Comment pubs.acs.org/JPCA
Reply to “Comment on ‘Accurate Thermochemistry of Hydrocarbon Radicals via an Extended Generalized Bond Separation Reaction Scheme’” Matthew D. Wodrich,*,† Jérôme F. Gonthier,† Clémence Corminboeuf,*,† and Steven E. Wheeler*,‡ †
Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland Department of Chemistry, Texas A&M University, College Station, Texas 77842, United States
‡
J. Phys. Chem. A 2012, 116 (13), 3436−3447. DOI: 10.1021/jp212209q J. Phys. Chem. A 2012, 116. DOI: 10.1021/jp304377u
I
n his comment, Fishtik1 raises several issues with our earlier work2 in which we extended Wheeler, Houk, Schleyer, and Allen’s original generalized bond separation reaction (GBSR) scheme to include hydrocarbon radicals.3 To refresh the reader’s memory, GBSRs are a series of chemical reactions offering routes to increasingly accurate hydrocarbon thermochemistry. The GBSRs extend over five reaction classes (RCs), where RC1 is the least refined approach (based on isogyric equations) and RC5 the most refined (based on hyperhomodesmotic reactions). The intermediate classes correspond to isodesmic (RC2), hypohomodesmotic (RC3), and homodesmotic (RC4) approaches. Each of the GBSR RCs is associated with a series of reactants and products, predefined to ensure that only a single chemical reaction can be constructed in each class. The use of these predefined compound sets allows the facile construction of the relevant chemical equations corresponding to each RC. Moreover, the use of these predefined molecular sets necessitates
highly accurate thermochemistry for the minimum plausible number of reference compounds since only one chemical reaction per RC can be written for a compound of interest.3 The RCs are arranged into a hierarchy (RC1 → RC5), which, when ascended, increases the matching of various bonding and hybridization elements. As such, the GBSRs represent a logical route to increasingly accurate thermochemical data based on ideas that are very familiar to chemists (i.e., hybridization, bonding, etc.). Note that the “unique” aspect of the GBSRs contrasts Fishtik’s approach, which, ideally, would include an infinite number of reactions used to derive the thermochemical properties of a compound of interest. In his contribution,1 Fishtik correctly points out that some errors exist regarding the uniqueness of the GBSR schemes. To correct these shortcomings, we employed the same mathematical methods based on matrix manipulation as Fishtik. The new reactant (Chart 1) and product (Chart 2) lists provided here
Chart 1. Elemental Reactants for the GBSRs (Isogyric, Isodesmic, Hypohomodesmotic, Homodesmotic, and Hyperhomodesmotic) of Closed-Shell (Black) and Open-Shell (Blue) Hydrocarbons
Received: June 22, 2012 Revised: August 8, 2012 Published: August 9, 2012 © 2012 American Chemical Society
8794
dx.doi.org/10.1021/jp3061653 | J. Phys. Chem. A 2012, 116, 8794−8796
The Journal of Physical Chemistry A
Comment
Chart 2. Elemental Products for the GBSRs (Isogyric, Isodesmic, Hypohomodesmotic, Homodesmotic, and Hyperhomodesmotic) of Closed-Shell (Black) and Open-Shell (Blue) Hydrocarbons
yield a single chemical reaction for a closed-shell or open-shell doublet hydrocarbon (a few aberrant systems do exist, see ref 3 for details) based upon the rules defining each reaction class. A comparison of the lists in our original article2 with the refinements here show a considerable reduction in the number of reference species needed to obtain the appropriate chemical reactions. Indeed, this was based upon the overly complex nature of the original set of reaction schemes. Ascending the various reaction classes (isogyric → hyperhomodesmotic) causes the chemical environment surrounding a particular atom to be increasingly matched. While the systems and choice of equations
is straightforward for isogyric and isodesmic approaches, the higher GBSR rungs require larger numbers of reference species. For instance, the product list for (hypo)homodesmotic reactions is based on carbon chains consisting of three atoms (e.g., propane, propene, etc.). These reference species are sufficiently large to match the environment around the middle carbon atom. Thus, for propane, the hybridization, C(sp3), and the number of C−H bonds, two, are preserved for a methylene group in the molecule being examined. Other reactant species match the hybridization and C−H bonding of other carbon atoms. In hyperhomodesmotic reactions, the basic concept is extended 8795
dx.doi.org/10.1021/jp3061653 | J. Phys. Chem. A 2012, 116, 8794−8796
The Journal of Physical Chemistry A
Comment
