exo-Substituent effects in halogenated icosahedral (B12H122–) and octahedral (B6H62–) closo-borane skeletons: chemical reactivity studied by experimental and quantum chemical methods

2009 ◽  
Vol 74 (1) ◽  
pp. 1-27 ◽  
Author(s):  
Martin Lepšík ◽  
Martin Srnec ◽  
Drahomír Hnyk ◽  
Bohumír Grüner ◽  
Jaromír Plešek ◽  
...  
Keyword(s):  
Quantum Chemical ◽  
Chemical Shifts ◽  
Chemical Reactivity ◽  
Substituent Effects ◽  
Calculated Data ◽  
Reaction Pathways ◽  
Activation Barriers ◽  
Quantum Chemical Methods ◽  
B3lyp Method ◽  
Experimental Values

The exo-substituent effects in halogenated icosahedral B12H122– (B12) and octahedral B6H62– (B6) closo-borane skeletons were studied both experimentally and theoretically. Firstly, the equilibrium geometries of exo-substituted B12 and B6 clusters were obtained using quantum chemical calculations at the MP2/def2-SVP level. A comparison with the available X-ray crystallographic data revealed a very good agreement between the theoretical and experimental values. Secondly, other descriptors of the molecular structure of these borane compounds – 11B NMR chemical shifts – were experimentally determined and compared with the calculated values obtained by the ab initio/GIAO approach at the MP2/def2-TZVP level. It was shown that the calculated data reproduced the experiment very closely. Thirdly, we investigated experimentally the halogenation reactions of B12 and attempted to explain the observed ratios between the two obtained disubstituted products (meta/ortho ~ 4:1) by calculating their thermodynamic stabilities using the DFT/B3LYP method. These calculations showed the enhanced stability of the meta disubstituted B12 but did not explain why the para product had not been observed in the experiment. We thus turned our attention to the kinetic aspects of exo-substitution reactions by exploring the possible reaction pathways and transition states. In spite of the complexity of the plausible reaction mechanisms, reasonable agreement was obtained between the calculated activation barriers and the experimental observations concerning the halogenation reactions of the B6 and B12 molecules. It also allowed to exclude from considerations certain reaction pathways leading to the mono- and dihalogenated B12 and B6 species.

Measurement and calculation of 13C chemical shift tensors in α-glucose and α-glucose monohydrate

2011 ◽  
Vol 89 (7) ◽  
pp. 737-744 ◽  
Author(s):  
Darren H. Brouwer ◽  
Kevin P. Langendoen ◽  
Quentin Ferrant
Keyword(s):  
Solid State ◽  
Chemical Shift ◽  
Quantum Chemical ◽  
Density Functional ◽  
Chemical Shifts ◽  
Chemical Calculation ◽  
Hydrogen Atoms ◽  
Chemical Shift Tensors ◽  
Experimental Values ◽  
Crystalline Forms

The 13C chemical shift tensors of two crystalline forms of glucose (α-glucose and α-glucose·H2O) were determined from one-dimensional (1D) and two-dimensional (2D) solid-state nuclear magnetic resonance (NMR) spectroscopy experiments. The experimental values determined from 1D and 2D methods are in very good agreement. Quantum chemical calculations were also carried out using the gauge-including projector augmented wave (GIPAW) method for plane-wave density functional theory (DFT) as implemented in the CAmbridge Serial Total Energy Package (CASTEP). The calculated 13C chemical shifts were found to be in excellent agreement with experimental values for crystal structures that had their hydrogen atoms optimized and after an appropriate calibration was applied to convert calculated chemical shieldings into chemical shifts. The work presented here lays an important foundation for future solid-state NMR and quantum chemical calculation investigations of the various crystalline forms of cellulose.

Predicting Feasible Organic Reaction Pathways Using Heuristically Aided Quantum Chemistry

2018 ◽  
Author(s):  
Dmitrij Rappoport ◽  
Alan Aspuru-Guzik
Keyword(s):  
Quantum Chemistry ◽  
Reaction Mechanisms ◽  
Quantum Chemical ◽  
Organic Reactions ◽  
Reaction Pathways ◽  
Empirical Knowledge ◽  
Organic Reaction ◽  
Chemical Methods ◽  
Quantum Chemical Methods ◽  
Wide Range

Studying organic reaction mechanisms using quantum chemical methods requires from the researcher an extensive knowledge of both organic chemistry and first-principles computation. The need for empirical knowledge arises because any reasonably complete exploration of the potential energy surfaces (PES) of organic reactions is computationally prohibitive. We have previously introduced the Heuristically-Aided Quantum Chemistry (HAQC) approach to modeling complex chemical reactions, which abstracts the empirical knowledge in terms of chemical heuristics—simple rules guiding the PES exploration—and combines them with structure optimizations using quantum chemical methods. The HAQC approach makes use of heuristic kinetic criteria for selecting reaction paths that are not only plausible, that is, consistent with the empirical rules of organic reactivity, but also feasible under the reaction conditions. In this work, we develop heuristic kinetic feasilibity criteria, which correctly predict feasible reaction pathways for a wide range of simple polar (substitutions, additions, and eliminations) and pericyclic organic reactions (cyclizations, sigmatropic shifts, and cycloadditions). In contrast to knowledge-based reaction mechanism prediction methods, the same kinetic heuristics are successful in classifying reaction pathways as feasible or infeasible across this diverse set of reaction mechanisms. We discuss the energy profiles of HAQC and their potential applications in machine learning of chemical reactivity.<br>

