Equilibrium structures have been determined for s-trans-1,3-butadiene and ethylene after adjusting the rotational constants obtained from rotational spectroscopy by vibration-rotation constants calculated from the results of quantum chemical calculations. For butadiene, the formal C=C bond length is 1.338 A, and the formal C-C bond length is 1.454 A. For ethylene, the C=C bond length is 1.3305 A. These values appear to be good to 0.001 A. It is shown for the first time that pi-electron delocalization has the structural consequences of increasing the length of the formal double bond by 0.007 A and decreasing the length of the formal single bond by 0.016 A. Comparisons are made with structures computed with several quantum chemical models. The MP2/cc-pVTZ results agree best with the new re structure.

High level ab initio calculations, utilizing coupled cluster theory with quasi-perturbative triple excitations and augmented quadruple zeta level basis sets, have been used to determine the structures and relative energies of the four stationary points on the 1,3-butadiene torsional potential curve. Corrections were applied in order to minimize the residual basis set error, as well as account for core/valence correlation and scalar relativistic effects. Higher order correlation recovery was also included to improve our estimate of the relative energies. The transition state separating the trans and gauche rotamers lies 26.8 kJ/mol above the trans global minimum. The gauche rotamer lies 12.6 kJ/mol above the trans rotamer and the s-cis form is a transition state 2.0 kJ/mol higher than the gauche rotamer (excluding zero point energies).

There are difficulties with the concept of work (1-9), particularly for irreversible processes. A thoughtful writer on that topic has said (9) "For mechanically irreversible volume changes, we usually cannot calculate the work from thermodynamic considerations." We believe that conclusion is incorrect. In this paper we give definitions of work and heat that overcome earlier difficulties. Both quantities are defined in terms of experimental quantities. The definitions are simple and general. They apply equally well to reversible and irreversible processes. Using these definitions, we are able to resolve several important problems as well as to obtain fresh insights into the First Law. Some of the results are surprising. They force a reevaluation of one's thinking about work and heat.

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