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How can a C≡C bond change the shift downfield?

Question asked by nuzakfarnas on Nov 1, 2019
Latest reply on Nov 1, 2019 by nuzakfarnas

I took a 1H-NMR spectra of 1,4-bis(trimethylsilyl)- buta-1,3-diyne. Because there are triple bonds present, i expected for the protons of the methyl groups to be slightly shifted upfield, relative to TMS. But instead the signal shifted a little bit downfield (+0.19 ppm). Now i am curious on how this can happen, because i thought that due to the magnetic anisotropy of the triple bonds, the signal would be shifted upfield. In contrast the 13C-NMR showed for the carbons of the methyl groups the expected shift upfield, in to the negative area.

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