Theoretical Models of Chemical Bonding: Molecular Spectroscopy, Electronic Structure and Intramolecular Interactions Part 3 PDF
Preview Theoretical Models of Chemical Bonding: Molecular Spectroscopy, Electronic Structure and Intramolecular Interactions Part 3
Theoretical Models of Chemical Bonding Part 3 Molecular Spectroscopy, Electronic Structure and Intramolecular Interactions Editor: Z. B. Maksic With contributions by G. Alagona, F. Bernardi, 1. E. Boggs, R. Bonaccorsi, R. Cammi, E. R. Davidson, M. Eckert-Maksic, D. Feller, H. P. Figeys, P. Geerlings, C. Ghio, Y. Harada, E. Heilbronner, E. Honegger, C.1. Jameson, K. Jug, L. Klasinc, M. Klessinger, 1. Kowalewski, A. Laaksonen, K. T. Leung, Z. B. Maksic, S. P. McGlynn, K. Ohno, M. Olivucci, T. M. A. Robb, 1. Tomasi, K. Wittel Pătter, With 172 Figures and 126 Tables Springer-Verlag Berlin Heidelberg GmbH Professor Dr. Zvonimir B. Maksic Theoretical Chemistry Group The "Rudjer Boskovic" Institute 41001 Zagreb, Bijenicka 54, Croatia/Yugoslavia and Faculty of Natural Sciences and Mathematics University of Zagreb 41000 Zagreb, Marulicev trg 19, Croatia/Yugoslavia ISBN 978-3-642-63493-2 ISBN 978-3-642-58179-3 (eBook) DOI 10.1007/978-3-642-58179-3 This work is subject to copyright. Ali rights are reserved, whether the whole of part of the material is concemed, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in other ways, and storage in data banks. Duplication of this publication or parts thereof is only permitted under the provisions ofthe German Copyright Law of September 9, 1965, in its current version, and a copyright fee must always be paid. © Springer-Verlag Berlin Heidelberg 1991 Originally published by Springer-Verlag Berlin Heidelberg New York in 1991 The use of registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. VA51j3020-543210 - Printed on acid-free paper To the memory ofmy parents Olivera and Branko Maksic Preface The renowned theoretical physicist Victor F. Weisskopf rightly pointed out that a real understanding of natural phenomena impliesacleardistinction betweentheessentialand the peripheral. Only when we reach such an understanding - that is to say when we are able to separate the relevant from the irrelevant, will the phenomena no longer appear complex, but intelectually transparent. This statement, which is generally valid, reflects the veryessence ofmodelling in the quantum theory ofmatter, on the molecular level in particular. Indeed, without theoretical models one would beswamped by too manydetails embodied in intricate accurate molecular wavefunctions. Further, physically justified simplificqtions enable studies ofthe otherwise intractable systems and/or phenomena. Finally, a lack of appropriate models would leave myriads of raw experimental data totally unrelated and incomprehensible. Thepresentseriesofbooksdwellsonthemostimportantmodels ofchemical bonding and on the variety of its manifestations. In this volume the electronic structure and properties of molecules are considered in depth. Particular attention is focused on the nature of intramolecular interactions which in turn are revealed byvarioustypesofmolecularspectroscopy.Emphasisisputonthe conceptual and interpretive aspects ofthe theory in line with the general philosophy adopted in the series. The book commences with the theoretical treatments ofvibra tions of nuclei, calculations of the force constants and infrared intensities. Then the theoretical basis of photoelectron spec troscopy is laid down followed by the interpretation of the PE spectraprovidedbythebondorbitalmodel.Considerableattention is paid to the important concepts ofthrough-space and through bond interactions between more or less separated molecular fragments as mirrored in PE spectroscopy. Penning ionization technique yielding information on the shape of the periphery of molecules is discussed next. The fundamental but elusive concept ofatomic charge in molecular environments deserved a separate chapter. It serves as an introduction to the article on ESCA spectroscopywhich probablygives the mostdirect insight into the VIII Preface atomic point-charge distributions in molecules. The next chapter showswhatcanbelearnedfromstudiesofmoleculesinmomentum