Today's world is increasingly magnetic.  From the simple compass to superfast complex computers, and from credit cards to CAT scans and MRIs, there is not a day we go about where we do not have some contact with magnets and their applications.  For this reason, many chemists are striving to create new and innovative magnets using organic materials (carbon, oxygen, nitrogen, and hydrogen).  The possible applications for these types of magnets are endless and as yet unknown; but possible uses include ambient temperature superconductors, higher density data storage, MRI contrast agents, and many others.  This summer I was fortunate enough to be involved in this research with the Rajca group at the University of Nebraska.  I was afforded this opportunity by a National Science Foundation grant to the Research Experience for Undergraduates program supported by the Center for Materials Research and Analysis at the University of Nebraska, Lincoln.  The following report is a short account of what I learned and did while working in the Rajca lab.

                 Many years ago the first radical was observed and was noted to behave differently than other molecules.  This was because a radical is a molecule that contains one or more unpaired electrons.  All electrons have a spin direction that is seen as a vector with a value of ½.  When in a magnetic field, we then assign them a relative value of ± ½ in relation to the direction of the field present. In normal molecules all these electrons are paired with another electron of the opposite spin in the same orbital giving the molecule a net spin (S) value of 0.  Therefore, if there is one extra electron the molecule will have a net spin (S) value of ½.  Because magnetism is caused by an excess S in a molecule or atom, a molecule with S = ½ would be slightly magnetic, but so slightly so that it is of no use anywhere.  In an attempt to raise the value of S, chemists have been trying to increase the number of unpaired electrons on a molecule without having them pair up and negate each other.  This problem involves many factors including how much the electrons interact and the resultant energy levels of the singlet and triplet states.  The singlet state is when two radical electrons are paired such that they have opposite spins and an S = 0, resulting in a usually favored lower energy state in many molecules.  The triplet state is when two electrons are paired such that their spins are parallel and S = 1, resulting in a usually unfavored higher energy state.  This is illustrated in Figure1¹.  One effort of current research is to increase the energy difference (DEST) between the singlet and triplet states.  Once we know we can control the energy difference, we can design a molecule with the triplet state at a much lower energy and thus give the molecule a high spin at ambient (room) temperature.  It has been theorized that by increasing the number of pathways between the unpaired electrons the energy difference will increase by a factor equal to the number of pathways².  To test this, two complementary diradicals must be synthesized and tested.
                    Before beginning an explanation of the synthesis, a short explanation about the multiple pathway theory is in order. In reference to Figure 2; as is seen there is an exchange interaction value of J between the two phenyl rings over the Exchange Coupling Pathway (ECP).  Because J = J_ij^eff r_ir_j over one ECP, where J_ij^eff is the effective exchange integral between sites i and j, and ri and rj are the spin densities on sites i and j, then over two pathways J = 2J_ij^eff r_ir_j.  This effectively doubles J in a two ECP system relative to a one ECP system.  Because the energy difference between the singlet and triplet states is equal to 2J over one pathway, a doubling of pathways would make the energy difference equal to 4J, or twice what it was with only one pathway².

                 In order to test this, molecules such as 1 and 2 have been synthesized, but both were so unstable they could only be worked on at extremely low temperatures and could have no contact with air.  It was this reason that I was striving to create two molecules, 3 and 4, which were stable at room temperature and easy to handle in air.  Unfortunately, because of time constraints I was unable to synthesize 4 but was very successful synthesizing molecule 3.
                 Although the synthesis of compound 3 was successful, the process was not easy and took almost the whole ten weeks to complete.  On the following page is the synthesis scheme (Scheme 1) used to create this diradical.  Despite the fact that the final compound is stable, many of the steps along the way are very moisture sensitive and therefore were performed under vacuum in special evacuatable reaction vessels using a vacuum line, septums, needles and dry solvents (no moisture).  In particular, it was step 3 that I had trouble with, performing it four times and successful only twice.   This reaction is performed with a very moisture sensitive compound, the t-BuNO compound, and is easily affected by contact with air and light.  I am unsure of the exact problems encountered in the failed trials, but it was probably a result of these sensitivities.  In the process of this synthesis I learned and performed many valuable skills including performing reactions under vacuum, handling of dry solvents, handling t-BuLi and n-BuLi, weighing air sensitive compounds under nitrogen, and how to run a column.  In addition to these skills, I also learned how to operate a NMR machine (200, 300, and 500 MHz) which I used on a daily basis to identify and check the purity of the compounds I synthesized after each step.  These skills I am sure will be an asset to me in my career ahead.

