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Indian Journal of C hemistry Vol. 39A, August 2000, pp. 792 - 801 MCSCF calculations on metal ion (Li+, Na+, Be2+) affinities of a few carbonyl molecules in the ground and 1,3 n1t:* excited states o Dey Adhikari , T Thakuria & C Medhi* , Department of Chemistry, Gauhati University, Guwahati 781014, India Received 16 Jun e 1997; accepted 9 No vember 1999 Metal affinities (MAs) of form al dehyde, acetaldehyde and formamide molecules have been calculated by MCSCF (OGM ) method. Ground state metal affinities of the mol ecules are larger th an excited state MAs. M n+ induced proximity effect is prominent in acetaldehyde-metal ion and formamide-metal ion complexes. Bond orders and net charge densities on different atoms of th e complexes are used to analyse geometrical relaxatio n du e to complex formation . Li+, Na+ and Be 2+ show spectral shift in th e i.3 nn * excited states. Introduction Interaction of group IA and group IIA metal ions with ligands is important in solution chemistry and transportation through biological membranes . There are reports on the binding of N2 and CO 2 with Na+. Hinton et ai.1 have calculated binding energies of Lt 2 with a number of molecules ; Gutmann established so lvent donicity and chemical shift due to Na+ in a series of solvents . Berthod et al. 3 calculated Li+, Na+, 46 Be 2+ affinities of NH, molecule. Rao et ai. - have observed shift in nrr* and rrrr* transi tions of carbonyl mol ecules due to metal ions. Marschoff et aC reported that M+ interaction with H 20 is a charge tran sfer interaction . However, experimental study on 2 solvation (Gutmann ) and variations of chemical shift due to environment do not give any direct clue to the dominating factor of such changes. Hence a systematic theoretical study on metal ion affinity towards carbonyl group along with the structural analysis in the ground and I" nrr* excited states on the basis of the MCSCF(OGM) method are presented in thi s paper. Theory Within the basic framework of single determinant LCAO-MO-SCF model have the following description in terms of a set of n occupied molecular spin orbitals (<1>;) which are linear combinations of say, N ( N~n) atom centered non-ortho ~o nal function s (Xj) : \If ( I ,2, .. . . ..... , n) =A { <l>j ... , ... •.. ... , <lln } <ll (<\>1<\>2' ... .. . .... <\>n)=X (XI ........ .... Xn) T, < \If I \If >= S , rST= I occ P = TT+, P = L njTjTj+ = 2P, j= 1 If all the n occupied spin orbitals are paired, P is a representation of the one-electron density matrix (OEDM) in the X basis. Clearly, this representation corresponds to the closed shell SCF model or the open shell unrestricted Hartee-Fock theory of the molecules depending upon the spin state. Within the limitations of the models mentioned above, one can define the atomic charge qA on an atom A in the molecules as the expectation value of an LCAOatomic density operator (ZA-q) 1-2 such that ZA being the nuclear charge of the atom A. <qA> is clearly the Mulliken gross charge density on the atom A and is an atomic invariant. The chemist's notion of the parti al charges on the atoms of a molecule can thu s be recovered from the OEDM represented in the X basis . Along with the so-called partia l charges on atoms in a molecule, a chemist's model of molecules involves the very useful notion of bond order between a pair of atoms in a molecule, a notion which practicall y corresponds to the notion of the multiplicity (or the number) of the bonds holding the pair of atoms together. The quantum chemical analogue of this quantity, the so-called quantum 793 ADHIKARI et af.: MCSCF CALCULATIONS ON METAL ION AFFINITIES Table I -Computed geometrical parameters of formaldehyde - Mn+( M n+== Li+, Na+, Be + ) complexes in the ground and t.3 mt * states. 