Synthesis, EPR Spectroscopy, and Matrix-Isolation Decay Kinetics of Triplet Cyclobutanediyls

Citation

Sponsler, Michael Bradley (1987) Synthesis, EPR Spectroscopy, and Matrix-Isolation Decay Kinetics of Triplet Cyclobutanediyls. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/me26-sd58. https://resolver.caltech.edu/CaltechTHESIS:08082025-174515566

Abstract

Chapter 1

Bicyclo[1.1.1]pentanone (11) has been prepared in two steps, the key reaction being the ozonolysis of 2-phenylbicyclo[1.1.1)pentan-2-ol (24). The microwave spectra of five isotopic species of 11 have been obtained, allowing a complete r_s structure determination for the heavy atoms. Analysis of Stark-effect measurements has shown the dipole moment to be along the a principle inertial axis with a magnitude of 3.164(5) D. These results are compared with those obtained using molecular mechanics (MM2), MNDO, and Hartree-Fock ab initio theory with STO-3G and 3-21G basis sets. On heating, 11 undergoes cycloreversion to allylketene (31). The activation parameters and solvent effects for this process suggest that the reaction is concerted and that the transition state is relatively nonpolar. The predominant photochemical pathway for 11 is decarbonylation to bicyclobutane (34). Cycloreversion to 31 is a minor reaction mode. Both the thermal and photochemical results are rationalized by considering the high strain energy and novel geometrical features of 11 and, in the latter case, the unusually high energy of its ¹(nπ*) state.

Chapter 2

Ab initio theoretical methods have been applied to cyclobutanediyl. The geometry of the triplet biradical was optimized under the constraints of D_2h symmetry. At the optimum geometry, the singlet-triplet energy gap was found to be 1.74 kcal/mol, with the ground state being the triplet. The closure reaction to bicyclo[1.1.0]butane (34) was investigated using a linear synchronous transit reaction coordinate. A barrier of 6.6 kcal/mol was obtained for the singlet surface, representing an upper limit to the actual barrier. Calculations on 1,3-dimethylcyclobutanediyl (12) show that the methyl groups do not strongly affect the singlet-triplet gap, reducing it to 1.47 kcal/mol.

Chapter 3

The synthesis and EPR spectroscopy of 1,3-divinyl- and 3-ethyl-1-vinylcyclobutanediyl (46-Vin and -EV) are described and compared with results for other cyclobutanediyls. The triplet biradicals were generated by photolysis of the appropriate 2,3-diazabicyclo[2.1.1]hex- 2-enes (47) in frozen-solvent matrices at cryogenic temperatures. A general synthetic scheme allowing the preparation of many diazenes has been developed. The key step is the photochemical addition of methyltriazolinedione to 1,3-dicarbomethoxybicyclo[1.1.0]butane (49), giving the dicarbomethoxyurazole 15. The triplet EPR spectra offer valuable insights into the electronic structure of the cyclobutanediyls and the role played by the substituents. Both zero-field splitting and hyperfine coupling emerge as sensitive gauges for determining the distribution of spin density in the biradicals.

Chapter 4

The nonexponential decay kinetics of 1,3-divinyl- and 3-ethyl-1-vinylcyclobutanediyl (46-Vin and -EV) has been quantitatively analyzed in the 20-54 K temperature range using two methods based upon a distribution of first-order rate constants. A numerical procedure for fitting of decay traces based upon an assumption of distribution shape has been modified to allow fitting of the signal growth during photolysis as well as the decay. This method successfully reproduced our data using a variety of distribution shapes. A new method for analysis of dispersive kinetics, called distribution slicing, has been developed. This method allows detailed determination of distribution shapes and is complementary to decay-trace fitting in several other ways. The two methods have been combined to eliminate several problems encountered in the separate methods, allowing the determination of approximate activation parameters. An approximate log A value of 7.5 was determined for the decay of 46-Vin along with most-probable activation energies in the range 1.2 to 1.7 kcal/mol, depending on the matrix material. Using the same log A value, 46-EV gives most-probable activation energies of 1.4 and 0.9 kcal/mol in MTHF and heptane, respectively. The stability of these biradicals relative to the dialkyl cyclobutanediyls was rationalized in terms of hypothetical potential energy surfaces.

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