Coherent X-Ray Scattering for the Hydrogen Atom in the Hydrogen Molecule

Stewart, Robert F.; Davidson, Ernest R.; Simpson, William T.

doi:10.1063/1.1696397

Cited by 3,657 publications

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“…A correction for secondary extinction was applied. Neutral atom scattering factors for non-H-atoms were taken from [22a], and the scattering factors for H-atoms were taken from [23]. Anomalous dispersion effects were included in F c [24]; the values for f' and f" were those of [22b].…”

Section: Synthesis Of 1-mentioning

confidence: 99%

Synthesis and Selected Reactions of Hydrazides Containing an Imidazole Moiety

Mlostoń

Pieczonka

Kowalczyk

et al. 2011

Helvetica Chimica Acta

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The preparation of two types of imidazole derivatives bearing a hydrazide group was achieved by treatment of the corresponding esters with NH2NH2·H2O in MeOH at room temperature. In the case of 4-(ethoxycarbonyl)-1H-imidazole 3-oxides 3, hydrazides of type 1 were formed with retention of the N-oxide structure (Scheme 1). Interestingly, due to a strong H-bonding, no deoxygenation of the N-O function could be achieved even by treatment of 3 with Raney-Ni. The second type, 2-[(1H-imidazol-2-yl)sulfanyl]acetohydrazides 2, was obtained from 1H-imidazole-2(3H)-thiones 4 in two steps via S-alkylation with methyl bromoacetate, followed by treatment with NH2NH2·H2O (Scheme 2). An imidazole 7, containing both types of hydrazide groups, was prepared analogously from ethyl 2,3-dihydro-2-thioxo-1H-imidazole-4-carboxylate 4d (Scheme 4). Both types of hydrazides, 1 and 2, were transformed successfully to the corresponding acylhydrazones 8 and 9, respectively (Scheme 5). Furthermore, it has been shown that hydrazides of type 1 are useful starting materials for the synthesis of 1,2,4-triazole-3-thiones 11 and 1,3,4-thiadiazole-2-amines 12, bearing an imidazole 3-oxide moiety (Scheme 7). The preparation of two types of imidazole derivatives bearing a hydrazide group was achieved by treatment of the corresponding esters with NH 2 NH 2 ·H 2 O in MeOH at room temperature. In the case of 4-(ethoxycarbonyl)imidazole 3-oxides 3, hydrazides of type 1 were formed with retention of the N-oxide structure (Scheme 1). Interestingly, due to a strong hydrogen bonding, no deoxygenation of the N→O function could be achieved even by treatment of 3 with Raney-nickel. The second type, 2-[(imidazol-2-yl)sulfanyl]acetohydrazides 2, was obtained from 1H-imidazole-2(3H)-thiones 4 in two steps via S-alkylation with methyl bromoacetate followed by treatment with NH 2 NH 2 ·H 2 O (Scheme 2). An imidazole 7, containing both types of hydrazide groups, was prepared analogously from ethyl 2,3-dihydro-2-thioxoimidazole-4-carboxylate 4d (Scheme 4). Both types of hydrazides, 1 and 2, were transformed successfully into the corresponding acylhydrazones 8 and 9, respectively (Scheme 5). Furthermore, it has been shown that hydrazides of type 1 are useful starting materials for the synthesis of 1,2,4-triazole-3-thiones 11 and 1,3,4-thiadiazole-2-amines 12, bearing an imidazole 3-oxide residue (Scheme 7).

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Section: Synthesis Of 1-mentioning

confidence: 99%

Synthesis and Selected Reactions of Hydrazides Containing an Imidazole Moiety

Mlostoń

Pieczonka

Kowalczyk

et al. 2011

Helvetica Chimica Acta

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“…It is presumed that they are artefacts caused by the inferior data quality. Neutral atom scattering factors for non-H-atoms were taken from [37a], and the scattering factors for H-atoms were taken from [38]. Anomalous dispersion effects were included in F c [39]; the values for f ' and …”