2 Electro nic state System H2COLi+ H 2CONa+ 2 H2COBe + Bond lengths ( in A) 6 OM n+ CO CHI CH 2 ground 2.45 1.26 1.16 1.12 In1t* 2.47 1. 26 1.15 1.12 3 n1t * 2.45 1.27 1.16 1.12 Angles (in degr~s) CO H2CO q, 73.0 120.5 127.0 0.0 75.4 114.2 123.2 0.0 75.4 11 3.0 124.0 0.0 180.0 121.0 121.0 0.0 Mn+OC HI ground 1.94 1.26 1.12 1.12 In1t* 2.24 1.27 1.12 1.12 137.8 117.6 118.0 43.2 117.1 117. 1 45.0 J n1t * 2.21 1.26 1. 12 1.12 180.0 ground 1.66 1.28 1.12 1.12 180.0 118.9 118.9 0.0 116.9 116.9 12.2 116.7 116.7 45 .0 In1t* 1.91 1.26 1.13 1.13 180.0 J n1t * 1.90 1.27 1.13 l.I3 180.0 Table 2-Computed metal affinities (MA) of formaldehyde, acetaldehyde and formamide ( M n+== Li+, Na+, Be2+ ) in the ground and I.3 n1t* states Metal affi nities ( in a. u. ) Electronic states H 2CO-Li+ H2CO-Na+ H 2CO-Be 2+ Ground 0. 117 0.115 0.402 In1t* 0.062 0.060 0.342 3n1t * 0.070 0.063 0.330 CH 3CHO-Lt CH 3CHO-Na+ CH 3CHO-Be 2+ Ground 0. 164 0. 158 0.479 In1t* 0.075 0.060 0.350 3n1t * 0.065 0.055 0.343 NH 2CHO-Li+ NH 2CHO-Na+ NH 2CHO-Be 2+ Ground 0.268 0. 161 0.672 In1t* 0. 122 0.068 0.655 3n1t * 0. 149 0.063 0.667 Formaldehyde-Mn+ Acetaldehyde-M n+ Formamide-M n+ Table 3-Computed bond orders between different atoms and net charge densities on different atoms of formaldehyde-Mn+ ( M n+== Lt, Na+, Be 2+ ) complexes in the ground and I.3 n1t * states System Electronic H2CO-Li+ H2CO-Na+ H 2CO-Be + Net charge on OM n+ CO CHI CM n+ HIM n+ M n+ 0 C H ground 0.22 1.96 0.77 0.22 0.16 0.67 -0. 12 0.36 0.04 In1t' 3 n1t' 0.09 1.91 0.83 1.16 0.09 -0. 18 0.25 0.41 0.21 0. 11 1.60 0.79 0.22 0. 14 0.66 0.08 0.42 0.21 0.35 1. 90 0.95 0.72 -0.25 0.44 0.11 0.1 1 1.16 0.92 0.88 -0.08 0.41 0.2 1 0.10 1.81 0.89 -0.03 0.22 0.4 1 0.21 0.97 1.61 0.95 1.40 -0. 19 0.55 0.12 0.37 1.71 0.85 0.78 0.19 0.50 0.27 0.37 1. 70 0.78 0.20 0.49 0.28 grou nd In1t ' J 2 Bond orders state n1t ' ground I , n1t 3 n1t ' CO bond order in H2CO 0.85 n1t ' ground In1t' 3 2.030 1. 150 1. 190 794 INDIAN 1 CHEM, SEC. A, AUGUST 2000 -+---25·60 (b) H2CO-U+ compiex _._-- H2CO-Na+ complex - . - - H2CO-Be++ complex :J o t: ::. -25·80 ... 1/1 (]I .......t: W "0 -26·00 (5 I-- Fig. I-Interactions of M"+ (M"+ == Li+, Na+, 8e 2+) with formaldehyde molecu le in different electronic states. (a) H 2COLt in the ground and I.3 nrc* states. (b) H 2CO-Na+ in the ground and 3 nrc* states; H 2CO-Be 2+ in the ground and I.3 nrc * states. (c) H 2CO-Na+ in the I nrc* state. chemical bond order has been defined for systems representable by closed shell single determinant wave functions as follows : -25·40 ---+--- H2Co.U+ complex ------ H2CO • Na+ complex -t--- H2CO· Se++complex -25 · 60 A more general definition that covers open-shell (UHF) states has also been successfully used: -'25·80 d ; [b) ~ - 26 ·00 "* __- - - , - -- -- . - -- -, . ._ ---; ~ BAB thus defined has been an interesting statistical interpretation, viz., BAB measures the correlation between the fluctuation of atomjc charges on atoms A and B from their respective average values 9 . If the motions of electron are totally uncorrelated the bond order BAB vanishes. It may be noted that even at the single determinant level of representation the electronic motions are geared through Fermi (spin) correlation which thus seems to play an important role in shaping the magnitude of the quantum 11 cherrucal bond order (BAB)' In fact, Mayer8. identifies BAB with the normalization of the relevant diatorruc component of the exchange part of the reduced two electron den sity matrix. A qualitative description of the role pl ayed by Fermi-correlation in -2 5·40 ___ H2C O-LI'· co mplex .