Synthesis , Crystal and Molecular Structure Studies and DFT Calculations of Phenyl Quinoline-2-Carboxylate and 2-Methoxyphenyl Quinoline-2-Carboxylate ; Two New Quinoline-2 Carboxylic Derivatives

The crystal and molecular structures of the title compounds, phenyl quinoline-2-carboxylate and 2-methoxyphenyl quinoline-2-carboxylate, two new derivatives of quinolone-2-carboxylic acid, are reported and confirmed by single crystal X-ray diffraction and spectroscopic data. Compound (I), C16H11NO2, crystallizes in the monoclinic space group P21/c, with 8 molecules in the unit cell. The unit cell parameters are a = 14.7910(3) Å; b = 5.76446(12) Å; c = 28.4012(6) Å; β = 99.043(2)°; V = 2391.45(9) Å3. Compound (II), C17H13NO5, crystallizes in the monoclinic space group P21/n with 4 molecules in the unit cell. The unit cell parameters are a = 9.6095(3) Å; b = 10.8040(3) Å; c = 13.2427(4) Å; β = 102.012(3)°; V = 1344.76(7) Å3. Density functional theory (DFT) geometry optimized molecular orbital calculations were performed and frontier molecular orbitals of each compound are displayed. Correlation between the calculated molecular orbital energies (eV) for the surfaces of the frontier molecular orbitals to the electronic OPEN ACCESS Crystals 2015, 5 101 excitation transitions from the absorption spectra of each compound has been proposed. Additionally, similar correlations observed among six closely related compounds examining small structural differences to their frontier molecular orbital surfaces and from their DFT molecular orbital energies, provide further support for the suggested assignments of the title compounds.


Structural Study of (I): Phenyl Quinoline-2-Carboxylate
The asymmetric unit of In (I), C16H11NO2, contains two molecules.Bond lengths are in normal ranges [15] (Table 1).The dihedral angle between the mean planes of the quinoline and phenyl rings is 55.3(9)° in molecule A and 56.4(9)° in molecule B (Figure 2a).The carboxylate group is twisted by 3.8(2)° (A) and 3.5(3)° (B), respectively from the mean plane of the quinoline group.A weak C12B---H12B O1A intermolecular interaction is observed between a carboxyl oxygen atom and a phenyl hydrogen atom from nearby molecules within the asymmetric unit forming dimers stacked along the a axis of the unit cell (Figure 2b).No classical hydrogen bonds were observed.Additional weak C---H O intermolecular interactions and π-π stacking interactions between the two rings of the quinoline group and carboxyl oxygen atoms of nearby molecules are also observed (Table 2).Notes: Symmetry transformations used to generate equivalent atoms: After a DFT geometry optimization calculation for (I), the dihedral angle between the mean planes of the quinoline and phenyl rings becomes 46.9(1)°, a decrease of 8.4(8)° in molecule A or 9.5 (8)° in molecule B (Figure 2a).The mean plane of the carboxylate group (O2/C1/O1/C2) is twisted by 1.3(4)°, from that of the quinoline group, an increase of 3.8(2)° in molecule A or 3.5(3)° in molecule B, respectively.These changes as well as changes in the C11/O2/C1/O1 torsion angle from −3.2(2)° to 0.2(1)° in molecule A or from −2.9(3)° to 0.2(1)° in molecule B after the geometry optimization DFT calculation suggests that while the observed C---H…O and π-π weak intermolecular interactions (Table 2) may influence somewhat the packing arrangement, only a high ΔEconfig value representing energy differences between the optimized and experimental electronic transitions would be indicative of a significant departure from the ideal molecular conformation in the gas phase and, therefore, influence the crystal packing.

Structural Study of (II): 2-Methoxyphenyl Quinoline-2-Carboxylate
In (II), C17H13NO3, the dihedral angle between the mean planes of the quinoline and phenyl rings is 67.4(6)° in the molecule (Figure 3a).The carboxylate group is twisted by 79.5(0)° from the mean plane of the phenyl group.Bond lengths are in normal ranges [15] (Table 3).A weak C9---H9…O2 intermolecular interaction and weak π-π stacking interactions (Figure 3b) between the two rings of the quinoline groups of nearby molecules are also observed (Table 4).No classical hydrogen bonds were observed.
After a DFT geometry optimization calculation he dihedral angle between the mean planes of the quinoline and 2-methoxyphenyl rings becomes 72.2(8)°, an increase of 4.8(2)° (Figure 3a).The mean plane of the carboxylate group (O2/C1/O1/C2) is twisted by 0.3(1) o , from that of the quinoline group, a decrease of 13.3(5)°.These changes as well as changes in the C11/O1/C1/O2 torsion angle from 1.7(3)° to 4.0(8)° after the geometry optimization DFT calculation suggests that while the observed C---H…O and π-π weak intermolecular interactions (Table 4) influence somewhat the packing arrangement, only a high ΔEconfig value representing energy differences between the optimized and experimental electronic transitions would be indicative of a significant departure from the ideal molecular conformation in the gas phase and, therefore, influence the crystal packing.

