What is it about?
Here we study using femtosecond disruptive probing the molecular dynamics associated with the well-known McLafferty rearrangement. By tracking the product yield for 2-pentanone and two other methyl substituted 2-pentanones, we are able to determine the mechanisms including the timescale for each step. We find that the first step associated with formation of a cyclic intermediate that facilitates H-transfer to the oxygen is fast, taking place in ~135 fs. We were very surprised that the second step, involving the pi-electron rearrangement, takes place in ~16 ps. The timescale of the second step is about 2X faster for the methyl substituted compounds.
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Why is it important?
The McLafferty rearrangement is taught in every class that deals with mass spectrometry. Most people assume that the process is concerted, or if stepwise, that the second step is just as fast as the first step. In this publication, we provide the first time-resolved measurements that reveal the surprising dynamics of the McLafferty rearrangement.
Perspectives
Finding that the pi-electron rearrangement is controlled by molecular vibrations is reminiscent of electron transfer in biology. It is very significant to find that this is the case in an isolated radical cation. It points to delayed fragmentation in mass spectrometry and the emergence of ergodicity.
Marcos Dantus
Michigan State University
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This page is a summary of: The Surprising Dynamics of the McLafferty Rearrangement, The Journal of Physical Chemistry Letters, November 2023, American Chemical Society (ACS),
DOI: 10.1021/acs.jpclett.3c02102.
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