Subject: HYBRIDIZATION REAL? SUMMARY

From: elewars <[email-masked]>
Date: Tue, 19 Sep 2000 13:20:35 -0400

2000 Sept 19

Hello,
here is the summary of replies and associated correspondence concerning
my question. Obviously there is no simple, general agreement. I thank
all who replied and made me rethink my view that it was generally agreed
that hybridization is a purely mathematical "trick". Yet I remind us
that it is possible to do electronic structure calculations (e.g.
standard ab initio) without invoking hybridization (explicit
hybridization anyway).

The Question:
2000 Sun Sept 10

Hello,

We know that hybridization is a mathematical procedure and not an actual

physical process; for example, an s and a p orbital
(orbital=one-electron function) are vectors in Hilbert space, which can
be mathematically combined to give two new orbitals (two sp orbitals).

QUESTIONS:

(1) Did Linus Pauling _invent_ hybridization? He certainly popularized
it.

(2) Did Pauling accept that hybridization is "only" a mathematical
procedure?

===========
The replies (there are 28 letters below); thanks again.  EL
===============
#1
[email-masked] (Alan Shusterman)

-- You wrote:
(1) Did Linus Pauling _invent_ hybridization? He certainly popularized
it.
--- end of quote ---

A lecture that I recently heard on this topic suggested that Pauling and
Slater arrived at the sp3 hybrid model for
methane independently, but Pauling is usually credited with the
invention. You might find some information in Mary Jo Nye's book, "from
chemical philosophy to theoretical chemistry".

What chemists seldom mention is the fact that Pauling didn't seem to
care for alternative hybridization schemes for
carbon (sp2 C in alkenes, sp C in alkynes). The Nature of Chemical Bond
describes CX double bonds in terms of sp3
hybrids on C.

On the other hand, Coulson, in "Valence", describes all three
hybridization schemes for C, but I don't know if he is the
inventor of these schemes.

As for your second question, I don't have any of Pauling's writings in
front of me, but I'm sure that he accepted (and
even stated) the idea that hybridization was only a mathematical scheme
for thinking about bonding.

It seems to be mainly non-theoretical chemists who get confused on this
point. I've just spent 2 hours going through
web sites on chemical bonding and hybrid orbital theory, and I find that
most of these propose that one set of orbitals
or another (hybrid, molecular, atomic, you name it) are "real" and the
others are not. Or, equally awful, they say that
one set of orbitals is the "right" set of orbitals and the others are
wrong. The fact that these people (and their students)
are productive, successful chemists suggests to me that a basic
understanding of molecular quantum mechanics is
irrelevant for the successful practice of chemistry.

-Alan

----------------------------
Alan Shusterman
Department of Chemistry
Reed College
Portland, OR 97202
===============
#2
"Isaac B. Bersuker" <[email-masked]>

I believe that you are wrong in your statement that hybridization is a
mathematical
procedure. The point is that there is no hybridization in free atoms,
but it takes
place UNDER THE INFLUENCE OF THE ENVIRONMENT, THE BONDING. In this
formulation
hybridization is quite a legitimate physical phenomenom (see, e.g., my
latest book
>from Wiley, New York, 1996)

Bersuker

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

#3
Paulo E. Abreu

"Isaac B. Bersuker" wrote:

> I believe that you are wrong in your statement that hybridization is a
mathematical
> procedure. The point is that there is no hybridization in free atoms,
but it takes
> place UNDER THE INFLUENCE OF THE ENVIRONMENT, THE BONDING. In this
formulation
> hybridization is quite a legitimate physical phenomenom (see, e.g., my
latest book
> >from Wiley, New York, 1996)

What do you mean by a legitimate physical phenomenom ?  Can you detect
it
experimentally ?
I thought it was a way to explain chemical bonding in the VB approach.
Like for
instances in CH4,
where the the bonds are made of orbitals with the suitable symmentry and
not a priori
with a recipe.
I dont know your book, but I would like to read very much. Can you
provide more
details ?

Paulo E. Abreu

--
Departamento de Quimica                      e-mail:[email-masked]
Universidade de Coimbra                      TEL:351 239 852080
3049 Coimbra Codex                           FAX:351 239 827703
Portugal
=============
#4
[email-masked]

I believed that Slater arrived at the hybridization scheme
independently of Pauling and at around the same time.

Pauling's own interprettion of hubridization has been the subject of
much debate.
There is also the related issue of the Russian 'ban' on quantum
mechanics brought about because of what they saw as the implications
of hybridization and resonance.  I could provide some references to
the philosophical literature if anyone is interested.

Also there was an interesting debate between Ogilvie, Pauling and
others  in the pages of Journal of Chemical Education about 8 years
ago.

