VIBRONIC EFFECTS ON THE 3500 {\AA} BAND IN AROMATIC CARBONYL COMPOUNDS
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Date
1964
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Ohio State University
Abstract
The polarization of the 0, 0-band of the phosphoresence of benzaldehyde, acetophenone, benzophenone and anthrone were measured relative to their near ultraviolet absorption. For these molecules it is accepted that the lowest triplet and the lowest singlet states are $(n, \pi^{\ast})$ in assignment. Positive polarization of the most intense progression in the $^{1}U(n, \pi^{\ast})\leftarrow A_{1}$ system, except for anthrone, indicates that this transition is allowed through perturbation by an $a_{2}$ vibration and hence intensity is stolen from an $A_{1}$ state. A second weaker progression having negative polarization indicates perturbation by a $b_{2}$ vibration with intensity stolen from a $B_{1}$ state. In anthrone the situation is reversed. Negative polarization of the principal progression indicates that the transition in this molecule is mainly allowed through a $b_{2}$ vibration. These vibronic effects are discussed in terms of $Pople-Sidman's^{1}$ and $Callomon-Innes^{2}$ scheme for formaldehyde. In benzophenone and anthrone, the coupling of the dipole moment vectors of two benzene rings causes the $L_{b}$ state to split into $A_{1} (L_{b} + L_{b})$ and $B_{2} (L_{b} - L_{b})$ states. The polarization of the second absorption band of these molecules indicates that this band actually consists of two transitions $A_{1}\leftarrow A_{1}$ and $B_{2}\leftarrow A_{1}$.
Description
$^{1}$ J. A. Pople and J. W. Sidman, J. Chem. Phys. 27, 1270 (1957). $^{2}$ J. H. Callomon and K. K. Innes, J. Mol. Spectroscopy, 10, 166 (1963).
Author Institution: Whitmore Chemical Laboratory, The Pennsylvania State University
Author Institution: Whitmore Chemical Laboratory, The Pennsylvania State University