What about the spin preferences of diradicals?

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1 What about the spin preferences of diradicals? What are the electronic structure factors that control spin states in diradical organic molecules? (i.e., systems that have two weakly interaccng electrons). TME TMM Singlet or triplet? What about the spin preferences of diradicals? General results: Weakly interaccng radicals will produce a weak preference for the singlet state. owever, when the nodal properces of the system are co- extensive in space significant exchange interaccons results and this can produce the triplet ground state (und s rule). Disjoint (=non co- extensive) alternate hydrocarbons will have small singlet- triplet energy gaps, with the singlet likely to be the ground state. Non- disjoint (co- extensive) alternate hydrocarbons will have large singlet- triplet gaps, with a triplet ground state. The pair of half- occupied near degenerate nonbonding MOs have their lobes coinciding significantly (co- extensive, the exchange integral is large). The lobes coincide to very small extent (disjoint MOs), the spin coupling is small (S=1 and S=0) are almost degenerated). 1

2 What about the spin preferences of diradicals? Are the linear combinacon of nonbonding molecular orbitals (NBMOs) disjoint (i.e., they occupy different sets of atoms) or not? = MO22 MO23 = MO22 MO23 What about the spin preferences of diradicals? The Trick of Borden and Davidson: to decide if the two nonbonding molecular orbitals are disjoint or not for alternate hydrocarbons by starring alternate carbons. If the difference between the number of starred (n) and unstarred (n) carbons is zero, then the molecule will have disjoint NBOs. n - n = 0 n - n = 2 2 C C 2 2 C C 2 2 C C 2 2 C C 2 C 2 RepresentaCve alternate hydrocarbons with starred and unstarred carbon atoms. 2

3 Tetramethyleneethane (TME) and Trimethylenemethane (TMM) TME is a singlet but the triplet state is very close in energy. TMM has a triplet ground state Tetramethyleneethane (TME) 1970 (Dowd, ESR): TME was assigned a triplet ground state...computaeons in disagreement with this result Clifford, gas- phase negacve ion photoelectron spectroscopy: the electron loss gives two different states of TME. Based on the populacon distribucon, they found the singlet state to be about 2 kcal mol - 1 below the triplet state. 3

4 Tetramethyleneethane (TME) The triplet state ( 3 B 1 ) can be described by a single configuracon. The singlet 1 A 1 state require at least two configuracons. Any computacons that does not account for the mulcconfiguraconal nature of the singlet will run into difficulces. Most computacons at mulcreference levels predict: For all conformacons, the singlet is lower than the triplet but those states are very close in energy. The smallest singlet- triplet gap, 1.3 kcal mol - 1 is obtained at an angle of 45 (with mulcreference coupled cluster methods) The gas phase PES experiment agrees with the computacons. The ground state of TME is a singlet but metastable triplet could be trapped explaining the ESR results of Dowd. Trimethylenemethane (TMM) Is TMM a ground states singlet or triplet? 4

5 Trimethylenemethane (TMM) Thermal rearrangement of methylenecyclopropane derivaeves MeO 2 C CO 2 Me MeO 2 C CO 2 Me MeO 2 C CO 2 Me Proposal: biradical TMM is produced Resonance stabilizaeon PreparaEon of TMM and selected reaceons Trimethylenemethane (TMM) Thermal rearrangement of methylenecyclopropane derivaeves Berson s trick to avoid rapid ring closure DemonstraEon of the different natures of singlet and triplet biradicals 5

6 The world of Benzyne The Bergman cyclizaeon The unusual chemical structural feature of potent ancbioccs: an enedyine fragment within the ring What is the mechanism underlying the biological acevity? The world of Benzyne The Bergman cyclizaeon The diradical abstract hydrogen atoms from the DNA molecule leading to its scission. ow to convert 12 into 13? 6

