Theoretical and Experimental Characterization of Cr−L Multiple Bonds (L = O, N, and C)

Abstract

A combined experimental and theoretical study on the bond characterization of Cr-L (L ) O, N, C) multiple bonds is applied to a series of Cr-complexes: [(CO) 4 (Cl)Cr (I) ; and [(TPPOMe)Cr (IV) O] 4, where [TPPOMe ) (5,10,15,20-p-methoxyphenyl)porphyrin)]. Compounds 1 and 2 were investigated by accurate single-crystal X-ray diffraction. Detailed descriptions of Cr-C carbyne , Cr-N nitrido , Cr-N imido , and Cr-O oxo bonds will be given based on the natural bonding orbital (NBO) analyses and Fermi hole function. The bonding feature of all these multiple bonds is essentially a triple bond character consisting of one σ and two π bonds. The σ character of the Cr-L multiple bonds is a highly polarized one with the electron density strongly polarized toward L (O, N, C) ligands. The π character of the Cr-L multiple bonds depends on the nature of L ligand, i.e., with electron density polarized toward the Cr and O center for Cr-C carbyne and Cr-O oxo bond, respectively, whereas electron density is roughly equally distributed at both Cr and N for the Cr-N nitrido and Cr-N imido bonds. Bond characterizations are also shown in terms of Laplacian of electron density where the inner valence shell charge concentration (i-VSCC) is embedded. The isovalue surface of zero Laplacian of electron density reveals the shape of such i-VSCC at each chromium atom. Shapes around the Cr atom are a pressed disklike for 1 and 3 and inverted square pyramid for 2 and 4. The topological properties associated with the bond critical point (BCP) of Cr-L multiple bonds in these compounds indicate a strong covalent bond character. The order of the binding interaction is Cr-N nitrido > Cr-O oxo > Cr-N imido > Cr-C carbyne . The combined study of experiment and theory on 1 and 2 demonstrates good agreement between experiment and theory.

Effect of temperature on isotopic mass dependence of excitonic band gaps in semiconductors: ZnO

“…The sharp signatures observed by using photomodulation allowed us to extend the temperature range of experimental data to 400 K and deduce their zero-point renormalizations in nat Zn nat O ͑ZnO with natural isotopic composition of Zn and O, i.e., 65.4 Zn 16 O͒. In addition, the temperature dependence obtained for the C excitonic band gap in an isotopically engineered 68 Zn 18 O, together with similar data in nat Zn nat O, provide convincing experimental evidence for a vanishing isotopic dependence of the band gap at high temperatures. However, the effect of the isotopic mass on the thermal expansion has been shown to tend to zero at high T in the case of germanium.…”

“…14 Previously, the temperature dependence of its A, B, and C excitonic band gaps, associated with the lowest conduction band and the top three valence bands at the center of the Brillouin zone, has been studied from 4.2 to 300 K employing reflectivity, 15 photoluminescence ͑PL͒, [15][16][17] and photoluminescence excitation spectroscopy ͑PLE͒. 18 In addition, the isotopic dependence of the ZnO band gaps has been investigated at low temperature by wavelength-modulated reflectivity 19,20 and PL. 17 In this study we present results on the A, B, and C excitonic band gaps of ZnO as a function of temperature.…”

Effects of resonant excitation, pulse duration and intensity on photoelectron imaging of a dianion

“…35 The situation is, however, significantly different in multiplycharged anions (MCAs). [9][10][11][12][13][14] Take for example a dianion, A 2-. Photoemission from A 2-leads to the formation of an anion and the PE: A 2-+ h → A -+ e -.…”

“…The net interaction leads to the repulsive Coulomb barrier (RCB) in MCAs, first probed experimentally by the Wang group using PE spectroscopy. [10][11][12] The tell-tale sign of an RCB in a PE spectrum is a region at low electron kinetic energy (eKE) over which no PE 45 signal can be observed. PE emission is then suddenly allowed at some cut-off, above which the emitted PE has sufficient energy to Figure 1.…”

“…surmount the RCB. [10][11][12] The RCB also has dramatic consequences on the PAD of MCAs in PE imaging experiments. [15][16][17][18][19][20][21][22] In contrast 55 to anions and neutrals, the PADs from MCAs have received virtually no theoretical attention.…”