                Once we have analyzed the NMR data from the hydroxyl amine, we can then look at the NMR data taken for the product of step 4.  There are several indicators that this NMR was given by the diradical compound I was attempting to synthesize.  The most obvious indicator is that, other than the chloroform peak, there is nothing in the aromatic region. This absence of aromatic peaks is caused by the unpaired electrons interacting with the aromatic protons, causing the aromatic protons to have a high spin density and relax too fast for the NMR probe to detect them.  The interaction with the unpaired electrons also shifts them so far down field (»-500 PPM) we are unable to see them at all.   The second thing to notice is the very low, broad peak at approximately -5.3 PPM.  This peak is caused by the protons on the t-Butyl group attached to the nitrogen (and right next to the radical sites).  Again, the unpaired electrons are interacting with these protons causing them to relax faster than normally, broadening the peak and shifting it to below zero PPM.  The third important peak is the sharp singlet at approximately 2.2 PPM.   This peak is the signal from the other t-Butyl groups attached directly to the phenyl rings.  Because they are relatively far away from the radical sites these t-Butyl groups are not affected as drastically by the unpaired electrons.  The effects that are seen are a result of the unpaired electrons is a shift to the left, although it is a much smaller shift compared to the t-Butyl groups attached to the nitrogen.  The reason for this difference in direction of shift and broadening are explained briefly in the next paragraph.

                 To explain the reason for the shifting of the t-Butyl peaks a little background must be discussed beforehand.  First, electron spins in radicals (and all molecules) must be explained.   In the external magnetic field of the NMR instrument, every unpaired electron has a certain spin direction.  Because these unpaired electrons are in the p-orbitals of the atoms they affect the electrons in p-orbitals on neighboring atoms in such a way that the neighboring electrons spins are opposite that of the unpaired electron.  This opposite spin is then related to the next electron in a similar way throughout the whole molecule as in Figure 4.  (Note: A single arrow is placed between the oxygen and the nitrogen because this is where the unpaired electron is, instead of on just the oxygen or nitrogen.)  The only exception in this molecule is in between the two phenyl rings.  Here we can see that the electrons between the phenyl rings have the same spin, and therefore the unpaired radical electrons also have the same spin, indicating a triplet state.  I have put my compound in this triplet state in order to explain the reason behind the shifts in the NMR spectrum in Figure 3.  Because the NMR is taken while the sample is at room temperature there are larger amounts of the compound in the triplet state than there are in the singlet state.  Only at very low temperatures will all the molecules in a sample be in the singlet state.  In the triplet state though, the two phenyl rings act independent of each other allowing the two sides to have the same direction spins on corresponding atoms.   As a result, the spins on the two different t-Butyl types are opposite, and affect the shift of their peaks inversely i.e. one peak shifts right, one peak shifts left.
                    In relation to the molecule being in the singlet and triplet state, it would now be appropriate to discuss what testing will be done with my compound, after I am gone.  One of the first things that must be done is that the cyclic molecule must be synthesized.  A synthesis scheme has been formed and seems fairly simple, but I am sure that it will also take several weeks to create a usable sample.  Once both compounds are attained and are found to be pure, testing on energy levels will begin using ESR (Electron Spin Resonance) spectroscopy and SQUID Magnetometry (Superconducting QUantum Interference Device).  Although I have very little idea of how these machines work, or how to read the data from them, I do know that when all results are received, the energy level difference between singlet and triplet states will be found for both cyclic and acyclic molecules.  In order to do this, the molecules are cooled down to 2-5° K in order to put all the molecules in a singlet state.  The temperature is then raised slowly until it is determined that a triplet state has been reached and the energy difference between the two states can be determined.
                     After determining the energy difference between the singlet and triplet states for both molecules, the theory of multiple pathways increasing the energy difference by the same number of pathways will be proven or disproved.  If proven, this knowledge will greatly help chemists in the future in designing organic magnets in order to take a place in our ever-growing magnetic world.
                     In conclusion, what I have done this summer was a miniscule part of this on the edge research, but nevertheless, a vital part.  Knowing the energy differences between the different states is essential if organic magnets are ever to take a place among our world.  I am sure that in time, we will not be using magnets made from iron, cobalt and manganese, but instead will be integrating magnets made out of carbon, oxygen and nitrogen in everything we do.
                     I must add, this summer has been a wonderful experience for me and I can not imagine a better way of spending a summer.  I have learned as much chemistry in a summer as I did in a whole year of school.  Not only did I gain extra knowledge and skill in organic synthesis and testing, but was also treated like one of the group members and given the opportunity to present on a regular basis.  I know that these skills will be an asset as I continue in my education and on through my career.  My most sincere thanks to Dr. Andrzej Rajca and the whole Rajca Group.
 

References:
1. A. Rajca, Chemical Reviews, 1994, Vol. 94, No. 4
2. A. Rajca, S. Rajca, J. Wongsriratanakul,  Chem. Commun., 2000, 1021-1022.
3. A. Rajca, S. Rajca, J. Am. Chem. Soc. 1996, 118, 8121-8126.
4. A. Rajca, K. Lu, S. Rajca, C.R. Ross II, Chem. Commun., 1999, 1249-1250.
5. D. A. Dougherty, Acc. Chem. Res., 1991, Vol. 24, No. 3.
6. P. M. Lahti, 1995 ACS Symposium Series,  Ó 1996, No. 644.

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