Angelo R. Rossi

One-Step Hydrothermal Synthesis of Manganese-Containing MFI-Type Zeolite, Mn–ZSM-5, Characterization, and Catalytic Oxidation of Hydrocarbons

Manganese-containing MFI-type Mn-ZSM-5 zeolite was synthesized by a facile one-step hydrothermal method using tetrapropylammonium hydroxide (TPAOH) and manganese(III)-acetylacetonate as organic template and manganese salts, respectively. A highly crystalline MFI zeolite structure was formed under pH = 11 in 2 days, without the need for additional alkali metal cations. Direct evidence of the incorporation of Mn in the zeolite framework sites was observed by performing structure parameter refinements, supported by data collected from other characterization techniques such as IR, Raman, UV-vis, TGA, N2-adsorption, SEM, TEM, EDAX, and XPS. UV-vis spectra from the unique optical properties of Mn-ZSM-5 show two absorption peaks at 250 and 500 nm. The absorption varies in different atmospheres accompanied by a color change of the materials due to oxygen evolution. Raman spectra show a significant and gradual red shift from 383 cm(-1) to 372 cm(-1) when the doping amount of Mn is increased from 0 to 2 wt %. This suggests a weakened zeolite structural unit induced by the Mn substitution. The catalytic activity was studied in both gas-phase benzyl alcohol oxidation and toluene oxidation reactions with remarkable oxidative activity presented for the first time. These reactions result in a 55% yield of benzaldehyde, and 65% total conversion of toluene to carbon dioxide for the 2% Mn-ZSM-5. Temperature programmed reduction (TPR) using CO in He demonstrates two reduction peaks: one between 300 and 500 °C and the other between 500 and 800 °C. The first reduction peak, due to manganese-activated oxidation sites shifted from higher temperature to lower temperature, and the peak intensity of CO2 rises when the dopant amount increases. For the first time, calculated photophysical properties of a model Mn(O-SiH3)4(-) compound, an Mn-embedded zeolite cluster, and model Mn oxides help to explain and interpret the diffuse reflectance spectroscopy of Mn-ZSM-5 zeolites.

Theoretical studies of the electronic structure and spectra of low‐lying states of NH+3

We report on ab initio calculations of the energies and geometries of the 2A″2 ground state and the 2E and 2A′1 excited states of NH+3. The Jahn–Teller distortion of the 2E is treated in detail. By global geometry optimization we determine that the Jahn–Teller stabilization energy is between 1.2 and 1.4 eV and that the 2A″ state is the lowest energy Jahn–Teller component. We find that angular distortions provide the major contribution to the stabilization energy and that the JT distortion is a static one. Nonlinear electronic‐vibrational coupling appears to be important but intermode coupling is small. Finally, we discuss the second photoelectron band of NH3 and the observed photofragmentation patterns in terms of the computed state energies and geometries.

Effective-core-potential calculations of sulphur, selenium and tellurium dioxides and dihydrides

Abstract The ground-state electronic structures of SO 2 , SeO 2 , TeO 2 , SH 2 , SeH 2 and TeH 2 have been calculated with effective core potentials. Satisfactory agreement with experimental molecular geometries was achieved in the dioxides only after d-functions were included in the basis sets for S, Se and Te; however, these d-functions were not essential for the dihydrides. The importance of electron correlation to the determination of dissociation energy is also evident from these calculations.

Excitation and ionization at surfaces: CO on metals

We discuss the energetics, electronic structure, and stability of neutral excited and ionic states of adsorbates on metals using CO as a prototype. By ab initio cluster calculations we show that two types of negative ion states, bonding and antibonding, exist. We then show that the valence 4σ, 5σ→2π* excitations of chemisorbed CO are essentially unshifted from their free CO positions, while a charge‐transfer excitation appears at ∼5–7 eV. Intra‐adsorbate polarization effects are shown to be responsible for the spectral shifts of the corresponding core (C1s, O1s→2π*) excitations. The relation between screened positive ion states and neutral excited states is discussed. Finally, the connection between the above spectroscopic results and photon‐ or electron‐stimulated desorption is examined.

Dissociating states of the H−3 system

Single determinant Hartree–Fock calculations for the lowest singlet and triplet potential energy surfaces of the H−3 system are presented over a broad range of isosceles triangular configurations of the nuclei. The addition of a diffuse s function to the four‐term Gaussian expansion of Huzinaga for H(1s) together with p type polarization functions produces results which are in agreement with experiments on double electron capture by H+3 to form H−3. The present calculations predict that capture to the ground singlet state produces H2+H−, with a dissociation energy in reasonable agreement with the experimental findings. Capture to the triplet state is predicted to resulted in the three body dissociation H+H+H− with small dissociation energy. This is consistent with, but not positively confirmed by, the experimental data.

