Semi-experimental equilibrium and high-level quantum-chemical structures are reported for three hydrocarbons containing the vinyl (H2C = CH-) group: vinyl acetylene, vinyl diacetylene and vinyl cyanide. Semi-experimental equilibrium structures have been obtained through a combination of new experimental results from Fourier transform microwave spectroscopy combined with force field calculations performed at the coupled-cluster singles and doubles model augmented by a perturbative correction for triple excitations, CCSD(T), level of theory. Comparison against equilibrium structures obtained by high-level CCSD(T) computations employing large correlation-consistent basis sets with up to 1090 basis functions yields excellent agreement. From this combined study, we find the carbon atom backbone in the vinyl group is largely unaffected when bound to different substituents, such as -H, -CCH, -C4H, -CN.
