1. Structure and Dynamics of Hydrogen-Bonded and Related Interactions
In Collaboration with R.R. Lucchese
Current and past Support: Robert A. Welch Foundation,
National Science Foundation, ARP Texas Higher Education Coordinating Board
The proposed spectroscopic investigations are based on high resolution pulsed-nozzle FT microwave (in collaboration with NIST and University of Exeter, England), infrared and submillimeter/terahertz spectrometers. Broadband spectral coverage from 10 cm-1 to 10,000 cm-1 is also available using a highly sensitive Fourier-transform infrared (FTIR) spectrometer with both i) multi-pass static gas phase and ii) supersonic slit jet sample source capabilities. This feature of the spectrometer greatly facilitates the systematic observation of the wide range of intermolecular and intramolecular rovibrational band spectra necessary for the analyses proposed here. A high frequency wavelength modulation diode laser spectrometer with cw supersonic slit jet sample source developed in our group is also available for higher resolution studies[5,59]. A submillimeter/Terahertz spectrometer has recently come on line providing a powerful new tool for extending these investigations[6,9]. Sub-Doppler static gas phase spectra can can now be recorded with linewidths < 16kHz ( 5x 10-7 cm-1) and precision up to 30 Hz (1x10-9 cm-1) using a phase and frequency stabilized backward wave oscillator FASSST. Corresponding supersonic jet spectra can also be recorded with linewidths < 37 kHz, taking advantage of a coaxial absorption instrumental configuration. This now gives us the capability of recording rovibrational spectra with generation of molecular parameters having the highest microwave accuracy and precision in both ground and vibrational excited states.
Theoretical Methods: Morphing potentials for molecular complexes
Widely used ab-initio calculations have been notoriously ineffective in generating potential energy surfaces and thus prediction of the fundamental properties of intermolecular interactions, even for the smallest dimers and clusters. In 1999, Meuwly and Hutson and Lucchese, Bevan and colleagues developed similar methods of shifting and scaling ab-initio to generate potential energy surfaces of intermolecular interactions that accurately reproduce available experimental data (particularly spectroscopic) to nearly experimental accuracy. Such potentials (now coined with the name morphed potentials) are currently being developed for higher dimensional treatments of complexes and quantitative estimations of reliability of the generated potentials for accurate prediction of the properties of such intermolecular interactions.
Potential Morphing [10-14]
Rovibrational Dynamics [15-18]
The rovibrational energy levels are obtained using the vibrational configuration interaction (VCI) in the laboratory frame.
|The lack of accurate vibrationally complete potential energy surfaces for neutral closed shell hydrogen bonded and related intermolecular interactions has been the driving force behind our initial investigations of morphed potentials. We have so far investigated selected systems of the type Rg:HX (Rg = Ne, Ar, or Kr; X= Br or I) to generate such potentials using a combination of high resolution spectroscopy, and computational modeling. As can be seen in Table I, the results of prior investigations on Rg-HF and Rg-HCl gave no indication of the multiplicity of structures subsequently found in the Rg:HBr and Rg:HI systems. Following our initial demonstration that the ground state structure and global minimum potential of Ar:HI were actually Ar-IH, we confirmed similar behavior in Kr-IH. A further investigation of a morphed potential of Ne:IH demonstrated the existence of a delocalized ground state with a Ne-IH global minimum..|
|Extending this work, we have determined the complete vibrational morphed potential of Ar:HBr, demonstrating that the ground state structure is hydrogen bound, but the global minimum is the van der Waals structure. In the case of Ar:HBr, zero point energy effects are sufficient to cause such structural changes. This modeling was based on unequivocal ordering of the isomeric energy levels found from precisely measured combination frequency differences. There are very few cases that challenge the oft-held tenet that the ground state of a molecular species is the same as its global minimum. In Kr:HBr, a similar effect may have been expected. Our recently generated morphed potential confirms this expectation. Kr:HBr, thus, also belongs to a restricted group of molecular species in which the ground state isomeric structure differs from that at the global minimum. Our recent investigation of the morphed potential of Ne:HBr is consistent with a delocalized ground state with a Ne-BrH global minimum. Further extensions of this work will also be presented and illustrated below.|
Results of Specific Spectroscopic Investigations
Near and far infrared rovibrational spectra of the dimers Rg:HX (Rg=Ar, Kr; HX=HBr, HI) have been investigated. Examples of recorded spectra and generated potential functions are illustrated below.
Resolved quadrupole substructure for Ar:HI and Ar:DI associated with the Ar-IH and Ar-ID isomeric forms of the complex.
These investigations should provide additional sources of precise data for characterizing the nature of their nearly equal energy isomers, Rg--HX and Rg--XH, and the corresponding vibration-rotation-tunneling intermolecular dynamics. These dimer studies will also form the basis for characterization of solvation effects in the clusters RgnHBr and RgnHI and their relevance to cage effects in photoinitiated reactions in these clusters.
A fully vibrational morphed potential of Ar:HBr. This demonstrates a ground state Ar-HBr structure but a corresponding Ar-BrH equilibrium structure. Such results challenge the oft-held tenet that the ground state structure of a molecular species is the same as its structure at the global minimum.
Hydrogen Iodide Dimers Can Be Different
HI dimer species can offer unusual opportunities for probing the nature of competing intermolecular forces in the limit of weak electrostatic interactions. Such dimers are of considerable interest because of their relative accessibility for laser induced HI photodissociation and for infrared photoconversion in matrices. In contrast to many HF and HCl dimers that are relatively well characterized, there had been relatively few HI dimers that have been investigated by high resolution spectroscopy prior to our studies. Furthermore, it is not possible to predict the ground state structures and intermolecular potential energy surfaces of HI interactions based on prior investigations of corresponding HF and HCl dimers. In addition, ab-initio calculations for HI dimers are invariably inadequate so they provide stringent tests of such theory.
Observation of such a variety of structures is indicative of an intricate multi-minimum potential energy surface even in a simple complex such as in OC:HI
Investigations of HI dimers using pulsed nozzle FT microwave spectroscopy and high frequency wavelength modulation infrared diode laser spectroscopy in slit jet expansions are of particular interest. The structures and dynamics of some simple complexes involving HI that have/are currently being considered including: Ne:HI, Ar:HI, Kr:HI, N2:HI, OC:HI, CO2:HI and HI dimer. Model potential energy surfaces have been completed for the Ne:HI, Ar:HI, Kr:HI dimers; and contrasted with information for corresponding members of homologous series involving HX(X=F, Cl and Br). Ne complexes are known to exhibit unusual characteristics and the Ne:HI complex has been shown to be no exception. In addition, the role of low lying isomeric forms in the complexes N2:HI and OC:HI are being investigated. A detailed investigation of the ground state structure of CO2:HI is also currently being modeled, and the results will be correlated with interpretation of previous UV photodissociation experiments carried out on this complex.
The intermolecular potential energy surface of N2:HI. Both ground and equilibrium structures involve van der Waals bonding , N2-IH
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