RESUMEN
A series of six 1,5-(ethylmethyl)semibullvalenes (1a <==> 1a', 2 <==> 2', 3 <==>3') and two 4(2)-substituted semibullvalenes (4 <==> 4'), each undergoing Cope equilibria between nondegenerate valence tautomers, was investigated by carbon-13 NMR spectroscopy at a range of temperatures in several different solvents. Gompper's treatment of substituent perturbation was extended, specifically accounting for the effects of the substituents on chemical shifts, to allow the determination of the thermodynamic parameters for these skewed equilibria. These new treatments were used to determine the population difference (f - f ') between the valence tautomers and the perturbation thermodynamic quantities DeltaH(P), DeltaS(P), and DeltaG(P). The slow-exchange limit was reached for the parent 1,5-(ethylmethyl)semibullvalenes 3a <==> 3a' from which it was established that the preferred valence tautomer is 3a with the ethyl group on the cyclopropane ring. Despite considerable effort, the slow-exchange limit could not be reached in any of our other remotely substituted semibullvalenes. Provided that the ethyl group always prefers the cyclopropyl position as in 3a, the 1-ethyl-5-methylsemibullvalenes 1a, 2, and 3 are more stable by DeltaH(P) = 0.7-1.7 kJ mol(-1) than their valence tautomers 1a', 2', and 3'. In the directly substituted semibullvalenes (4 left harpoon ovet right harpoon 4'), the preferred valence tautomers 4a and 4b have the bromine atom or the nitrile group on the vinyl position (C(4)) rather than on the cyclopropane ring (C(2)) and are more stable than 4a' and 4b' by DeltaH(P) = 4.8 and 7.0 kJ mol(-1), respectively.
RESUMEN
The UV/Vis spectra of thermochromic semibullvalenes 1 and barbaralanes recorded at various temperatures yield enthalpy differences between the two degenerate classical structures 1 and 1' and the less stable species (1*) that absorb at long wavelengths. The latter species are interpreted in terms of higher, delocalized states that are located just above the flat potential energy barrier between the ground states (ΔH(0) =11 (R=H; in butyronitrile), 1 kJ mol(-1) (R=Ph)).
RESUMEN
The position of the equilibrium between localized and delocalized states of thermochromic semibullvalenes and barbaralanes (see the Equation) depends strongly on the solvent. Dipolar aprotic solvents, particularly N,N'-dimethylpropylene urea, favor the delocalized, bishomoaromatic state (ΔH(0) =8 kJ mol(-1) (cyclohexane), ΔH(0) <0 kJ mol(-1) (N,N'-dimethylpropylene urea)).
RESUMEN
The first carbon-13 shift thermometer for the temperature range of 100-300 K is based on the very rapid equilibration of a pair of semibullvalene valence tautomers. The temperature dependence of the equilibrium constant is reflected in strongly temperature-dependent shift differences Deltadelta between averaged signals, e.g., d(Deltadelta)/dT = 0.051 ppm K-1 at 300, 0.087 ppm K-1 at 200, and 0. 175 ppm K-1 at 110 K for the quaternary carbon atoms C2 and C6. At 37 temperatures T, which were measured with calibrated platinum resistance thermometers, shift differences Deltadelta were taken from nondecoupled carbon-13 spectra recorded from solutions of 1 in mixtures of chlorodifluoromethane and deuterated dimethyl ether without spinning. The least-squares fit of these Deltadelta vs T data to a polynomial equation of the fourth degree (Eq. [5], r2 = 0. 9999) allows the calculation of temperatures from measured shift differences with a standard deviation of 0.46 K and an estimated error of about 1 K. The heating effects of WALTZ-16 decoupling and the influence of solvents on Deltadelta are investigated. A comparison with existing NMR thermometers demonstrates the superior performance of the new carbon-13 shift thermometer with respect to precision and the accessible temperature range. Copyright 1998 Academic Press.
