Determination Of Molecular Geometry Research Articles

Reconstructing perspectives: investigating how molecular geometry cards (MGCards) and molecular model building (MMB) disrupt students' alternative notions of molecular structure – a qualitative study

The range of abstract concepts encountered when learning chemistry and the inability of students to make connections between the macroscopic, sub-microscopic, and symbolic representations, used in chemistry teaching, are believed to be the main reasons for students’ difficulty when learning chemistry. Prediction and determination of molecular geometry using the theory of valence shell electron pair repulsion (VSEPR) is a sample of the abstract concept that is hard to understand by students who learn chemistry. Students may comprehend these ideas better if the learning process is supplemented with cutting-edge, interactive learning aids. To address the conceptual difficulties that students encounter when learning how to predict the shapes of molecules, a card game (MGCards) has been developed which is supported by simple molecular model building (MMB). The card game allows students to work through the steps required to predict the shape of a molecule in an engaging format. The student learning process is supported by feedback at all stages (if students make a mistake, they receive hints that will help them in the next step of the game). Action research with qualitative methods has been used to design, develop, and evaluate the MGCards. The MGCards and MMB were piloted at the University of Leicester with year one Natural Sciences students and modified based on the feedback received. Both MGCards and MMB were then used as part of the first-year chemistry education programme at Tanjungpura University in Indonesia. The findings of students’ answer analysis (pre- and post-test) in both cycles showed that students had a better understanding after learning with MGCards and MMB. The positive feedback for MGCards and MMB confirmed that these resources were effective in delivering an engaging learning experience. The results suggest that MGCards and MMB play a significant role in enhancing students’ understanding while also keeping them engaged.

Quantum mechanical study of the solvent-dependence of electronic energy transfer rates in a Bodipy closely-spaced dyad

The ability of Förster theory to describe electronic energy transfer rates, and their solvent-dependence, have been studied theoretically in a series of 15 solvents of varying degrees of polarity for a rigid closely-spaced dyad, constituted by two boron dipyrromethene dyes, which was recently studied experimentally by Harriman & Ziessel, Photochem. Photobiol. Sci., 2010, 9, 960. We use time-dependent density functional theory calculations coupled to the polarizable continuum model to analyse the solvent-dependence of the spectroscopic and energy transfer properties of the system. This methodology allows us to examine the impact of the solvent on both electronic (solvent screening) and structural (dipole separation and orientation) factors by consistently incorporating solvent effects in the determination of molecular geometries, transition densities, and electronic couplings. In addition, we analyse the impact of bridge-mediated contributions to the electronic interaction between the dyes. We are therefore able to assess whether a Förster-type point-dipole approximation is valid for the molecular system studied.

Software design of ACES III with the super instruction architecture

AbstractThe Advanced Concepts in Electronic Structure (ACES) III software is a completely rewritten implementation for parallel computer architectures of the most used capabilities in ACES II, including the calculation of the electronic structure of molecular ground states and excited states, and determination of molecular geometries and of vibrational frequencies using many‐body and coupled cluster methods. To achieve good performance on modern parallel systems while simultaneously offering a software development environment that allows rapid implementation of new methods and algorithms, ACES III was written using a new software infrastructure, the super instruction architecture comprising a domain‐specific language, super instruction assembly language (SIAL), and a sophisticated runtime environment, super instruction processor (SIP). The architecture of ACES III is described as well as the inner workings of SIAL and SIP. The execution performance of ACES III and the productivity of programming in SIAL are discussed. © 2011 John Wiley & Sons, Ltd. WIREs Comput Mol Sci 2011 1 895–901 DOI:10.1002/wcms.77This article is categorized under: Software > Quantum Chemistry

Growth of pentacene on Ag(1 1 1) surface: A NEXAFS study

Thin films of pentacene (C 22H 14) have become widely used in the field of organic electronics. Here films of C 22H 14 of thickness ranging from submonolayer to multilayer were thermally deposited on Ag(1 1 1) surface. The determination of molecular geometry in pentacene films on Ag(1 1 1) studied by X-ray absorption at different stages of growth up to one monolayer is presented. XAS spectra at the C K-edge were collected as a function of the direction of the electric field at the surface. The different features of the spectra were assigned to resonances related to the various molecular unoccupied states by the comparison with the absorption coefficient of the pentacene gas phase. The transitions involving antibonding π states show a pronounced angular dependence for all the measured coverages, from submonolayer to multilayer. The spectra analysis indicates a nearly planar chemisorption of the first pentacene layer with a tilt angle of 10°.

