HuaJian Testing | nmr analyzing material structure 

Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for analyzing material structure. Previously, we explored its principles of operation and which atoms can undergo NMR. So, how exactly do NMR spectra reflect material structure? In this issue, we’ll use NMR spectra to get to the bottom of this and delve deeper into the mysteries of material structure.

Introduction to NMR Spectra

The figure below shows an NMR spectrum of hydrogen. The spectrum has horizontal and vertical axes; the horizontal axis represents chemical shift, while the vertical axis represents signal intensity. The signal intensity on the vertical axis is easy to understand. But what exactly is this chemical shift on the horizontal axis? If you look closely, the unit is ppm—but what do these numbers actually represent?

PPM actually represents an order of magnitude, specifically one part per million. But what about chemical shift? This parameter is crucial for structural analysis in nuclear magnetic resonance (NMR). Below, we’ll explain in detail how chemical shift is generated.

Chemical Shift

To discuss chemical shift, we must first revisit the principles of nuclear magnetic resonance. A nuclear magnetic resonance (NMR) spectrum is a graphical representation of the nuclear magnetic resonance phenomenon occurring in specific atomic nuclei within a magnetic field. The nuclear magnetic resonance phenomenon occurs when specific atomic nuclei are exposed to radiofrequency (RF) radiation of a certain frequency within a magnetic field. In other words, the atomic nuclei are situated within a magnetic field.

To illustrate: Let’s take the simplest example—the hydrogen nucleus. As shown in the figure, this is a hydrogen nucleus. For it to exhibit the nuclear magnetic resonance phenomenon, it must be placed in a magnetic field and exposed to radiofrequency radiation.

Protons carry a positive charge and, like the Earth, rotate around their own axes—a property known as “spin.” The rotation of charged particles generates a tiny magnetic field; therefore, each proton acts as an extremely small bar magnet, possessing a magnetic moment (which can be understood as the strength and direction of magnetism). When we place a sample inside the powerful superconducting magnet of an NMR spectrometer, the situation changes: these tiny “proton magnets” can no longer arrange themselves freely within the external magnetic field. The directions of their magnetic moments can only assume two specific energy states (quantization): a low-energy state and a high-energy state. At this point, we transmit electromagnetic waves of a specific frequency into the magnetic field. When the radiofrequency (RF) frequency exactly matches the frequency corresponding to the energy required for a proton to transition from the low-energy state to the high-energy state, “resonance” occurs. The ppm value represents the difference between the sample’s resonance frequency and that of the standard.

ppm, or parts per million, represents a difference of one part in a million, and chemical shifts can typically be measured with an accuracy of 0.01 ppm; even if the effect on the target nucleus is extremely slight, it can still be resolved in the NMR spectrum. Through extensive collection and verification of experimental data, it is now possible to determine the relationship between the shift in chemical shift produced by a specific nucleus and that of neighboring nuclei. By synthesizing information from all nuclei that produce the nuclear magnetic resonance phenomenon, along with data from their neighboring nuclei, the structure of the substance can be determined.

How does NMR analyze the structure of a substance?

1. Specific nuclei exhibit nuclear magnetic resonance under specific conditions.

2. The shift between the measured field strength and the standard field strength is called chemical shift. (TMS is generally used as the standard reference for this shift.)

3. When a specific atomic nucleus combines with other atoms to form a molecule, the field strength of the nuclear magnetic resonance signal differs due to interference from the other atoms—that is, the chemical shift differs. Because nuclear magnetic resonance spectra are extremely precise, even a difference of one millionth results in a significant discrepancy in the resulting spectrum.

4. By analyzing the chemical shifts and other information, we can obtain information about the atoms surrounding that particular atom. By synthesizing the information from all atoms, we can determine the structure of the substance.

Frequently Asked Questions

Q1：What are the x- and y-axes on a hydrogen spectrum? What does ppm stand for?

A: X-axis: Chemical shift; Y-axis: Signal intensity. ppm is a unit of chemical shift, representing the relative frequency difference.

Q2: What is chemical displacement?

A: The atomic nucleus is influenced by surrounding groups, causing the resonance position to shift relative to the standard (TMS); this shift is known as chemical shift.

Q3: Why can chemical shift be used to infer molecular structure?

A: Different functional groups place hydrogen atoms in different chemical environments, resulting in different chemical shifts; based on this, we can identify the attached groups and deduce the structure.

Q4: What is the core logic behind NMR structural determination?