The reaction of the cyano radical (CN) with hydrogen was studied by time-resolved infrared absorption spectroscopy of individual rovibrational states of HCN. The initial vibrational level distribution of HCN(v10v3) was determined by plotting the time dependence of the fractional population of a vibrational level and extrapolating these curves to the origin of time. The experiments were carried out at two temperatures, 293 and 324 K, with similar results. It was estimated that about 50% of the available reaction exothermicity was deposited as vibrational excitation of the HCN product. Surprisingly, the HCN(101) vibrational level received a significant fraction of the observed vibrational population, implying that the CN vibration was not really a spectator bond in the reaction dynamics. Furthermore, the observed HCN(v10v3) vibrations only account for about 27% of the initial HCN population produced in the title reaction. A significant fraction of the product HCN molecules must have been produced with the bending mode excited, likely in combination with the H–C stretch vibrations.

The reaction of the cyano radical (CN) with hydrogen was studied by time-resolved infrared absorption spectroscopy of individual rovibrational states of HCN. The initial vibrational level distribution of HCN(v(1)0v(3)) was determined by plotting the time dependence of the fractional population of a vibrational level and extrapolating these curves to the origin of time. The experiments were carried out at two temperatures, 293 and 324 K, with similar results. It was estimated that about 50% of the available reaction exothermicity was deposited as vibrational excitation of the HCN product. Surprisingly, the HCN(101) vibrational level received a significant fraction of the observed vibrational population, implying that the CN vibration was not really a spectator bond in the reaction dynamics. Furthermore, the observed HCN(v(1)0v(3)) vibrations only account for about 27% of the initial HCN population produced in the title reaction. A significant fraction of the product HCN molecules must have been produced with the bending mode excited, likely in combination with the H-C stretch vibrations. (C) 1998 American Institute of Physics.

An extensive quasiclassical trajectory study of the dynamics of the CN+H2→HCN+H reaction has been undertaken on two of the potential energy surfaces reported by ter Horst, Schatz, and Harding [J. Chem. Phys. 105, 558 (1996)] with the goal of converging product state distributions. The effect of zero-point energy violations on the behavior of the reactive cross section near threshold has been examined leading to an improved estimate of the thermal rate constant on ter Horst–Schatz–Harding potential energy surface 3 (3.01±0.24×10−14 cm3/s at 300 K). The calculated HCN vibrational product state distribution is not statistical and exhibits a systematic over population in the stretching vibrations of the ground state bend manifold indicating that the –C–N does not behave like a “spectator bond” in this reaction. There is also significant population in modes with bending excitation, but these vibrations are under populated relative to prior statistical expectations. The sensitivity of the distribution on the size of the barrier and its location in the entrance channel has been undertaken by comparing results on the ter Horst–Schatz–Harding potential energy surfaces 2 and 3. Similar to the case of exoergic atom-diatom reactions, it is found that the earlier barrier on ter Horst–Schatz–Harding potential energy surface 3 gives rise to more excitation in the –C–H stretching vibration. The rotational distributions of the HCN product appear similar to the thermal distribution of CN reagents from which they are born indicating that the abstraction of the light H atom perturbs the rotational motion of the cyano radical very little. The dependence of the average HCN rotational quantum number, 〈J〉, on the bending quantum number, v2, exhibits an interesting alternation such that the points for even values of v2 are larger than those for odd. There is a corresponding alternation in the dependence of the average scattering angle, 〈θ〉, on v2 in the opposite sense. These observations suggest that for the odd bending states (which are primarily l=1) the energy diverted into exciting motion perpendicular to the reaction path at the transition state is not available to excite product rotation or to produce reactive trajectories with large impact parameters which lead to small scattering angles.

