Special interest was directed to boron-based substituents in which the electron-donating ability is dependent on the coordinational environment of the boron atom.
The Journal of Organic Chemistry 05/2009; 74(9):3316-22. · 4.45 Impact Factor
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ABSTRACT: A new hierarchy of augmented basis sets optimized for the calculation of molecular properties such as indirect spin-spin coupling constants is presented. Based on the Dunning hierarchy of cc-pVXZ (X = D, T, Q, and 5) basis sets augmentation functions with tight exponents have been optimized for coupled-cluster calculations of indirect spin-spin coupling constants. The optimal exponents for these tight functions have been obtained by optimizing the sum of the absolute values of all contributions to the coupling constant. On the basis of a series of test cases (CO, HF, N(2), F(2), H(2)O, NH(3), and CH(4)) we propose a set of tight s, p, and d functions to be added to the uncontracted Dunning basis sets, and, subsequently, to recontract. The resulting ccJ-pVXZ (X = D, T, Q, and 5) basis sets demonstrate excellent cost efficiency in benchmark calculations. These new basis sets should generally be applicable for the calculation of spin-spin coupling constants and other properties that have a strong dependence on powers of 1r or even contain a delta distribution for correlated ab initio methods.
The Journal of chemical physics 09/2008; 129(6):064111. · 3.09 Impact Factor
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ABSTRACT: Benchmark calculations of (19)F nuclear magnetic shielding constants are presented for a set of 28 molecules. Near-quantitative accuracy (ca. 2 ppm deviation from experiment) is achieved if (1) electron correlation is adequately treated by employing the coupled-cluster singles and doubles (CCSD) model augmented by a perturbative correction for triple excitations [CCSD(T)], (2) large (uncontracted) basis sets are used, (3) gauge-including atomic orbitals are used to ensure gauge-origin independence, (4) calculations are performed at accurate equilibrium geometries [obtained from CCSD(T)/cc-pVTZ calculations correlating all electrons], and (5) vibrational averaging and temperature corrections via second-order vibrational perturbation theory (VPT2) are included. For the CCSD(T)/13s9p4d3f calculations corrected for vibrational effects, mean and standard deviation from experiment are -1.9 and 1.6 ppm, respectively. Less elaborate theoretical treatments result in larger errors. Consideration of relative shifts can reduce the mean deviation (through an appropriately chosen reference compound), but does not change the standard deviation. Density-functional theory calculations of absolute and relative (19)F nuclear magnetic shielding constants are found to be, at best, as accurate as the corresponding Hartree-Fock self-consistent-field calculations and are not improved by consideration of vibrational effects. Molecular systems containing fluorine-oxygen, fluorine-nitrogen, and fluorine-fluorine bonds are found to be more challenging than the other investigated molecules for the considered theoretical methods.
The Journal of chemical physics 07/2008; 128(24):244111. · 3.09 Impact Factor
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ABSTRACT: In this publication, a characterization of different azidoalkyne compounds using high-level ab initio quantum chemical methods is presented. For this purpose, the molecular structures and the 13C NMR chemical shifts have been calculated at the MP2 and CCSD(T) level of theory and the influence of zero-point vibration as well as the solvent on the chemical shifts are discussed. Furthermore, a comparison of the energy barriers of the decomposition under N2 separation for a set of 1-azidoalkynes with different functional groups has been carried out. The molecular structures and properties of the resultant decomposition products have been investigated. It is remarkable that large deviations of the NMR chemical shifts of ethylthioethynyl azide occur in comparison to the experiment. These deviations are far outside of the error bars. Electron correlation effects are of high importance if an accurate description of the chemical shifts of 1-azidoalkynes shall be obtained. A comparison of the energy barriers of the decomposition under N2 separation of 1-azidoalkynes with different functional groups indicates that the stability of 1-azidoalkynes is not increased by typical donor or acceptor groups but rather by silyl or phenyl substituents. The molecular geometries of the decomposition products indicate that the equilibrium structures are of carbene character. Some of the results presented here are in contradiction to previous experimental publications and cast a new light on some open challenges for experimentalists.
The Journal of Physical Chemistry A 11/2007; 111(39):9945-51. · 2.95 Impact Factor
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ABSTRACT: A series of stilbeneboronate pinacol cyclic esters, containing none to three nitro groups, have been synthesized by various olefination reactions and characterized by X-ray single-crystal structure analysis. A stilbeneboronate ester bearing electron-acceptor groups experiences transition to a push-pull pi-electron system upon complexation with one fluoride ion at the boron atom. The UV-vis absorption maxima of the presented nitro-substituted stilbeneboronate esters are red-shifted upon addition of fluoride ions, indicating this binding event. The enhancement of the polarity of the investigated compounds and the changes in the electronic system were investigated by UV-vis absorption spectroscopy and solvatochromism. Additionally, studies were performed by natural bond orbital (NBO) analysis and RI-CC2 calculations of the vertical excitation energies. The synergism of fluoride ion complexation and solvation upon the UV-vis band shift is interpreted in terms of linear solvation energy relationships (LSERs) using the Kamlet-Taft solvent parameter set. It is found that the UV-vis absorption of the fluoro-boronates is strongly dependent on the solvents hydrogen-bond donating ability.
