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

Abstract

2-Chloro-4-fluorotoluene was investigated using a combination of molecular jet Fourier transform microwave spectroscopy in the frequency range from 5 to 21 GHz and quantum chemistry. The molecule experiences an internal rotation of the methyl group, which causes a fine splitting of all rotational transitions into doublets with separation on the order of a few tens of kHz. In addition, hyperfine effects originating from the chlorine nuclear quadrupole moment coupling its spin to the end-over-end rotation of the molecule are observed. The torsional barrier was derived using both the rho and the combined-axis-method, giving a value of 462.5(41) cm -1 . Accurate rotational constants and quadrupole coupling constants were determined for two 35 Cl and 37 Cl isotopologues and compared with Bailey's semi-experimental quantum chemical predictions. The gas phase molecular structure was deduced from the experimental rotational constants supplemented with those calculated by quantum chemistry at various levels of theory. The values of the methyl torsional barrier and chlorine nuclear quadrupole coupling constants were compared with the theoretical predictions and with those of other chlorotoluene derivatives.

Large Amplitude Motions in 2,3-Dimethylfluorobenzene: Steric Effects Failing to Interpret Hindered Methyl Torsion

“…36 These terms are not necessary to fit separately the (00) species, which follows a semirigid rotor Hamiltonian model. The standard deviations obtained for the (00), ( 01), ( 01), (11), and (12) species are close to 4 kHz, confirming that all frequencies are correctly assigned and the measurement accuracy is about 4 kHz. The results of the five separate fits are summarized in Table 2; those of the XIAM global fit in Table 3.…”

The LAM of the Rings: Large Amplitude Motions in Aromatic Molecules Studied by Microwave Spectroscopy

“…Some examples are 2-butynoic acid [16], tetrolyl fluoride [123], 1-chloro-2-butyne [124], 3-pentyn-1-ol [125], and 4-hexyn-3-ol [126]. If the methyl group is located in close proximity of an atom or a group of atoms, then the barrier to methyl internal rotation increases to about 400-600 cm −1 due steric hindrance, as observed in o-methylanisole [127], 2-halogenotoluene [128], 2-fluoro-4-chlorotoluene [129], 2-chloro-4-fluorotoluene [130], 1,2-dimethylnapthalene [131], 3,4-dimethylfluorobenzene [79], or 3,4-dimethylbenzaldehyde [81]. Predicting whether steric effects occur seems evident, but sometimes the estimation of values for methyl torsional barriers fails because of electronic effects, as in the cases of the 2-isomers of methylthiazole (3) [98,99], methyloxazole (11) [107], and methylimidazole (21) [114] mentioned above.…”

“…The fluoro-substituted toluene derivatives form a large body with many previous investigations, for example the fluorotoluene family with its three isomers [161][162][163][164], the difluorotoluenes with six isomers [165][166][167][168][169], and trifluorotoluenes with two isomers [170] are reported in the literature. The number of investigations on chloro-substituted toluenes is smaller and limited to the three isomers ortho- [128], meta- [171], and para-chlorotoluene [172] and the two mixed halogen-substituted toluenes, 2-fluoro-4-chlorotoluene [129] and 2-chloro-4-fluorotoluene [130]. For the o-and p-isomers, studies on bromo-and iodotoluene also exist [128,172].…”

“…Barriers to methyl internal rotation in halogen-substituted toluene derivatives (in cm −1 ). (1) ofluorotoluene [161], (2) m-fluorotoluene [162,163], (3) p-fluorotoluene [164], (4) 2,3-difluorotoluene [165],(5) 2,4-difluorotoluene[166], (6) 2,5-difluorotoluene[167], (7) 2,6-difluorotoluene[168], (8) 3,5difluorotoluene[168], (9) 3,4-difluorotoluene[163,169], (10) 2,3,4-trifluorotoluene[170], (11) 2,4,5trifluorotoluene[170], (12) 2-fluoro-4-chlorotoluene[129], (13) 2-chloro-4-fluorotoluene[130], (14) o-X-toluene[128], (15) m-chlorotoluene[171], (16) p-X-toluene[172], X = Cl, Br, I.…”

Local vs global approaches to treat two equivalent methyl internal rotations and 14N nuclear quadrupole coupling of 2,5-dimethylpyrrole

“…While high barriers result in rather small torsional splittings in the order of a few tens of kHz to a few MHz as found for 4-and 5-methylthiazole, 23,24 or 2-chloro-4-fluorotoluene, 25 in the case of low barriers, the splittings can be up to several GHz, [26][27][28] making the spectral assignment difficult. If the molecule contains also a 14 N nucleus which causes a quadrupole hyperfine structure with splittings also from a few tens of kHz to 2 MHz in addition, the spectral analysis is even challenging for high barrier cases, since the hyperfine patterns overlap with the torsional splittings.…”