. INTERMOLECULAR POTENTIAL FUNCTIONS AND HIGH RESOLUTION MOLECULAR SPECTROSCOPY OF WEAKLY BOUND COMPLEXES Final Progress Report John S. Muenter Department of Chemistry University of Rochester Rochester, N.Y. 14627 Prepared for the U. S. Department of Energy Under Grant DE-FG0293ER6 1596 This report describes accomplishments over the past year in research supported by this grant. Two papers published in this period are briefly discussed. The general goal of the work is to consolidate the understanding of experimental results through a theoretical model of intermolecular potential energy surfaces. Progress in the experimental and theoretical phases of the program are presented and immediate goals outlined. The ability to construct analytic intermolecular potential fhctions that accurately predict the energy of interaction between small molecules will have great impact in many areas of chemistry, biochemistry, and biology.
DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liability or respoirsibilily for the accuracy, completeness, or usefulness of any information, apparatus, product, or proms disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
PROGRESS REPORT The work done under DOE grant DE-FG0293ER61.596 is contained in the annual report, which is included here. This project has occupied, and will continue to occupy, approximately 15% of the principal investigator s efforts. PROPOSED TECHNICAL SCOPE The purpose of the proposed work is to study weakly bound molecular complexes to better understand intermolecular interactions. Intermolecular potential functions play major roles in every aspect of chemistry, including both atmospheric and biological systems. Techniques involving infrared laser and microwave excitation and absorption oe molecular beams will be developed. Theoretical models to analyze experimental data in terms of potential functions will be created. FINANCIAL STATEMENT The funds in the present budget for DE-FG0292ER61596 will be expended by 6/30/1994. STATEMENT OF OTHER GOVERNMENT AGENCY SUPPORT Similar, but not identical research is supported by NSF grant CHE-9121534. The work supported by this grant is related to this NSF supported project but the actual expense items, essentially all salaries, are distinct.
3 INTRODUCTION This report covers the first ten months of- this project, funded for three years by DE- FG0293ER61596. This report will emphasize the results of a useful intermolecular potential function model. PREVlOUS RESULTS Much oe the past year has been spent making improvements to experimental apparatus and refining our potential function model. Two papers have appeared in the past year, and three papers are currently being written. The two published papers present both experimental and theoretical results for small van der WaaIs cluster molecules. The three manuscripts in preparation report on calculations carried out with a substantially improved intermolecular potential function model. The first paper (J. Chem. Phys. 97, 8850 (1992)) describes an extensive experimental and theoretical study of the nuclear quadrupole hyperfine structure of perdeuterated acetylene dimer, This work, done in collaboration with French theoretician Laurent Coudert, is the seminal work on hyperfine properties of molecular complexes undergoing large amplitude tunneling motions. These results relate directly to our potential function calculations, because experimental hyperfine data provide direct measures of the curvatures of the potential with respect to angular coordinates. One of the main reason we have confidence in our current potential function model is its ability to predict bending force constants that have been measured from experimental nuclear hyperfine studies. The second paper (J. Molec. Spec. 160, 422 (1993)) deals with accurate electric dipole moment measurements in rare gas-carbonyl sulfide complexes. A central theme of this project is to ' understand the effects of large amplitude internal motions on experimentally accessible dipole moments. Since these motions can be calculated from our potential functions, this work is again directly connected to modelling efforts. Experimental efforts have been directed toward improving the sensitivity and resolution of
4 0 our molecular beam, infrared laser absorption spectrometer. The first step in this process was having our laser upgraded to current commercial specifications. The modifications will extend the wave length tuning range and ease of single mode scanning. The process of getting this accomplished, however, has been very time consuming. The laser manufacturer, Burleigh Instruments, was not able to meet its promised delivery date for the return of the laser, and even after the work was completed, the agreed upon specifications have not been met. This has wasted much time and has been a frustrating experience. A second major project has been the development of a new type of rotating slit nozzle. Successful development of this component will significantly improve sensitivity and resolution. A prototype has been designed and constructed. The initial design has proved the principle, but substantial modifications are required to achieve a practical device. This work is important and ongoing. Several developments have been made to our potential function model. Basic changes to the model include replacing R-I2 repulsion with exponential repulsion and adding centers of dispersion located within chemical bonds. These changes produce significantly improved results or interactions between slightly anisotropic partners. Application programs using the potential function have also been improved. A major effort has been made to develop automated searching programs to minimize the eneru. This capability has permitted our first studies of fully relaxed, nonpianar trimeric species. These procedures optimize all nine degrees of freedom and have been applied to (HCCH),, (CO,),, (HCCH),CO, and HCCH(CO,),. A manuscript describing this work is currently being written. Papers are also being prepared on studies of hydrogen halide dimers and HX-C02 complexes.