Conformational Properties of 2-Fluoroanisole in the Gas Phase

Abstract

Molecular structure and conformational properties of 2-fluoroanisole have been studied by gas electron diffraction and quantum chemical methods (HF/6-31G*, MP2/6-31G*, and B3LYP/6-31G*). All methods predict the existence of two minima on the potential to internal rotation around the C(sp 2 )-O bond corresponding to planar and nonplanar forms, but they give rather different results on the geometry and the relative energy of the nonplanar conformation. The electron diffraction data were treated with two different procedures for obtaining the scattering intensities (scanner and densitometer) and two different procedures of structural analysis used in laboratories of Moscow and Tu ¨bingen. With all data sets and procedures used in analysis, geometrical parameters and conformational composition were found to be very close. The preferred conformation of 2-fluoroanisole is a planar anti form, 70 ( 12%, whereas the minor conformation is a nonplanar form with the CH 3 group rotated toward the F atom with φ(CdC-O-C) ) 57( 8)°. The structural parameters are compared with those obtained for related compounds. Stereochemical peculiarities in ortho derivatives of anisole, which result in the existence of a nonplanar conformer, are discussed in terms of a natural bond orbital (NBO) analysis.

Experimental and computational study on the molecular energetics of the three monofluoroanisole isomers

“…The methoxy group is a medium p electron donor, reaching its maximum when it is coplanar with the aromatic ring. In the case of the 2-fluoroanisole, the global effect of stabilization due to the methoxy group, adopting a perpendicular conformation yielding to less steric effects, would be lower than the effect of conjugation between the lone electron pairs of the oxygen atom and the p electron system of the benzene ring, in a planar conformation [20]. Taking into account the optimization calculations performed at the B3LYP/6-311++G(d,p) (tables S7-S9 of the Supplementary Information), the methoxy group was found to be coplanar with the aromatic benzene ring in the most stable conformations for all the three monofluoroanisole isomers, with the methoxy group in anti orientation with respect to fluorine atom for the 2-and 3-fluoroanisole.…”

“…Some researchers groups have investigated the conformal properties of anisole molecule, by electron diffraction [13], microwave spectroscopy [14,15], high resolution spectroscopy [16], fluorescence spectroscopy [17], and high-level ab initio calculations and DFT calculations [15][16][17][18][19], and have conclude that anisole exists only as a single conformer with planar heavy atom skeleton, being this sterically unfavourable structure stabilized by the electron delocalization between the oxygen lone pairs and the electron system of the ring. The molecular geometry of 2-fluoroanisole has been studied using gas-phase electron diffraction, low-temperature matrix isolated FT-IR spectroscopy and quantum chemical methods [20,21], and it was demonstrated that the preferred conformation is a planar conformer with anti orientation of the methyl group with respect to fluorine, u = 180°, while the minor conformer is non-planar with the CH 3 group rotated toward the fluorine atom by u % 60°. Lister et al [22][23][24] have done some studies using microwave spectroscopy aiming the understanding of the molecular conformation of 3-and 4-fluoroanisole, but the molecular conformation of the 3-and 4-fluoroanisoles, was only better understood more recently, with the work developed by Oberhammer et al [25,26], using gas-phase electron diffraction and quantum chemical methods.…”

Existence of a Preferred Orientation for the Methoxy Group on an Extended Aromatic System.

“…The preferred conformation of a methoxy (‐O‐CH 3 ) group attached to a phenyl ring in anisole is well known and established . In the absence of substituents in the ortho position, the oxygen‐carbon bond places itself in the plane of the aromatic ring.…”

Second-harmonic generation and the influence of flexoelectricity in the nematic phases of bent-core oxadiazoles

“…These two states are equivalent if the aromatic group is a benzene, [38] whereas in the presence of fluorosubstituents the conformation having the O-CH 2 bond on the opposite side of the fluorine atoms is found to be more stable (by about 3 kJ mol −1 at the B3LYP/6-31G** level and 7 kJ mol −1 at the B3LYP/6-31 + G* level), in agreement with calculations reported in the literature. [42,43] In MC sampling of conformers, the structures having pairs of atoms closer than a cut-off distance equal to 0.82 σ, where σ is the sum of their van der Waals radii, were discarded. Van der Waals radii equal to 0.185 nm (C), 0.15 nm (N and O), 0.135 nm (F) and 0.1 nm (H) were assumed.…”