A SINGLE-CRYSTAL MOLECULAR-STRUCTURE DETERMINATION AND THEORETICAL CALCULATIONS ON ALKYNYL CARBOXYLATE ESTERS

A single crystal molecular structure is reported for propynyl p-nitrobenzoate, a member of the new class of alkynyl esters. An experimentally determined Csp-O bond length of 1.366 (9) Å is observed for the first time. The C≡C bond length is 1.155 (9) Å. The structural features of the ester moiety in the alkynyl ester are compared to the analogous saturated and unsaturated (vinyl, aryl) esters. Ab initio molecular orbital calculations are reported for hydroxyacetylene, ethynyl formate (7), propynyl formate, ethynyl acetate, vinyl formate (10), and methyl formate (11). at the 6-31G* level the most stable conformation of 7 is Z (or syn) and the calculated C≡C—O and C≡C bonds are 1.312 Å and 1.179 Å, respectively. By use of model compounds, it is estimated that the addition of correlation energy would increase these bond distances by 0.01-0.02 Å. Possible reasons for the experimental/theoretical discrepancies are discussed, and it is concluded that the theoretical values are probably closer to reality. In agreement with this conclusion, good agreement is observed between the experimental and theoretical geometries, for vinyl formate (10). At the 6-31G* level the calculated energies of hydrolyses for 7, 10, and 11 are computed to be -8.9, -4.3, and +1.5 kcal/mole, respectively. The calculated electronic structures, charge distributions, and dipole moments for acetylenic esters are also discussed. © 1990, American Chemical Society. All rights reserved.