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Comment on “Enthalpy Difference between Conformations of Normal Alkanes: Raman Spectroscopy Study of n-Pentane and n-Butane” Gyula Tasi* and Bala´zs Nagy Department of Applied and EnVironmental Chemistry, UniVersity of Szeged, Rerrich B. te´r 1, H-6720 Szeged, Hungary ReceiVed: April 20, 2010 Very recently, several papers have been concerned with the thermochemistry of conformational equilibria of n-alkanes.1-3 Balabin has reported benchmark enthalpy differences determined by Raman spectroscopy for the conformers of n-butane and n-pentane.1 The Raman spectroscopic studies have revealed that n-butane is one of the worst substances from an experimental point of view; the trans conformer has no contamination-free Raman-active vibrations.1 For this reason, we do not consider the case of n-butane here. The first column of Table 1 shows the experimental enthalpy differences1 in kJ/mol for the conformers of n-pentane. In Table 1, the following notation4,5 is used for the conformers: g( for gauche torsional angles around (60°, x( for cross or perpendicular torsional angles around (90°, and t for trans. It can be seen that ∆Hgg is less than 2∆Htg, that is, the enthalpy difference for the conformers of n-pentane is nonadditive. Due to experimental difficulties, Balabin could not determine the relative enthalpy of the gx (denoted (incorrectly) as g+g- in the paper) conformer.1 By comparing the high-quality experimental values to computed data, the following conclusion was drawn.1 “More reliably, ab initio data is expected in the next 5-10 years”. About ten years ago,6 we parametrized an all-electron tight binding method (SEOEM, scaled effective one-electron method) for alkanes using RMP2(fc)/6-311G** equilibrium molecular geometries as reference geometries, RHF/6-311++G** permanent electric dipole moment vectors obtained at the reference geometries, and then high-quality G2 total molecular energies7 at 0 K including zero-point vibrational energy corrections. It was found that the reference molecular geometries and dipole moments for alkanes were very close to the experimental values.8 In the 1998 paper6 mentioned above, Table 1 presented the reference molecular properties used in the parametrization process along with the standard enthalpies of formation obtained within the rigid rotor-harmonic oscillator (RRHO) approxima* To whom correspondence should be addressed. E-mail:
[email protected].
TABLE 1: Relative Enthalpies and Energies (in kJ/mol) of the Conformers of n-Pentane conformers tt tg gg gx
experimenta 0 2.59 ( 0.02 3.93 ( 0.09
G2b
W1h-valc
0 2.58 3.92 11.97
0 2.57 4.02 11.77
a Experimental relative enthalpies from ref 1. b Computed relative enthalpies from ref 6. c Computed relative energies from ref 2.
tion (see the last column of the table in question). Subtracting the standard enthalpy of formation of the tt conformer of n-pentane (-147.9 kJ/mol) from those of the conformers of n-pentane with higher energies (tg: -145.3, gg: -144.0, and gx: -135.9 kJ/mol), the following enthalpy differences can be obtained: tg: 2.6; gg: 3.9; and gx: 12.0 kJ/mol. The second column of Table 1 presented here shows the previous numbers with two decimal figures. It can be seen that our computed enthalpy differences precisely match the benchmark experimental values1 of Balabin, and the missing number for the gx conformer is also available. Table 1 also presents the relative energies (not enthalpies) of the conformers of n-pentane obtained by Martin et al. via the more sophisticated W1h-val method.2 Of course, we cannot state that the computed G2 enthalpy differences for conformers in general are exact; however, the careful selection of model chemistry for alkanes resulted the same numbers6 in 1998 as the best experimental values1 in 2009. Furthermore, with the help of then very time consuming quantum chemical computations, it was possible to derive and verify rules4,5 for a more correct enumeration of the conformers of n-alkanes. These rules have been successfully applied in recent research projects.2,3,9,10 References and Notes (1) Balabin, R. M. J. Phys. Chem. A 2009, 113, 1012. (2) Gruzman, D.; Karton, A.; Martin, J. M. L. J. Phys. Chem. A 2009, 113, 11974. (3) Bakowies, D. J. Phys. Chem. A 2009, 113, 11517. (4) Tasi, G.; Mizukami, F.; Pa´linko´, I.; Csontos, J.; Gyo¨rffy, W.; Nair, P.; Maeda, K.; Toba, M.; Niwa, S.; Kiyozumi, Y.; Kiricsi, I. J. Phys. Chem. A 1998, 102, 7698. (5) Tasi, G.; Mizukami, F.; Csontos, J.; Gyo¨rffy, W.; Pa´linkoˇ, I. J. Math. Chem. 2000, 27, 191. (6) Tasi, G.; Mizukami, F. J. Chem. Inf. Comput. Sci. 1998, 38, 632. (7) Curtiss, L. A.; Raghavachari, K.; Trucks, G. W.; Pople, J. A. J. Chem. Phys. 1991, 94, 7221. (8) Tasi, G.; Mizukami, F.; Pa´linko´, I. THEOCHEM 1997, 401, 21. (9) Vansteenkiste, P.; Pauwels, E.; Van Speybroeck, V.; Waroquier, M. J. Phys. Chem. A 2005, 109, 9617. (10) Krinas, C. S.; Demetropoulos, I. N. Chem. Phys. Lett. 2007, 433, 422.
JP103549E
10.1021/jp103549e 2010 American Chemical Society Published on Web 05/28/2010
