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

Abstract

We have measured gas phase vibrational spectra of the bimolecular complex formed between methanol (MeOH) and dimethylamine (DMA) up to about 9800 cm(-1). In addition to the strong fundamental OH-stretching transition we have also detected the weak second overtone NH-stretching transition. The spectra of the complex are obtained by spectral subtraction of the monomer spectra from spectra recorded for the mixture. For comparison, we also measured the fundamental OH-stretching transition in the bimolecular complex between MeOH and trimethylamine (TMA). The enthalpies of hydrogen bond formation (ΔH) for the MeOH-DMA and MeOH-TMA complexes have been determined by measurements of the fundamental OH-stretching transition in the temperature range from 298 to 358 K. The enthalpy of formation is found to be -35.8 ± 3.9 and -38.2 ± 3.3 kJ mol(-1) for MeOH-DMA and MeOH-TMA, respectively, in the 298 to 358 K region. The equilibrium constant (Kp) for the formation of the MeOH-DMA complex has been determined from the measured and calculated transition intensities of the OH-stretching fundamental transition and the NH-stretching second overtone transition. The transition intensities were calculated using an anharmonic oscillator local mode model with dipole moment and potential energy curves calculated using explicitly correlated coupled cluster methods. The equilibrium constant for formation of the MeOH-DMA complex was determined to be 0.2 ± 0.1 atm(-1), corresponding to a ΔG value of about 4.0 kJ mol(-1).

Similar Strength of the NH···O and NH···S Hydrogen Bonds in Binary Complexes

“…Unlike ab initio calculated equilibrium constants, calculated oscillator strengths show significantly less variation toward the computational level of theory. 40 Therefore, we expect the calculated oscillator strength to be in good agreement with the experimental oscillator strength and, thus, provide an accurate value for the abundance of the complex. The linear fit corresponding to eq 6 for the DMA−DME and DMA−DMS complexes is shown in Figure S7 in the Supporting Information.…”

“…The enthalpy of complex formation for the MeOH−DMA complex was predicted to be −28.3 kJ mol −1 with the wB97XD/aVTZ method, in reasonable agreement with experimental results of −25.9 ± 1.0 and −35.8 ± 3.9 kJ mol −1 in two separate experiments, taking into account that the enthalpy is difficult to both calculate and determine experimentally. 40,41 In comparison, the corresponding value predicted with the B3LYP/aVTZ method is −19.8 kJ mol −1 . 40 The structures of the compounds were also optimized at the CCSD(T)-F12a 42 /VDZ-F12 43 (F12) level of theory using Molpro.…”

“…40,41 In comparison, the corresponding value predicted with the B3LYP/aVTZ method is −19.8 kJ mol −1 . 40 The structures of the compounds were also optimized at the CCSD(T)-F12a 42 /VDZ-F12 43 (F12) level of theory using Molpro. 44 The CCSD(T)-F12 methods have been shown to give very accurate intermolecular distances and interaction energies of hydrogen-bonded complexes, 45 as well as accurate XH-stretching (X = O, C, F, Cl) fundamental and overtone transition wavenumbers and intensities.…”

“…We find that the B3LYP binding energies are underestimated compared to wB97XD and F12 binding energies, in agreement with previous studies and the lack of dispersion in the B3LYP approach. 9,40 The calculated ZPVE corrected binding energies, −BE ZPVE , at the F12 level of theory for A and B of the DMA−DME complex are of comparable size. Conformer B of the DMA− DMS complex has a higher absolute −BE ZPVE than conformer A, but in general the complexes have similar binding energies, indicating similar strength.…”

Weak hydrogen bonding competition between O–H⋯π and O–H⋯Cl

“…Practically, a pressure "calibration" procedure was performed. 14 We measured a pure TFE spectrum at a certain pressure (nominal) immediately aer lling the vapor into the cell. In the spectral subtraction, a weighting factor was applied to the pure TFE spectrum so that the TFE transitions matched in regions where only TFE was absorbed.…”