April 17, 2008
When faced with a crowded region in a 1H NMR spectrum, a 2D NMR experiment can assist in removing the ambiguity and in narrowing down the proton count.
For the 1H NMR spectrum below, Multiplet F (the region between 1.4 to 1.8 ppm) displays an integral of 6.71. Although several factors may contrbute to the disparity of the integral as a whole number, the question remains does the integral 6.71 represent 6 or 7 protons assuming Multiplet E corresponds to 1 proton?
A 1H-13C DEPT-HSQC, shown below, illustrates the correlation between a 1H and 13C that are separated by one bond. In addition, the correlations that are negative (commonly displayed in blue) signify a methylene (CH2) group. The 1D projection along the F1 dimension represents 4 carbons at 26, 27, 31, and 39 ppm and each carbon correlates with 2 inequivalent proton resonances.
Based on the information from the DEPT-HSQC spectrum, one can say with a higher degree of certainty that there are 7 protons within the 1H region of 1.4 to 1.8 ppm.