Conformation-specific spectroscopy and dynamics of flexible bichromophores and combustion-related molecules

Nathan R Pillsbury, Purdue University

Abstract

This thesis has as its theme the conformation-specific spectroscopy and isomerization dynamics of flexible benzene derivatives. These substituted benzenes were studied because of their relation to combustible fuels. Studying the conformation-specific spectroscopy and dynamics of these types of molecules is relevant to the isomerspecific chemistry and rates of reaction of large molecules that make up the pathways to soot formation during the process of combustion. In our experiment, a supersonic expansion was utilized to cool the gas-phase molecules into their respective zero-point levels where various laser-based spectroscopic methods were implemented to obtain their conformation-specific infrared and ultraviolet spectral signatures. The ultraviolet and infrared spectra of 5-phenyl-1-pentene and bis(2-hydroxyphenyl)methane were obtained. Ultraviolet hole-burning spectroscopy revealed that multiple conformational isomers exist in free jet environment for both molecules. The different conformations were assigned to specific structures by comparing experimental spectra to the results of density functional and ab initio calculations. This information was then used to study the dynamics of isomerization between conformations with stimulated emission pumping –population transfer spectroscopy (SEP-PTS). SEPPTS was previously developed in our laboratory as a means to directly measure the barriers to isomerization between specific X→Y conformer pairs. When applied to 5-phenyl-1-pentene and bis(2-hydroxyphenyl)methane, we obtained bounds on the isomerization barriers, relative energies of the minima, and were able to deduce from this data key aspects of the preferred pathways to isomerization. In the case of bis(2-hydroxyphenyl)methane, SEP-PTS was also utilized to elucidate the rate of collisional cooling in the supersonic expansion. The second part of this work is aimed at understanding the complicated spectroscopy of flexible bichromophores. Here the bichromophores of bis(2-hydroxyphenyl)methane and diphenylmethane are discussed. Both molecules possess two ultraviolet chromophores in close proximity to one another, resulting in fascinating spectroscopy depending on the relative orientation and distance between the two chromophores. Diphenylmethane was particularly interesting because very detailed information about the interchromophore electronic coupling was extracted. These are the first state-to-state studies of internal mixing, which uncovered a break-down in vibronic coupling selection rules by virtue of the close energy spacing between the two electronic states.

Degree

Ph.D.

Advisors

Zwier, Purdue University.

Subject Area

Physical chemistry

Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server
.

Share

COinS