INTRODUCTION:

The organotin(IV) chemistry has drawn considerable attention to the researchers in recent decades owing to their structural diversity and wide range of synthetic, industrial and biological applications.¹ Organotin(IV) derivatives of ligands containing hetero donor sites holds importance as potential biologically active metallopharmaceuticals owing to their unique structural features and also due to their potential biological applications as anti-tumour,^2-5 and antiviral activity.²

In the last decade, several of the di- and triorganotin(IV) derivatives of dipeptides have been modelled for metal-protein interactions and also been shown to exhibit wide range of biological activities.^6-11These studies have motivation to understand the electronic properties of such derivatives, so as to formulate a theoretical basis for the experimental observations. In the contemporary research, the density functional theory (DFT) based quantum-mechanical methods holds special significance in order to understand the detailed electronic structure of the molecules and to calculate the properties, so as to correlate the structural features with the probable biological activity.

The DFT has been successfully utilized to account for the experimental observations for several organotin(IV) derivatives with hetero donor atoms.^12-14As a part of our systematic efforts to study the electronic structure of organotin(IV)-peptide system,¹⁵ the present study reports the DFT based quantum-mechanical calculations on the previously synthesized di-n-butyltin(IV) derivative of glycyltryptophane (Figure 1).¹⁶

Figure 1: Structure (along with atom number) of (a) gly-trp (H₂L), and (b) di-n-butyltin(IV) derivative of gly-trp.

COMPUTATIONAL DETAILS:

All the quantum chemical calculations have been performed using the Gaussian 09 program package.¹⁷The molecular geometries of glycyl-tryptophane (H₂L) and n-Bu₂SnL were fully optimized in the gas phase at the DFT level using the B3LYP functional which is a combination of Becke’s three parameter (B3) gradient corrected hybrid exchange functional,¹⁸ with the dynamical correlation functional of Lee, Yang and Parr (LYP).¹⁹All the atoms except Sn were described by 3-21G basis set, and the Sn atom was described by LANL2DZ basis set in which Sn inner shells are described by effective core potential ECP46MWB (1s²2s²2p⁶3s²3p⁶3d¹⁰4s²4p⁶4d¹⁰) along with the basis set (3s3p)/[2s2p].²⁰The geometry optimization was carried out through an algorithm of iterative steps to locate true global minima on the potential energy surface (PES) with default parameters for convergence is met. The absence of an imaginary frequency in a harmonic frequency calculation carried out at the same level of theory indicates that the calculated geometry is a true global minimum on the PES. The optimized geometrical parameters and the atomic charges at all the atoms in the studied systems were calculated using Mulliken Population Analysis (MPA), Hirshfeld Population Analysis (HPA), and Natural Population Analysis (NPA) at the same level of theory. The energies of frontier molecular orbitals and conceptual-DFT based global and local reactivity descriptors based on MPA, HPA and NPA have been calculated using finite difference approximation.^21,22The visualization of all results have been performed using Gauss View 5.0.²³

RESULTS AND DISCUSSION:

Geometry Optimization:

The ground state geometry optimization and harmonic frequency calculations (in the gas phase) of n-Bu₂Sn(IV) derivative of gly-trp (H₂L) was achieved using DFT at B3LYP/3-21G/LANL2DZ(Sn) basis set. The ground state optimized geometry in the gas phase for n-Bu₂SnL is presented in Figure 2 (cf. Figure 1 for atom number notation). The selected bond lengths and bond angles in the studied system are presented in Table 1.The standard 3-21G basis set accurately predicts the geometric parameters for organic part of the compounds, and the LANL2DZ pseudo-potential sufficiently accounts for the relativistic effects on the tin(IV) atom. The calculated C-Sn, O-Sn, N-Sn and N®Sn bond lengths are comparable to the reported bond lengths for various diorganotin(IV) derivatives which possess coordination of organotin(IV) moiety with the hetero donor atoms.^10,12-14,24The distortion of the molecule is evident from the axial angle N_amino-Sn-O17 of 154.47°, as reported for other diorganotin(IV)-dipeptide system.^10,24The calculated geometric parameters suggest that the complex adopts distorted trigonal bipyramidal arrangement around the tin(IV) atom with two n-butyl groups and peptide nitrogen occupying equatorial positions and the amino nitrogen and carboxylic oxygen atoms occupying the axial positions, as reported for other diorganotin(IV) derivatives of dipeptides.^{7,8,10,16,24,25}

Figure 2: Ground state optimized geometry (in gas phase) of n- Bu₂SnL calculated at B3LYP/3-21G/LANL2DZ(Sn) level of theory.

