Subject: experimental geometry --> summary

From: <[email-masked]>
Date: Mon, 29 Aug 1994 17:49:45 +0000 (O)

Dear Netters;

Thank you very for all the replay I got concerning the comparison between 
different experimental geometries. My original question was:

< Can someone elaborate on the difference of the geometry by microwave,
< electron diffraction and X-Ray. Can you give references. My understanding
< is that the microwave geometry is better than the others but I do not know
< exactly why.

The following is a summary of all the replies I got. 

Adel El-Azhary
Cairo University

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From:	IN%"[email-masked]"  "AL LOWREY" 22-AUG-1994 15:57:40.30
To:	IN%"[email-masked]"
CC:	
Subj:	experimental geometries

Gas Phase Electron Diffraction, Microwave Spectroscopy and Xray Diffraction
determine different expoerimental geometries because they are measuring 
different internuclear separations.  Broadly speaking, Xray diffraction 
measures the separation between centroids of electron density, electron 
diffraction 
measures average distances between nuclei, and microwave spectrospocy measures 
effective legnths of dipoles.  Thermal efffects ore critically important for
all these techniques and change experiemntally observed distances in different
ways.  None of these techniques directly determine an equilibirum molecular
structure  nor to they directly correspond to the structures produced by
computational techniques.  The method of combined techniques uses all
this information.  Good references are Accurate Molecular Structure by
Domenicano and Hargittai  published by Oxford  and two voluems
on gas phase electron diffraction published by VCH  and edited by Hargittai
and Hargittai.

                                            Bewst Wished
                                            Best Wishes,

                                                   Alfred Lowrey
[email-masked]

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From:	IN%"[email-masked]"  "Wayne Steinmetz" 23-AUG-1994 
        01:34:24.56
To:	IN%"[email-masked]"
CC:	
Subj:	microwave structures

The full story is somewhat complicated and is discussed in detail in
D. R. Lide, Jr. and M. A. Paul, ed. Critical Evaluation of chemical and
Physical Structural Information, National Academy of Sciences, US,
Washington, D.C., 1974.  ISBN 0-309-02146-4  .

A complete microwave study is the most reliable for the following reasons:
a) The structure is based on frequencies measured to 1 part in ten
million rather than intensities.  If the spectroscopic transitions can
be correctly assigned, the entire study can be done with a mixture.  A mixture
of species is a source of systematic error when the structural method
such as electron diffraction depends on the measurement of intensities.

b) The frequencies of the individual spectroscopic transitions are related
to a well known, well defined quantum mechanical model.  Molecules with
large amplitude motion can be difficult to handle.

c) In the cases where the contributions from vibrational motion can be
removed, the structure corresponds exactly to that calculated from
ab initio methods.  This can be done easily in the case of diatomics and
with considerable effort in the case of triatomics.

d) With polyatomics, a good structure is based on data from isotopically
substituted species.  Most but not all of the contributions from
vibrations have been removed in the resulting rs structure.

The Allinger group which has produced the MM2 and MM# force fields has
to worry about the issue which you raised and has empirical correction
factors for structural data derived from theoretical and experimental
methods.  Biosym also has a first-class force field and has to use
structural data to test it and would naturally be concerned about the
integrity of the data.

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From:	IN%"[email-masked]" 23-AUG-1994 02:46:32.32
To:	IN%"[email-masked]"  "netters"
CC:	
Subj:	CCL:Accurate Molecular Structures

For a very comprehensive discussion of the accuracy and meaning of geometry
parameters I suggest the book "Accurate Molecular Structures
Their Determination and Importance." Ed. A. Domenicano and I Hargattai.
 published by the International Union of Crystallography and Oxford Univ.
 Press 1992. The paper by Kozo Kuchitsu (pages 14-46)on the meaning of internuc
lear distances and a number of papers following it are essential information
for anyone attempting to use experimental geometry in their calculations.
    See also Calloman et al Molecular Constants. Landolt-Bornstein. Numerical
data and functional relationships in science and tech. New Series Group II
vol 14b Springer, Berlin, also vol 15 and maybe later vols. Bankrupt your
university by asking them to buy the set.
 (The OUPress book has ISBN number 0-19-855556-3)
                        John E. Bloor,Univ of Tenn(Pa13808 at UTKVM1.utk.edu)

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From:	IN%"[email-masked]" 24-AUG-1994 07:08:38.45
To:	IN%"[email-masked]"
CC:	
Subj:	CCL:experimental geometry

Adel El-Azhary  writes:

> Can someone elaborate on the difference of the geometry by microwave,
> electron diffraction and X-Ray. Can you give references. My ynderstanding
> is that the microwave geometry is better than the others but I do not know
> exactly why.

What's "better" depends on the molecule and what information you want.
First off, ED and XRay give you gas-phase, i.e. isolated molecule, info,
which may or may not be what you want.  Also, neither can handle large
species.

When both ED and MW can give you a structure, bond distance differences
are likely to be at the .00x Angstrom level.  It's important to remember
vibrational amplitudes are .0x Angstroms!  Both ED and MW measure some
sort of average distance, and just what average that is depends both on
the experimental method and on the way the distances were deduced from the
data.  And common methods of data treatment, for both ED and MW, can go
pathological in the hands of the naive or careless user.  [But of course,
that sort of thing never happens to theorists. :) ]

So in terms of the real world and the measurements we can make on it,
all those digits (past, say, .01 Angstrom and 1 degree) in geometries
we get from Gaussian 92 don't have a lot of significance.

There's an extensive literature on just what ED and MW distances "mean."
I really don't recommend it to those outside the fields unless you badly
need to know!

                    | H. Bradford Thompson   [Brad]
[email-masked]       | Scholar in Residence, Chemistry & Physics
                    | Gustavus Adolphus College, St. Peter MN 56082-1498

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From:	IN%"[email-masked]" 24-AUG-1994 14:52:04.08
To:	IN%"[email-masked]"
CC:	
Subj:	CCL:experimental geometry (correction)

Sorry.  In my message on "exerimental geometry" this morning I said:

> What's "better" depends on the molecule and what information you want.
> First off, ED and XRay give you gas-phase, i.e. isolated molecule, info,
> which may or may not be what you want.  Also, neither can handle large
> species.

I meant to say,

... ED and MW, in contrast to XRay, give you gas-phase, i.e., isolated
molecule, info. ....

A classic case of the fingers getting behind the brain.  Thanks to Gary
Newton for calling this to my attention promptly.

                    | H. Bradford Thompson   [Brad]
[email-masked]       | Scholar in Residence, Chemistry & Physics
                    | Gustavus Adolphus College, St. Peter MN 56082-1498

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From:	IN%"[email-masked]" 24-AUG-1994 17:58:35.77
To:	IN%"[email-masked]"
CC:	
Subj:	RE: CCL:experiemental geometry by microwave spectroscopy

Adel,
	This is a common question that several people have. Look at the book :

Molecular Mechanics by Ulrich Burkert and Norman Allinger ACS monograph#177

where this question has been dealt with (see page 6 to 8 for example.)

Good Luck.

Adi M Treasurywala,Sterling Winthrop Inc,1250 South Collegeville Road,
PO Box 5000, Collegeville, PA 19426-0900,Voice (215)983-6610 FAX
(215)983-5559, INTERNET [email-masked]