LOW RESOLUTION MICROWAVE SPECTROSCOPIC STUDIES OF ALLYL ESTERS

Abstract

Low resolution microwave spectra of allyl cyanoformate and chloroformate display a-type band series associated with four conformational forms designated EG, CG, CG^{\prime} and IG. The band series are characterized by $B + $C values of 1838.1(3), 2019.8(3), 2188.0(7), and 2069(2) MHz, respectively, for allyl cyanoformate and 1834.5(3) (1799.5(5), $^{37}Cl)$, 2015(1) (1988(1), $^{37}C1)$, 2213(2) and 2081(2) MHz, respectively, for allyl chloro-formate. Allyl fluoroformate displays band series from four rotamers designated EG, ES, CG and IG having B + C values of 2406(1), 2764(1), 3075(5), and 2815(5) MHz, respectively. Allyl formate displays one band series having a B + C value of 3298(3) attributed to an EG conformer. Each E conformer is consistent with a syn-anti $[\tau_{1}(OCOC) = O^\circ, \tau_{2}$ (COCC) $= 180.^\circ]$ heavy atom planar structure and each C species is consistent with a syn-gauche $[\tau_{1} (OCOC) = 0^\circ, \tau_{2}$ (COCC) $\sim 90^\circ]$ structure. The I conformers are consistent with a gauche-gauche $[\tau_{1}$ (OCOC) $\sim 90^\circ \tau_{2}$ (COCC) $\sim 240^\circ]$ structure. The ES conformer of allyl fluoroformate has the vinyl group syn-eclipsed with the C-O bond, $[\tau_{3}(OCCC) = 0^\circ]$. Rotational constants of all other conformers are consistent with structures having the vinyl group eclipsed with a methylene C-H bond, $[\tau_{3} (OCCC) = 120^\circ]$. E and C conformers are of nearly equal energy in each case. I conformers are approximately 2-3 kcal/mole higher in energy. These results parallel previous findings for ethyl, propargyl and n-propyl esters, demonstrating that the potential functions for internal rotation of primary esters are similar.""