Coupling of torsion and OH-stretching in tert-butyl hydroperoxide. I. The cold and warm first OH-stretching overtone spectrum

Abstract

The infrared (IR) spectrum of tert-butyl hydroperoxide (TBHP) in the region of the first OH-stretching overtone has been observed under jet-cooled and thermal (300 K, 3 Torr) conditions at ∼7017 cm−1. The jet-cooled spectrum is recorded by IR multiphoton excitation with UV laser-induced fluorescence detection of OH radical products, while direct IR absorption is utilized under thermal conditions. Prior spectroscopic studies of TBHP and other hydroperoxides have shown that the OH-stretch and XOOH (X = H or C) torsion vibrations are strongly coupled, resulting in a double well potential associated with the torsional motion about the OO bond that is different for each of the OH-stretching vibrational states. A low barrier between the wells on the torsional potential results in tunneling split energy levels, which leads to four distinct transitions associated with excitation of the coupled OH-stretch-torsion states. In order to interpret the experimental results, two theoretical models are used that include the OH-stretch-torsion coupling in TBHP. Both methods are utilized to compute the vibrational transitions associated with the coupled OH-stretch-torsion states of TBHP, revealing the underlying transitions that compose the experimentally observed features. A comparison between theory and experiment illustrates the necessity for treatments that include OH-stretch and COOH torsion in order to unravel the spectral features observed in the first OH-stretching overtone region of TBHP.

Hydrogen Delocalization in an Asymmetric Biomolecule: The Curious Case of Alpha-Fenchol

“…An error in the calculated asymmetry of the potential is likely not to blame for this deviation, as the agreement cannot be substantially improved by varying the asymmetry in a concerted way for the OH stretch ground and excited state ( Figure S36 ). This underestimation of transition intensities connecting torsional wave function with different number of nodes by the Franck–Condon model is reminiscent of the situation for OH stretching spectra of symmetric hydroperoxides [ 114 , 115 , 116 ]. It was explained by a neglected component of the transition dipole moment being an antisymmetric function of the torsional coordinate, which needs to be factored into the symmetry selection rules.…”

“…Analogous transitions for hydroperoxides were shown to have non-negligible infrared activity for multi-quanta OH stretching excitation [ 114 , 115 , 116 ], contrary to the symmetry-selection rules from the simple vibrational Franck–Condon model; for single-quantum OH, stretch excitation even intensities comparable with the symmetry-conserving transitions are predicted [ 114 , 116 ]. Considering the structural similarities between alkyl hydroperoxides and alcohols, we extend here our investigation of propargyl alcohol to FTIR detection.…”

Hydrogen Delocalization in an Asymmetric Biomolecule: The Curious Case of alpha-Fenchol

“…No Raman activity for conceivable symmetry-changing transitions (u 1 ← l 0 and l 1 ← u 0 ) at 3680 or 3640 cm −1 was observed. Analogous transitions for hydroperoxides were shown to have non-negligible infrared activity for multi-quanta OH stretching excitation [93][94][95], contrary to the symmetryselection rules from the simple vibrational Franck-Condon model; for single-quantum OH stretch excitation even intensities comparable with the symmetry-conserving transitions are predicted [93,95]. Considering the structural similarities between alkyl hydroperoxides and alcohols, we extend here our investigation of propargyl alcohol to FTIR detection.…”

“…Due to known shortcomings of the Franck-Condon model [93][94][95] and the sensitivity on the details of the potential, the latter hypothesis however requires further investigation, especially explicit theoretical modelling of relative IR and Raman intensities. Further experimental insights might come from microwave characterization of the second and perhaps also the third torsional state populated in a milder jet expansion or at ambient temperature, a direct measurement of the mm-wave or THz torsional transitions expected at 16(1) (OH) and 7(1) cm −1 (OD), as well as analysis of OH and OD stretching overtone spectra.…”

“…An error in the calculated asymmetry of the potential is likely not to blame for this deviation, as the agreement cannot be substantially improved by varying the asymmetry in a concerted way for the OH stretch ground and excited state (Figure S36). This underestimation of transition intensities connecting torsional wave function with different number of nodes by the Franck-Condon model is reminiscent of the situation for OH stretching spectra of symmetric hydroperoxides [93][94][95] It was explained by a neglected component of the transition dipole moment being an antisymmetric function of the torsional coordinate, which needs to be factored into the symmetry selection rules.…”