Table 1: Selected bond lengths (Å) and bond angles (0) inn-Bu₂Sn(IV) derivative of gly-trp (H₂L)in the gas phase.

Parameter^a	n-Bu₂SnL
Parameter^a	B3LYP/3-21G/LANL2DZ(Sn)
Bond Length
Sn-C34	2.152
Sn-C37	2.154
Sn-O17	2.010
Sn-N_peptidic	2.064
Sn-N_amino	2.339
N_amino-C3	1.506
N_peptidic-C5	1.356
O17-C13	1.356
CO_peptide	1.229
Bond angle
C34-Sn-C37	120.60
C34-Sn-N_peptidic	117.85
C37-Sn-N_peptidic	119.52
N_amino-Sn-N_peptidic	74.73
N_amino-Sn-O17	154.47
N_peptidic-Sn-O17	80.03

Thermo Chemical Studies:

The various energetic and thermo chemical parameters for n-Bu₂SnL in the gas phase have been obtained at 1 atm and 298.15 K at B3LYP/3-21G/LANL2DZ(Sn) level of theory, and the results are presented in Table 2. The calculated molecular mass for n-Bu₂SnL is 491.13834 a.m.u.. Since, for n-Bu₂SnL T >>q_r (Table 2), it will show classical behavior. Further, for n-Bu₂SnL the calculated zero point vibrational energy (ZPVE) in gas phase, which is the difference in the energy of the bottom of the internuclear potential energy well (V_BOT) and the energy of the vibrational ground state (v = 0) is 315.16 Kcal/mol. Furthermore, the total energy of n-Bu₂SnL in gas phase has been calculated after zero-point correction, and thermal correction to energy, enthalpy and Gibbs free energy, and the results (Table 2) indicate that all of the electronic energy of n-Bu₂SnL is exothermic. For instance, the calculated total energy of n-Bu₂SnL after thermal correction to Gibbs free energy is -1205.282048 a.u. The electronic heat capacity and the internal thermal energy due to electronic motion are both zero, because electronic partition function does not contain temperature dependent terms. The value of E_tot, S_tot and CV_tot for n-Bu₂SnL in gas phase is 334.246 Kcal/mol, 202.253 Cal/Mol-Kelvin and 111.929 Cal/Mol-Kelvin, respectively (Table 2). As evident from the results, the maximum contribution to E_tot, S_tot and CV_tot of n-Bu₂SnL in gas phase is due to vibrational motion of the molecule. Further, the individual contribution due to each component to the partition function has also been calculated for n-Bu₂SnL in gas phase. The results indicate that the maximum contribution is due to vibrational partition function computed with the zero of energy being the bottom of the well (q_{vib, BOT}).

Table 2. Calculated thermo chemical parameters for the ground state optimized geometries (in gas phase) of n-Bu₂SnL.

Parameter	Level of Theory
Parameter	B3LYP/3-21G/ LANL2DZ(Sn)
Molecular mass (a.m.u.)	491.13834
Rotational temperature^a (q_r) (Kelvin)	0.01096 0.00365 0.00331
Rotational constant^b (GHz)	0.22836 0.07600 0.06899
ZPVE^c (kcal/mol)	315.32
Zero-point correction^d (Hartree/Particle)	0.502497
Thermal correction to Energy^e (Hartree/Particle)	0.532671
Thermal correction to Enthalpy^f (Hartree/Particle)	0.533615
Thermal correction to Gibbs free energy^g (Hartree/Particle)	0.437376
Sum of electronic and zero-point energies^h(a.u.)	-1205.216927
Sum of electronic and thermal energiesⁱ(a.u.)	-1205.186753
Sum of electronic and thermal enthalpies^j(a.u.)	-1205.185809
Sum of electronic and thermal free energies^k (a.u.)	-1205.282048
log₁₀Q^l q^E q^T [log₁₀(q^T)] q^R [log₁₀(q^R)] q^{V, BOT}[log₁₀(q^{V, BOT})] q^{V, v = 0} [log₁₀(q^{V, v = 0})]	0 8.631261 7.399348 -201.178396 29.953792
E_tot^m (kcal/mol) (E_tot = E_elec + E_trans + E_rot + E_vib)	334.256 (0 + 0.889 + 0.889 + 332.479)
S_totⁿ (Cal/Mol-Kelvin) (S_tot = S_elec + S_trans + S_rot + S_vib)	202.553 (0+ 44.462 + 36.838 + 121.253)
CV_tot^o(Cal/Mol-Kelvin) (CV_tot = CV_elec + CV_trans + CV_rot + CV_vib)	111.929 (0 + 2.981 + 2.981 + 105.968)

