* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download I believe the chemical bond is not so simple as people seem to think
Survey
Document related concepts
Franck–Condon principle wikipedia , lookup
Rutherford backscattering spectrometry wikipedia , lookup
Woodward–Hoffmann rules wikipedia , lookup
Homoaromaticity wikipedia , lookup
Aromaticity wikipedia , lookup
Coupled cluster wikipedia , lookup
Ultrafast laser spectroscopy wikipedia , lookup
Electron scattering wikipedia , lookup
Rotational–vibrational spectroscopy wikipedia , lookup
Rotational spectroscopy wikipedia , lookup
Heat transfer physics wikipedia , lookup
Metastable inner-shell molecular state wikipedia , lookup
Atomic orbital wikipedia , lookup
Atomic theory wikipedia , lookup
Molecular orbital wikipedia , lookup
Physical organic chemistry wikipedia , lookup
Transcript
Birth of a bond Lecture 2 I believe the chemical bond is not so simple as people seem to think - R. S. Mulliken concentration (though not of ion activity), since this assumption is made in the calculation from the freezing points, but not in that from the electromotive forces. Happy Centenary! Bos’roN, MASS. [CONTRIBUTION FROM THE CHEMICAL LABOR~TORY OF THE UNIVERSITY OF CAGIFORNIA. I TRE ATOM APQD THE MOLECULE. BY GILBSRT N. LBWIS. Received January 26, 1916. In a paper entitled “Valence and Tautomerism”l I took occasion THISJOURXAL,35,1448 (1913);see also the important article of Bray and Branch, IEd., 35, 14-40 (19x3). To localize or not to localize ‘tis the question • Lewis dots,Valence bond (Heitler-London, Pauling,..) • Molecular Orbitals (Hund, Mulliken,..) • Concept of a bond - a dream or reality? Seeing is believing!? Others are more ambivalent. ‘At present I don’t see a clear way to make a new definition of the chemical bond that is at the same time practical and all-encompassing,’ says Alvarez. ’Probably we need to develop new ways of looking at interatomic interactions, or maybe the improvement of visualisation techniques at the atomic scale will provide relevant information to help us develop a new systematic scheme.’ But he agrees that the Lewis model still ‘works well enough for the largest part of the "real" chemistry, and is a good starting point for the analysis of anomalous cases’. In other words, ‘classifying bonds into "covalent" and "noncovalent" is still something we can teach in high school and early courses at college, since this provides a useful first approximation’. Just don’t believe everything you learn there. Philip Ball, What is a bond? chemistry world 2014 V2s O O2 " O !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! " V*2p 6+$&$'()*+," N2 -.(/".%/)%".0"7"28҂85" S*2p 2p 2p V2p smaller 'E =(>+?)"31@"*A)"V1B"$(/"S1B". ;C+*,A)/"+(")()%':"2;))"()D* " E3F3"4"V1B@"G=F3"4"SH1 S2p V*2s 2s 2s V2s N " " N2 9)%:";*%.('"88"<.(/" N ntal wave ould urier ame elds, pence of rent ep in ame. the the l be N2 . ts as rom muhigh that rate ion n is cket at angle v through the two-A complete inversion of a general orbital requires knowledge of aArgon projection v is equal in aitscut response to strong laser fields, is veryPsimilar to N2to dimensional Fourier of the object. This is the basis the of relative phase and amplitude of each harmonic for two orthohaving nearly the same transform ionization Fpotential and intensity-depen6 computed tomography the inverse transform. . This on is confirmed byRadon the dependence of Our dent ionization probabilitybased 29 dipole is the Fourier transform projection of the atoms, and for wavefunction, different the instantaneous ionization ratesof afor and so canofbeNinverted. orientations 2 (ref. 30). That means that the first, critical, step in the We three-step harmonic generation process is the same.recondescribehigh the mathematical details of the tomographic Because theinlaser field dominates motion the struction the Methods section.wave This packet procedure can in reconstruct direction the laser field, the secondsuch step, which pg and puthe , using orbital of shapes with symmetries as jg, determines be 08 chirp of the re-colliding packets seen by and Ar or 2, will from harmonics 17–51 of anwave 800-nm laser field 25Nangles the Thus, a[k(q)] willneeded be the same. to same. 1808 (fewer angles are for symmetric molecules). The continuum wave packet will also be similar 1,2 1,3 1 for Ar and N2. A complete inversion of a general requires knowledge of, H. Pépin3, J. C. Kieffer3, P. B. Corkum1 & D. M. Villeneuve1 J. Itatani , J. Levesque , D.orbital Zeidler , Hiromichi Niikura1,4 The which the electron tunnels thenarrow relativesaddle phasepoint and through amplitude of each harmonic for acts twoasortho- article Tomographic imaging of molecular orbitals 1 a spatial filter that removes structure the orbital from Ontario K1A 0R6, Canada National Research much Councilofofthe Canada, 100 of Sussex Drive, Ottawa, 2 the continuum waveofpacket. can be seen Ottawa, in numerical University Ottawa,This 150 Louis Pasteur, OntariosimuK1N 6N5, Canada 31 3 measuring the ellipticity dependence of the high lations . By INRS- Energie et Materiaux, 1650 boulevard Lionel-Boulet, CP 1020, Varennes, Québec J3X 1S2, Canada 24 4 by and N2 and argon,Agency, we confirmed thatKawaguchi Saitama, 332-0012, Japan harmonic signal PRESTO,produced Japan Science Technology 4-1-8 Honcho the lateral spread of the wave packets is similar. The ionization rate 30,32 , but is readily measured from the ion of N2 is angle-dependent .................................................................................................................................................................................................................... yield, and varies only by 25% for N2 (ref. 33). This variation is almost cancelled by the angular dependence of the wave-packet Single-electron wavefunctions, or orbitals, are the mathematical constructs used to describe the multi-electron wavefunctio molecules. Because the highest-lying orbitals are responsible for chemical properties, they are of particular interest. To obs these orbitals change as bonds are formed and broken is to observe the essence of chemistry. Yet single orbitals are difficu observe experimentally, and until now, this has been impossible on the timescale of chemical reactions. Here we demonstrate the full three-dimensional structure of a single orbital can be imaged by a seemingly unlikely technique, using high harmo generated from intense femtosecond laser pulses focused on aligned molecules. Applying this approach to a series of molec alignments, we accomplish a tomographic reconstruction of the highest occupied molecular orbital of N2. The method also all us to follow the attosecond dynamics of an electron wave packet. The electrons that make up molecules are organized by energy in alignment19,20. It results in field-free alignment of linear molec orbitals1,2. Although total electron density in molecules is routinely well after the aligning pulse has terminated. A 60-fs laser p measured by X-ray diffraction or electron scattering, only two produces a rotational wave packet in N2. This wave packet peri methods are able to ‘see’ the highest occupied molecular orbitals cally rephases, giving periods when the molecular axes are aligne (HOMOs) — electron momentum spectroscopy and scanning tun- space. The probe pulse coincides with the first half-revival at 4 an (e,4 2e) nelling microscopy. Electron momentum spectroscopy3 isFigure (ref. 20). The degree angular alignment Molecular orbital wavefunction of N2. of a, Reconstructed wavefunctionisofgood the enough to s scattering technique that can determine the radially HOMO averaged the harmonic 58 rotation of N2. The difference reconstructionin is from a tomographicspectra inversion for of thea high harmonic of the alignm thewere outermost electrons. Scanning direction. spectra taken at 19 projection angles. Both positive and negative values are present, so Figure 3 Highdensity harmonic of spectra recorded forvalence N2 molecules aligned at 19 different tunnelling 4 gives thepolarization electronaxisdensity, distorted microscopy this isstates. a wavefunction, not the square of the wavefunction, up to an arbitrary phase. b, The angles between 0 and 908 relative to the of the laser. For clarity,by onlysurface 5–11 of the some of the angles have been The highemission harmonic spectrum from argon is shape jg orbital from an ab initio calculation. The colour scales are the same allows theN2 2pHigh We show thatplotted highabove. harmonic from molecules harmonic generation from molecules also shown; argon is used as the reference atom. Clearly spectra depend on both theorbital for both three-dimensional shape of thethe highest electronic toimages. be c, Cuts along the internuclear axis for the reconstructed (dashed) and High harmonics are produced by ionizing atoms or molecules VOLUME 85, NUMBER 11 PHYSICAL REVIEW LETTERS 11 SEPTEMBER 2000 Determination of the Bond Length