Photolytic Studies of Norbornadiene Derivatives under High-Intensity Light Conditions.

The photoconversion of a norbornadiene (NBD) derivative was studied under high-intensity mono- and polychromatic light conditions at high concentrations. The photoisomerization quantum yield (ϕNBD→QC), proceeding from NBD to its quadricyclane (QC) isomer, was determined using a tunable OPO laser and a solar simulator light source. The solar simulator was designed to mimic the AM1.5G solar spectrum between 300 and 900 nm. Using the OPO laser, ϕNBD→QC was measured at discrete values between 310 and 350 nm in steps of 10 nm, and a variation between 0.81 and 0.96 was observed. Weighting these values of ϕNBD→QC with the spectral profile of the solar simulator, an averaged value of 0.87 ± 0.03 was obtained. Determination of ϕNBD→QC was also performed directly in the solar simulator providing a value of 0.97 ± 0.14, in good agreement with the weighted values from the OPO. Photoisomerization quantum yields were found to decrease slightly at higher concentrations. At high concentrations, we found that correcting for the presence of QC was important due to similar absorption coefficients of the NBD and QC isomers at the absorption tail. Cyclability of the forward and backward NBD/QC conversion was studied over several cycles. The NBD/QC couple exhibited excellent thermal stability, but a slight photodegradation per cycle was observed, increasing with the concentration of the sample. This result indicates that the molecules undergo some intermolecular reactions.

Coupling of torsion and OH-stretching in tert-butyl hydroperoxide. II. The OH-stretching fundamental and overtone spectra.

The vibrational spectra of gas phase tert-butyl hydroperoxide have been recorded in the OH-stretching fundamental and overtone regions (ΔvOH = 1-5) at room temperature using conventional Fourier transform infrared (ΔvOH = 1-3) and cavity ring-down (ΔvOH = 4-5) spectroscopy. In hydroperoxides, the OH-stretching and COOH torsion vibrations are strongly coupled. The double-well nature of the COOH torsion potential leads to tunneling splitting of the energy levels and, combined with the low frequency of the torsional vibration, results in spectra in the OH-stretching regions with multiple vibrational transitions. In each of the OH-stretching regions, both an OH-stretching and a stretch-torsion combination feature are observed, and we show direct evidence for the tunneling splitting in the OH-stretching fundamental region. We have developed two complementary vibrational models to describe the spectra of the OH-stretching regions, a reaction path model and a reduced dimensional local mode model, both of which describe the features of the vibrational spectra well. We also explore the torsional dependence of the OH-stretching transition dipole moment and show that a Franck-Condon treatment fails to capture the intensity in the region of the stretch-torsion combination features. The accuracy of the Franck-Condon treatment of these features improves with increasing ΔvOH.

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

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.

Room Temperature Gas-Phase Detection and Gibbs Energies of Water Amine Bimolecular Complex Formation.

We have detected the H2O·DMA and H2O·TMA (DMA, dimethylamine; TMA, trimethylamine) bimolecular complexes at room temperature in the gas phase using Fourier transform infrared spectroscopy. For both complexes, five vibrational bands associated with the H2O molecule are observed and assigned. Within a reduced dimensional local mode framework, we set up a six-dimensional model, including the three H2O vibrational modes and three of the six intermolecular modes, all described with internal curvilinear coordinates. The single points on the potential energy surface and Eckart corrected dipole moment surface are calculated with the CCSD(T)-F12a/cc-pVDZ-F12 method. Combining the measured and calculated transition intensities, we determine the Gibbs energy of complex formation of both complexes from each of the observed bands. The multiple determinations give similar Gibbs energies, for each complex, and increase the confidence in the combined experimental and theoretical approach, and improve the accuracy of the determined Gibbs energies. The average Gibbs energies of complex formation are found to be 5.0 ± 0.2 and 3.8 ± 0.2 kJ/mol for H2O·DMA and H2O·TMA, respectively. In addition to the experimental uncertainty, there is a potential error on the calculated intensities corresponding to 0.4 kJ/mol. However, the small spread among the four determinations suggests that this error is even less. The Gibbs energies of these complexes serve as accurate benchmarks for theoretical approaches that are prevalent in hydrogen bonding and nucleation studies.

