Article pubs.acs.org/JPCA
Enthalpy of Formation of Anisole: Implications for the Controversy on the O−H Bond Dissociation Enthalpy in Phenol Ricardo G. Simões,† Filipe Agapito,† Hermínio P. Diogo,‡ and Manuel E. Minas da Piedade*,† †
Centro de Química e Bioquímica e Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal ‡ Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa,1049-001 Lisboa, Portugal S Supporting Information *
ABSTRACT: Significant discrepancies in the literature data for the enthalpy of formation of gaseous anisole, ΔfHom(PhOCH3, g), have fueled an ongoing controversy regarding the most reliable enthalpy of formation of the phenoxy radical and of the gas phase O−H bond dissociation enthalpy, DHo(PhO−H), in phenol. In the present work ΔfHom(PhOCH3, g) was reassessed using a combination of calorimetric determinations and high-level (W2-F12) ab initio calculations. Static-bomb combustion calorimetry led to the standard molar enthalpy of formation of liquid anisole at 298.15 K, ΔfHom(PhOCH3, l) = −(117.1 ± 1.4) kJ·mol−1. The corresponding enthalpy of vaporization was obtained as, ΔvapHom(PhOCH3) = 46.41 ± 0.26 kJ·mol−1, by Calvet-drop microcalorimetry. These results give ΔfHom(PhOCH3, g) = −(70.7 ± 1.4) kJ·mol−1, in excellent agreement with ΔfHom(PhOCH3, g) = −(70.8 ± 3.2) kJ·mol−1, obtained from the W2-F12 calculations. The ΔfHom(PhOCH3, g) here recommended leads to ΔfHom(PhO•, g) = 55.5 ± 2.4 kJ· mol−1 and DH°(PhO−H) = 368.1 ± 2.6 kJ·mol−1.
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ΔfHom(PhOCH3, l) and ΔvapHom(PhOCH3) results. There are, however, significant discrepancies between the published ΔfHmo(PhOCH3, l)15−18 and ΔvapHmo(PhOCH3)18−23 data used by various authors to derive ΔfHom(PhOCH3, g) at 298.15 K. This leads to substantially different ΔfHom(PhO•, g) and DH°(PhO−H) values, depending on the particular combination of data chosen. For example, based on a review of literature data and on predictions from a group contribution scheme, Mulder et al.2 selected ΔfHom(PhOCH3, g) = −(76.7 ± 1.0) kJ·mol −1 , which diverges by 8.8 kJ·mol −1 from ΔfHom(PhOCH3, g) = −(67.9 ± 0.8) kJ·mol−1 given in Pedley’s compilation.6 This choice leads to DH°(PhO−H) = 362.8 ± 2.9 kJ·mol−1,2 a value 8.5 kJ·mol−1 lower than the previously recommended DH°(PhO−H) = 371.3 ± 2.3 kJ·mol−1,1 which is compatible with Pedley’s selection for ΔfHom(PhOCH3, g). This conflict has not been resolved by theory, since the results of high-level methods, such as G3, G3B3, CBS-APNO, CBS extrapolated CCSD(T), and CASPT2, span a range of 22 kJ· mol−1 (see ref 24 and references cited therein). The present work describes the redetermination of the standard molar enthalpies of formation and vaporization of liquid anisole at 298.15 K, using combustion calorimetry and Calvet-drop microcalorimetry. From the obtained values a new experimental value for ΔfHom(PhOCH3, g) is proposed. The reliability of this value was further supported by high-level ab initio calculations (W2-F12 procedure).
INTRODUCTION The enthalpy of formation of anisole, 1, came under close scrutiny in recent years since it is implicated in an ongoing controversy regarding the “best” values for the enthalpy of formation of the phenoxy radical, ΔfHom(PhO•, g), and of the gas-phase O−H bond dissociation enthalpy in phenol, DHo(PhO−H).1−4 The experimental measurement of DHo(PhO−H) through a direct study of the O−H bond homolysis PhOH(g) → PhO•(g) + H•(g)
(1)
has been hampered by the presence of secondary reactions involving the tautomerization of phenol to cyclohexa-2,4- and cyclohexa-2,5-dienones. The O−H bond dissociation enthalpy in phenol may, however, be derived using Hess’ law,5 if ΔfHom(PhO•, g) is obtained from a process other than (1) and combined with ΔfHom(PhOH, g) and ΔfHom(H•, g), which are accurately known.6,7 Several authors chose to determine ΔfHom(PhO•, g) via gas-phase kinetic studies of reaction:8−14 PhOCH3(g) → PhO•(g) + CH3•(g)
(2)
Received: July 21, 2014 Revised: October 22, 2014
a strategy which requires a reliable value of ΔfHom(PhOCH3, g). This quantity can be derived by combining experimental © XXXX American Chemical Society
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dx.doi.org/10.1021/jp507267f | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
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Article
MATERIALS AND METHODS Sample Purification and Characterization. Anisole (Fluka, 99.9% GC) was purified by fractional distillation at 383 K and 46.7 kPa using a Vigreux column, dried over calcium hydride, and redistilled at the same reduced pressure. The obtained sample was stored and handled under an oxygen and water free (
