These experimental and computational studies characterize the dynamics and,thermochemistry of several species that are important in atmospheric, combustion, and,interstellar chemistry. The species are selected both as interesting problems in their own right,,and as proxies that give insight into even more complex problems. In crossed laser-molecular,beam experiments, the uv photochemistry of BrCH2CH2ONO and CH2Cl2 are probed, revealing,rich excited-state dynamics including the unexpected photoelimination of HNO from,BrCH2CH2ONO and an unusual trimodal kinetic energy release in HCl photoelimination from,CH2Cl2. In the case of CH2Cl2, C-Cl photofission produces the CH2Cl radical, allowing a first,characterization of its photoionization cross section. BrCH2CH2ONO also generates the openshell,nitrite CH2CH2ONO, which is found to dissociate predominately via its lowest-barrier,pathway to NO2 + ethene; however, this radical evinces significant dissociation to HNO +,vinoxy, even though the barrier to this process (calculated at the G4/B3LYP/6-311++G(3df,2p),level of theory) is prohibitively high. This result implies a non-intrinsic reaction coordinate,pathway to HNO that circumvents the classical transition state; thus, HNO production from,nitrite radicals is likely a more important channel than was previously believed. In a theoretical,study, a new model of energy partitioning in dissociation reactions is developed that estimates,the distribution of vibrational energy based solely on the intrinsic reaction coordinate surface; the,predictions of this model are found to agree quite well with experimental measurements in the,case of HCl photoelimination from acryloyl chloride. Finally, in support of future dynamics,studies, a novel composite thermochemical protocol is assessed by calculating the singlet-triplet,gap of CH2 and the dissociation threshold of (H2O)2. The protocol agrees exceptionally well with,experimental values, giving confidence that it may be applied to other systems.