Crystal Structure and Solution Structural Dynamic Feature of 1 , 8-Dibenzoyl-2 , 7-Dimethoxynaphthalene

The crystal structure and the dynamic feature of molecular structure in solution for 1,8-dibenzoyl-2,7-dimethoxynaphthalene are revealed by X-ray crystallographic analysis and VT-NMR measurements. In crystal, the molecule of the title compound is located on a twofold rotation axis. The two benzoyl groups are situated in an opposite direction. The dihedral angle between the mean planes of the phenyl ring and the naphthalene ring system is 80.25(6) ̊. The benzene ring and carbonyl moiety in each benzoyl group are almost coplanar. The molecular packing is stabilized by weak C–H...O hydrogen bonds and a π-π stacking interaction between the benzene rings [centroid-centroid and interplanar distances of 3.6383(10) and 3.294 Å, respectively]. In solution, the temperature-dependent rotation behavior of the C–C bond between the benzene ring and the ketonic carbonyl group has been observed by H VT-NMR measurements. Furthermore, comparison of the C–C bond rotation behavior between the benzene ring and the carbonyl group with 1-benzoyl-2,7-dimethoxynaphthalene has clarified that the C–C bond between the ketonic carbonyl group and the naphthalene ring rotates slower than the 1,8-dibenzoylated homologue.

In this article, the crystallographical structural charac-teristics and the dynamic feature in solution structure of a 1,8-diaroylated naphthalene derivative having two methoxy groups at the 2,7-positions are described as the most simple homologue of monoaroylated derivative.

Experimental
All reagents were of commercial quality and were used as received.Solvents were dried and purified using standard techniques.Pentoxide-methanesulfonic acid (P 2 O 5 -MsOH) was prepared according to literature [27].

1
H NMR spectra were recorded on a JEOL JNM-AL300 spectrometer (300 MHz) and a JEOL ECX400 spectrometer (400 MHz).Chemical shifts are expressed in ppm relative to internal standard of Me 4 Si (δ 0.00). 13C NMR spectra were recorded on a JEOL JNM-AL300 spectrometer (75 MHz).Chemical shifts are expressed in ppm relative to internal standard of CDCl 3 (δ 77.0).IR spectra were recorded on a JASCO FT/IR-4100 spectrometer.High-resolution FAB mass spectra were recorded on a JEOL MStation (MS700) ion trap mass spectrometer in positive ion mode.

Synthetic Procedure of the Title Compound
The title compound was synthesized via direct condensation mediated by P

X-Ray Crystallography
For the crystal structure determination, the single-crystal of the compound C 26 H 20 O 4 was used for data collection on a four-circle Rigaku R-AXIS RAPID diffractometer (equipped with a two-dimensional area IP detector).The graphite-mono-chromated Cu Kα radiation (λ = 1.54187Å) was used for data collection.The lattice parameters were determined by the least-squares methods on the basis of all reflections with F 2 > 2σ(F 2 ).The data collection and cell refinement were performed using PROC-ESS-AUTO [29] software.The data reduction was performed using CrystalStructure [30].The structures were solved by direct methods using SIR2004 [31] and refined by a full-matrix least-squares procedure using the program SHELXL97 [32].All H atoms were found in a difference map and were subsequently refined as riding atoms, with the aromatic C-H = 0.95 Å and methyl C-H = 0.98 Å, and with U iso (H) = 1.2U eq (C).Molecular structure of the compound showing the atomic numbering scheme is shown in Figure 1.The crystallography details for the structures determination of the compound are displayed presented in Table 1.

Variable Temperature NMR
Variable temperature 1 H NMR spectra were recorded on an FT-NMR operating at 400 MHz.Chemical shift values were reported in parts per million (ppm) relative to (CH 3 ) 4 Si (TMS).The solvent used in all cases was CDCl 3 and CS 2 (1:4 v/v) solution.Low temperature spectra were obtained with the use of a JEOL cooling system.

