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J. Phys. Chem. 1993,97, 10250
Reply to Comment on "Internal Rotation in Conjugated Molecules: Substituted Ethylenes and Benzenes" Martin Head-Gordon' Department of Chemistry, University of California, Berkeley, California 94720
John A. Pople Department of Chemistry, Carnegie-Mellon University, Pittsburgh, Pennsylvania 15213 Received: June 23, I993 The Comment by Meier' raises questions about our recent study'of barriers to internal rotation in 12 substituted ethylenes and benzenes. On the basis of recent MCSCF calculations on benzaldehyde," which show a substantial nondynamicalcorrelation effect on the barrier, he questions the accuracy of our MP2/63 1 lG**//HF/6-31G* calculations and the conclusions we have drawn from them. We agree that our calculations on the substituted benzenes are open to challenge since those results were not shown to be stable with respect to improvements in the theoretical model. In reply, we note the following points. 1 . Beyond-MP2 Correlation Effects. The performance of MP2 theory for this class of problem was investigated by performing QCISD(T) calculations4on the substituted ethylenes. (Such calculations were not attempted on the substituted benzenes for computationalreasons.) QCISD(T) gives excellent agreement with experimental relative energies even for highly correlated systems.' For rotational barriers, we found beyond-MP2 correlation effects were less than 1 kcal mol-' for all substituted ethylenes,Zsuggestingthat MP2 theory is adequate and that there are no large nondynamical correlation effects. Since the MP2 contributions to barriers are quite similar for the substituted benzenes (see Table I of ref 2), this suggests (but does not prove) that beyond-MP2 correlation effects may also be in the f l kcal mol-' range for these species.
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2. MP2 versus MCSCFResultsfor Benzaldehyde. Our MP2 calculations2 suggest a small -0.5 kcal mol-' correlation effect on the rotational barrier, while MCSCF calculations3 show a much larger effect of up to -3.5 kcal mol-'. This difference is far outside the uncertainty of f 1 kcal mol-' estimated for our calculations. It could be due to a failure of MP2 theory for benzaldehyde, although our results are very similar for the analogous substituted ethylene, acrolein, where no large beyond MP2 correlation effect occurs, and good agreement with experiment is found. Alternatively, the difference might be due to an unbalanced MCSCF description of electron correlation in the planar and perpendicular conformations, due to correlating only up to 14 of the 40 valence electrons. Further calculations are necessary to definitively resolve the present difference. 3. Theory versus Experiment for Benzaldehyde. Since the MCSCF calculations3 agree well with experiment,"' Meier considers our questioning of the experimental result of 4.6 kcal mol-' to be unjustified. However, this value' can be queried2 independently of our barrier calculations because of thesurprising associatedresult of a positivevalue for the V4 Fourier term. Related molecules2 typically have negative V4 values, corresponding to a flattening of the torsional potential about the planar minimum. While we also have plausible reasons why our value of 8.3 kcal mol-' might be accurate to f l kcal mol-', confirmation of this larger value for the rotational barrier of benzaldehyde awaits higher level calculations and new experiments. We hope both will be forthcoming. References and Notes (1) Meier, R. J. J . Phys. Chem., preceding p p e r in this issue. (2) Head-Gordon, M.; Pople, J. A. J. Phys. Chem. 1993,97, 1147. (3) Coussens, B.; Pierloot, K.; Meier, R. J. J . Mol. Struct. 1992, 259, 331. (4) Pople, J. A.; Head-Gordon, M.; Raghavachari, K. J. Chem. Phys. 1987, 87, 5968. ( 5 ) Raghavachari, K. Annu. Rev. Phys. Chem. 1991.42.615. (6) Kakar, R. K.; Rinehart, E.A.; Quade, C. R.; Kojima, T. J. Chem. Phys. 1970.52, 3803.
( 7 ) Durig, J. R.;Bist, H. D.; Furic, K.; Qiu, J.; Little, T. S.J. Mol. Struct. 1985, 129, 45.
0 1993 American Chemical Society
