Showing papers on "HOMO/LUMO published in 1992"

A theoretical study of linear carbon cluster monoanions, C−n, and dianions, C2−n (n=2–10)

Abstract: A large number of carbon cluster monoanions, C−n, have now been detected by negative ion photoelectron spectroscopy. In addition, evidence for carbon cluster dianions, C2−n, as small as C2−7 has been obtained mass spectrometrically. In this research we report results of theoretical calculations of structures and energetics of formation of linear carbon cluster monoanions and dianions containing up to ten carbon atoms. A number of different electronic states have been investigated. Self‐consistent field (SCF) theory, many‐body perturbation theory, and coupled‐cluster theory including triple excitations have been used with basis sets containing polarization and diffuse functions. Considerably larger basis sets have also been used in calculations on some of the smaller species. For the monoanions, the observed electron detachment energies and the even–odd alternation thereof are well reproduced by the calculations. For the dianions, the even numbered species are found to be more easily formed than the odd numbered species, in accord with the intensity pattern observed in the mass spectrometric experiments, and with the availability of partially occupied π orbitals. C2−10 is established to be vertically and adiabatically stable to electron loss, while C2−8 is found to be vertically stable but adiabatically unstable to electron loss. Improved calculations may be sufficient to make C2−8 also stable to adiabatic electron loss. C2−7 and C2−9 are both found to be unstable to vertical electron loss, although both have negative highest occupied molecular orbital (HOMO) eigenvalues and C2−9 is stable to vertical electron loss at the SCF level. The geometry changes resulting from the addition of two electrons are significant, especially for the even numbered clusters. Addition of two electrons to the partially occupied π orbitals of the latter leads to strong single–triple bond alternation, which may be rationalized by noting that the dianions are products of double deprotonation of HC2nH. Such an ‘‘accordion’’ mechanism may have a role in the ability of carbon clusters to conduct electricity.

Spectroscopic and theoretical studies on the excited state in diimine dithiolate complexes of platinum(II)

Abstract: The photophysical properties of a series of Pt(N-N)(S-S) complexes have been studied where (N-N) is either an [alpha],[alpha][prime]-diimine or saturated diamine chelating ligand and (S-S) is either a dithiolate chelating ligand or two monothiolate ligands in order to determine the orbital composition of the excited state. The solvent dependence of the absorption spectra of these complexes and the temperature dependence of their emission intensities and lifetimes have been examined while the ligands have been systematically varied. The electronic spectra are found to be dependent on whether or not the nitrogen chelating ligand is unsaturated (contains a vacant [pi]* orbital). On the basis of the spectroscopic data, the lowest energy absorption band in the diimine complexes is assigned as a metal-dithiolate to [pi]*(diimine) transition, whereas in the diamine complexes it is assigned as a metal-to-dithiolate MLCT transition. The only room-temperature emissive complexes are those that contain an [alpha],[alpha][prime]-diimine chelating ligand. The nature of the emission in these complexes at all temperatures depends on the dithiolate ligand, and the temperature dependence of the emission spectra has been examined. The nature of the HOMO and LUMO has been examined experimentally using cyclic voltammetry. On the basis of the electrochemical and spectroscopic data, themore » emission from all of the Pt(diimine)(S-S) complexes except those of 1,2-dithiolate maleonitriledithiolate (mnt) is assigned as a [sup 3](d(Pt)/p(S)-[pi]*(diimine)) transition, while, for the mnt complexes, it corresponds to a [sup 3](d(Pt)/p(S)-[pi]*(mnt)) transition. These assignments are supported by extended Hueckel molecular orbital calculations.« less

On Pearson's HSAB principle

Abstract: Some 35 examples of the exchange reaction AB+CD=AC+BD (type 1), for which Pearson's HSAB (hard-soft acid-base) principle was originally devised, are examined in terms of the experimental or calculated hardness (η) values of the various species involued. The calculation of the η values has been done at the MNDO level by using η=(∈ LUMO -∈ HOMO )/2, where ∈ is energy, LUMO indicate the lowest unoccupied molecular orbital, and HOMO is the highest occupied molecular orbital

Local density of states of aperiodic polymers using the localized orbitals from an ab initio elongation method

Abstract: An elongation method, which was already proposed to calculate efficiently the electronic states of aperiodic polymers, is developed to provide the local density of states. Elongation method is carried out by repeating the uniform localization of wave functions with inclusion of the interaction between the cluster and an attacking molecule. At first, we develop this method to the ab initio level. Next, we show that the local density of states (LDOS) is obtained efficiently following the elongation step without treating the whole system directly. The validity of our method is examined via application to a hydrogen molecule cluster by comparing the results with those by the conventional ab initio method for the whole system. This approach makes it possible to investigate the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), and their density of states on each site of a polymer chain, which will provide a novel approach to examine intra‐ and intermolecular reactivities within and between aperiodic polymers.

