Subject: Summary: geometry optimizations of isotopic substituted molecules

From: jochen <[email-masked]>
Date: Mon, 23 Sep 1996 18:17:14 +0200 (MET DST)

Dear Netters,
Some days ago I asked the following question:

> recently a friend came up with the problem of geometry optimization of
> deuterated species. He is expecting slighthly different geometries for
> the different isotopic species that are not only originated in the 
> zero point vibrations.
> Packages like GAMESS and gaussian only use isotopic information in
> frequency calculations. Are there any other packages that use this
> information in geometry optimizations.

The answers are:

1.) From: Jan Hrusak <[email-masked]>

Bin Chemiker und aufgewachsen mit Massenspektrometrie, also habe enormes 
interesse
an den isotopenmarkierten verbindungen. ZPVE ist recht OK obwohl auch eine 
geometrie-
optimiereung mit einschluss dieser nicht trivial ist. Aber der 
elektronische anteil bleibt unberuecksichtigt. Wie gesagt bin ich nicht in 
der teilchenphysik sehr bewandert aber es
hat mich schon immer fasziniert dass das H anderes IP hat wie D. Da es aber 
ein
zweitelchenproblem ist laesst sich es wohl loesen in dem man die korrekte
reduzierte masse einsetzt. Interssant in diesem zusammenhang ist auch dass
das IP von H im nicht-BO fall nicht genau 0.5 Hartree ist, sondern muss den
faktor Me+MH/MeMH korigiert werden. Fur groesere systeme als H wird es noch
schwieriger. Ich glaube nicht, dass das angesprochen problem fuer andere 
systeme als
H2+, HD+ und D2+ im moment zu loesen ist.

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2.) From [email-masked] Mon Sep  9 13:58:09 1996

First, I don't know your answer.  But the reason GAMESS and Gaussian
do not include isotopic discrimination is because most (if not all) of
the levels of theory in the packages only include e-e, e-n, and n-n
interactions through nuclear charge, i.e., not including nuclear mass
effects.  I believe from my advanced courses, that these effects are
much smaller than ZPE effects that you can take into consideration,
perhaps even smaller in effect than the accuracy of the rest of the
Hamiltonian.

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3.) From [email-masked] Mon Sep  9 18:28:25 1996

Dear Jochen,
Due to the Born-Opperheimer approximation used in most quentum chemical 
packages, the geometry of isotopically subsituted compounds will be 
identical to the parent species.  However, sometimes the variations in 
bond lengths that do in fact occur in nature due to isotopic subsitutions 
do in fact influence experiments.  We bumped into such a phenomenon while 
studying the ESR properties of isotopically substituted pyrenes, and 
rationalized the experimental data using the differences in bond length 
between C-H and C-D bonds.  This was confirmed by quantum chemical 
computations, in which we fixed the C-D bond length to be a certain 
fraction shorter than the optimized C-H bond length, and re-optimized the 
whole system again with this fixation of the C-D bond length.
Reference: Perturbation of spin density distribution due to deuterium 
substitution
Zuilhof, H; Lodder, G.; Van Mill, R.P.; Mulder, P.P.J.; Kage, D.E.; Reiter,
R.C.; Stevenson, C.D. - J. Phys. Chem. 1995, 99, 3461-3464.

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4.) From: [email-masked]

this is an interesting question: please summarize!

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5.) From: Jens Spanget-Larsen <[email-masked]>

There are subtle differences in the electronic structures of atomic 
hydrogen and deuterium.  Due to the increased mass of the deuterium 
nucleus, the reduced mass of the electron-nuclear two-body system is 
slightly larger for deuterium than for hydrogen. This means that the 
electronic energy is slightly more negative, corresponding to 
slightly larger ionization energy, and the orbital wavefunction is 
slightly more contracted for deuterium.  I would guess that the 
equilibrium structures for the diatomic molecules corresponds to a 
slightly shorter bond length for di-deuterium, D-D, than for 
di-hydrogen, H-H.  However, I have not looked up the literature for 
experimental data (Gmelin?).  In any case, the difference should be 
really small. Factors due to the different zero point energy levels 
are probably more significant, and I have not heard of any 
computational package that treats the electronic systems of H and D 
differently.    

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Thanks to everybody who responded,
Jochen

+-----------------------------------------------------------------------+
|  Jochen Kuepper                                                       |
|                                                                       |
|  Heinrich-Heine-Universitaet Duesseldorf   [email-masked]  |
|  Institut fuer Physikalische Chemie I                                 |
|  Universitaetsstrasse 26.43.02.29          phone ++49-211-8113681     |
|  40225 Duesseldorf                         fax   ++49-211-8115195     |
|  Germany                                                              |
|                                                                       |
+-----------------------------------------------------------------------+

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From: A J Turner <[email-masked]>
Date: Mon, 23 Sep 1996 21:06:43 +0100 (BST)

Hi!

I am sorry not to have replied firt time around.  Clearly the anharmon
nature of the c-d vibration will make the c-d bond shorter when temp > 0
K.  The best way of truely finding this owuld be to get the mean bond
length from a dynamics trajectory.  Is is easly done - but the cost is
prohibitive for large theory or large molecules.  

If you are looking at super computers and three heavies I guess MP2 is
well within reach.

Hope it helps

Alex

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|Alexander J Turner         [email-masked]                  |
|Post Graduate              |http://www.bath.ac.uk/~chpajt/home.html|
|School of Chemistry        |+144 1225 8262826 ext 5137             |
|University of Bath         |                                       |
|Bath, Avon, U.K.           |Field: QM/MM modeling                  |
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