Determination of Molecular Geometry in Solid-State NMR: Rotational-Echo Double Resonance of Three-Spin Systems

The precise measurement of internuclear distances in solids by NMR has been widely explored using experiments that measure the dipolar coupling between labeled spin pairs. The rotational-echo double-resonance (REDOR) experiment is one of the most successful techniques for quantifying distances between heteronuclei. In the present work, REDOR is applied to the precise determination of the angle between internuclear vectors in triply labeled spin systems. The time domain REDOR signal for two heteronuclear coupled spin pairs sharing a common partner (an I2S spin system) is derived and analyzed with the aid of dipolar transforms. A two-step experimental approach for the structural analysis of I2S spin systems is then developed. Independent θ-REDOR and traditional REDOR measurements are used to obtain the heteronuclear dipolar coupling constants and the angle between them. Demonstrations are carried out on labeled polycrystalline samples of glycine and uracil. The REDOR experiment is also examined using altern...

Accessible Geometrical Changes

The molecular geometry model has been a fruitful concept in chemistry. Although it is unambiguous for relatively rigid systems in the gas phase, where interactions with other molecules can be ignored, with increasing fluxionality there is increasing ambiguity of the molecular geometry model. A different kind of complication arises in situations where molecules are closely packed, such as in a supermolecule or in a molecular crystal. With enhanced intermolecular interactions the delineation of individual molecular structures may be increasingly difficult. In the early days, there was little concern whether the determination of molecular geometry was carried out on gaseous or crystalline molecules. It was generally assumed that the molecule had one structure regardless of its environment. This was a workable assumption as long as the experimental uncertainty was greater than the differences arising from the absence or presence of intermolecular interactions. Those determining gas-phase molecular structures had expressed concern for small structural differences due to intermolecular interactions before it became a more general concern. The reason was that there were relatively few structure determinations in the gas phase and, more often than not, crystal structures were invoked for comparison. Curiously, the interest in molecular structure dominated crystal structure determinations for

Electronic structure and photoelectron spectra of substituted anilines

The spectroscopically parametrized CNDO/S3 model is used to provide a quantitative description of the valence electron photoemission spectra of a series of substituted anilines. The CNDO/S3 density of valence states was used to determine the twist angles in these molecules via analysis of gas-phase photoemission spectra. These twist angles are compared with prior estimates. Finally, a discussion of the use of photoelectron spectroscopy in the determination of molecular geometries is presented.

X-ray refinement of the crystal and molecular structure of 1,12-dimethylbenz[a]anthracene.

The molecular and crystal structure of 1,12-dimethylbenz[a]anthracene, one of the least carcinogenic of the benz[a]anthracenes, has been refined, from new X-ray diffractometric data collected at room temperature, to an R index 0.047 over 1217 independent reflections. Improved determination of molecular geometry shows that steric strain arising from the presence of two bay-methyl substituents causes even greater molecular distortion than in the highly carcinogenic 7,12-dimethylbenz[a]anthracene, with the benzo A ring inclined at about 29 degrees to the furthest C and D rings. Methyl carbon atoms are displaced 1.0 and 1.3 A on opposite sides of the mean molecular plane and the beach bond of the bay region is 1.480(4) A long, flanked by C-C-C angles of 124.3 and 125.0 degrees. The shortest carbon-carbon bond is C5-C6 = 1.327(5) A at the K region, with the next shortest C8-C9 = 1.346(6) A. Close intramolecular approaches of methyl hydrogens across the bay and to nearest aromatic hydrogens are 2.3-2.6 A.

