Nitromethane proton chemical shifts

The proton chemical shift of the cyclopropenium carbocation 173 was reported as S 11.1-11.2 in either sulphur dioxide, nitromethane or acetonitrile two coupling constants were observed, Jqh — 265 1 Hz and Jqch — At about the same time Olah s... 

What makes FHF an attractive species to investigate by nmr spectroscopy is that it consists of three nuceli each with spin 1/2 bonded directly. Also, the proton of the strong hydrogen bond should have an unusual chemical shift. Early work failed to detect the expected F doublet and H triplets (e.g. Soriano et al., 1969), and it was not until the importance of the solvent was appreciated that coupling was observed (Fujiwara and Martin, 1971, 1974a,b). Suitable media were found to be the dipolar aprotic solvents acetonitrile, nitromethane and dimethylformamide. 

The proton noise-decoupled 13c-nmr spectra were obtained on a Bruker WH-90 Fourier transform spectrometer operating at 22.63 MHz. The other spectrometer systems used were a Bruker Model HFX-90 and a Varian XL-100. Tetramethylsilane (TMS) was used as internal reference, and all chemical shifts are reported downfield from TMS. Field-frequency stabilization was maintained by deuterium lock on external or internal perdeuterated nitromethane. Quantitative spectral intensities were obtained by gated decoupling and a pulse delay of 10 seconds. Accumulation of 1000 pulses with phase alternating pulse sequence was generally used. For "relative" spectral intensities no pulse delay was used, and accumulation of 200 pulses was found to give adequate signal-to-noise ratios for quantitative data collection. 

The proton NMR of the hydrogen dichloride ion has been studied in chloroform solution, and a limiting t value of -2,7 observed, relative to tetramethyl silane as 10,0. A similar value has been obtained in nitromethane solution. The concenuation dependence of the shift, in chloroform, is shown in fig. 2. The chemical shift for the dichloride ion is fat to low field of the values for hydrogen chloride in chloroform, and for liquid hydrogen chloride. We therefore conclude that the shielding of the proton is much reduced in the dichloride ion. 

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