and Binding Energy of the Helium Dimer by Diffraction from a Transmission Grating R. E. Grisenti, W. Schöllkopf, and J. P. Toennies Max-Planck-Institut f ür Strömungsforschung, Bunsenstraße 10, 37073 Göttingen, Germany G. C. Hegerfeldt, T. Köhler, and M. Stoll Institut f ür Theoretische Physik, Universität Göttingen, Bunsenstraße 9, 37073 Göttingen, Germany (Received 5 June 2000) A molecular beam consisting of small helium clusters is diffracted from a 100 nm period transmission grating. The relative dimer intensities have been measured out to the 7th order and are used to determine the reduction of the effective slit width resulting from the finite size of the dimer. From a theoretical analysis of the data which also takes into account the van der Waals interaction with the grating bars, the bond length (mean internuclear distance) and the binding energy are found to be !r" ! 52 6 4 Å and jEb j ! 1.1 1 0.3# 2 0.2 mK. PACS numbers: 33.15. – e, 03.75.Be, 21.45. + v, 36.40.Mr Virtually allMeasuring our current knowledge ofathe structure of Focus: molecules is based on either NMR, microwave, infrared, visible, or Raman spectroscopy—or on x-ray and electron Fragile Molecule scattering. Since these techniques all in one way or another disturb the system, they are not suitable for very weakly September 2000• Phys. Focus 6, 9 bound 6,systems withRev. large scattering lengths. Such apparA team of physicists has precisely length and ently exotic systems have measured recently the attracted considerable strength of the exceedingly delicate formed between attention in connection withbond Bose-Einstein condensation two of helium atoms. ultralow temperature gases [1]. © 2000 Boson Photodisc, Inc. An outstanding example of a fragile molecule 4 Butterfly of molecules. catching measuring the is the helium Like dimer Hea2 .butterfly, Its existence was for a long delicate molecule formed by two helium atoms requires a light touch. time disputed because of its extremely small binding enThe helium dimer is the largest two-atom molecule and has the 27 ergy which is now thought to be about 1 mK ($10 eV). weakest chemical bond known. Presently, theory predicts the He-He potential to support only a single s wave bound state [2]. While a first indica- of the dimers at the edges of the holes in a quantum mechanical theory [10]. This theory predicts that transmission experiments provide an upper limit on !r" only. The present experiments overcome these difficulties by measuring only the coherently scattered dimers which make up the intensity in the small angle low order peaks in the diffraction from a transmission grating. From the data, !r" is determined by a quantum mechanical theory which includes the surface potential of the grating bars and breakup effects. Then from !r" the binding energy is determined to be jEb j ! 1.1 1 0.3# 2 0.2 mK, which is the most precise determination to date. The cluster beam diffraction apparatus is the same as recently described in Ref. [11]. The cryogenic 4 He source pressures P0 and temperatures T0 were varied between HF exchange, are the procedures that are presently used to obtain approximate wave functions for the prediction of expectation values for an atom, molecule, or crystal.Invited The eminently useful and predictive models that come from MO theory, the Review crystal field=ligand field descriptions of electronic structures of metal complexes and H€ uThe ckel’s 4n þ 2 rule of aromaticity, for example, exemplify the proper use of Quantum Mechanical Basis MO theory – the prediction of a molecule’s electronic structure by the successive of Conceptual Chemistry occupation of the orbitals. Orbital ordering forms the basis for the application of the ‘second-order Jahn-Teller’ symmetry rule [8] that underlies Fukui’s frontier orbital theory [9]. Richard F. W. Bader! Department of Chemistry, McMaster University, Hamilton ON, L8S 4M1 Canada Impact of Quantum Mechanics on Conceptual Chemistry Received this November 30, 2004; accepted December 6, 2004 Unfortunately linking of quantum mechanics with experiment did not extend Published online May 4, 2005 # Springer-Verlag 2005 to the basic concepts essential to the molecular structure hypothesis – of atoms with characteristic and additive properties and of the bonding underlying molecular Summary. experimentalist approaching literature theory for anstatements understandingtoofthe conceptual chemistry structure. IndeedAnone can find numerous contrary: ‘that