Relative reactivity of three oxygen lone pairs of an α-nucleophile in SN2 reactions

1990 ◽  
Vol 68 (7) ◽  
pp. 1182-1185 ◽  
Author(s):  
J. L. Wolk ◽  
M. R. Hajnal ◽  
S. Hoz ◽  
R. M. Tarkka ◽  
E. Buncel
Keyword(s):  
Transition State ◽  
Lone Pair ◽  
Am1 Method ◽  
Activation Barriers ◽  
Molecular Plane ◽  
Relative Stabilities ◽  
Sn2 Reactions ◽  
Lone Pairs ◽  
Stability Order ◽  
Reactivity Order

The SN2 reaction of H2C=NO− with CH3Cl was investigated at the AM1, STO-3G, and 6-31 + G* levels. Three modes of approach of CH3Cl to the oxygen of the nucleophile were examined: syn and anti to the lone pair on the nitrogen in the molecular plane, and an approach along the π system of the oximate anion. The AM1 method differed markedly from the two abinitio methods, both in the reactivity order of the three geometries as well as in the relative stabilities of the products. The reactivity order calculated at the STO-3G and the 6-31 + G* levels was syn > π > anti (AM1 order: anti > syn > π). Product stability order calculated by the two abinitio methods was syn > anti > π (AMI order: anti > syn > π). Excellent agreement was obtained between percent bond elongations at the transition state predicted by the equation suggested by Shaik, Schlegel, and Wolfe, and the activation barriers calculated at the 6-31 + G* level. The relevance of the calculations with respect to the α-effect is also discussed. Keywords: oxygen lone pairs, aximate, SN2, abinitio.

COMPUTATIONAL INSIGHTS ON THE LONE PAIR INDUCED BARRIER MODULATION IN THE THERMAL REARRANGEMENT OF 6-HALO-2-PYRONES

2007 ◽  
Vol 06 (02) ◽  
pp. 233-243 ◽  
Author(s):  
LAKSHMINARAYANAN AKILANDESWARI ◽  
PONNAMBALAM VENUVANALINGAM
Keyword(s):  
Transition State ◽  
Ring Opening ◽  
Natural Bond Orbital ◽  
Lone Pair ◽  
Thermal Rearrangement ◽  
Lone Pairs ◽  
Sigmatropic Shift ◽  
Tandem Process

2-pyrones undergo intramolecular thermal rearrangement resulting in the migration of groups at 3-position to 5-position and vice-versa. Rearrangement of 6-halopyrone is a tandem process involving electrocyclic ring opening and closure (ERO & ERC), rotation, sigmatropic shift. It has been modeled at MP2/6-31g (d) level to understand the migratory aptitude of the halogens. Computations show that electrocyclic transition state and the corresponding intermediate which could not be located in 2-pyrone rearrangement have been located for 6-fluoropyrone and 6-chloropyrone. Halogens effectively modulate the barriers of all the steps involved and NBO (Natural Bond Orbital) analyses clearly reveal the involvement of lone pairs in activating rotation and sigmatropic shift, and deactivating ERO and ERC.

An NBO-TS predictive approach for torquoselectivity of ring opening in 3-substituted cyclobutenes

2017 ◽  
Author(s):  
Arpita Yadav ◽  
Dasari L V K Prasad ◽  
Veejendra Yadav
Keyword(s):  
Transition State ◽  
Ring Opening ◽  
Natural Bond Orbital ◽  
Theoretical Calculations ◽  
Lone Pair ◽  
Product Distribution ◽  
Acceptor Substituent ◽  
Important Contributor ◽  
Predictive Approach ◽  
Donor Substituent

<p>The torquoselectivity, the inward or outward ring opening of 3-substituted cyclobutenes, is conventionally guided by the donor and/or acceptor ability of the substituent (S). It is typically predicted by estimating the respective ring opening transition state (TS) barriers. While there is no known dissent in regard to the outward rotation of electron-rich substituents from the approaches of TS calculations, the inward rotation was predicted for some electron-accepting substituents and outward for others. To address this divergence in predicting the torquoselectivity, we have used reliable orbital descriptors through natural bond orbital theoretical calculations and demonstrated that (a) interactions <i>n</i><i><sub>S</sub></i>→s*<sub>C3C4</sub> for a lone pair containing substituent, s<sub>S</sub>→s*<sub>C3C4</sub> for a s-donor substituent, s<sub>C3C4</sub>→p*<sub>S</sub> for a resonance-accepting substituent and s<sub>C3C4</sub>→s*<sub>S</sub> for a s-acceptor substituent constitute the true electronic controls of torquoselectivity, and (b) reversibility of the ring opening event is an additional important contributor to the observed product distribution.</p>

Theoretical perspective of the lone pair activity influence on band gap and SHG response of lead borates

RSC Advances ◽  
2015 ◽  
Vol 5 (97) ◽  
pp. 79882-79887 ◽  
Author(s):  
Danni Li ◽  
Qun Jing ◽  
Chen Lei ◽  
Shilie Pan ◽  
Bingbing Zhang ◽  
...  
Keyword(s):  
Band Gap ◽  
Theoretical Perspective ◽  
Lone Pair ◽  
Red Shift ◽  
Lone Pairs

Metal lone pairs play an important role in determining the SHG enhancement and bandgap red shift.

