Subject: Interpreting the quadrapole for aromatics

From: Jeremy Greenwood <[email-masked]>
Date: Sat, 3 Mar 2001 16:05:02 +0100

Dear all,

I'm studying series of closely-related planar 
(halogen,oxo,methyl)-substituted azines. I'm interested in how I can 
interpret the multipoles, particularly the quadrapole. 
Unfortunately, they are anions.

1) Is the quadrapole as confounded as the dipole as regards choice
of origin for an ion? The mass difference between F and Br will
disturb the centre-of-mass origin of otherwise identical anions.

2) Using MP2/6-311+G(3df,2p) set I get quadrapoles like

XX = -97 YY = -83 ZZ = -66
XY = -5.1 XZ = 0 YZ = 0

(orientation X, Y, 0)

Can comparisons of e.g. ZZ and XY across the series give me an idea 
about the degree of pi-density/aromaticity, and an idea about electron 
density in within the ring vs. on the electronegative substituents?

3) Given the size, my options are HF, DFT or MP2, with large basis
set including diffuse functions. Any reason to choose one over the 
other when comparing multipoles, electron density and aromaticity? 
I seem to recall B3LYP has a reputation for over-estimating aromaticity.

4) EPS-derived atomic charges from Gaussian (e.g. MKS) seem hard to interpret 
because fitting to the (origin dependent) dipole swamps the effects I am 
looking at. Would multipole fitting be a good way to go, or should
I be looking at AIM or even Mullikan charges instead, bearing in
mind that these are aromatics.

Thanks for any enlightenment,

Jeremy
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Jeremy Greenwood                                 [email-masked]
Department of Medicinal Chemistry                      bh +45 35306117
Royal Danish School of Pharmacy                        fx +45 35306040
Universitetsparken 2, DK-2100 Copenhagen, Denmark      ah +45 32598030
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