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Results of two field experiments in the North Sea are presented. Pressure was measured at two fixed heights above the mean water level and correlated with simultaneous wave height measurements. Roughly 90 hours of data have been analysed and the results are in agreement with earlier results obtained by Snyder et al. (1981). Measurements over swell give no indication of wave decay or growth for waves travelling faster than the wind or against the wind.

Synthesis and kinetic study of (I?)-( +)-1 ,l-dideuterio-2-(cyclohex-3-enyl)ethene (labeled 4vinylcyclohexene) has revealed new reactions of racemization and deuterium exchange, the former of which fixes the heat of formation of cis,trans-(and/or cis,cis) octa-3,6-diene-l,7-diyl diradical more firmly than heretofore. Redetermination of the kinetic parameters for the thermal reactions of cycloocta-l,5-diene establishes the heat of formation of the cleavage of the trans,trans (or, less probably, the cis,cis) diradical to butadiene to be 81.1 5 2.0 kcal/mol, in good agreement with that (82.3 kcal/mol) deduced for the same reaction in the transformations of trans-divinylcyclobutane investigated by Hammond and DeBoer. In relation to the transition state for the Diels-Alder reaction of butadiene with itself to give Cvinylcyclohexene, the cleavage of the diradical appears, within the present limitations of experimental results, to be somewhat higher in energy. The mechanism of this Diels-Alder reaction is tentatively concluded to be concerted, perhaps two stage, but not two step.ontroversy over the mechanism of the Diels-Alder C reaction has centered on the alternatives of a onestep, concerted process and a two-step, unconcerted process. The former, allowed by orbital symmetry,2 envisages the simultaneous formation of both bonds (A),3 whereas the latter involves the rate-determining formation of one bond, followed by a closure of the second bond more rapid than internal rotation (B).435To these widely disputed alternatives, Woodward and Katz have added a third, a merger of the two mechanisms into a two-stage process (C)I (Figure 1).One generally applicable approach, particularly appropriate to the study of mechanism among thermal reorganizations, involves the placement of the interrelated isomeric elements of the system on an energy surface with a view t o including or excluding certain

Dedicated to Professor Wolfgang Kirmse on the occasion of his 75th birthdayThe controversies over concerted versus stepwise diradical mechanisms of potentially pericyclic reactions have subsided with improvements in the understanding of stereoselectivity in thermal rearrangements that involve modestly stabilized diradical intermediates. The thermal rearrangements of vinylcyclopropanes [1] and vinylcyclobutanes, [2] including bicyclo[3.2.0]hept-2-ene, [3] involve diradical intermediates that lack a deep potential energy well, and their outcomes deviate from statistical predictions. The thermal rearrangement of 6-methylenebicyclo[3.2.0]hept-2-ene (1) to 5-methylenenorbornene (3) is more intriguing, as it yields a nonrandom distribution of products despite the stabilizing effect of the 6-methylene substituent on the intermediate 2.Dideuterio labeling has revealed a preference for [1,3] over [3,3] shifts, [4] and the stereochemistry observed with methyl labels has led to the hypothesis that diradical intermediates do not equilibrate rotationally before collapsing to form the bicyclic products.[5] New experimental studies of monodeuterated species currently show modest levels of both regioand stereoselectivity (Table 1), [6] and DFT and ab initio calculations provide a theoretical framework for understanding these results.UB3 LYP and CASSCF calculations indicate that two modes of C1ÀC7 cleavage produce stereoisomeric diallyl intermediates 2 from 1 (Figure 1). The favored transition state, TS-12 n, moves C7 toward C3 and places the 7x substituent in an E configuration. In contrast, TS-12 x moves C7 away from C3 and puts the 7x substituent in the Z position. The preference (1.8 kcal mol À1 ; CASPT2//UB3 LYP) for TS-12 n over TS-12 x is analogous to torquoselectivity in electro- Table 1: Deuterium label distribution in product 3 from the thermolysis of 7x-, 8E-, and 8Z-1, extrapolated to t = 0 min.

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