Infrared Photon Initiated Conformer Interconversion, Fourier Transform Microwave Spectroscopy of Hydrogen Bonded Trimers, and Study of Conformer Cooling in Supersonic Expansions

Abstract

C-H stretch fundamentals were excited by an IR OPO laser in 2 molecules having conformers. Because the barrier for interconversion is lower (higher) in methyl vinyl ether (methyl nitrite) than IR photon energies, IR fluorescence spectra showed conformational interconversion occurred (did not occur). For the first time, well defined energy in collisionless conditions was deposited in a ground electronic state molecule with subsequent intramolecular energy flow perturbing the equilibria between conformers (methyl vinyl ether).

A FT-MW spectrometer is used for assignment of spectra of H-bonded trimers HCN-HCN-HCN, (HCN)$\sb2$Y (Y = HF, HCl, HCF$\sb3$, CO$\sb2$) and X-(HCN)$\sb2$ (X = NH$\sb3$, H$\sb2$O, CO, N$\sb2$). Ground vibronic state geometries and shrinkages of the H-bonds (compared to dimers) are obtained. Also obtained is the spectrum of Ar(HCN)$\sb2$, a T-shaped trimer with Ar about equidistant from the c.m. of each HCN, and the (HCN)$\sb2$ subunit essentially equivalent to free (HCN)$\sb2$.

Theoretical treatment of measured induced dipole moments in (HCN)$\sb2$ and (HCN)$\sb3$ show they are well predicted by the first 4 nonzero molecular electrical multipole moments and bond polarizabilities of HCN. Equations are derived for the induced moment per HCN in an infinite chain of HCN's. These allow separation of contributions to the induced dipole moment per HCN from same-chain and off-chain sites in HCN crystal.

$\spƒ$Kr quadrupole coupling constants for $\spƒ$Kr-pyridine and $\spƒ$Kr-CO$\sb2$ are calculated from proposed representations of charge on pyridine and CO$\sb2$. Experimental coupling constants therefore test descriptions of charge distributions of pyridine and CO$\sb2$.

Dependence of conformational relaxation in ethanol, isopropanol, and ethyl formate in supersonic expansions on noble carrier gas type show He will not relax the high energy conformers, Ar and Kr will relax them completely, and Ne will relax the alcohols but not ethyl formate. Results for the high energy isomer of CO$\sb2$-HCN and of DF-HF(HF-DF) show the number ratio of high:low energy isomer is perturbed in different carrier gases, with He yielding a "freeze-in temperature" equal to the nozzle temperature, but Ar causing more relaxation. Mechanisms for such relaxation are suggested.