Reactivity and rotational spectra: the old concept of substitution effects

Abstract

The internal rotation of methyl groups and nuclear quadrupole moments of the halogens Cl, Br, I in o-halotoluenes result in complex spectral fine and hyperfine structures in rotational spectra which can be related to qualitative concepts in chemistry.

Internal rotation and chlorine nuclear quadrupole coupling in 2-chloro-4-fluorotoluene explored by microwave spectroscopy and quantum chemistry

“…The χab NQCC could not be determined experimentally with sufficient accuracy in the case of 2-chloro-4fluorotoluene. A similar observation was made for the37 Cl isotopologue with respective cc values of -25.157 MHz, -53.889 MHz[17], and -47.913 MHz[18] for the fluorine-substituted and the two unsubstituted chlorotoluenes.…”

“…The cc NQCC of the chlorine nucleus of 2-chloro-4fluorotoluene can be compared directly with that of other molecules with Cs symmetry, because the principal c-axis is perpendicular to the molecular plane and collinear with one principal axis of the coupling tensor. The value cc = -39.432 MHz found for the35 Cl isotopologue of 2-chloro-4-fluorotoluene is much smaller than that found for 2-chlorotoluene (cc = -67.972 MHz)[17] and 3-chlorotoluene (cc = -60.325 MHz)[18]. This shows that not only the position of the methyl group but also the presence of a fluorine atom on the aromatic ring affects the nuclear quadrupole coupling of the chlorine nucleus, whereby the latter substituent has a much greater impact.…”

“…Correlations also explain the unusually large deviation between the experimental angle (i,a) and its predicted value, changing significantly when using different sets of parameters. Comparing the V3 potential barrier of 2-chloro-4-fluorotoluene (462 cm -1 with the data set reported in Table 2) to that of 2-chlorotoluene (469 cm -1 ) [17], it is interesting to note that while the fluorine substitution at the para-position significantly affects the NQCCs of the chlorine nucleus, its effect on the potential barrier is small. Investigations of other halogenated toluenes will increase the empirical data required to provide a global view of the internal rotation and quadrupole coupling effects in this important class of compounds.…”

“…The study was subsequently extended to the millimeter wave region [16]. Recently, a reanalysis included both the microwave and millimeter wave data, and a barrier to internal rotation of about 469 cm 1 has been reported [17].…”

“…This work aims at determining the potential barrier hindering the methyl torsion when a chlorine atom is in close proximity of the methyl top as well as the NQCCs of the chlorine nucleus. We expect that the torsional barrier is largely influenced by electronic or steric effects and therefore would be similar to that of o-chlorotoluene [13][14][15][16][17].…”