Electric Field Effects on Aromatic and Aliphatic Hydrocarbons: A Density-Functional Study Dhurba Rai, ² Harshad Joshi, ²,‡ Anant D. Kulkarni,* ,§,| Shridhar P. Gejji, § and Rajeev K. Pathak ², ⊥ Department of Physics, UniVersity of Pune, Pune-411007, India, Department of Chemistry, UniVersity of Pune, Pune-411007, India, and Department of Physics, Tulane UniVersity, New Orleans, Louisiana 70118 ReceiVed: May 25, 2007; In Final Form: July 12, 2007 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 π 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. I. Introduction With the advent of molecular electronics, recent years have witnessed a remarkable progress on the understanding of current conduction through prototype molecular electronic devices such as a single or, at best, a countable number of molecules. 1,3,4 The conductance spectra of molecules and molecular wires have a direct bearing on their response to applied external electric fields. 1-4 Electric fields modify the molecular geometry, drasti- cally alter the electric dipole moments, and bring out a redistribution of molecular orbitals (MOs) as well as a reduction in the energy gap between frontier molecular orbitals. Further, appreciable shifts in their infrared (IR) vibrational spectra accompanied by a redistribution of the corresponding spectral intensities as a function of the field strength are observed, an effect that is aptly termed the molecular vibrational stark effect (VSE). Choi et al. 1 studied the role of molecular orbitals in the conductance spectra of benzene using the hybrid density functional calculations employing the 6-311++G(d,p) basis set. A set of threshold electric fields was observed beyond which the energy of the LUMO + 2 gets lowered below the energy of LUMO (lowest unoccupied molecular orbital) rendering the molecule electronically active (F th | ) 0.2 V/Å and F th ⊥ ) 0.7 V/Å, where F th | and F th ⊥ denote the threshold electric field strengths of the external electric field F B applied parallel and perpendicular to the aromatic plane, respectively). More recently, Li et al. 2 found an appreciable reduction of the HOMO-LUMO gap on increasing the length of molecular chain constituting the molecular wires. Diminishing the gap is also evinced with increasing external electric field strengths leading to spatial distributions of frontier molecular orbitals that vary from fully delocalized form to partly localized. Electrical conduction through molecules essentially requires “promoting” an electron to erstwhile virtual, unoccupied molecular orbitals (UMOs). Contribution to the current conduction from higher UMOs and the lower occupied ones has been investigated by Wang, Fu, and Luo. 4 On the basis of their studies on benzene-1,4-dithiol and R,R′-xylyldithiol, they inferred that the upper UMOs (LUMO + n, where n ) 10, 11, 12, 13, ...) contributed significantly to the electron conduction compared to the low- lying ones. Within the density functional theory (DFT), To ´bik et al. 5 investigated the changes in the electronic structure of small slabs of benzene (six layers) and anthracene (four layers) due to electric field applied perpendicular to the slabs. They found that the HOMO-LUMO band gap in benzene and anthracene slabs “closed down” at the field values of 0.005 and 0.014 au, respectively (1 au of electric field strength = 51.42 V/Å). All these foregoing features form important descriptors in molecular electronic architecture with molecular wire ele- ments 6 and in modeling the electrical characteristics of molecular devices. 7 A property of any molecule that is of primal importance in the description of its response to an external electric field is its electric polarizability, R ij , and higher order polarizabilities. These electrical properties enter in the expansion * To whom correspondence should be addressed. E-mail: anantkul@ chem.unipune.ernet.in, anantkul@dcci.unipi.it. ² Department of Physics, University of Pune. ‡ Present address: Max Planck Institute for Biophysical Chemistry, Am Fassberg 11 D- 37077 Go ¨ttingen, Germany. § Department of Chemistry, University of Pune. | Present address: Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, Via Risorgimento 35, 56126 Pisa, Italy. ⊥ Department of Physics, Tulane University. 9111 J. Phys. Chem. A 2007, 111, 9111-9121 10.1021/jp074051v CCC: $37.00 © 2007 American Chemical Society Published on Web 08/28/2007