Ftir, Ft-Raman and Dft Calculations of 5-Nitro-1,3-Benzodioxole

Introduction

The benzodioxole ((methylenedioxy)benzene) group is present in a number of endothelin receptor antagonists. Endothelins are 21-amino acid bicyclic peptides originally isolated from the supernatant of cultured porcine endothelial cells¹. There have been many studies about the molecular conformations of indan-like benzene fused ring molecules^2-4. These molecules have received considerable attention due to their interesting conformational properties. Laane et al. reported the infrared, Raman, electronic UV absorption, and laser induced fluorescence excitation spectra of two oxygen containing indan-like molecule, phthalan^5-7  and 1,3-benzodioxole^8,9. 1,3-benzodioxole has been studied extensively by several spectrographic and theoretical methods owing to its propensity for large amplitude motion^10-12. It is believed that the molecule has a nonplanar structure in its electronic ground state. While the aromatic ring is planar, the five member ring is puckered, with the oxygen atoms and the CH₂ group on opposite side of the aromatic plane. In the present work, the vibrational spectroscopic analysis of 5-nitro-1,3-benzodioxole is reported, experimentally and theoretically.

Experimental

The FT-IR spectrum was recorded on a DR/Jasco FT-IR 6300 spectrometer in KBr pellets, number of scans 16, resolution 2 cm^-1. The FT-Raman spectrum was obtained on a BRUKER RFS 100/S, Germany. For excitation of the spectrum the emission of a Nd:YAG laser was used, excitation wavelength 1064 nm, maximal power 150 mW. One thousand scans were accumulated with a total registration time of about 30 min. The spectral resolution after apodization was 2 cm^-1.

Computational Details

Calculations of the title compound were carried out with Gaussian03 software program¹³ using the B3LYP/6-31G* basis sets to predict the molecular structure and vibrational wavenumbers. The DFT hybrid B3LYP functional tends also to overestimate the fundamental modes; therefore scaling factors have to be used for obtaining a considerably better agreement with experimental data. Therefore, a scaling factor of 0.9613 was uniformly applied to the DFT calculated wavenumbers¹⁴.  The assignment of the calculated wavenumbers is aided by the animation option of MOLEKEL program, which gives a visual presentation of the vibrational modes^15,16.

Results And Discussion

The observed IR, Raman and calculated (scaled) wavenumbers and the assignments are given in Table 1. In nitro compounds, υ_asNO₂ are located in the region 1580 ± 80 cm^-1. The nitro benzene derivatives¹⁷ shows υ_asNO₂ in the region 1535 ± 30cm^-1 and the 3-nitropyridine at 1530 ± 20cm^-1. In the present case, υ_asNO₂ are obtained at 1596 theoretically and at 1609 cm^-1 in IR spectrum. In nitro compounds, υ_sNO₂ are located in the region 1400 ± 40 cm^-1.  For the title compound, υ_sNO₂ is observed at 1427 cm^-1 (DFT), 1437 cm^-1 (IR) and 1430 cm^-1 (Raman). In the aromatic nitro compounds, bands are usually seen at 855 ± 40cm^-1 (NO₂ scissors deformation δNO₂), 760 ± 30cm^-1  (NO₂ out of plane wag ωNO₂), 580 ± 30cm^-1 (NO₂  in-plane rock ρNO₂) and 70 ± 20cm^-1 (τNO₂). In the present case, the observed values for δNO₂ are 917 (DFT), 920 (IR) cm^-1; ωNO₂ is 803 cm^-1 (DFT), ; ρNO₂ are 666 (DFT) and τNO₂ is at 21cm^-1 (DFT). Panicker et al. [18] reported the NO₂ deformation bands at 800, 724, 534 cm^-1 (theoretically) and 809, 727, 717, 524 cm^-1 (experimentally). Sundaraganesan et al.¹⁹ reported the NO₂ deformation bands at 839, 744 and 398cm^-1 experimentally and 812, 716,703 and 327  cm^-1 theoretically.

