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ScienceDirect Physics Procedia 86 (2017) 32 – 36

International Conference on Photonics of Nano- and Bio-Structures, PNBS-2015, 19-20 June 2015, Vladivostok, Russia and the International Conference on Photonics of Nano- and MicroStructures, PNMS-2015, 7-11 September 2015, Tomsk, Russia

Structure and electronic properties of nano-complex CCl4…Cr(AcacCl)3 on evidence derived from vibrational spectroscopy S.N.Slabzhennikov, L.A.Kuarton, O.B.Ryabchenko* Far Eastern Federal University, Sukhanova str. 8, Vladivostok, 690091, Russia

Abstract In order to specify influence of intermolecular interaction on IR spectrum of interacting species, an investigation of a process CCl4 + Сr(АcacCl)3 → CCl4…Сr(АcacCl)3 has been performed by means of Hartree–Fock–Roothaan method in MIDI basis set with p- and d- polarization functions. An estimation of intermolecular interaction in geometrical parameters, electron density function both between interacting particles and inside themselves, frequencies and intensities of normal modes has been carried out. Chemical bonds with the most significant shifts of characteristics under formation of nano-complex CCl4…Сr(АcacCl)3 have been noted. 2016The TheAuthors. Authors. Published by Elsevier ©2017 © Published by Elsevier B.V. B.V. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of PNBS-2015 and PNMS-2015. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of PNBS-2015 and PNMS-2015. Keywords: nano-complex, vibrational spectroscopy, quantum-chemical calculations

1. Introduction “Amongst the methods that enable to get information about structure of solutions optical ones belong to the most reliable, however their results are not always unambiguous in interpretation” (Krestov G.A. (1984)). IR spectra of

* Corresponding author. Tel. +7-902-506-2319; E-mail address: [email protected]

1875-3892 © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of PNBS-2015 and PNMS-2015. doi:10.1016/j.phpro.2017.01.012

S.N. Slabzhennikov et al. / Physics Procedia 86 (2017) 32 – 36

both a solute and a solvent differ from ones in gas phases and these discrepancies contain information about physical and chemical properties of the both components of solution that govern origin and power of intermolecular interactions in a system. Yet, for spectrum interpretation (qualitative analysis) intermolecular forces are usually supposed infinitesimal: characteristics of calculated modes for a single molecule are related to its spectrum in dilute solution in a latent solvent. Nevertheless ignoring of mutual influence of solution components, for example, in quantitative analysis, may result in various errors due to frequency shifts or intensity shuffle in spectra of a solute as well as a solvent. Consideration of molecular interaction impacts on IR spectrum may be performed by quantum mechanical calculations in “supermolecular” approach: at the first step a simple model includes one solute molecule and one solvent molecule (Burshtein L.Ya. et. al. (1989)). This paper presents results of such investigation by the example of a process CCl4 + Сr(АcacCl)3 → CCl4…Сr(АcacCl)3 , where an impact of intermolecular forces on geometric, electronic and dynamic characteristics (mode frequencies and intensities) of supramolecular formation CCl 4…Сr(АcacCl)3 (which is reasonable to call “nano-complex”) has been obtained. 2. Model Nomenclature amu D Å au

atomic mass unit =1.660·10-27 kg debye unit for molecular dipole moments = 3.336·10-30 C·m angstrom – unit of length = 10-10 m atomic unit for charge = 1.602·10-19 C

Optimization of geometrical parameters of CCl4 molecule, coordination complex Сr(АcacCl)3, nano-complex CCl4…Сr(АcacCl)3 and its conformational analysis as well as normal mode and intensity calculations were carried out by GAMESS program (Schmidt M.W. et.al., 1993), http://www.msg.ameslab.gov/gamess/) Hartree–Fock– Roothaan method in MIDI basis set. For H and Cl atoms basis sets one polarization function p-type was added; basis sets for carbon, oxygen and chlorine atoms were supplemented with d-type one polarization function. Multiplicities of Сr(АcacCl)3 complex and nano-complex CCl4…Сr(АcacCl)3 equaled four. Quantum chemical calculations for CCl4 molecule, Сr(АcacCl)3 complex and CCl4…Сr(АcacCl)3 nano-complex were performed in frames of their group symmetry Td, D3, С3, respectively. Hydrogen atom mass was supposed 1,088 (spectroscopic mass) to correct for anharmonicity. Visualization of normal modes was made by MOLEKEL program (Flükiger P. et.al (2000-2002), Portmann S. (2000)). Charges on atoms and overlap populations were expressed in D2Å-2 amu-1. Scaling coefficient for mode frequencies recommended in documentation for GAMESS program complex was equal to 0,89. IR-spectra of complexes in CCl4 were registered by spectrometer Specord M80 at room temperature with resolution of 1 – 3 cm-1. 3. Results and discussion In symmetry group C3 nano-complex CCl4…Сr(АcacCl)3 has two conformations. One of them (A), which has chlorine atom of CCl4 molecule closer to atom Cr, is shown in the figure 1. In this conformation nano-complex has minimum of total energy. Total energy of conformation obtained from A by reflection of CCl4 molecule in perpendicular to axis С3 plane exceeds total energy of conformation A by 1.25 kcal/mole. A-type conformation is built up by means of electrostatic attraction of negatively charged atom Cl1 with its charge equal -0.130 au to positively charged methyl atoms H1, H2, H3 where charge is 0.136 аu on each of them.