ChemInform Abstract: STUDY ON PROPARGYL FLUORIDE BY NMR IN NEMATIC PHASE: DETERMINATION OF MOLECULAR GEOMETRY AND SIGNS OF INDIRECT COUPLING CONSTANTS

Chemischer InformationsdienstVolume 14, Issue 7 Physical Organic Chemistry ChemInform Abstract: STUDY ON PROPARGYL FLUORIDE BY NMR IN NEMATIC PHASE: DETERMINATION OF MOLECULAR GEOMETRY AND SIGNS OF INDIRECT COUPLING CONSTANTS E. HALOUI, E. HALOUISearch for more papers by this authorC. BEN NASR, C. BEN NASRSearch for more papers by this authorD. CANET, D. CANETSearch for more papers by this author E. HALOUI, E. HALOUISearch for more papers by this authorC. BEN NASR, C. BEN NASRSearch for more papers by this authorD. CANET, D. CANETSearch for more papers by this author First published: February 15, 1983 https://doi.org/10.1002/chin.198307061AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume14, Issue7February 15, 1983 RelatedInformation

Axis Switching in the Transition of HCO: Determination of Molecular Geometry

The [Formula: see text] band systems of HCO and DCO have been reinvestigated with higher resolving power than in earlier work. Some weak lines have been assigned to K′ = 0 – K″ = 0 and K′ = 0 – K″ = 2 subbands; these lines derive their intensity from axis switching. The new data give values for the A rotational constants of HCO and DCO and permit a more reliable determination of the ground state geometry, viz, r0(CH) = 1.125(5) Å, r0(CO) = 1.175(1) Å, [Formula: see text]. The sign of the spin–rotation constant εaa has been shown to be positive.

Determination of molecular geometry, rotational diffusion constants and coupling constants between quadrupolar nuclei and protons by means of proton N.M.R.

Expressions are given for the longitudinal and transverse relaxation rates of the individual lines in a HRNMR proton AX2 spectrum. The relaxation mechanisms considered are dipolar and scalar interactions. Measurements are performed on the proton spectra of dilute degassed solutions of 2-amino pyrimidine in acetone-d6/D2O. The relative values are determined of the interproton distances in the AX2 system and of three rotational diffusion constants, Dxx , Dyy and Dzz . Two coupling constants between the ring protons and the 14N ring nitrogens are found to be 10·3 and 2·2 Hz.

Anisotropy of Nuclear Spin-Spin Coupling in Methyl Fluoride

Equations for indirect nuclear spin coupling tensors are developed within the framework of Hartree-Fock perturbation theory. Calculated values for CF, CH, HF, and HH spin-spin coupling anisotropies (pseudodipolar couplings) in methyl fluoride are compared with the results suggested by recent experimental work. An estimate of the importance of including pseudodipolar coupling in the determination of molecular geometry by NMR in liquid crystals is made.

Determination of the Symmetry of the First Excited Electronic State of Benzyl by Rotational Contour Analysis of Vibronic Bands of the Emission Spectra of C6H5CH2, C6H5CD2, and C6D5CD2

Theoretical calculations have differed in their predictions as to the symmetry of the first excited state of benzyl. Furthermore, conflicting interpretations have been given to previous experimental results. Rotational band contour analysis of high resolution gas phase emission spectra of C6H5CH2, C6D5CD2, and C6H5CD2 are here used to determine this electronic symmetry. A discussion is given of the treatment of new problems which arise in the band contour analysis of emission spectra and of free radicals as compared with previous contour work on absorption spectra of stable molecules, in particular, as regards the determination of molecular geometry, temperature effects and contour program modifications. Qualitative and quantitative criteria for matching experimental and calculated contours are made explicit. Nonmatching features are assigned to hot bands, using supporting assignments. The effect on calculated contours of the interpolation procedure used in Parkin's program is discussed in the Appendix. Good fit of calculated to experimental contours for type A and type B vibronic bands of the three isotopic benzyl radicals, including the vibronic origin band of the C6H5CH2 species, leads to the conclusion that the first doublet-doublet electronic transition in benzyl is 1 2A2–1 2B2. This agrees with an earlier experimental assignment, based on the observation of a2 vibrations in the fluorescence spectrum of benzyl in crystalline cyclohexane solution but is in disagreement with the 2 2B2–1 2B2 assignment of Johnson and Albrecht based on three-step photoselection studies in glassy solutions.

Additions and Corrections - Determination of Molecular Geometry by Quantitative Application of the Nuclear Overhauser Effect

Additions and Corrections - Determination of Molecular Geometry by Quantitative Application of the Nuclear Overhauser Effect

Determination of molecular geometry by quantitative application of the nuclear Overhauser effect

Determination of molecular geometry by quantitative application of the nuclear Overhauser effect