Active Thermochemical Tables: Sequential Bond Dissociation Enthalpies of Methane, Ethane, and Methanol and the Related Thermochemistry.

Branko Ruscic

doi:10.1021/acs.jpca.5b01346

Abstract

Active Thermochemical Tables (ATcT) thermochemistry for the sequential bond dissociations of methane, ethane, and methanol systems were obtained by analyzing and solving a very large thermochemical network (TN). Values for all possible C-H, C-C, C-O, and O-H bond dissociation enthalpies at 298.15 K (BDE298) and bond dissociation energies at 0 K (D0) are presented. The corresponding ATcT standard gas-phase enthalpies of formation of the resulting CHn, n = 4-0 species (methane, methyl, methylene, methylidyne, and carbon atom), C2Hn, n = 6-0 species (ethane, ethyl, ethylene, ethylidene, vinyl, ethylidyne, acetylene, vinylidene, ethynyl, and ethynylene), and COHn, n = 4-0 species (methanol, hydroxymethyl, methoxy, formaldehyde, hydroxymethylene, formyl, isoformyl, and carbon monoxide) are also presented. The ATcT thermochemistry of carbon dioxide, water, hydroxyl, and carbon, oxygen, and hydrogen atoms is also included, together with the sequential BDEs of CO2 and H2O. The provenances of the ATcT enthalpies of formation, which are quite distributed and involve a large number of relevant determinations, are analyzed by variance decomposition and discussed in terms of principal contributions. The underlying reasons for periodic appearances of remarkably low and/or unusually high BDEs, alternating along the dissociation sequences, are analyzed and quantitatively rationalized. The present ATcT results are the most accurate thermochemical values currently available for these species.

Highlights

Bond dissociation enthalpy (BDE) is one of the most fundamental concepts in chemistry
BDET is defined as the change in standard[1] enthalpy at temperature T that occurs upon cleavage of a particular chemical bond, assuming ideal gas behavior of the dissociating chemical species and its dissociation products
The two quantities are truly synonymous only in the limit of 0 K, at which point BDE0 is identical to the 0 K bond dissociation energy, a spectroscopic quantity often denoted as D0

Summary

Introduction

Bond dissociation enthalpy (BDE) is one of the most fundamental concepts in chemistry. BDET is defined as the change in standard[1] enthalpy at temperature T that occurs upon cleavage of a particular chemical bond, assuming ideal gas behavior of the dissociating chemical species and its dissociation products. D0 is defined as the difference in energy between a chemical species and the appropriate dissociation asymptote, where both the reactant and its dissociation products are in their lowest existing rovibrational energy level of the lowest electronic state.[3] For regular chemical species, the lowest electronic state is obviously the ground electronic state,[4] that is not necessarily the case for nonstandard thermochemical species, such as those that are assumed to be thermodynamically equilibrated only within a manifold of electronic states of a particular spin multiplicity possibly different than the ground-state multiplicity e.g., singlet methylene, quartet methylidyne, singlet oxygen, etc. The lowest electronic state is obviously the ground electronic state,[4] that is not necessarily the case for nonstandard thermochemical species, such as those that are assumed to be thermodynamically equilibrated only within a manifold of electronic states of a particular spin multiplicity possibly different than the ground-state multiplicity e.g., singlet methylene, quartet methylidyne, singlet oxygen, etc. (vide inf ra)

Methods

Results

Conclusion