Experimental and Theoretical Studies of the Gas-Phase Protonation of Vinyl Ethers, Vinyl Sulfides, and Vinyl Selenides

Article abstract of DOI:10.1021/ja00196a002

A series of nine chalcogen-substituted ethylenes (chalcogen = O, S, Se) have been synthesized, and their gas-phase proton affinities (PA) were determined experimentally by measuring gas-phase basicities (GB) in an ion cyclotron resonance (ICR) spectrometer and theoretically by means of ab initio MO calculations at the STO-3G and 3-21G^* levels.A satisfactory correlation (r = 0.978, slope = 1.41) has been obtained between the experimental and calculated 3-21G(^*) values.In contrast with a number of previous reports, we consistently found that third- (SMe) and fourth-row (SeMe) substituents do not stabilize better the adjecent positive charge than does the second-row substituent OMe, even in the gas phase.In fact, comparison of experimental proton affinity value of ethylene with that of mono(methylchalco)ethylenes indicates that OMe, SMe, and SeMe groups stabilize the corresponding ethyl cations to very much the same extent.In 2-propyl cations the trend is O > S > Se, but the differences (δΔ in Table III) are quite small: 1.2 kcal mol^-1 between O and S and 1.4 kcal mol^-1 between S and Se.The superior ability of oxygen in carbenium ion stabilization appears the most clearly in the protonation of bis(methylchalco)ethylenes: dimethoxyethyl cation is more stable than the corresponding thio species by ca. 5.5 kcal mol^-1, whereas dithio- and diselenocarbenium ions again have very similar stabilities.These conclusions are supported and extended by ab initio results on optimized geometries.