of t D yas 7; ine of tem) and their O–Li counterparts were performed with complete geometry optimisations. The gas phase O–Li complexation turns out 6]. The applicability of the method depends on the Li+ ion hyde containing both electron withdrawing and electron releasing groups. Recently, the ground state basicities of unsaturated conjugated carbonyls, namely, crotonalde- unsubstituted to the substituted crotonaldehydes. 2. Computational details Calculations were performed using Gaussian 03W soft- ware [18] and B3LYP (DFT) method with 6-31G(d) basis * Corresponding author. Tel.: +91 03222 276554; fax: +91 03222 275329. E-mail address:
[email protected] (B.R. De). Journal of Molecular Structure: THEOC affinity of the samples.Since then Li+ ion affinities of per- fluorocarbons and other compounds have been determined [7–15]. Recently the Li+ ion affinities of a series of substi- tuted acetophenones have been studied theoretically [16]. The purpose of the present study is (1) to calculate the same for a series of aliphatic unsaturated conjugated sys- tems, viz., substituted crotonaldehydes which are carcino- genic, mutagenic air, water and food pollutants and this study may help their removal (2) to examine the relative lithium ion affinities (DE) of the substituted crotonalde- hyde, we have calculated the gas phase ground state Li+ affinities (DE) of a number of substituted crotonaldehydes by the hybrid B3LYP DFT method using Gaussian 03W program [18]. We have then analysed the computed Li+ affinity values to understand whether the pre-complex for- mation charge distribution local to the chromophore or post-complex relaxation of charge density or both are important in shaping the overall Li+ affinity of the croton- aldehydes. We have also looked into the possible origin of the small shift in the Li+ affinities as one goes from the to be the exothermic case and the local stereochemical disposition of the Li+ is found to be almost the same in each case. The presence of substituent is seen to cause very little change of the Li+ affinity (DE) relative to the unsubstituted crotonaldehyde. Electron releasing or electron withdrawing substituents change it by 0.002–0.01 hartree. Computed Li+ affinities are sought to be correlated with a number of computed system parameters such as the net charge on the Li+ and the carbonyl oxygen of the Li+ complexes and the net charge on the carbonyl oxygen of the free bases. The energetics, structural and electronic properties of the complexes indicate that the interaction between the Li+ ion and a carbonyl base is predominantly an ion–dipole attraction and the ion-induced dipole interaction as well rather than a covalent interaction. � 2007 Elsevier B.V. All rights reserved. Keywords: B3LYP DFT; GAUSSIAN; Crotonaldehyde; Charge distribution; Gas phase 1. Introduction Li+ ion attachment mass spectrometry was being widely used in quantitative analysis of the atmospheric species [1– a series of substituted crotonaldehyde were reported in the literature [17]. In an effort to understand the nature and origin of variation in the relative magnitude of the Li+ affinities (DE) to be expected in a series of aliphatic The Li+ affinities of a series in the ground sta S. Pandit, D. Department of Chemistry and Chemical Technology, Vid Received 20 March 200 Available onl Abstract DFT (B3LYP6-31G(d)) calculations of Li+ affinities on a series + 0166-1280/$ - see front matter � 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2007.05.039 substituted crotonaldehyde e: A DFT study e, B.R. De * agar University, Midnapore 721102, West Bengal, India accepted 30 May 2007 7 June 2007 substituted crotonaldehyde (aliphatic unsaturated conjugated sys- + www.elsevier.com/locate/theochem HEM 819 (2007) 160–162 sets. In all calculations complete geometry optimisation has been carried out on the molecules both before and after Li+ complex formation. Li+ affinities were computed as DE for the reaction Li+ + Bfi BLi+. 3. Result and discussion The molecules studied are listed in Table 1 along with their respective abbreviated names and total energies (har- tree) of free base (B) and the complex (BLi+) and computed Li+ affinities (DE). Table 2 reports the computed net charge on the carbonyl oxygen atoms of the free base molecules and the Li+ complexes both in their equilibrium ground state as well as the computed net charge carried out by Li+ at the equilibrium ground state of the Li+ complexes. have an opposite effect as expected. From Table 2 it is also clear that the charge density on O-atom before complex formation is higher (with the exception of ACR and AMCR where it is slightly lower) when x is an electron releasing group. This favours complex formation. The reverse is the case with electron attracting group. This may be one of the reasons of the slight increase and decrease of DE values relative to unsubstituted crotonaldehydes. 