(re my Q on Fock) There was a recent article on the life of Fock in
Chemical Intelligencer.

eric

eric scerri
=================
#5

            "Paulo E. Abreu" <[email-masked]>
            To:
            "Isaac B. Bersuker" <[email-masked]>

"Isaac B. Bersuker" wrote:

> Yes, you can: hybridization changes the charge distribution which is,
in principle
> observable

Sorry to disagree (again) with you but I dont think that is a correct
answer. Bonding
changes the charge
distribution not hybridization. Besides what do you mean by changing the
charge
distribution ?
Comparing it with to the free atoms ? Orbital are models, and within
these models we
can describe
chemical bonding with various approaches (MO or VB). Hybridization is
only used in VB
theory, because
in MO the symmetry of the molecule plays an important role in
determining the
coefficients of the orbitals.
Remember that to use hybrid-orbitals you must a priori know the geometry
of the
molecule.
Paulo E. Abreu

--
Departamento de Quimica                      e-mail:[email-masked]
Universidade de Coimbra                      TEL:351 239 852080
3049 Coimbra Codex                           FAX:351 239 827703
===========
#6

         [email-masked] (Alan Shusterman

Before people get too worked up about the "reality" of hybridization,
they might pause and enjoy this quote from
Coulson's "Valence", p. 219, section 8.14 Status of hybridization:

"The previous discussion has brought out clearly the very wide context
in which hybridization seems to be relevant.
We must not, however, allow ourselves to believe that it represents any
real 'phenomenon', any more than resonance
between different structures such as the covalent and ionic ones of a
polar bond may be called a 'phenomenon'. But we
can at least say:
(a) that hybridization is the most efective way of preserving the
concept of a localized bond with perfect pairing of
orbitals on the two atoms of the bond;
(b) that it is a restricted form of resonance, so that a residual
resonance, due to alternative schemes of pairing, still
exists."

I don't see anything here to disagree with.

-Alan

====
Alan Shusterman
Department of Chemistry
Reed College
Portland, OR 97202
============
#7

            [email-masked]
        To:
            "Isaac B. Bersuker" <[email-masked]>
        CC:
            "Paulo E. Abreu" <[email-masked]>, [email-masked]

My $0.02:

Perturbations on a system that change electronic distribution can be
_rationalized_
post-facto in terms of changes in [presumed] hybridization....[ I can't
get away from
the fact that eigenvalues SCF canonical orbitals are the ones that best
correlate with
experiment, not the _attendant_ ones derived from a hybridization
process. They render
the Fock matrix non-diagonal]

John McKelvey
=============
#8

            "Isaac B. Bersuker" <[email-masked]>
        To:
            "Paulo E. Abreu" <[email-masked]>
        CC:
            [email-masked]

"Paulo E. Abreu" wrote:

>
> What do you mean by a legitimate physical phenomenom ?  Can you detect
it
> experimentally ?

Yes, you can: hybridization changes the charge distribution which is, in
principle
observable
I. B.

--
Dr. Isaac B. Bersuker
Institute for Theoretical Chemistry
Department of Chemistry & Biochemistry
The University of Texas at Austin
Austin, TX 78712, USA
Ph: (512) 471-4671
Fax: (512) 471-8696
Email: [email-masked]
==========
#9

            "Isaac B. Bersuker" <[email-masked]>
        To:
            "Paulo E. Abreu" <[email-masked]>
        CC:
            "Isaac B. Bersuker" <[email-masked]>,
[email-masked]

Hybridization is present in any theory (VB, MO, etc.), it reflects the
redistribution of
charge by bonding (the mixing of orbitals of different symmetry takes
place under the
influence of the bonding field). Sorry, I cannot continue this
discussion until you read
the basics that I cannot repeat here...
Regards
I. B.
========
#10

             maurice cafiero <[email-masked]>
 Organization:
             University Of Arizona Dept. Of Chemistry
          To:
             [email-masked]

> Hello:

    A proccess called Hybridization does not occur. No atom other than
hydrogen has
the familiar s,p,d, etc orbitals. A molecule is characterized by a
molecular
wave function that can be poorly approximated by a linear combination of
atomic orbitals.

It seems really silly that we are arguing over bad approximations....

mauricio cafiero
==============
#11
"Shobe, Dave" <[email-masked]>

In my opinion, orbitals, hybridization, etc. are auxiliary constructs to

help us think about what is really a complicated wavefunction (itself a
mathematical construction to explain the behavior of waves/particles).
As
for which of these are "real", that discussion goes back to Plato at
least...

--David Shobe
Süd-Chemie Inc.
phone (502) 634-7409
fax     (502) 634-7724
email  [email-masked]

Any opinions herein are not necessarily representative of Süd-Chemie.
============
#12

 maurice cafiero <[email-masked]>
 Organization:
   University Of Arizona Dept. Of Chemistry
   To:
    [email-masked]

True enough, I may have been a bit rash...
But I was trying to address the original question, that
hybridization is not `physical, ' but theoretical
and mathematical.

