From owner-chemistry@ccl.net Tue Nov 22 07:14:00 2016
From: "Daniel Morales Salazar danielmoralessalazar91^^gmail.com" <owner-chemistry%a%server.ccl.net>
To: CCL
Subject: CCL: About virtual orbitals in oligomeric pi-conjugated system
Message-Id: <-52503-161122071102-3954-ea5Dt9q/HLqFTCtdMK6FSw%a%server.ccl.net>
X-Original-From: "Daniel  Morales Salazar" <danielmoralessalazar91(!)gmail.com>
Date: Tue, 22 Nov 2016 07:11:00 -0500


Sent to CCL by: "Daniel  Morales Salazar" [danielmoralessalazar91[*]gmail.com]
Dear all,

I am performing calculations on a pi-conjugated monomer system (83 atoms 
of which 43 are hydrogens) with a Donor-Acceptor-Donor type of electronic 
structure. At the B3LYP/6-311G(d,p) level of theory the structural and 
optical parameters compared well with experimental X-ray data and UV-Vis-
NIR spectroscopy. In the monomer case the HOMO density is located on the 
Donor fragments whereas the LUMO density is found on the acceptor units, 
naturally. 

I started modelling oligomeric systems, up to the tetramer. In this 
latter one, for example, I noticed that the LUMO, LUMO+1 LUMO+2 LUMO+3 
were localized on different acceptor units. The same was true for the 
dimer (LUMO, LUMO+1) and trimer (LUMO, LUMO+1,LUMO+2). At first sight , 
these orbital densities made me think that the role of the acceptor was 
"lost" upon increasing the conjugation length/size. 

However, when I looked at the energies of these frontier virtual 
orbitals, the differences on the dimer (LUMO to LUMO+1), trimer (LUMO to 
LUMO+2) and tetramer (i.e. LUMO to LUMO+3) are very low. Less than 0.1 eV 
in the tetramer system. 

Since these energy differences are so low, can I consider all of these 
virtual orbitals (i.e. LUMO to LUMO+3 in the tetramer model) to be an 
intrinsic part of the molecular electronic structure of the system; that 
is of course, understanding the fact that they are unoccupied orbitals, 
and what I mean by intrinsic is that either three of them may be 
accessible and/or act as acceptors. For example if a donor of the right 
symmetry or energy approaches the molecular motif; or if there is an 
intramolecular charge transfer with the right symmetry. Therefore, one 
could conclude that the role of the acceptor fragments remained intact in 
the oligomers as compared the monomers.

 Is this interpretation a valid one? 

I would like some confirmation as I have read the virtual orbitals 
obtained in DFT have no physical significance (apart from the HOMO, and 
perhaps LUMO).

Thank you very much for your help and I hope the question was somehow 
clear.

Sincerely,

Daniel Morales Salazar