Robert E. Rehwoldt and Elinor Boynton Vassor College Poughkeepsie, N e w York
A Molecular Complex Suitable for Spectrophotometric Investigation
Spectraphotometric methods of investigation of complex species in solution are often described or suggested in instrumental analysis laboratory manua1s.lJ Molecular complexes of aromatic and polyaromatic systems are well known and in some cases well classified. There are many techniques which may be used for investigation of these complexes, and several excellent review articles are available which discuss the nature of the molecular complex.3-" We have found that 2,3-dichloro-5,6-dicyanobenzoquinone6 ( D D Q ) forms a pi complex with naphthalene C1
C1
equation' can be used. The heat of formation of the complex can be calculated with the van't Hoff equation, assuming AH t,o be independent of temperature. Consider the following donor-acceptor interaction: where the equilibrium expression is defined as
under the assumption that the concentration (a) of the acceptor naphthalene minus that of the complex (c), is essentially (a). If one substitutes Beer's law, Abs = rbc, into the last expression for K,,, the Foster equation follows: Kde
in methylene chloride which is quite suitable for spectrophotometric investigation. The complex is green, with maximum absorbance a t 625 mp. Solutions of the complex can be stored under nitrogen and show no change in spectra over a 48-hour period. Since the complex absorption band does not overlap either of the component bands, i t is not necessary to correct for component absorption; therefore the Foster 1 EWINR,G. W., "Instrumental Methods of Chemical Analysis," 2nd ed., MoGraw-Bill Book Co., Ino., New York, 1960. REILLEY,C. R., AND SAWYER,D. T., "Experiments far Instmmental Analysis," McGraw-Hill Book Co., Inc., New York. - .-. ,.1961. ANDRES, L. J., Chem. Reu., 54,713 (1954). P. S., C h m . Rev$., 58,1113 (1958). MCGLYNN, J. Angezo. Chem., 72, 401 'BRIEGLIEB,G. AND CZEKELLA, (1960). 6 Available from Arapahoe Chemicals, Inc. 'FOSTER,R. J., J. C h m . Sac., 3817 (1953). ~~
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Journol o f Chemicd Education
- KAbs
= Abala
Abs/a was plotted against Abs, and the slope and intercept were determined by a least squares method, thus giving the two parameters e and K.,. The table contains the calculated value of the extinction coeEcient, equilibrium constants, and the heat of formation for the DDQ-naphthalene complex. The DDQ complex system and the Foster approach to the evaluation of this type of system both adapt well to student research projects. Two students working together can evaluate a system in three laboratory periods. Tem-
K (molar)
AH (kcal mole-')
