Configuration interaction, ionization potentials and valence-bond aspects of the rotation barrier for dinitrogen tetroxide

1981 ◽  
Vol 34 (2) ◽  
pp. 231 ◽  
Author(s):  
RD Harcourt
Keyword(s):  
Bond Order ◽  
Photoelectron Spectrum ◽  
Valence Bond ◽  
The Other ◽  
Unexpected Result ◽  
Rotation Barrier ◽  
Empirical Estimate ◽  
Planar Conformation ◽  
Dinitrogen Tetroxide ◽  
Ci Calculations

The results of recent He II studies of the N2O4 photoelectron spectrum are used to obtain an empirical estimate for a cis O-O core resonance integral, β°oo'. This integral is associated with the overlap between pairs of cis oxygen in-plane (2pπ) orbitals (which are doubly occupied in valence-bond structures of types (A) and (B)), and is therefore relevant for an analysis of the cis O-O overlap contribution to the barrier to rotation around the NN bond of N2O4. Limited SCF-CI calculations for the planar and skew conformations give a rotation barrier of 7.2 kJ mol-1 (cf. expt., 8.4-12.6 kJmol-1) when the NN σ-bond order is 0.525 for the planar conformation. The CI expression for the rotation barrier is expressed as a sum of cis O-O overlap, Coulomb and exchange terms, of which the cis O-O overlap contribution has the largest magnitude. The sensitivity of the magnitude of the rotation barrier to variation in the value of the NN σ-bond order is also demonstrated. A valence-bond analysis of the cr wavefunction provides further support for an earlier conclusion that resonance between covalent (NO2NO2) and ionic (NO2+ NO2- and NO2- NO2+) structures provides the valence-bond explanation for the origin of the cis O-O overlap contribution to the rotation barrier. An unexpected result obtained from this analysis is that, although resonance between covalent and ionic structures of types (A) and (B) alone is not cis O-O overlap dependent, this type of resonance does provide a contribution to the cis O-O overlap stabilization of the planar conformation when it is coupled to the other types of covalent-ionic resonance, which arise when the oxygen 2pσ-electrons of (A) and (B) delocalize. Each of the latter types of resonance alone is cis O-O overlap dependent.

Three-electron bonds and a valence-bond study of the rotation barrier for N2O4

1978 ◽  
Vol 31 (8) ◽  
pp. 1635 ◽  
Author(s):  
RD Harcourt
Keyword(s):  
Molecular Orbital ◽  
Significant Contribution ◽  
Valence Bond ◽  
Rotation Barrier ◽  
Atomic Orbitals ◽  
Planar Conformation ◽  
Valence Bond Structure ◽  
Oxygen Atoms

From molecular orbital studies, a number of previous workers have concluded that a most significant contribution to the barrier to rotation around the NN bond of N2O4 arises from the overlap of pairs of atomic orbitals located on the cis oxygen atoms of the planar conformation. A valence-bond study of this overlap contribution is reported. It is calculated that the 'long OO bond' formed by overlap of singly occupied 2pπ-orbitals in the valence-bond structure (3) for the planar conformation has negligible strength, and that insufficient stabilization is obtained when this structure participates in resonance with ionic structures of the type (5a). Diagram The primary overlap stabilization for the planar conformation is calculated to arise from resonance between structures of the types (2a) and (7). The O. :O ↔ 0: .O resonance that pertains here is equivalent to the formation of a Pauling 'three-electron bond' O...O between the two atoms. Therefore, the development of this type of bond in the planar conformation can be associated with the cis OO overlap contribution to the barrier to rotation around the NN bond. Another type of Pauling three-electron bond resonance, namely (4) ↔ (6), is calculated to produce a smaller but not insignificant stabilization of the planar conformation.

The photoelectron spectrum of dinitrogen tetroxide

1977 ◽  
Vol 10 (3) ◽  
pp. 293-303 ◽  
Author(s):  
D.C. Frost ◽  
C.A. McDowell ◽  
N.P.C. Westwood
Keyword(s):  
Photoelectron Spectrum ◽  
Dinitrogen Tetroxide

scholarly journals q-DEFORMATIONS OF QUANTUM SPIN CHAINS WITH EXACT VALENCE-BOND GROUND STATES

1994 ◽  
Vol 08 (25n26) ◽  
pp. 3645-3654 ◽  
Author(s):  
M.T. BATCHELOR ◽  
C.M. YUNG
Keyword(s):  
Condensed Matter ◽  
Valence Bond ◽  
Ground States ◽  
Quantum Spin ◽  
Total Spin ◽  
Spin Chains ◽  
The Other ◽  
Condensed Matter Physics ◽  
Spin States ◽  
Quantum Spin Chains

Quantum spin chains with exact valence-bond ground states are of great interest in condensed-matter physics. A class of such models was proposed by Affleck et al., each of which is su(2)-invariant and constructed as a sum of projectors onto definite total spin states at neighboring sites. We propose to use the machinery of the q-deformation of su(2) to obtain generalisations of such models, and work out explicitly the two simplest examples. In one case we recover the known anisotropic spin-1 VBS model while in the other we obtain a new anisotropic generalisation of the spin-½ Majumdar-Ghosh model.