~ ___ - H2CO-N a:+ complex ~ ---t- - ., H2C O-Be++compiex t; (3 I- -25·60 ! -25, 80· Fig. 2-Plots of total energies versus O-M"+ distances of H 2COM"+, (M"+ == Li+, Na+, 8e 2+) complexes in the (a) ground, (b) Inrc* and (c) 3nrc* states. 795 ADHIKARl et at.: MCSCF CALCULATIONS ON METAL ION AFFINITIES 5 Table 4---Calculated chemical shifts for 1.3nn* transitions of forrnaldehyde-Mn+, acetaldehyde-Mn+ and forrnamide-M n+ ( M n+'" Li+, Na+, Be 2+ ) complexes determining bond orders was suggested even earlier . The idea of a molecular orb ital picture of valency was first introduced by Wibergl 2. If the gross population A of an AO bas is function (X Il ) on an atom A in the molecul e is qA Il ' the potential bonding power of that particular AO in the mo lecul e is supposed to be bAil =2 (q\) - Chemical shifts (in a.u.) System (qAlli 3nn H 2CO-Lt 0 . 140 0 . 150 H2CO-Na+ 0.074 0.070 H 2CO-Be 2+ Summing over, all the basis functions ce ntered on atom A we get what has been called valency (V A) of the atom A in the molecul e. * Inn* 0.063 0.032 C H 3CHO-Lt 0.063 0.049 C H 3CHO-Na+ 0.D25 0 .015 2 C H3CHO-Be + 0 .062 0.068 NH 2CHO-Li+ 0 .045 0.019 NH 2C HO-Na+ 0 .085 0.079 NH 2CHO-Be 2+ 0.010 0 .005 n 2 Table 5-Co mputed geomelrical parameters of aceta ldehyde - Mn+( M +'" Li +, Na+, Be + ) complexes in the ground and 1.3nn * states Electroni c Bond lengths ( in A) ,. OM n + CO CC CH, CHI ground 2.23 1.29 1.44 1.13 Inn* 2A7 2A5 1.30 1.1 3 1.29 IA4 IA3 gro und 1.90 1.29 Inn* 2. 14 1.28 3ni1 * 2.15 state Angles (in odegrees) <'> , 7 \ ceo CH 2C H 3 Mn+OC 1.1 2 1.13 163.0 125.1 5.5 1.1 5 1.13 95.3 124.2 5.0 1.1 3 1.15 1.13 98. 1 120.2 5.6 1.43 1.13 1.12 1.1 3 170.5 124.9 4.6 IA2 1.1 3 1.1 4 1.1 3 90.8 125.3 12A 1.30 IA2 1.14 1.1 4 1.13 88 .0 126.9 12. 1 IAI l AO lAO 1.1 3 1. 12 1.1 3 180.0 126.3 5.6 1. 13 1.14 1.13 180.0 121.3 5.0 1.1 3 1.14 1.1 3 180.0 127 .7 4.8 <I> H3CC HO-Li + 3nn * H3CCHO-Na+ H3CCHO-Be2+ ground 1.64 1.32 Inn* 1. 86 1.3 1 3nn * 1. 86 1.32 a: aldehydic Tab le 6- Computed bond orders between different atoms of acetaldehyde-M n+( M n+'" Lt, Na+, Be 2+ ) complexes in th e grou nd and 1.3 nn * states System Electroni c state Bond orders OM n + CI H, CIO C 2 HJ C IC 2 C Hs C 2 M n+ H,;M n+ HsC I C 2 H~ CHJCHO-Li + C H3C HO-Na+ CH3C HO-Be2+ Gro und I . nn Jnn ' OAO 1.74 0.93 1.11 0 .94 0.96 0. 11 1.58 0.87 1.09 0 .85 0 .91 0 . 17 0. 11 0. 12 1.57 0.88 1.09 0.84 0.92 0 . 18 0 .11 Ground 0.75 1.55 0 .94 1.15 0.90 0 .97 Inn' 0 .08 1.58 0 .86 1.11 0 .87 0 .93 0 . 10 0.08 1 . nn • 0.08 1.52 0 .87 1.1 3 0 .87 0 .93 0.11 0 . 11 Ground I . 1.11 1.31 0.94 1.24 0.89 0.96 OA6 1.39 0.84 1.19 0.92 0 .88 0.51 1.33 0. 82 1. 25 0.92 0.90 1l1t 1 ' nn • CO bond order in CH 3CHO a : aldehydi c 0. 12 0. 16 ground Inn' 3nn ' 1.903 1.076 1.114 796 INDIAN J CHEM, SEC. A, AUG UST 2000 Table 7--Computed net charge densities on different atoms of acetaldehyde - Mn+( M n+== Li +, Na +, Be 2+ ) complexes in the ground and I.3 mt * states System Electronic 0 Ca C Ha HI H2 H3 -0.22 0.41 -0.04 0.07 0 .66 0.07 0.07 -0.03 - 0 .23 0.13 0.48 0 .003 0.16 0.18 -0. 16 0.14 -0.23 0 . 