Several acyclic acetylenic precursors to multidimensional carbon networks, including tetraethynylbutatriene (C12H4), tetraethynylmethane (C9H4) derivatives, and the yet unknown molecules tetraethynylallene (C11H4) and hexaethynyl[3]radialene (C18H6), have been studied using ab initio molecular quantum mechanics. Their equilibrium geometries, vibrational frequencies, thermochemical properties, and nonlinear optical responses have been predicted. To allow direct comparisons with experiment, the recently synthesized tris(trimethylsilyl)tetraethynylmethane molecule was also studied quantum mechanically. Excellent agreement with the experimental geometry and vibrational frequencies for the tetraethynylbutatriene (C12H4) molecule has been achieved. However, the C=C bond contraction, which was found by X-ray diffraction in the tris(trimethylsilyl)tetraethynylmethane crystal, is not reproduced by our research. We suggest that a novel mechanism, which we call "pi electron compression", might be responsible for part of the deviation of the X-ray structure from our theoretical results. Therefore, it may be advisable to reexamine the structure by both X-ray and neutron diffraction. The pi electron compression effects have been employed to explain the negative nonadditivity of the (hyper)polarizability of the C9H4 molecule and the effects of the substitution of hydrogen atoms by lithium atoms, fluorine atoms, cyano groups, and acetylenic groups. The HOMO energy and nonadditivity of the (hyper)polarizability for the C11H4 molecule are lower than those for other planar molecules. The heats of formation for the precursors are evaluated. They are 236, 247, 289, 317, and 437 kcal mol(-1) for the C9H4, C10H4, C11H4, C12H4, and C18H6 molecules, respectively. The heat of formation of the tris(trimethylsilyl)tetraethynylmethane decreases to 76 kcal mol(-1), partially due to the hyperconjugation effect of the TMS groups.