Garg, P P

Garg, S C

Jaiswal, R M P

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Thermodynamic functions of para-halogenated benzaldehydesP, P. Gaeg*, S. C. Gabo** and R. M. P. Jaiswal Department of Physics, KurukAetra University, Kuruksh€ira-\^2M  ^{Received 81 March 1978)Iniian J. Phy». 52B, 391-896 (1978)New measurements of the infrared (3650-80 cm^ )^ and Laser Raman spectra, and complete vibratioi|al assignments availabk  ^ in literatun  ^for para-halogenatod benzaldehjdes (p~x CHO; X  =  F, Cl, Br) have been utilised to compute thermodynamic functions for the ideal gases of para-fluoro, -chloiro- and -bromobenzaldehydes at the pressure of one atmosphere under the rigid rotor harmonic oscillator approximation.1. TntbobfotionRecently Green et al (1976) have proposed interpretation of new measurements o f the infrared and laser Raman spectra of para-halogeuated benzald(‘hydes along with benzaldehyde and its other halogenatcd derivatives. On the basis of the interpretation of the spectra, they have also suggested complete vibrational assignments for the above mentioned molecules. Earlier Miller et al (1967) made a detailed study of torsional vibrations in these molecules in tht^  far infrared in vapour and liquid phases. Vibrational spectra of benzaldehyde and halo- genated benzaldehydes have also been studied extensively by Singh et al (1970) in our laboratory hy infrared and Raman methods. Those spectroscopic data are examined and utilised to compute thermodynamic functions of halogenated benzladehydes statistically as these are expected to be much more accurate than the direct experimental measurements. Present paper reports briefly results of the computations of thermodynamic functions in case of para-halo- genated benzladehydes on a TDC-316 computer.2. Caotjlations an d  R esultsPara-halogenated benzaldehydes possess a single asymmetric top {-C«H4X,X  =  F Cl Br) attached to an essentially rigid frame (-CHO group). Botationo f the top about the common C-C bond is hindred by a two-fold potential barrier. Thus the vibrational degree of freedom corresponding to hindred internal rota- tion has been omitted from the vibrational partition function and instead an• Department of Physics. G.H.S.S., University Campus, Kurukdietra. •• Department of Physios, Government CoUege, Jmd.391392 P. P. Garg, S. C. Garg and R. M. P. Jaiswalappropriate term has been added. In case of completely free internal rotation, an additional factor Q , given by Herzborg (1945).Qs(%nU,nkT)^(Tihwhere Im is the reduced moment of inertia of t.oi), (Ti is number of potential minima per revolution and other terms having their usual meanings, appears with the overall rotational partition function. However, in case of hindred rotation as for the present molecules, the corrections to various thermodjmamic functions are applied with the help of Pitzer and Gwinn’s tables (1942) with the knowledge of Qf and barrier height hindering internal rotations, given by (Miller et al 1967)^^0 = 4Fhwhere F — cm~  ^ and vt is observed torsional frequency in cm” .^Considering each molecule a planer one and having symmetry, principal momcjits of inertia /^, ly  and Iz havt^  bocui computed where X  and Z axes are in the plane and Y axis is perpendicular to the plane of the molecule. The structure parameters of the molecules have bt^ en taken from International tables for X-ray Crystallography (1962). Th(^  rcduci>d moments o f inertia have been calculated by the formula given by Pitzer (1946) for asymmetric top attached to a rigid frame. Full programrm  ^ has boon developed for a TDC-316 computer reduced moments of inertia, F  and potential barritu* height as well as thermo­dynamic functions for such molecules.Table I gives computed values of moments of inertia, reduced moments of