further; instead of matching only the environment of the carbon atom, the environment of the entire carbon−carbon bond is matched. As such, the hyperhomodesmotic product species tend to be based on longer chains consisting of four carbon atoms (e.g., nbutane, 1-butene, etc.). This ensures that the central carbon−carbon bond of the four atom chain matches a bond in the molecule being examined. The main shortcoming of the previous product set of molecules (see Charts 1 and 2 in ref 2) was the unnecessarily long chain length of the terminal radical species. In fact, examination of any terminal radical will show that the bonding environment of the radical can be matched with a carbon chain consisting of one fewer atom for the (hypo)homodesmotic and hyperhomodesmotic reaction classes. A chain of only two atoms is required for (hypo)homodesmotic reactions, while hyperhomodesmotic equations require a three atom chain. This occurs precisely because the radical is located at the terminus of a chain! In (hypo)homodesmotic reactions, a terminal C(sp3)H2• can be matched using only the ethyl radical. Adding another carbon atom to the chain (to form the n-propyl radical) does not change the atomic environment of a terminal radical located in a geminal position. Similarly, a terminal C(sp3)H2−C(sp3)H2• bond can be matched by the n-propyl radical, larger species based on a four carbon chain are not necessary. These facts eliminate a large number of product species from the previous product list, as illustrated in Chart 2. Because many of the product species have changed with the simplification of the GBSR hierarchy provided here, we reexamined the effect brought on by an alteration in the reference species for homodesmotic and hyperhomodesmotic reactions. As the problems mentioned above mainly affect terminal radicals, a majority of the reactions presented in our previous work remain valid with these trimmed reactant/product sets. However, four molecules from the HCR27 set (see Chart 4 in ref 2) require refinement. As an example, eqs 1−8 represent “old” and “new” homodesmotic/hyperhomodesmotic reactions (RC4/RC5) for 5TS and 8TS from the HCR27 test set. Similarly, eqs 9 and 10 are old and new hyperhomodesmotic reactions for 9TS. As alluded to earlier, the length of the terminal radicals located on the product side of the new reactions are reduced in comparison to the old products, yet the reaction remains balanced according to the rules defining their respective reaction classes. Table 1 illustrates the reaction energy deviations
very small differences are evident, in general, the changes are less than 0.1 kcal/mol, which makes the modification of the specific chemical reactions inconsequential from a thermochemical perspective.
Table 1. Deviations from Estimated CCSD(T)/CBS for Old and New Homodesmotic/Hyperhomodesmotic Reactions of Selected Compounds; Values in kcal/mol
■
5TS
8TS
9TS 27TS
equation number
B3LYP/6311+G(d,p)
M06-2X/6311+G(d,p)
B97-dDsC/ 6311+G(d,p)
1 2 3 4 5 6 7 8 9 10 11 12
1.02 0.89 0.29 −0.16 0.37 0.31 0.05 −0.01 0.12 −0.27 −0.22 −0.19
0.36 0.23 0.54 −0.02 0.30 0.38 0.07 0.04 0.07 −0.16 0.18 0.27
0.08 0.09 0.02 0.14 0.05 0.20 −0.03 −0.02 0.13 −0.20 0.09 0.17
CCSD(T)/ cc-pVTZ −0.06 −0.13 0.05 0.04 −0.10 −0.07 −0.03 −0.01 0.01 0.10 −0.29 −0.33
In contrast to 5TS, 8TS, and 9TS ,the problem with 27TS does not arise from the reference compounds used to describe the radical, but in another small inadequacy of the old product set, as described in the Supporting Information of Fishtik’s contribution.1 As with the previous examples, evaluation of the reaction energies using the new homodesmotic equation yields no significant difference in the computed thermochemistry from the old reaction.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: matthew.wodrich@epfl.ch (M.D.W.); clemence. corminboeuf@epfl.ch (C.C.);
[email protected] (S.E.W.). Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We thank Professor Fishtik for introducing systematic ways to improve our earlier work. The Swiss NSF and EPFL are acknowledged for financial support (to C.C.), as well as the ACS Petroleum Research Fund (ACS PRF 50645-DNI6, to S.E.W.). REFERENCES
(1) Fishtik, I. J. Phys. Chem. A 2012, 116, DOI: 10.1021/jp304377u. (2) Wodrich, M. D.; Corminboeuf, C.; Wheeler, S. E. J. Phys. Chem. A 2012, 116, 3436. (3) Wheeler, S. E.; Houk, K. N.; Schleyer, P. v. R.; Allen, W. D. J. Am. Chem. Soc. 2009, 131, 2547.
from benchmark extrapolated CCSD(T)/CBS values for the HCR27 set compounds that required modification. While some 8796
dx.doi.org/10.1021/jp3061653 | J. Phys. Chem. A 2012, 116, 8794−8796