D---H d(D---H) d(H…A) d(D…A)
Notes: Symmetry transformations used to generate equivalent atoms:

Computational Details
A density functional theory (DFT) molecular orbital calculation (WebMoPro) [16] with the GAUSSIAN-03 program package [17] employing the B3LYP (Becke three parameter Lee-Yang-Parr) exchange correlation functional, which combines the hybrid exchange functional of Becke [18,19] with the gradient correlation functional of Lee, Yang and Parr [19] and the 6-31 G(d) basis set [20] was performed on each of the two molecules (I and II) studied.No solvent corrections were made with these calculations.Starting geometries were taken from X-ray refinement data.The optimized results in the free molecule or gas phase state are, therefore, compared to those in the crystalline state.Experimentally determined oscillator strengths (f) were determined by use of the equation relating them to the molar decadic absorption coefficient (e) (f = 4.32 × 10 −9 emax•Δx1/2) [21,22].The molar decadic absorption coefficient measures the intensity of the optical absorption at a given wavelength.Deconvolution of the spectra to obtain the emax and Δx1/2 values was carried out by the IGOR program [23].Discrepancies between the experimental and calculated band centers and band intensities exist.However, this does not prohibit us from making informed decisions on the observations since it is generally known that DFT often underestimates HOMO-LUMO gaps, thereby having a tendency to give excitations far too low in energy.All calculations were performed on a workstation PC using default convergence criteria.

Theoretical Density Functional Theory (DFT) Calculations for (I) and (II)
From a DFT molecular orbital calculation for each molecule (I) and (II), surface plots for the two highest occupied molecular orbital (HOMO and HOMO−1) and four lowest unoccupied molecular orbitals (LUMO, LUMO+1, LUMO+2, LUMO+3) are displayed to provide visual evidence of the molecular orbitals involved in the spectroscopic electronic energy transitions examined.Based on correlation of the energies of these HOMO-LUMO frontier surfaces to the UV-Vis absorption spectra (Table 5), electronic excitation transition predications are suggested.

DFT Frontier Molecular Orbitals for (I)
Calculated molecular orbital energies (eV) for the surfaces of the frontier molecular orbitals for (I) are shown in Figure 4a and Table 5.In HOMO−1 and HOMO, electronic clouds are distributed primarily on the phenyl and quinoline groups, respectively.In LUMO, LUMO+1, electronic clouds are delocalized primarily on the quinoline ring while In LUMO+2 and LUMO+3 electron clouds are delocalized on the phenyl ring.The observed experimental absorption spectrum shows one intense band envelope at λmax = 322 nm.Electronic transitions are generally paired between the various molecular orbitals of the ground and excited states corresponding to this single band envelope as indicated in Table 5.Therefore, the absorption band envelope at 322 nm is assigned to overlapping contributions from each of HOMO→LOMO, HOMO−1→LUMO, HOMO→LOMO+1, HOMO−1→LUMO+1, HOMO→LUMO+2, HOMO−1→LUMO+2 and HOMO→LUMO+3, respectively.The energy differences (ΔEconfig) between the optimized and experimental electronic transitions for (I) are 0.79, 0.90, 1.69, 1.81, 2.57, 2.86 and 2.71 eV, respectively, that are associated with the broad band envelope at 322 nm.However, this comparison while suggestive of some interaction, is inconclusive in relation to an extension of their effects on crystal packing.

DFT Frontier Molecular Orbitals for (II)
Calculated molecular orbital energies (eV) for the surfaces of the frontier molecular orbitals for (II) are shown in Figure 4b and Table 6.In HOMO−1 and HOMO, electronic clouds are distributed primarily on the quinoline and methoxyphenyl groups, respectively.In LUMO, LUMO+1 and LUMO+3 electronic clouds are delocalized primarily on the quinoline ring while in LUMO+2 electron clouds are delocalized on the 2-methoxyphenyl ring.The observed experimental absorption spectrum shows one intense band envelope at λmax = 254 nm and a second less intense band envelope at 319 nm.Electronic transitions are generally paired between the various molecular orbitals of the ground and excited states corresponding to these two band envelopes as indicated in Table 6.Therefore, the absorption band envelope at 254 nm is assigned to overlapping contributions from each of HOMO→LOMO, HOMO−1→LUMO and HOMO→LOMO+1 while the band envelope at 319 nm is assigned to HOMO−1→LUMO+1, HOMO→LUMO+2, HOMO−1→LUMO+2 and HOMO→LUMO+3, respectively.The energy differences (ΔEconfig) between the optimized and experimental electronic transitions for (II) are 0.23, 0.83 and 1.16 eV for the band envelope at 319 nm and 0.76, 1.11, 1.35 and 1.68 eV associated with the band envelope at 254 nm, respectively.As with (I), this comparison while suggestive of some interaction, it is also inconclusive in relation to an extension of their effects on crystal packing.

