Structural properties of some monosubstituted cyclobutanes, C 4H 7X, determined by ab initio calculation II. X = OH, NH 2 and CH 3

Abstract

Complete equilibrium geometries have been determined for all stable conformations of cyclobutanol, C 4H 7OH, cyclobutylamine, C 4H 7NH 2, and methyl cyclobutane C 4H 7CH 3, by ab initio Hartree—Fock gradient calculation with a 4–21 basis augmented by d functions on O and N. Conformations differ in having the substituent equatorial ( eq) or axial ( ax) on the puckered ring and by gauche ( g) or anti ( a) orientation of the substituent hydrogen with respect to the ring hydrogen. For C 4H 7NH 2 four stable conformers exist, a—g—eq being the most stable with Δ E for the others being: g—g—eq, 3.2; g—g—ax, 4.2; and a—g—ax, 6.5 kJ mol −1. For C 4H 7OH only three conformers are found with a—eq most stable, Δ E for g—eq, 2.1, Δ E for g—ax, 8.1 kJ mol −1, and no minimum found for a—ax. For C 4H 7CH 3, only two conformers can exist when the CH 3 group staggers the adjacent CCH group. The eq form is 1.7 kJ mol −1 more stable than the ax. These results extend our earlier series and show that the equatorial position is most favored by the most electronegative substituent. The axial conformer becomes increasingly more stable in the series F < OH < NH 2 < Cl < CH 3 < Li. Ring structural parameters show only small differences, but these vary quite systematically with substituent electronegativity. Trends are established for differences in adjacent vs. opposite CC bond lengths, ring puckering angle, distortions of the CH 2 groups, ∠HCX and ∠XC 2C 1C 4.