As far as science goes, quantum mechanics is the model
most removed from anything `physical.'

mauricio cafiero
====================
#13

            Jim Kress <[email-masked]>
        To:
            [email-masked]

Quantum Mechanics is a mathematical methodology which best (at this
time) reproduces existing knowledge.  As such it is an approach
that we can use to understand our physical universe.  While it describes
what we perceive as reality, it is no more "real" than the theory
of phlogiston nor are its constructs (i.e orbitals - hybridized or not)
any more real than the billiard ball representation of atomic structure.

These are all artificial constructs that help us humans find a frame of
reference in which we can understand the reality in which we are
immersed.  They have no 'reality' beyond that we assign them to enhance
our understanding ...

Jim Kress

     ----- Original Message -----
     From: maurice cafiero
     To: [email-masked]
     Sent: Monday, September 11, 2000 5:13 PM
     Subject: CCL:HYBRIDIZATION NOT "REAL";V. FOCK

     True enough, I may have been a bit rash...
     But I was trying to address the original question, that
     hybridization is not `physical, ' but theoretical
     and mathematical.

     As far as science goes, quantum mechanics is the model
     most removed from anything `physical.'

     mauricio cafiero
====================
#14

      Eric Scerri <[email-masked]>
     To:
         [email-masked]

Pauling's views on hybridization and resonance have been much
debated.  Among other things his writings were the cause of the "ban"
on quantum mechanics which occurred in the 1950's in the former
Soviet Union.

Articles dealing with the reality or otherwise of Pauling's concepts
include,

Vermeeren, 1986, Controversies and Existence Claims in Chemistry, The
Theory of Resonance,
Synthese, 69, 273-90.

Pauling himself debated the reality of orbitals, and hybridization
with Ogilvie among others in the pages of Journal of Chemical
Education.

see Ogilvie, J, 1990, The Nature of the Chemical Bond-1990, Journal
of Chemical Education, 67, 280-90.  and follow up letters to the
editor  and a full article reponse by pauling.

Pauling, L. 1992, The nature of the Chemical Bond- 1992, Journal of
Chemical Education,
65, 519-21.

>  -----
>  By the way, does anyone know where Vladimer Fock got his Ph.D., and
>  where and when he died? I think he worked at the University of St.
>  Petersburg/Leningrad.
>

A recent article on the life and work of Fock appeared in Chemical
Intelligencer.
==================

#15
From:
 Eric Scerri <[email-masked]>
 To:
 [email-masked]

[Quantum Mechanics is a mathematical methodology which best (at this
time) reproduces existing knowledge. As such it is an approach that
 we can use to understand our physical universe. While it describes what
we perceive as reality, it is no more "real" than the theory of
    phlogiston nor are its constructs (i.e orbitals - hybridized or not)
any more real than the billiard ball representation of atomic structure.
--Jim Kress]

While I agree with the jist of this statement I wonder whether you might
be overstating the case?

Phlogiston theory was long ago refuted and is therefore not even
candidate for a true scientific entity.

Not all the entities discussed by quantum mechanics have the same status
surely?
Most people would want to say that electrons and protons are real
entities.

Your statement could be taken to mean that QM is a theory (epistemology)
and therefore may not give us direct access
to real entities (ontology).

But there is a problem that we can only get at the micoworld via quantum
mechanics so the neat distinction between
the world and our description of the world is blurred.

I wonder whether you also intended the first sentence literally, namely
that QM makes no true predictions? If so I think
this is debateable although it is probably true that there are few
genuine predictions, in the temporal sense, made by
QM.

To get back to the main issue we can distinguish between atomic orbitals
(non real) and electron density (real). Each
case can be dealt with separately whereas a general statement about
quantum mechnics as a whole, as above, would
suggest that all entities are lacking physical reality which to repeat
is going too far.

eric scerri

   [ These are all artificial constructs that help us humans find a
frame of reference in which we can understand the reality in which we
are
    immersed. They have no 'reality' beyond that we assign them to
enhance our understanding ...

Jim Kress ]
===================
#16
            Keith Refson <[email-masked]>
        To:
            [email-masked]

The question of the "reality" of hybrid orbitals seems moot since
even unhybridized orbitals are not physical observables.

I would like to raise what seems to me to be a more pertinent question
-- that of the usefulness of that concept.  I ran up against this
recently in
teaching a course in quantum mechanics of mineral structures and
bonding, and found myself questioning whether hybridization has any
explanatory power at all.  Here's my reasoning.

"Hybridization" is not derivable from quantum mechanics.  An sp2 or
sp3 orbital is not an eigenstate of any Hamiltonian of which I am
aware. One can certainly construct linear combinations of eigenstates
in quantum mechanics, and a linear combination of eigenstates is also
an eigenstate if its components are degenerate.  But of course an s
and a p state are non-degenerate in an isolated atom, so the hybrid is
meaningless in the framework of quantum-mechanics.

