M. KUMRU, M. KOCADEMİR, H. M. ALFANDA Fatih University, Faculty of Arts and Sciences, Physics Department, Büyükçekmece, Istanbul

Quinoline is an aromatic nitrogen compound characterized by a double–ring structure contains a benzene fused to pyridine at two adjacent carbon atoms. Quinoline and its derivatives have been the research area of many scientists due to their presence in the areas : Biological medicine Chemicals Many other industrial processes Electronic Spectra of these molecules were investigated.

Quinoline carboxaldehyde (QC) is a derivative of quinoline. In QC, aldehyde group is attached to quinoline. It has molecular formula of C 10 H 7 NO. Quinoline carboxaldehyde exhibits seven different positional isomers relative to aldehyde group namely; n-quinoline carboxaldehyde or quinoline – n - carboxaldehyde where n = 2, 3, 4, 5, 6, 7, 8.

Material Benzen Quinoline (C 9 H 7 N) (C 6 H 6 )

Material Quinoline Quinolinecarboxaldehyde C 10 H 7 NO

Material In the present study, the theoretical UV–Vis spectra in gas phase and water would be compared with the experimental spectra also in water for the seven isomers of quinoline carboxaldehyde. Conformational analysis, frontier molecular orbitals (FMOs), global chemical reactivity indices and molecular electrostatic potential (MESP) map would also be discussed.

THEORETICAL STUDY Computations were performed using GAUSSIAN03 software package. The initial structure of QC molecules were modeled with the GaussView program Geometry were optimised in gas phase at B3LYP level; hybrid DFT functional with the 6-31G++ (d,p) basis set. These optimised geometries were then subjected to 40 state TD- B3LYP to simulate the electronic spectra. The bulk solvent effects were treated using the standard polarisable continuum model (PCM).

EXPERIMENTAL STUDIES The entire samples were obtained from Alfa Aeser, USA except Q7C which is obtained from Tokyo Chemical Industry Co. Ltd. and were used as received without further purification. Measurements were completed using Thermo Scientific Evolution 300 spectrophotometer at room temperature in the region of nm using water as solvent. Quartz cells with a path length of 1cm and a 3cm 3 volume were used for all measurements.

RESULTS AND DISCUSSIONS MOLECULAR GEOMETRY QC has a planer structure; When C=O is farther away than N atom of quinoline we call the conformation as rotamer 1 (Rot 1). When it is closer to the nitrogen, we call it as rotamer 2 (Rot 2). The Rot1 are found to be more stable than Rot2. All ab initio results reported herein are of the more stable rotamers; Rot1

UV-VIS SPECTRAL ANALYSIS ExperimentalCalculated Water λ (nm) E (eV)fMO contribution H-1→L+2 H→L H-4→L H→L H-1→L+1 H→L H→L+1 H-1→L H-1→L H→L H-1→L+1 Gas Expt. and Theoret. Spectra for Q2C

ExperimentalCalculated Water λ (nm) E (eV)fMO contribution H→L+2 H→L H→L H-1→L H-1→L H→L H→L H-1→L Gas Expt. and Theoret. Spectra for Q3C

Expt. and Theoret. Spectra for Q4C ExperimentalCalculated Water λ (nm) E (eV)fMO contribution H-1→L+1 H→L H-4→L H-1→L H→L+1 H-1→L H-1→L H→L H-4→L Gas

Expt. and Theoret. Spectra for Q5C ExperimentalCalculated Water λ (nm) E (eV)fMO contribution H-2→L+2 H-5→L H→L+2 H-2→L H-2→L H→L H→L H-2→L Gas

Expt. and Theoret. Spectra for Q6C ExperimentalCalculated Water λ (nm) E (ev)fMO contribution H→L+2 H-4→L H→L+2 H-1→L H-1→L H→L H→L Gas

ExperimentalCalculated Water λ (nm) E (eV)f MO contribution H →L+2 H-4→L H-4 →L H →L H-1 →L H→L H →L Gas Expt. and Theoret. Spectra for Q7C

Expt. and Theoret. Spectra for Q8C ExperimentalCalculated Water λ (nm) E (eV)fMO contribution H-4→L H-1→L H-1→L H-2→L Gas

FRONTIER MOLECULAR ORBITALS (FMOs) The frontier molecular orbitals (FMOs) are the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) They are the most important orbitals in a molecule. Many properties of molecules such as chemical reactivity, kinetic stability and site of electrophilic attack can be understood fully through HOMO- LUMO analysis.

FMOs for Q2C

FMOs for Q3C

FMOs for Q4C

FMOs for Q5C

FMOs for Q6C

FMOs for Q7C

FMOs for Q8C

MOLECULAR ELECTROSTATIC POTENTIAL Molecular electrostatic potential of a molecule is the net electrostatic effect, produced by the total charge distribution (electrons and nuclei) of the molecule. MESP predict the sites for electrophilic and nucleophilic attacks.

MESP for Q2C MESP for Q3C

MESP for Q4C MESP for Q5C

MESP for Q6C MESP for Q7C

MESP for Q6C

CONCLUSION In the current contribution, the experimental and theoretical electronic absorption spectra of QC derivatives were studied. Theoretical calculations were performed at B3LYP/6311++G(d,p) level. Conformational properties of the molecules, geometric parameters (bond length and bond angle), FMOs and MESP analyses for the tittle molecules were also presented. QC exhibits two different conformers depending on the orientation of CHO which we called as Rot1 and Rot2. Theoretical calculations showed Rot1 to be more stable and therefore all computational results of it were reported.

Theoretical spectra were simulated in gas phase and in water solvent. Absorption maxima for lower-lying singlet states were calculated by TD-DFT/B3LYP/ G(d,p). The absorption bands were observed to show slight red-shift in the present of solvent. Analysis of the molecular orbitals shows that the absorption maxima of these molecules correspond to electon transitons between FMOs such as HOMO to LUMO and so on. Plots of FMOs show that the FMOs are mainly composed of π and π* orbitals and thus electronic transitions are can be designated as π→π*.

The experimental spectra were recorded in the range of nm with the aid of UV-Vis spectrophometer. Comparisons of the experimental and theoretical spectra show a very good agreement. Hence TD-DFT proves to be powerful computational tool for investigation of electronic absorption spectra. MESP plot indicates oxygen and nitrogen as sites for electrophilic attack whereas hydrogen atoms as sites of nucleophilic attack.

Related Our Some Articles V. Küçük, A. Altun, M. Kumru, "Combined experimental and theoretical studies on the vibrational spectra of 2-quinolinecarboxaldehyde", Spectrochim Acta Part A 85(2012)92–98 M. Kumru, V. Küçük, T. Bardakçı, “Theoretical and experimental studies on the vibrational spectra of 3-quinolinecarboxaldehyde", Spectrochim Acta Part A 90(2012)28–34 M. Kumru, V. Küçük, M. Kocademir, “Determination of structural and vibrational properties of 6- quinolinecarboxaldehyde using FT-IR, FT-Raman and Dispersive-Raman experimental techniques and theoretical HF and DFT (B3LYP) methods” Spectrochim Acta Part A, 96 (2012) 242–251 M. Kumru, V. Küçük, P. Akyürek, “Vibrational spectra of Quinoline-4-carbaldehyde: Combined experimental and theoretical studies”, Spectrochimica Acta Part A, 113 (2013) 72–79