Sang Tae Park

Observation of conformation-specific pathways in the photodissociation of 1-iodopropane ions.

Many molecules can rotate freely around single bonds and thereby interconvert between different conformations, such as gauche and anti 1,2-disubstituted ethane, a classic example of conformational isomerism. Even though rotation occurs rapidly at room temperature, the product selectivity seen in some reactions has been explained by conformation-dependent reaction mechanisms: if reactant molecules differing only in their conformation are located at different positions on the reaction path, they may undergo different reactions. But a direct verification of this effect is difficult, because the energy barrier separating conformational isomers is so low that under ambient conditions reactants with more than one conformation will be present. But by using temperatures low enough to suppress the interconversion between different conformations, gauche-1-iodopropane ions and anti-1-iodopropane ions have been selectively generated. Here we show that the kinetic energy released during the photodissociation of 1-iodopropane ions depends strongly on the conformation of the ions. Thermodynamic arguments and ab initio calculations indicate that this difference in kinetic energy release results from differences in the reaction mechanism, with gauche-1-iodopropane ions forming 2-propyl ions and anti-1-iodopropane ions forming protonated cyclopropane ions. These findings suggest that the well-known concept of conformation selection forms the basis of a simple scheme for reaction control, thus providing in some cases an attractive alternative for more involved schemes that utilize the phase and pulse shape of laser beams to control chemical reactions.

One-photon mass-analyzed threshold ionization spectroscopy of 1- and 2-iodopropanes in vacuum ultraviolet

One-photon mass-analyzed threshold ionization ~MATI! spectroscopy of 1- and 2-iodopropanes has been studied using coherent vacuum ultraviolet ~VUV! radiation generated by four-wave mixing in Kr gas. Accurate ionization energies to the lower and upper spin‐orbit states of the molecular ions have been determined. These are 9.175560.0005 and 9.690360.0017 eV for the lower and upper spin‐orbit states, respectively, of 2-iodopropane ion. For 1-iodopropane ion, gauche and trans peaks were resolved in the MATI spectra. Ionization energies to the lower spin‐orbit states are 9.256760.0005 and 9.271860.0005, respectively, for the gauche and trans conformers. The corresponding values are 9.833260.0017 and 9.846660.0017 for the upper spin‐orbit states. The pure ion beam of the gauche-only or that of trans-only could be selectively generated by tuning the VUV wavelength. Dissociation of 1- and 2-iodopropane ions, C 3 H 7 I 1 !C 3 H 7 1I, occurring in the ion core of highly excited Rydberg neutrals has been observed. Fragmentation thresholds for these reactions have been determined. This has led to an accurate potential energy diagram for the dissociation of the C 3 H 7 I 1 system in the threshold region. The heat of formation a t0Ko f 2-C 3 H 7 has been determined, 821.763.8 kJ mol 21 , together with the proton affinity a t0Ko f C 3 H 6 , 741.663.9 kJ mol 21 . Plausible mechanisms for the production of 2-C3H7 1I from 1-C3H7I 1 have been proposed.

Vacuum-ultraviolet mass-analyzed threshold ionization spectra of iodobutane isomers: Conformer-specific ionization and ion-core dissociation followed by ionization

Mass-analyzed threshold ionization (MATI) spectra using coherent vacuum ultraviolet radiation have been obtained for t-butyl iodide, iso-butyl iodide, 2-iodobutane, and 1-iodobutane. The ionization energy to the lower spin–orbit state of t-butyl iodide ion has been determined to be 8.9984±0.0006 eV, while the threshold for fragmentation to t-C4H9+ and I has been estimated to be 9.1762±0.0047 eV. Heat of formation of t-butyl cation, ΔfH0(t-C4H9+), has been re-estimated, 733.7±3.3 kJ mol−1. Peaks due to two different conformers of iso-butyl iodide ion, PH and PC, are clearly resolved in the MATI spectra, enabling the measurement of ionization energies to the lower spin–orbit states of respective conformers, 9.1725±0.0006 and 9.1972±0.0006 eV. Corresponding values for the upper spin–orbit states have been determined from the MATI spectra for the C4H9+ fragments generated by dissociation in the ion core of neutral as 9.7394±0.0024 and 9.7649±0.0023 eV. Only two out of three possible 2-iodobutane conformers ha...

Theoretical and experimental studies of the dissociation dynamics of methaniminium cation, CH2NH2+→CHNH++H2: Reaction path bifurcation

The unimolecular dissociation of CH2NH2+ has been investigated experimentally and theoretically. Kinetic energy release distribution was obtained by analyzing the mass-analyzed ion kinetic energy profile. Critical configurations along the reaction path were investigated by electronic structure calculations at the HF, MP2, QCISD, and B3LYP levels using the 6-31G** and 6-311+G** basis sets. Reaction path bifurcation was observed at all the levels. The bifurcation point was in the entrance region (before the transition state) at the HF level. This point moved to the exit region when the electron correlation effect was included at the MP2, QCISD, and B3LYP levels. A global potential energy surface incorporating this bifurcation feature was constructed by interpolation at the MP2/6-311+G** level. Classical trajectories were calculated on this surface and product mode-specific energies were evaluated. Based on these data, various experimental observations, lack of hydrogen scrambling in particular, could be ade...

