On the magnitudes and origins of the “anomeric effects”, “exo-anomeric effects”, “reverse anomeric effects”, and CX and CY bond -lengths in XCH 2YH molecules

Abstract

Ab-initio molecular-orbital calculations have been performed at the STO-3G level on the series of molecules XCH 2YH in which X  H 2N, CH 3, OH, F, and Cl, and Y  O and S, and on the series of fluoromethanes CH nF 4-n. The CX and CY bond lengths in the gauche and antiperiplanar conformations of XCH 2YH have been optimized. In all cases, the experimental trends in the bond lengths from conformation to conformation and from molecule to molecule, and in the torsional behavior as a function of X and Y have been reproduced. Thus, the “anomeric effects” are found to be Cl > F > OH, as a function of X, and O > S, as a function of Y. Molecules in which X  CH 3 and NH 2 exhibit “reverse anomeric effects”; in methanediol, the “anomeric effect” and the “exo-anomeric effect” are equal. The results have been analyzed by a quantitative, perturbational molecular-orbital (PMO) treatment that calculates orbital interactions between XCH 2 and YH in the cases of the “anomeric” and “exo-anomeric effects”, and between X and CH 2YH in the case of the “reverse anomeric effect”, by using fragments and fragment orbitals generated from the ab-initio wavefunction. Attention has been focused, especially, upon the stabilizing interactions between the lone pair of Y and antibonding orbitals of XCH 2, and between the highest-lying orbital of X and antibonding orbitals of CH 2YH. The justification for this choice is that these orbitals make a dominant contribution to the highest occupied molecular orbital (HOMO) of XCH 2YH, and the stereochemical behavior of the HOMO parallels that of the total energy. In all cases, the trends in these stabilizing orbital interactions parallel the trends in the “anomeric” and “reverse anomeric effects”, suggesting that, within the framework of the PMO model, such interactions may be regarded as the “origin” of these effects. Although both σ* and π* antibonding orbitals have to be taken into account to achieve quantitative agreement between the calculated orbital interactions and the total energy-differences, only those interactions associated wit σ* vary significantly as X and Y are varied, as suggested originally by Lucken and by Altona. In the series of fluoromethanes, a linear relationships exist between the stabilizing orbital interactions and the experimental CF bond energies, and also between stabilizing orbital interactions and the number of double bond-no bond resonance structure. This result provides quantitative support for a description of these various effects in terms of the classical concept of double bond-no bond resonance. Finally, the in the CX and CY bond lengths in XCH 2YH and XCH 2X molecules have found to be the result of the combination of coulombic effects and the principle of maximization of overlap between fragments.