Dielectric function of solid C70 films

Abstract: Using variable‐angle ellipsometry and normal incidence reflection/transmission optical techniques we have measured the dielectric function e(ω) at room temperature for solid C70 films over the photon energy range 0.5≤E≤5.3 eV. The onset of absorption across the highest‐occupied‐molecular‐orbit (HOMO)–lowest‐unoccupied‐molecular‐orbit (LUMO) gap is measured to be 1.25 eV. Furthermore, structure in the interband absorption at 2.41, 3.10, 3.50, and 4.45 eV is observed. The refractive index at zero frequency is estimated to be n(0)=1.94 as compared to the value n(0)=1.90, which we obtained for solid C60 from our previous study.

Low-lying electronic excited states of Buckminsterfullerene anions

Abstract: The authors report Pople-Pariser-Parr (PPP) calculations of the energies of the lowest electronically excited states of six C{sub 60} anions ranging from the monoanion to the hexaanion. Multiplicities up to septet are included. The results for the monoanion are compared with the existing absorption spectrum. The stability of various anions and the possibility of obtaining them by electrochemical reduction are discussed. In most of the species studied, the authors find a remarkable density of electronic levels close to the electronic ground state. This density is due to the existence, in neutral C{sub 60} of a triply degenerate lowest unoccupied molecular orbital (LUMO), together with a close-lying LUMO+1 of the same degeneracy. 26 refs., 2 figs., 6 tabs.

Synchrotron radiation and low-energy electron-diffraction studies of ultrathin C60 films deposited on Cu(100), Cu(111) and Cu(110)

Abstract: High resolution K-edge photoemission and X-ray absorption spectroscopies have been used to study the chemical properties of the interface formed with C60 and Cu. We find that for very thin (0.24–1 monolayers) C60 films, deposited in ultrahigh vacuum, the Cu substrate plays a role similar to that of K in K3C60 films. Thus Cu donates charge to partially fill the π* LUMO band, and the C (1s) core level shifts due to charge transfer. For monolayer coverage the C (1s) core-level line shape is broad. Low energy electron diffraction studies of C60 on Cu(100), Cu(111) and Cu(110) show that well ordered overlayers of thickness >2 monolayers can be formed on all three Cu surfaces, but single-domain epitaxy is achieved only on Cu(111). The substrate temperature during C60 deposition is critical for the ordering of the film.

Organometallic Chemistry of the N=N Group

Abstract: Publisher Summary This chapter reviews the types of complexes obtained from the reactions of metal carbonyls with cyclic diazenes of different steric and electronic properties. It describes thermal and photochemical reactions of some diazene iron complexes with alkynes and 1,3-dienes. Complexes of di-imine and bis(trimethylsily1)diazene with metal carbonyls and cyclopentadienyl metal fragments, respectively, have been recently reviewed. 1,2-Diphenyldiazenes (azobenzenes) usually react with metal carbonyls by orthometallation. The N=N bond character of the n1 orbital remains zero whereas for the n2 orbital it increases from 9 to 12 and 16%, respectively, in the diazene sequence, in contrast, the N=N character decreases from 85 to 80 and 74%, respectively, for the π MO but remains almost constant (96–100%) for the lowest unoccupied molecular orbital (LUMO). The initial interaction of a diazene with a metal is mainly determined by the nature of the highest occupied molecular orbital (HOMO) that has a more or less pronounced lone pair character. This explains the good ability of diazenes to form mononuclear σ complexes of types I and II wherein the diazene ligand acts as two-electron donor.

