Molecular Organization in a Nematogen: PBPCN - A Computational Analysis Based on Quantum Mechanics Durga Prasad Ojha, Devesh Kumar, and V. G. K. M. Pisipati Centre for Liquid Crystal Research and Education (CLCRE), Faculty of Physical Sciences, Nagarjuna University, Nagarjuna Nagar (A.P.) - 522 510, India Reprint requests to Prof. V. G. K. M. P.; E-mail: venkata_pisipati@hotmail.com Z. Naturforsch. 56 a, 730-734 (2001); received September 7, 2001 A computational analysis has been carried out to determine the configurational preference of a pair of 4'-n-pentyloxy-4-biphenylcarbonitrile (PBPCN) molecules with respect to translatory and orientational motions. The CNDO/2 method has been employed to evaluate the net atomic charge and atomic dipole components at each atomic centre of the molecule. Modified Rayleigh- Schrödinger perturbation theory along with multicentered-multipole expansion method has been employed to evaluate long-range intermolecular interactions, while a ‘6-exp’ potential function has been assumed for short-range interactions. On the basis of stacking, in-plane and terminal interaction energy calculations, all possible geometrical arrangements of molecular pair have been considered. It has been observed that the molecule has a strong preference for stacking through a particular face, while the other configurations, such as stacking through the other face, in-plane and terminal interactions show, in general, an aligned structure along molecular axis. The results are discussed in the light of experimental as well as other theoretical observations. Keywords: PBPCN; Nematogen; Interaction Energy; Computer Simulation. Introduction Liquid crystalline phases have always partial ori entational order of the molecules, and some have also partial positional order of the molecules. Since this occurs frequently in natural and synthetic materi als, liquid crystals are of considerable fundamental and applied interest [1 - 3]. The peculiar changes that ooccur at phase transitions are primarily governed by intermolecular interactions between sides, planes and ends of a pair of molecules [4]. The molecular interac tions of mesogenic compounds have been calculated by several workers [5 - 7] basing on the Buckingham potential with modified Rayleigh-Schrödinger pertur bation theory. Attempts were also made to correlate the mesogenic properties with molecular interactions. It has been observed that the pair potential between such molecules is anisotropic and that the minimum energy configuration corresponds to the crystallo- graphic structure of the molecule in the solid state. Recently [8 , 9] the results of theoretical investi gations on the role of intermolecular interactions in mesogenic compounds have been reported. One of the limitations of the work was that interaction energy calculations were carried out only through one face. In the present communication, an attempt has been made to interpret the results of interaction energy cal culations through both faces, sides and ends, of a molecular pair of PBPCN molecules at an interme diate distance of ±7 A for stacking and ±7 A for in-plane interactions. Similarly, a distance of ±20 A has been kept for terminal interactions. The thermodynamic parameters reveal that PBPCN shows a nematic phase at 47.5 °C and ex hibits an isotropic melt at 67.5 °C [10]. Computational Details The CNDO/2 method [11] has been employed to compute the net atomic charge and dipole moment at each atomic centre of the molecule. The molecu lar geometry of PBPCN has been constructed on the basis of the published crystallographic data with stan dard values of bond lengths and bond angles [10]. The length of the most extended conformation was found to be 17.5 A, which is the calculated length of the 0932-0784 / 01 / 1100-0730 $ 06.00 © Verlag der Zeitschrift für Naturforschung, Tübingen • www.znaturforsch.com