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.

Increased-Valence or Electronic Hypervalence for a Diatomic One-Electron Bond

2005 ◽  
Vol 58 (10) ◽  
pp. 753 ◽  
Author(s):  
Richard D. Harcourt
Keyword(s):  
Excited State ◽  
Molecular Orbital ◽  
Valence Bond ◽  
Ground States ◽  
Bond Valence ◽  
Atomic Orbitals ◽  
Valence Bond Structure ◽  
The One ◽  
Increased Valence

With a and b as overlapping atomic orbitals to form the A–B bonding molecular orbital ψab = a + kb, it is deduced that for k ≠ 0, 1, or ∞, either the A atom or the B atom in the one-electron bond valence bond structure (A · B) exhibits increased-valence or electronic hypervalence, namely its valence exceeds unity. The result is illustrated using the results of STO-6G valence bond calculations for the one-electron bond of LiH+ and an excited state for H2CN. Valencies for the ground-states of H2+, H2, and H2− are also considered.

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.

A molecular orbital treatment of diborane as a four-centre, four-electron problem

1956 ◽  
Vol 235 (1202) ◽  
pp. 395-407 ◽  
Keyword(s):  
Molecular Orbital ◽  
Valence Bond ◽  
Field Equations ◽  
Net Charge ◽  
Atomic Orbitals ◽  
Plane Normal ◽  
Self Consistent Field ◽  
Actual Configuration ◽  
Resonance Hybrid ◽  
Better Than

The Roothaan self-consistent field equations in their linear combination of atomic orbitals form have been solved for a system composed of four electrons in the field of a framework of two asymmetric boron cores of net charge +1 each and two protons in the 'bridge' positions as in diborane. Exact values for all one and two-centre integrals were used, and approximations were made for three- and four-centre integrals. The molecular orbitals of lowest energy are the completely symmetric orbital and the π -orbital with a nodal plane normal to the line between the protons. In terms of the molecular-orbital coefficients, there is a small residual net negative charge on the hydrogens, i. e. the bridge hydrogens have a hydridic character. In terms of configurational expansions, a localized three-centre bond description of the molecule is very close to the actual configuration and is somewhat better than a description as a resonance hybrid between either ordinary covalent valence bond structures or localized two-centre molecular orbital structures.

Resolving the Quadruple Bonding Conundrum in C2 Using Insights Derived from Excited State Potential Energy Surfaces: A Molecular Orbital Perspective

2019 ◽  
Author(s):  
Ishita Bhattacharjee ◽  
Debashree Ghosh ◽  
Ankan Paul
Keyword(s):  
Excited State ◽  
Potential Energy ◽  
Molecular Orbital ◽  
High Spin ◽  
Potential Energy Surfaces ◽  
Valence Bond ◽  
Deep Minimum ◽  
State Potential ◽  
Energy Surfaces ◽  
Mo Theory

The question of quadruple bonding in C<sub>2</sub> has emerged as a hot button issue, with opinions sharply divided between the practitioners of Valence Bond (VB) and Molecular Orbital (MO) theory. Here, we have systematically studied the Potential Energy Curves (PECs) of low lying high spin sigma states of C<sub>2</sub>, N<sub>2</sub> and Be<sub>2</sub> and HC≡CH using several MO based techniques such as CASSCF, RASSCF and MRCI. The analyses of the PECs for the<sup> 2S+1</sup>Σ<sub>g/u</sub> (with 2S+1=1,3,5,7,9) states of C<sub>2</sub> and comparisons with those of relevant dimers and the respective wavefunctions were conducted. We contend that unlike in the case of N<sub>2</sub> and HC≡CH, the presence of a deep minimum in the <sup>7</sup>Σ state of C<sub>2</sub> and CN<sup>+</sup> suggest a latent quadruple bonding nature in these two dimers. Hence, we have struck a reconciliatory note between the MO and VB approaches. The evidence provided by us can be experimentally verified, thus providing the window so that the narrative can move beyond theoretical conjectures.

