Analytical Instrument Documents

NM250001E

1 D spectrum

For a spin with spin quantum number I, there are 2I+1 energy levels and 2I possible single-quantum transitions. In most cases, these single-quantum transitions are observed in NMR. The transitions between the ±1/2 levels of half-integer spins (I = 3/2, 5/2, 7/2, 9/2) are called central transitions (CT), while other transitions are called satellite transitions (ST). ST undergoes first-order quadrupolar perturbation, causing the signal to broaden in the MHz range. In contrast, CT only undergoes second-order quadrupolar perturbation and the signal is distributed over a narrower range than ST. ST signals are broad, have lower sensitivity, and often exhibit more complex patterns. Therefore, in the case of half-integer quadrupolar nuclei, CT signals are mostly observed and analyzed.

Figure 1. The ²³Na one-dimensional spectrum of Na₄P₂O₇ shows that the ST signals are distributed over a very wide range (left), with low sensitivity and complex signal patterns. In contrast, the CT signals are distributed over a narrower range and have higher sensitivity (right).

* denotes the background signal of ⁶³Cu from the coil.

The linewidth of the CT signals (in Hz) is proportional to the inverse of the resonance frequency, and therefore, the resolution (in ppm) is proportional to the square of the inverse of the resonance frequency. Since the resonance frequency is proportional to the strength of the magnetic field, using a stronger magnetic field results in sharper and higher-resolution spectra. However, in many cases, it is difficult to completely separate the signals even with a high magnetic field.

Figure 2. Na₄P₂O₇ ²³Na spectra, Magnetic field dependence of CT signals: By using a high-field instrument, the signals become relatively sharper. and the resolution is improved.

MQMAS spectrum

The technique of MQMAS (Multiple Quantum Magic Angle Spinning) provides a high-resolution spectrum with the Isotropic shift on the vertical axis. The Isotropic shift axis combines two pieces of information: chemical shift and the magnitude of the quadrupolar interaction. Therefore, unlike typical NMR two-dimensional measurements (such as COSY, HMQC, etc.), the position of the signal along the vertical axis (in ppm) changes with the magnetic field. Fig. 3 shows the ²³Na MQMAS spectra of Na₄P₂O₇ obtained at multiple magnetic fields. It can be seen that the signal position along the vertical axis changes, and this change is nonlinear.

Figure 3. The ²³Na MQMAS spectra of Na₄P₂O₇, obtained under magnetic field strengths of 400, 500, 600, 700, and 800 MHz, from left to right.

In the Isotropic axis of MQMAS, a larger magnetic field does not necessarily result in better signal separation. Note that multiple signals may overlap on the Isotropic axis, or the signal positions may be swapped when different magnetic fields are used. For a rigorous analysis, it is advisable to acquire and compare MQMAS spectra of the same sample at multiple magnetic fields. Fig. 4 shows the ⁸⁷Rb MQMAS spectra of RbNO₃ obtained at several magnetic fields. This sample contains three different chemical species of ⁸⁷Rb, but at 500 MHz, two of the signals overlap. Additionally, at 400 MHz and 600, 700 MHz, the signal positions are swapped.

Figure 4. The ⁸⁷Rb MQMAS spectra of RbNO₃, obtained under magnetic field strengths of 400, 500, 600, 700, and 800 MHz, from left to right.

Reference

• Duer, M. J. (Ed.). (2001). Solid‐State NMR Spectroscopy: Principles and Applications. Blackwell Science Ltd