^a, where, h is Planck’s constant, I is moment of inertia, and k_B is Boltzmann constant; ^b, where h and I have their usual meaning; ^cZero point vibrational energy; ^dZero-point energy (E_ZPE), is the vibrational energy which a molecule possess in the vibrational ground state (v = 0); ^eCorrection to internal thermal energy (E_tot= E_elec + E_trans +E_rot + E_vib); ^fH_corr = E_tot + k_BT, and it includes zero-point energy; ^gG_corr= H_corr –TS_tot, where, S_tot= S_elec + S_tran + S_rot + S_vib, and includes ZPE; ^hSum of electronic and zero point energy (= E₀ + E_ZPE); ⁱSum of electronic and thermal energies (= E₀ + E_elec + E_transl + E_rot + E_vib); ^jSum of electronic and thermal enthalpies (= E₀ + H_corr); ^kSum of electronic and thermal free energies (= E₀ + G_corr); ^lQ is total molecular partition function, where q(V,T) is the partition function for the corresponding component i.e., electronic (q^E = w₀, where w₀is the electronic spin multiplicity of the molecule ), translational (q^T = , where m is the mass of the molecule, and k_B, h, P and T have their usual meaning), rotational (q^R = , where I is the moment of inertia along three axis, s_R is the symmetry number), vibrational computed with the zero of energy being the bottom of the well (q^{V, BOT} = ), and vibrational computed with the zero of energy being the first vibrational level (q^{V, v =
0} = ); ^mInternal thermal energy with contribution from electronic, translational, rotational and vibrational motion, respectively, as E_elec = 0 (because partition function doesn’t contain temperature dependent term); E_trans = ; E_rot = ; ; ⁿEntropy with contribution from electronic, translational, rotational and vibrational motion, respectively, as S_elec = Rlnq^E = Rlnw₀, where w₀ has its usual meaning; S_trans = R(lnq^T + ); S_rot = R(lnq^R + ); and S_V = ; ^oConstant volume molar heat capacity, with contribution from electronic, translational, rotational and vibrational motion, respectively, as CV_elec= 0, CV_trans = , CV_rot = , and CV_vib = .

Atomic charges:

In molecular systems, the attribution of net atomic charges is allowed by population analysis, though the atomic charge is not a physical reality. In order to understand the probable interaction of n-Bu₂SnL with macromolecular receptors, an electron density distribution analysis has been performed on the basis of atomic charges determined by three different population schemes viz., MPA, HPA and NPA in the gas phase within the n-Bu₂SnL at B3LYP/3-21G/LANL2DZ(Sn) level of theory. The calculated atomic charges based on MPA, HPA and NPA at the selected atoms in n-Bu₂SnL are presented in Table 3. The population of the atomic orbitals suggest that in n-Bu₂SnL the natural electron configuration of central Sn atom is [core]5s^0.865p^0.996p^0.02, which differs significantly from n-Bu₂Sn(IV)²⁺cation configuration [core]5s². The absolute value of the natural charge of Sn in n-Bu₂SnL is about 2.133 (Table 3) on the basis of NPA, whereas on the basis of MPA and HPA the charge is about 1.454 and 0.701 (Table 3), respectively. The NPA charge suggests that charge transfer is very minor and interaction of n-Bu₂Sn(IV) moiety and dipeptide unit is mostly ionic. Similar, results are reported for diorganotin(IV) systems with ligands having hetero donor atoms.^12-14 Further, the natural electron configuration of coordinating atoms in dipeptide unit viz., N1, N9 and O17 is [core]2s^1.432p^4.48, [core]2s^1.352p^4.463p^0.01and [core]2s^1.702p^5.06, respectively. The absolute value of natural charge of N1, N9 and O17 on the basis of NPA is -0.909, -0.817 and -0.761, respectively (Table 3), whereas the charge on these atoms on the basis of MPA is about -0.073, -0.793 and -0.632, respectively, and on the basis of HPA is about 0.143, -0.220 and -0.329, respectively. Moreover, the natural electron configuration of two carbon atoms (covalently bonded to the central Sn atom) viz., C34 and C37 is [core]2s^1.182p^3.813p^0.01. The absolute value of natural charge of C70 and C73 on the basis of NPA is -1.001 and -1.000, respectively (Table 3), whereas the charge on these atoms on the basis of MPA is about -0.301 and -0.322, respectively, and on the basis of HPA is about -0.107 and -0.119, respectively. The existence of such oppositely charged centres around the central Sn atom further confirms the ionic interaction in the Sn-O and Sn-N bonds.