12 0.47 0 .01 0. 15 0.19 0.16 0 .13 ground 0.76 -0 .33 0.43 - 0 .05 - 0.01 0.04 0 .08 0.07 In1t • 3 n1t " -0.15 0 .10 0.47 -0.11 0 . 15 0 .17 0. 15 0.12 -0 .16 0 .09 0.47 -0.004 0. 15 0 . 18 0 . 16 0.12 - 0. 13 0.38 0.46 0 .08 0.31 0 .27 0 .39 0.25 0 .72 0 .06 0 .51 -0.05 0.20 0.15 0 .2 1 0.19 0 .70 0.05 0.50 -0.03 0 .20 0 . 17 0 .22 0. 19 state CH.lCHO-Li' ground In1t ' ,. n1t CH 3CHO-Na+ CH, CHO-Be 2+ Net charge on M n+ Ground In1t " 3 n1t " a : aldehydic Table 8--Computed geometri cal parameters of formamide - Mn+( M n+== Li+, Na+, Be 2+ ) complexes in the ground and Electronic Bond lengths ( in state H2NCHO-Li + ground Imt * J n1t * H2NCHO-Na+ ground In1t* 3 n1t * H2NCHO-Be 2+ ground In1t* 3n1t * A) OM n+ CO CN CHa NH I NH2 " oc rvin+ 2.21 2.42 2.46 1.30 1.35 1.34 1.37 1.30 1.3 2 1.12 1.12 1.12 1.09 1.24 1.28 1.07 L08 1.08 1.9 1 2.09 2.24 1.29 1.30 1.33 1.36 1.3 2 1.33 1.12 1.13 I. II 1.08 1.08 1.09 1.65 1.81 1.92 1.31 1.33 1.31 1.40 1.36 1.37 1.13 1.14 1.14 1.13 1.13 1.14 1.3 n1t * states3 An gles (in degrees) ' .. < ..... OCN CNH I $ 106.2 100.1 97.2 122.1 114.9 115.8 12 1.7 11 3.0 114.4 0.0 34.0 32.0 1.07 1.07 1.07 100.0 100.9 89.2 124.0 120.6 126 .5 121.0 11 9.6 11 6. 6 0.0 26.3 22.1 1.08 1.10 1.09 81.2 76.6 75 .3 124.1 127.3 128 .2 12 1.4 11 6 .6 11 8. 7 0.0 24.2 33.1 a: aldehydic L ·t ••••••• 1 • •• • •••• ' 0. H'~"" , : :' /":'0 ",' . " /~-c;"" H4 ~ H3 H ;: Li+,Na+,Be2+ (a) (b) (c) (d) Fig. 3-Interaction s of M n+ (M n+ == Li+, Na+, Be 2+) with acetaldehyde molecule in different electronic states. (a) CH 3CHO-M n+ (M"+ == Li+, Na+, Be 2 +) in the ground state, (b) CH 3CHO-Li+ in the 1,3 n1t * state, (c) CH 3CHO-Na+ in the I.3 n1t * state and (d) CH3C HO-Be2+ in the 1.3 111t* state Wiberg's definition of valency was proposed in connection with ZDO theori.es of 1t electron systems. The ab initio generalization of the concept by Mayer is of a rather recent vintage. Mayer's9 definition of atomic valence (V AM) is a useful definition', which is a straightforward generalization of Wiberg's valence when the basis set of expansion is non-orthogonal. V AM = 2 L (PS) ILE A L (PS)f1v (PS)vf1 IlVEA If the system is in a open-shell state and represented by an UHF wave function, we have 797 ADHIKARl et at. : MCSCF CALCULATIONS ON METAL ION AFFINITIES -34'00 __ ~ ____ ~ Cfi;3CHO-U+ complex -+- ~ CH3CHO-Be++ complex H"" CH3CHO-Na+ compl.x ~~o :. ' ) . . .' . ............. .'::~, ~ -34·20 ,.,. HI ::l "C: . ~ III -34'40 M-"E Li+, Na+,Bel+ ~ c ., (a) UJ ~ M-".= Li+, Na+ -34·60 := (a) - 3 4 .8 O+..-'TT'TT. . . .--.-.-.--.-.--...,..,,..,..,.-rr-rr--,--,--,--.-, 2·00 lOO O-M n+ JOO 4'0{J (b) Fig, 5-lnteractions of M"+ (M"+ '" Li +, Na+, Be 2+) with formamide molecule in different electronic states, (a) NH 2CHO-M"+ (M o+ '" Li +, Na+, Be 2+) in the ground state and i.3 nn * states of NH 2CHO-Be h (b) NH 2CHO-M o+ (Mo+:; Li+, Na+ ) in the 1.3nn * state 0 Bond L~ng'h (A) P = pU + pP , pS _ _ - CH3CHO-Li+ complex _ _ - CH3CHO -Na+ complex -3400 ___ CH3CHO-BeH complex = pU _ pP It can be shown that this automatically leads to a residual or free valence (<I>A) for the atom A given by ~ s S <I>A = L ~ (P S)~v (P S)\,~ JJVEA -34 ·20 r:::l o I: III .~ (b) -34'60:f--,-~-.--,--,--,--,-,.....~-~-.--r-.