space. Awealth ofinformation on thefine details ofthe electronic charge distributions in molecules is provided by the experimental data and the theoretical parameters of NMR and ESR spec troscopies which are considered in the following two articles. Theimportanceofadetailedknowledgeofthemolecularpotential energy surfaces is stressed in discussions of the influence of rovibrationsontheaveragepropertiesandinadescriptionofsome salient features of molecules exhibited in excited states. This is followed by an extensive and illuminative survey of the semi classical methods designed for understanding and interpretation of intramolecular interactions. Finally, the last chapter offers a brief presentation of the diabatic model in analysing potential energy surfaces and its use in treating chemical reactions. It represents a transition to the fourth volume of the series, where intermolecular interactions and reactivity will be elaborated in much more detail. Ceterum censeo, it is perfectly clear by now that the best description ofmolecular systems is obtained by combined use of experimentaltechniquesandtheoreticalmethods.Thewholeseries is constructed in a way to build bridges between these two traditionally separated approaches. Ifwe have succeeded, at least partly,inachievingthisgoal,oureffortswillbegreatlyrewarded. I would like to express by sincere gratitude to all the authors ofthisbookfortheirvaluablecontributions.ApartoftheEditorial workhasbeenperformedat theOrganisch-chemischesInstitutder Universitat Heidelberg and I would like to thank the Alexander von Humboldt-Stiftung for financial support and Professor R. Gleiter for his hospitally. Z. B. Maksic Table of Contents Nuclear Vibrations and Force Constants 1. E. Boggs . Some Aspects ofthe Quantumchemical Interpretation ofIntegrated Intensities ofInfrared Absorption Bands H. P. Figeys and P. Geerlings . . . . . . . . . . 25 The Orbital Concept as a Foundation for Photoelectron Spectroscopy S. P. McGlynn, K. Wittel and L. Klasinc . . . . . . 63 The Equivalent Bond Orbital Model and the Interpretation ofPE Spectra E. Honegger and E. Heilbronner . . . . . . . . . .. 99 Through-space and Through-bond Interactions as Mirrored in Photoelectron Spectra M. Eckert-Maksic . . . . . . . . . . . . . .. . 153 Penning Ionization - The Outer Shape ofMolecules K. Ohno and Y. Harada . . . . . . . . . . . . 199 TheMeaningandDistributionofAtomicChargesinMolecules K. Jug and Z. B. Maksic . . . . . . . . . . . .. 235 Electron Spectroscopy for Chemical Analysis (ESCA) Basic Features and Their Model Description Z. B. Maksic . . . . . . . . . . . . . . . .. .. 289 Experimental Momentum-Space Chemistry by (e,2e) Spectroscopy K. T. Leung . . . 339 Theoretical Parameters of NMR Spectroscopy 1. Kowalewski and A. Laaksonen 387 x Table ofContents Theoretical Approaches to ESR Spectroscopy D. Feller and E. R. Davidson . . . . . . . 429 Rovibrational Averaging ofMolecular Electronic Properties C.1.Jameson . . . . . . . . . . . . . . . . . . . . 457 Properties ofMolecules in Excited States M. Klessinger and T. Potter ..... . 521 Semiclassical Interpretation ofIntramolecular Interactions 1.Tomasi,G. Alagona,R. Bonaccorsi,C. GhioandR. Cammi 545 The Analysis ofPotential Energy Surfaces in Terms ofthe Diabatic Surface Model F. Bernardi, M. Olivucci and M. A. Robb . . . . . . . . 615 Nuclear Vibrations and Force Constants James E. Boggs Department ofChemistry, The University ofTexas, Austin, Texas 78712, U.S.A. While valuable information on chemical bonding can be obtained from studies of the static structure of a bound system, even more understanding can be obtained from the internal restoring forces which control molecular vibrational motions and the mechanical response of molecules to external forces. This chapter deals with the description ofsuch motions in terms ofmolecularharmonicand anharmonic vibrational force constants. Methodsaredescribedfor the experimental evaluation ofthese parameters from spectroscopy, but the primary emphasis isontheircomputationaldeterminationsincethisapproachcangivemorecompleteandaccurate informationformedium-sized polyatomicmolecules.Considerationisgiventotechniqueswhich can provide the highest possible accuracy within the harmonic oscillator approximation for polyatomicmoleculesandalsotheprogressthat has been made infully anharmonicevaluation ofthe molecular vibrational potential energy hypersurfaces and the anharmonic energy levels on these surfaces. 