INDUCTIVE AND STERIC EFFECTS ON THE GAS-PHASE STRUCTURE OF TERT-BUTYLACETATE - ELECTRON-DIFFRACTION AND AB-INITIO MO INVESTIGATIONS

We have studied the molecular structure of tert-butyl acetate, (3)H3C( =O(2))(1)O(4)C(C(6)H-3)(C(8)H-3)(5)C(7)H-3, by gas electron diffractio n and HF/4-21G calculations on geometrical parameters and harmonic for ce constants. Experimental data are consistent with the assumptions th at the C1=02 bond is cis to the O4-C5 bond and that the tert-butyl gro up is staggered with respect to the C1-O4 bond. The structural paramet ers (r(g)/Angstrom and angle(alpha)/deg) with estimated limits of erro r (3 sigma) are r(C1=O2) = 1.198(5), r(C1-O4) = 1.334(6), r(O4-C5) = 1 .456(8), [r(C-C)] = 1.522(2), [r(C-H)] = 1.101(4), angle O2C1C3 = 120. 4(24), angle O2C1O4 = 126.1(5), angle C1O4C5 = 122.3(9), angle O4C5C6 = 101.5(10), angle O4C5C7 = 110.0(8), angle C6C5C7 = 111.4 (fixed at t he HF/4-21G value), and [angle CHHMe] = 108.0(10). The C6 and C7 atoms are trans and gauche to the C1-O4 bond, respectively, and [] denotes average values. The differences between structural parameters of simil ar types have been taken from the HF/4-21G calculations. The C1-O4 bon d length in tert-butyl acetate is about 0.03 Angstrom shorter than tha t in methyl acetate. This shortening can be rationalized in terms of t he electron-releasing inductive effect of substituents and the resonan ce effect. The steric effect of the tert-butyl group is seen in angle C1O4C5, angle O2C1C3, angle O2C1O4, and the tilt angle of the tert-but yl group.