UV photoelectron spectroscopic and computational study of 7-substituted cycloheptatrienes

The He(I) photoelectron (PE) spectra of cycloheptatriene (1), 7-methylcycloheptatriene (2), 7-methoxycycloheptatriene (3), 7-methylthiocycloheptatriene (4), and 7-diemthylaminocycloheptatriene (5) were recorded and interpreted using MO energies and ionization potentials acquired from B3PW91 calculations. Partial simulated PE spectra were in good agreement with the experimental results. The axial and equatorial conformers have distinct PE spectra as illustrated by simulation. The PE spectra of 2, 3, and 5 are representative of the equatorial conformers, while the PE spectrum of 4 was in accord with a 1:1 mixture of equatorial and axial conformational isomers based on spectral simulation. The calculated free energy differences between the equatorial and axial conformers are in the order of 2 kcal mol–1 (1 cal = 4.184 J) for compounds 2, 3, and 5, where the equatorial conformational isomer is lowest in energy. In the case of 4, the equatorial conformer was only marginally lower in energy than its axial counterpart, where the free energy difference is 0.1 kcal mol–1.Key words: 7-substituted cycloheptatrienes, He(I) photoelectron spectroscopy, B3PW91, spectral simulation.

Anomeric effects of sulfones

The anomeric effect of the sulfone group in tetrahydropyrans has been determined. The value is >2 kcal mol–1, which is larger than the A-value of a methyl group but less than the A-value of the sulfone in a tetrahydropyran. Hence, in an unsubstituted tetrahydropyranyl sulfone, the equatorial conformer predominates, whereas in a properly substituted methyltetrahydropyranyl sulfone, an axial sulfone is preferred over an axial methyl group.Key words: sulfone, tetrahydropyran, anomeric effect.

On the purported axial preference in 2-methylthio and 2-methoxycyclohex-anones: steric effects versus orbital interactions

The value for the equilibrium constant representing the ratio of equatorial to axial conformers for 2-methylthio- and 2-methoxycyclohexanone has been measured in five solvents using 500 MHz 1H nuclear magnetic resonance. In cyclohexane solvent, the methylthio group is found to have a large axial preference while that for the methoxy is negligible. For both compounds the equilibrium shows a strong solvent dependence. Comparison of these results with those known for the halo derivatives shows the amount of axial to increase in the order: fluoro, methoxy, chloro, bromo, and methylthio. With the aid of molecular mechanics calculations it is concluded that the order of conformational preference can be attributed to a variation in the van der Waals interaction between the substituent and the carbonyl group in the equatorial conformer. Keywords: α-substituted cyclohexanones, conformation, MMX calculations, steric and dipolar effects.

4,4-Dimethyl-A-homocholestane derivatives

The synthesis of epimeric bromo ketones VII, VIII, XXV and XXVI, derived from 4,4-dimethyl-A-homo-4a-cholesten-3-one (I) and 4,4-dimethyl-A-homo-5-cholesten-3-one (XIII) is described in this paper. In the case of bromo ketones VII, VIII and XXV the IR data demonstrate the presence of conformers both with a quasi-axial and a quasi-equatorial bromine atom in the equilibrium mixture, the quasi-equatorial conformer being the predominant component. On the basis of IR, CD and 1H-NMR data the conformation of the ring A in compounds VII, VIII, XXV and XXVI is discussed.

Microwave spectrum of morpholine

The microwave spectra of morpholine and N-deuteromorpholine have been investigated within the 8 to 40 GHz region. For both the normal and the deuterated species, strong a-type spectra which are due to the chair conformer with an equatorial imino group were recorded. The ground vibrational state rotational constants and quadrupole coupling constants are (in MHz), for normal morpholine, A = 4924.88 ± 0.05, B = 4625.15 ± 0.05, C = 2684.25 ± 0.05, χaa = 2.22 ± 0.02, χbb = 2.51 ± 0.04, χcc = −4.73 ± 0.02, and for N-deuteromorpholine, A = 4925.39 ± 0.05, B = 4399.67 ± 0.05, C = 2607.09 ± 0.05, χaa = 1.92 ± 0.02, χbb = 2.59 ± 0.04, χcc = −4.51 ± 0.02. The rs coordinates of the imino hydrogen are (in Å) aH = 2.373 ± 0.005 and cH = 0.10 ± 0.03. For the normal species the dipole moment components are (in D) μa = 1.68 ± 0.01, μc = 0.30 ± 0.01, μ = 1.71 ± 0.02. The value of aH and the direction of μ with respect to the a and c principal axes confirm that the observed spectrum is that of the chair equatorial conformer of morpholine.