The valence-bond theory of molecular structure II. Reformulation of the theory

1954 ◽  
Vol 223 (1154) ◽  
pp. 306-323 ◽  
Keyword(s):  
Valence Bond ◽  
Closed Shell ◽  
Numerical Illustration ◽  
Atomic Orbitals ◽  
Spin Eigenfunctions ◽  
Spin Pairing ◽  
Vb Structures ◽  
Bond Theory ◽  
Strong Repulsion ◽  
A Charge

The construction of spin eigenfunctions and the evaluation of matrix elements between ,them are discussed generally in preparation for a development of the valence bond (VB) theory along the lines indicated in I. The customary approximation of considering explicitly only the electrons outside a ‘closed shell’ is shown to be defensible. The reformulation of the VB theory is now straightforward, but its final description of bonding is quite new. Atomic orbitals (AO’s) are replaced, whenever they appear, by orthogonalized atomic orbitals (AO’s); but when the assumptions of the conventional theory are rigorously validated in this way the ‘covalent’ structures (now ‘VB’ structures) are found, quite generally, to indicate only strong repulsion between the ‘bonded’ atoms, and formal descriptions of bonding and of bond orders, in terms of ‘spin-pairing’, become nonsensical. Bonding can be described only by admitting into the wave functions polar VB structures; a bond between two atoms demands the appearance (with considerable weight) of pairs of structures differing by a ‘charge hop’ between the atoms concerned. The conventional VB structures are found to be equivalent to certain groupings of VB structures (non-polar and polar) and do, indeed, predict bonds between spin-paired atoms and repulsion between the atoms of different pairs. It is then possible to make full use of chemical intuition, using a plausible combination of conventional structures as a starting approximation in the more rigorous theory. A numerical illustration is provided by a discussion of the Kekulé structures of benzene. Some important characteristics of energy calculations in the VB theory are pointed out. Quantities of intra - and inter -atomic origin are well separated, and the method is apparently well suited to development along either ab initio or empirical lines.

Valence Bond Structure and Mechanical Properties of CNxFilms Prepared by Glow Discharge Assisted Pulsed Laser Deposition

2013 ◽  
Vol 40 (11) ◽  
pp. 1107002
Author(s):  
郑晓华 Zheng Xiaohua ◽  
宋建强 Song Jianqiang ◽  
杨芳儿 Yang Fanger ◽  
陈占领 Chen Zhanling
Keyword(s):  
Mechanical Properties ◽  
Glow Discharge ◽  
Pulsed Laser Deposition ◽  
Valence Bond ◽  
Pulsed Laser ◽  
Laser Deposition ◽  
Valence Bond Structure

Orthogonalized atomic orbitals and the interpretation of valence bond wavefunctions

1987 ◽  
Vol 137 (5) ◽  
pp. 437-440 ◽  
Author(s):  
Gilles Ohanessian ◽  
Philippe C. Hiberty
Keyword(s):  
Valence Bond ◽  
Atomic Orbitals

Plasma sterilization. Methods and mechanisms

2002 ◽  
Vol 74 (3) ◽  
pp. 349-358 ◽  
Author(s):  
Michel Moisan ◽  
Jean Barbeau ◽  
Marie-Charlotte Crevier ◽  
Jacques Pelletier ◽  
Nicolas Philip ◽  
...  
Keyword(s):  
Significant Contribution ◽  
Genetic Material ◽  
Reactive Species ◽  
Gas Pressure ◽  
Survival Curves ◽  
Plasma Sterilization ◽  
Physicochemical Processes ◽  
Inactivation Process ◽  
Uv Photons ◽  
Oxygen Atoms

Utilizing a plasma to achieve sterilization is a possible alternative to conventional sterilization means as far as sterilization of heat-sensitive materials and innocuity of sterilizing agents are concerned. A major issue of plasma sterilization is the respective roles of ultraviolet (UV) photons and reactive species such as atomic and molecular radicals. At reduced gas pressure (£10 torr) and in mixtures containing oxygen, the UV photons dominate the inactivation process, with a significant contribution of oxygen atoms as an erosion agent. Actually, as erosion of the spore progresses, the number of UV photons successfully interacting with the genetic material increases. The different physicochemical processes at play during plasma sterilization are identified and analyzed, based on the specific characteristics of the spore survival curves.

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