Axial–equatorial isomerism and semiexperimental equilibrium structures of fluorocyclohexane

Abstract

An experimental-computational methodology combining rotational data, high-level ab initio calculations and predicate least-squares fitting is applied to the axial-equatorial isomerism and semiexperimental equilibrium structure determination of fluorocyclohexane. New supersonic-jet microwave measurements of the rotational spectra of the two molecular conformations, together with all C isotopologues of both isomeric forms are reported. Equilibrium rotational constants are obtained from the ground-state rotational constants corrected for vibration-rotation interactions and electronic contributions. Equilibrium structures were determined by the mixed estimation (ME) method. Different computational methods were tested for the evaluation of predicate values of the structural parameters, and a computationally effective procedure for estimating reliable dihedral angles is proposed. Structural parameters were fitted concurrently to predicate parameters and moments of inertia, affected with appropriate uncertainties. The new structures of the title compound are regarded as accurate to 0.001 Å and 0.2°, illustrating the advantages of this methodology. Structural comparisons are offered with the cyclohexane prototype, revealing subtle substituent effects. For comparison purposes the equilibrium structures for the two fluorocyclohexane isomers and cyclohexanone are computed from high-level ab initio theory with inclusion of adjustments for basis set dependence and correlation of the core electrons.

The S−S Bridge: A Mixed Experimental‐Computational Estimation of the Equilibrium Structure of Diphenyl Disulfide

“…Compared to the equilibrium value the MP2/V(T+d)Z value is too small (82.60°), whereas at the B3LYP/6‐311+G(3df,2pd) level, it is too large (89.79°). This behavior is somehow expected as the equilibrium value cannot be estimated from the empirical correction devised for the first‐row atoms . The result from the B2PLYPD3/6‐311+G(3df,2pd) level is disappointing (84.45°).…”

“…In conclusion, this method, although it seems to be straightforward, should be used with care Linear regression approach: This method can be applied to derive structural parameters from a series of previously evaluated data, as the CO bond lengths from the MP2/cc‐pVQZ values or the CF bond lengths from the MP2/6‐311+G(3df,2pd) results . Alternatively, the systematic errors on the torsional angles, τ(XCCY), computed at the MP2/cc‐pVTZ level may be estimated from the difference τ[MP2/cc‐pVTZ]−τ[B3LYP/6‐311+G(3df,2pd)] …”

“…However, as the number of accurate data for the C−S and S−S bond lengths is small, the accuracy might be worse, as will be checked below. The case of the dihedral angles is more difficult because it is established that the MP2 method fails to deliver accurate dihedral angles, the error being sometimes as large as several degrees . There is however one way to solve this problem because the rotational constants are sensitive to the torsional angles.…”

Computational study of inversion‐topomerization pathways in 1,3‐dimethylcyclohexane and 1,4‐dimethylcyclohexane: Ab initio conformational analysis

“…As the study of cycloalkanes, focusing on their molecular preferences, conformational behaviors, and relative energies, has long been a topic of interest for chemistry and some interrelated application, they have been investigated by theoretical calculations and experimental measurement [1‐11]. The adopted experimental methods briefly include X‐ray diffraction, Raman, infrared, and NMR spectrum detection, [9, 11‐14] and theoretical approaches contain ab initio, molecular force field, and dynamical density functional theory [7, 8, 10, 15]. These previous studies usually cover two mainly aspects: (1) conformational determination of the simple neutral cycloalkane compounds from three‐membered ring to the larger ones, like cyclohexane, cyclooctane, and cyclotridecane [10, 15, 16]; (2) conformational changing investigation of cyclohexane and its substitution derivatives [1, 5, 12, 14, 17‐19].…”

“…The adopted experimental methods briefly include X‐ray diffraction, Raman, infrared, and NMR spectrum detection, [9, 11‐14] and theoretical approaches contain ab initio, molecular force field, and dynamical density functional theory [7, 8, 10, 15]. These previous studies usually cover two mainly aspects: (1) conformational determination of the simple neutral cycloalkane compounds from three‐membered ring to the larger ones, like cyclohexane, cyclooctane, and cyclotridecane [10, 15, 16]; (2) conformational changing investigation of cyclohexane and its substitution derivatives [1, 5, 12, 14, 17‐19]. Since cyclohexane has a fluxional and most symmetrical structure, it serves as the archetype molecule to understanding the energetics and conformational preferences for other related cycloalkanes and yielded derivatives [20, 21].…”