Computational studies of organic biradicals
Abstract
The research discussed in this dissertation involves the computational determination of the thermochemistry of biradicals, specifically benzynes, $\alpha$,n-dehydrotoluenes and carbyne anions. Ab initio molecular orbital theory is applied to determine the singlet-triplet splittings and heats of formation for these species. A new correlation consistent configuration interaction (CCCI) methodology has been developed to treat electron correlation in these systems in a balanced manner, thus achieving experimental accuracy. For the benzynes the CCCI/pVTZ method gave calculated heats of formation of 107, 125 and 138 kcal/mol for the ortho-, meta- and para-benzyne isomers, respectively. The agreement with CID experimental measurements is exact. The same calculations give singlet-triplet energy splittings of 37, 17 and 2.2 kcal/mol, respectively. The MCSCF(8,8)/cc-pVDZ calculations on the $\alpha$,n-dehydrotoluenes gave heats of formation of 105 kcal/mol for each isomer, agreeing well with the experimental value of 103 $\pm$ 3 kcal/mol. These calculations also explained puzzling experimental results of CID measurements on halobenzyl anions. The ground-state assignments, singlet-triplet splittings and stabilization energies of all 1st and 2nd row substituted carbyne anions were computed at high, single-reference levels and corrected to the "best" values with MCSCF and CCCI calculations with a large basis set. The computed properties are accurate enough to discern trends in carbyne anion behavior, but larger, more computationally intensive calculations need to be performed to achieve experimental accuracy and to derive absolute heats of formation for these species. The calculations in this thesis show the utility of computational methods in characterizing biradicals and assisting experimental research. The CCCI method developed here is a generally useful tool for studying biradical thermochemistry. The technique can be applied to a large number of molecules and offers numerous advantages over other techniques.
Degree
Ph.D.
Advisors
Squires, Purdue University.
Subject Area
Organic chemistry
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.