Journal of M olecular Structure, 76 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLK (1981) 219-226 THEOCHEM Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands zyxwvutsrqponmlkjih FORCE FIELD FOR IN-PLANE VIBRATIONS OF TETRACYANO- QUINODIMETHANE (TCNQ) JUA N MURGICH* and YOSSLEN ARAY** Centro de Quimica, Instituto Venezolano de Znvestigaciones Cientlyicas (ZVZC), Apartado 182 7, Caracas 101 O-A (Venezuela) (Received 20 June 1980) ABSTRACT The force field for the in-plane vibrations of TCNQ and TCNQ-d, has been calculated by the iterative self consistent method of Koptev et al. (Opt. Spektrosk., 25 (1968) 350). With the new set of force constants an improved matching was obtained between the observed and calculated vibration frequencies in TCNQ and TCNQ-d4. INTRODUCTION The vibrational and electronic spectra of the salts of tetracyanoquinodi- methane (TCNQ) have been analyzed extensively [l---5] . Several calculations of the force constants have been described for the neutral TCNQ [6, 71. These results were used as a starting point for the calculations of force con- stants in TCNQ- and other anions even in cases where the force fields used gave large differences (up to 19% [6] ) between calculated and observed fre- quencies. A more precise force field would be helpful in the assignment of the fundamental vibrations of the TCNQ anions and will allow the calcula- tions of other quantities [8,9] with a higher degree of accuracy. Thus, a new calculation of the TCNQ force constants using an alternative method was undertaken. Several semiempirical methods have been proposed to solve the inverse vibrational problem. However, when the necessary condition that the force constants correctly interpret the vibrational spectrum of the isotope analogue (Born-Oppenheimer) is applied, the number of calculation methods available is greatly reduced [lo]. Duncan [ll] has also pointed out that the commonly used Urey-Bradley method does not give consistent results and there are some controversial statements in its formulation [ 121. Koptev and co-workers [ 13, 141 formulated an iterative method free of the above restrictions. *To whom all correspondence should be addressed. **Present address: Dept. de Quimica, Facultad Experimental de Ciencias, Universidad de1 Zulia, Maracaibo, Edo. Zulia, Venezuela. 0166-1280/81/0000-0000/$02.50 0 1981 Elsevier Scientific Publishing Company