Role of Intersystem Crossing in the Dynamics of the O(3P) + (CH3)2CHI, (CH3)3CI Reactions

Intersystem crossing in the entrance channel of the reaction of O(3P) with pyridine

Two quantum effects can enable reactions to take place at energies below the barrier separating reactants from products: tunnelling and intersystem crossing between coupled potential energy surfaces. Here we show that intersystem crossing in the region between the pre-reactive complex and the reaction barrier can control the rate of bimolecular reactions for weakly coupled potential energy surfaces, even in the absence of heavy atoms. For O(3P) plus pyridine, a reaction relevant to combustion, astrochemistry and biochemistry, crossed-beam experiments indicate that the dominant products are pyrrole and CO, obtained through a spin-forbidden ring-contraction mechanism. The experimental findings are interpreted-by high-level quantum-chemical calculations and statistical non-adiabatic computations of branching fractions-in terms of an efficient intersystem crossing occurring before the high entrance barrier for O-atom addition to the N-atom lone pair. At low to moderate temperatures, the computed reaction rates prove to be dominated by intersystem crossing.

Reactions of CF3CH2I+O(3P): competing mechanisms of HF elimination

Ab initio density functional and molecular orbital calculations provide singlet and triplet electronic potential energy surfaces for the reactions of CF3CH2I+O(3P) leading to OI and HF eliminations, reactions which have been the subject of recent experimental studies. A barrier to OI formation occurs on the triplet potential energy surface; there is no reverse barrier to OI formation on the singlet pathway. Findings suggest that two competing pathways may form HF. One is an addition-insertion-elimination process involving insertion of O into the C-I bond. The alternate path involves OI elimination, addition of an O atom to CF3CH2, and subsequent HF elimination. The computed reactant pathways and energetics are discussed in relation to recent experiments.

Submillimeter-wave spectra of hypoiodous acid

Pure rotational spectra of hypoiodous acid, HOI, and its deuterated species, DOI, were measured in the frequency range of 320-670 GHz. The molecule was efficiently produced by a reaction of atomic oxygen with iodoethane. Rotational constants and centrifugal distortion constants for the molecule were determined accurately. The vibrationally averaged structure for HOI was obtained by taking the isotopic difference of the OH bond length into consideration: r(z)(OH)=0.967(8) A, r(z)(OI)=1.9941(3) A, and theta(z)(HOI)=103.89 degrees, where the errors were estimated from the residual inertial defect. Equilibrium bond lengths for the OH and OI bonds were derived as 0.959(8) A and 1.9874(3) A, respectively, by assuming anharmonic constants of the corresponding diatomic molecules. Electric-quadrupole interaction constants and nuclear-spin-rotation coupling constants for the iodine nucleus were obtained. Nonaxial terms of the electric-quadrupole constant for HOI can be determined as well, which enabled us to derive the principal values of the coupling tensor. The values obtained were used to gauge the ionicity of the X-O bond in the HOX molecular system. The nuclear-spin-rotation coupling constant along the a inertial axis is found to be significantly smaller than others, which may be explained by a contribution from two low-lying singlet excited states.