Absolute Intensities of NH-Stretching Transitions in Dimethylamine and Pyrrole

Abstract

Vibrational spectra of vapor-phase dimethylamine (DMA) and pyrrole have been recorded in the 1000 to 13000 cm(-1) region using long path conventional spectroscopy techniques. We have focused on the absolute intensities of the NH-stretching fundamental and overtone transitions; Δν(NH) = 1-4 regions for DMA and the Δν(NH) = 1-3 regions for pyrrole. In the Δν(NH) = 1-3 regions for DMA, evidence of tunneling splitting associated with the NH-wagging mode is observed. For DMA, the fundamental NH-stretching transition intensity is weaker than the first NH-stretching overtone. Also, the fundamental NH-stretching transition in DMA is much weaker than the fundamental transition in pyrrole. We have used an anharmonic oscillator local mode model with ab initio calculated local mode parameters and dipole moment functions at the CCSD(T)/aug-cc-pVTZ level to calculate the NH-stretching intensities and explain this intensity anomaly in DMA.

Fundamental and overtone vibrational spectroscopy, enthalpy of hydrogen bond formation and equilibrium constant determination of the methanol–dimethylamine complex

“…S10, ESI †), two bands located at 9656 and 9410 cm À1 are observed in the 9000-9800 cm À1 region. 57 Similar to the complex, we assign the 9656 cm À1 band to the 3ñ NH transition of DMA and the 9410 cm À1 band to a hot band combination of a low frequency methyl torsion with a wavenumber of B246 cm À1 with the 3ñ NH transition. 57 The B3LYP/aug-cc-pVTZ calculated normal mode harmonic frequency of the methyl torsion in DMA is 226 cm À1 , 57 which support our assignment of the 9410 cm À1 band.…”

“…57 Similar to the complex, we assign the 9656 cm À1 band to the 3ñ NH transition of DMA and the 9410 cm À1 band to a hot band combination of a low frequency methyl torsion with a wavenumber of B246 cm À1 with the 3ñ NH transition. 57 The B3LYP/aug-cc-pVTZ calculated normal mode harmonic frequency of the methyl torsion in DMA is 226 cm À1 , 57 which support our assignment of the 9410 cm À1 band. It is not possible to record spectra with even higher vapor pressures of the two gases because the 3ñ NH transition of DMA will be too strong and saturate the detector.…”

“…The NH-stretching transition shows, similar to DMA, that the first overtone is stronger than the fundamental transition. 57 The OH-stretching fundamental transition intensity of the MeOH-DMA and MeOH-TMA complexes are calculated to be 51 and 61 times stronger than that of methanol. The intensity increase ofñ XH is considered to be a criterion for hydrogen bonding.…”

“…55 Results of this method have previously been found to be in good agreement with results obtained using much larger basis sets using conventional CCSD(T) theory and where possible with experiment. 38,[56][57][58][59][60] The OH-and NH-stretching transition wavenumbers and intensities for the complexes and monomers were calculated with an anharmonic oscillator local mode model. [61][62][63][64][65]…”

The effect of large amplitude motions on the vibrational intensities in hydrogen bonded complexes

“…Studies in the literature suggest that experimental intensities of molecules can be predicted within at least a factor of 2 for XH-stretches. 24,25 From an experimental point of view, the fundamental XH b -stretching transition is ideal for the determination of the equilibrium constant, as it is often redshifted and its intensity is significantly enhanced compared to the corresponding monomer transition. However, it is a challenge to calculate accurately the transition frequency and oscillator strength of this vibrational transition.…”