Predicting Feasible Organic Reaction Pathways Using Heuristically Aided Quantum Chemistry

2018 ◽  
Author(s):  
Dmitrij Rappoport ◽  
Alan Aspuru-Guzik
Keyword(s):  
Quantum Chemistry ◽  
Reaction Mechanisms ◽  
Quantum Chemical ◽  
Organic Reactions ◽  
Reaction Pathways ◽  
Empirical Knowledge ◽  
Organic Reaction ◽  
Chemical Methods ◽  
Quantum Chemical Methods ◽  
Wide Range

Studying organic reaction mechanisms using quantum chemical methods requires from the researcher an extensive knowledge of both organic chemistry and first-principles computation. The need for empirical knowledge arises because any reasonably complete exploration of the potential energy surfaces (PES) of organic reactions is computationally prohibitive. We have previously introduced the Heuristically-Aided Quantum Chemistry (HAQC) approach to modeling complex chemical reactions, which abstracts the empirical knowledge in terms of chemical heuristics—simple rules guiding the PES exploration—and combines them with structure optimizations using quantum chemical methods. The HAQC approach makes use of heuristic kinetic criteria for selecting reaction paths that are not only plausible, that is, consistent with the empirical rules of organic reactivity, but also feasible under the reaction conditions. In this work, we develop heuristic kinetic feasilibity criteria, which correctly predict feasible reaction pathways for a wide range of simple polar (substitutions, additions, and eliminations) and pericyclic organic reactions (cyclizations, sigmatropic shifts, and cycloadditions). In contrast to knowledge-based reaction mechanism prediction methods, the same kinetic heuristics are successful in classifying reaction pathways as feasible or infeasible across this diverse set of reaction mechanisms. We discuss the energy profiles of HAQC and their potential applications in machine learning of chemical reactivity.<br>

scholarly journals Substituent Effects on the σ···π Interactions Between Au6 Cluster and Substituted Benzene

2021 ◽  
Author(s):  
Qiang Zhao
Keyword(s):  
Quantum Chemical ◽  
Decomposition Analysis ◽  
Substituent Effects ◽  
Interaction Region ◽  
Energy Decomposition Analysis ◽  
Energy Decomposition ◽  
Chemical Methods ◽  
Π Interactions ◽  
Quantum Chemical Methods ◽  
Indicator Analysis

Abstract The σ···π interactions in the Au6···PhX (X=H, CH3, OH, OCH3, NH2, F, Cl, Br, CN, NO2) complexes are studied using quantum chemical methods. The present study focuses on the different effects of electron-donating and -withdrawing substituent. The structure and binding strength of the complexes are examined. The interactions between Au6 cluster and various substituted benzene become strengthened relative to the Au6···benzene complex. The interaction region indicator analysis was performed, and the interaction region and interaction between the substituent and Au6 cluster are discussed. It is found that the substituent effects on the σ···π interactions between Au6 cluster and substituted benzene are different from π···π interactions of benzene dimer. Energy decomposition analysis was carried out to study the nature of σ···π interactions, and the substituent effects are mainly reflected on the electrostatic interaction and dispersion.

SYNTHESIS AND GIAO NMR CALCULATIONS FOR TWO DIASTEREOISOMERS OF 2′-ACETYLOXY-2′-PHENYLSPIRO[INDENO[1,2-b]QUINOXALIN-11,1′-CYCLOPROPANE]

2012 ◽  
Vol 11 (06) ◽  
pp. 1227-1236 ◽  
Author(s):  
MASOUD SHAABANZADEH ◽  
HAMID HASHEMIMOGHADDAM ◽  
MARYAM BIKHOF TORBATI ◽  
TAHEREH SOLEYMANI AHOEE
Keyword(s):  
Nmr Spectra ◽  
Chemical Shifts ◽  
Atomic Orbital ◽  
Basis Sets ◽  
Computational Results ◽  
Nmr Chemical Shifts ◽  
H Nmr ◽  
Chemical Structures ◽  
B3lyp Method ◽  
Experimental Values

Two diastereoisomers of 2′-acetyloxy-2′-phenylspiro[indeno[1,2-b]quinoxalin-11,1′-cyclopropane] were synthesized and their 1 H NMR spectra were recorded. Their chemical structures were fully optimized at B3LYP/6-311+G(d,p) level of theory using the Gaussian 03W program package. The 1 H NMR chemical shifts were calculated for geometry-optimized structures of the diastereoisomers with the gauge independent atomic orbital (GIAO) and B3LYP method with the 6-311+G(d,p), 6-311++G(d), 6-31++G(d,p) and 6-31+G(d) basis sets. The computational results were then compared with the experimental values and the structures associated with each spectrum were assigned.

