Theoretical and experimental structures of vinyl fluoride and vinyl alcohol

Abstract

A new theoretical r0 structure for vinyl fluoride has been derived using ab initio molecular orbital procedures. In addition, new experimental r0 and rs structures have been obtained through a reanalysis of existing experimental microwave data. Significant discrepancies are found between the new theoretical (125.4°) and experimental (127.6°) r0 estimates of the CCHu angle involving the hydrogen atom geminal to the substituted center. This is attributed to the relative insensitivity of the experimental structural fits to the precise value of this bond angle, an insensitivity that is associated with the proximity of the relevant atoms to inertial axes. Because of these difficulties, the best estimate of the experimental r0 structure for vinyl fluoride is obtained by taking the theoretical r0 value for the CCHu angle and determining the remaining parameters by a weighted-least-squares fit to the experimental data. Weighted-least-squares fits are found generally to provide more consistency in the derived structural parameters than unweighted procedures. It is found that fitting of isotopic differences in rotational constants (as opposed to the conventional fitting of rotational constants themselves) provides the structure for vinyl fluoride most similar to the theoretical r0 structure. Similar observations are found to hold for vinyl alcohol, for which new theoretical and experimental results are presented.

A comparison of semiempirical and ab initio transition states for HF elimination in unimolecular decompositions

“…level, can provide a more accurate estimate of the structure of this molecule than can microwave spectral results [42]. Similar calculations on vinyl bromide and vinyl chloride produce results in good agreement with experimentally derived structures (it is relatively more difficult to determine the vinyl fluoride structure experimentally because some atoms lie very close to its inertial axes) [43].…”

The equilibrium OH bond length

“…The equilibrium OH bond length 407 dismissed the experimental r s structure [122]. We recalculated the structure at the CCSD(T) level of theory.…”

Computational chemistry: A useful (sometimes mandatory) tool in mass spectrometry studies

“…Another important illustration of the reliability of MP2‐optimized geometries can be found in a paper that deals with the structure of vinyl alcohol (Smith & Radom, 1990), where it has been shown that the previous disagreement between the ab initio r 0 structure and the r s obtained from MW could be solved through a reanalysis of previously reported experimental data. Two years later, the same kind of weighted‐least‐square fit of the existing experimental microwave data resulted in a new r 0 structure for vinyl fluoride (Smith, Coffey, & Radom, 1992). It must be indicated, however, that in some specific cases the MP2 method leads to unreliable geometries.…”