Classical trajectory study of CD2OH+→CDO++HD on potential energy surfaces constructed at various quantum chemical levels: Scaling of product mode-specific energies

Potential energy surfaces for the title reaction have been constructed by interpolation of 40 local potentials along the intrinsic reaction coordinate obtained by quantum chemical calculations at the semiempirical, Hartree–Fock (HF), post-HF, and density functional thoery (DFT) levels of theory. Classical trajectory calculations have been carried out and the average product mode-specific energies originating from the reverse barriers have been estimated. When normalized to the reverse barrier heights, the mode-specific energies evaluated at the HF, post-HF and theory DFT levels have been found to be comparable, indicating that the energy partitioning data are scaled to the barrier heights at these levels of theory. Namely, dynamical results obtained on potential energy surfaces constructed at moderate quantum chemical levels were similar (within ∼5%) to those at higher levels. It has been found theoretically that classical dynamics on two potential energy surfaces related by simple scaling results in scal...

Dissociation dynamics of gauche and anti conformations of 1-iodopropane ions prepared selectively by vacuum-ultraviolet mass-analyzed threshold ionization spectrometry: Photodissociation at 426 and 355 nm

Mass-analyzed threshold ionization with coherent vacuum ultraviolet radiation was used to generate the gauche, or alternatively anti, ion beam of 1-iodopropane ion. Conformation-selective photodissociation of these ion beams was studied at 426 and 355 nm which correspond to excitation to the second and third electronic states appearing in the photoelectron spectrum, respectively, and compared with dissociation in the first excited state reported previously. Conformation-specificity was not observed at 426 and 355 nm unlike the dissociation in the first excited state. Plausible mechanisms for dissociations from the excited electronic states are discussed based on the presence/absence of conformation-specificity, dissociation anisotropy, and kinetic energy release.

Rotational energy analysis for rotating–vibrating linear molecules in classical trajectory simulation

A method has been developed to evaluate the rotational energy of a rotating–vibrating linear molecule in classical trajectory simulation. The method is based on our finding that the component of the angular momentum perpendicular to the figure axis which closely approximates the pure rotational angular momentum is a fairly good constant of motion. Classical kinetic energy of the system has been reorganized to separate the rotational and vibrational parts according to the above concept. Time evolution of the rotational energy thus evaluated shows much less irregular behavior than the ones evaluated with the previous methods over a wide range of rotational and vibrational energies. Combined with the method for mode-specific vibrational energy analysis reported previously, the present method allows a reliable separation of the total energy into each degree of freedom. In particular, the accuracy of the present method seems to be good enough for the rotational energy determination at an instantaneous configur...

Classical dynamics of bend–stretch vibrational recurrence in HOD

Abstract Intramolecular dynamics of HOD was investigated by the classical trajectory method and detailed mechanisms of vibrational energy exchange were studied. The time evolution of mode energies in each vibrational mode along the trajectories was calculated. A periodic energy exchange, that is, a vibrational recurrence between the bending and OD stretching modes, is observed. The centrifugal force by the angular bending motion and the coriolis force due to stretching of the OD bond undergoing angular motion are responsible for the recurrence. Implication of the present finding in intramolecular vibrational relaxation and mode selective chemistry is discussed.

Scaling of classical rate constants on scaled potential-energy surfaces

The scaling relation for the classical rate constants on the scaled potential-energy surfaces has been derived using the scaling theorem in classical dynamics reported previously. This applies to the classical rate constants, both for unimolecular and for bimolecular reactions, that can be obtained by the classical trajectory method and the transition state theory. Validity of the theory has been tested for the prototype reactions, H2CO→H2+CO and Cl+H2→HCl+H. Exact scaling of the rate constants obtained by the classical trajectory calculations has been demonstrated. The rate-energy relations for the former reaction calculated with the statistical Rice–Ramsperger–Kassel–Marcus theory also displayed excellent scaling in the high-energy limit. The scaling relation does not hold rigorously near the reaction threshold due to the quantum mechanical zero-point energy effect. Regardless, the order of magnitude prediction of the threshold rate constant by scaling was possible even in extreme cases. The present met...

Reply to comment on ''Checking the influence of numerically induced chaos in the computational study of intramolecular dynamics using trajectory equivalence''

Abstract Time evolutions of mode energies averaged over equivalent trajectories are presented for an uncoupled two-dimensional harmonic oscillator. The mode energies hardly show any significant damping. Hence, our previous interpretation that the excessive damping observed for HOD was due to the onset of numerically induced chaos caused by the coupling terms in the Hamiltonian is upheld.