What QM *does* imply is that if we have a Hamiltonian with, for
example C3 or Td symmetry then the eigenstates *will* be
symmetry-adapted functions. These can be expressed as linear
combinations of spherical harmonics which might look somewhat like
(but are not) the sp2 and sp3 hybrid orbitals respectively.

I can not find any sound argument in the textbooks to identify in
what circumstances an sp2 hybrid might be "stable" over sp3 or vice
versa or over unhybridized orbitals.  The undergraduate physical
chemistry texts I have read simply assert without evidence that
hybridization can occur. ( Interestingly enough the small sample of
"real" quantum mechanics books on my shelf do not mention
hybridization at all!)

The semi - implicit assumption in elementary discussions of
hybridization is that the character of hybridization depends on the
number of atoms to which bonds are being formed.

To paraphrase the argument:

   An sp3 hybrid orbital has something of the character of
   an eigenstate of a Hamiltonian with Td symmetry.  (I can not come
   up with a better justification for its existence than this.  Anyone
   else?).   Elements such as C and Si which form sp3 bonds tend to
   prefer tetrahedral co-ordination environments in molecules and
   solids.

Electron-counting and valence arguments won't do either, as they
can not explain why C can bond as either graphite or diamond.

I can not see how to teach these concepts without resorting to
unsubstantiated assertion or circular reasoning.  I would really like
to give my students some idea about the bonding "preferences" of Si
and C which may be non-rigorous but can not be simply wrong.

There is no way I am aware of to discuss energetics other than with
full MO theory -- but the fact that a full HF or DFT calculation
predicts this does not really count as an "explanation" either. I would
appreciate any suggestions, especially those which can be treated at
an elementary level.

Keith Refson

This message is copyright Keith Refson (2000), and is distributed with
my permission by the Computational Chemistry email list (CCL) to its
subscribers.  Permission is also granted to reproduce this message on
the official archives of the CCL list and authorised mirrors thereof.
Any other reproduction is NOT authorised except where explicit
permission is obtained from me.

--
Dr Keith Refson,        "Paradigm is a word too often used by those who
would
Dept of Earth Sciences      like to have a new idea but cannot think of
one."
Parks Road,                  -- Mervyn King, Deputy Governor, Bank of
England
Oxford OX1 3PR, UK
Keith.Refson@                       Tel: 01865 272026
             earth.ox.ac.uk         Fax: 01865 272072

============
#17

         "Oscar N. Ventura" <[email-masked]>
     To:
         Eric Scerri <[email-masked]>
     CC:
         [email-masked]

On Mon, 11 Sep 2000, Eric Scerri wrote:

> To get back to the main issue we can distinguish between atomic
> orbitals (non real) and electron density (real).  Each case can be
> dealt with separately whereas a general statement about quantum
> mechnics as a whole, as above, would suggest that all entities are
> lacking physical reality which to repeat is going too far.

It seems to me there's a confussion here. Atomic orbitals (or molecular,

localized, canonical, Kohn-Sham, etc, etc....orbitals) are not less
"real"
than total, electron, differential, etc, etc....densities, provided we
could arrive to an agreement of what "real" means. Both orbitals
and densities can be obtained from the same theory and (to a point)
even offer similar descriptions of the microworld. Physics tells us
however that density is observable, i.e. that we can also construct some

experimental devices which measure this property. So we should be
talking
about observability (i.e. the ability of an observer to obtain
information
>from an object which can be the subject of measurement) and not reality

(which I would not dare to define, but seems to imply the object to
exist
independently of any observer and as such it is metaphysics, not
physics).

Total density (STM), differential density (X-ray), energies of single
electrons within a molecule (photoelectron spectroscopy) are observable.

Molecular or atomic orbitals, energies of the molecular orbitals, etc,
are
constructs of the theory and are not observable.

Just my $ 0.015.

Oscar

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

Prof.Dr.Oscar N. Ventura             Tel.+(5982)9248396 Fax
+(5982)9241906
MTC-Lab,Facultad de Quimica          MTC-Lab
http://bilbo.edu.uy/MTC-Lab.html
Universidad de la Republica
Gral.Flores 2124, C. C. 1157         All opinions are my own, and not
Montevideo 11800, URUGUAY            necessarily those of my employer

"Violence is the last refuge of the incompetent." --- Salvor Hardin
=======
#18
         Guido Raos <[email-masked]>
     To:
         [email-masked]

The discussion about the reality of hybrid orbitals reminds me a lot
of the one on the reality of molecular orbitals, which was on this
list a few weeks ago.

