A hydrogen-bonded complex of OH with CO is identified along the reaction coordinate for the OH+CO↔HOCO→H+CO2 reaction. The existence of this linear OH–CO complex is established by infrared action spectroscopy, which accesses vibrational stretching and bending modes of the complex. Complementary electronic structure calculations characterize the OH–CO and OH–OC complexes, the transition state for HOCO formation, and the reaction pathways that connect these complexes directly to the HOCO intermediate.
The application of single photon ionization in combination with mass-selective detection by time-of-flight mass spectrometry is described for the rapid detection of the nitro-containing explosives and explosives-related compounds nitrobenzene, 1,3-dinitrobenzene, o-nitrotoluene, 2,4-dinitrotoluene, and 2,4,6-trinitrotoluene, as well as the peroxide-based explosive triacetone triperoxide in the gas phase. The technique is demonstrated to be a plausible approach for laser-based rapid detection of explosives. The limits of detection for nitrobenzene and 2,4-dinitrotoluene using SPI were also measured and determined to be 17-24 (S/N approximately 2:1) and approximately 40 ppb (S/N approximately 2:1), respectively.
The decay dynamics of the OH–CO reactant complex have been examined following infrared excitation in the OH overtone region using various IR pump–UV probe methods. The time scale for overall decay of the OH–CO (2vOH) complex has been bracketed between 0.19 and 5 ns through linewidth and direct time-domain measurements. The inelastically scattered OH (v=1) fragments exhibit a bimodal internal energy distribution, which reveals that vibrational predissociation proceeds through two pathways. The dominant inelastic decay channel involves vibrational energy transfer from OH to CO with little excess energy remaining for rotational excitation of the OH fragment, while a slower secondary channel releases most of the excess energy as OH rotational excitation. Intermolecular bending excitation of the OH–CO complex through combination bands results in increased rotational excitation of the OH fragments. The most probable OH product states display a strong lambda-doublet preference indicating that the singly occupied pπ orbital of OH is aligned perpendicular to the OH rotation plane following vibrational predissociation of the complex. These product states also minimize the translational recoil of the fragments and maximize the rotational angular momentum of the OH fragment. Abrupt cutoffs in the OH (v=1) fragment internal energy distributions are utilized to determine an upper limit for the ground state binding energy of OH–CO, D0⩽410 cm−1, which is in good accord with ab initio predictions. Finally, a comparison of infrared band intensities obtained using action and depletion detection methods suggests that geared bend and H-atom bend excitation may promote reactive decay of the OH–CO reactant complex.
Intermolecular vibrations of the linear OH–CO reactant complex have been observed as combination bands in the OH overtone region using infrared action spectroscopy. Rotational analyses and simulations of the band structures have been carried out for transitions to geared bend, excited spin–orbit, and H-atom bend states with 50–250 cm−1 of intermolecular excitation. The projection quantum number associated with each of these upper states is identified through the intensity profile of the band contour, missing rotational lines, and/or parity splitting of individual rotational lines. Intermolecular states with projection quantum numbers P=1/2 and 5/2 are observed for each of the two bending modes, arising from coupling of the unquenched angular momentum of OH with the vibrational angular momentum associated with the bending motion of the complex. An additional P=1/2 state is attributed to spin–orbit excitation, which shifts to higher energy than in free OH and gains infrared transition strength through the spin-decoupling interaction. The intermolecular energy level pattern is also examined in the context of the Renner-Teller interaction and spin–orbit coupling. The intermolecular bends of the OH–CO complex are of special interest because they probe portions of the reaction path leading to trans-HOCO formation.
A theoretical framework has been developed to describe the bending levels associated with an intermolecular potential of moderate anisotropy between an open-shell diatom and a diatom partner, such as OH–CO or OH–N2. The model explicitly allows for coupling between the electronic and spin angular momenta of the open-shell OH radical and the vibrational angular momentum arising from intermolecular bending motion of the complex. The energies and wave functions of the intermolecular bending levels for the OH–N2 complex have been computed based on a dipole–quadrupole interaction. The model is used to interpret the infrared spectrum of the linear OH–N2 complex in the OH overtone region, which has been recorded by detecting the OH fragments from vibrational predissociation. The pure OH overtone band at 6973.54(2) cm−1 and several combination bands, which involve the simultaneous excitation of OH stretching and geared bending modes, have been observed, analyzed, and assigned within the context of the model. In addition, the time evolution and quantum state distribution of the OH fragments yield the lifetime for vibrationally activated OH–N2 of 30±4 ns and an upper limit for the ground state binding energy of OH–N2, D0⩽277 cm−1.
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