Section: N-(4-mentioning

confidence: 99%

Synthesis of 1,3‐Selenazol‐2(3H)‐imines

Atanassov

Linden

Heimgartner

2010

Helvetica Chimica Acta

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The reaction of N,N′‐diarylselenoureas 16 with phenacyl bromide in EtOH under reflux, followed by treatment with NH3, gave N,3‐diaryl‐4‐phenyl‐1,3‐selenazol‐2(3H)‐imines 13 in high yields (Scheme 2). A reaction mechanism via formation of the corresponding Se‐(benzoylmethyl)isoselenoureas 18 and subsequent cyclocondensation is proposed (Scheme 3). The N,N′‐diarylselenoureas 16 were conveniently prepared by the reaction of aryl isoselenocyanates 15 with 4‐substituted anilines. The structures of 13a and 13c were established by X‐ray crystallography.

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“…This displacement only applies to vdW interactions and was optimized to better reproduce different water dimer configurations. The use of reduction factors to move hydrogen sites off of the nuclear position dates from early work on the hydrogen molecule by Stewart et al 57 Detailed analysis of the X-ray structures of crystalline glycylglycine and sulfamic acid also supports the need to shift hydrogen centers toward the attached heavy atom. 58 A similar scheme has been used successfully by the MM3 force field.…”

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Polarizable Atomic Multipole Water Model for Molecular Mechanics Simulation

Ren¹,

2003

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A new classical empirical potential is proposed for water. The model uses a polarizable atomic multipole description of electrostatic interactions. Multipoles through the quadrupole are assigned to each atomic center based on a distributed multipole analysis (DMA) derived from large basis set molecular orbital calculations on the water monomer. Polarization is treated via self-consistent induced atomic dipoles. A modified version of Thole's interaction model is used to damp induction at short range. Repulsion-dispersion (vdW) effects are computed from a buffered 14-7 potential. In a departure from most current water potentials, we find that significant vdW parameters are necessary on hydrogen as well as oxygen. The new potential is fully flexible and has been tested versus a variety of experimental data and quantum calculations for small clusters, liquid water, and ice. Overall, excellent agreement with experimental and high level ab initio results is obtained for numerous properties, including cluster structures and energetics and bulk thermodynamic and structural measures. The parametrization scheme described here is easily extended to other molecular systems, and the resulting water potential should provide a useful explicit solvent model for organic solutes and biopolymer modeling. IntroductionEmpirical potential energy functions derived from classical molecular mechanics are central to computational modeling at the atomic level. Molecular mechanics has long enjoyed great success in application to many classes of isolated, gas-phase organic compounds. 1 Beginning with the pioneering work of Bernal and Fowler, 2 water has probably been the target of more potential energy models than any other substance. An interesting overview and historical perspective on the development of water models was recently presented by Finney. 3 Simple nonpolarizable pairwise-additive models that describe the average structure and energetics of liquid water have been in wide use for many years (e.g., TIP3P 4 and SPC 5 ). The recently developed TIP5P potential exhibits excellent agreement with the experimental internal energy, density, and O‚‚‚O radial distribution at room temperature. 6 These models typically use fixed atom-based partial charges to model electrostatics and include polarization response to the environment only in an averaged, mean-field sense. As a result, nonpolarizable potentials that provide excellent descriptions of the homogeneous bulk phase are poor models for gas phase clusters and for nonpolar solutes in polar solvents. For example, the gas phase binding energy of the water dimer is overestimated by more than 30% in the TIP5P model. In application to large biomolecular systems, there is concern that such models cannot correctly account for situations where the same nonpolarizable moiety is exposed to different electrostatic environments, either within a single large static structure or during a course of simulation. In addition, there is an inherent inconsistency in most nonpolarizable models related to thei...

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Coherent X-Ray Scattering for the Hydrogen Atom in the Hydrogen Molecule

Cited by 3,657 publications

References 16 publications

Synthesis and Selected Reactions of Hydrazides Containing an Imidazole Moiety

Synthesis and Selected Reactions of Hydrazides Containing an Imidazole Moiety

Synthesis of 1,3‐Selenazol‐2(3H)‐imines

Polarizable Atomic Multipole Water Model for Molecular Mechanics Simulation

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