that can concepts be related toof measurable properties, focuses the electron density One finds to in the while the atoms and structure areonundeniably usefuldistribution. if not essential topology the not electron density the from definition of an atom, of the bonding atoms, and of the chemistry, theyofare recoverable quantum mechanics’ andbetween the link between boundary condition for the extension of quantum mechanics to an open system – to an atom in a experiment and conceptual theory was not established. Such statements come at the molecule. This paper describes this approach, as it evolved from the failure of existing models to a end of study arbitrary or failed attempts to define the elements of conceptual chemistry of molecular charge distributions and of how these studies resulted in the extension of quantum using either valence bondsystem or molecular orbital theory and are frequently expressed mechanics to an open using the action principle. in terms of the ‘atomic orbital’ basis functions used in their expansion. This is Keywords. Electron density; Atoms; Molecular structure. an acknowledged failure of the orbital approach. Libit and Hoffmann [10], for example, after stressing the logic of substituent effects that made possible the great strides in synthetic and mechanistic organic and inorganic chemistry state: Introduction ‘‘Nothing like this logic comes out of molecular orbital calculations. Every molecule is Conceptual treated as aChemistry whole and no set of transferable properties associated with a functional groupis emerges.’’ Notexperiment, only are the properties of functional groups Science observation, and theory. This is the path that known led to the experimentally to beofcharacteristic, the properties can in somea molecule instancesisappear to development the molecular structure hypothesis – that a collection exhibitofperfect transferability [11], giving rise to by thea network existenceof of experimental atoms with characteristic properties linked bonds that impart a mad me n gy ed he al h rH sad id as r, o ). raht y Real-Space Identification of Intermolecular Bonding with Atomic Force Microscopy Jun Zhang,1* Pengcheng Chen,1* Bingkai Yuan,1 Wei Ji,2† Zhihai Cheng,1† Xiaohui Qiu1† We report a real-space visualization of the formation of hydrogen bonding in 8-hydroxyquinoline (8-hq) molecular assemblies on a Cu(111) substrate, using noncontact atomic force microscopy (NC-AFM). The atomically resolved molecular structures enable a precise determination of the characteristics of hydrogen bonding networks, including the bonding sites, orientations, and lengths. The observation of bond contrast was interpreted by ab initio density functional calculations, which indicated the electron density contribution from the hybridized electronic state of the hydrogen bond. Intermolecular coordination between the dehydrogenated 8-hq and Cu adatoms was also revealed by the submolecular resolution AFM characterization. The direct identification of local bonding configurations by NC-AFM would facilitate detailed investigations of intermolecular interactions in complex molecules with multiple active sites. Controversial? I ntermolecular bonding has been experimentally characterized mainly through crystallography via x-ray and electron diffractions, as well as through infrared, Raman, nuclear magnetic resonance, and near-edge extended absorption fine-structure spectroscopy (1, 2). At the single-molecule level, state-of-the-art scanning tunneling microscopy (STM) is a technique widely used to elucidate the molecular structure and chemical specificity of surface-immobilized spe- high-resolution AFM images, and the local bonding configuration was determined with the atomic precision. We also observe the coordination complex composed of dehydrogenated 8-hq and Cu adatoms. The observations were validated with ab initio density functional theory (DFT) calculations. The 8-hq molecules deposited on Cu(111) at LHe temperature appeared as individual molecules or randomly assembled aggregates (fig. S1) (13). For the single 8-hq molecules, DFT calcu- image (Fig. the heterocy primarily se near the Fer with a COmolecular s range Pauli calculated e itatively rep quency shif AFM senso components electrostatic negative Df attractive v uted to the without ato range Pauli atomic cont spect to the was decreas proportion enhanced co a quantitativ mechanism tween the A ture of a mo case, the he group of 8-h idine ring in which may