METHYLENECYCLOPROPENE AND RELATED COMPOUNDS: SYNTHESIS, SPECTROSCOPY AND STRUCTURE (MICROWAVE)

Methylenecyclopropene (1) was synthesized by passing 2-bromo- or 2-chloro-methylenecyclopropane through a column containing potassium tert-butoxide on Chromosorb W at 110(DEGREES)C and 100 mtorr. The (\u271)H and (\u2713)C NMR chemical shifts of 1 are indicative of its highly dipolar nature. The untraviolet spectrum of 1 in pentane at -78(DEGREES)C exhibits two (pi) (---\u3e) (pi)* bands at 206 and 309 nm and a (pi) (---\u3e) (sigma)* band at 242 nm. The (pi) (---\u3e) (pi)* (HOMO to LUMO) band for 1 is red shifted relative to that for butadiene. The majority of this shift is attributed to differences in electron-electron repulsion terms for the two molecules owing to the fact that 1 is a nonalternate hydrocarbon. The photoelectron spectrum of 1 gives (pi) ionization potentials of 8.41 and 13.11 eV while the electron transmission spectrum gives an electron affinity of -0.87 eV for the lowest (pi)* orbital. The electric dipole moment and heavy-atom molecular structure were collaboratively determined for 1 by microwave spetroscopy. In addition, a revised microwave structure for cyclopropenone (2) is presented. Analysis of the MP2/6-31G* charge distributions for 1 and 2 led to the conclusion that the contribution of (pi) dipolar resonance forms to the ground states of 1 and 2 are 1/5 and 1/2, respectively. The calculated (MP2/6-31G*) heat of formation for 1 is 93.4 kcal/mol. The chemical shifts of the ring double bond carbons (C(,1,2)) and the attached protons in a number of 3-substituted cyclopropenes correlated linearly with the 6-31G*//6-31G* (pi)-electron densities at C(,1,2). The slopes of these correlations were 481 ppm/e(\u27-) and 18.3 ppm/e(\u27-) for the (\u2713)C and (\u271)H chemical shifts, respectively. These values are discussed in light of ab initio calculations and comparisons to charge-chemical shift correlations for other molecules. The chemical shifts of the ring protons of 3-substituted cyclopropenes exhibit large changes as a function of the solvent. Evidence is presented which shows that these solvent-induced shifts result from hydrogen bonding of these protons with the solvent