Gas-Phase Room-Temperature Detection of the tert-Butyl Hydroperoxide Dimer

Abstract

We have detected the tert-butyl hydroperoxide dimer, (t-BuOOH) 2 , in the gas phase at room temperature using conventional FTIR techniques. The dimer is identified by an asymmetric absorbance band assigned to the fundamental hydrogen-bound OH b -stretch. The weighted band maximum of the dimer OH b -stretch is located at ∼3452 cm −1 , red-shifted by ∼145 cm −1 from the monomer OH-stretching band. The gas-phase dimer assignment is supported by Ar matrix isolation FTIR experiments at 12 K and experiments with a partially deuterated sample. Computationally, we find the lowest energy structure of (t-BuOOH) 2 to be a doubly hydrogen bound six-membered ring with non-optimal hydrogen bond angles. We estimate the gas-phase constant of dimer formation, K, to be 0.4 (standard pressure of 1 bar) using the experimental integrated absorbance and a theoretically determined oscillator strength of the OH b -stretching band.

Effect of Temperature on the OH-Stretching Bands of the Methanol Dimer

“…The dimer spectra were obtained by subtracting spectra recorded at a low pressure from those recorded at a higher pressure with a suitable scaling factor. As the formation of the dimer scales with the square of the monomer pressure, intensity of the OH-stretching band from the dimer remains after subtracting the monomer signals. , An example of subtraction is shown for the OH-stretching region of CH 3 OH in Figure . Apart from the bound OH-stretching band associated with the dimer, the subtraction leaves some monomer residuals in the high-energy tail of the observed dimer band due to differences in the pressure broadening of the individual ro-vibrational transitions in the monomer OH-stretching band.…”

Room Temperature Gas Phase Equilibrium Constants of the Methanol Dimer, Trimer, and Tetramer

“…The experimental setup and approach has been described previously ,,, and is summarized. Spectroscopic grade methanol, MeOH ≥ 99.9%, was purchased from Sigma-Aldrich.…”

“…The main challenge in obtaining equilibrium constants is to determine the small partial pressures of the clusters. To do this, we use a hybrid theoretical/experimental spectroscopic approach, , which we have used previously to determine small partial pressures of molecular complexes in gas mixtures. ,,,,, The partial pressure of a cluster can be determined from the ratio of a measured integrated absorbance and the corresponding calculated oscillator strength of the band: p c l u s t e r = 2.6935 × 10 − 9 Torr m cm normalK T f c a l c · l · prefix∫ A ( ν̃ ) .25em normald ν̃ where p cluster is the cluster partial pressure, f calc is the calculated oscillator strength, l the path length of the cell, and A (ν̃) the wavenumber dependent base-10 absorbance of the band. We focus on the absorption band associated with the bound OH b -stretch, thus taking advantage of the OH-stretching redshift and intensity enhancement upon hydrogen bonding. , We approximate the oscillator strength of the band as the oscillator strength of the ground-state OH-stretching transition .…”

“…The coupling leads to mixing of the OH-stretching modes and a redistribution of intensity, but does not have a large effect on the intensity of the total OH b -stretching band. The frequency and oscillator strength of the OH b -stretching transition were calculated with a 1D local mode model programmed in Python 3. , The PES was sampled along the OH-bond displacement coordinate: q = [−0.5 Å ; 1.5 Å] in 0.05 Å increments from the equilibrium geometry. The points were interpolated to Δq = 0.00002 Å using a cubic spline to ensure convergence within the used Simpson’s integration method.…”

“…An alternative hybrid approach is to obtain the partial pressure of the clusters from a combination of experimental spectra and theoretical oscillator strengths. , The ratio of the measured integrated absorption band and the corresponding calculated oscillator strength of the band is proportional to the partial pressure of the cluster. This approach is especially suited for the detection and quantification of hydrogen bound complexes. ,− Upon hydrogen bonding, the OH-stretching band is red-shifted and the intensity of the band is enhanced, typically by a factor 20 or more. ,, The detection of an isolated OH b -stretching band usually relies on accurate spectral subtraction of the monomer species. ,,, The OH b -stretching band is a very sensitive and quantitative probe for the minute amounts of complex that forms at room temperature. Recently, this approach was used to quantify equilibrium constants of (MeOH) 2–4 , but oversaturation of the monomer signal drastically influenced the band shape and area of the clusters .…”