Crystal and molecular structure studies of (Z)-N-methyl-C-4-substituted phenyl nitrones by XRD, DFT, FTIR and NMR methods

Abstract ( Z )- N -methyl- C -4-substituted phenyl nitrones – O + N(Me)=C(H)R ( Z -2a R = 4-ClC 6 H 4 , Z- 2b R = 4-NO 2 C 6 H 4 , Z- 2c R = 4-CH 3 OC 6 H 4 ) were synthesized and characterized by elemental analyses, FTIR, 1 H, 13 C and DEPT-135 NMR spectroscopy and also by single crystal X-ray diffraction (in the case of Z -2a and Z -2b ). The geometries of the nitrone molecules Z - 2a , Z- 2b and Z- 2c and their E- isomers; ( E )- N -methyl- C -4-chlorophenyl nitrone E -2a , ( E )- N -methyl- C -4-nitrophenyl nitrone E -2b and ( E )- N -methyl- C -4-methoxyphenyl nitrone E -2c were optimized using density functional theory (DFT) at the B3LYP/6-311++G(d,p) level of theory. The theoretical vibrational frequencies obtained by DFT calculations are in good agreement with the experimental values. The electronics structures were described in terms of the distribution of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Gauge independent atomic orbital (GIAO) method was used to calculate the NMR spectra, the correlation between the calculated and experimental chemical shifts is mostly in the range of 0.94–0.97 for 1 H, whereas, the correlation for 13 C is 0.99. Thermodynamics study showed that the Z- isomer is favoured than E -isomer with energy barrier of 7.1, 7.2 and 7.1 kcal/mol for Z- 2a , Z- 2b and Z- 2c , respectively. The abundance of the most stable species Z -isomers is equal to 99.99% for all three compounds at 298 K in gas phase.