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This is evident from the fact that at a distance between Cl1 and atoms H1, H2, H3 of 3.545 Å overlap populations of orbitals equals zero. As a result covalent energy constituent of interaction between Cl1 and H1, H2, H3 atoms is nil. Molecular interaction of CCl4 molecule and Сr(АcacCl)3 complex is followed by electron density shuffle both inside interacting particles and between them. In consequence of these changes nano-complex acquires dipole moment of 0.55 D directed from CCl4 molecule to Сr(АcacCl)3 complex. Existence of dipole moment in such a system is consistent with experimental data (Sahai R. et.al. (1981)). Discrepancy of theoretical and experimental magnitudes is due to simplicity of the model. Polarization of the nano-complex is a result of tree processes. First, electron charge transfer (-0.015 au) from Сr(АcacCl)3 complex towards CCl4 molecule occurs. Magnitude of this charge is low due to large distance between Cl1 and H1, H2, H3 atoms. Second, Сr(АcacCl) 3 complex polarizes CCl4 molecule. Localized on CCl4 molecule electron charge shifts from atom Cl1 to a fragment CCl 3. Charge at the atom Cl1 rises by 0.012 au. Third, CCl4 molecule polarizes Сr(АcacCl)3 complex. Charge at Cr atom is built up by 0.006 au and reaches as much as 1.730 au. Charges at ligand atoms besides O1 are growing as well. Charge at O1 atom, like two other equivalent with it, decays by 0.007 au to a value of -0.630 au. Thus, electron density localized on Сr(АcacCl)3 complex, shifts to CCl4 molecule and governs dipole moment direction of the whole system.

Fig. 1. Configuration of atoms in nano-complex CCl4…Сr(АcacCl)3 in ground conformational state: (a) — side view perpendicular С3 axis, (b) — view from C1 atom along С3 axis.

Upon CCl4…Сr(АcacCl)3 nano-complex formation changing of geometrical parameters both in the molecule and in the complex occurs. In CCl4 molecule C1– Cl1 bond undergoes the most changing, it shrinks by 0.006 Å and reaches 1.760 Å that means C1– Cl1 bond strengthening. The rest of geometrical parameters are significantly lower. For example, C1– Cl2 bond length rises by 0.002 Å to the value of 1.760 Å. The extension of C1– Cl2 bond is a result of its weakening. Valence angle Сl1-С1-Сl2 is grown by 0,2° and reached 109.7°. Geometrical parameters of Сr(АcacCl)3 complex, besides С3-О1 bond, as well as two other equivalent ones and relative positions of chelate rings are essentially invariable. Torsion angle changes does not exceed 0.2°. C1– Cl2 bond length bond length rises by 0.002 Å to the value of 1.253 Å. Interligand distance between H1, H2, H3 atoms of methyl groups reduces by 0.008 Å and reaches 5.712 Å. Interligand distance of oxygen atoms closest to Cl1 atom reduces by 0.008 Å to 2.752 Å. Interligand distance between remote from Cl1 oxygen atoms rises by 0.007 Å and reaches 2.767 Å. Cr atom is found beneath a plane generated by C atoms in γ-position at a distance of 0.018 Å from it. These noticed changes in relative ligand and Cr positions show that as a result of electrostatic attraction between H1, H2, H3 atoms of methyl groups and Cl1 atom each chelate ring rotates about axis having a cross point with the


chelate ring plane. Such insignificant bond length and angle changes cannot influence IR band shifts of Сr(АcacCl)3 complex. Nevertheless influence on spectrum is possible by means of electron density distribution change which was evaluated by changing of overlap population of atomic orbitals (AO) both in CCl 4 molecule and in Сr(АcacCl)3 complex upon combining them into nano-complex. Further we analyzed changing of dynamic characteristics of chemical bonds in CCl 4 upon nano-complex formation. In an isolated molecule thrice degenerated mode of T 2 symmetry type with calculated frequency of 831 cm-1 (see Figures 2) is a valence vibration with C–Cl bonds involved. In IR spectrum of the nano-complex degeneracy factor is partially decreased due to symmetry group degradation of CCl 4 molecule from Td to C3, so that there should two bands appear at 780 and 769 cm-1. The first band, according to calculations, belong to normal mode of C1–Cl1 bond A-type symmetry and frequency of 839 cm-1 which is 8 cm-1 higher than that of isolated CCl4 molecule. The second band is related to degenerate mode of С1-Сl2 bond E-type symmetry with a frequency of 827 cm-1, which is 4 cm-1 lower than that in isolated molecule. From all these data it follows that predicted by calculations band splitting of mode belonging to С-Сl bonds due to nano-complex foundation is as much as 11 cm-1. Besides frequency changes essential changes are observed in intensities of normal modes. Intensity of E-type symmetry mode, which belongs to С1-Сl2 bond vibration reduces by 13%, while normal mode intensity of type A related to C1–Cl1 bond vibration rises by 47%. This effect is indicative of asymmetrical redistribution of electron density in CCl4 molecule placed in electric field of Сr(АcacCl)3 complex. As a parameter for estimation of electron density redistribution in CCl4 molecule an overlap population of atomic orbitals of C and Cl atoms was chosen. From the data of the Figures 2 it follows that both normal mode frequencies and intensities correlate with AO overlap populations of C and Cl atoms. As AO overlap population of C–Cl bond increases by 0.013 au, like in case of C1–Cl1 bond, two other characteristics increase also: both frequency and intensity of the related normal mode. As AO overlap population of C–Cl decreases by 0.006 au, like in case of C1– Cl1 bond, two other characteristics decrease as well. From these data it follows that increase of the three parameters occurs only for C–Cl bond which is located closer to Сr(АcacCl)3 complex.