I believe we all agree that no orbitals can ever be observed. For
this reason, some would then go on to argue that orbitals (of any
kind: hybrids, canonical or localized MO's, natural orbitals,
Kohn-Sham orbitals...) should be banned from our discussion of
chemical phenomena. I do not share this view. The userfulness of the
orbital concept should be judged from its "effectiveness" and
"economy" in the explanation of particular phenomena. In
particular, the canonical Hartree-Fock orbitals and energies help the
discussion of photoelectron spectra and of a number of symmetry
selection rules. However, hydrid orbitals do a much better job when
it comes to chemical bonding.

Incidentally, what "chemical" insights have been gained from
"exact" quantum chemical calculations (full CI, diffusion Monte
Carlo, and the like)? Often, approximate methods are much better at
producing some understanding of particular chemical phenomenona.
If I remember correctly, Dirac (quoted by Feynman) once said: "I
understand an equation, if I don't have to solve it in order to know
what the solution is going to look like". I think this applies
also to our case: just replace "equation" with "molecular structure",
and "solve it" with "run a Gaussian job".

Coming back to the hydrids, I agree with Alan Shusterman that
Coulson's discussion provides an excellent starting point:

> "The previous discussion has brought out clearly the very wide
> context in which hybridization seems to be relevant. We must not,
> however, allow ourselves to believe that it represents any real
> 'phenomenon', any more than resonance between different structures
> such as the covalent and ionic ones of a polar bond may be called a
> 'phenomenon'. But we can at least say:
>
> (a) that hybridization is the most efective way of preserving the
> concept of a localized bond with perfect pairing of orbitals on the
> two atoms of the bond;
>
> (b) that it is a restricted form of resonance, so that a residual
> resonance, due to alternative schemes of pairing, still exists."

Since then, one major advance has come from the applications of the
GVB method of Goddard and the spin-coupled method of Gerratt, Cooper
and Raimondi. Building on the Coulson-Fisher wavefunction for H_2,
they both generalize and bridge the gap between MO and VB theories on
a "one electron per orbital" basis.  I shall not dwell on the
differences between these methods. In simple cases, such as CH_4,
they give very similar answers. The eight valence electrons turn out
to be localized. Four of them are found on the hydrogens in distorted
1s H orbitals. The other four orbitals are centered on the carbon,
and they are strikingly similar to Pauling's sp^3 hybrids.
The point is that the shape of the orbitals in not given "a priori"
(as in the classical VB treatment), but it is a _unique_ outcome of
the energy minimization. The N-electron spin-coupled wavefunction is
_not invariant_ under some unitary/linear transformation of the
orbitals (as the Hartree-Fock wavefunction: a principle exploited in
the construction of localized MO's).

Sorry for the lengthy message. Thanks for the attention.

Guido.

------------------------------------------------------
Dr. Guido Raos
Dipartimento di Chimica, Politecnico di Milano
Via L. Mancinelli 7, I-20131 Milano, Italy
E-mail: [email-masked], [email-masked]
phone: +39-02-2399-3051       fax: +39-02-2399-3080
===========
#19

         Steve Cabaniss <[email-masked]>
     To:
         elewars <[email-masked]>

Dear Prof. Lewars,

        My colleague Edwin Gould has debated the desirability of
teaching
hybridization with some of our younger organic chemists.  When I showed
him
your note, he replied

        "The earliest significant presentation of the hybridization
model (to my
knowledge) was by Pauling;  see The Nature of the Chemical Bond, section

III-14, ref. #1.  I think he regarded this as a mathematical tool to
accomodate a given pattern of electron density."

        This has certainly touched off some interesting notes.  I hope
you can
track down a more definitive answer (at least in terms of history, even
if
you do not resolve the ultimate nature of reality...)

        Regards,

                Steve Cabaniss

Steve Cabaniss
Professor of Environmental and Analytical Chemistry
Department of Chemistry
Kent State University
Kent, OH 44242          USA
(330) 672-3731  FAX 672-3816
===============
#20

            "Isaac B. Bersuker" <[email-masked]>
        To:
            Eric Scerri <[email-masked]>
        CC:
            [email-masked]

I agree with your statement.

I entered this discussion just to remind people that hybridization may
be
obtained from perturbation theory (mixing of s, p, d,... orbitals under
the
influence of the field of other atoms, bonding), and hence it is not
just an
arbitrary assumption (as it may appear at first sight). It leads to
quite
observable charge redistribution (s type admixture in different
situations can
be obtained also from spectroscopic data).

This simple remark lead some people to global philosophycal deduction
like
"orbitals are not real", "quantum mechanics is not real", etc. I believe
that
such statements were of some interest 50 to 60 years ago, not now. It is
well
known that our knowledge is relative (not absolute) truth, but as a
relative
truth quantum mechanics is excelent with its predictions which are
confirmed
by experimental data.