7 The world of Benzyne The Bergman cyclizaeon The original experiment (1972) ReacCons typical of a free radical behavior suggescng the formacon of 41. Experimental reaccon and accvacon enthalpies (Δ ( ) in kcal mol - 1 ): 39è 41 : 28.2 (470 K) 42è 43 : è 42 : è 44 : 10.0 The cyclizacon has a large accvacon barrier and depend on the cd distance The world of Benzyne TheoreEcal ConsideraEons The inherent mulcconfiguraconal nature of 41 and 40 was early recognized: CASPT2, large CI or DFT appears as the most promising opcons but without potencal difficulces. Method TS Product CASSCF(12,12)/TZ2P CASPT2// CASSCF(12,12)/ANO CCSD(T)/6-31G9d,p) with MP2 Frequencies Composite Approach UB3LYP/6-311+(3df, 3dp) Expt

8 The world of Benzyne The cd Criteria and Cyclic Enediynes cd Δ cd Δ NA NA (C 2 ) n a) values computed at the BPW91/6-311G level. a-c, n = 3-5 b) c) The cyclizacon has a large accvacon barrier, which depend on the cd distance and on the strain in the resulcng product. Benzynes Benzynes are challenging for both experiment and theory. All three isomers are ground state singlets, but with a differing extent of diradical character. diradical character: 11% 20% 65% ΔE ST (in kcal mol - 1 ) o- benzyne m- benzyne p- benzyne artree- Fock (Triplet ground state) MulEreference 33 (singlet ground state) Expt. 37.7±0.6 (singlet ground state) 21± ±0.5 8

9 Chemical ProperCes of p- Benzyne The S- T gap control the reaccvity of p- Benzyne ΔE ST (in kcal mol - 1 ) CASPT2/cc- pvdz Efficiencies for - transfer from tetrahydrofuran 0.007% none 8% 13% The decrease radical reaccvity result from the need to parcally uncouple the biradical electrons in the transicon state. Kenqamaa et al. J. Am. Chem. Soc. 2002, 124, Chemical ProperCes of p- Benzyne B3LYP/6-31G R R E a (C 1 - C 6 ) E a (C 1 - C 5 ) in kcal mol Ph C 1 - C 6 TS Pascal et al. J. Am. Chem. Soc. 2008, 130, C 1 - C 5 TS 9

10 RelaCve Energies of Benzynes RelaCve energies (kcal mol- 1 ) Method otrho meta para CASPT B3LYP/cc- pvtz CCSD(T)/cc- pvtz Latest Expt Structures of m- Benzyne Does m- Benzyne exist as the monocyclic biradical or as bicyclic closed- shell species? Or Comparisons of Spectroscopic Data 10

11 Mini Quiz 5 1. Using what you have learned about the difference between the number of starred (n) and unstarred (n) carbons, give the spin state of the following molecule. 2. What is the biggest challenge in the computaeon the electronic structure and reaceon mechanisms of diradicals and carbenes? 3. Propose an intermediate for the formaeon of cyclopentenones from vinylallene oxides. O 4. The ESR signal of 17 depends on the temperature. What does it mean? O An interescng case: the phenalenyl radical The phenalenyl radical is a prototypical open- shell graphene fragment, proposed for use in organic electronics. 9b- azaphenalene is a closed- shell analogue of phenylalenyl radical. N Phenalenyl 9b-azaphenalene 1. Which physical forces dominate the interaccons of the 9b- azaphenalene dimer? 2. What about the phenalenyl- radical dimer(s)? Which dimer is bound more strongly? 3. Which X- Ray structures do you expect for 2,5,8- t Bu- phenalenyl- radical and the unsubsctuted molecule? 4. Which computaconal level would you choose to compute the interaccon energy profile of 9b- azaphenalene? Do you expect the same methods to be accurate for the phenalenyl dimers? 5. If you remove one electron from the phenalenyl- radical dimer (giving a doublet caconic dimer), do you expect the interaccon energy to increase or decrease? ow could you verify your prediccon? 11