Cooperative Institute for Research in Environmental Sciences

A winning proposal for the Innovative Research Program, 2006:

Sunlight initiated chemistry by low energy vibrational overtone excitation of oxidized organics in the atmosphere

Investigators: V. Vaida, R. Skodje, K. Takahashi, K. Plath


Objectives. In a collaboration between experiment (Prof. V. Vaida) and theory (Prof. R. Skodje) we propose to investigate a new paradigm for sun light initiated chemical reactions occurring by low energy by excitation of vibrational overtones in the ground electronic state. The target for this study are oxidized atmospheric organics (acids, alcohols). An important objective of this work is to investigate the role of water in catalyzing environmental photoreactions. The contribution of such chemistry to photochemical processing of atmospheric aerosols, arctic ice and other environmental media will be investigated with expertise available in CIRES.

Background and Importance. The incoming beam of photons from the Sun, approximates the emission of a black body at about 5800K with a maximum flux in the red. The vast majority of photochemical reactions known in the atmosphere involve a molecule’s excited electronic states at sufficiently high energy to rupture covalent bonds. This process requires UV wavelengths > 400nm. Some of us have shown that photochemical reactions can occur in the ground electronic state by absorption of radiation by vibrational overtone transitions. The cross sections for such transitions near chemical thresholds are very low, yet in the Earth atmosphere, excitation is effected near the solar wavelengths maxima. Visible light, obviously abundant in the solar spectrum, is sufficiently energetic to excite high overtone states of chromophores such as the hydroxyl-group (OH). If the overtone transition is sufficiently intense, and if the vibrational energy is effectively transferred to the reaction coordinate, vibrational overtone absorption may provide an important new mechanism for atmospheric chemistry. Previous studies by Vaida and coworkers suggest that vibrational overtone absorption likely plays a role in a number of atmospheric reactions.

This work is important in establishing the fundamental foundations for a new paradigm for processing organics in the atmosphere by sun light. Environmental circumstances where such chemistry could be important are photochemical processing on atmospheric aerosols, on ice, especially arctic ice where observations suggest additional mechanisms are needed to explain sun-rise promoted emissions from snowpacks.

What makes this innovative? This study has two innovative aspects:

A new paradigm is proposed for photochemistry in the atmosphere by solar pumping of vibrational overtone transitions of the ground electronic state. For organic acids and alcohols under investigation, the excited state photochemistry normally considered is irrelevant since the electronic states absorb in the UV at wavelengths filtered by stratospheric ozone.

An interesting possibility is the lowering of the transition states for reaction by water. If this investigation determines the viability of such water-catalyzed reactions, water becomes an important environmental catalyst for processing of oxidized organics not previously considered.

How might this be interdisciplinary? This study requires a synergistic approach based in chemical theory, laboratory experiments and atmospheric science. We described in the initial stage the collaboration between theory and experiment. Once the results are available, expertise with atmospheric aerosols, field observations regarding sun-rise emissions from arctic ice and atmospheric modeling are necessary and available in CIRES.

Research Plan. This study chooses for illustration the molecule methane-diol, CH2(OH)2 and its hydrated form CH2(OH)2·H2O. The dehydration reaction

CH2(OH)2 + hv → CH2O + H2O (A)
CH2(OH)2·H2O + hv → CH2O + 2H2O (B)

will be used as prototype reactions to study the vibrational overtone absorption mechanism. We already computed the reaction paths for A and B. A very interesting result was obtained. While the barrier to reaction was approximately 45 kcal/mol for reaction A, the reaction of the van der Waals complex (B) takes place with a barrier of only approximately 29 kcal/mol suggesting the possibility of catalysis of reaction (A) by water. The main focus of the theoretical research is the dynamics and kinetics of the vibrational overtone absorption induced chemical reactions using the method of direct dynamics.

The experimental work will involve two related studies:

A spectroscopic study of the vibrational spectrum of this molecule never before obtained. A Fourier-Transform spectrometer available in the Vaida group will be used to obtain the mid IR spectrum. A newly developed cavity-ring-down spectrometer will allow the investigation of high OH vibrational overtones, which are at the barrier for reaction.

A photochemical study will be performed to obtain the quantum yields and wavelengths dependent photochemical cross sections. With this information the J value for CH2(OH)2 can be obtained.

Expected Outcome and Impact. The goal of this inquiry is to acquire a valid theoretical description confirmed by experiment of how water catalyzed photochemical reactions occur. The outcome will be to introduce in atmospheric models new classes of photo-processing reactions relevant to atmospheric aerosols, ices and other water-air environmental interfaces.