James T. (Casey) Hynes of the University of Colorado Boulder is supported by an award from the Theoretical and Computational Chemistry program to carry out research on the development of theoretical and computational methods for the investigation of chemical reactions in solutions and at surfaces. Three topics are being investigated: the environment's influence on conical intersections is being studied; water molecule dynamics near ions and in the grooves of DNA are being investigated; and a simulation of the formation of amino acid molecules on the surface of icy particles, of interest in the field of astrobiology, is being carried out.
This work is having a broader impact on many areas of science from biology to astrobiology as well as through the training of graduate student researchers. Topic 1 will shed light on and provide a practical, useful description for excited to ground state transitions via conical intersections in polar environments, a very general phenomenon in all areas of chemistry, including biophotochemical phenomena such as vision. Topic 2 on water dynamics in hydration shells and DNA grooves will bring new insight into hydrogen bond dynamics and the microscopic details of e.g. ionic reactions and drug insertion into DNA. Topic 3 deals with aerosols of astrobiological and environmental significance, respectively related to a possible mechanism for synthesizing amino acids in the Interstellar Medium and to atmospheric ozone depletion. Each topic will provide training of students in the multi-faceted combination of techniques necessary to deal with complex condensed phase reactivity, and each provides features suitable for incorporation into modern interdisciplinary courses centered on physical chemistry.
The theoretical research supported by this grant resulted in a number of outcomes of importance for understanding and predicting the rates and mechanisms of (A) chemical reactions in water and (B) related dynamics of water, including molecular reorientation and vibrational energy flow. The outcomes for Topic (A) divide into three subjects. The first subject was nitric acid, which is a strong acid in water. We determined by computer simulation whether it is still a strong acid at the surface of water at the low temperatures relevant for the upper troposphere; the answer to this question is important for tropospheric ozone depletion. Our simulations showed that, and explained why, nitric acid remains a strong acid, provided that it can penetrate slightly below the top of the water surface. The second subject was the oxidation of water, which is related to splitting of water into oxygen and hydrogen gases, and is an important topic for solar energy conversion. We found that the catalysis, i.e. the acceleration, of this reaction by a well-known metal-containing complex(the ruthenium blue dimer) proceeds by a mechanism quite similar to that we found previously for the reactions occurring on ice responsible for the Antarctic Ozone Hole. We also found that if the reaction could be carried out in an environment where the amount of surrounding water was reduced, the reaction could speed up significantly. Since water oxidation needs to be very considerably speeded up for solar energy conversion to be practical, this finding may have important consequences. The third subject for Topic (A) was the influence of a water solvent (and other solvents) on a photo-induced isomerization, of the type which occurs in the vision process. It has been known that the presence of a special feature called a conical intersection is crucial to allow such reactions to be very rapid and efficient, but the influence of a solvent upon this has not been well understood. We could show via theoretical modeling that in fact the solvent’s influence can be to accelerate the photoreaction and to increase the yield of the desired reaction product. These conclusions, if they carry over to similar photo-reactions occurring in biomolecules, should prove important in understanding and controlling such reactions. The outcomes for Topic (B) divide into two subjects. The first subject was the mechanism and rate of reorientation of a water molecule next to different types of solutes dissolved in water. This phenomenon is important for a wide range of issues, including the rates of chemical reactions in water and the folding of proteins into their natural state. Building on our recently proposed novel mechanism involving large, sudden angular jumps of the molecule, we determined ---and characterized by simple mathematical models ---the influence of different types of solutes on this reorientation rate. A key aspect here is our realization that the water jump involves a switching of hydrogen-bond partners (hydrogen-bonds are responsible for the very special nature of water). For hydrophobic ("water-hating") solutes, the reorientation slows due to the difficulty of the exchange caused by the physical size of the solute. For hydrophilic ("water-loving") solutes, the reorientation can be speeded up by a switch between a weak hydrogen bond to a strong one. For solutes containing both hydrophobic and hydrophilic portions, there is a competition between the two effects. We also showed from this work that several recent ultrafast spectroscopic experiments needed to be reinterpreted. The second subject for Topic (B) concerned the rate and mechanism of energy transfer from a vibrationally excited water molecule in water. Such transfer can play an important part in e.g. the details of chemical reactions in water. Modern ultrafast spectroscopy experiments had shown that, when a water molecule’s bending motion was excited, this energy flow was extremely fast, but could not establish the flow mechanism. We determined this mechanism in detail and explained its rapidity, tracing the energy transfer from the water bend to the rotation of the excited water itself and to its nearest water molecule neighbors. The broader impacts of our research concerning various scientific issues and relevance has been indicated for each of the subjects discussed above. The research has involved the training of one undergraduate, two doctoral students and three postdoctoral students at Boulder. Dissemination of the research supported by this grant resulted in 24 papers. It has been presented by the PI in 23 invited lectures at national/international meetings and in 6 departmental seminars; the PI’s students have presented 4 invited lectures, 10 contributed lectures and 13 contributed posters at national/international meetings and 9 invited departmental seminars. Aspects of this research have been incorporated in a mathematical methods graduate course and an Honors General Chemistry undergraduate course at Boulder, as well as in a popular science lecture (K-10) in the Univ. of Colorado ’CU Wizards’ series.