The results (Table 3) indicate that the most negative atomic charges are attributed to oxygen, nitrogen and organotin(IV) carbon atoms in n-Bu₂SnL derivative. Further, in the trigonal bipyramidal arrangement of ligand around n-Bu₂Sn(IV) moiety, the charge density increases on all the coordinating atoms viz. carboxylic oxygen atom (upon deprotonation), amino nitrogen atom and peptide nitrogen atomupon coordination to the tin(IV) atom. Furthermore, the calculated charge on the selected atoms in the neutral complex is observed probably due to the high positive charge of tin(IV) atom and also due to the shift of electron density towards coordinating atoms, as reported for other organotin(IV)-hetero donor system.^12-14

Table 3. Atomic charge (a.u.) at the selected atoms for neutral (N), anionic (N+1) and cationic (N-1) species of n-Bu₂SnL, based on MPA, HPA and NPA charge schemes calculated at B3LYP/3-21G/LANL2DZ(Sn) level of theory.

Atom (k)^a	MPA^b,c			HPA^d			NPA
Atom (k)^a	q_N(k)	q_N+1(k)	q_N_-₁(k)	q_N(k)	q_N+1(k)	q_N_-₁(k)	q_N(k)	q_N+1(k)	q_N_-₁(k)
Sn	1.4536	1.0542	1.2676	0.7007	0.5224	0.7016	2.1332	1.6600	1.9824
C34	-0.3008	-0.3489	-0.2503	-0.1071	-0.1960	-0.0784	-1.0011	-0.9773	-0.9401
C37	-0.3218	-0.3535	-0.2496	-0.1194	-0.1760	-0.0775	-1.0003	-0.9794	-0.9578
N1	-0.0733	-0.1148	-0.0401	0.1425	-0.0242	0.0850	-0.9089	-0.8335	-0.8518
C3	0.1597	0.0689	0.2017	0.1122	0.0376	0.1405	-0.3543	-0.3351	-0.3338
C5	0.6585	0.5256	0.5637	0.1517	0.1262	0.1807	0.6071	0.5304	0.5915
O8	-0.5023	-0.5217	-0.4502	-0.2890	-0.3072	-0.2295	-0.5598	-0.5770	-0.4997
N9	-0.7929	-0.6152	-0.6353	-0.2204	-0.2529	-0.2246	-0.8168	-0.7768	-0.7769
C10	0.1366	0.0382	0.0707	0.0685	0.0261	0.0608	-0.1667	-0.1879	-0.1780
C13	0.6775	0.5912	0.6367	0.1853	0.1748	0.2056	0.7220	0.6696	0.6914
O17	-0.6316	-0.5910	-0.5352	-0.3285	-0.3391	-0.2841	-0.7608	-0.7306	-0.6759
O18	-0.4902	-0.5069	-0.4420	-0.2771	-0.2798	-0.1990	-0.5391	-0.5602	-0.4673
C16	-0.0128	0.0257	0.0496	-0.0559	-0.0464	0.0483	-0.1283	-0.0938	0.0553
N22	-0.5032	-0.3782	-0.2616	0.0639	-0.0203	0.0987	-0.5617	-0.5938	-0.5232

^aAtom number as represented in Figure 1; ^bMulliken charges with hydrogens summed into heavy atoms; ^cN is the number of electrons for neutral (N), anionic (N+1) and cationic (N-1) species; ^dHirshfeld charges with hydrogens summed into heavy atoms.