--r'-----'---' -34,00 II --- CH3CHO-Li+ complex .. __ - CH3CHO-Na+ co mplex ~ \ __ -+-- CH3CHO-Be++ compl ex '~ - I: UJ a Clearly <I>A=O for any closed shell system at the HF level of description. With this theoretical background in mind we propose to examine the usefulness of the quantum chemical valence parameters in the context of structural modelling of some molecules in the ground and excited states. We represent the molecule in its lowest singlet and triplet excited states by the fo llowing wave functions (S=O corresponds to singlet, S= I to triplet) U\jli~j ~ I = 12 [( I</>,</>, - -I .. " </>i</>j .. " </>n</>n ) -J4 ·20 + (-1)\ I</>,¢, .. " </>j¢i , ... </>n¢n I)] The variational optImIzation of the orbitals <1>1 .. " ....... <1>n' which the so luti on of the master . 0 fM cweeny 13. equatIOn -3440 v = (hTP, + Z) = STA ". (I) (c) -34 6O t=====~~~o:;:_:-::_:-::_==_-=_~==:-===== ~_. ' .00 2·00 O-M n+ 300 4·00 and the CI problem requires one to solve the equation q Bond Lwglh (A) HC=CE Fig. 4-Plots of total energies versus O_M"+ di stances of CH 3CHO-M"+, (M"+ '" Li +, Na+, Be 2 +) complexes in the (a) ground (b) Inn* and (c) 3nn* states. Equation orthogonal .. . (2) is solved iteratively by a variant of gradient method'4.' 6 which involves 798 [NDIAN J CHEM, SEC. A, AUGUST 2000 construction of the following sequence iteratively until the desired degree of convergence is achieved The CI coefficients at convergence are used to construct the one-electron density matrix (PI) in the MO (<1» basis . Once PI is determined, the optimized orbital expansion coefficient (T) can then be condensed into a representation of the density matrix in the X basis as follows . For the simplified choice of 'l' where we have a fixed (invariant) set of doubly occupied orbitals (core) and set of singly occupied orbitals, P = pO +ps, pO represents the component of the OEDM coming from the doubly occupied orbitals and pS, the component contributed by the singly occupied orbital. The pO and pS matrices can now be used for the determination of the quantum chemical valence parameters B AB , V A , PA etc . Results and Discussion Formaldehyde and metal ion (M"+-=Lt, Na+, Bi+) interactions The optimised geometries of form,aldehyde - M"+ (Mn+-=Li+, Na+, Be 2+) are given in Table 1. Li+ interacts with the oxygen atom of the formaldehyde molecule at an angle <90 0 in the ground and l.3 n1t * excited states resulting decrease in C=O bond length. 2 Similarly, Be +, Na+ interact along the C=O bond axis 2 where O---Be + distance is found to be much less compared to other O ___ Mn+ distances. Unlike proton17 formaldehyde system , C=O bond length decreases in the metal-formaldehyde system. This may be due to charge transfer interaction between metal ion and oxygen lone pair electrons. The decreased electron density is compensated from the other atoms in the l8 molecule . So it is important to rationalize cation interacting power towards formaldehyde molecule and also Lewis acidities of the ions. From the computed metal affinities (MA) of formaldehyde (Table 2) in the ground and several excited states, Be 2+ affinity is found to be more than those of Li+ and Na+. Ground state MA is higher than excited state MAs. MA increases with each methyl substitution and agrees with reported data I. There is no good correlation between MA and net charge on oxygen atom in (he ground and excited states. Metal ion, on the other hand, increases C=O bond order and decreases C-H bond order in the · i.3 n1t * states of formaldehyde molecule (Table 3). The observation is found to be contrary to that in the case 17 of proton-formaldehyde system . The net charge densities (Table 3) together with the bond order data can be taken to imply the structures [Fig. I (a,b,c)] in the ground