The HNC molecule was generated by the reaction of translationally hot H atoms with either ClCN or BrCN. The energetically rich HNC products were probed by time-resolved infrared laser absorption spectroscopy. This allowed for the spectroscopic analysis of 16 vibrational bands in the wavelength region 2.6 to 3.1 μm and the identification of 8 new bands for HNC. The dependence ofGv,Bv, andDvwas fit to appropriate polynomials in the vibrational quantum numbers andl. No bands were identified in which the bend, ν2, and CN stretch, ν3, were simultaneously excited so that the spectroscopic constants depending on the interaction between these two vibrations could not be experimentally measured. Nevertheless, the observations of this work allow for an almost complete experimental determination of the quadratic spectroscopic constants of this simple but important molecule.

The reaction of the cyano radical (CN) with methane was studied by time-resolved infrared absorption spectroscopy by monitoring individual rovibrational states of the HCN and CH3 products. The initial vibrational level distribution of the bendless vibrational levels of HCN(v1,0,v3) was determined by plotting the time dependence of the fractional population of a vibrational level and extrapolating these curves to the origin of time. About 20% of the HCN products were observed to be initially produced in the HCN(v1,0,v3) vibrational levels, with v1 and v3=0,1,2. The CN radical was created by laser photolysis of three different precursors. Each photolyte provided a different initial vibrational level distribution of CN; however, similar initial HCN(v1,0,v3) vibrational level distributions were obtained independent of the CN radical precursor. This may indicate that the CN radical does not act as a spectator bond during the course of a reactive encounter for this system. The time dependence of the CH3 (00000) ground state was also followed using time-resolved infrared absorption spectroscopy. Preliminary data indicates that a large fraction, if not all, the CH3 radicals are produced in their ground state in the title reaction.

The reaction of the cyano radical (CN) with methane was studied by time-resolved infrared absorption spectroscopy by monitoring individual rovibrational states of the HCN and CH3 products. The initial vibrational level distribution of the bendless vibrational levels of HCN(upsilon(1),0,upsilon(3)) was determined by plotting the time dependence of the fractional population of a vibrational level and extrapolating these curves to the origin of time. About 20% of the HCN products were observed to be initially produced in the HCN(upsilon(1),0, upsilon(3)) vibrational levels, with vl and upsilon(3) = 0,1,2. The CN radical was created by laser photolysis of three different precursors. Each photolyte provided a different initial vibrational level distribution of CN; however, similar initial HCN(upsilon(1),0,upsilon(3)) vibrational level distributions were obtained independent of the CN radical precursor. This may indicate that the CN radical does not act as a spectator bond during the course of a reactive encounter for this system. The time dependence of the CH3 (000(0)0) ground state was also followed using time-resolved infrared absorption spectroscopy. Preliminary data indicates that a large fraction, if not all, the CH3 radicals are produced in their ground state in the title reaction. (C) 1996 American Institute of Physics.

Author Institution: Depatment of Chemistry, The University of Akron; Chemistry Division, Argonne National Laboratory

In applying modern theories (RRKM) of unimolecular reaction, it is necessary to decide the volume of phase space in which the energy is assumed to be randomized. The question of whether the K rotational quantum number is conserved impacts on that choice. The conceptual sequence from experimental spectra, through analysis, and interpretation in terms of K relaxation is described for ethanol and 1-butyne in the 3 micron region. The interpretation of molecular eigenstate spectra involves identification of the bright state from the coherent excitation of part of the spectrum, evaluation of the rate of energy transfer out of the bright state, deducing the mechanism of the coupling of the bright state to the bath states, and modeling the spectra in order to determine the average coupling parameters for anharmonic coupling and Coriolis interactions.

