CCL: sp2 or sp3 for the single-bonded ester oxygen atom

 Sent to CCL by: Guosheng Wu [wu_guosheng2002!^!]
 Dear Kalju,
 Thanks a lot for your nice and informatic response.
 It is obvious that you think sp3 hybridization is more appropriate to describe
 that oxygen atom in
 ester.  However, I have different understandings over some of your reasons.
 For CH3C(=O)OCH3, I name the carbonyl O as O1, and the other O(also in the
 topic) as O2.
 > One justification behind ester oxygen being sp3 is in bond lengths and
 > angles in esters.  The C(sp2)-O bond in esters is about 1.33-1.34 Ang.
 >  This is closer to the C-O(sp3) bond in alcohols and ethers
 > (1.41-1.42) than to the C=O(sp2) bond in carbonyl compounds
 > (1.20-1.21).
 It is not "fair" arrive the conclusion based on comparing the bond
 lengths of those two O atoms
 (the C=O and the C-O-C in ester), since O1 is double-bonded to a carbon, but O2
 is single-bonded
 to atoms.  Similar argument applies for the comparison for pyridine N and amide
 N, since both N
 atoms are sp2, but quite different situations.
 > Also, the QM potential-based partial charge on this
 > oxygen is more similar to alcohols than to carbonyls.
 You are right, but still it can not justify the hybridization.  I think O2 in
 ester is much less
 polar than the alcohol O, certainly not comparable.
 Also, point charge model might be one of the most misleading component in
 molecular modelling, no
 matter how it is calculated. So I think polarized charge models could make lots
 of improvements
 for modelling, although it takes much time for its development and in
 >Liquid simulations with the OPLS-AA force field suggest that Lennard-Jones
 > parameters of ether oxygen should be similar to ether and not to
 > carbonyl oxygen.  However, many force fields reconize the uniqueness
 > of this oxygen by using a special atom type for the ether and
 > carboxylic acid oxygens.
 Typing O2 as sp2 or O3 does not necessary mean that one has to use all of the
 parameters of
 carbonyl O or ether O. The L-J parameter does not have much to do with sp2 or
 sp3, which is more
 about electrostatic interaction.  Also, the simulations are dependent on many
 factors, so the
 conclusion from OPLS-AA has its limitations.  (I feel the last "ether"
 in your email above is a
 typo for "ester")  Certainly the O2 is very unique, quite different
 from carbonyl or ether O, so
 it's appropriate to name it specially, but it seems not clean what kinds of its
 properties are captured in the force fields.
 > It in not necessarily true that the coplanar structure of esters
 > arises only from p-pi conjugation.  The repulsion of negative charges
 > on the carbonyl oxygen and the two tetrahedrally-arranged electron
 > pairs on sp3 oxygen would also predict planar structures, with
 > Z-conformer (in which both e-pairs point away from the C=O oxygen)
 > much more favorable than the E conformation (e-pairs surrounding C=O
 > oxygen).  This is somewhat similar to glyoxal (HOC-CHO), which is
 > planar with a *long* C-C bond.  Also note that the classical Lewis
 > resonance structure would make the ester oxygen positively charged,
 > something that would not fit well with the electronegative nature of
 > the oxygen.
 I agree with you that steric effect and lone-pair interactions have a role in
 the structure of
 ester. (Lewis structure has lots limitations, so I do not even mention it.)
 > The ester (sp3) oxygen is known to be weak H-bond acceptor so your
 > finding of limited number of structures is OK.  I wonder if anybody
 > has done a gas phase QM study to see the difference between
 > tetrahedrally positioned H-bond acceptor and coplanar one?
 > P.S. Some details about ester force fields are hidden in our own
 > "Parameterization of OPLS-AA force field for the conformational
 > analysis of macrocyclic polyketides", in J. Comp. Chem. 23, 977 (2002)
 In order to get deep understanding of the electronic structure of ester, I think
 one has to use
 very high level of QM theories and do very careful analysis. Something like
 NBO(natural bond
 orbital) analysis might be helpful and more relevant, which was reminded by one
 of the responses I
 got from this list.  However I am limited to the resources, so I posted it out
 and hope someone
 would be interested in it.  Certainly it could be a good porject for the
 students if anyone is
 teaching QM or Comp. Chem.
 Nevertheless, I think it would be more reasonable to assign sp2 hybridization
 for O2.
 Electronically, O in ester is much like NH in amide, which is popular in protein
 structures. I
 can not believe anyone would say that the N atom in amide is sp3.
 Other p-pi conjugation examples can be found in PhOH(Ph=benzene), PhOCH3, furan,
 pyrrole, etc. The common feature for all of them is that O or N atom is
 connected to a pi system,
 either C=O or benzene, or whatever, so that one of the lone-pairs is more like
 pi electrons,
 becoming less polar, and not good for being a H-bond acceptor.  One special case
 is substituted
 anilines that are often away from coplanar structures because the steric effect
 of the substituted
 groups, while aniline itself is complete flat.
 > -------------------
 > > Sent to CCL by: Guosheng Wu [wu_guosheng2002]^[]
 > > Hi there,
 > >
 > > For the first O atom in ester like CH3OC(=O)CH3, what kind of hybrid
 > state we should assign?
 > >
 > > Certainly it's within the context of molecular mechanics, although I
 > tried a little QM study with
 > > electrostatic potential and electron density calculation, and did
 > not get a clue.  (Any QM expert
 > > can help me on it ?)
 > >
 > > I looked up the literature or the web(google), but all I can find
 > out for this problem is that it
 > > has been called sp3 at many occasions.  One paper (Kresimir Molcanov
 > et al, Acta Crys. B, 2004,
 > > B60, 424) and some force fields (CVFF, AMBER,...) explicitly
 > call/type it as sp3.
 > >
 > > However, that conflicts my chemistry intuition, since 2 electrons of
 > that O should form so-called
 > > p-pi conjugation with C=O, so it should be in sp2 hybrid state.
 > That also explains the coplanar
 > > structure of ester or simple organic acids.
 > >
 > > Like the paper mentioned above, I did some CSD search for that O as
 > H-bond acceptor, there are
 > > only ~200 hits (dependent on parameter such as resolution, bond
 > length etc), so it's certainly not
 > > much statistically meaningful, but still what I found out is that
 > most of the O..H-X
 > > (X=O or N) H-bonds are more or less in the same plane of the ester.
 > So shall we call it sp2
 > > oxygen in the future when we talk about molecular mechanics, and
 > apply this concept in any related
 > > study?
 > >
 > > I find this an interesting study, and hope to receive your insights,
 > especially those who work in
 > > QM areas.
 > >
 > > Thanks for your attention,
 > > -Guosheng
 > >
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