200mμ band, which is an allowed π-π* transition has been studied
in five monovalent nitrates. The blue shift of the band has been found to be proportional to the inverse of the cationic radius

Mookherji, A

Tandon, S P

English

IACS Institutional Repository

LIGHT ABSORPTION IN NO, ION IN STATE  OF SOLUTION (Part III.—Effect of Cation)A MOOKHERJI a n d  S. P. TANDON**PnysicAL Laboratories, U niversity or B uudwan, Burdwan, (W.B.)IndiaABSTRACT. 200 band, which is an hUowchI *rr  ^tt* transition, has boon studied in five mt)iH)Vrtlont nitrates. The blue shift of tho band has boon found to be proporl ional to the 111 verse of the cationic radius.I N T R O D U C T I O NIn tho preijoding pari (Part II) of the present si r^ios of papers, a systematii^ study of the eharacteriFtics and assignment of the 200m// band of nitrate ion lias Ixum reported (Mookherji and Tandon, 1965).Alisorption hands are oliserved to suffix* a shift towards sJiorter wavelengtli due to the effei t^ of environment known as blue shift. In the static of solu­tion the factors which presumalily affo(!t the position of the (doctronic hands are ion-ion and ion-solvent interactions. The former lias not hf^n studied in tlu* ease of 200m//band of the nitrate ion while the latter has been studiwl in some details (McConnell, 1952, Strickler, 1961, Strickler and Kasha, 1961) and will he disijussed in the next paper of th(  ^ sories.In dilute solutions ion-ion interactions are expected to bo very weak and are usually masked by the ion-solvent effect. Ion-ion interactions are of two tyj>es, anion-cation and anion-anion. The later interactions an  ^ so weak that they can not be recordiMl. So there is a possibility of studying only the former by investi­gating the spectral shifts, keeping all other parameters constant and varying the cation. The importance of this lies in the expectation that it may become a tool for making direct measurements of intormolocular interai^tions.The present communication reports a systematic study of the effect of cation on the energy of the 200m// band, which has lioon assigned to symmetry allowed 7r-> tt* transition of the nitrate ion (Mookherji and Tandon, 1962, 1965) in state of aqueous solution. The results have been discussed in the light of the work of Mott and Gurney, (1948) , Meyerstein and Treinin, (1961) and Smith and Boston (1961).♦♦Present address ; Physical Laboratories, University of Jodhpur, Jodhpur, Rajasthan,44India.396E X P E K 1 M E N T A L R E S U L T SThe results of measurements of absorption with UVISPEK spectrophoto­meter reporte<l earlier (Mookliorji and Tandon, 1965) have been used. Energy in electron volts of thci-o peaks- (A B, C, D and E of 200m/«) against inverse of the radius of five monovalent cations, ■—lithinm, sodium, potassium, silver and ammonium, have boon given in Fig, 1.Light Ahsjrptioii in no.^  Ion in State of Solution 3 9 ?Pig. 1. Variation of ont'rgy of jjoaks A. Ih U, 1) and E of 200m(x band of nitrato ion witii (fiatioiiio radms)“ i .I) TS ( J US  S I O NA study of P’ig. J ri^voals a bhu' shift with de(;reaso of (;ation radius for the nitrate’s studied. To explain sue-h band shifts duo to onvironmont several theories have boon proposed (Griffiths and Symons, lb(>Oa) in (‘onnoetion with the eloetron transf(F?r spectra. Tn an aqueous solution there is also possibility of ion-pairs formation. An internal transition in an anion, brings about only a limited ex- pansion of the electronic orbital, and will be probably affected only by theexternal polarizing effect of tho cations (Kortum, 1944; Griffiths and Symons, 1960b) suggesting that the cation would affect the spectrum of anion only at contact approach. But in tho present case of nitrate ion, it is jiossible for the cationic effect to bo convoyed (Meyorstfdn and Trtunin, 1961) through the first hydration layer. In dilute aqueous solutions solvatcnl cations whi(;h have the nitrato ions as nearest neighbours may be attached quite Rrmiy to tho cation. This is tho case of local hydrolysis (Diamond, 1958), and is an extreme case of polarization. The polarized water must be more H-bonded to nitrate ion and thus will act as a more polar solvent than ordinary water. The magnitude of the ionic effects6398 A Mookerjee and S. P. Tandonwill dopend on the cdiargo on the cation, distance and mode of approach to the nitrate i(ni. Thus the (nations having only one water molecule separating them from nitrate km are (*,xp(*cted to liavo an effect decreasing with increasing cationic; radii. This is what lias ])oen observed. But this