Understanding the Limitations of Electron Spin Resonance

[sha1000]

TL;DR Summary

Why Electron Spin Resonance (ESR) is reportedly unable to detect radicals with short lifetimes, particularly within dynamic systems like high-temperature liquid sulfur, where reactions occur at rapid rates (e.g., every 100 picoseconds)

Hello,

I have a question regarding the limitations of Electron Spin Resonance (ESR). I've read somewhere that ESR cannot detect radicals with short lifetimes.

I'm trying to understand why is that?

For example: a highly dynamic system like liquid sulfur at high temperatures, where sulfur chains with two radical chain-ends are constantly reacting with other sulfur molecules at an average rate of every 100 picoseconds. Will ESR be able to detect any signal in this case?
Even though radicals are fast to react, we have a steady-state concentration of radicals (on average radical concentration remain the same).

I would really appreciate any paper regarding this.

 

[Nugatory]

I've read somewhere

Where? You will get better and more helpful responses if we know where to start.

 

[sha1000]

Where? You will get better and more helpful responses if we know where to start.

Hi,

Thank you for your reply.

I read in some papers about the radical trap methods developed because short-lived radicals are impossible to detect using ESR. Below, you can find a couple of them.

But I can't understand the exact reason:
- Is it because with short-lived radicals the steady-state concentration is too low for detection?
- Or does the problem lie in the transient nature of the free radicals?
When dealing with a highly dynamic system like liquid sulfur above 300°C, the steady-state concentration is high, even though the average lifetime of a radical is small.



1) doi: 10.3164/jcbn.10-13R
"The spin-trapping method was originally proposed by Janzen et al. and several other researchers in the late 1960s [7–10]. The spin label method was reported by Ohnishi et al. in 1965 [11], and the reagents for spin labeling are used for ESR imaging. Fundamentally, spin-trapping reagents react with short-lived radicals, which are subsequently changed to long-lived radicals called spin-adducts. By observing the ESR spectra of spin-adducts, the ESR characteristics can be obtained, including the g-value, hyperfine coupling constant (hfcc) and spin concentration."

2) https://doi.org/10.1021/jacs.2c03618
"Short-lived radical intermediates play a key role in many chemical processes, including synthetic chemistry (e.g., polymerization (1) and photoredox catalysis (2)), biochemistry (e.g., oxidative stress (3)), and atmospheric chemistry (e.g., photochemical oxidation cycles (4) and secondary organic aerosol formation (5)). However, their detection is challenging due to their short lifetimes and hence low concentrations in real systems, which are often below the detection thresholds of conventional analytical techniques. In addition, one may want to detect radicals in environments where deployment of complex instrumentation can be difficult, for example, in atmospheric field measurements.
Electron paramagnetic resonance (EPR) spectroscopy detects radicals directly, but this can be very challenging for short-lived radicals, and gaseous radicals can only usually be observed at reduced pressure"

 

[f95toli]

There are two main types of ESR/EPR: Pulsed and continuous wave (CW).
CW-ESR is a relatively sensitive technique, but does not give you any information about the dynamics of the system. Moreover, it typically can't give you as much "chemical" information as pulsed ESR, nearly all of the clever techniques (ESEEM, ENDOR etc) that do that require pulsed schemes .
In CW-ESR, you also very often end up with say a very broad g=2 peak which is the superposition of lots of different signals with g "almost" equal to 2; unless you can then switch to a pulsed technique it is almost timpossible to figure out what is going on.

Unfortunately. conventional tulsed-ESR is also limited by the ring-down time of the cavity; if you want high sensitivity you need a high-Q cavity; but that also means that any process faster than the cavity ring-down time can't be seen. . This ring down time can be quite long;, say a few microseconds, which if slow compared to many processes.

There are ways around all of the problems above, but what is written should apply to most "standard" spectrometers.

 

[sha1000]

There are two main types of ESR/EPR: Pulsed and continuous wave (CW).
CW-ESR is a relatively sensitive technique, but does not give you any information about the dynamics of the system. Moreover, it typically can't give you as much "chemical" information as pulsed ESR, nearly all of the clever techniques (ESEEM, ENDOR etc) that do that require pulsed schemes .
In CW-ESR, you also very often end up with say a very broad g=2 peak which is the superposition of lots of different signals with g "almost" equal to 2; unless you can then switch to a pulsed technique it is almost timpossible to figure out what is going on.

Unfortunately. conventional tulsed-ESR is also limited by the ring-down time of the cavity; if you want high sensitivity you need a high-Q cavity; but that also means that any process faster than the cavity ring-down time can't be seen. . This ring down time can be quite long;, say a few microseconds, which if slow compared to many processes.

There are ways around all of the problems above, but what is written should apply to most "standard" spectrometers.

Thank you for your response. Its very helpfull.

Would you have any

There are two main types of ESR/EPR: Pulsed and continuous wave (CW).
CW-ESR is a relatively sensitive technique, but does not give you any information about the dynamics of the system. Moreover, it typically can't give you as much "chemical" information as pulsed ESR, nearly all of the clever techniques (ESEEM, ENDOR etc) that do that require pulsed schemes .
In CW-ESR, you also very often end up with say a very broad g=2 peak which is the superposition of lots of different signals with g "almost" equal to 2; unless you can then switch to a pulsed technique it is almost timpossible to figure out what is going on.

Unfortunately. conventional tulsed-ESR is also limited by the ring-down time of the cavity; if you want high sensitivity you need a high-Q cavity; but that also means that any process faster than the cavity ring-down time can't be seen. . This ring down time can be quite long;, say a few microseconds, which if slow compared to many processes.

There are ways around all of the problems above, but what is written should apply to most "standard" spectrometers.

Hi again,
Looks like my last message didn't go through properly. Once again, thank you for your response.

I'm on the lookout for any papers that talk about the limitations we're discussing. I couldn't really find any publications that go into detail on those specific issues. Do you think this method can accurately measure unpaired electrons in highly dynamic systems, like liquid sulfur over 170°C?

The radical displacement is fast (around 1 ns.) I've also heard that EPR can not capture short-lived radicals, but I'm a bit confused about what exactly "short-lived" means here. Are we talking about how fast the displacement reaction (propagation) or the lifespan from the initiation to termination?

Thanks!

 

[f95toli]

I think you need to be a bit clearer about what you want to measure. If you are asking if pulsed ESR would be able to do time-resolved measurements of a process that takes 1 ns the answer is no. However, that does not mean that you couldn't use ESR to measure a "time-averaged" signal; as long as there are enough relevant spins at any given time to give you a good signal.

My suggestion would be to have look in a good book. For example

Principles of Pulse Electron Paramagnetic Resonance by Schweiger and Jeschke.​