Correction: Acid-catalyzed carboxylic acid esterification and ester hydrolysis mechanism: acylium ion as a sharing active intermediate via a spontaneous trimolecular reaction based on density functional theory calculation and supported by electrospray ionization-mass spectrometry

Abstract

Correction for ‘Acid-catalyzed carboxylic acid esterification and ester hydrolysis mechanism: acylium ion as a sharing active intermediate via a spontaneous trimolecular reaction based on density functional theory calculation and supported by electrospray ionization-mass spectrometry’ by Hongchang Shi et al., Phys. Chem. Chem. Phys., 2015, DOI: 10.1039/c5cp02914g.

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In the published article, the activation energy E_a in Table 1 is incorrect. The E_a data actually are the activation energies that the proton (H⁺) transfers to hydroxyl-oxygen or alkyl-oxygen, not those of generating acylium ions. For example, the C–O bond distance is 1.527 and 1.538 Å in the Fig. 1 (2) and Fig. 2 (2), respectively. The bond has been partly, but not completely disconnected, so the E_a (9.0 and 3.4 kcal mol^-1) cannot lead to the formation of the corresponding acylium ions. The E_a that can generate acylium ions must be recalculated in accordance with a new scheme.

Scheme 5 (1) shows that an initial bimolecular system of RCOOR₂ + R′OH₂⁺ goes through an effective Lewis collision to complete the hydroxyl-oxygen or alkyl-oxygen protonation. Because the water or alcohol molecule is a good leaving group, this collision caused the C–OR₂ bond disconnection, which generates an acylium ion and a water or an alcohol molecule, as shown in Scheme 5 (2)

Scheme 5 1 is the initial bimolecular system, 2 is the system after an effective Lewis collision. The bond distance is in angstroms.

In the optimization of system 2, the R–C⁺(O)----O(H)R₂ bond distance set here is equal to 2.500 Å. In this spacing, the interaction between the acylium ion and O(H)R₂ molecule is slight, that is, the C–OR₂ bond is completely disconnected. The 2.500 Å is frozen in the optimization. The E₂ of system 2 is higher than the E₁ of 1. The elevated energy is the E_a of the hydroxyl-oxygen or alkyl-oxygen protonation: E_a = E₂ − E₁.

For the protonation of CH₃COOH + CH₃OH₂⁺ (configuration A), the calculation result is shown as Fig. 1.

Fig. 1 The optimized initial bimolecular system 1 and the energy E₁ of CH₃COOH + CH₃OH₂⁺ (configuration A); the optimized system 2 and the energy E₂ of CH₃C⁺O + H₂O + CH₃OH. The bond distance is in angstroms.

E_a = E₂ − E₁ = −345.192684 − (−345.218981) = 0.026297 a.u. = 16.5 kcal mol⁻¹.

For the alkyl-oxygen protonation of C₆H₅COOCH₃ + H₃O⁺, the calculation result is shown as Fig. 2.

E_a = E₂ − E₁ = 0.017276 a.u. = 10.8 kcal mol⁻¹.

Fig. 2 The optimized initial bimolecular system 1 and the energy E₁ of C₆H₅COOCH₃ + H₃O⁺ (configuration B); the optimized system 2 and the energy E₂ of C₆H₅C⁺O + CH₃OH + H₂O. The bond distance is in angstroms.

For acetic acid, benzoic acid and their methyl ester, the E_a of their hydroxyl-oxygen and alkyl-oxygen protonations are changed as in the following Table 1.

Table 1 The hydroxyl-oxygen and alkyl-oxygen protonation E_a of acetic acid, benzoic acid and their methyl esters

System	Config.	E 1/a.u.	E 2/a.u.	E a
CH3COOH + CH3OH2^+	A	−345.218981	−345.192684	16.5
	B	−345.217477	−345.190683	16.8
CH3COOCH3 + H3O^+	A	−345.220936	−345.196416	15.4
	B	−345.217162	−345.192777	15.3
C6H5COOH + CH3OH2^+	A	−536.945912	−536.919611	16.5
	B	−536.944778	−536.920010	15.5
C6H5COOCH3 + H3O^+	A	−536.946688	−536.922924	14.9
	B	−536.940190	−536.922914	10.8

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The E_a in Table 1 are much higher than that in the Table 1 of the published article, which is reasonable because the C–OR₂ bond is completely disconnected, and this requires more energy.

The E_a values of the hydroxyl-oxygen and alkyl-oxygen protonation in the Abstract, Introduction, Scheme 7 and Scheme 8 should be accordingly changed in accordance with Table 1.

The Table 5 in the published article has been changed as following Table 5. It is also the E_a calculations of hydroxyl-oxygen or alkyl-oxygen protonation.

Table 5 E _a of hydroxyl-oxygen or alkyl-oxygen protonation by CH₃OH₂⁺ or H₃O⁺ in 2,4,6-trimethylbenzoic acid and its methyl ester

Molecular system	Config.	E 1/a.u.	E 2/a.u.	E a/kcal mol^−1
(CH3)3C6H2COOH + CH3OH2^+	A	−654.839323	−654.827446	7.4
	B	−654.839660	−654.827455	7.7
(CH3)3C6H2COOCH3 + H3O^+	A	−654.842084	−654.830307	7.4
	B	−654.837079	−654.829814	4.6

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For the E_a calculation of carbonyl-oxygen protonation in the published article, the calculation scheme and result (Table 2) are correct because the protonation is a simple proton transfer.

The change mentioned above not only does not affect the conclusions in the published article, but also helps to support these conclusions.

The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.

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