Conceptual-DFT based global and local reactivity descriptors:

The molecular properties and conceptual-DFT based global reactivity descriptors for the ground state optimized geometries in the gas phase of n-Bu₂SnL, calculated using finite difference approximation are presented in Table 4. The results indicate that the dipole moment that accounts for the existence of charged separated regions within the system is high for n-Bu₂SnL (9.344 Debye). Since, the band gap (DE) measures the stability and reactivity of the system,^21,26 therefore the observed DE value (14.828 eV) indicates that the n-Bu₂SnL complex is stable. The frontier molecular orbital analysis for n-Bu₂SnL has been performed through the Koopman’s approximation within the molecular orbital theory.^21,26 The E_HOMO and E_LUMO energies for n-Bu₂SnL in gas phase are presented in Table 4. The E_HOMO and E_LUMO plots along with the band gap (DE) for n-Bu₂SnL in the gas phase are presented in Figure 3. As evident from the HOMO and LUMO plots (Figure 3), in n-Bu₂SnL, the HOMO is concentrated over the dipeptide unit, whereas the LUMO is concentrated over the Sn centre. The global reactivity descriptors calculated based on frontier molecular orbital analysis are also presented in Table 4. The global hardness (h) is a global property that can be regarded as a resistance to charge transfer, whereas global softness (S) measures the ease of charge transfer and is associated with high polarizability.²⁶The value of h for n-Bu₂SnL is 14.8277 eV, whereas the value of S is 0.0674/eV. Since finite difference approximation to global hardness equates it to band gap, therefore these results indicate that n-Bu₂SnL with large band gap is hard and not easily polarizable. Moreover for n-Bu₂SnL, the electrophilicity index (w), which measures the electrophonic power of a system,²⁷and which can be described as the maximum ability of a molecule to accept electrons in the neighborhood of an electron reservoir,²⁸ has value 0.3126 eV. Since, w is positive, therefore the charge transfer is an energetically favorable process, and assuming the system as electron donor, the complex has a greater electrophonic power during partial electron transfer to an electron acceptor.

The conceptual-DFT based local reactivity descriptors, which are considered to predict the regioselectivity of the atoms in n-Bu₂SnL, have been calculated at all the atoms with three different population analysis schemes viz., MPA, HPA and NPA. The results based on NPA scheme at the selected atoms of n-Bu₂SnL are presented in Table 5. The Fukui function measures the reactivity towards the nucleophilic attack on the system, its large value at a particular site indicates that the site is capable of accepting electron density, similarly, since the Fukui function measures the reactivity towards an electrophilic attack on the system, its large value at a particular site indicates that the site is an electron donating site.²¹ Further, Parr and Yang had proposed that larger value of Fukui function indicates more reactivity,²⁹ and hence, for a particular atomic centre in the molecule, greater the value of the condensed Fukui function, more reactive it will be.²⁶ As evident from the results (Table 5), in n-Bu₂SnL, the trends based on the values of , and with NPA scheme suggest that Sn is the most reactive site towards nucleophilic attack. The results obtained are in accordance to the frontier molecular orbital analysis, which indicates that in n-Bu₂SnL the HOMO is concentrated around the coordinating atoms in the dipeptide moiety and LUMO is concentrated around the central Sn atom. These results outline the general behavior of the central Sn atom in the organotin(IV) complexes in accordance to the previously reported experimental observations, that R₂Sn(IV)²⁺ is an active species in the biological medium.^1,30The two variants of the hardness potential viz., the electrophilic hardness potential () and nucleophilic hardness potential (), measures the reactivity towards an approaching nucleophilic and electrophilic reagent, respectively. These descriptors have also been calculated at all the atoms of n-Bu₂SnL, and the values for these two descriptors at the selected atoms are presented in Table 5.

A nucleophilic attack on an atom increases the electron density on it, resulting in a higher absolute value of V_el^N+1(k) thereby making a positive quantity. Normally, the higher the value of at an atom, higher should be the reactivity (i.e., electrophilicity) of that atom towards an approaching nucleophile.³¹Similarly, in case of an electrophilic attack on an atom the electron density gets reduced over it, resulting in a positive value of as the absolute value of V_el(k) of the atom decreases. Also, higher the positive value of for an atom, higher should be the reactivity (i.e., nucleophilicity) of that atom towards an approaching electrophile. As evident from the results (Table 5), in n-Bu₂SnL, the trends based on the values of suggest that O18 is the most reactive site towards an approaching nucleophilic reagent. Similarly, the trends based on the values of suggest that, in n-Bu₂SnL N1 is the most reactive site towards an approaching electrophilic reagent.