Theoretical study of the hydrogen chemisorption on a ZnO surface

Abstract: Reactions of a hydrogen molecule with a ZnO surface are studied by an ab initio method. For simulating the ZnO (10 1 0) surface, one ZnO molecule both with and without a Madelung potential is used. Since the electrostatic potential due to the ionic layer decreases exponentially, the effect of the layers deeper than the second one can be neglected. The Madelung potential is, therefore, expressed by the 32 point charges of ±0.5 situated on the first and second layers. Several low-lying states of ZnO and the ZnO + H2 system have been calculated by the symmetry-adapted cluster (SAC) and SAC–CI methods. It is found that the 1Σ+ state of ZnO is the ground state and catalytic active and the other states are inactive. ZnO (1Σ+) reacts with H2 and dissociatively adsorbs it with making ZnH and OH bonds. This occurs both with and without the Madelung potential. Without the Madelung potential, the heat of reaction is 81.3 kcal/mol and the reaction barrier is 14.0 kcal/mol. With the Madelung potential, the heat of reaction decreases to 73.5 kcal/mol and the barrier decreases to 11.5 kcal/mol. The mechanism of this reaction is the electron donation from the 2pπ orbital of O to the antibonding σu MO of H2 and the back-donation from the bonding σg MO of H2 to the LUMO of ZnO. In the intermediate stage of the reaction, the dipole of ZnO works to increase the overlap of the active MOS to make the reaction easier. Throughout the reaction, the in-plane 2pπ orbital of O and the HOMO of ZnO are inactive and work to keep the ZnO bond stable during the catalytic process.

Low-energy electron impact spectroscopy of [1.1.1.]propellane: electron attachment energies and singlet and triplet excited states

Abstract: Electron transmission spectra and energy and angular dependence of vibrational excitation by electron impact were recorded and used to characterize different states of the short-lived negative ion (resonances) of the title compound. The first attachment energy, corresponding to electron capture into the 3a 2 LUMO, is 2.04 eV, exceptionally low in comparison with a typical value of -6 eV for a saturated hydrocarbon. Observation of (faint) vibrational structure indicates a lifetime broadening of the order of a vibrational spacing for this resonance, much less than is typical for σ * resonances of saturated hydrocarbons (several electron volts)

Charge Donation by Calcium into the t1g Band of C60.

Abstract: Photoemission spectra of compounds prepared by the reaction of C60 films with calcium show two distinct metallic phases, whereas alkali-doped C60 films have only one. In the first phase the bulk t 1u band, derived from the lowest unoccupied molecular orbital of C60, is partially occupied. This is followed by an insulating phase that has the composition Ca3C60 in which the t 1u band is filled and has properties analogous to those of K6C60. Continued exposure to calcium produces a second metallic phase in which electrons are donated into the t 1g band. The superconductivity of Ca5C60 is associated with the t 1g band.

On the asymptotic behavior of Hartree-Fock orbitals

Abstract: Handy et al. have shown that the asymptotic behavior of Hartree-Fock orbitals is controlled by the energy of highest occupied molecular orbital, except for the atomic case in which onlys-orbitals are occupied. However, their proof is not complete at one point. This point is clarified, and a more unified derivation is given. Further, we discuss the preexponential factorrα of the leading asymptotic termrα exp[−βr], where β=(−2eHOMO)1/2 and eHOMO is the orbital energy of HOMO. New results are obtained for linear molecules, and the results of several authors for atoms and non-linear molecules are reproduced.

Structural and electronic properties of MX compounds related to TA100 mutagenicity. A semi-empirical molecular orbital QSAR study.

Abstract: The structural and electronic properties of chlorofuranones including MX and its anhydride were calculated using the semi-empirical AM1 method to elucidate the key features related to the strong mutagenic activity of MX. Significant correlations were found between Ames TA100 mutagenicity and the following electronic parameters of chlorofuranones: LUMO energy ( r = 0.9607, n = 17), electron affinity ( r = 0.9557), LUMO electron density at the α-carbon ( r = 0.8855) and partial charge of the α-carbon ( r = 0.8812). Based on these results, a molecular orbital QSAR model for the mutagenic activity of 17 MX analogues is presented. The controversial role of the open-chain tautomers of MX compounds, chlorinated butenoic acids, is discussed briefly.