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.

scholarly journals Expression and interactions of stereochemically active lone pairs and their relation to structural distortions and thermal conductivity

IUCrJ ◽  
2020 ◽  
Vol 7 (3) ◽  
pp. 480-489 ◽  
Author(s):  
Kasper Tolborg ◽  
Carlo Gatti ◽  
Bo B. Iversen
Keyword(s):  
Thermal Conductivity ◽  
Density Functional ◽  
Lone Pair ◽  
Density Functional Theory Calculations ◽  
Orbital Interaction ◽  
Void Space ◽  
Metal Compounds ◽  
Bond Angles ◽  
Lone Pairs ◽  
Bonding Effect

In chemistry, stereochemically active lone pairs are typically described as an important non-bonding effect, and recent interest has centred on understanding the derived effect of lone pair expression on physical properties such as thermal conductivity. To manipulate such properties, it is essential to understand the conditions that lead to lone pair expression and provide a quantitative chemical description of their identity to allow comparison between systems. Here, density functional theory calculations are used first to establish the presence of stereochemically active lone pairs on antimony in the archetypical chalcogenide MnSb2O4. The lone pairs are formed through a similar mechanism to those in binary post-transition metal compounds in an oxidation state of two less than their main group number [e.g. Pb(II) and Sb(III)], where the degree of orbital interaction (covalency) determines the expression of the lone pair. In MnSb2O4 the Sb lone pairs interact through a void space in the crystal structure, and their their mutual repulsion is minimized by introducing a deflection angle. This angle increases significantly with decreasing Sb—Sb distance introduced by simulating high pressure, thus showing the highly destabilizing nature of the lone pair interactions. Analysis of the chemical bonding in MnSb2O4 shows that it is dominated by polar covalent interactions with significant contributions both from charge accumulation in the bonding regions and from charge transfer. A database search of related ternary chalcogenide structures shows that, for structures with a lone pair (SbX 3 units), the degree of lone pair expression is largely determined by whether the antimony–chalcogen units are connected or not, suggesting a cooperative effect. Isolated SbX 3 units have larger X—Sb—X bond angles and therefore weaker lone pair expression than connected units. Since increased lone pair expression is equivalent to an increased orbital interaction (covalent bonding), which typically leads to increased heat conduction, this can explain the previously established correlation between larger bond angles and lower thermal conductivity. Thus, it appears that for these chalcogenides, lone pair expression and thermal conductivity may be related through the degree of covalency of the system.

Rabbit ears concepts of water lone pairs: a reply to comments of Hiberty, Danovich, and Shaik

2015 ◽  
Vol 16 (3) ◽  
pp. 694-696 ◽  
Author(s):  
A. D. Clauss ◽  
M. Ayoub ◽  
J. W. Moore ◽  
C. R. Landis ◽  
F. Weinhold
Keyword(s):  
Lone Pair ◽  
Lone Pairs ◽  
Rabbit Ears

We respond to recent comments (Hibertyet al., 2015) on our earlier article (Clausset al., 2014) concerning “rabbit ears” depictions of lone pair orbitals in water and other species.

Étude théorique des réactions d'abstraction d'hydrogène , avec R, X≡H, CH3, NH2,OH et F

1985 ◽  
Vol 63 (7) ◽  
pp. 1447-1456 ◽  
Author(s):  
Georges Leroy ◽  
Michel Sana ◽  
Anne Tinant
Keyword(s):  
Transition State ◽  
Rate Constants ◽  
Transition State Theory ◽  
Hydrogen Abstraction ◽  
State Theory ◽  
Arrhenius Behavior ◽  
Activation Barriers ◽  
Empirical Procedure ◽  
Semi Empirical ◽  
Theory And Experiment

Hydrogen abstraction reactions [Formula: see text] with R, X≡H, CH3, NH2,OH, and F have been studied at the abinitio 6-31G – UHF level. However, energetic properties were computed at the CI level. Rate constants and Arrhenius parameters have been obtained using the transition state theory formalism with Eckart's tunneling correction. The discrepancy between theoretical and experimental results led us to elaborate a semi-empirical procedure to calculate activation barriers, in which the bonds R—H and X—H are represented by Morse curves. Thus, the agreement between theory and experiment is much better. Moreover, the results obtained by this procedure demonstrate the non-Arrhenius behavior of all the reactions under consideration and allow us to rationalize a large number of experimental facts.

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