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

 Sent to CCL by: Kalju Kahn [kalju-]
 Dear Guosheng,
 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).  Also, the QM potential-based partial charge on this
 oxygen is more similar to alcohols than to carbonyls.  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.
 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.
 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)
 Hope this helps,
 > 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
 > __________________________________________________
 > Do You Yahoo!?
 > Tired of spam?  Yahoo! Mail has the best spam protection around
 Dr. Kalju Kahn
 Department of Chemistry and Biochemistry
 University of California, Santa Barbara