mechanism of band shift is analogous to that of electron transfer spectra. I t  has already been proved (Me Kwen, 1961; Strickler and Kasha, 1961; M(»oklierji and Tandon, 1962, 1965) that the Fpectrum of the nitrate^ ion is not due to (>l(H^ tron transfer, but due to an allowed tt—> n* transition, which shifts tlu^  ^ (‘lecdronic; charge density from the region round oxygen atoms to the region around nitrogen atom. Consequently the blue shift may bo expJain<^d as follows :In solution (Mott and Gurney, 1948) like crystal and medt (Smith and Boston, 1961) tliere exist two types of forceps, Coulomb and Ovt^rlap forces. The Coulomb forces of the othc;r ions give risc‘, to the external electtrostatic; potcuitial, V, arising from pairwise  ^ cdectrostatic interaction botw^ ecai ions. For solutions containing A  ions, wc5 have thenN'-lV{roNO,) -  V  " f dV,^  D J rNOs—r, . . .  ( 1)Where is the vector position of the volume efemont (c^Fjvo.J nitrateion that becomes ox(;ited, is tlu» vector position of a volume element dVi in the t-th ion, p%(ri) is the (;harge density function of the i-th ion, c is the electronic charge and 1) is effective dielectric; constant. The term is the scalardistance betw^ecn and dV^.The external electrost«ati(* potential F is positive bot;ause iiitrat*  ^ ion is nega­tive. The dipole and higher terms in F are produced primarily by neighbouring ions. Therefore it is useful to divide the ions into twci groups : tlios<; ions wdiich lie nearest to a given nitrate ion and those ions wdii(;h are more remote. Tin; nearast neighbours are cations and are assumed to have rare-gas-electronic configurations, with small polarization terms ignored. These cations can be represented as point charges and the following change made in equation (1) :- r 7\ , f ' - d y N O . +  V^ >,er ... (2)Where is the not t;harge or electrostatic valence of the /-tli cation, scalar distance from the centre of the i-th cation to the volume element dVi^o^ of the nitrate ion, the summation is over the G-cations of the nearest neighbour shell, and F is the term similar to equation (1) but is summed from (C + l) through {N—l). The dipole and higher terms in equation (2) are of greatest magnitude near the neighbour cations where they increase as the cationic Z/tq increase.Light Absorption  ^ in no.^  Ion in State of Solution 399The potential V as discussed above contribuU's to tho transition energy in a way approximately described by (Smith elal, Ihfil).(hwU, = (Av)o-i-rj(1 ifv12 - I <lr,r\‘^)VdV (3)where (Av)# represents the transition energy for a nitrate ion separated to infinity intiluding the effect of n'pulsive overlap fbtX'Os and the integral nipresonts the change in interaction energy between V and the electronic, charge (ilond of the nitrate ion in going from the gromid to the excited states | | | V^«r | *are the electronic density functions of the ground and the excited stat(W respec­tively, and tho integral is over the volume of the nitrate ion sin(;e dii«)le and liigh(»r terms in oquatioti (2) are of groat magnitude lu'ar the luaghobouring cations, when* they increase as the cationic iimrease. Consiupiontly the interaction integral in equation (2) changes when /jjr„ changes. This explairus the observed dependence of energy of tho 20()in/< hand on l/r#.K K F K i: E N 0 E ISTniimond, R. TVf., I ' j r .S ,Anin- Ohun. Sor., 80, 480SGrifiiith. T K and Symon.s, M. C R , lOfiOa. Tram. Faradaii Sor . homlon, 56, 1125.GrifRlh, T. R. and ,Symons, M, C. R., MMiOb, Mol. Phtp.. 3, 00.Korium, (»., 15)44, Z. Elerlrorhem., 50, 144.McConnell, H., 15)52,./, Chern. Php  , 20, 700.McEwen, K. L.. 15)61../. CTiem. Php.. 84. 547.Myerstein, 1). anti Troiiim. , 15)61, TranK. Faraday Hoc., Lorulon, 67, 2104.Mooknrji, A and Tandon, S. P , 15)62, Ind. ./. Phys., 36. 211.Mooklierji. A and Tandon, IS. I’ , 15)65, Inil. .!■ Phyn.. 89, 137Mott, E. F. and Gurney. R- W., 15)4S, Eloctroiiic Procossos m Ionic crystals Oxford Cniversity Pruss, New York, 2nd Ed.Smith, G. P. and Boston. C. R„ 15)61. d. Chem. Phyn., 84, 135)6.Strickler, 8. .T„ 1961 I’li.l) Dissortal ions, Florida State Univorsil v-Stricklor, S. J. and Kasha, ,M.. 1961../. Gliem Phys.. 34, 1077.

Light Absorption in NO2 Ion in State of Solution (Part III- Effect of Cation)

http://arxiv.iacs.res.in:8080/jspui/bitstream/10821/2544/1/Light%20Absorption%20in%20NO2%20Ion%20in%20State%20of_A.Mookherji%20and%20S.P.Tandon.pdf

Light Absorption in NO2 Ion in State of Solution (Part III- Effect of Cation)

Abstract

Similar works

Full text

Available Versions

IACS Institutional Repository