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Abstract-The temperature dependent spectra of several mono-, di-and trialkylthioureas have been recorded. Free energy barriers to internal rotation about the C-N bonds have been calculated. In thioureas that were unsymmetrically substituted, free energy barriers were found to be different for each C-N bond with the more substituted amino group exhibiting the higher barrier. The monosubstituted thioureas showed different rotational barriers for the NH, groups of the cis and trunsisomers,respectively. The free energy barriers for the trans isomers were found to be substituent dependent and substantiate the reassignment of the high and low field substituent signals to the trans and cis isomers, respectively. The spectrum of I-methyl-3-t-butylthiourea indicates restricted rotation of the t-butyl group at temperatures below 200 K.

Anal. Calcd. for n Z 4 D 1.5380. . Redistillation afof XV,, CljHzlCI: C, 76.09; H, 8.94; C, 75.94; H, 8.85; C1, 14.82. A solution of 7.3 g. (0.031 mole) of XV in 30 ml. of ethanol was hydrogenated over 5 % palladium on charcoal at room temperature and several atmospheres of pressure. Hydrogen was slowly absorbed for 20 hr. The solution was filtered, the solvent was distilled at atmospheric pressure, and the residue (5.74 g.) was subjected to preparative gas chromatography (205 O , 20 % silicone on Chromosorb W, Perkin-Elmer Model 154 vapor fractometer). The major component (of six) was collected; it had an infrared spectrum identical with that of VI11 obtained from cyclization of X.The n.m.r. spectrum of racemic 1,1,4,4-tetramethyl-6,7 benzocycloheptene at low temperatures is that of a mixture of two racemic conformational isomers called d,l-B and d,l-C. Evidence is presented that the sevenmembered ring is in the boat conformation in the Bisomer, and in the chair conformation in theC-isomer. The equilibrium constant, K = [CJ/[B], was obtained from the n.m.r. data and is represented by the equation, log K = -0.026 + 69.21T; AH" = -317 cal.; AS" = -0.12 e.u. From observations of proton exchange, two separate conformational changes could be characterized. The j r s t is isomerization without racemization, d-C d-B and I-C + 1-8. Kinetic results are: log kC-B = 10.919 -21341T; AH* = 9.3 kcal.; and ASx = -IO e.u. The second is racemization of the C-isomer, d-C e I-C. Kinetic results are: log kd-c-r-c = 12.847 -2870/T; AH* = 12.6 kcal.; and

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