Theoretical Methods That Help Understanding the Structure and Reactivity of Gas Phase Ions
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Chemistry 2000 Slide Set 1: Introduction to the Molecular Orbital Theory
Chemistry 2000 Slide Set 1: Introduction to the molecular orbital theory Marc R. Roussel January 2, 2020 Marc R. Roussel Introduction to molecular orbitals January 2, 2020 1 / 24 Review: quantum mechanics of atoms Review: quantum mechanics of atoms Hydrogenic atoms The hydrogenic atom (one nucleus, one electron) is exactly solvable. The solutions of this problem are called atomic orbitals. The square of the orbital wavefunction gives a probability density for the electron, i.e. the probability per unit volume of finding the electron near a particular point in space. Marc R. Roussel Introduction to molecular orbitals January 2, 2020 2 / 24 Review: quantum mechanics of atoms Review: quantum mechanics of atoms Hydrogenic atoms (continued) Orbital shapes: 1s 2p 3dx2−y 2 3dz2 Marc R. Roussel Introduction to molecular orbitals January 2, 2020 3 / 24 Review: quantum mechanics of atoms Review: quantum mechanics of atoms Multielectron atoms Consider He, the simplest multielectron atom: Electron-electron repulsion makes it impossible to solve for the electronic wavefunctions exactly. A fourth quantum number, ms , which is associated with a new type of angular momentum called spin, enters into the theory. 1 1 For electrons, ms = 2 or − 2 . Pauli exclusion principle: No two electrons can have identical sets of quantum numbers. Consequence: Only two electrons can occupy an orbital. Marc R. Roussel Introduction to molecular orbitals January 2, 2020 4 / 24 The hydrogen molecular ion The quantum mechanics of molecules + H2 is the simplest possible molecule: two nuclei one electron Three-body problem: no exact solutions However, the nuclei are more than 1800 time heavier than the electron, so the electron moves much faster than the nuclei. -
More Insight in Multiple Bonding with Valence Bond Theory K
More insight in multiple bonding with valence bond theory K. Hendrickx, B. Braida, P. Bultinck, P.C. Hiberty To cite this version: K. Hendrickx, B. Braida, P. Bultinck, P.C. Hiberty. More insight in multiple bonding with va- lence bond theory. Computational and Theoretical Chemistry, Elsevier, 2015, 1053, pp.180–188. 10.1016/j.comptc.2014.09.007. hal-01627698 HAL Id: hal-01627698 https://hal.archives-ouvertes.fr/hal-01627698 Submitted on 21 Nov 2017 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. More insight in multiple bonding with valence bond theory ⇑ ⇑ K. Hendrickx a,b,c, B. Braida a,b, , P. Bultinck c, P.C. Hiberty d, a Sorbonne Universités, UPMC Univ Paris 06, UMR 7616, LCT, F-75005 Paris, France b CNRS, UMR 7616, LCT, F-75005 Paris, France c Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), 9000 Gent, Belgium d Laboratoire de Chimie Physique, UMR CNRS 8000, Groupe de Chimie Théorique, Université de Paris-Sud, 91405 Orsay Cédex, France abstract An original procedure is proposed, based on valence bond theory, to calculate accurate dissociation ener- gies for multiply bonded molecules, while always dealing with extremely compact wave functions involving three valence bond structures at most. -
Theory and Technique
Lawrence Berkeley National Laboratory Lawrence Berkeley National Laboratory Title COMPUTATIONAL METHODS FOR MOLEUCLAR STRUCTURE DETERMINATION: THEORY AND TECHNIQUE Permalink https://escholarship.org/uc/item/3b7795px Author Lester, W.A. Publication Date 1980-03-14 Peer reviewed eScholarship.org Powered by the California Digital Library University of California CONTENTS Foreword v List of Invited Speakers vii Workshop Participants viii LECTURES 1 Introduction to Computational Quantum Chemistry Ernest R. Davidson 1-1 2/3 Introduction to SCF Theory Ernest R. Davidson 2/3-1 4 Semi-Empirical SCF Theory Michael C. Zerner 4-1 5 An Introduction to Some Semi-Empirical and Approximate Molecular Orbital Methods Miehael C. Zerner 5-1 6/7 Ab Initio Hartree Fock John A. Pople 6/7-1 8 SCF Properties Ernest R. Davidson 8-1 9/10 Generalized Valence Bond William A, Goddard, III 