Fig. 2. Comparing of C–Cl valence mode characteristics in isolated CCl4 molecule and this molecule in nano-complex. In parenthesis normal mode’s type of symmetry, lower – calculated and experimental frequencies are shown, I — intensity. * - (Herzberg G. (1945))

We also carried out analysis of dynamic characteristics of chemical bonds in Сr(АcacCl)3 complex upon nanocomplex foundation. Dynamic characteristics of each chemical bond of Сr(АcacCl) 3 complex were governed by that normal mode which contained the highest contribution from a certain chemical bond. In comparing dynamic characteristics of chemical bonds of Сr(АcacCl)3 complex and CCl4…Сr(АcacCl)3 were taking into account frequency closeness of modes. Calculation result analysis has shown that for the most of chemical bonds there is no correlation between dynamic characteristics and overlap populations of atomic orbitals. The lack of correlation is

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due to the fact that normal modes of Сr(АcacCl)3 complex, unlike that for CCl4 molecule, consist of a number of different types natural coordinate contributions (bonds, valence and plane angles), which are mutually dependent. In general upon CCl4…Сr(АcacCl)3 nano-complex foundation changes of chemical bond modes frequencies of Сr(АcacCl)3 complex do not exceed 3 cm-1, i.e. stay within error range. For example, mode frequency of С3–О1 bond decreases by 2 cm-1 and mode frequency of H1–С2 bond increases by 3 cm-1. These frequency changes of chemical bond modes are determined by closer position of atoms H1 и О1 to Cl1 atom of CCl 4 molecule, therewith О1 atom is closer to Cl1 atom by 0.177 Å than atom H1. Mode frequencies of the rest of chemical bonds either do not change or decrease just by 1 cm-1. Conclusion from these analysis results is that the highest changes of mode frequencies exhibit those chemical bonds of Сr(АcacCl) 3 complex which atoms are placed closer to CCl4 molecule. From comparing mode intensities of isolated Сr(АcacCl)3 complex and when inside CCl4…Сr(АcacCl)3 nanocomplex no regularity is found which could describe in general intensity changes of chemical bond modes upon nano-complex foundation. Analysis of changes of normal mode intensities related to C–H bonds of methyl groups cannot be performed because their values are close to zero: they do not exceed 0.5 D2Å-2 amu-1. Particular group of chemical bonds with general tendency of changing mode intensities is represented by chelate rings. For example, intensity of normal mode with Cr–O bond participation equals 1 D2Å-2 amu-1 and it decreases by 5%. Intensity of normal mode related to C–O bond is high and equals 32 D2Å-2 amu-1. In nano-complex it decreases by 3%. Intensities of normal modes related to С3-С4, С4-С5 bonds equal relatively 3 and 7 D2Å-2 amu-1 and they decrease by 12% and 5% in the same order. It follows from the data that upon nano-complex foundation normal mode intensities related to chelate rings decrease. C–Cl bonds are independent on chelate rings because related to them mode intensity that is equal to 6 D2Å-2 amu-1 increases just by 1%. References Burshtein K.Ya., Shorygin P.P., 1989. Quantum-Chemical Calculations in Organic Chemistry and Molecular Spectroscopy. Moscow, Nauka, 104 p. [in Russian] Flükiger P., Lüthi H.P., Portmann S., Weber J., 2000–2002. MOLEKEL 4.3. Manno (Switzerland): Swiss Center for Scientific Computing. Herzberg G., 1945. Infrared and Raman Spectra of Polyatomic Molecules. New York, D. Van Nostrand. Krestov G.A., 1984. Thermodynamics of Processes in Solutions. Leningrad, Khimia. 272 p. [in Russian]. Portmann S., Lüthi H.P., 2000. MOLEKEL: An Interactive Molecular Graphics Tool. CHIMIA.. V.54. P. 766. Sahai R., Verma R., 1981. J. Indian Chem Soc. V. 58. P. 640 - 642. Schmidt M.W., Baldzidge K.K., Boatz J.A. et al., 1993. J. Comput. Chem. V.14. P. 1347. http://www.msg.ameslab.gov/gamess/