Isaac Bersuker

Eric Scerri wrote:

>      Quantum Mechanics is a mathematical methodology which best (at
this
> time) reproduces existing knowledge. As such it is an approach that we
can use
> to understand our physical universe. While it describes what we
perceive as
> reality, it is no more "real" than the theory of phlogiston nor are
its
> constructs (i.e orbitals - hybridized or not) any more real than the
billiard
> ball representation of atomic structure.
>
> While I agree with the jist of this statement I wonder whether you
might be
> overstating the case?
>
> Phlogiston theory was long ago refuted and is therefore not even
candidate for a true
scientific entity.
>
> Not all the entities discussed by quantum mechanics have the same
status surely?
> Most people would want to say that electrons and protons are real
entities.
>
> Your statement could be taken to mean that QM is a theory
(epistemology) and
> therefore may not give us direct access to real entities (ontology).
>
> But there is a problem that we can only get at the micoworld via
quantum
> mechanics so the neat distinction between the world and our
description of the
> world is blurred.
>
> I wonder whether you also intended the first sentence literally,
namely that
> QM makes no true predictions? If so I think this is debateable
although it is
> probably true that there are few genuine predictions, in the temporal
sense,
> made by QM.
>
> To get back to the main issue we can distinguish between atomic
orbitals
> (non real) and electron density (real). Each case can be dealt with
separately
> whereas a general statement about quantum mechnics as a whole, as
above, would
> suggest that all entities are lacking physical reality which to repeat
is
> going too far.
>
> eric scerri
>
>      These are all artificial constructs
> that help us humans find a frame of reference in which we can
understand the
> reality in which we are immersed. They have no 'reality' beyond that
we assign
> them to enhance our understanding ...
>
> Jim Kress
>
>
-------------------------------------------------
> Dr. Eric Scerri,
> Department of Chemistry & Biochemistry,
> Charles E. Young Drive,
> UCLA,
> Los Angeles, CA 90095
> USA
>
> E-mail: [email-masked]
>
> Editor of "Foundations of Chemistry"
> http://www.wkap.nl/journalhome.htm/1386-4238
>
> Also see,
> International Society for the Philosophy of Chemistry
> http://www.georgetown.edu/earleyj/ISPC.html

--
Dr. Isaac B. Bersuker
Institute for Theoretical Chemistry
Department of Chemistry & Biochemistry
The University of Texas at Austin
Austin, TX 78712, USA
Ph: (512) 471-4671
Fax: (512) 471-8696
Email: [email-masked]
======================================
#21
Eric Scerri <[email-masked]>

Guido Raos writes,

>The discussion about the reality of hybrid orbitals reminds me a lot
>of the one on the reality of molecular orbitals, which was on this
>list a few weeks ago.
>
>I believe we all agree that no orbitals can ever be observed. For
>this reason, some would then go on to argue that orbitals (of any
>kind: hybrids, canonical or localized MO's, natural orbitals,
>Kohn-Sham orbitals...) should be banned from our discussion of
>chemical phenomena. I do not share this view. The userfulness of the
>orbital concept should be judged from its "effectiveness" and
>"economy" in the explanation of particular phenomena. In
>particular, the canonical Hartree-Fock orbitals and energies help the
>discussion of photoelectron spectra and of a number of symmetry
>selection rules. However, hydrid orbitals do a much better job when
>it comes to chemical bonding.

etc.

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

I am surprised to see this argument even needs to be presented to
this forum given the prevalence of all kinds of very successful
orbital methods in computational chemistry.

Of course orbitals are supremely useful even though they are not
'real'.  Did anyone seriously propose banning orbitals from
discussion of chemical phenomena just on the grounds that they are
not real?  I should add that I am a relative newcomer to this list. *

Does that someone not accept that the use of the square root of minus
one is also very useful in many branches of mathematical physics,
chemistry and engineering regardless of the lack of 'reality' of the
square root of minus one as opposed to -1?

I hope this will not lead to a discussion of whether numbers are
'real' although this might also prove interesting.  The question has
a long tradition in philosophy and philosophy of mathematics and
many believe that numbers ARE real objects although not observable of
course.

This leads me to the earlier comment, by Oscar Ventura, who suggested
that the whole discussion on orbitals etc. could be simplified by
just taking "real" to mean observable.  Such a 'Marxist' approach has
some appeal but I believe is far too crude for the kinds of issues
under discussion even without drifting off into philosophy of
mathematics.

For example quarks are considered to be 'real' particles in
elementary particle physics but the theory itself now ensures that
they cannot be directly observed.  This may be a perverse aspect of
modern particle theory but it cannot be swept aside too easily.

eric scerri

* I joined CCL after A. Hocquet mentioned my article "Have Orbitals
Really Been Observed?'  which is due to appear in Journal of Chemical
Education.
============
#22

         "Shobe, Dave" <[email-masked]>
     To:
         [email-masked]

I for one have been enjoying the philosophical discussion.

Perhaps more useful(?) questions to discuss are:
1. Is hybridization a well-defined concept within quantum theory?  (I
suspect this is what the original posting "really" meant by "real")?
2. Is hybridization a useful concept in rationalizing or thinking about
wave
functions, or about the properties of molecules?

