Structures, intramolecular rotation barriers, and thermochemical properties of radicals derived from h atom loss in mono-, di-, and trichloromethanol and parent chloromethanols

Document Type

Article

Publication Date

5-10-2001

Abstract

Structures and thermochemical properties, ΔHf298,° S298° and CP(T) of three (mono-, di-, and tri-) chloromethanols; three chloromethoxy and the two hydroxychloromethyl radicals are determined by ab initio and density functional calculations. The molecular structures and vibration frequencies are determined at the B3LYP/ 6-31G(d,p) density functional level, with single point calculations for the energy at the B3LYP/6-311+G(3df,2p), QCISD(T)/6-31G(d,p), and CBSQ//B3LYP/6-31G(d,p) levels. The vibration frequencies are scaled for zero-point energies and for thermal corrections. The enthalpies of formation (ΔHf298°) are determined at each calculation level using several isodesmic reactions. Standard entropy (S298°) and heat capacity (Cp(T)'s, 300 ≤ T/K ≤ 1500) from vibrational, translational, and external rotational contributions are calculated using the rigid-rotor-harmonic-oscillator approximation based on the vibration frequencies and structures obtained from the density functional study. Potential barriers for internal rotation of the hydroxyl group are calculated at the B3LYP/6-31G(d,p) level, and hindered internal rotational contributions to entropy and heat capacity are calculated by summation over the energy levels obtained by direct diagonalization on the Hamiltonian matrix of hindered internal rotations. An evaluation of data from the isodesmic reactions at each calculation level results in the enthalpy of formation values: -58.07 ± 0.69, -65.88 ± 0.76, and -65.96 ± 0.76 kcal/ mol for mono-, di-, and trichloromethanol, respectively. The standard enthalpies for the mono-, di-, and trichloromethoxy radicals are -5.13 ± 2.18, -7.65 ± 2.25, and -9.05 ± 2.24 kcal/mol, respectively. The standard enthalpies for the hydroxymono- and hydroxydichloromethyl radicals are -14.46 ± 1.75 and -22.54 ± 1.83 kcal/mol. Bond energies for the RO-H bond are 105.04, 110.33, and 109.01 kcal/mol, respectively. Bond energies for the R-OH bonds are 95.20, 98.81, and 94.39 kcal/mol, respectively. Groups for use in Benson type additivity estimations are determined for the carbon bonded to oxygen and chlorine(s). The enthalpy values for the C/Cl/H2/O, C/Cl2/H/O, and C/Cl3/O groups are -20.17, -27.98, and -28.06 kcal/ mol, respectively. Hydrogen bond increment groups for the chloromethoxy and hydroxychloromethyl radicals are also developed. The bond energies and ΔHf298° values suggest that the electronegative CI(s) on the methyl serve to increase and RO-H bond energy. ΔHf298° for CH2OH, CH3CHOH, and C2H5O are also determined and compared with literature data, and recommended values are -3.97 ± 0.22, -13.34 ± 0.84, and -3.90 ± 1.27 kcal/mol, respectively. © 2001 American Chemical Society.

Identifier

0035837975 (Scopus)

Publication Title

Journal of Physical Chemistry A

External Full Text Location

https://doi.org/10.1021/jp003877t

ISSN

10895639

First Page

4504

Last Page

4516

Issue

18

Volume

105

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