Molecular orbitals from group orbitals. IX. The problem of hybrid lone pairs

1979 ◽  
Vol 57 (7) ◽  
pp. 729-732 ◽  
Author(s):  
Daniel Kost ◽  
H. Bernhard Schlegel ◽  
David John Mitchell ◽  
Saul Wolfe
Keyword(s):  
Ab Initio ◽  
Molecular Orbitals ◽  
Congeneric Species ◽  
Matrix Elements ◽  
Orbital Interactions ◽  
Lone Pairs ◽  
Conformational Properties ◽  
Group Orbitals

The quantitative PMO analysis of the ab initio wavefunction of a molecule A—B is based upon a partitioning of the Fock matrix elements of this wavefunction to obtain the fragments A and B, followed by computation of the stabilizing and destabilizing orbital interactions between the orbitals of these fragments that contribute to the HOMO of A—B. However, when one or both of the fragments is NH2 or a congeneric species, neither the 3a1 nor the 1b1 orbital of this fragment is appropriate for overlap with the second fragment, and the PMO analysis cannot be performed. A solution to this problem is proposed, and has been tested by application to various conformational properties of methylamine.

Configurational Assignment and Conformational Study of Methylglyoxal Bisdimethylhydrazones Derived from the 2-Ethoxypropenal Precursor

2006 ◽  
Vol 59 (3) ◽  
pp. 211 ◽  
Author(s):  
Leonid B. Krivdin ◽  
Lyudmila I. Larina ◽  
Kirill A. Chernyshev ◽  
Natalia A. Keiko
Keyword(s):  
Ab Initio Calculations ◽  
Ab Initio ◽  
Experimental Measurements ◽  
Conformational Study ◽  
Lone Pairs ◽  
Configurational Assignment ◽  
Out Of Plane ◽  
High Level

A configurational assignment of the isomeric methylglyoxal bisdimethylhydrazones derived from the 2-ethoxypropenal precursor has been performed based on experimental measurements and high-level ab initio calculations of 1J(C,C) and 1J(C,H) couplings. The results reveal the marked stereochemical dependence upon the orientation of the lone pairs of both nitrogen atoms in different isomers. Methylglyoxal bisdimethylhydrazone is shown to exist in a mixture of the EE and ZE isomers (ca. 75:25), both of which adopt predominant s-trans conformations with minor (up to 8°) out-of-plane deviations.

scholarly journals The influence of exocyclic lone pairs on the bonding and geometry of type A mesoionic rings

2021 ◽  
Author(s):  
Christopher Antony Ramsden ◽  
Wojciech Piotr Oziminski
Keyword(s):  
Ab Initio ◽  
Type A ◽  
Nbo Analysis ◽  
Ring Systems ◽  
X Ray ◽  
X Ray Crystallography ◽  
Bond Model ◽  
Lone Pairs ◽  
Mp2 Calculations ◽  
Good Agreement

AbstractBased on structures determined by X-ray crystallography, ab initio MP2 calculations on type A mesoionic rings give geometries in good agreement with observed values. A study of four mesoionic ring systems, each with exocyclic oxygen, nitrogen or carbon groups, shows that the presence and configuration of exocyclic lone pairs significantly influences the geometry and configurational preference. Using a localised bond model and NBO analysis, these effects are rationalised in terms of an anomeric interaction of lone pairs with the antibonding orbitals of adjacent σ bonds. In agreement with experiment, similar effects are calculated for pyran-2-imines.

Ab Initio NMR Chemical Shift Calculations Using Fragment Molecular Orbitals

2009 ◽  
Author(s):  
Satoshi Yokojima ◽  
Qi Gao ◽  
Shinichiro Nakamura ◽  
Dong-Qing Wei ◽  
Xi-Jun Wang
Keyword(s):  
Chemical Shift ◽  
Ab Initio ◽  
Molecular Orbitals ◽  
Nmr Chemical Shift ◽  
Chemical Shift Calculations

Anomalous stabilizing and destabilizing effects in some cyclic π-electron systems

1993 ◽  
Vol 71 (8) ◽  
pp. 1123-1127 ◽  
Author(s):  
Peter Politzer ◽  
M. Edward Grice ◽  
Jane S. Murray ◽  
Jorge M. Seminario
Keyword(s):  
Ab Initio ◽  
Lone Pair ◽  
Electrostatic Potentials ◽  
Electron Delocalization ◽  
Isodesmic Reaction ◽  
Computational Studies ◽  
Electron Systems ◽  
Molecular Electrostatic Potentials ◽  
Lone Pairs

Ab initio computational studies have been carried out for three molecules that are commonly classed as antiaromatic: cyclobutadiene (1), 1,3-diazacyclobutadiene (7), and 1,4-dihydropyrazine (6). Their dinitro and diamino derivatives were also investigated. Stabilizing or destabilizing energetic effects were quantified by means of the isodesmic reaction procedure at the MP2/6-31G*//HF/3-21G level, and calculated molecular electrostatic potentials (HF/STO-5G//HF/3-21G) were used as a probe of electron delocalization. Our results do not show extensive delocalization in the π systems of any one of the three parent molecules. The destabilization found for 1 and 7 is attributed primarily to strain and to repulsion between the localized π electrons in the C=C and C=N bonds, respectively. However, 6 is significantly stabilized, presumably due to limited delocalization of the nitrogen lone pairs. NH2 groups are highly stabilizing, apparently because of lone pair delocalization. NO2 is neither uniformly stabilizing nor destabilizing.

ChemInform Abstract: Cyclynes. Part 5. Pericyclynes: "Exploded Cycloalkanes" with Unusual Orbital Interactions and Conformational Properties.

1986 ◽  
Vol 17 (10) ◽  
Author(s):  
K. N. HOUK ◽  
L. T. SCOTT ◽  
N. G. RONDAN ◽  
D. C. SPELLMEYER ◽  
G. REINHARDT ◽  
...  
Keyword(s):  
Orbital Interactions ◽  
Conformational Properties
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