In lab 9, we measured^1H NMR spectrum and T_1 relaxations for 1-butanol sample,

Question

In lab 9, we measured^1H NMR spectrum and T_1 relaxations for 1-butanol sample, CH_3CH_2CH_2OH. Use the spectra to answer the following questions. i) List the chemical shifts of^1 H at these sites: CH_3; ___ ppm; CH_2 (a); ____ ppm; CH_2 (b); _____ ppm; CH_2 (c); ____ ppm; OH; ___ ppm. ii) From the fine structure i.e. peak splitting, the J-coupling of^1H at CH_3 or CH_2 sites can be determined as about ___ ppm, and ____ Hz. iii) At which site, there is the strongest^H nuclear shielding sigma? ____ At which site, there is the weakest^1H nuclear shielding sigma? ____ (a) CH_3 site (b) CH_2 (a) site (c) CH_2 (b) site (d) CH_2 (c) site (e) OH site iv) The signal changes in T_1 experiment can be described by the equation: M_z = M_0 (1-2 e^-tau/T_1). According to the T_1 spectra obtained from the lab, sketch the curve of spin magnetization (M_z) i.e. peak intensities versus varied time tau. v) From the experiment, what are the T_1 values of^1H nucleus at eh different sites of CH_3CH_2CH_2CH_2OH molecule? CH_3: T_1 = _____; CH_2(a): T_1 = ____; CH_2 (b): T_1 = _____; CH_2 (c): T_1 = ____; OH: T_1 = ____.

Explanation / Answer

A. According to Proton NMR spectra of 1-butanol, CH3 shows peak at chemical shift of 1.0 ppm, CH2(a) at 1.5 ppm, CH2 (b) at 3.3 ppm and CH2(c) at 4.0 ppm.

The sequence is due to the electronegative O that increases the chemical shift value by deshielding it.

B. The coupling constant is about 7 Hz as observed from the fine structure, whose value in ppm depends on the frequency at which instruments is operating.

C. Strong shielding at CH3 and weakest shielding at CH2(c).

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