Results and Discussion
The title compound was synthesized via direct condensation Table 1 shows the crystallographic data of the title compound.Table 2 shows selected bond lengths and angles.Table 3 gives selected torsion angles.Figure 1 gives ORTEP representation of the molecular structure of the title compound, as determined by the structured X-ray analysis [33].The molecule of the title compound lies across a crystallographic 2-fold axis so that the asymmetric unit contains one half of the molecules.Thus, the two benzoyl groups are situated in an opposite direction (anti-orientation).The benzoyl groups are twisted away from the naphthalene moiety, and the dihedral angle is 80.25(6)˚.
The dynamic behavior of the title compound in solution was estimated by variable temperature NMR (VT-NMR) measurement performed in chloroform-d and CS 2 (1:4, v/v) solution from 293 to 173 K. Figure 4(a     The sequent signal changes for the protons at o-and m-positions of the benzoyl group as shown in the VT-NMR spectra of the title compound suggest that the C-C bond rotation between benzene ring and ketonic carbonyl group is gradually slowed down with decreasing temperature then the magnetically non-equivalent environment is made on the benzene ring.The rotation barrier of the C(benzene)-C(carbonyl) bond was calculated as ∆G = 9.63 kcal/mol on the basis of the detailed VT-NMR measurements (Figure 5) [34].
In a similar manner, VT-NMR measurement of the homologous compound of 1-benzoyl-2,7-dimethoxynaphthalene was carried out (Figure 4(b)).For spectrum at 293 K, the signals of δ 6.80, 7.01, 7.17 The title compound and the homologue have two kinds of the C-C bonds allowed to rotate, i.e., C(benzene)-C(carbonyl) and C(naphthalene)-C(carbonyl) bonds.The steric environment around the neighboring carbonyl groups in the title compound is highly congested as shown in Figure 1.Naturally, the rotation behavior of C(naphthalene)-C(carbonyl) bond should be slower than 1-benzoylnaphthalene homologue.In the consequence, the temperature-dependent C(benzene)-C(carbonyl) bond rotation behavior might be observed.In other words, C(naphthalene)-C(carbonyl) bond in the title compound rotates slowly in the temperature range that the C(benzene)-C(carbonyl) bond is allowed to rotate freely.

Conclusion
Conclusively, the crystal structural shapes and the dy- namic feature of solution structure of 1,8-dibenzoyl-2,7dimethoxynaphthalene are clarified.In crystal, the aroyl groups of the compound are perpendicularly attached to the naphthalene ring core and situated in an opposite direction.two types of C-H…O interactions of ketonic carbonyl group with benzene ring and methoxy group and π-π interactions between benzene rings mainly stabilize the molecular packing.According to VT-NMR study, the split signals assigned to the o-and m-protons of the benzene ring are observed from 193 to 173 K. Comparison of the dynamic behavior with 1-benzoylnaphthalene homologue shows that the C-C bond rotation between ketonic carbonyl group and naphthalene ring in the title compound is enough slow to detect the rotation of the C-C bond rotation between benzene ring and ketonic carbonyl group.The structural information of the title compound in solid state and solution affords some hitherto-unknown aspects in molecular formula and intramolecular rotation properties relationship of these non-coplanarly accumulated aromatic rings molecules.

Figure 1 .
Figure 1.Molecular structure of the title compound, with the atom-labeling scheme and displacement ellipsoids drawn at the 50% probability level.The symbol "_2" refers to symmetry code: −x, y, −z + 1/2.

) shows the 1 H
VT-NMR spectra of the title compound in the aromatic region.At 293 K, two signals of δ 7.21 and 7.95 ppm are assigned the protons at 3(6)-and 4(5)-positions of the naphthalene ring, respectively.The signals of δ 7.34, 7.49, and 7.70 ppm are assigned to the protons at m-positions, p-position, and o-positions of benzoyl groups.The signals of δ 7.21 and 7.95 ppm (at 293 K) are scarcely changed from 293 to 173 K. On the other hand, the figures of the three signals of δ 7.34, 7.49, and 7.70 ppm (at 293 K) are drastically changed.The signal of δ 7.49 ppm (at 293 K) is broadened from 293 to 233 K then sharpened from 213 to 173 K.The signals of δ 7.34 and 7.70 ppm (at 293 K) are broadened from 293 to 213 K and each of them splits into two broad signals at 193 K.The both of the pairs of broad signals are sharpened again at 173 K.
, and 7.72 ppm are assigned to the protons at 8-, 6-, 5-, and 3-positions of the naphthalene ring, respectively.The two signals of δ 7.43 and 7.57 ppm are assigned to the protons at m-positions and p-position of the benzoyl groups.The signals from δ 7.84 to 7.89 ppm are overlapped by the two kinds of signals, the proton at 4-position of the naphthalene ring and the protons at o-positions of the benzoyl group.The signals assigned to the protons of the benzoyl group are broadened in the temperature range from 293 to 173 K.However, no split behavior is observed at 173 K.