9/10-1 11 Open Shell KF and MCSCF Theory Ernest R. Davidson 11-1 12 Some Semi-Empirical Approaches to Electron Correlation Miahael C. Zerner 12-1 13 Configuration Interaction Method Ernest R. Davidson 13-1 14 Geometry Optimization of Large Systems Michael C. Zerner 14-1 15 MCSCF Calculations: Results Boaen Liu 15-1 lfi/18 Empirical Potentials, Semi-Empirical Potentials, and Molecular Mechanics Norman L. Allinger 16/18-1 iv 17 CI Calculations: Results Bowen Liu 17-1 19 Computational Quantum Chemistry: Future Outlook Ernest R. Davidson 19-1 V FOREWORD The National Resource for Computation in Chemistry (NRCC) was established as a Division of Lawrence Berkeley Laboratory (LBL) in October 1977. The functions of the NRCC may be broadly categorized as follows: (1) to make information on existing and developing computa tional methodologies available to all segments of the chemistry community, (2) to make state-of-the-art computational facilities (both hardware and software] accessible to the chemistry community, and (3) to foster research and development of new computational methods for application to chemical problems. -
An Experimental Chemist's Guide to Ab Initio Quantum Chemistry
J. Phys. Chem. 1991, 95, 1017-1029 1017 FEATURE ARTICLE An Experimental Chemist’s Guide to ab Initio Quantum Chemistry Jack Simons Chemistry Department, University of Utah, Salt Lake City, Utah 84112 (Received: October 5, 1990) This article is not intended to provide a cutting edge, state-of-the-art review of ab initio quantum chemistry. Nor does it offer a shopping list of estimates for the accuracies of its various approaches. Unfortunately, quantum chemistry is not mature or reliable enough to make such an evaluation generally possible. Rather, this article introduces the essential concepts of quantum chemistry and the computationalfeatures that differ among commonly used methods. It is intended as a guide for those who are not conversant with the jargon of ab initio quantum chemistry but who are interested in making use of these tools. In sections I-IV, readers are provided overviews of (i) the objectives and terminology of the field, (ii) the reasons underlying the often disappointing accuracy of present methods, (iii) and the meaning of orbitals, configurations, and electron correlation. The content of sections V and VI is intended to serve as reference material in which the computational tools of ab initio quantum chemistry are overviewed. In these sections, the Hartree-Fock (HF), configuration interaction (CI), multiconfigurational self-consistent field (MCSCF), Maller-Plesset perturbation theory (MPPT), coupled-cluster (CC), and density functional methods such as X, are introduced. The strengths and weaknesses of these methods as well as the computational steps involved in their implementation are briefly discussed. 1. What Does ab Initio Quantum Chemistry Try To Do? quantitative predictions to be made. -
Introduction to Molecular Orbital Theory
Chapter 2: Molecular Structure and Bonding Bonding Theories 1. VSEPR Theory 2. Valence Bond theory (with hybridization) 3. Molecular Orbital Theory ( with molecualr orbitals) To date, we have looked at three different theories of molecular boning. They are the VSEPR Theory (with Lewis Dot Structures), the Valence Bond theory (with hybridization) and Molecular Orbital Theory. A good theory should predict physical and chemical properties of the molecule such as shape, bond energy, bond length, and bond angles.Because arguments based on atomic orbitals focus on the bonds formed between valence electrons on an atom, they are often said to involve a valence-bond theory. The valence-bond model can't adequately explain the fact that some molecules contains two equivalent bonds with a bond order between that of a single bond and a double bond. The best it can do is suggest that these molecules are mixtures, or hybrids, of the two Lewis structures that can be written for these molecules. This problem, and many others, can be overcome by using a more sophisticated model of bonding based on molecular orbitals. Molecular orbital theory is more powerful than valence-bond theory because the orbitals reflect the geometry of the molecule to which they are applied. But this power carries a significant cost in terms of the ease with which the model can be visualized. One model does not describe all the properties of molecular bonds. Each model desribes a set of properties better than the others. The final test for any theory is experimental data. Introduction to Molecular Orbital Theory The Molecular Orbital Theory does a good job of predicting elctronic spectra and paramagnetism, when VSEPR and the V-B Theories don't. -