Asymmetric Diels‐Alder Cycloadditions with Chiral Carbamoyl Dienophiles

Abstract: Chiral acylnitroso dienophiles 14, which were obtained from L-proline and from D-mandelic acid, reacted with cyclohexa-1,3-diene to give the expected diastereoisomers 15 and 16 (Scheme 2 and Table 1). The d.e. values for these Diels-Alder reactions were moderate; they are related to the molecular stiffness of the dienophiles. The absolute configuration of the major cycloadducts was interpreted in terms of HOMO/LUMO interactions, the approach being ‘endo’ and the acylintroso dienophiles reacting from their s-cis-conformation.

Associations of the bacterial mutagenicity of halogenated 2(5H)-furanones with their MNDO-PM3 computed properties and mode of reactivity with sodium borohydride.

Abstract: Electrophilicity as a general basis for both the mutagenicity and nucleophile inactivation of halogen-substituted 2(5H)-furanones was tested. Lowest unoccupied molecular orbital (LUMO) energy levels and stabilities of 2(5H)-furanone radical anions and C-2, C-3, and C-4 anionic hydride adducts were computed with MNDO-PM3 for each of 10 compounds. These three computed sets of values were considered electrophilicity indicators. Each individual value from a given indicator set was plotted against the logarithm of the Salmonella typhimurium (TA100) mutagenicities (log Mm) for each of the corresponding compounds. Highest occupied molecular orbital (HOMO) energy levels for the 10 compounds were also computed. Strong negative correlations were obtained from the plots of LUMO and radical anion stability against log Mm of the 10 compounds. Also, a negative correlation was observed for the plot of the stability of the C-4 anionic hydride adduct for a smaller set of six compounds possessing a 4-(chloromethyl)-2(5H)-furanone structure and having the same HOMO characteristics. HOMO energies failed to correlate with mutagenicity. Neither computed atomic charge nor 13C chemical shift values for the larger compound group of 10 correlated well with mutagenicity or with each other. Sodium borohydride and borodeuteride reductions of 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone demonstrated experimentally that hydride became attached to C-4.

15N‐, 1H‐, and 13C‐NMR chemical shifts and electronic properties of aromatic diamines and dianhydrides

Abstract: SYNOPSIS We measured the 15N -, 1 H -, and l1C_ NMR chemIcal shIfts for a senes of aromatIc dlammes and aromatIc tetracarboxyhc dlanhydndes dIssolved m DMSO-d b , and dIscuss the rela­ tIOnshIps between these chemIcal shIfts and the rate constants of acylatIOn (k) as well as such electronIc-property-related parameters such as IOnIzatIOn potentIal (IP), electrOnIc affinIty (EA), and the energy E of the hIghest occupIed molecular orbItal (HOMO) and the lowest unoccupIed molecular orbItal (LUMO) The 15N chemIcal shIfts of the ammo group of dlammes ( ON) depend monotonIcally on the loganthm of k (log k) and on IP We mferred the reactIvItIes of dlammes whose acylatIOn rates have not been measured from theIr ON, and we propose an arrangement of dlammes m the order of theIr reactIvIty The 1 H chemIcal shIft of ammo hydrogens (OH) and the 11C chemIcal shIft of carbons bonded to nItrogen ( oc) are roughly proportIOnal to ON, but these shIfts are not as closely correlated WIth log k and IP Although the llC chemIcal shIfts of the carbonyl carbon of dlanhydndes (oc') vanes much less than the Oc and ON of dlammes, Oc ' can be an mdex of acylatIOn reactIVIty for dJanhydndes because It IS closely correlated WIth ELUMO These facts mdlcate that the chemIcal shIfts of dlammes and dianhydndes are dIsplaced accordmg to theIr electron­ donor and electron -acceptor propertIes, and that these chemIcal shIfts can be used as mdlces of the electrOnIC propertIes of monomers Changes m reactIVIty caused by the mtroductlOn of tnfiuoromethyl groups mto dlammes and dlanhydndes are mferred from the dIsplace­ ments of ON and oc' ~\ 1992 John Wiley & Sons, Inc

Quantum mechanical calculations of amino pyrazole derivatives as corrosion inhibitors for zinc, copper and α-brass in acid chloride solution

Abstract: Quantum mechanical calculations have been applied to a series of pyrazole derivatives used as corrosion inhibitors for zinc, copper and α-brass in order to assess quantum chemistry as a means of evaluating effectiveness of corrosion inhibitors. The corresponding structures have been optimized and the energies and coefficients of their molecular orbitals (HOMO and LUMO) have been computed using the semi-empirical method, MNDO. The theoretical results are then compared with experimental data.