Theoretical study of structure of alkali metal cyanates and isocyanates and their related ion pair SN2 reactions

Leung, Simon Shun-Wang; Streitwieser, Andrew

doi:10.1002/(sici)1096-987x(199809)19:12<1325::aid-jcc1>3.3.co;2-r

Cited by 2 publications

(3 citation statements)

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“…The above order reveals that the more stable CH 3 NCO will be the main product in the view of thermodynamics and kinetics if the reactions involve the inversion mechanisms. These results are somewhat different from those of Leung and Streitwieser 9, where it was predicted that 3a ‐X is lower in terms of Δ H

_{≠}^{ovr}

(298 K) than 3b ‐X (X = F and Cl) at the level of RHF/6‐31G( d,p ).…”

Section: Resultscontrasting

confidence: 82%

“…From the theoretical point of view, much less attention in the past has been paid to the ion pair S N 2 reactions that involve isocyanates. Leung and Streitwieser 9 examined the structures of alkali metal cyanates and isocyanates and their related ion pair S N 2 reactions with methyl halides Y + CH 3 X [Y = MNCO, MOCN, (MNCO) 2 (M = Li and Na); X = F and Cl]. Their results showed that methyl cyanate should form preferentially via a six‐member ring transition‐state structure on analysis of energetics of the transition structures if the reaction involves a monomer ion pair inversion pathway.…”

Section: Introductionmentioning

confidence: 99%

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Comprehensive mechanistic study of ion pair S_N2 reactions of lithium isocyanate and methyl halides

Sun¹,

Ren²,

Wong³

et al. 2006

Int J of Quantum Chemistry

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ABSTRACT:The anionic S N 2 reactions NCO Ϫ ϩ CH 3 X and ion pair S N 2 reactions LiNCO ϩ CH 3 X (X ϭ F, Cl, Br, and I) at saturated carbon with inversion and retention mechanisms were investigated at the level of MP2/6-311ϩG(d,p). There are two possible reaction pathways in the anionic S N 2 reactions, but eight in the ion pair S N 2 reactions. Calculated results suggest that the previously reported T-shaped isomer of lithium isocyanate does not exist. All the retention pathways are not favorable based on the analysis of transition structures. Two possible competitive reaction pathways proceed via two six-member ring inversion transition structures. It is found that there are two steps in the most favorable pathway, in which less stable lithium cyanate should be formed through the isomerization of lithium isocyanate and nucleophilic site (N) subsequently attacks methyl halides from the backside. The thermodynamically and kinetically favorable methyl isocyanate is predicted as major product both in the gas phase anionic and the ion pair S N 2 reactions. In addition, good correlations between the overall barriers relative to separated reactants, ⌬H ovr , with geometrical looseness parameter %L and the heterolytic cleavage energies of the COX and LiON (or LiOO) bonds are observed for the anionic and ion pair S N 2 reactions. The trend of variation of the overall barriers predicts the leaving ability of X increase in the order: F Ͻ Cl Ͻ Br Ͻ I. The polarized continuum model (PCM) has been used to evaluate the solvent effects on the two inversion pathways with six-member transition structures for the reactions of LiNCO ϩ CH 3 X. The calculations in solution indicate that solvent effects will retard the rate of reactions and the predicted product, methyl isocyanate, is same as the one in the gas phase.

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_{≠}^{ovr}

(298 K) than 3b ‐X (X = F and Cl) at the level of RHF/6‐31G( d,p ).…”

Section: Resultscontrasting

confidence: 82%

Section: Introductionmentioning

confidence: 99%

Comprehensive mechanistic study of ion pair S_N2 reactions of lithium isocyanate and methyl halides

Sun¹,

Ren²,

Wong³

et al. 2006

Int J of Quantum Chemistry

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“…Streitwieser et al5 extended the work to the higher alkyls employing different levels of theory (RHF, MP2, and B3LYP) with a 6‐31+G(d) basis set and discussed some steric effects for the ion‐pair displacement reactions. More recently, Leung and Streitwieser6 reported a theoretical study on ion‐pair S N 2 reactions of metal cynates and methyl halides.…”