Characterization of Tris(Diselenocarbamato)Cobalt(III) and Pentakis(Diselenocarbamato)Dicobalt(III) Complexes by Electrochemical, Cobalt-59 N.M.R. and Mass-Spectrometric Techniques. Comparisons With Dithiocarbamate Analogs

1986 ◽  
Vol 39 (9) ◽  
pp. 1385 ◽  
Author(s):  
AM Bond ◽  
R Colton ◽  
DR Mann ◽  
JE Moir
Keyword(s):  
Electrochemical Oxidation ◽  
Chemical Shifts ◽  
Chemical Reactivity ◽  
Mercury Electrode ◽  
Substituent Effects ◽  
Reduction Process ◽  
Reduced Form ◽  
Platinum Electrodes ◽  
Redox Potentials ◽  
Mercury Electrodes

A series of Co(RR′dsc)3 and [Co2(RR′dsc)5]+ complexes (R, R′ = two alkyl groups or one heterocyclic group; dsc = NCSe2) have been synthesized and their redox behaviour, chemical reactivity and spectroscopic properties compared with the corresponding dithiocarbamate (RR′dtc) complexes. Electrochemical oxidation of Co(RR′dsc)3 in dichloromethane at platinum electrodes occurs at potentials about 0.34 V less positive than for Co(RR′dsc)3. The formally cobalt(IV) complexes [Co(RR′dsc)3]+ can be identified as a product which is then converted into [Co2(RR′dsc)5]+ via dimerization and an internal redox reaction. Despite the enhanced thermodynamic stability implied by the redox potentials, [Co(RR′dsc)3]+ has similar kinetic stability to the analogous dithiocarbamate complexes. Co(RR′dsc)3 is reduced at fairly negative potentials on both platinum and mercury electrodes with extremely rapid loss of [RR′dsc]-. [Co(RR′dsc)3]- is therefore thermodynamically and kinetically more unstable than [Co(RR′dtc)3]- . The [Co2(RR′dsc)5]+ complexes are also more readily oxidized and harder to reduce than the sulfur analogues. Oxidation of [Co2(RR′dsc)5]+ produces [Co2(RR′dsc)5]2+ at low temperatures and fast scan rates, but no stable reduced form of the dimer is accessible on the voltammetric time scale examined. The reduction process for the dimer is consistent with the reaction [Co2(RR′dsc)5]+ +e- → Co(RR′dsc)3+ Co(RR dsc)2. Electrochemical oxidation data obtained at mercury electrodes for the diselenocarbamate complexes are complicated by adsorption but are similar to that found at platinum electrodes. This contrasts with the dithiocarbamates where a mercury electrode specific pathway is observed. Cobalt-59 n.m.r. spectroscopy in dichloromethane shows the non- equivalence of the two cobalt atoms in [Co2(RR′dsc)5]+. The chemical shifts for Co(RR′dsc)3 complexes exhibit similar substituent effects to the dithiocarbamates in cobalt-59 n.m.r. measurements as was the case in oxidative electrochemistry. Cobalt-59 n.m.r. spectroscopy and mass spectrometry demonstrate that exchange, substitution and redox reactions can lead to the formation of mixed ligand diselenocarbamate complexes and mixed dithiocarbamate/diselenocarbamate complexes for both the cobalt(III) monomers and dimers.

Experimental and theoretical investigation of molecular structure, vibrational analysis, chemical reactivity, electrostatic potential of benzyl methacrylate monomer and homopolymer

2016 ◽  
Vol 94 (9) ◽  
pp. 853-864 ◽  
Author(s):  
Feride Akman
Keyword(s):  
Electrostatic Potential ◽  
Density Functional ◽  
Chemical Shifts ◽  
Chemical Reactivity ◽  
Atomic Orbital ◽  
Vibrational Analysis ◽  
Calculated Data ◽  
Basis Set ◽  
Geometrical Parameters ◽  
Molecular Approaches

Until now, a number of new polymers have been discovered with the aid of experimental and computational molecular approaches and indicated to have potential applications. All the computational molecular approaches provide information helpful to further study. So, monomer and homopolymer of benzyl methacrylate (BzMA), which is a popular methacrylate ester monomer, were synthesized and investigated based on density functional theory (DFT) and Hartree–Fock (HF) methods. The monomer and homopolymer were characterized by FTIR, 1H, and 13C NMR techniques. The molecular geometry, geometrical parameters, Mulliken atomic charges, and vibrational frequencies of BzMA monomer and homopolymer (in dimer form) were calculated by using the DFT and HF methods with 6-31G (d, p) as basis set. The molecular electrostatic potential maps and molecular orbitals properties of monomer and homopolymer were calculated using the 6-31G (d, p) basis set of theories. Besides, 1H and 13C chemical shifts were calculated by the gauge–including atomic orbital approach. The results demonstrated that the theoretical values were in good agreement with the experimental values. The calculated data are important to providing insight into molecular analysis and may be used in technological applications.

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