The assumption that the internal energy of a molecule is randomised on a timescale that is short compared with the reaction time is at the heart of modern theories of unimolecular reaction. In applying such theories it is necessary to decide the volume of phase space in which the energy is assumed to be randomised. The question of whether the K rotational quantum number is conserved has an impact on that choice. The conceptual sequence from experimental spectra, through analysis, and interpretation in terms of K relaxation is described below.At low resolution, intramolecular vibrational energy randomisation results in the broadening of the features of IR absorption spectra. At high resolution in bound systems, such broadened features are revealed to be clumps of discrete lines, each of which is a transition to a molecular eigenstate. Since the discrete lines can be assigned by spectroscopic means, the erroneous assignment of inhomogeneous broadening to rate processes can be avoided. Each clump of eigenstates is characterised by its dilution factor, interaction width and effective level density. Examples include the IR spectra of ethanol and but-1-yne in the 3 µm region.The interpretation of molecular eigenstate spectra involves several conceptual stages: (1) identification of the bright state which would be prepared by the coherent excitation of a certain section of spectrum, (2) evaluation of the rate of energy transfer out of the bright state, (3) use of the rotational quantum number dependence of the spectra and the trends among related systems to deduce the mechanisms by which the bright state is coupled to the bath, and (4) modelling the spectra with random matrix calculations in order to determine the average coupling parameters for anharmonic coupling and x, y and z-type Coriolis interactions.Random matrix simulations provide the opportunity to address the title questions. The simulations focused particularly on rotationally mediated vibrational relaxation and were constrained to obey the rotational quantum number dependence of the Coriolis interaction. For ethanol, when the system is prepared with a specific K quantum number, one finds that K is not conserved but neither is the population completely randomised among the 2J+ 1 available K states even at long times. The time needed for the final (non-random) distribution among K-states to be achieved is typically of the order of 1 ns, even though the energy leaves the bright state an order of magnitude more quickly.

Previous work by the present authors (Can. J. Chem. 72, 652 (1994)) pointed out the acceleration of IVR in flexible molecules when the prepared vibration is close to the centre of flexibility (COF). A COF was defined as a bond about which hindered internal rotation can occur thereby giving rise to molecular flexibility. Here, it is shown that the rate of IVR is inversely correlated with the height of the barrier to internal rotation for systems in which the prepared vibration is adjacent to the COF. In ethanol and hydrogen peroxide the barriers are low (approximate to 380 cm(-1)) and the relaxation of the adjacent O - H vibration is fast (4 to 26 ps). On the other hand, higher barrier torsions (1100 to 1700 cm(-1)) adjacent to the chromophore in 1-butyne, 2-fluoroethanol, and 1,2-difluoroethane give rise to much longer IVR lifetimes (270 to 565 ps).Most of the IVR lifetimes used in this paper were derived from discretely resolved spectra in which a bright state transition is fragmented into a clump of molecular eigenstates; the remainder are from rotationally selected double resonance spectra. An algorithm is described for the derivation of consistent IVR lifetimes for coupling cases ranging from intermediate down to the very sparse limit where only a few perturbing states are explicitly observed in the spectrum.

The reaction of the cyano radical (CN) with ethane was studied using time-resolved infrared absorption spectroscopy to monitor individual rovibrational states of the HCN product. A method is described that can be used to determine the initial vibrational state distribution at pressures of several Torr. This technique was applied to the title reaction to determine that the vibrational states of HCN(v1,0,v3), where v1, v3=0, 1, and 2, were not directly populated in the title reaction to any significant extent. The initial vibrational energy content of the CN radical was also varied but did not influence the initial population in the HCN vibrational levels probed in this experiment. The time dependence of HCN(v1,0,v3) was followed and interpreted in terms of bimolecular rate constants for vibrational relaxation with ethane. The title reaction is mode specific in its energy disposal in that at least every HCN product appears to have at least one quantum of bending excitation, likely in combination with stretching vibrations.

A direct measurement of the transition dipole moment, μ3, of the degenerate v3 in-plane asymmetric C–H stretching vibration of the methyl radical has been made. The measurements were carried out in a flow reactor using laser-photolysis transient infrared absorption spectroscopy. Cyano (CN) radicals (and Cl atoms) were produced by laser photolysis of BrCN (or ClCN) at 193 nm and reacted with methane to give both CH3 and HCN (and HCl). The intensities of 18 rotational lines of the v3 fundamental band were measured relative to the R(8) line of the C–H stretching vibration (v3) of HCN(001←0). The best estimate of the transition dipole moment of the CH3 (00110←0) transition was provided by the measured line intensity for the CH3 (00110←0)rR(3,3) transition and was determined to be μ3=0.0327±0.0021 D.