--David Shobe
Süd-Chemie Inc.
phone (502) 634-7409
fax     (502) 634-7724
email  [email-masked]

Any opinions herein are not necessarily representative of Süd-Chemie.

===============
#23

   Jens Spanget-Larsen <[email-masked]>

Oscar N. Ventura:

> Total density (STM), differential density (X-ray), energies of single
> electrons within a molecule (photoelectron spectroscopy) are
observable.
> Molecular or atomic orbitals, energies of the molecular orbitals, etc,
are
> constructs of the theory and are not observable.

Dear Oscar,

I object to the statement that "energies of single electrons within a
molecule
(photoelectron spectroscopy) are observable".  In photoelectron
spectroscopy
you observe differences in energy between the ground state of the
neutral
molecule and the different electronic states of the molecular radical
cation.
That is, you observe differences in energy between many-electronic
states, not
single electron energies.  If you assume the validity of Koopmans'
approximation (briefly: adopt the MO model and assume the same set of MO

functions for the neutral molecule and the radical cation), the
ionization
energies of a closed-shell molecule can be set equal the negative of the

energies of the occupied canonical MOs.  This result is usually referred
to as
"Koopmans' theorem", but it is only valid within the above-mentioned
approximations.  The errors involved in these approximations tend to
cancel
each other out and "Koopmans' theorem" is thus very useful in the
interpretation of photoelectron spectra.  But the underlying assumptions

frequently break down, leading to inapplicability of the approximate
one-electron picture of ionization.

Yours, Jens >--<

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
JENS SPANGET-LARSEN         Phone:  +45 4674 2000  (RUC)
Department of Chemistry             +45 4674 2710  (direct)
Roskilde University (RUC)   Fax:    +45 4674 3011
P.O.Box 260                 E-Mail: [email-masked]
DK-4000 Roskilde, Denmark   http://www.rub.ruc.dk/dis/chem/psos
===========
#24

  "Oscar N. Ventura" <[email-masked]>

Dear Jens:

Of course, you are right. I was trying to make another point and
simplified too much. Actually your argument reinforces what I wanted to
point out with my previous (so calld "marxist" :-) argument.

Regards. Oscar

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

Prof.Dr.Oscar N. Ventura             Tel.+(5982)9248396 Fax
+(5982)9241906
MTC-Lab,Facultad de Quimica          MTC-Lab
http://bilbo.edu.uy/MTC-Lab.html
Universidad de la Republica
Gral.Flores 2124, C. C. 1157         All opinions are my own, and not
Montevideo 11800, URUGUAY            necessarily those of my employer

"Violence is the last refuge of the incompetent." --- Salvor Hardin
---------------------------------------------------------------------
=============
#25

    Eric Scerri <[email-masked]>

THe original question which started this thread was partly about
Pauling's
OWN views about hybridization and orbitals.

I think Pauling was surprisingly naive philosophically when it came to
interpretation of what he was doing.  I base this on the following
personal
anecdote.  When the debate with Ogilvie was taking place in J. Chem. Ed.

the same issue that carried the Pauling response also included three
other
reponses including a brief piece by myself.  I used this as an excuse to

get in touch with Pauling to try to explore his view further.  In his
reply
he said almost verbatim,

"Orbitals clearly exist since Mulliken and I have been writing about
them
for the past 60 years"

I could not resist replying that storytellers have also been writing
about
unicorns for many years.

Dr. Eric R. Scerri

Department of Chemistry & Biochemistry,
UCLA,
607 Charles E. Young Drive East,
Los Angles,
CA 90095-1569,
USA

tel:  310 206 3708
--------------------------------------------------------------------------

Editor: Foundations of Chemistry,

An Interdisciplinary journal for Philosophical, Historical and
Educational
Aspects of Chemistry.

http://www.wkap.nl/journals/foch

See also International Society for the Philosophy of Chemistry (ISPC)
http://www.georgetown.edu/earleyj/ISPC.html
=============
#26

         From: Eric Scerri <[email-masked]>
          To:  elewars <[email-masked]>

>Hello,
>
>that was an interesting anecdote. Most of us don't worry too much about
the
>philosophical implications of what we do.

As you might guess I think this is a serious omission.

>I am coming around to the view (and I may repeat this in my summary
>to CCL) that
>the question is really:  _In what sense_ do orbitals exist?  After
>all, in what
>sense do, for example, pure numbers exist?

I look forward to seeing the summary and perhaps further discussion
on these topics.
eric scerri

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

#27
        Wai-To Chan <[email-masked]>
         To:  [email-masked]

  I don't see how Pauling's statement, when considered in the context
of his J Chem Ed article, can be viewed as naive philosophicaly or
scientificaly.

