Oxygen: how does it affect your T1 times?

In this blog post, I will talk about how T₁ times can vary depending on how you prepare your samples, as well as the role of oxygen during relaxation. To refresh your memory, please feel free to take a look at one of our previously published blog posts about T₁ times here.

Although there are many factors that play a role in T₁ relaxation, such as spin-spin dipolar interactions, paramagnetic interactions, chemical shift anisotropy, etc., we will be focusing on how the paramagnetic molecule, oxygen, can affect T₁ relaxation.^[1] As you have learned in your first-year chemistry class, oxygen has two unpaired electrons in its highest occupied molecular orbital, classifying it as a paramagnetic molecule. This plays a major factor in the relaxation of an NMR active nuclide as an unpaired electron has a high magnetic moment, which promotes relaxation.^[2] With oxygen’s unpaired electrons, it ends up helping the nuclide relax faster, thereby, shortening relaxation times.

It is probably not enough to just tell you that oxygen shortens T₁ times, but also to show you an example of this phenomenon. For ethyl trans-2-butenoate (Figure 1), I will be measuring the T₁ value of each peak after preparing the sample under ambient conditions (oxygen is present in the sample) and after deoxygenating it via vacuum pump. While a more rigorous approach would involve deoxygenating the sample by bubbling another gas in the solution or submitting the sample to free-pump-thaw cycles, this method will suffice in showing how the presence of oxygen (or lack thereof) can drastically affect experiment times. By understanding how oxygen can affect T₁ times, you can use this knowledge to your advantage when designing an experiment. For example, integral ratios will likely not be accurate due to longer relaxation times when the sample is placed under vacuum.

Figure 1. Molecular structure and ¹H NMR spectrum (60 MHz) of ethyl trans-2-butenoate in CDCl₃.

Table 1 depicts the T₁ values obtained before and after sealing the sample under vacuum. As you can see, the T₁ increases by around 2-5 times after being placed under vacuum, which can drastically effect experiment times. So, next time you decide to remove oxygen from your samples, make sure you have come to terms with having longer T₁ times!

Table 1. T₁ measurements for each peak region of ethyl trans-2-butenoate before and after vacuum sealing.

Oxygen also plays an important role in NOE (Nuclear Overhauser Effect) measurements. In this case, the presence of paramagnetic molecules adds an extra relaxation mechanism, decreasing the NOE experiment sensitivity, and degassing the sample is then recommended. ^[2] This is especially true when a small NOE effect is expected. Stay tuned for a future blog post elaborating on NOE-based experiments.

If you have any questions or comments about anything NMR related, do not hesitate to contact us!

References

[1] D’Agostino, C.; Bräuer, P.; Charoen-Rajapark, P.; Crouch, M.D.; Gladden, L.F. RSC Adv. 2017, 7, 36163-36167.

[2] Keeler, J. in Understanding NMR Spectroscopy, 2nd ed.; John Wiley & Sons Ltd: Chichester, West Sussex, England, 2010.