Some semi-empirical molecular orbital studies of the origin of conformational preferences for free S2O42- and N2O4

1984 ◽  
Vol 37 (7) ◽  
pp. 1553 ◽  
Author(s):  
RD Harcourt ◽  
BJ Smith ◽  
CJ Marsden
Keyword(s):  
Molecular Orbital ◽  
Rotation Barrier ◽  
Coulombic Repulsion ◽  
Barrier Analysis ◽  
Trans Conformer ◽  
Conformational Preferences ◽  
Semi Empirical ◽  
Ci Calculations

The results of some semi-empirical MO-CI calculations of the barriers to rotation around the S-S and N-N bonds of S2O42- and N2O4 are reported. The calculated barriers are expressed in terms of atomic, cis O-O overlap, coulombic repulsion and exchange contributions. The barrier analysis for N2O4 shows that cis O-O overlap stabilizes the planar relative to the skew conformer, whereas for free S2O42-, reduced coulombic repulsions stabilize the trans conformer relative to the eclipsed. Some CNDO/2 estimates of the rotation barrier for S2O42- are also reported.

Valence–Bond Order (VBO): A New Approach to Modeling Reactive Potential Energy Surfaces for Complex Systems, Materials, and Nanoparticles

2009 ◽  
Vol 5 (3) ◽  
pp. 594-604 ◽  
Author(s):  
Meiyu Zhao ◽  
Mark A. Iron ◽  
Przemysław Staszewski ◽  
Nathan E. Schultz ◽  
Rosendo Valero ◽  
...  
Keyword(s):  
Potential Energy ◽  
Complex Systems ◽  
Potential Energy Surfaces ◽  
Bond Order ◽  
Valence Bond ◽  
New Approach ◽  
Reactive Potential ◽  
Energy Surfaces

Excited states of four hemes in a c-type cytochrome subunit of the photosynthetic reaction center of Rhodopseudomonas viridis: SAC-CI calculations

2001 ◽  
Vol 05 (03) ◽  
pp. 256-266 ◽  
Author(s):  
K. OHKAWA ◽  
M. HADA ◽  
H. NAKATSUJI
Keyword(s):  
Reaction Center ◽  
Excited States ◽  
Photosynthetic Reaction Center ◽  
Linear Dichroism ◽  
The Other ◽  
Oscillator Strengths ◽  
Energy Separation ◽  
Ligand Effect ◽  
Ci Calculations ◽  
Homo Lumo

The excited states of four hemes, c-552, c-554, c-556, and c-559, in a c-type cytochrome subunit of the photosynthetic reaction center (PSRC) of Rhodopseudomonas (Rps.) viridis were studied using the symmetry-adapted cluster (SAC)-configuration interaction (CI) method. c-552 has two imidazole ligands, while the other three hemes have imidazole and methionine ligands. The electronic states of these four hemes are affected significantly by both the ligands and protein and water environments. The ligand effect classifies the four hemes into two groups, while the environmental effect is unique in each heme. The HOMO–LUMO energy separation of c-552 is much smaller than those of the other hemes, and therefore, the low-lying excited states of c-552 are more stable than those of the other hemes. The low-lying excited states and their oscillator strengths of c-559 are significantly modified by the environment when compared with c-556 and c-554. Some observed UV peaks of the PSRC of Rps. viridis were found to have been assigned reasonably to the excited states of these four hemes in a previous report. The calculated linear dichroism angles also supported our assignment.

The U.S. Assistance to the MENA Arab Countries under the Trump Administration: The Logic of a Diff erentiated Approach

2020 ◽  
Vol 12 (4) ◽  
pp. 131-170
Author(s):  
V. I. Bartenev
Keyword(s):  
United States ◽  
The United States ◽  
Executive Branch ◽  
Arab Countries ◽  
The Other ◽  
Unexpected Result ◽  
Donald Trump ◽  
Mena Region ◽  
The U.S ◽  
Trump Administration

This paper identifi es and explains key changes in the U.S. aid policies towards Arab countries of the Middle East and North Africa (MENA) under Donald Trump. It seeks to validate two widespread arguments — the one about the current administration’s revision of pivotal principles of providing foreign assistance, and the other one — about an accelerated disengagement of the United States from the MENA region since 2017. The paper consists of four sections. The fi rst section explores the transformation of the U.S. strategic thinking and regional context under the Trump administration and then posits fi ve hypotheses about possible changes in the volume and composition of the U.S. assistance to the MENA region (in comparison with the fi nal two years of the Obama administration), as well as the diff erences in the executive branch and the Congress’s positions. The second section explains particularities of the statistical data and the methods of its exploration, the third section presents the results of hypothesis testing using aggregated data on aid fl ows to the region, and the fi nal section explains these results, sometimes unexpected, using the data disaggregated by country. Three of fi ve hypotheses proved wrong based on the aggregate data. First, the Trump administration did not cut assistance to the MENA more substantially than to other regions of the globe. Second, it did not ringfence aid accounts which helped yield direct dividends to the U.S. businesses. Third, the Republican Congress was clearly less willing to support the executive’s aid chocies under a new Republican President than during the last years of a Democrat Barack Obama’s second term. Only two hypotheses proved correct — one about a prioritization of security and military assistance under Donald Trump and the other one — about disproportionate cuts of democracy promotion assistance. Such an unexpected result calls for refi ning both aforementioned arguments and taking into account the dissimilarities in the dynamics of assistance to diff erent countries. The United States tends to practice a diff erentiated approach in dealing with two largest Arab aid recipients (Egypt and Jordan) and with other Arab countries. The assistance to Cairo and Amman is ringfenced and protected, while aid to other recipients, including security assistance and FMF grants, is prone to quite drastic cuts. This diff erentiation is explained by the fact that cooperation with Egypt and Jordan rests not only on more solid strategic foundations but also on a strong support within the United States — both from the defense contractors interested in large export contracts and from an infl uential pro-Israel lobby. The U.S. will not abandon this highly diff erentiated approach after the 2020 elections but the structure of assistance to the MENA region might undergo quite a dramatic transformation.

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