and i.3 n1t * excited states. It is observed · that considerable electron density has migrated from oxygen atom to metal ions. This stabilises the charge transfer interactions, resulting in substantial reduction of C-H bond order. The minimum energy plots corresponding to standard geometries with respect to Table 9---Computed net charge densities on different atoms of formamide - Mn+( Mn+;: Li+ , Na+, Be 2+ ) complexes in the ground and 1.3 nn:* states System NH 2CHO-Li+ NH 2CHO-Na+ NH 2CHO-Be 2+ a : aldehydic Electronic Net charge on state M n+ 0 C N Ha HI H2 ground 0.67 -0.32 OA7 -0. 18 0.03 0.16 0.18 Inn:' 0.62 -0.08 0.37 -0.01 0.05 -0. 12 0.16 3n n:' 0.6 1 -0.09 0.33 0.02 0.06 -0.10 0.17 ground 0.71 -0.34 0.45 -0.20 0.02 0.15 0.19 Inn:' -0.10 0.D3 0. 11 0.27 0.24 -0.09 0.03 OA6 OA7 -0.01 3nn:' -0.02 0. 11 0.27 0.23 ground 0.99 -0.13 0.53 -0.13 0.16 0.25 0.32 Inn:' OAI 0.16 0.51 0.04 0.22 0.33 0.34 3n n:' 0.44 0. 15 0.52 -0.01 0.24 0.33 0.33 799 ADHIKARI et a/.: MCSCF CALCULATIONS ON METAL ION AFFINITIES O---M+ distances are given in Figs [2(a,b,c)]. At the equilibrium geometry both in the ground and excited states total energies (T.E.) vs ro---8/+ curves are found to be steeper than the corresponding curves for Li+, Na+ ions, which reflect t~e stability of Be2+ ion complexed with the formaldehyde molecule in all the electronic states . It has been reported 19 that in aqueous solutions, stability of Li+ is less than that of Na+ and we obtained similar observations for these metal ions interacted with formaldehyde molecule also. We have analysed special shift (Table 4) in l.3n7t* transitions. Li+ gives large blue shift, whereas Be2+ and Na+ show a small shift4 in the 1,3 n1t * transitions. Acetaldehyde and metal ion (Mn+-=Li+, Na+, Be 2 +) interactions Study on the geometrical structure (Table 5) and energetics of Mn+ interaction with acetaldehyde shows that M n+ interacts with the oxygen atom of the carbonyl groupl . Metal affinities of the metal ions Lt, Na+ and Be2+ in the ground state are higher than excited state MAs and Be 2+ affinity is more than Lt and Na+ affinitie s (Table 2) . The computed Li+ affinities in the ground and l.Jn1t * states are found to be higher th an the corresponding Na+ affinities 4. The computed MAs of acetaldehyde are found to be higher than those of formaldehyde molecule in almost all the cases. These observation agree with the reported data that with each methyl substitution MA mcreases. Structural changes of acetaldehyde molecule in Mn+-acetaldehyde complexes are less prominent if one considers MA values. There is no good correlation between MAs and net charge density on oxygen atom in the ground and I.3 n1t * states for CH 3CHO-M n+ complexes. Minimum energy search for the position of M n+ relative to acetaldehyde molecule shows that C=O-Mn+(Mn+-=Lt, Na+) angles are less than 100° in all the exci ted electronic states (Table 5) . rO_Be2+ is much shorter than ro_ .. I./ and rO---N/ . Be2+ interacts linearly with the >C=O group. This may be due to strong 1t decolazation in >C=O group . Methyl group enhances such an effect. From the bond orders (Tab le 6) and net charge densities (Table 7) we analyse structural re-organi sations in acetaldehyde molecule due to M n+ ion s. Structure in Fig 3(a) is expected in the ground state of CH)CHOMn+ interactions-(Mn+-=Lt, Na+, Be 2+). In the I.3 n1t * states of CH 3CHO-Lt complex, Lt-C 2 and Lt-H5 bond orders ,are quite large and structure in Fig. [3(b)] is expected in the excited