Low‐lying excited states of N3−

A b i n i t i o SCF molecular‐orbital calculations on low‐lying states of linear, symmetric N3 − were performed with the Hunt–Hay–Goddard open‐shell program. Both a [3s, 2p] basis contracted from a primitive (9s, 5p) Gaussian basis, and the same basis augmented by polarization functions [3s, 2p]+d, were employed at a fixed internuclear distance r N–N=2.225 a.u. in calculating vertical excitation energies for the following states: ...4σ g 2 3σ u 2 1π g 4, 1Σ+ g ; ...1π g 3 2π1 u , 1,3Σ+ u , 1,3Σ− u , and 1,3Δ u ; ⋅⋅⋅3σ u 1 ⋅⋅⋅2π1 u , 1,3Π g ; and ⋅⋅⋅4σ1 g ⋅⋅⋅2π u 1, 1,3Π u . The smaller basis [3s, 2p] was used in calculating energies of the 1Σ+ g ground state and the 3Σ+ u , 1,3Σ− u , and 1,3Δ u states of linear, symmetric N− 3 as a function of internuclear distance. The [3s, 2p] basis was also used in the ibmol program of Clementi and Mehl to calculate energies of the 1 A 1(1Σ+ g ) ground state and the following low‐lying triplet states of bent N3 −: 3 B 2(3Σ u +), 3 A 2(3Δ u ), 3 B 2(3Δ u ), and 3 A 2(3Σ− u ). The triplet‐state energies of bent N3 − were corrected by incorporating approximate configuration mixing. Comparison of calculated energies with absorption spectra of azide compounds shows only qualitative agreement, but supports the interpretation of McDonald, Rabalais, and McGlynn.

The bent RC N linkage: An AB initio study of NH2CN, NF2CN and PF2CN

Abstract The ab initio energies, nuclear and electron repulsions and charge distributions have been calculated using moderately large basis sets as a function of the RC  N angle (R  NH 2 , NF 2 or PF 2 ). The optimum RC  N angles were calculated to be 178.9°, 176.6°, and 175° for NH 2 CN, NF 2 CN, and PF 2 CN, respectively. A rationalization of the differing bends is presented in terms of nuclear-nuclear and electron-electron repulsions.

Comparison of the electronic spin-orbit splitting in CH3O using UHF and RHF wavefunctions

The electronic spin-orbit splitting λe of methoxy was calculated using RHF wavefunctions and a semiempirical spin-orbit hamiltonian. The result, λe = -158 cm-1, agrees well with a calculation that used a RHF wavefunction and the relativistic Breit hamiltonian (λe = -142 cm-1). However, this result does not agree with a calculation that used the same semiempirical spin-orbit hamiltonian and an UHF wavefunction (λe = -78 cm-1). The disagreement arises from the form of the UHF and RHF wavefunctions. The experimental evidence is not sufficiently conclusive for one to decide which wavefunction gives a better description of methoxy.

A molecular orbital study of alkoxy allyl and vinyl anions

The substitution of an alkoxy group for hydrogen is to stabilize by inductive effects both the vinyl and ally1 anions. Since the vinyl carbanion center is more localized, substitution of an OR group adjacent to this center produces a more dominant stabilizing effect than for the ally1 system. Superimposed upon the inductive stabilization is a trend of destabilization of the n system of the ally1 moiety and to a lesser extent the carbanion center of the vinyl anion by the oxygen lone pair orbitals. An interaction diagram for the T orbitals of an ally1 fragment interacting with oxygen lone pair orbitals of the -OR group is shown in Figure 1. The lower molecular orbitals of S and A synzaetry are localized predominantly on the oxygen lone pair orbitals while the high energy orbitals are essentially on the ally1 fragment with some oxygen lone pair character mixed in an antibonding manner. The overall effect is a four-electron destabilizing interaction between the w-donor orbitals on the oxygen with the filled II orbitals of the ally1 moiety. The OR group also has a destabilizing effect in the vinyl anion. In 1 is shown an alkoxy vinyl anion along with optimized values for some of the important structural parameters. The optimized value of 1.45 /! for the C-O bond length in the alkoxy vinyl anion represents an increase

AB initio calculations of the electronic properties of N4− in KN3

Abstract The energies of the ground state and low-lying excited states of the N 4 − defect in KN 3 have been calculated using ab initio techniques. A rectangular equilibrium geometry with dimensions X = 2.76 and Z = 2.47 a.u. and ground state symmetry of Γ 4 + was determined by calculating N 4 − as a free radical. For this ground state the unpaired electron is in a π orbital which is consistent with the experimental hyperfine tensor only if one edge of the N 4 − radical is parallel to the c axis in KN 3 . These results were used to calculate the X 2 Γ 4 + state of N 4 − in the crystal field of KN 3 , yielding an energy of −217.899 Hartrees. The isotropic hyperfine constant was calculated to be a = 2.1 G and the components of the anisotropic hyperfine tensor as B xx = −3.4 G , B yy = 7.0 G and B zz = −3.6 G, in good agreement with experimental and INDO results. Several excited states were calculated for the N 4 − defect in KN 3 . When an estimate was made of the correlation energy, the transition energy of the X 2 Γ 4 + → A 2 Γ 3 − transition agreed well with the peak energy of the 780 nm absorption band which has been attributed to N 4 − .