Semiexperimental Equilibrium Structure of the Lower Energy Conformer of Glycidol by the Mixed Estimation Method Jean Demaison,* ,† Norman C. Craig,* ,‡ Andrew R. Conrad, § Michael J. Tubergen,* ,§ and Heinz Dieter Rudolph* ,∥ † Laboratoire de Physique des Lasers, Atomes et Molé cules, Universite ́ de Lille I, 59655 Villeneuve d’Ascq Cedex, France ‡ Department of Chemistry and Biochemistry, Oberlin College, Oberlin, Ohio 44074, United States § Kent State University, Kent, Ohio 44242, United States ∥ Department of Chemistry, University of Ulm, D-89069 Ulm, Germany ABSTRACT: Rotational constants were determined for 18 O-substituted isotopologues of the lower energy conformer of glycidol, which has an intramolecular inner hydrogen bond from the hydroxyl group to the oxirane ring oxygen. Rotational constants were previously determined for the 13 C and the OD species. These rotational constants have been corrected with the rovibrational constants calculated from an ab initio cubic force field. The derived semiexperimental equilibrium rotational constants have been supplemented by carefully chosen structural parameters, including those for hydrogen atoms, from medium level ab initio calculations. The combined data have been used in a weighted least-squares fit to determine an equilibrium structure for the glycidol H-bond inner conformer. This work shows that the mixed estimation method allows us to determine a complete and reliable equilibrium structure for large molecules, even when the rotational constants of a number of isotopologues are unavailable. 1. INTRODUCTION Glycidol, or 2-oxiranemethanol, C 3 H 6 O 2 , is a chiral and bifunctional compound with a great variety of uses. 1 The molecule has internal hydrogen bonding expressed in two different conformers. Determining an equilibrium structure for this asymmetric 11 atom molecule from rotational constants for only heavy atom substitution (plus OD) depends on new methods. The microwave spectrum of glycidol has been investigated several times. 2−4 Two conformers were confirmed. 3 The lowest energy conformer, called H-bond inner (see Figure 1), has an internal hydrogen bond formed between the oxirane ring oxygen atom and the hydroxyl group hydrogen atom from above. This conformer is measured to have 3.6 kJ mol −1 lower energy than the H-bond outer conformer (see Figure 1), 3 which has an internal hydrogen bond formed between the hydroxyl group hydrogen atom and the pseudo-π-electrons of the oxirane ring. Recently, the microwave spectrum of the glycidol−water complex was measured, and the rotational constants of the 13 C isotopologues of glycidol were determined as part of this study permitting the determination of a partial substitution (r s ) structure. 4 However, this structure for the glycidol H-bond inner is inaccurate because several atoms of this molecule have small Cartesian coordinates. 5 For a medium-sized molecule, such as glycidol, it is not possible to obtain a structure good to 0.001 Å by midlevel quantum chemical (QC) calculations. However, it is now possible to reduce the errors in structures by using the semiexperimental technique whereby equilibrium rotational constants are derived from experimental ground state rotational constants and rovibrational corrections derived from a quantum chemical cubic force field. 6 Nonetheless, the Kraitchman’s equations 7 used to determine the Cartesian coordinates of the substituted atoms are quite sensitive to the remaining errors. One way to, at least partially, obviate this difficulty is to use the least-squares technique, which smoothes the errors. However, the determined parameters will be affected by the uncertainty of any fixed parameters. It is still better to use the mixed estimation method where no parameters are fixed, but auxiliary information is added directly to the data matrix for the least- squares fit. 8,9 This auxiliary information, usually called predicate observations, consists in carefully chosen values for the internal coordinates of the unsubstituted atoms, together with their corresponding uncertainties. This approach is relatively easy because QC calculations at a medium level of theory permit determining the internal coordinates of many light atoms with a reasonable accuracy. These favorable predictions are partic- ularly true for the hydrogen atoms. Furthermore, it is advantageous, when possible, to also use predicate observations for the substituted atoms in order to offset the possible harmful Received: June 5, 2012 Revised: July 25, 2012 Published: August 15, 2012 Article pubs.acs.org/JPCA © 2012 American Chemical Society 9116 dx.doi.org/10.1021/jp305504x | J. Phys. Chem. A 2012, 116, 9116−9122