Six-fold-symmetry internal rotation in toluenes: the low barrier challenge of 2,6- and 3,5-difluorotoluene

Abstract

Pure six-fold symmetry (V 6 ) internal rotation poses significant challenges to experimental and theoretical determination, as the very low torsional barriers result in huge tunneling splittings difficult to identify and to model. Here we resolved the methyl group internal rotation dynamics of 2,6-and 3,5-difluorotoluene using a newly developed computer code especially adapted to V 6 problems. The jet-cooled rotational spectra of the title molecules in the 5-25 GHz region revealed internal rotation tunneling doublings of up to 3.6 GHz, which translated in methyl group potential barriers of V 6 = 0.14872(24) and 0.0856(10) kJ mol À1 , respectively, in the vibrational ground-state. Additional information on Stark effects and carbon isotopic species in natural abundance provided structural data and the electric dipole moments for both molecules.Ab initio calculations at the MP2 level do not reproduce the tiny torsional barriers, calling for experiments on other systems and additional theoretical models.

Microwave spectra, molecular geometries, and internal rotation of CH3 in N-methylimidazole⋯H2O and 2-methylimidazole⋯H2O Complexes

“…It is known that there is a negligible barrier to the internal rotation of CH 3 in toluene 25 while the addition of substituents to the benzene ring introduces an asymmetry into molecular orbitals that leads to an increased barrier. 26–29 The ( V 3 ) barrier to internal rotation of CH 3 has been determined for different structural isomers of methylthiazole, 30–32 methyloxazole 33 and methylimidazole. 9 The magnitude of the parameter depends on the asymmetry of the molecular orbitals with which the CH 3 group interacts.…”

Two Equivalent Internal Rotations in the Microwave Spectrum of 2,6‐Dimethylfluorobenzene

“…Only in 2,6-difluorotoluene, the potential is V 6 due to symmetry. [13] As mentioned, the barrier hindering a methyl rotation is quite sensitive to many structural and electronic effects. In a systematic investigation on a series of methyl alkyl ketones, Andresen et al proposed a link between the torsional barrier of the acetyl methyl group and the molecular structure at the other side of the carbonyl bond.…”

“…(1) 26DMFB (this work), (2) 2-fluorotoluene, [6] (3) 2,4,5-trifluorotoluene, [14] (4) 2,3,4trifluorotoluene, [14] (5) 2,3-difluorotoluene, [9] (6) 2,4-difluorotoluene, [10] (7) 2,5difluorotoluene, [11] and (8) 2,6-difluorotoluene. [13]…”

“…Especially for spectroscopy, the rotational spectrum of toluene has always attracted attention since its microwave spectrum was recorded and analyzed for the first time by Rudolph et al [1] Not only the structure of toluene was determined to great accuracy, several theoretical models and program codes have been also developed to reproduce the V 6 potential arising from the internal rotation of the C 3v methyl group attached to a phenyl frame with C 2v symmetry. [2][3][4][5] To gain insights into the substitution effect on this large amplitude motion (LAM) of toluene, studies on many fluorinated derivatives have been performed, such as the investigations on three isomers of fluorotoluene, [6][7][8] a systematic microwave investigation on the six isomers of difluorotoluene, [9][10][11][12][13] and the work on two isomers of trifluorotoluene. [14] All these studies have shown a variety of the potentials of the methyl torsion in both shape and height, which depend on the substituted position(s) of the fluorine atom(s).…”

The low-barrier methyl internal rotation in the rotational spectrum of 3-methylphenylacetylene

“…An example is seen in the change of the potential barrier of toluene for various levels of halogenation around the benzyl ring. In toluene, the V 6 barrier is reported as 4.837920599(11) cm −1 ,[2] and increases for p-chlorotoluene (4.872(14) cm −1 )[3], 3,5-difluorotoluene (7.156(84) cm −1 )[4] and 2,4-difluorotoluene (12.432(20) cm −1 ), [4] but decreases slightly for p-fluorotoluene (4.8298(64) cm −1 ) [5]. In a more recent study, an investigation of p-halotoluenes found a p-chlorotoluene V 6 barrier of 4.836(29) cm −1 , which was also found to be a good barrier to describe the tunneling motion in p-bromotoluene and p-iodotoluene.…”