3-MO Theory(U).Pptx
Molecular Orbital Theory Valence Bond Theory: Electrons are located in discrete pairs between specific atoms Molecular Orbital Theory: Electrons are located in the molecule, not held in discrete regions between two bonded atoms Thus the main difference between these theories is where the electrons are located, in valence bond theory we predict the electrons are always held between two bonded atoms and in molecular orbital theory the electrons are merely held “somewhere” in molecule Mathematically can represent molecule by a linear combination of atomic orbitals (LCAO) ΨMOL = c1 φ1 + c2 φ2 + c3 φ3 + cn φn Where Ψ2 = spatial distribution of electrons If the ΨMOL can be determined, then where the electrons are located can also be determined 66 Building Molecular Orbitals from Atomic Orbitals Similar to a wave function that can describe the regions of space where electrons reside on time average for an atom, when two (or more) atoms react to form new bonds, the region where the electrons reside in the new molecule are described by a new wave function This new wave function describes molecular orbitals instead of atomic orbitals Mathematically, these new molecular orbitals are simply a combination of the atomic wave functions (e.g LCAO) Hydrogen 1s H-H bonding atomic orbital molecular orbital 67 Building Molecular Orbitals from Atomic Orbitals An important consideration, however, is that the number of wave functions (molecular orbitals) resulting from the mixing process must equal the number of wave functions (atomic orbitals) used in the mixing -
B3.1 Quantum Structural Methods for Atoms and Molecules
Quantum structural methods for atoms and molecules 1907 B3.1 Quantum structural methods for atoms and molecules Jack Simons B3.1.1 What does quantum chemistry try to do? Electronic structure theory describes the motions of the electrons and produces energy surfaces and wave- functions. The shapes and geometries of molecules, their electronic, vibrational and rotational energy levels, as well as the interactions of these states with electromagnetic fields lie within the realm of quantum structure theory. B3.1.1.1 The underlying theoretical basis—the Born–Oppenheimer model In the Born–Oppenheimer [1] model, it is assumed that the electrons move so quickly that they can adjust their motions essentially instantaneously with respect to any movements of the heavier and slower atomic nuclei. In typical molecules, the valence electrons orbit about the nuclei about once every 10−15 s (the inner-shell electrons move even faster), while the bonds vibrate every 10−14 s, and the molecule rotates approximately every 10−12 s. So, for typical molecules, the fundamental assumption of the Born–Oppenheimer model is valid, but for loosely held (e.g. Rydberg) electrons and in cases where nuclear motion is strongly coupled to electronic motions (e.g. when Jahn–Teller effects are present) it is expected to break down. This separation-of-time-scales assumption allows the electrons to be described by electronic wavefunc- tions that smoothly ‘ride’ the molecule’s atomic framework. These electronic functions are found by solving ˆ a Schrodinger¨ equation whose Hamiltonian He contains the kinetic energy Te of the electrons, the Coulomb repulsions among all the molecule’s electrons Vee, the Coulomb attractions Ven among the electrons and all of the molecule’s nuclei, treated with these nuclei held clamped, and the Coulomb repulsions Vnn among all of these nuclei, but it does not contain the kinetic energy TN of all the nuclei. -
