Molecular mechanics (MM4) calculations on carbonyl compounds. I–IV.

Abstract

Several years ago we began a study of aldehydes and ketones using the MM4 molecular mechanics program. Sometime later, recognizing the strong effect that induced dipole moments have in carbonyl compounds, we developed a method for calculating induced dipole moments, 1 which was subsequently incorporated it into the MM4 program. Unfortunately, when we prepared the write-up for the MM4 carbonyl work, 2-5 some of the earlier calculations which did not include induced dipoles were accidentally mixed in with the later correct calculations that did include them. This mix-up led to some errors in our aldehydes/ketone papers. The errors are all quite small, but we want to correct them here. First, the dipole moments themselves are not quite right (errors of a few percent). More importantly perhaps, energies are not calculated quite correctly due to the absence of the induced dipole energies. This makes a small difference in conformational energies, which in principle changes conformational equilibria slightly (the conformational energy differences are typically in error by a few hundredths of a kcal/mol). But the induced dipoles do change the total molecular energies more significantly, and this leads to small changes in the calculated heats of formation of molecules of this class. (Most of the latter changes become insignificant after reparameterization of the heats of formation.)Additionally, there are small errors (about 10 cm Ϫ1 ) in the C™H stretching frequencies for hydrogens located in the ␤ position to the carbonyl group. These lead to small errors in the calculated energies of those molecules which have ␤ hydrogens, and also in the moments of inertia. Again, the errors are small. However, we plan to distribute the MM4 program in the near future, and we want others who use it to be able to reproduce results that we have reported. The program calculates everything correctly now, and it did earlier, for these compounds. It was only in transcribing the data to the publication that an error was made, in that the wrong data set was used. This communication is to report corrections to the induced bond moment and heat of formation parameters for aldehydes and ketones in the MM4 program, and the subsequent changes that resulted from the MM4 calculations for these compounds published in our MM4 series of papers. 2-5The dipole moments of the carbonyl compounds are generally smaller than the published values, 2,3 by about 0.01-0.05 Debye (except for cyclohexanone, where the error was 0.30 D). In order to correct for this, the types 156-7, 157-7, and 3-7 induced bond moment parameters for the C¢O bond in formaldehyde, aldehydes and ketones, respectively, had to be scaled up by a similar amount. The correct values for the induced bond moment parameters are presented in Table 1. The permanent bond moment parameters, as well as the induced bond moment parameters not listed in the table, remain unchanged from those previously published. 2,3 The new values for total induced dipole moments, along with the previous reported MM4 va...

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