Molecular structure and conformation of methyl methacrylate determined by gas electron diffraction

The molecular structure of methyl methacrylate (MMA, H2C=C(CH3)-C(=O)-OCH3), a monomer of an acrylic resin, has been determined by gas-phase electron diffraction combined with vibrational spectroscopy and HF/6-31G** ab initio calculations. At room temperature (25 degrees C), the vapor of MMA exists as a mixture of s-trans and s-cis conformers and the mole fraction of the s-trans conformer is 0.64(17). The principal structure parameters of the s-trans form with the estimated limits of error (3 sigma) are: r(g)(C=C)=1.341(6) Angstrom; r(g)(C-C(=O))=1.494(2) Angstrom; r(g)(C-CH3)-r(g)(C-C(=O))=0.011 Angstrom (assumed); r(g)((O=)C-O)=1.348(5) Angstrom; r(g)(O-CH3)=1.433(3) Angstrom r(g)(C=O)=1.209(3) Angstrom; [r(g)(C-H)]=1.106(4) Angstrom; angle(alpha)C=C-C(=O)=121.2(7)degrees; angle(alpha)C=C-CH3=123.3(9)degrees; angle(alpha)C-C-O=113.3(8)degrees; angle(alpha)C-C=O=123.4(7)degrees; angle(alpha)C-O-C=116.8(9)degrees, where [] denotes an averaged value. The difference between the structures of MMA and methyl acrylate has been discussed in terms of steric repulsion and the electron releasing property of the methyl groups. (C) 1999 Elsevier Science B.V. All rights reserved.