Conformational studies of benzyl alcohol and benzyl fluoride

Abstract

ABSTRACT:The conformations of several benzyl compounds, C 6 H 5 CH 2 X, have been characterized by rotational spectra and theoretical structure calculations. We have observed the microwave spectrum of benzyl alcohol and its OD isotopomer at high resolution in a pulsed-jet Fourier transform microwave spectrometer. The spectrum is consistent with an asymmetric stable conformation characterized by a COCOCOO dihedral angle of approximately 60°. The spectrum is strongly perturbed by tunneling interactions. The transitions are split into doublets consistent with tunneling interactions between two equivalent conformational minima. The observed splittings diminish upon deuterium substitution. These observations are similar to those previously reported by us for benzyl fluoride. However, the minimum energy conformation for benzyl fluoride has the COCOF plane orthogonal to the plane of the phenyl ring. This conformation for benzyl fluoride is consistent with a model that minimizes steric repulsion between the fluorine of the OCH 2 F group and the closest hydrogens located on the phenyl ring. For benzyl alcohol, previous studies have suggested a weak attraction between the electrons of the phenyl ring and the substituent OOH group as the explanation for the observed stable conformation. A theoretical analysis of the atomic charges in benzyl alcohol suggests another possible explanation of the observed structure. Atomic charges generated by fits to the electrostatic potential indicate a relatively strong dipole-dipole coupling between the OCOH group in the methylene side chain and the closest OCOH group in the phenyl ring. This results in a nearly planar orientation of the OCOH group in the methylene side chain with the phenyl ring, a conformation that yields the observed COCOCOO dihedral angle.

7‐Arylnorbornanes: Model Compounds for the Study of CH···π and OH···π Interactions

“…According to these studies, the most stable conformer is the gauche disposition, a result that is interpreted in terms of CH···π intramolecular hydrogen bonds. Furthermore, ab initio HF, [14,[16][17][18] MP2, [15][16][17][18][19][20] and DFT (B3LYP) [19] calculations have been used to investigate the conformers and interactions involved in benzyl alcohol. These studies suggest the existence of the gauche conformer as the most stable in benzyl alcohol.…”

“…[5] Therefore, the design of preorganised model compounds in which the interaction studied is favoured is highly desirable and, in some cases, remains a challenge. In this respect, benzylic derivatives such as benzyl methyl ether (1; Figure 1) [6] and benzyl alcohol (2) [7][8][9][10][11][12][13][14][15][16][17][18][19][20] have been used to study CH···π and OH···π interactions, respectively. Both spectroscopic data and theoretical calculations point to stabilisation of the gauche cis rotamers 1a and 2a in comparison to the trans rotamers 1b and 2b by a five-membered CH···π hydrogen bond in the case of 1 and a four-membered OH···π hydrogen bond in the case of 2 ( Figure 1).…”

“…However, the study of these interactions in model compounds 1 and 2 is somewhat distorted by competition with the planar conformations 1c and 2c, which are stabilised by CH···O interactions (Figure 1). [14,[16][17][18][19][20] In benzyl methyl ether 1, the energy difference between conformations 1a and 1c has been calculated to be only -1.03 kJ/mol. [6b] In the case of benzyl alcohol, the two characteristic stretching absorption bands in the IR spectrum (3636 and 3617 cm -1 ) have been traditionally interpreted as being the result of rotamers about the C α -O bond.…”

“…[8b,8c,9] Subsequent spectroscopic and theoretical studies support the presence of several rotamers, the most stable being the gauche disposition, with a C ortho -C ipso -C α -O torsional angle around 60°and the OH bond directed towards the aryl ring. [12][13][14][15][16][17][18][19][20] However, the role played by a OH···π interaction in the relative stability of rotamers remains controversial. Some of these studies provide no support for the existence of an intramolecular hydrogen bond [12] whereas ab initio calculations and spectroscopic data point to a very weak interaction, suggesting that the preferred gauche conformation cannot be fully explained by the presence of OH···π interactions.…”

“…Some of these studies provide no support for the existence of an intramolecular hydrogen bond [12] whereas ab initio calculations and spectroscopic data point to a very weak interaction, suggesting that the preferred gauche conformation cannot be fully explained by the presence of OH···π interactions. [15][16][17][18][19][20]…”