Vibrationally induced interconversion of H-bonded NO2(-) x H2O isomers within NO2(-) x H2O x Ar(m) clusters using IR-IR pump-probe through the OH and NO stretching vibrations.
We introduce a method based on sequential application of vibrational predissociation spectroscopy to explore the high-amplitude rearrangements available in a small H-bonded complex that is vibrationally excited within a larger Ar cluster. The weakly bound Ar atoms play the role of a solvent in mediating the energy content of the embedded system, ultimately quenching it into local minima through evaporation. We demonstrate the approach on the NO(2)(-) x H(2)O binary hydrate, which is known to occur in two nearly isoenergetic isomeric forms. The scheme involves three stages of mass separation to select a particular NO(2)(-) x H(2)O x Ar(m) parent ion cluster prior to vibrational excitation and then isolate the NO(2)(-) x H(2)O x Ar fragment ions for interrogation using resonant vibrational predissociation with a second infrared laser. The initial vibrational excitation selectively energizes one of the isomers through one of its characteristic resonances while the predissociation spectrum of the NO(2)(-) x H(2)O x Ar fragment encodes the distribution of isomers present after Ar evaporation. Isomerization from the front- to backside form is found to occur upon excitation of the NO stretch near 1200 cm(-1); although the reverse reaction is not observed upon excitation of the NO stretch, it is observed upon excitation of the higher-energy OH stretching fundamental near 3000 cm(-1). We discuss these observations in the context of the calculated isomerization energetics, which focus on the minimum energy structures for the isomers as well as the transition states for their interconversion.
Relph
Rachael A
RA
Sterling Chemistry Laboratory, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, USA.
Elliott
Ben M
BM
Weddle
Gary H
GH
Johnson
Mark A
MA
Ding
Jing
J
Jordan
Kenneth D
KD