Origins of Small Proton Chemical Shift Differences in Monodeuterated Methyl Groups

4 years ago

Origins of Small Proton Chemical Shift Differences in Monodeuterated Methyl Groups

Lynda J. Brown, Sebastian Cevallos, Stuart J. Elliott, Stuart Sawyer, O. Maduka Ogba, Daniel J. O’Leary, Malcolm H. Levitt, David A. Kolin

We have recently shown that the small proton chemical shift difference in 2-methyl-1-(methyl-d)piperidine supports a long-lived nuclear spin state. To identify additional candidate molecules with CH₂D groups exhibiting accessible long-lived states, and to investigate the factors governing the magnitude of the shift differences, we report a computational and experimental investigation of methyl rotational equilibria and proton chemical shifts in a variety of 2-substituted 1-(methyl-d)piperidines. The polarity and size of the 2-substituent affect the 1,2-stereoisomeric relationship, and consequently, the strength of the rotational asymmetry within the CH₂D group. Nonpolar and large 2-substituents prefer the equatorial position, and relatively large shift differences (i.e., > 13 ppb) are observed. Polar and small substituents, however, increasingly prefer the axial position, and medium to small shift differences (i.e., 0 to 9 ppb) are observed. In addition, the diastereotopic CH₂D proton chemical shift difference for tricarbonyl(1-chloro-2-deuteriomethylbenzene) chromium(0) was computed, showing that reasonable predictions of these small shift differences can be extended to more complex, organometallic species.

Publisher URL: http://dx.doi.org/10.1021/acs.joc.7b01356

DOI: 10.1021/acs.joc.7b01356