Hyperconjugation, Steric hindrance, Chemical bonds, Carbon compounds, Bond length, Charge perturbation
Section 1 Calculations indicate that the C-Cl bond length in organic molecules varies between 1.73 and 1.93 Å. In determination of the bond length, three interactions are of particular importance: charge perturbation, primary and secondary hyperconjugation, and steric hindrance. To establish their relative strength, a variety of molecules of the Cl-C-C/O-X type were optimized using ab initio calculations in seven different conformations of central bond rotation. The X substituents were selected to exemplify each interaction. Charge perturbation is particularly strong; not only does it dictate the C-Cl bond length when the X substituent is charged or polarized, it is also detectable when primary hyperconjugation from the oxygen lone pair in the O-X compounds is present. Secondary hyperconjugation together with steric hindrance are the weakest interactions; they are not observable unless charge perturbation and primary hyperconjugation are negligible. In all molecules, the C-Cl bond length correlates with the chlorine atom charge. Although quantification of the three interactions was unsuccessful, qualitative analysis including an expression that describes the magnitude of charge perturbation provides an important insight into the effect of intramolecular interactions on bond length. Section 2 The Natural Bond Orbital program (NBO) is widely used as a tool that, among other computations, provides a quantitative insight into hyperconjugation. In our case, it is used to predict the magnitude of hyperconjugative interactions (expressed as the E2 xvi value) for two molecules: 2-chloromethanol and 1-chloro-2-ethoxy ion. We describe all interactions mentioned in the NBO output and predict their effect on the C-Cl bond length. Besides the expected bond lengthening interactions from the oxygen lone pairs in 2-chloromethanol, a variety of other interactions were discovered. Among the most notable are the bond shortening interactions from the chlorine atom lone pairs to the central C-O or C-C and C-H antibonding orbitals. It is therefore impossible to use the simple, but qualitatively accurate, model from Section 1 to quantify hyperconjugation with NBO. We also employed a new feature of the NBO program, the deletion function, to establish the trends in the C-Cl bond length upon removal of all hyperconjugation interactions. In such case, all bonds in the two subject molecules stretched. The trend in the C-Cl bond length with torisonal angle in 1-chloro-2-ethoxy ion is preserved; that in 2-chloromethanol corresponds to that for molecules with a positively charged substituent. Charge perturbation thus plays the dominant role in molecules with all hyperconjugation removed. Further, deletion was employed to determine the contribution of the individual hyperconjugative interactions to the C-Cl bond length. While bond lengthening interactions and the change in the C-Cl bond length are directly proportional, the relationship between bond shortening interactions and the C-Cl bond length is less intuitive. In fact, the stronger the hyperconjugation, the smaller the C-Cl bond length change. In addition, bond shortening interactions have a bigger effect on the C-Cl bond: for a given E2 value, the C-Cl shortens more than it lengthens. The C-Cl bond length is clearly more sensitive to bond shortening interactions. xvii Using NBO to quantify hyperconjugation has proved very difficult since not all interactions contribute to the C-Cl bond length equally. Also, the deletion of the effect leads to compensatory changes in the molecule, therefore charge perturbation and steric effect, although observable after deletion, are not as large in magnitude as when hyperconjugation is present. For this reason, the magnitude of charge perturbation and steric hindrance after deletion cannot be used to quantify the two effects. Nevertheless, this study provided an important insight into the use of NBO to quantify hyperconjugation and confirmed our qualitative conclusions from Section 1.
Reznickova, Anna, "Computational study of the C-CI bond length" (2010). Honors Project, Smith College, Northampton, MA.
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