state. Similarly, in the CH 3CHO-N'a+ complex, Na+-O, Na+-C 2 bond orders are found to be the same in the I.3 n1t * states . Fig. [3(c)] is expected to contribute in these states . In the CH3CHO-Be2+ interaction, C-C bond order is much larger in the excited states than the single bond and expected structure is given in Fig. [3(d)]. Figs [,4(a, b, c)] show the correlation between total energy and rOoM n+ in different states. It is obvious that M n+ ions show small effect in I ,J n1t * states and agree with experimental findings4 . It appears that M n+ induced proximity effect Table I O--Computed bond orders between different atoms of formamide - Mn+( Mn+;: Li+, Na+, Be2+ ) complexes in the ground and 1.3 n1t * states System NH 2CHO.Li + NH 2CHO.Na+ NH 2CHO.Be2+ Electronic state OM n+ CO CN ground 0.3 5 1.62 1.24 CHa 0.93 0.07 'n1t' 0.19 1.07 1.80 0.94 0.06 3nn:' 0.19 1.10 1.73 0.93 0.06 ground 0.38 1.59 1.28 0.94 'n1t' 3 n1t ' 0.14 1.33 1.57 0.91 0.09 1.26 1.61 0.91 0.03 0.87 ground 0.80 1.40 1.15 0.92 0.28 0.72 'n1t' 3 n1t ' 0.40 1.22 1.42 0.86 0.23 0.68 0.31 1.33 1.34 0.82 0.21 0.70 CO bond order in NH 2CHO a : aldehydic Bond orders CM n+ ground 'n1t' 1.753 0.977 NM n+ Ha Mn + 0.87 NH2 0.94 0.01 0. 11 0.24 0.89 0.22 0.13 0.27 0.91 0.21 0.13 0.05 0.91 0.94 0.01 0.06 0.03 0.88 0.91 0.18 0.07 0.92 0. 17 0.07 0.88 0.15 0.42 0.86 0.00 0.42 0.87 0.00 0.43 NH, 3 n1t ' 0.998 800 INDIAN J CHEM, SEC. A, AUGUST 2000 contributes in producing the MA of acetaldehyde molecule. However, dielectric continuum may not be the appropriate model for metal ion affect in aprotic l4 solvent . Thorough evaluation of intermolecular interactions is required in large carbonyl molecules. Li+, Na+ and Be 2+ show blue shift in the 1.3 n1t * transition energies 20 (Table 4). -37045 -.-~ H2NCHO-Li+ complex -fI-~ H2NCHO-Na+ complex _~ H2NCHO-BeH complex -37-65 II! Formamide and metal ion (Mt/+ == Li+, Na+, Be 2+) interactions Geometrical parameters of NH 2CHO-M"+ (Table 8) and metal affinities of formamide molecule at the oxygen atom (Table 2) have been calculated for the metal ions Li+, Na+ and Be 2+. The calculated values provide information about the strength of metal carbonyl binding as well as geometrical changes due to metal ion. From the computed data (Table 2), Mn+ ion affinities of formamide molecule are found to be more than those of other carbonyl molecules 7 . It appears that extensive electronic reorganization must have occurred in the metal complex due to charge transfer interaction between oxygen atom and metal ion which, on the other hand, influences the electron density distribution in the other atoms of formamide molecule (Table 9). Alkali metals have strong affinity towards amide:! I and Li+ affinity is found to be more than Na+ affinity whereas Be 2+ affinity is the largest. In the Li+-formamide complex, Li+-Ha bond order is found to be much more than Li+-0 bond order in the excited states (Table 10). Small positive charge on H. and large negative charge on oxygen atom confirm these observations. Structure in Fig. 5(a) is expected in the ground state for all formamide-M"+ complexes and structure in Fig. 5(b) is expected in the 1.3 n1t * states of Li+ aIld Na+ complexes with formamide. But observations are different in Be 2+-formamide complexes in these states. In the Be2+-formamide complex, Be 2+-N bond order is found to be considerably large (almost half the Be2+-Q bond order in the ground state and greater than those in the excited states) (Table 10). It may be expected due to the large charge transfer interactions between oxygen atom and metal ion which influence the lone pair electron on nitrogen atom. This can be referred from the large negative charge on nitrogen atom, and structure identical to Fig. 5(a) is expected in the excited states. The presence of metal-oxygen charge transfer interaction and the absence of simple oxygen to metal (donor to acceptor) interaction may be the reason for the absence of correlation between Li+ .