1 Introduction . . . . . . . . . . . . . . 2 2 Vibrational Force Fields and Energy Levels 2 2.1 Potential Energy Surfaces for Nuclear Motions 3 2.1.1 Diatomic Molecules . . . . . 3 2.1.2 Polyatomic Molecules . 4 2.1.3 Vibrational Coordinate Systems . . . . 4 2.2 Harmonic OscillatorVibrational Energy Levels 6 3 Theoretical Determination ofthe Potential Surface 8 3.1 Computation ofEnergy Derivatives 8 3.1.1 Energy Gradients 8 3.1.2 Higher Derivatives . . . 8 3.2 Accuracy and Source ofErrors 10 3.3 Empirical Corrections . . . . 13 3.3.1 Scale Factors . . . . . 13 3.3.2 Semi-Empirical Methods 15 3.4 Recommendations ..... 16 4 Determination ofAnharmonic Vibrational Energy Levels 17 4.1 Anharmonic Vibrational Potential Energy Surfaces 17 4.2 Anharmonic Vibrational Energy Levels and Spectra 18 4.3 Utility ofAnharmonic Calculations 19 5 Applications . . . 20 6 Acknowledgements 23 7 References . . . . 23 2 JamesE. Boggs 1 Introduction Chapter 7 ofVolume 1 ofthis series discusses theoretical methods for determining the equilibrium structure of polyatomic molecules. The result of such studies is information on the static, minimum-energy molecular geometry, not the time dependent structure of the twisting, stretching, distorting, real molecule with all of the vibrational motions it has in even its ground vibrational state. In Chapter 6 volume 1, the common experimental approaches for determining structurearesurveyed.Thesedo not give direct information on the equilibrium state ofthe molecule, but rather provide a structure which is some sort ofa time average over all the vibrational motions. The type of average produced depends on the experimental method used, and may be taken over only the molecular motion in some particular quantum state or it may be over all of the states occupied by an assembly ofmolecules at the temperature ofthe experiment. In any case, even ifthe experiment does involve a time average, the average is again a static quantity. Deeperinsightintomolecularbondingmustinvolvemorethananunderstandingof the bond distances and angles that would be present if the system were in some frozen conformation. Consider how a molecule would be examined if it were of macroscopic size so that it could be picked up and handled. Aside from noting its average shape and size, we would try to analyzeits spontaneous squirming motions, deduce characteristic frequencies and amplitudes of motion, squeeze it in different waystoseehowitresistedsuchpressureandhowitsprangbackafterbeingdeformed, and generally poke it and see how it responded. All of this information, as well as staticaverages,can help in drawing the generalizations about structure and bonding that are at the heart ofchemistry. 2 Vibrational Force Fields and Energy Levels Within the Born-Oppenheimer approximation, the structure and internal dynamics of a molecule are described by the properties of a system of electrons and nuclei moving subject to a potential energy surface which is a function of the position coordinates ofthe nuclei. This multi-dimensional energy surface can be calculated by the methods ofquantum chemistry with varying degrees ofaccuracy depending onitsdimensionality.Thepositionsoflocalenergyminimagiveequilibriumstructures ofthe system while energy saddle points provide reaction transition states. While theory can provide information regarding the energy surface with relative ease, it is considerably more difficult to evaluate the steady-state vibrational eigenfunctions of the system and a great deal more difficult to determine the time-dependentsolutionswhichgivethereactiondynamicsofthesystem.Experiment, on the otherhand, providesdirectinformation on the difference betweenvibrational eigenvalues, as observed in the transition frequencies seen in infrared absorption or Raman scattering. For reactions, experiment reveals full information about the reactants and products and some newer state-selected experiments give important
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