Experimental and theoretical justifications for the observed discriminations between enantiomers of prochiral alcohols by chirally blind EI-MS

Abstract

To all appearances, electron impact mass spectrometer (EI-MS) is considered a "chirally blind" instrument. Yet, numerous non-identical R (right) and S (left) configurations of prochiral alcohols' mass spectra alcohols have appeared in the literature with almost no justification. Such observations are often attributed to impurities, experimental circumstances, inaccurate measurements, etc. In an experimental attempt to explain this phenomenon, here we have avoided the above mentioned pitfalls by conducting control experiments using different pure enantiomers under the same circumstances. Hence, we report the mass spectra of R- and S-enantiomers of 2-octanol (1R, 1S) and 1-octyn-3-ol (2R, 2S) collected by running 20 independent experiments for each R- and S-enantiomer. Statistical analyses confirmed that the peak intensities were significant to an acceptable level of confidence. The 1R and 1S enantiomers were separated reasonably in the PC space, implying that the chirally blind EI-MS is able to discriminate between R and S prochiral alcohols. Theoretically, self-complexation through H-bonding for S (or R) appears to give a new chiral center at the H-bonded oxygen atom, producing a new dimeric pair of diastereomers SRS and SSS (or RRR and RSR) before ionization, and SRS.+ and SSS.+ (or RRR.+ and RSR.+) after ionization. The results of our calculations have explicitly shown that these hydrogen bonds formed. Interestingly, the latter four ionized diastereomers appear with different structural and thermodynamic parameters at the M06-2X/6-311++g (d,p) level of theory.