~ Ol :;; -38,05 ~ o o I- -38.25 -3 8·45-1' ...,.,..-.-:-,-,-"...,,....,.....,~--,---,-r..,....,,....,.,.,I-;-. .-'-;-...,-, . I 20'0 3·00 4·00 1· 00 O-M -37'40 n+ Bond 0 Length (A ) -+-- H;2NCH~U+ complex -e- ..... H2NCHO-Na+ complex --+-- HzNCHO-S.ei'+ comolex - 37·60 -37-80 -38 ·00. _ -38 ·20 ::r r:i ~~ -38 40 ~~~~~~~~~~~~~ (b) ·~ -37·'O .. __ - H;zNCHO-li+- compler w -.- ...... H:zNCHO-Be+-+ complex -+-- H24'JCHO-Na + complex c ...~ -376 (c) -38'4 ~+O.,...O~·C'T"'--.-r""'i~.00"-- ' , .. , rio n o- M -'"-Bond Lenglh (Ao) Fig. 6-Plot of total energies versus O_Mn+ distances of NH 2CHO-Mn+, (M n+ == Li+, Na+, Be 2+) complexes in the (a) ground, (b) 'n7t* and (c) 3n7t* states .. binding energy and Gutmann number of Li+l formamide system . It seems that theoretical calculations may be of great importance to classify geometrical relaxations due to complex formation which cannot be obtained experimentally. Like Mn+_ ) ADHIKARJ el al. : MCSCF CALCULATIONS ON METAL ION AFFINITIES fonnaldehyde and Mn+-acetaldehyde complexes, there is no good correlation between MA and net charge density on oxygen atom of the Mn+-fonnamide complexes. The plots of total energy vs rOoM n+ distance [Fig. 6(a, b, c)] reflect the stability of metal fonnamide complex compared to other carbonyl l compounds . From Table 4 we find that Lt and Na+ show favourable blue shift in Inn* state whereas in CH 3CHO-Be2+ complex indicates a small blue shift in 1,3 nn * states 20 • Conclusions From the computed data observations can be made: 3. References I 2 3 4 5 6 interesting 1. The observed affinities of the carbonyl molecules toward Li+, Na+ and Be 2+ ions are in the order: Be 2+> Lt> Na+. 2. Department of Atomic Energy, Mumbai for providing partial financial support. 7 following 8 9 10 II 12 13 Metal affinIty value is state dominant and ground state affinity is higher than excited state affinities for all the carbonyl molecules. IS Nature of the molecule is one of the factors, determining metal affinities . 16 4 . There is no good correlation between MA and net charge on oxygen atom of the carbony l molecules because of the proximity effect from fI neighbouring atoms of M + ion in the complex . 14 17 18 19 20 Acknowledgement CM acknowledges CSIR, New Delhi for financial support to carry out the work and DA thanks 801 21 Hinton J F, Beeler A, Harpool 0 & Briggs R W , Chern Phys Lett, 47 (1977) 411. Gutmann V , Fortcher Chern, Foech , 27 ( 1972) 59, Berthod H & Pullman A, Chem Phys Lett, 70 (1980) 434. Rao C N R, Rao K G & Reddy N V R, J Am chern Soc, 97 (1975) 2918. Gupt a A & Rao C N R , J phys Chern, 77 (1973) 2888. Rao C N R, UV alld visible speclroscopy-chem applications, (B utterworths, London), 1975. Almadeck S H, Marschoff C M , Cachau R E & Castu E A, Chem Phys Lett, 4 (1987) 133 . Mayer I, Chem Phys Lett, 97 ( 1983) 270. Mayer I, Th eoret chern Acta , 67 ( 1985 ) 315. Mayer I, lnt J quallt Chem, 29 (1986) 477. Mayer I, llIt J quant Chem, 29 (1986) 73. 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