The Journal of Organic Chemistry 07/2007; 72(12):4328-39. · 4.45 Impact Factor
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ABSTRACT: In this paper we present a local coupled cluster approach based on a dynamical screening scheme, in which amplitudes are either calculated at the coupled cluster level (in this case CCSD) or at the level of perturbation theory, employing a threshold driven procedure based on MP2 energy increments. This way, controllable accuracy and smooth convergence towards the exact result are obtained in the framework of an a posteriori approximation scheme. For the representation of the occupied space a new set of local orbitals is presented with the size of a minimal basis set. This set is atom centered, is nonorthogonal, and has shapes which are fairly independent of the details of the molecular system of interest. Two slightly different versions of combined local coupled cluster and perturbation theory equations are considered. In the limit both converge to the untruncated CCSD result. Benchmark calculations for four systems (heptane, serine, water hexamer, and oxadiazole-2-oxide) are carried out, and decay of the amplitudes, truncation error, and convergence towards the exact CCSD result are analyzed.
The Journal of Chemical Physics 08/2006; 125(2):24104. · 3.33 Impact Factor
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ABSTRACT: Various approximations combining coupled-cluster (CC) and many-body perturbation theories have been derived and implemented into the parallel execution programs that take into account the spin, spatial (real Abelian), and permutation symmetries and that are applicable to closed- and open-shell molecules. The implemented models range from the CCSD(T), CCSD[T], CCSD(2)(T), CCSD(2)(TQ), and CCSDT(2)(Q) methods to the completely renormalized (CR) CCSD(T) and CCSD[T] approaches, where CCSD (CCSDT) stands for the CC method with connected single and double (single, double, and triple) cluster operators, and subscripted or parenthesized 2, T, and Q indicate the perturbation order or the excitation ranks of the cluster operators included in the corrections. The derivation and computer implementation have been automated by the algebraic and symbolic manipulation program TENSOR CONTRACTION ENGINE (TCE). The TCE-synthesized subroutines generate the tensors with the highest excitation rank in a blockwise manner so that they need not be stored in their entirety, while enabling the efficient reuse of other precalculated intermediate tensors defined by prioritizing the memory optimization as well as operation minimization. Consequently, the overall storage requirements for the corrections due to connected triple and quadruple cluster operators scale as O(n(4)) and O(n(6)), respectively (n being a measure of the system size). For systems with modest multireference character of their wave functions, we found that the order of accuracy is CCSD<CR-CCSD(T) approximately CCSD(2)(T) approximately CCSD(T)<CCSDT approximately CCSD(2)(TQ)<CCSDT(2)(Q), whereas CR-CCSD(T) is more effective in cases of larger quasidegeneracy. The operation costs of the TCE-generated CCSD(2)(TQ) and CCSDT(2)(Q) codes scale as rather steep O(n(9)), while the TCE-generated CCSD(T), CCSD(2)(T), and CR-CCSD(T) codes are near operation minimum [a noniterative O(n(7))]. The perturbative correction part of the CCSD(T)/cc-pVDZ calculations for azulene exhibited a 45-fold speedup upon a 64-fold increase in the number of processors from 8 to 512.
The Journal of Chemical Physics 01/2005; 121(24):12197-207. · 3.33 Impact Factor
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ABSTRACT: A two-electron basis-set completeness profile is proposed by analogy with the one-electron profile introduced by D. P. Chong (Can J Chem 1995, 73, 79). It is defined as Y(alpha, beta) = sigmam sigman (Galpha(1)Gbeta(2)/(1/r12)/ psim(1)psin(2)) (psim(1)psin(2)/r12/Galpha(1)Gp(2)) and motivated by the expression for the basis-set truncation correction that occurs in the framework of explicitly correlated methods (Galpha is a scanning Gaussian-type orbital of exponent alpha and [psim] is the orthonormalized one-electron basis under study). The two-electron basis-set profiles provide a visual assessment of the suitability of basis sets to describe electron-correlation effects. Furthermore, they provide the opportunity to assess the quality of the basis set as a whole--not only of the individual angular momentum subspaces, as is the case for the one-electron basis-set profiles. The two-electron completeness profiles of the cc-pVXZ (X = D, T, Q), aug-cc-pVTZ, cc-pCVTZ, and SVP-auxiliary basis sets for the carbon atom are presented as illustrative examples.
Journal of Computational Chemistry 03/2002; 23(3):420-5. · 4.58 Impact Factor
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ABSTRACT: A two-electron basis-set completeness profile is proposed by analogy with the one-electron profile introduced by D. P. Chong (Can J Chem 1995, 73, 79). It is defined as Y(α,β)=∑m∑n〈Gα(1)Gβ(2)∣(1/r12)∣Ψm(1)Ψn(2)〉〈Ψm(1)Ψn(2)∣r12∣Gα(1)Gβ(2)〉 and motivated by the expression for the basis-set truncation correction that occurs in the framework of explicitly correlated methods (Gα is a scanning Gaussian-type orbital of exponent α and {Ψm} is the orthonormalized one-electron basis under study). The two-electron basis-set profiles provide a visual assessment of the suitability of basis sets to describe electron-correlation effects. Furthermore, they provide the opportunity to assess the quality of the basis set as a whole—not only of the individual angular momentum subspaces, as is the case for the one-electron basis-set profiles. The two-electron completeness profiles of the cc-pVXZ (X=D, T, Q), aug-cc-pVTZ, cc-pCVTZ, and SVP-auxiliary basis sets for the carbon atom are presented as illustrative examples. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 420–425, 2002
Journal of Computational Chemistry 01/2002; 23(3):420 - 425. · 4.58 Impact Factor
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01/2000; 2(10):2235-2238.
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ABSTRACT: Full configuration-interaction calculations of the indirect spin–spin coupling constant of the BH molecule have been carried out in order to investigate the performance of various coupled-cluster (CC) methods in the treatment of electron-correlation effects, while the corresponding basis set convergence is analyzed in CC singles and doubles calculations. Assuming additivity of correlation and basis set effects, a theoretical estimate of 50.67 Hz is obtained for the spin–spin coupling constant.
Chemical Physics Letters.