Attenuated Deuterium Stabilization of Hydrogen-Bound Complexes at Room Temperature.

We have observed nine bimolecular hydrogen- or deuterium-bound complexes at room temperature using Fourier transform infrared (FTIR) spectroscopy. The complexes were formed using methanol or ethanol as hydrogen bond donors, as well as deuterated isotopologues of these, in order to study isotopic effects on hydrogen bonds. The complexes were formed using either a dimethylether- (O) or trimethylamine (N) acceptor, to facilitate comparison of two different types of hydrogen or deuterium bonds, OH(D)·O and OH(D)·N. For each complex, the characteristic OH- or OD-stretching fundamental band in the bimolecular complex was observed. The Gibbs energy of complex formation was determined at room temperature for each complex to compare the relative stability of hydrogen- and deuterium-bound bimolecular complexes. It is well known that deuterium-bound complexes are more stable at low temperatures because of the lower frequency of its intermolecular modes and thus a lower zero point vibrational energy. However, at room temperature, entropic contributions to the stability should also be considered. At room temperature, we find the Gibbs energy of complex formation for each pair of corresponding hydrogen- and deuterium-bound complex to be similar. The similar values of the Gibbs energies at room temperature is explained from a difference in the entropy, upon complexation, which favors the formation of the hydrogen-bound complex more than the deuterium-bound complex at higher temperatures.

Room Temperature Gibbs Energies of Hydrogen-Bonded Alcohol Dimethylselenide Complexes.

A number of hydrogen-bonded complexes, formed between an alcohol donor and dimethylselenide, have been detected experimentally, at room temperature in the gas phase using FTIR spectroscopy. The Gibbs energy of complex formation has been determined from the measured integrated absorbance of the hydrogen-bonded OH stretching band and the calculated oscillator strength of the associated transition. The OH stretching frequency and Gibbs energy of the selenium hydrogen-bonded complexes are compared to those found in complexes with the same donor molecule and either dimethylether (O) or dimethylsulfide (S) as the acceptor molecule. For a given donor, we found a similar OH stretching frequency in the complexes for each of the three acceptors O, S, and Se. However, the Gibbs energies were found to be less positive (i.e., stronger bound) for the dimethylether complexes (OH·O), as compared to the dimethylsulfide (OH·S) and dimethylselenide (OH·Se) complexes, with the latter two having comparable Gibbs energies.

Hybridization of Nitrogen Determines Hydrogen-Bond Acceptor Strength: Gas-Phase Comparison of Redshifts and Equilibrium Constants.

Gas-phase Fourier transform infrared spectroscopy and quantum chemical calculations are combined to illustrate the effect of hybridization on the hydrogen-bond acceptor strength of nitrogen by a comparison of nine bimolecular complexes. We present gas-phase results for the complexes of methanol, ethanol, and 2,2,2-trifluoroethanol with acetonitrile (sp-hybridized N) and find that the structure of these complexes is nearly linear and dominated by the OH···N hydrogen bond with no experimental indication of an OH-π bonded structure. We compare experimental redshifts and equilibrium constants, obtained by combining experiments and theory, for these complexes to the corresponding complexes with pyridine (sp2-hybridized N) and trimethylamine (sp3-hybridized N). The comparison clearly illustrates that increasing the s-character of the nitrogen lone pair decreases the hydrogen-bond acceptor strength (sp3 > sp2 > sp). The observed trend correlates with the basicity of the acceptors and can be explained by the partial charge on the accepting nitrogen atom and the degree of localization of the nitrogen lone pair.