4. Conclusion From the present theoretical study it can be well con- cluded that the gas phase lithium ion affinity of crotonalde- AMCR,x = �NH(CH3) �325.8820 �333.2525 �0.0860 Molecule q � q þ S. Pandit et al. / Journal of Molecular Struct NCR,x = �NO2 �435.6087 �442.9662 �0.0730 CxCR,x = �CN �323.4654 �330.8300 �0.0801 OxCR,x = �OCL �765.9722 �773.3374 �0.0807 CX H C C H C O H H O Li From Table 2, it is seen that the computed net charge on the Li+ is medium in each case and is in the range 0.7707–0.7919 showing that some migration of electron density to the added Li+ has taken place. That this migra- tion is not local and originates from all over the molecule is clearly reflected in the computed net charges on the car- bonyl oxygen atoms of the Li+ complexes as seen from Table 2. The oxygen atom still carries a net negative char- ge,little increased relative to the free base molecules. The magnitudes of charges of the complexes indicate that the interaction between Li+ and the carbonyl base is predomi- nantly an ion–dipole attraction and ion-induced dipole interaction as well rather than a covalent interaction. This also shows that both pre- and post-complex correlations with local charge densities in the immediate neighbourhood of the complex formation site are weak. It can therefore be anticipated that the Li+ affinities of these carbonyl bases can not be modelled or described by local properties of the carbonyl moiety only.The charge on oxygen in ECR is highest and its lithium affinity is also highest,the same in NCR is lowest and its lithium affinity is lowest in the ser- ies.This correlation is not followed by other substituents. Therefore, the lithium affinity of the series must be shaped Table 1 Computed total energies (hartree) of free base (B) and Li+ complex (BLi+) and lithium ion affinities ½DE ¼ ðEBLiþ � EB � ELiþÞ; hartree� at the equilibrium geometry of the ground state ðELiþ ¼ �7:284905 hartreeÞ by B3LYP 6-31G(d) method Molecule Total energy DE B BLi+ CR,x = �H �231.2342 �238.6062 �0.0875 ECR,x = �C2H5 �309.8602 �317.2347 �0.0900 ACR,x = �NH2 �286.5727 �293.9441 �0.0869 H H H C H X C C H C H strongly by distant atom contribution in addition to the contribution from the carbonyl group. The local characteristics at or around the carbonyl moi- ety are very nearly identical in each case. This is revealed from the data reported in Table 3 where some of the selected computed geometrical parameters are listed. The O–Li+ bond length has a variation in the range 1.73– 1.75A˚ for all the substituted complexes. The C–O–Li+ bond angle is in the range 170–173� in all the cases. Simi- larly the torsion angle s (C–C–O–Li+) shows a small vari- ation (176.9–182.1�) The carbonyl ring near invariant stereochemistry round the complex formation site of each base tends to suggest that the entire contribution from sub- stituent effects to Li+ affinity (DE) can not be modelled properly unless contribution from far away centres are taken into account. The C–X bond length is little decreased (with the exception of CR where it is slightly increased) in each case upon lithium ion complexation. From Table 1 it is seen that the DE values of all the substituted crotonaldehydes are in the range �0.07 to �0.09 hartree. The DE for the unsubstituted base is �0.087 hartree indicating that the gas phase Li+ complex formation turns out to be exothermic in each case. The electron releasing substituents are seen to increase the com- puted DEs (with the exception of ACR and AMCR where it is slightly decreased) while electron withdrawing groups O Li B BLi+ CR �0.4153 �0.5257 0.7793 ECR �0.4158 �0.5288 0.7757 ACR �0.4143 �0.5281 0.7755 AMCR �0.4164 �0.5317 0.7707 NCR �0.3967 �0.5159 0.7901 CxCR �0.3990 �0.5118 0.7919 OxCR �0.3997 �0.5188 0.7874 Table 2 Computed net charge on O-atom ðqO�Þ of free base (B) and Li+ complex (BLi+) and computed net charge ðqLiþÞ on lithium in the equilibrium ground state of the complex (BLi+) and free base (B) by B3LYP 6-31G(d) method ure: THEOCHEM 819 (2007) 160–162 161 hydes and its para-substituted counterparts is spontaneous irrespective of their electron releasing or withdrawing nature. The electronic properties of the complexes indicate that the interaction is predominantly an ion–dipole attrac- tion and ion-induced dipole interaction as well rather than a covalent interaction. The overall reactivity is fully explained by distant atom contribution in addition to the contribution from the carbonyl group. References [13] E. Buncel, A. Chen, M. Decouzon, S.A. Fancy, J.-F. Gal, M. Herreros, P.-C. Maria, J. Mass. Spectrom. 33 (1998) 757. [14] Y.-N. Su, M.-S. Tsai, S.-Y. Chu, Int. J. Quant. Chem. 59 (1996) 487. [15] I. Corral, O. Mo, M. Yancz, Int. J. Mass Spectrom. 255 (2006) 20. [16] U. Senapati, D. De, B.R. De, J. Mol. Struct. (Theochem) 808 (2007) 157. [17] S. Pandit, D. De, B.R. De, J. Mol. Struct. (Theochem) 760 (2006) 245. [18] (a) M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Table 3 Geometrical features of the free base (B) and the complex (BLi+) (Length in A˚ and angle in �) by B3LYP 6-31G(d) method Molecule Free base (B) r(C–X) Complex (BLi+) r(C–X) r(O–Li+)