Comparison of the Frontier Molecular Orbitals from Six Related Quinoline-2-Carboxylate Derivatives to the Title Compounds
A display of the DFT frontier molecular orbitals from six similar and related quinoline-2-carboxylate derivatives previously published in this laboratory is shown in Figure 5a-f.The absorption spectra for each of the six molecules shown above display two intense absorption maxima with λmax between 251-253 nm and 316-320 nm indicating slight shifts in band maxima being related most likely to the various substituted groups on the phenyl moiety.The oscillator strengths of each of these two band envelopes in (III-VIII) are also in similar ranges as recorded for molecules (I) and (II) (see Table S1-S6 for DFT HOMO-LUMO assignments of molecules III-VIII).From the DFT calculated frontier surface molecular orbitals for compounds (III-VIII) in Figure 5, it is therefore suggested that each of these molecules can be assigned to similar transitions within the band envelopes as described for molecules (I) and (II) providing further support for these collective HOMO-LUMO assignments in the title molecules.

Synthesis of Phenyl Quinoline-2-Carboxylate and 2-Methoxyphenyl Quinoline-2-Carboxylate
The two quinoline-2-carboxylates (I) and (II) were prepared by the following method (Scheme 1).To a mixture of quinaldic acid (1.73 g, 10 mmole) and o-methoxyphenol (1.24 g, 10 mmole) (I) or phenol (0.9 g, 10 mmole) (II) in a round-bottomed flask fitted with a reflux condenser and a drying tube, (0.150 g, 10 mmole) of phosphorous oxychloride was added.The mixture was heated with occasional swirling maintaining the temperature at 348-353 K.At the end of eight hours (I) or six hours (II) the reaction mixture was poured into a solution of 2 g of sodium bicarbonate in 25 mL of water.The precipitated esters were collected on a filter and washed with water.The yield of crude, air dried 2-methoxyphenyl quinoline-2-carboxylate (I) was (80%-90%) and air dried phenyl quinoline-2-carboxylate (II) was (55%-60%).X-ray quality crystals of both (I) and (II) were obtained by recrystallization from absolute ethanol.The melting points were determined on SELACO melting point in open capillary tubes and are uncorrected.Reactions were monitored by Thin-layer chromatography (TLC) using pre-coated sheets of silica gel G/UV-254 of 0.25 mm thickness (Merck 60F254) using UV light for visualization.All the solvents and reagents used for the synthesis were of analytical grade and procured from Sigma Chemical Co.(St.Louis, MO, USA).NMR spectra ( 1 H and 13 C) for the compound was recorded on a 500 MHz NMR Spectrometer (Bruker advance, Reinstetten, Germany) using deuteriated DMSO as the solvent.The chemical shift values (ppm) and coupling constants (J) are given in δ and Hz respectively.Mass spectral analysis was carried out in the ESI positive mode using HRMS mass spectrometer (Waters Q-Tof Utima, Manchester, UK).OD of the samples was measured using UV/Vis spectrometer, UV-1800 Shimadzu, Tokyo, Japan.P = (Fo 2 + 2Fc 2 )/3, min./max.∆ρ = −0.18,+0.26e Å −3 .The quoted wR(F 2 ) values are for all data or give the applied sigma limit for observed data.Cambridge Database deposition number: CSD-1039092.

Conclusions
The crystal and molecular structure of two new derivatives of quinolone-2-carboxylic acid have been determined along with the frontier molecular orbitals of each compound displayed through density function theory (DFT-B3LYP 6-31G(d)), geometry optimization and molecular orbital calculations.Correlation between the calculated molecular orbital energies (eV) for the surfaces of the frontier molecular orbitals to the electronic excitation transitions from the absorption spectrum of each compound has been determined.In each compound, the DFT molecular orbital calculation, supported by a geometry optimization calculation confirmed that the excitation energies of the surfaces of the frontier molecular orbitals from the HOMO−1 and HOMO to LUMO, LUMO+1, LUMO+2 and LUMO+3 electronic excitations closely match the λmax values of the absorption spectra in overlapping contributions from three to five of these excitations within each band envelope.In the crystal structures of both (I) and (II) no classical hydrogen bonds were observed.In (I) weak C---H…O intermolecular interactions and π-π stacking interactions between the two rings of the quinoline group and carboxyl oxygen atoms of nearby molecules are present, while in (II), weak C---H…O and π-π stacking interactions between the two rings of the quinoline groups of nearby molecules are observed.While the energy differences (ΔEconfig) between the optimized and experimental electronic transitions for (I) and (II) associated with the band envelopes for each structure are suggestive of some interaction, it is inconclusive in relation to an extension of their effects on crystal packing.

Figure 2 .
Figure 2. (a) Molecular structure of C16H11NO2 (I), showing the atom numbering scheme with 30% probability ellipsoids; (b) Packing diagram for (I) viewed along the b axis.H atoms not involved in hydrogen bonding have been removed for clarity.

Figure 3 .
Figure 3. (a) Molecular structure of C17H13NO3, (II), showing the atom numbering scheme with 30% probability ellipsoids; (b) Packing diagram for (II) viewed along the c axis.H atoms not involved in hydrogen bonding have been removed for clarity.

Table 5 .
Experimental and Calculated Energy of Molecular Orbitals of (I) and Associated Transitions.