Q and A

Q1: why 1D C13 spectrum of CDCl3 shows triplet peaks?

A: Because C13 couples with D, it follows 2nI+1 rule, 2*1*1 +1 = 3; since it’s not first-order splitting, the ratio of three peaks intensities won’t follow Pascal triangle (not 1:2:1, but 1:1:1). Similarly, for DMSO-d6, 2*3*1 +1 = 7; (but the intensity ratio is not 1:6:15:20:15:6:1).

Q2: why 1D H1 spectrum of CDCl3 exists two small satellite peaks around 7.24ppm?

A: It’s not because of the sample spinning! H1 of the residual CHCl3 couples with natural abundance C13 (1.1%), follows 2nI+1 rule, so: 2*1*(1/2) + 1 = 2. This can be validated by 1JCH = 209.0 Hz for H-CCl3. Similarly, the peak at 0 ppm for DSS and TMS shows two small satellite peaks too.

Q3: why Bruker Topspin uses ZG30 but not 90 or other tilt angles for 1D proton measurement?

A: Based on Ernst Angle equation: Cos(θtilt) = e-(D1 + AQ)/T1 , after balance pulse tilt angle, recycle delay D1, acquisition time AQ, and sample molecule relaxation time T1, it turns out that for a small molecule (MW is several hundreds Da with T1 around 1 or 2 sec), use a 30° tilt angle pulse and D1 = 1 sec, AQ = ~3 sec, NS = 8 or more, it’s the most efficient 1D proton measurement with shorter expt. time and relatively stronger signals. For a sample with long T1, 90° pulse is used, and the length of D1 does matter. If you are further interested, look here.

Hongwei edit on 6/27/2023