The photodissociation dynamics of 1,2- and 2,2-dibromopropane at 234 and 265 nm have been investigated using the velocity-map ion imaging technique. The experimental data and theoretical calculations provide information on the translational energy distributions of the photoproducts $(Br and Br^\ast)$.
$\bold{1,2-dibromopropane (CH_3CHBrCH_2Br)}$
At both wavelengths, structured images of Br have been obtained for the spin-orbit ground state $Br(^2P_{3/2})$, while no structures was observed on the images of the spin-orbit excited state $Br^\ast(^2P_{1/2})$. The obtained translational energy distributions of Br have been reproduced by at least three Gaussian functions and those of $Br^\ast$ show a single Gaussian distribution. The secondary dissociation of Br atom from the bromopropyl radical, following the first C-Br bond rupture of 1,2-dibromopropane has been considered to explain the observed results, represented by $CH_3CHBrCH_2Br\to(1)C_3H_6Br+ Br^\ast(^2P_\frac{1}{2})^{1st}\to C_3H_6+Br(^2P_\frac{3}{2})^{2nd}+ Br^\ast(^2P_\frac{1}{2})^{1st}$ and $(2)C_3H_6Br+Br(^2P_{3/2})^{1st}\to C_3H_6 +Br(^2P_\frac{3}{2})^{2nd}+Br(^2P_\frac{3}{2})^{1st}$. The dissociation energy of the C-Br bond for each process is obtained by ab initio calculation and is given to be $D_0(Br-C_3H_6Br)=59.02$ and $D_0(Br-C_3H_6)=10.98$ kcal/mol. The recoil anisotropy parameter (β) is determined to be 0.53 and 1.26 for $Br(^2P_\frac{3}{2})^{1st}$ and $Br^\ast(^2P_\frac{1}{2})^{1st}$, respectively, at 234 nm, while the values are 0.49 and 1.73, respectively, at the longer wavelength of 265 nm. $Br(^2P_\frac{3/2})^{2nd}$ produced by the secondary dissociations gives the value of β =0.87 as high as $Br(^2P_\frac{3}{2})^{1st}$ implying that the anisotropic angular distributions are originated from the vector sum of the isotropic velocities of $Br^{2nd}$ and the velocity of the center of mass determined by $Br^{2nd}$ and propene, $C_3H_6$, product.
$\bold{2,2-dbromopropane (CH_3CBr_2CH_3)}$
At both wavelengths, the experimental data are well fitted by three Gaussian functions for $Br^\ast$, while five Gaussian functions are required to reproduce the experimental data for Br. The two C-Br bond breaking events occur concertedly for 2,2-dibromopropane. Minor contribution of the synchronous concerted reaction pathway is also observed for 2,2-dibromopropane. The primary bond dissociation gives rise to the fast Br (or $Br^\ast$) fragments, while the slower Br product originates from the secondary bond dissociation from the vibrationally hot intermediate. The translational energy distributions and anisotropy parameters are interpreted on the basis of ab initio calculations. The dissociation energy of the C-Br bond for each process is obtained by ab initio calculation and is given to be $D_0(Br-C_3H_6Br)=57.26$ and $D_0(Br-C_3H_6)=11.8$ kcal/mol. The recoil anisotropy parameter (β) is determined to be 0.45 and 0.20 for $Br(^2P_\frac{3}{2})^{1st}$ and $Br^\ast(^2P_\frac{1}{2})^{1st}$, respectively, at 234 nm, while the values are 0.32 and 0.98, respectively, at the longer wavelength of 265 nm.
본 실험에서는 1,2-dibromopropane $(CH_3CHBrCH_2Br)$ 과 2,2-dibromopropane $(CH_3CBr_2CH_3)$의 광분해 동역학에 대한 연구를 234 nm 와 265 nm 의 영역에서 수행하였다. 브롬(bromine)과 같은 할로겐(halogen)을 포함하고 있는 분자들은 성층권에서 오존층(ozone) 의 파괴의 주원인으로써 작용한다. 따라서 이런 할로겐을 포함하고 있는 분자를 연구하는 것은 매우 중요하다. 본 실험에서 사용한 분자들은 이 영역의 에너지를 흡수하여 들뜨게 되고 n→σ* 전이에 의한 C-Br 결합의 해리가 일어난다. C-Br 결합해리에너지는 이론 계산방법인 ab initio 계산을 이용하여 얻었다. 그리고 이를 통해 우리는 브롬의 병진 에너지 분포와 비등방성 계수(β)를 얻었다. 이 두 분자는 단지 하나의 브롬원자(bromine)의 위치가 다른데, 이들의 반응경로는 많은 차이가 있다. 우선 1,2-dibromopropane의 경우에는 step-wise 메커니즘에 의해 반응이 일어나고 2,2-dibromopropane의 경우에는 concerted 메커니즘에 의해 반응이 일어난다. 이것은 실험을 통해 얻은 이미지들을 통해서도 알 수 있다.
또한, 고분해 이온 영상장치를 이용하면 주어진 반응이나 광화학과정에서 생성물이 무엇인지, 분자에서 어느 부분이 가장 반응성이 좋은지, 어떤 접근구조들이 중요한지, 그리고 반응이 일어날 때 에너지가 어떻게 진행되는지 등을 알 수 있다.