Infrared and Raman Spectral Signatures of Aromatic Nitration in Thermoplastic Urethanes

Abstract

The spectral signatures of nitro attack of the aromatic portion of thermoplastic urethanes (TPU) were determined. Eight fragment molecules were synthesized that represent the nitrated and pristine methylenediphenyl section common to many TPUs. Infrared (IR) and Raman (785 nm illumination) spectra were collected and modeled using the B3LYP/6-31G(d)//B3LYP/6-31G(d) model chemistry. Normal mode animations were used to fully assign the vibrational spectra of each fragment. The vibrational assignment was used to develop a diagnostic method for aromatic nitro attack in thermoplastic urethanes. The symmetric NO₂ stretch coupled out of phase with the C–NO₂ stretch (1330 cm⁻¹) was found to be free from spectral interferences. Spectral reference regions that enable correction for physical differences between samples were determined. The carbonyl stretch at 1700 cm⁻¹ was the best IR reference region, yielding a limit of quantitation (LOQ) of 0.66 ± 0.02 g N/100 g Estane. Secondary IR reference regions were the N–H stretch at 3330 cm⁻¹ or the urethane nitrogen deformation at 1065 cm⁻¹. The reference region in the Raman was a ring stretching mode at 1590 cm⁻¹, giving an LOQ of 0.69 ± 0.02 g N/100 g Estane. Raman spectroscopy displayed a larger calibration sensitivity (slope = 0.110 ± 0.004) than IR spectroscopy (slope = 0.043 ± 0.001) for nitration determination due to the large nitro Raman cross-section. The full spectral assignment of all eight molecules in the infrared and Raman is presented as supplemental material.