Section: Introductionmentioning

confidence: 99%

Modified Gaussian‐2 level investigation of the identity ion‐pair S_N2 reactions of lithium halide and methyl halide with inversion and retention mechanisms

Ren

Chu

2004

J Comput Chem

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Identity ion-pair S(N)2 reactions LiX + CH(3)X --> XCH(3) + LiX (X = F, Cl, Br, and I) have been investigated in the gas phase and in solution at the level of the modified Gaussian-2 theory. Two possible reaction mechanisms, inversion and retention, are discussed. The reaction barriers relative to the complexes for the inversion mechanism [DeltaH(cent) ( not equal )(inv)] are found to be much higher than the corresponding values for the gas phase anionic S(N)2 reactions, decreasing in the following order: F (263.6 kJ mol(-1)) > Cl (203.3 kJ mol(-1)) > Br (174.7 kJ mol(-1)) > I (150.7 kJ mol(-1)). The barrier gaps between the two mechanisms [DeltaH(cent) ( not equal ) (ret) - DeltaH(cent) ( not equal ) (inv)] increase in the order F (-62.7 kJ mol(-1)) < Cl (4.4 kJ mol(-1)) < Br (24.9 kJ mol(-1)) < I (45.1 kJ mol(-1)). Thus, the retention mechanism is energetically favorable for fluorine and the inversion mechanism is favored for other halogens, in contrast to the anionic S(N)2 reactions at carbon where the inversion reaction channel is much more favorable for all of the halogens. The stabilization energies for the dipole-dipole complexes CH(3)X. LiX (DeltaH(comp)) are found to be similar for the entire set of systems with X = F, Cl, Br, and I, ranging from 53.4 kJ mol(-1) for I up to 58.9 kJ mol(-1) for F. The polarizable continuum model (PCM) has been used to evaluate the direct solvent effects on the energetics of the anionic and ion-pair S(N)2 reactions. The energetic profiles are found to be still double-well shaped for most of the ion-pair S(N)2 reactions in the solution, but the potential profile for reaction LiI + CH(3)I is predicted to be unimodal in the protic solvent. Good correlations between central barriers [DeltaH(cent) ( not equal ) (inv)] with the geometric looseness of the inversion transition state %C-X( not equal ), the dissociation energies of the C-X bond (D(C-X)) and Li-X bond (D(Li-X)) are observed, respectively.

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Theoretical study of structure of alkali metal cyanates and isocyanates and their related ion pair SN2 reactions

Cited by 2 publications

References 0 publications

Comprehensive mechanistic study of ion pair S_N2 reactions of lithium isocyanate and methyl halides

Comprehensive mechanistic study of ion pair S_N2 reactions of lithium isocyanate and methyl halides

Modified Gaussian‐2 level investigation of the identity ion‐pair S_N2 reactions of lithium halide and methyl halide with inversion and retention mechanisms

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Theoretical study of structure of alkali metal cyanates and isocyanates and their related ion pair SN2 reactions

Cited by 2 publications

References 0 publications

Comprehensive mechanistic study of ion pair SN2 reactions of lithium isocyanate and methyl halides

Comprehensive mechanistic study of ion pair SN2 reactions of lithium isocyanate and methyl halides

Modified Gaussian‐2 level investigation of the identity ion‐pair SN2 reactions of lithium halide and methyl halide with inversion and retention mechanisms

Contact Info

Product

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About

Comprehensive mechanistic study of ion pair S_N2 reactions of lithium isocyanate and methyl halides

Comprehensive mechanistic study of ion pair S_N2 reactions of lithium isocyanate and methyl halides

Modified Gaussian‐2 level investigation of the identity ion‐pair S_N2 reactions of lithium halide and methyl halide with inversion and retention mechanisms