Why it is sometimes difficult to determine the accurate position of a hydrogen atom by the semiexperimental method: Structure of molecules containing the OH or the CH₃ group

Abstract

The semiexperimental (SE) technique, whereby equilibrium rotational constants are derived from experimental ground-state rotational constants and corrections based on an ab initio cubic force field, has the reputation to be one of the most accurate methods to determine an equilibrium structure ( reSE). However, in some cases, it cannot determine accurately the position of the hydrogen. To investigate the origins of this difficulty, the SE structures of several molecules containing either the OH or the CH3 group are determined and compared to their best ab initio counterparts. It appears that an important factor is the accuracy of the geometry used to calculate the force field, in particular when the least-squares system is not well conditioned. In this case, the mixed regression method is often an easy way to circumvent this difficulty.

Axial–equatorial isomerism and semiexperimental equilibrium structures of fluorocyclohexane

“…Furthermore, the rotational constants of the deuterated species have sometimes large systematic errors because of the large mass increase. 7 For these reasons, it is important to have reliable predicates for these torsional angles.…”

“…Moreover, it was observed that, in some cases, the rotational constants of deuterated species do not improve the fits. 7 Furthermore, the system of normal equations of the least-squares method may easily become ill-conditioned (i.e. sensitive to small errors of the input data), in particular for atoms close to the center of mass (in other words, their isotopic substitution does not bring new information).…”

Interplay of experiment and theory: high resolution infrared spectrum and accurate equilibrium structure of BF2OH

“…For molecules with an OH group, the situation is more complicated. 17 It was possible to obtain accurate SE structures for nitrous acid (syn and anti conformers), 18,19 formic acid (syn and anti conformers) 20 as well as glycolaldehyde. 17 On the other hand, for proline 21 and glycidol, 17 the SE rotational constants of the OD species were found to be incompatible with the best structure.…”

“…17 It was possible to obtain accurate SE structures for nitrous acid (syn and anti conformers), 18,19 formic acid (syn and anti conformers) 20 as well as glycolaldehyde. 17 On the other hand, for proline 21 and glycidol, 17 the SE rotational constants of the OD species were found to be incompatible with the best structure. Finally, although accurate SE structures could be determined for ethanol (anti conformer) 17 and nitric acid, 22 there was obviously a problem: the rotation-vibration interaction constants (a-constants) of the lowest fundamental vibration could not be reproduced accurately by the ab initio force field.…”

“…17 On the other hand, for proline 21 and glycidol, 17 the SE rotational constants of the OD species were found to be incompatible with the best structure. Finally, although accurate SE structures could be determined for ethanol (anti conformer) 17 and nitric acid, 22 there was obviously a problem: the rotation-vibration interaction constants (a-constants) of the lowest fundamental vibration could not be reproduced accurately by the ab initio force field. It might lead us to think that it is due to the large amplitude vibration.…”