Consider what you think how Gilbert Lewis should respond when asked
if electron-pairs exist.
Granted that Lewis could be excused for not knowing quantum
mechanics which preclude spatial localization of two electrons
he must have known the coulomb's law for repulsion between
electrons. Yet we have been using Lewis model to explain much of
descriptive chemistry to this day. We also use aufbau principle
to rationalize the periodic table when we know that these
K, L, M and N shells and S, P, D and F wavefunctions
don't exist; or at least they don't exist for all elements
beyond hydrogen.  But then our concept of valence shells/orbitals
based on aufbau principle is very much in line with the
chemical periodicity of elements. The Lewis electron-pair model
has a validity that seem to transcend our understanding
of QM. Hence, in spite of its not being a physical entity
we continue to apply it to interpretation of chemical reactivity
in such a manner as to imply its 'existence'.
In light of this I think it is not unreasonable to
say that electron-pairs/orbitals exist.

Wai-To Chan

Eric Scerri ([email-masked]) wrote:
<<<<<<<<<<<<<<<<<<<<<<<<<
THe original question which started this thread was partly about
Pauling's
OWN views about hybridization and orbitals.

I think Pauling was surprisingly naive philosophical ...etc.
==================
#28

         Victor Lua~na <[email-masked]>
     To:  [email-masked], [email-masked]
     CC: [email-masked]

] Eric Scerri ([email-masked]) wrote:
]> I think Pauling was surprisingly naive philosophically when it came
to
]> interpretation of what he was doing.  I base this on the following
personal
]> anecdote.  When the debate with Ogilvie was taking place in J. Chem.
Ed.
]> the same issue that carried the Pauling response also included three
other
]> reponses including a brief piece by myself.  I used this as an excuse
to
]> get in touch with Pauling to try to explore his view further.  In his
reply
]> he said almost verbatim,
]> "Orbitals clearly exist since Mulliken and I have been writing about
them
]> for the past 60 years"
]
]     I don't see how Pauling's statement, when considered in the
context
] of his J Chem Ed article, can be viewed as naive philosophicaly or
] scientificaly.

Certainly, it look more like the annoyed answer of someone not very
willing to discuss the concept at that time or in that circumstances.
Pauling's books and articles, usually written with great care and in a
very pedagogical way, emphasize the usefulness of sound but simple
concepts able to explain and predict very general trends. I don't think
Pauling was interested in the 'ultimate truth' behind chemistry,
whatever such a thing may be. Implying naivety from such an answer of
a genius that made ground-breaking contributions to structural
chemistry,
crystallography or biochemistry is, perhaps, a too strong statement.

[re W-T Chan, #27]:
] Consider what you think how Gilbert Lewis should respond when asked
] if electron-pairs exist.
] Granted that Lewis could be excused for not knowing quantum
] mechanics which preclude spatial localization of two electrons
] he must have known the coulomb's law for repulsion between
] electrons. Yet we have been using Lewis model to explain much of
] descriptive chemistry to this day. We also use aufbau principle
] to rationalize the periodic table when we know that these
] K, L, M and N shells and S, P, D and F wavefunctions
] don't exist; or at least they don't exist for all elements
] beyond hydrogen.  But then our concept of valence shells/orbitals
] based on aufbau principle is very much in line with the
] chemical periodicity of elements. The Lewis electron-pair model
] has a validity that seem to transcend our understanding
] of QM. Hence, in spite of its not being a physical entity
] we continue to apply it to interpretation of chemical reactivity
] in such a manner as to imply its 'existence'.
] In light of this I think it is not unreasonable to
] say that electron-pairs/orbitals exist.

Lewis neglected, at first, that Coulomb's law could be at work in
chemistry, such was his 'faith' in the explanatory power of pairs and
octaves. The interesting thing is that Lewis's concept of electron
pairs is essentially recovered within the framework of Quantum
Mechanically rigorous Atoms in Molecules theory, even though in a form
which departs from the naive orbital ideas propagated by most
General Chemistry manuals. The same can be said of the shell structure
of atoms. Both can be determined from the rigorous analysis of the
experimental or theoretical electron densities. Pauli's principle and
the fermionic nature of electrons provide the origin of Lewis's pairing.

Two recent references on this subject can help any interested reader:

@Article{         BDR-FAB99,
  author        = { X. Fradera and M. A. Austen and R. F. W. Bader },
  title         = { The Lewis Model and Beyond },
  journal       = { J. Phys. Chem. A },
  year          = { 1999 },
  volume        = { 103 },
  pages         = { 304--314 },
}

@Article{ BDR-BH99,
  author        = { R. F. W. Bader and G. L. Heard },
  title         = { The mapping of the conditional pair density onto the

        electron density },
  journal       = { J. Chem. Phys. },
  year          = { 1999 },
  volume        = { 111 },
  pages         = { 8789--8798 },
}

                 Regards,
                          Victor Lua~na
                          [email-masked]

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