For methylene groups²⁰, the CH₂ vibrations are observed in the region 2800-3000, 1200-1400, 875-1150 and 600-950 cm^-1. The vibrations of the CH₂ group (the asymmetric stretch υ_asCH₂, symmetric stretch υ_sCH₂, the scissoring vibration and wagging vibration) appear in the regions 2940-3005, 2870-2940, 1420-1480 and 1320-1380 cm^-1, respectively^{17, 21}.  The stretching bands of the CH₂ groups are observed at 2925 cm^-1 in the IR spectrum and at 2992, 2942 cm^-1 theoretically for the title compound. According to literature²² scissoring mode of the CH₂ group give rise to characteristic band near 1485 cm^-1 in IR and Raman spectra. This mode is observed at 1489 cm^-1 in the IR spectrum and at 1474 cm^-1 theoretically. The wagging mode is observed at 1378 cm^-1 in the Raman spectrum and at 1372 cm^-1 theoretically. The twisting and rocking vibrations of the CH₂ group appear in the region¹⁷ of 1200-1280 and 740-900 cm^-1, respectively. These modes are also assigned. For the title compound the twisting vibration is observed at 1115 cm^-1 in the IR spectrum and at 1100 cm^-1 theoretically. The rocking deformation is assigned at 907 cm^-1 theoretically.

The asymmetric and symmetric C-O-C stretching vibrations are expected to appear at 1150-1250 and 1000-1050 cm^-1 [²²]. In the present case the DFT calculations give these modes at 1224, 1160, 1046 and 1027 cm^-1. The bands at 1171, 1066, 1036 (IR) and 1051, 1028 cm^-1 (Raman) are assigned as C-O-C symmetric stretching modes. The C-O-C stretching modes are reported at 1250, 1073 cm^-1 for 2-mercaptobenzoxazole²³, 1263, 1055 cm^-1 for 5-methyl-2-(p-fluorophenyl)benzoxazole²⁴.

The existence of one or more aromatic rings in a structure is normally readily determined from the C–H and C=C–C related vibrations. The C–H stretching occurs above 3000 cm^-1 and is typically exhibited as a multiplicity of weak to moderate bands, compared with the aliphatic C–H stretch²⁵. In the present case, the DFT calculations give the υCH modes at 3145, 3134 and 3110 cm^-1. The band observed at 3119 cm^-1 in the IR spectrum is assigned as υCH mode of the benzene ring. The benzene ring possesses six ring stretching vibrations, of which the four with the highest wavenumbers (occurring near 1600, 1580, 1490 and 1440 cm^-1) are good group vibrations¹⁷. In the absence of ring conjugation, the band near 1580 cm^-1 is usually weaker than that at 1600 cm^-1. The fifth ring stretching vibration which is active near 1335 ± 35 cm^-1 a region which overlaps strongly with that of the CH in-plane deformation and the intensity is in general, low or medium high¹⁷.  The sixth ring stretching vibration or ring breathing mode appears as a weak band near 1000 cm^-1 in mono, 1,3-di and 1,3,5-trisubstituted benzenes. In the other wise substituted benzene, however, this vibration is substituent sensitive and difficult to distinguish from the ring in-plane deformation.      For tri-substituted phenyl ring the υPh modes¹⁷ are seen in the region 1640 – 1250 cm^-1 and these modes are observed at 1632,  1551, 1516, 1381, 1345 cm^-1 in the IR spectrum, 1620,  1333 cm^-1 in the Raman spectrum and at 1619,  1563, 1515, 1395, 1336 cm^-1 (DFT) theoretically. In asymmetric tri-substituted benzenes, when all the three substituents are light, the wavenumber interval of the breathing mode²⁶ is between 500 and 600 cm^-1.  When all the three substituents are heavy, the wavenumber appears above 1100 cm^-1. In the case of mixed substituents, the wavenumber is expected²⁶ to appear between 600 and 750 cm^-1. For the title compound the phenyl ring breathing mode is assigned at 796 cm^-1 theoretically.  Mary et al.²⁷ reported the ring breathing mode of tri-substituted benzene ring at 733 cm^-1 in the IR spectrum and at 738 cm^-1 theoretically. The in-plane bending δCH modes¹⁷ of the phenyl ring are expected above 1000 cm^-1. For the tri-substituted benzene ring these modes are observed at 1120,   1036 cm^-1 in IR,   1128, 1028 cm^-1 in Raman and at 1144, 1103, 1027 cm^-1 theoretically. The CH out-of-plane deformations¹⁷ are observed between 1000 and 700 cm^-1. Generally the CH out-of-plane deformations with the highest wavenumbers have a weaker intensity than those absorbing at lower wavenumbers. These γCH modes are observed at 872, 810, 719 (IR), 809, 725 (Raman), 864, 806, 721 cm^-1 (DFT). The in-plane and out-of-plane substituent modes of the phenyl ring are also identified and assigned (Table 1).

