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DFT studies of the conversion of four mesylate esters during reaction with ammonia.

Nowacki A, Sikora K, Dmochowska B, Wiśniewski A - J Mol Model (2013)

Bottom Line: Solvent effect corrections were computed using PCM/B3LYP/6-31+G** level.The bifurcation at C2 causes a significant activation barrier increase.The reaction 4 is the slowest one in all environments.

View Article: PubMed Central - PubMed

Affiliation: Faculty of Chemistry, University of Gdańsk, Sobieskiego 18, 80-952 Gdańsk, Poland. anowacki@chem.univ.gda.pl

ABSTRACT
The energetics of the Menshutkin-like reaction between four mesylate derivatives and ammonia have been computed using B3LYP functional with the 6-31+G** basis set. Additionally, MPW1K/6-31+G** level calculations were carried out to estimate activation barrier heights in the gas phase. Solvent effect corrections were computed using PCM/B3LYP/6-31+G** level. The conversion of the reactant complexes into ion pairs is accompanied by a strong energy decrease in the gas phase and in all solvents. The ion pairs are stabilized with two strong hydrogen bonds in the gas phase. The bifurcation at C2 causes a significant activation barrier increase. Also, bifurcation at C5 leads to noticeable barrier height differentiation. Both B3LYP/6-31+G** and MPW1K/6-31+G** activation barriers suggest the reaction 2 (2a + NH3) to be the fastest in the gas phase. The reaction 4 is the slowest one in all environments.

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Comparison of activation barriers for reactions 1 and 2 with different nucleophiles in the gas phase
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Fig7: Comparison of activation barriers for reactions 1 and 2 with different nucleophiles in the gas phase

Mentions: The calculated barriers are higher than those for the reactions with trimethylamine [21] and pyridine [22, 23], which corresponds to the lower basicity of ammonia in a vacuum (Fig. 7). The proton affinity of ammonia is 204.0 kcal mol−1, but is 225.1 kcal mol−1 and 220.8 kcal mol−1 for trimethylamine and pyridine respectively [38]. The energy barrier is the lowest for reaction 2 but the highest for reaction 4. Interestingly, the energy barrier for reaction 1 is higher than for reactions 2 and 3 according to both B3LYP and MPW1K methods. This stands in contrast to the reactions of the same mesylate derivatives with other nucleophiles studied earlier [21–23], where the barrier was the lowest for the reaction of methyl mesylate with the corresponding nucleophile (Fig. 7). Presumably, hydrogen bond formation between the endocyclic oxygen atom and the hydrogen atom attached to the nitrogen atom stabilizes the transition state geometry. Such an interaction cannot occur in reaction 1.Fig. 7


DFT studies of the conversion of four mesylate esters during reaction with ammonia.

Nowacki A, Sikora K, Dmochowska B, Wiśniewski A - J Mol Model (2013)

Comparison of activation barriers for reactions 1 and 2 with different nucleophiles in the gas phase
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3713272&req=5

Fig7: Comparison of activation barriers for reactions 1 and 2 with different nucleophiles in the gas phase
Mentions: The calculated barriers are higher than those for the reactions with trimethylamine [21] and pyridine [22, 23], which corresponds to the lower basicity of ammonia in a vacuum (Fig. 7). The proton affinity of ammonia is 204.0 kcal mol−1, but is 225.1 kcal mol−1 and 220.8 kcal mol−1 for trimethylamine and pyridine respectively [38]. The energy barrier is the lowest for reaction 2 but the highest for reaction 4. Interestingly, the energy barrier for reaction 1 is higher than for reactions 2 and 3 according to both B3LYP and MPW1K methods. This stands in contrast to the reactions of the same mesylate derivatives with other nucleophiles studied earlier [21–23], where the barrier was the lowest for the reaction of methyl mesylate with the corresponding nucleophile (Fig. 7). Presumably, hydrogen bond formation between the endocyclic oxygen atom and the hydrogen atom attached to the nitrogen atom stabilizes the transition state geometry. Such an interaction cannot occur in reaction 1.Fig. 7

Bottom Line: Solvent effect corrections were computed using PCM/B3LYP/6-31+G** level.The bifurcation at C2 causes a significant activation barrier increase.The reaction 4 is the slowest one in all environments.

View Article: PubMed Central - PubMed

Affiliation: Faculty of Chemistry, University of Gdańsk, Sobieskiego 18, 80-952 Gdańsk, Poland. anowacki@chem.univ.gda.pl

ABSTRACT
The energetics of the Menshutkin-like reaction between four mesylate derivatives and ammonia have been computed using B3LYP functional with the 6-31+G** basis set. Additionally, MPW1K/6-31+G** level calculations were carried out to estimate activation barrier heights in the gas phase. Solvent effect corrections were computed using PCM/B3LYP/6-31+G** level. The conversion of the reactant complexes into ion pairs is accompanied by a strong energy decrease in the gas phase and in all solvents. The ion pairs are stabilized with two strong hydrogen bonds in the gas phase. The bifurcation at C2 causes a significant activation barrier increase. Also, bifurcation at C5 leads to noticeable barrier height differentiation. Both B3LYP/6-31+G** and MPW1K/6-31+G** activation barriers suggest the reaction 2 (2a + NH3) to be the fastest in the gas phase. The reaction 4 is the slowest one in all environments.

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