Subject: CCL:protonation/deprotonation at the excited state: computational evidence

From: "Jens Spanget-Larsen" <spanget!at!virgil.ruc.dk>
Date: Fri, 21 Nov 2003 08:57:51 +0100

"Olga Dmitrenko" <odmitr!at!UDel.Edu>:

> I would like to suggest in my paper that nitrogen atom in my molecule
> (urocanic acid) becomes more basic upon excitation. Is it enough to
> demonstrate that a particular atom, in my case nitrogen, has
> significantly increased electron density at the excited state when
> comparing HOMO and LUMO molecular orbital pictures? I did literature
> search - not much outcome. Any references would be of great help -
> thanks in advance. Bests, Olga

Dear Olga,
increased electron density on the basic nitrogen center is 
certainly an important indication, but it is not generally 
sufficient to predict increased basicity. Poor correlations 
have been found between calculated nitrogen electronic 
populations and pKa values (e.g., pyridine, pyrimidine, s-
triazine [1]). These publications may be of interest to you:

[1] J. Spanget-Larsen, "A Structure-Reactivity Relationship 
for the Basicity of Aza-arenes", J. Chem. Soc. Perkin Trans 
II, 417-419 (1985)
[2] J. Waluk, W. Rettig, J. Spanget-Larsem, "Ground and 
Excited-State Protonation of Aminoquinoxalines", J. Phys. 
Chem. 92, 6930-6935 (1988)
[3] J. Spanget-Larsen, "Structure-Reactivity Correlations for 
Aza-arenes. Proton Affinities, pKa Values, Hydrogen-Deuterium 
Exchange Rates, and Radical Induced 13C Shifts", J. Phys. Org. 
Chem. 8, 496-505 (1995)
[4] E. Sikorska, H. Szymusiak, I.V. Khmelinskii, A. Koziolowa, 
J. Spanget-Larsen, M. Sikorski, "Spectroscopy and Photophysics 
of Alloxazines Studies in Their Ground and First Excited 
States", J. Photochem. Photobiol. A 158, 45-53 (2003)           
	
Good Luck, Jens >--<

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JENS SPANGET-LARSEN         Office:         +45 4674 2710
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