Effect of bond angle distortion on torsional potentials. Ab initio and CNDO/2 calculations on dimethoxymethane and dimethyl phosphate

Abstract

A "coupling" of RO-XTOR (X = C or P) bond angle to rotation about the O-X bond in dimethoxymethane (DMM) and dimethyl phosphate (DMP) has been established through ab initio and semiempirical CNDO molecular orbital calculations. Analysis of structural data (x ray and electron diffraction) on related molecules supports this conclusion. Specifically, rotation about the O-X bond from a gauche to a trans conformation results in ca. a 5°reduction in the O-X-O bond angle. Thus, the CNDO optimized O-C-O bond angle in DMM is 112°for the g,g conformation, 107°for the g,t conformation, and 102°for the t,t conformation (ab initio geometries are comparable). A similar change is observed for DMP. This bond angle distortion may be useful in identifying important interactions between lone electron pairs and bonds, which are suggested to be responsible for the observed bond angle variation. Reversal of the relative energies for the g,g and g,t conformations is possible without bond angle optimization in the calculations and establishes the importance of optimization of this geometric parameter in molecular orbital torsional barrier calculations.

3,4,4aα,10aα-Tetrahydro-7-nitro-2H,5H-[1]benzopyrano[2,3-b]pyran

“…The extra constraint of the third ring probably has only a minor effect on the bond lengths at the acetal centre; certainly the outer pair of C-O lengths of the C-O-C-O-C system are not significantly different from those in the bicyclic acetals (I) and (III). The low acetal bond angle [O(10)-C(10a)-O(1) = 105.9 °] is close to that observed for the two equatorial compounds (I, X = H, NO2) and in the range expected for an acetal adopting the gauche,trans conformation (Gorenstein & Kar, 1977).…”

2e-(4-Nitrophenoxy)-trans-1-oxadecalin

“…The introduction of the 4-nitro group has had little effect on the bond lengths and angles about the acetal centre of (I). The angle O(1)-C(2)-O(10) is unchanged at 107.2 ° (Jones et al, 1978b) and is consistent with the trans,gauche conformation adopted by both compounds (Gorenstein & Kar, 1977). (3) and 1.415 (3) A respectively; Jones et al, 1978b].…”

Lone pair interactions in dimethoxymethane and anomeric effect

“…The final geometries of the equilibrium conformers are shown in Table 4. Though the C-0 bond lengths for the ap,ap and sc,sc conformers seem to be shorter in comparison with the results of ab initio restricted minimization (38,39), the bond angle and bond lengths are in very good agreement with the experimental data (18) for the sc,sc conformation. The results show the great dependence of the 0-C-0 bond angle on conformation and confirm the PMO prediction of an increase in the 0-C-0 bond angle on going from the ap,ap to the sc,sc conformation.…”

“…the acyclic phosphate ester has been demonstrated (38). Since the structure of this compound is very similar to DMM we may speculate that in this case the origin of the coupling can also be ascribed to lone-pair orbital interactions.…”