The relative electrophilicity helps to locate the preferable site (or atom) in a molecule for a nucleophilic attack on it and it is the electrophilicity of any site as compared to its own nucleophilicity, whereas the relative nucleophilicity helps to locate the preferable site (or atom) in a molecule for an electrophilic attack on it and it is the nucleophilicity of any site as compared to its own electrophilicity.³²As evident from the results (Table 5) in n-Bu₂SnL, O8 has maximum relative nucleophilicity (3.5073), whereas O18 has maximum relative electrophilicity (0.2943).The dual reactivity descriptor (,³³ a selectivity index is able to characterize both the nucleophilic and electrophilic behaviours. As evident from the results (Table 5), in n-Bu₂SnL among the selected atoms Sn atom has the most positive value (16.98 eV). The positive value of indicate that the central Sn atom is favoured for the nucleophilic attack, whereas negative value of indicate that the coordinating atoms are favoured for an electrophilic attack.

Table 4. Calculated molecular properties and conceptual-DFT based global reactivity descriptors for the ground state optimized geometries (in gas phase) of n-Bu₂SnL at B3LYP/3-21G/LANL2DZ(Sn) level of theory.

Parameter / Property	System(n-Bu₂SnL)
E_N(a.u.)^a	-1205.719424
E_N+1(a.u.)	-1205.558859
E_N_-₁(a.u.)	-1205.335086
Dipole moment (Debye)^b	9.3435
I.P. (eV)^c	10.4585
E.A. (eV)^d	-4.3692
DE (eV)^e	14.828
E_HOMO (eV)^f	-10.4585
E_LUMO (eV)^g	4.3692
Electronic chemical potential (m) (eV)^h	-3.0446
Electronegativity (c) (eV)ⁱ	3.0446
Global hardness (h) (eV)^j	14.8277
Global softness (S) (/eV)^k	0.0674
Electrophilicity index (w) (eV)^l	0.3126

^aE_N, E_N+1 and E_N-1 are the total energies of the system containing respectively, N, N+1 and N-1 electrons; ^bDipole moment for the system containing N number of electrons; ^cI.P. is the ionization potential given by E_N1- E_N; ^dE.A. is the electron affinity given by E_N-E_N+1; ^eΔE is the band gap given by I.P.-E.A; ^fEnergy of the highest occupied molecular orbital as -E_HOMO = I.P.; ^gEnergy of the lowest unoccupied molecular orbital as -E_LUMO = E.A.; ^hElectronic chemical potential of the system given by (); ⁱElectronegativity of the system given by –µ; ^jGlobal hardness of the system given by E_LUMO- E_HOMO; ^kGlobal softness of the system given by 1/ɳ; ^lElectrophilicity index of the system given by µ²/2ɳ.

Figure 3: Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) plots along with band gap (ΔE) in the gas phase for n-Bu₂SnL calculated at B3LYP/3-21G/LANL2DZ(Sn) level of theory.

Table 5. Conceptual-DFT based local reactivity descriptors at the selected atoms in n-Bu₂SnL, based on NPA scheme calculated at B3LYP/3-21G/LANL2DZ(Sn) level of theory.^a

