Steric effects on two inequivalent methyl internal rotations of 3,4-dimethylfluorobenzene

Abstract

The microwave spectrum of 3,4-dimethylfluorobenzene was measured using a pulsed molecular jet Fourier transform microwave spectrometer operating in the frequency range from 2.0 to 26.5 GHz with the goal of quantifying the steric effects on the barriers to internal rotation of the two inequivalent methyl groups. Due to these torsional motions, splittings of all rotational transitions into quintets were observed and fitted with residuals close to measurement accuracy. The experimental work was supported by quantum chemical calculations, and the B3LYP-D3BJ/6-311++G(d,p) level of theory yielded accurate optimized geometry parameters to guide the assignment. The three-fold potential values of 456.19(13) cm −1 and 489.77(15) cm −1 for the methyl groups at the meta and para position, respectively, deduced from the experiments are compared with the predicted values and those of other toluene derivatives.

The two-top molecule 3-penten-2-one: Acetyl methyl torsion in α,β-unsaturated ketones

“…For the sp conformer, the combination of the B3LYP-D3BJ method, the 6-311 basis set, and the (d,p) polarization function as well as the combination of the wB97X-D method and the cc-pVDZ basis set also yield the best agreement, however, the use of diffuse function (+, ++, or aug) is necessary.Without Becke-Johnson damping, the B3LYP-D3 method performs well with the 6-311 basis set and the (df,pd) polarization function. Overall, regarding also the results for other molecules with a conjugated double bond system, such as 2,6-[25] and 3,4-dimethylfluorobenzene[26], or 2-acetylfuran[35], the B3LYP-D3BJ/6-311++G(d,p) level of theory yields Xe values of equilibrium rotational constants (with X = A, B, C), which are in almost exact agreement with the experimental X0 values. It is obvious that from a theoretical perspective, the Xe constants should not be compared with the X0 constants.…”

“…Due to a lack of reliable starting values for the barriers, multiple cycles of trial and error were necessary to assign the torsional species. We note that there are other techniques to assign spectra, such as using combination difference loops [24], or fitting the torsional species separately [11,[25][26][27]. But, for 3-penten-2-one, it worked best to switch between different XIAM fits floating various sets of parameters.…”

Microwave Spectrum of Two‐Top Molecule: 2‐Acetyl‐3‐Methylthiophene

“…[39] [a] C. Dindić Institute of Physical Chemistry RWTH Aachen University [39] LAM containing molecules with conjugated π-double bonds are of special interest as electronic effects can be transferred across a longer range, often enabling interactions between two internal rotors. Examples are investigations on a number of benzene derivatives featuring two methyl internal rotations such as three isomers of dimethylanisole, [40][41][42] three isomers of dimethylbenzaldehyde, [43] o-and m-xylene, [44,45] two isomers of dimethylfluorobenzene, [46,47] 4-methylacetophenone, [48] or some five-membered aromatic rings like the 2,5-isomers of dimethylthiophene, [49] dimethylfuran, [50] and dimethylpyrrole [51] or 2acetyl-5-methylfuran [52] and 4,5-dimethylthiazole. [53] In these studies, it was clearly proven that steric hindrance arising from a neighboring substituent increases the torsional barrier of a methyl group attached to the aromatic ring, but also that electrostatic effects can be transferred across the π-conjugated system and influence the methyl torsion.…”

Neighborhood matters: Steric effects on methyl internal rotation and chlorine nuclear quadrupole coupling in 2-fluoro-4-chlorotoluene

“…Though the predicted rotational constants are sufficiently reliable to guide the assignment of the microwave spectrum, the results are generally worse than in the case of the isomer 2-chloro-4-fluorotoluene where many levels of theory perform well [16]. For 2-fluoro-4-chlorotoluene, MP2/6-31G(d,p) has again proven itself to serve as a cost-efficient level yielding Be rotational constants calculated accidentally close to the experimental B0 constants due to error compensations, as it did for a number of aromatic ring containing molecules like coumarin [41], quinoline and isoquinoline [42], dimethylfluorobenzenes [11,12], phenetole [43], and 2-ethylfuran [44].…”

“…Among them, toluene and its fluoro-substituted derivatives (Figure 1) form a large body with more than 20 molecules reported in the literature, e.g. fluorotoluenes (molecules 5 and 6 in Figure 1) [2][3][4], difluorotoluenes (7) [5][6][7][8][9], trifluorotoluenes (8) [10], and dimethylfluorobenzenes (9) [11,12]. In contrast, the number of investigations on chloro-substituted toluenes is rather small and limited to the three isomers ortho- [13], meta- [14], and para-chlorotoluene [15].…”