Simple Molecular Orbital Theory Chapter 5
Simple Molecular Orbital Theory Chapter 5 Wednesday, October 7, 2015 Using Symmetry: Molecular Orbitals One approach to understanding the electronic structure of molecules is called Molecular Orbital Theory. • MO theory assumes that the valence electrons of the atoms within a molecule become the valence electrons of the entire molecule. • Molecular orbitals are constructed by taking linear combinations of the valence orbitals of atoms within the molecule. For example, consider H2: 1s + 1s + • Symmetry will allow us to treat more complex molecules by helping us to determine which AOs combine to make MOs LCAO MO Theory MO Math for Diatomic Molecules 1 2 A ------ B Each MO may be written as an LCAO: cc11 2 2 Since the probability density is given by the square of the wavefunction: probability of finding the ditto atom B overlap term, important electron close to atom A between the atoms MO Math for Diatomic Molecules 1 1 S The individual AOs are normalized: 100% probability of finding electron somewhere for each free atom MO Math for Homonuclear Diatomic Molecules For two identical AOs on identical atoms, the electrons are equally shared, so: 22 cc11 2 2 cc12 In other words: cc12 So we have two MOs from the two AOs: c ,1() 1 2 c ,1() 1 2 2 2 After normalization (setting d 1 and _ d 1 ): 1 1 () () [2(1 S )]1/2 12 [2(1 S )]1/2 12 where S is the overlap integral: 01 S LCAO MO Energy Diagram for H2 H2 molecule: two 1s atomic orbitals combine to make one bonding and one antibonding molecular orbital. -
Conjugated Systems, Orbital Symmetry and UV Spectroscopy
Conjugated Systems, Orbital Symmetry and UV Spectroscopy Introduction There are several possible arrangements for a molecule which contains two double bonds (diene): Isolated: (two or more single bonds between them) Conjugated: (one single bond between them) Cumulated: (zero single bonds between them: allenes) Conjugated double bonds are found to be the most stable. Ch15 Conjugated Systems (landscape).docx Page 1 Stabilities Recall that heat of hydrogenation data showed us that di-substituted double bonds are more stable than mono- substituted double bonds. H2, Pt Ho= -30.0kcal H2, Pt Ho= -27.4kcal When a molecule has two isolated double bonds, the heat of hydrogenation is essentially equal to the sum of the values for the individual double bonds. H2, Pt Ho= -60.2kcal For conjugated dienes, the heat of hydrogenation is less than the sum of the individual double bonds. H2, Pt Ho= -53.7kcal The conjugated diene is more stable by about 3.7kcal/mol. (Predicted –30 + (–27.4) = –57.4kcal, observed –53.7kcal). Ch15 Conjugated Systems (landscape).docx Page 2 Allenes, which have cumulated double bonds are less stable than isolated double bonds. H H H2, Pt C C C Ho= -69.8kcal H CH2CH3 Increasing Stability Order (least to most stable): Cumulated diene -69.8kcal Terminal alkyne -69.5kcal Internal alkyne -65.8kcal Isolated diene -57.4kcal Conjugated diene -53.7kcal Ch15 Conjugated Systems (landscape).docx Page 3 Molecular Orbital (M.O.) Picture The extra stability of conjugated double bonds versus the analogous isolated double bond compound is termed the resonance energy. Consider buta-1,3-diene: H2, Pt Ho= -30.1kcal H2, Pt Ho= -56.6kcal (2 x –30.1 = –60.2kcal). -
The Size-Consistency Problem in Configuration Interaction Calculation
The Size-Consistency Problem in Configuration Interaction calculation P´eter G. Szalay E¨otv¨osLor´and University Institute of Chemistry H-1518 Budapest, P.O.Box 32, Hungary [email protected] P.G. Szalay: Size-consistency corrected CI Rio de Janeiro, Nov. 27 - Dec. 2, 2005 Size-consistency Consider two subsystems at infinite separation. We have two choices: • treat the two system separately; • consider only a super-system. Provided that there is no interaction between the two systems, the two treatments should give the same result, a basic physical requirement. E¨otv¨osLor´and University, Institute of Chemistry 1 P.G. Szalay: Size-consistency corrected CI Rio de Janeiro, Nov. 27 - Dec. 2, 2005 Size-consistency Let us use the CID wave function to describe this system! For the