COMMENT pubs.acs.org/JPCA
Comment on “Rotational Barriers in Azobenzene and Azonaphthalene” Robert J. Meier* DSM Research, P.O. Box 18, 6160 Md Geleen, The Netherlands he paper by Klug and Burcl1 addresses the interesting problem of rotational barriers in some conjugated molecules, which seems somewhat problematic in several cases. The authors suggest that DFT methods are suitable for the evaluation of the rotational barriers for the named species. The barrier in benzaldehyde is considered to be over 32 kJ/mol, as they say is in agreement with the theoretical data reported by Speakman,2 which is the only justification that their values would be quantitatively reliable. Speakman et al., however, have only tentatively suggested that Pitzer’s model (applied when interpreting experimental data) would not be appropriate for benzaldehyde, but this has by no means been proven. The only “evidence” provided is the disagreement between the experimental value based on Pitzer’s model and MP2 results. It is the MP2 method that more often causes peculiar erroneous answers, and higher-level methods should be reported to confirm. Whereas Klug and Burcl1 refer to one of our papers dealing with these matters,3 originally, we reported data for benzaldehyde, employing higher levels of calculation much earlier.4 In that earlier work, we reported high-level ab initio data (MCSCF type), also cited in ref 3, where it was found that the best experimental data published then, comprised of the reliable gasphase microwave and IR techniques, yielded a rotational barrier for benzaldehyde of around 20 kJ/mol, in good agreement with the highest level of ab initio calculation reported in ref 4. In fact, by increasing the level of calculation, the calculated barrier started to lower, and finally the highest levels reached good agreement with experiment. The discussion on this result was further reported in a published discussion with John A. Pople.5,6 The status regarding the rotational barrier for benzaldehyde, with potential consequences for the calculated barriers for azobenzene and azonaphthalene, is thus that the currently highest published levels of ab initio calculation (MCSCF) for benzaldehyde are in good agreement with published interpretation of gas-phase infrared and microwave spectroscopic data, yielding a barrier of around 20 kJ/mol.4 Furthermore, currently available literature does not give appropriate evidence that Pitzer’s model should not be used in analyzing the rotational barrier for species including benzaldehyde. MP2 results have, in various studies, shown to overestimate the rotational barrier for benzaldehyde. This suggests that it is not the effect of dynamic correlation (MP2) that causes the discrepancy between theory and experiment but the effect of nondynamic correlation that is taken into account in MCSCF-type calculations, where we do find agreement with experiment.4 Also, DFT calculations appear to be doing no better than the MP2 level of calculations, as shown by the data in ref 3 and the paper by Klug and Burcl.1 Consequently, on the basis of the currently available literature, the values for azobenzene and azonaphthalene should be reconsidered after calculations
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at an appropriate level of theory have been performed, in particular, the contribution of nondynamic correlation with a sufficiently large basis set.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ REFERENCES (1) Klug, R. L.; Burcl, R. J. Phys. Chem. A 2010, 114, 6401. (2) Speakman, L. D.; Papas, B. N.; Woodcock, H. L.; Schaefer, H. F., III. J. Chem. Phys. 2004, 120, 4247. (3) Meier, R. J.; Koglin, E. Chem. Phys. Lett. 2002, 353, 239. (4) Coussens, B.; Pierloot, K.; Meier, R. J. J. Mol. Struct.: THEOCHEM 1992, 259, 331. (5) Meier, R. J. J. Phys. Chem. 1993, 97, 10248. (6) Head-Gordon, M.; Pople, J. A. J. Chem. Phys. 1993, 97, 10250.
Received: July 28, 2010 Revised: March 3, 2011 Published: March 24, 2011 3604
dx.doi.org/10.1021/jp1070659 | J. Phys. Chem. A 2011, 115, 3604–3604
