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Electric field effects on aromatic and aliphatic hydrocarbons: a density-functional study.

The influence of a uniform static external electric field on some aliphatic and aromatic molecular species is studied within the density functional theory (DFT) employing the 6-311++G(2d,2p) basis set with B3LYP exchange-correlation prescription. The electric field perturbs the molecular geometry but drastically alters the dipole moments and engenders, to a varying degree, the molecular vibrational Stark effect, i.e., shifts in the infrared (IR) vibrational frequencies accompanied by spectral intensity redistribution. For polar molecules, significant negative ("red") and positive ("blue") frequency shifts are observed for field orientations both parallel and antiparallel to their permanent dipole moments. Further, a selective reordering of frontier orbitals is observed to be brought about by moderately intense fields. In particular, molecules having a lowest unoccupied molecular orbital (LUMO) with predominant pi character possess a threshold field beyond which energy gap between the highest occupied molecular orbital (HOMO) and LUMO diminishes rapidly. A time-dependent (TD) DFT analysis reveals that an increase in the applied field strength by and large increases the excitation energies corresponding to significant electronic transitions among frontier MOs with a concomitant decrease in their oscillator strengths.