Weak interactions between molecules control a wide range of phenomena. Some examples are the following: the intricate folding of a protein delicately positions an enzyme and its substrate in a biochemical reaction. The attraction between water molecules in the upper atmosphere leads to cluster growth and eventual droplet and cloud formation. Even ordinary chemical reactions in solvents are often intimately affected by the weak forces which influence the solute-solvent interaction. Our research is directed at understanding these weak forces, which are commonly referred to as van der Waals forces. They include dispersion forces, electrostatic interactions, and hydrogen bonding. For this purpose a Fourier transform microwave spectrometer has been constructed in our laboratory which employs a pulsed supersonic nozzle to form a cold molecular beam.
A Fourier transform microwave (MW) spectrometer operates by pulsing MW radiation at molecules, followed by detection of a remitted signal (called the free induction decay). This probes the quantized rotational energy levels. The rotational spectrum is obtained from which detailed structural information can be deduced. By using a supersonic nozzle, weakly bound species (such as s water dimer or trimer) can be formed as a gas expands through a pinhole into a vacuum. This technique makes it possible to study the structures, dipole moments, internal dynamics and interaction forces for a variety of dimers, trimers, and small clusters held together by very weak forces.
Studies in our laboratory have focused on complexes of sulfur dioxide with amines and hydrocarbons. Complexes of water or methanol with a variety of partners have probed H-bonding interactions and solvent effects.Dimers and trimers involving greenhouse gases such as carbon dioxide or nitrous oxide have been a recent interest. Analysis of trends in the structural details of such complexes yields insights on the relative importance of classical electrostatic, dispersion, induction, and repulsive contributions to the intermolecular interaction potential. Knowledge of the potential surfaces for such prototype systems should be transferable when modeling more complex systems which are less accessible to experimental work.
S.A. Peebles and R.L. Kuczkowski, Rotational Spectrum, Structure and Internal Motions of the Ethylene-OCS Weakly Bound Dimer, Mol. Phys., 2001, 99, 225..
J.J. Oh, I. Park, R.J. Wilson, S.A. Peebles, R.L. Kuczkowski, E. Kraka, D. Cremer, Structure of the Chlorobenzene-argon Dimer: Microwave Spectrum and ab initio Analysis, J. Chem. Phys. 2000, 113, 9051.
R.A. Peebles and R.L. Kuczkowski, (N2O)2SO2: Rotational Spectrum and Structure of the First van der Waals Trimer Containing Sulfur Dioxide. J. Chem. Phys. 2000, 112, 8839.
R.J. McMahon, R.J.Halter, R.L. Fimmen, R.J. Wilson, S.A. Peebles, R.L. Kuczkowski and J.F. Stanton, Equilibrium Structure of cis-Hex-3-ene-1,5-diyne and Relevance to the Bergman Cyclization, J. Am. Chem. Soc. 2000, 122, 939.
R.A. Peebles, S.A. Peebles, Microwave Spectrum and Structure of the (CO2)2N2O Complex, Mol. Phys. 1999, 96, 1355.