scheme 1 Click here to View scheme

Conclusion

The FTIR and FT-Raman spectra of 5-nitro-1,3-benzodioxole were recorded and analyzed. The harmonic vibrational wavaenumbers were calculated theoretically using Gaussian03 set of quantum chemistry codes. The calculated wavenumbers (B3LYP) agree well with the observed wavenumbers. The small differences between experimental and calculated vibrational modes are observed. It must be due to the fact that hydrogen bond vibrations present in the crystal lead to strong perturbation of the infrared wavenumbers and intensities of many other modes. Also we state that the experimental results belong to solid phase and theoretical calculations belong to gaseous phase.

Table 1: Calculated (scaled) wavenumbers, IR, Raman bands and assignments

B3LYP/6-31G*υ(cm-1)	IR intensity	Raman activity	IRυ(cm-1)	Ramanυ(cm-1)	Assignments
3145	6.64	54.46			υCH
3134	2.48	77.13			υCH
3110	1.89	92.31	3119		υCH
2992	40.89	163.43			υasCH2
2942	115.42	250.34	2925		υsCH2
1619	25.45	11.34	1632	1620	υPh
1596	15.65	73.75	1609		υasNO2
1563	189.45	6.23	1551		υPh
1515	1.32	38.51	1516		υPh
1474	302.11	0.49	1489		δCH2
1427	20.57	33.63	1437	1430	υsNO2
1395	3.13	12.10	1381		υPh
1372	13.89	12.77		1378	ωCH2
1336	428.93	404.83	1345	1333	υPh
1260	375.73	26.55	1270	1272	υCN
1224	40.89	6.75			υCOC
1160	0.01	9.54	1171		υCOC
1144	4.87	2.59			δCH
1103	10.66	0.97	1120	1128	δCH
1100	16.73	1.37	1115		τCH2
1046	22.51	17.81	1066	1051	υCOC
1027	145.43	3.68	1036	1028	υCOC,δCH
936	32.58	8.05	930	932	γCH
917	0.56	2.32	920		δNO2
907	29.56	7.10			ρCH2
864	24.85	1.08	872		γCH
806	13.30	2.52	810	809	γCH
803	1.55	6.40		805	ωNO2
796	33.34	23.60			υPh
721	31.63	2.05	719	725	γCH
715	3.60	11.80			γPh
678	0.79	0.23	686		δPh(X)
666	10.98	2.14			ρNO2
570	5.99	7.14	574		γPh(X)
544	0.13	0.10	546		δPh(X)
528	3.82	0.92			γPh(X)
411	0.58	0.50			δPh(X)
394	2.81	3.46			δCX(X)
339	0.01	2.62			δCX(X)
316	0.36	2.33			γCX(X)
224	0.13	2.51			γCX(X)
203	1.18	1.15		208	δCX(X)
119	4.59	0.02			γCX(X)
63	0.17	0.17		75	tPh
21	6.43	0.69			tNO2

υ-stretching; δ-in-plane deformation; γ-out-of-plane deformation; ω-wagging; ρ-rocking; t-torsion; as-asymmetric; s-symmetric; Ph-phenyl ring; X-substituent sensitive.

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