Studies on the NMR contact shifts induced by hydrogen bonding with nitroxide radical. A correlation between 13C contact shifts induced by 13C-H...nitroxide hydrogen bond and 13C-H nuclear spin coupling constants

✍ Scribed by I. Morishima; K. Endo; T. Yonezawa

• Publisher: Elsevier Science
• Year: 1971
• Tongue: English
• Weight: 352 KB
• Volume: 9
• Category: Article
• ISSN: 0009-2614
• DOI: 10.1016/0009-2614(71)85029-7

✦ Synopsis

lIf and 13C NMR contact shifts have been observed for chloroform, methylene cblorido and phenylacetylene in the presence of di-tertbutyl nitroxido radical (DBNO). Upfield and downfield contact shifts were observed for 1~ and 13f.2 NMR resonances, respectively. The relative values of 13C contact shifts with respect to 1H contact shifts are linearly correlated with direcfly'bonded i&Z-H spin coupling constants. This correlation is interpreted In terms of finite perturbation theory of nuclear spin coupling constants in which the 13C-H coupling constant is related to the electron spin density on the 1% nucleus induced when spin density is placed finitely on the proton. The potential utility of this relation in the prediction of sign and magnitude of long-range l3C-H coupling constants is also stated. In previous papers [l, 21 we have reported the results of proton contact shifts for a variety of the proton donor molecules, XI-I, induced by the presence of stable radical, di-tert-butyl nitroxide (DBNO), which serves as the proton acceptor, Y, and stressed the importance of the covalent effect in the XII. . . Y hydrogen bonding. The observation of the XH proton contact shift presents direct evidence of the covalent character of the XH.. . Y hydrogen bond which induces electron spin densiiy on the XB proton interacting with yeNO [11 . The observed negative spin density *. ., upfield contact shifts) on the XH proton is most likely due to the spin polarization mechanism. The induced negative spin density on the * In the diamagnetic solution, the XH proton in the proton donor molecule experiences r downfieldshift by hydrogen bonding. In this paramagnetk soldtion,