Atom (k)^b	Fukui function^c,d		Local softness^e		Local electrophilicity^f		Hardness Potential^g		Relative nucleo- philicity^h	Relative electro- philicityⁱ	Dual descriptor^j
Atom (k)^b	f⁺(k)	f^-(k)	s⁺(k)	s^-(k)	w⁺(k)	w^-(k)	D⁺h(k)	D^-h(k)	Relative nucleo- philicity^h	Relative electro- philicityⁱ	Dual descriptor^j
Sn	12.8771	-4.1030	0.8679	-0.2765	4.0254	-1.2826	-6.1474	14.3169	-0.3186	-3.1385	16.9800
C34	-0.6476	1.6613	-0.0437	0.1120	-0.2025	0.5193	4.2411	2.7756	-2.5651	-0.3898	-2.3089
C37	-0.5679	1.1540	-0.0383	0.0778	-0.1775	0.3608	1.2579	5.2702	-2.0321	-0.4921	-1.7220
N1	-2.0518	1.5532	-0.1383	0.1047	-0.6414	0.4855	-50.6332	57.7100	-0.7570	-1.3210	-3.6050
C3	-0.5206	0.5578	-0.0351	0.0376	-0.1627	0.1744	-32.2478	38.9233	-1.0716	-0.9332	-1.0784
C5	2.0871	-0.4259	0.1407	-0.0287	0.6524	-0.1331	-29.3199	36.9273	-0.2040	-4.9010	2.5130
O8	0.4667	1.6368	0.0315	0.1103	0.1459	0.5117	-13.1179	20.6879	3.5073	0.2851	-1.1701
N9	-1.0887	1.0855	-0.0734	0.0732	-0.3403	0.3393	-25.0160	33.0379	-0.9970	-1.0030	-2.1742
C10	0.5758	-0.3078	0.0388	-0.0207	0.1800	-0.0962	5.2238	2.4234	-0.5345	-1.8709	0.8836
C13	1.4264	-0.8340	0.0961	-0.0562	0.4459	-0.2607	15.7435	-8.2114	-0.5847	-1.7103	2.2605
O17	-0.8210	2.3092	-0.0553	0.1556	-0.2566	0.7218	2.8451	4.5330	-2.8127	-0.3555	-3.1302
O18	0.5750	1.9538	0.0388	0.1317	0.1797	0.6108	36.3719	-28.4596	3.3980	0.2943	-1.3788
C16	-0.9374	4.9947	-0.0632	0.3366	-0.2930	1.5613	3.0124	5.2721	-5.3280	-0.1877	-5.9321
N22	0.8749	1.0455	0.0590	0.0705	0.2735	0.3268	-6.0999	13.5622	1.1950	0.8368	-0.1706

^aAll the values are in electron volt, except for relative electrophilicity and relative nucleophilicity; ^bAtom number as represented in Figure 1; ^cFukui function [f^±(k)]: f⁺(k) = [P_N+1(k) – P_N(k)] and f^-(k) = [P_N(k) – P_N–1(k)]; ^dP(k) is the electronic population on atom k in the neutral species (N), the cationic species (N-1) and the anionic species (N+1), where N is the number of electrons; ^eLocal softness [s^±(k)]: s⁺(k) = [P_N+1(k) – P_N(k)]S and s^-(k) = [P_N(k) – P_N–1(k)]S, where S is the global softness (= 0.0674 for n-Bu₂SnL); ^fLocal electrophilicity [w^±(k)]: w⁺(k) = [P_N+1(k) – P_N(k)]w and w^-(k) = [P_N(k) – P_N–1(k)]w, where w is the electrophilicity index (= 0.3126 for n-Bu₂SnL); ^gHardness potential [D^±h(k)]: D⁺h(k) = h_N+1(k) – h_N(k) and D^-h(k) = h_N(k) – h_N_-₁(k), where h(k) =-V_el(k) (V_el being the electronic part of the molecular electrostatic potential); ^hRelative nucleophilicity defined as ; ⁱRelative electrophilicity defined as ; ^jDual reactivity descriptor defined as: .

Table 6: Characteristic infrared vibrational frequencies (in cm^-¹) for n-Bu₂SnL calculated at B3LYP/3-21G/LANL2DZ(Sn) level of theory.

Type of band	Frequency (cm^-1)
ν(NH)_imidazole	3600
ν (NH)_amino	ν_asy: 3472 ν_sym: 3392
Amide I [ν (CO)_amide]	1690
ν (CN)	1342
ν (NH)	1707 (scissoring)
ν_asym(OCO)	1746
ν_sym(OCO)	1192
ν_asym(Sn-C)	625
ν_sym(Sn-C)	601
Sn -O Sn -N	421 520; 496

Figure 4: Theoretical infrared spectrum of n-Bu₂SnL calculated at B3LYP/3-21G/LANL2DZ(Sn) level of theory.

Figure 5: Theoretical Raman spectrum of n-Bu₂SnL calculated at HF/3-21G/LANL2DZ(Sn) level of theory.

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Received on 17.01.2016 Modified on 25.01.2016

Asian J. Research Chem. 9(2): Feb., 2016; Page 53-61

DOI: 10.5958/0974-4150.2016.00010.9