inertia, F , and potential barrier heights of para-fluoro, -chloro- and -bromo- benzaldehydes. Computed values of heat capacity, enthalpy function, free energy function and entropy at various temperatures ranging between 100-1500°K for the para-fluoro-, chloro- and -bromobenzaldehydes respectively are given inTable 1. The computed values of moments of inertia, reduced moments of inertia, F, and potential barrier heights of para-fluoro-, -chloro- and -bromo-benzaldehydes.CompoundPrincipal moments of inertia(10 -®®gm.cm2)Reduced moment of — inoi tia F(cm~ )^Barrier height Vq (Real/mole)lx ly h (K)-30gjjj cm )^p-Flurobenzaldehyde 85*98 102*37 16*39 1-6129 1*7339 3*6041p- Chlorobenzaldehyde 121*63 13818 16-56 1-7298 1*6553 2*8684p-Bromobonzladehyde 180*92 197-58 16-67 1-7467 1*6011 2*4119It'oaIThermodynamic functions of benzaldekydies 393-§oCQ CNICOb-STtH ©OiasX©»oas«cto©scb>-<''t9FH(NOlt-©<NzcbCOVitocbTftt'F©05rH©9Vi10Vi' f© ©©CO9r-wtor-%•§ 1(N Op 9 05Vi© to9 © 9049 to QO 9 © 9r-F.o X't t"|-F»1a §305CO'itI'F >ot'* cbi'- X s © cb© ©© s ob© G?4 cb 604 cb04 cb04oT -HI S ’1 059 05QO cc9 ©9 CO© ©© $ i>-co ©9 ©CO op ©© - Vi9 CO9 Vi 9noQ1ow o ob © cb04 g ©eo cbeo cbCO obCO cb -Ot cb XTt< ©Tta 00 <M 9 aseo ©9 i- 049 ©9 CO9 © © © ©CO ©9 «5VJ ''tp—( CO<N 05<N CO CO obCO T©© "itVi l>Vi ©to 04© 'it© cb© © X© ©tofcji|| wH(N <M —«O  "No2 .T©lOr-©04|FFCO MI-'©I-* s ©X5 ©<C4 © 1—9 ©CO XCO 04lO IFF©j£© GO ob (h04cb040504o04©asCOr-9Xt'- ©©4^©9t'FfH(h(Nob046CO©COobCO»b ©40O -H f-lr  9CC CDfH W <M 00F-t C©©  f-4 fHfHi-t <N(M©^COOOeO 1- 05Vi 05 ©  ©fH  1-4 05g g 2to-H 40 1—( ©© g © ©© g g ©© g o© g g g gcbtF ob© g ©© i § i g cb© i ©© s ©2 s ©04 04 CO 'St* © © t'* X © o 04 COI>H U5D4Table 2, and represented by curves in figures 1 and 2. Hero Eff> is the energy of one mole of an ideal gas at absolute zero and C / ,  and denote heatcapacity at constant pressure, enthalpy, free energy and entropy respectively.394 P. P. Garg, S. 0. Oarg and R. M. P. JaiswalABSOLUTE TEMPERATURE^ ®KFig. 1. Variation of enthalpy function and heat capacity with absolute temperatures for para-fluoro-, -chloro- and -bromo-benzaldehydes.Thermodynamic functions o f henzaldehydes 395180|I70|SOL "TW “IJJT ~slw”155 1000 \tW------------► A B S O L U T E  T E M P E R A T U R E / KFig. 2. Variation of froo energy function and entropy with absolute temperatures for para- fluoro-, -chloro- and -brorao-benzladehydos.A ck no w led gm entThe authors are thankful to Prof. N. Nath for his interest in the work, Shri D. P. Asht for recording Baman spectra o f some of the molecules and Shri P. K. Suri, Programmer, Computer Centre, Kurukshetra University, Kuruk- shetra for his help in developing the computer programme,396 P. P. Garg, S. C. Garg and R. M. P- JaiswalR b p b r e n c k sGropii J. H. S. & Hamaion D. J. 1976 Sp ectroch im ica  A c ta  23A, 1266.Horzborg G. 1945 In fra r ed  and R a m a n  S p ectra , D. Van Nostrand Co. Inc., N.Y,In tern a tion a l tables f o r  X ^ r a y  crysta llogra ph y, Vol. Ill, Physical and Chemical tables. Kynock In tern a tion a l tables f o r  X ^ r a y  crysta llogra ph y. Vol. TIT, Physical and Chemical tables (1962)Kynock Press Birmingham, England.Miller F. A., Fateley W. G. & Witkowski R. E. 1967 S p ectroch im ica  A cta  23A, 891.Pitzer K. S. & Gwimi W. 13. 1942 J, Chem. Phys. 10, 428.Pitzor K. S. 1946 J. C h em . P h y s . 14. 239.Singh B. & Jaiswal R. M. P. 1970 In d ia n  J .  P u r e  & A p p lie d  P h y s . 8, 432.Singh B. 1971 P h .D . T h e sis . D ep a rtm e n t o f  P h y s ic s , K n ru k sh etra  V n w e r s i ty , K u ru h sh etra ,

Thermodynamic Functions of Para-Halogenated Benzaldehydes

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Thermodynamic Functions of Para-Halogenated Benzaldehydes

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