Library

feed icon rss

Your email was sent successfully. Check your inbox.

An error occurred while sending the email. Please try again.

Proceed reservation?

Export
Filter
  • Theoretical, Physical and Computational Chemistry  (3)
  • 3-Dipolar cycloadditions  (1)
  • 1
    ISSN: 1432-2234
    Keywords: Key words: Spin-coupled approach ; 1 ; 3-Dipolar cycloadditions ; Reaction mechanisms ; Valence-bond theory
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract. The electronic mechanism for the gas-phase 1,3-dipolar cycloaddition of fulminic acid (HCNO) to ethyne is studied through a combination of modern valence-bond theory in its spin-coupled (SC) form and intrinsic reaction coordinate calculations utilizing a complete-active-space self-consistent field wavefunction. It is shown that the concerted reaction follows a “heterolytic” route, during which three orbital pairs corresponding to three distinct bonds in the reactants (an in-plane π bond in ethyne, and a C-N and an N-O in-plane bond in HCNO) shift simultaneously to create the two new bonds closing the isoxazole ring and a nitrogen lone pair. The analysis of the SC wavefunction strongly suggests that the reacting system remains nonaromatic throughout the most important part of the cycloaddition process. This investigation provides the first demonstration of an alternative SC description of a bond rearrangement, achieved through the movement of singlet orbital pairs through space, during which at least one of the orbitals within a pair becomes completely detached from the atomic centre with which it is associated initially and ends up localized about another centre.
    Type of Medium: Electronic Resource
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 439-451 
    ISSN: 0020-7608
    Keywords: CASVB ; modern VB ; nonorthogonal orbital optimization ; multiconfiguration spin-coupled ; CASSCF ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We outline the CASVB strategy, which may be used either to generate very compact modern valence bond representations of CASSCF wave functions or to optimize general types of modern VB wave function. A simple algorithm is presented for the elimination of redundant or constrained parameters from the appropriate second-order optimization problems. Selected results for three systems - benzene, the allyl radical, and LiH - are used to illustrate various ways in which CASVB procedures can be used to perform fully variational optimizations of nonorthogonal orbitals and structure coefficients in modern VB wave functions.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 439-451, 1997
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 637-650 
    ISSN: 0020-7608
    Keywords: excited states ; modern valence bond theory ; CASVB ; spin-coupled wave function ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The valence bond description of excited states is considered in the context of the CASVB approach, with emphasis on the simultaneous optimization of orbitals and structure coefficients for highly general types of wave function, as well as the close connection to the complete active space self-consistent field (CASSCF) method. In addition to fully variational optimization of modern valence bond (VB) wave functions, one may also generate compact representations of CASSCF wave functions, essentially irrespective of their nature. Consequences of generalizing the CASVB methodology to other than the lowest-lying state of a given symmetry are explored for both the overlap- and energy-based optimization criteria. The methodology is illustrated by means of applications to the 21Ag state of trans-1,3-butadiene, the c˜1A1 state of methylene, and the first excited singlet state of benzene.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 637-650, 1998
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 4
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electronic structure of borabenzene (C5H5B, known also as borinane, borinine, borine) is studied using modern valence bond theory in its spin-coupled (SC) form. Three different types of SC wave functions - with six active π orbitals and with four and eight active σ orbitals - are used to describe the π system of the molecule and the σ-bond framework around the boron atom. It is demonstrated that the SC picture of the π space in borabenzene is very similar to that in benzene: The spins of six distorted nonorthogonal 2pπ orbitals are combined in a spin-coupling pattern involving two dominating Kekulétype and three less important Dewar-type Rumer spin functions. This indicates that it is appropriate to consider the π-electron sextet in borabenzene as aromatic and that the reason for the reactivity of this molecule should lie with its σ framework. The two SC models of the σ bonding around B show that the boron-carbon σ bonds in borabenzene involve orbitals are “bent” to the outer side of the six-membered ring. This creates an orbital “hole” at the boron, which should represent the preferred attachment site for Lewis acids. © 1997 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
Close ⊗
This website uses cookies and the analysis tool Matomo. More information can be found here...