Subject: Summary: Gauche effect ab initio calculations

From: [email-masked] (M. Nicklaus)
Date: Wed, 27 Jul 94 11:32:25 -0400

Dear Net-Workers,

Some time ago I posted the following

QUESTION:
=========
> We are thinking about doing ab initio modeling (using Gaussian 92)
> on small organic molecules which possess, apart from C, H, N, O,
> a sulfur and a halogen atom (F, Cl, Br, I) in such an arrangement
> that we expect interactions of the gauche-effect type to occur
> between them.
>    We would like to ask the experts what level of theory, basis
> sets, electron correlation methods etc. they would recommend for
> obtaining reliable conformational energies when calculating
> local energy minima for such compounds.  Would different approaches
> be advisable for geometry optimization vs. the final energy
> calculation?  Would using non-standard basis sets (which ones? 
> obtainable where?) be necessary especially for the higher-period 
> halogens?

   I am summarizing the answers I received below.  The bottom line
seems to be that the more "**" and "++" the better (and hence the
more "$$", I fear).  Thanks to all who responded or even sent me 
preprints.

Marc C. Nicklaus                        Lab. of Medicinal Chemistry
e-mail: [email-masked]               National Cancer Institute, NIH
Phone:  (301) 402-3111                  Bldg 37, Rm 5B29
Fax:    (301) 496-5839                  Bethesda, MD 20892

======================================================================

ANSWERS:
========

From: [email-masked]   Bob Stolow

We have done ab initio computations using Gaussian for 1,2-difluoroethane
to find out what level of theory was adequate to reproduce the results of
published gas-phase experiments.  We can confirm that it is necessary
to use a level of theory approaching those reported in the following
articles:

	Wiberg, K.B.; Murcko, M.A. J. Phys. Chem. 1987, 91, 3616.

	Dixon, D.A.; Smart, B.E. J. Phys. Chem. 1988, 92, 2729.

For 1,2-difluoroethane, we found that MP2/6-311G**//6-311G** and lower
levels of theory were unsatisfactory.  Diffuse functions on all atoms
gave a reasonable result at the MP2/6-311++G**//6-311G** level of theory.
Electron correlation cannot be neglected.  Dixon also used MP2, while
Wiberg used MP3/6-311++G**//6-311G*.

This should give you some idea of the minimum level required for your
calculations to yield reasonable results.  Of course, the current
state-of-the-art would use even higher levels of theory.

---------------------------------------------------------------------

From: Patrick Bultinck <[email-masked]>

I am just finishing some work on ethylene glycol, ethylenediamine and 
ethane-dithiol....

My results can be trusted now, contrary to some of my early findings 
which were not so good. My calculations contain : 3-21 G optimizations, 
6-31++G** optimizations (full) and MP2 calculations...

-----------------------------------------------------------------------

From: [email-masked]    Igor Topol

Many recepies how to calculate accurately different conformational minima
including problems with gauche effects you can find in our recently published
paper:
T. Oie, I.A.Topol, S.K.Burt "Ab Initio and Density Functional Calculations on
Ethylene Glycol", J.Phys.Chem., 98 (1994) 1121-1128.

------------------------------------------------------------------------

From: [email-masked] (Dawn Friedman)

Use the best basis set you can, with plenty of polarization functions,
and diffuse functions (+) would be good too, as you will be looking at
systems whose resonance structures would include lots of lone pairs and
possibly anions; 3-21G*+ at the minimum, but preferably 6-31G**++.  

Use the
best basis set you can muster for the geometry optimization; energies
calculated at that geometry but at a higher basis set probably won't give
you much more information than you already have.  If you don't reoptimize
the geometry at each dihedral angle you try, your results will be deceptive,
but everyone knows that by now, right?
  
You'll want to do some post-Hartree-Fock, perhaps CISD; if you really
need to try for the absolute geometry changes between conformations, 
you'll have to
try optimizing with post-HF, but HF will give you the trends in geometry
changes.
    
If you're using Gaussian, which has it available as a keyword, the
Reed-Weinhold Natural Bond Orbitals are custom-made for this sort of thing.
They'll tell you which orbitals are interacting and how much (relative
to other systems in the set you're looking at.)

---------------------------------------------------------------------------

From: Patrick Bultinck <[email-masked]>

Have a look at the following :

1/      Wiberg and Murcko, J. Phys. Chem., 1987, Vol. 91, 3616-3620
	Title : Rotational barriers, 1. 1,2-Dihaloethanes

2/      Hirano et al., J; Chem. Soc. Chem. Comm., 1986
	An NMR study that has as aim the study of the gauche effect among 
	others.

3/      Wolfe, Acc. Chem. Res., Vol. 5, 1972
	Must reading ...

4/      Siam et al., J. Mol. Struct., 236 (1991), 1-13
	Maybe less interesting !!!!

5/      Radom et al., JACS, 95 (3), 1973, 693
	Albeit a little low level calculus (but then again this is 1994, 
	not 1973 !!) it gives a good comparison between different molecules.

	I think nr 1 would have to be very interesting for you, since it gives a 
	flavour of different results with different HF-basissets, and MPx results.

	If you would be interested, I have some results for 6-31++G** 
	optimizations on FCH2CH2F...  in which one clearly sees that the reason 
	for stabilisation of the G-conformer in the XCH2CH2X is governed by the 
	relative size of the 1-el energy lowering in the G-conformer resp. to the 
	increase of the N-N energy.

--------------------------------------------------------------------------------

From: [email-masked] (Bill Schneider)

We have recently submitted a paper to JACS in which we have looked at
(essentially) the gauche effect in halogenated methanols.  I will drop a
preprint of the paper in the mail to you.  Perhaps it will be of help.
[Thanks for the preprint. M.N.]

-------------------------------------------------------------------------------

From: [email-masked]   Bob Stolow

Since the time we completed our own calculations on 1,2-difluoroethane,
the following pertinent papers have appeared:

	Wiberg, K.B.; Murcko, M.A.; Laidig, K.E.; MacDougall, P.J.
	J. Amer. Chem. Soc. 1990, 94, 6956-6959.

	Durig, J.R.; Liu, J.; Little, T.S.; Kalasinsky, V.F.
	J. Phys. Chem. 1992, 96, 8224-8233.

--------------------------------------------------------------------------------