Crystallographic study and molecular orbital calculations of thiadiazole derivatives. 2. 3,4-diphenyl-1,2,5-thiadiazole 1-monoxide E.E. Castellano a , O.E. Piro b , J.A. Caram c , M.V. Mirõ ®co c , S.L. Aimone c , E.J. Vasini c , A. Ma Ârquez-Lucero d , D. Glossman-Mitnik d, * a Departamento de Fõ Âsica, Instituto de Quõ Âmica y Fõ Âsica de Sa Äo Carlos, Universidad de Sa Äo Paulo, C.P. 369, (13560) Sao Carlos (SP), Brazil b Departamento de Fõ Âsica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata and PROFIMO (CONICET), C.C. 67, (1900) La Plata, Argentina c INIFTA (CONICET), Universidad Nacional de La Plata, Casilla de Correo 16, Suc. 4 (1900) La Plata, Argentina d LAQUICOM, Centro de Investigacio Ân en Materiales Avanzados (CIMAV), Miguel de Cervantes 120, Complejo Industrial Chihuahua, Chihuahua, Chih, 31109 Mexico Received 31 October 2000; accepted 14 May 2001 Abstract Single-crystal X-ray diffraction studies are reported for 3,4-diphenyl-1,2,5-thiadiazole 1-monoxide (I). Ab initio MO calcu- lationsontheelectronicstructure,conformationandreactivityforthiscompoundarealsoreportedandcomparedwiththeX-ray results. A charge sensitivity analysis is performed on the results applying concepts derived from density functional theory (DFT), obtaining several sensitivity coef®cients such as the molecular energy, net atomic charges, global and local hardness, global and local softness and Fukui functions. With these results and the analysis of the dipole moment and the total electron densityandelectrostaticpotentialmaps,severalconclusionshavebeeninferredaboutthepreferredsitesofchemicalreactionof the compound. q 2002 Elsevier Science B.V. All rights reserved. Keywords: 1,2,5-Thiadiazole 1-oxide; Ab initio MO calculations; Single-crystal X-ray diffraction; DFT; Sensitivity analysis 1. Introduction Organic heterocycles are systems of growing interest in materials science in view of the potential technological applications in ®elds such as electro- nics, photonics, sensors or corrosion protection. Several potentially conducting polymers, optically nonlinear polymers and biomaterials contain hetero- cyclic structures [1]. Thiadiazoles, the nitrogen containing analogs of thiophene, and their derivatives are the structural basis of some of these polymeric materials [2,3]. One of the goals of computational chemistry is to understand chemical reactivity and to predict the outcome of molecular interactions [4]. The behavior ofatomsandmoleculesischaracterizedbysomepara- meters that chemists have extracted from their experi- ence and used for prediction of chemical reactivity. Electron population analysis [5,6], electronic density Journal of Molecular Structure 604 (2002) 195±203 0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII:S0022-2860(01)00656-1 www.elsevier.com/locate/molstruc * Corresponding author. Tel.: 152-14-39-1151; fax: 152-14-39- 1112. E-mail address: glossman@mail.cimav.edu.mx (D. Glossman- Mitnik).