super − system we have : ΨCID = ΦHF + ΦD (1) ΦD is the sum all double excitations out of ΦHF (including coefficients). For the subsystems we can write: A A A ΨCID = ΦHF + ΦD (2) B B B ΨCID = ΦHF + ΦD (3) The product of these two wave functions gives the other choice for the wave function of the super-system: A+B A B ΨCID = ΨCID ΨCID (4) A B A B B A A B = ΦHF ΦHF + ΦHF ΦD + ΦHF ΦD + ΦD ΦD A B = ΦHF + ΦD + ΦD ΦD E¨otv¨osLor´and University, Institute of Chemistry 2 P.G. Szalay: Size-consistency corrected CI Rio de Janeiro, Nov. 27 - Dec. 2, 2005 Size-consistency This simple model enables us to identify the origin of the size-consistency error: The difference of the two super-system wave functions: A B A B ΨCID ΨCID − ΨCID = ΦD ΦD (5) i.e. -
An Introduction to Molecular Orbital Theory
Objectives of the course • Wave mechanics / Atomic orbitals (AOs) – The basis for rejjgecting classical mechanics ( the Bohr Model) in treating An introduction to electrons – Wave mechanics and the Schrödinger equation Molecular Orbital Theory – Representatifion of atom ibilic orbitals as wave fifunctions – Electron densities and radial distribution functions – Understanding the effects of shielding and penetration on AO energies 6 Lecture Course • Bonding – Review VSEPR and Hybridisation Prof G. W. Watson – Linear combination of molecular orbitals (LCAO), bonding / antibonding Lloyd d Ins titut e 2 .36 – La be lling o f mo lecu lar or bital s (MO s) ( σ, π and)d g, u) [email protected] – Homonuclear diatomic MO diagrams – mixing of different AO’s – More complex molecules (CO , H 2O)O ….) – MO diagrams for Inorganic complexes 2 Lecture schedule Literature Lecture 1 Revision of Bohr model of atoms • Book Sources: all titles listed here are available in the Hamilton Library Lecture 2 Schrödinger equation, atomic wavefunctions and radial – 1. Chemical Bonding, M. J. Winter (Oxford Chemistry primer 15) distribution functions of s orbitals Oxford Science Publications ISBN 0 198556942 – condensed text, excellent diagrams Lecture 3 More complex wavefunctions and radial distribution functions and electron shielding – 2. Basic Inorggy(y),,anic Chemistry (Wiley) F.A.Cotton, G. Wilkinson, P. L. Gaus – comprehensive text, very detailed on aufbau principle Lecture 4 Lewis bonding, Hybridisation, and molecular orbitals – 3. Inorgan ic Chemi stry (P renti -
Acronyms Used in Theoretical Chemistry
Pure & Appl. Chem., Vol. 68, No. 2, pp. 387-456, 1996. Printed in Great Britain. INTERNATIONAL UNION OF PURE AND APPLIED CHEMISTRY PHYSICAL CHEMISTRY DIVISION WORKING PARTY ON THEORETICAL AND COMPUTATIONAL CHEMISTRY ACRONYMS USED IN THEORETICAL CHEMISTRY Prepared for publication by the Working Party consisting of R. D. BROWN* (Australia, Chiman);J. E. BOWS (USA); R. HILDERBRANDT (USA); K. LIM (Australia); I. M. MILLS (UK); E. NIKITIN (Russia); M. H. PALMER (UK). The focal point to which to send comments and suggestions is the coordinator of the project: RONALD D. BROWN Chemistry Department, Monash University, Clayton, Victoria 3 168, Australia. Responses by e-mail would be particularly appreciated, the number being: rdbrown @ vaxc.cc .monash.edu.au another alternative is fax at: +61 3 9905 4597 Acronyms used in theoretical chemistry synogsis An alphabetic list of acronyms used in theoretical chemistry is presented. Some explanatory references have been added to make acronyms better understandable but still more are needed. Critical comments, additional references, etc. are requested. INTRODUC'IION The IUPAC Working Party on Theoretical Chemistry was persuaded, by discussion with colleagues, that the compilation of a list of acronyms used in theoretical chemistry would be a useful contribution. Initial lists of acronyms drawn up by several members of the working party have been augmented by the provision of a substantial list by Chemical Abstract Service (see footnote below). The working party is particularly grateful to CAS for this generous help. It soon became apparent that many of the acron ms needed more than mere spelling out to make them understandable and so we have added expr anatory references to many of them.