Potential energy curves, electronic structure

Fig. 6.—Potential energy curves for hydrogen molecules. The electron-pair halide molecules. bond structures H F , etc., are...

Hydroxyl radical (OH) is a key reactive intermediate in combustion and atmospheric chemistry, and it also serves as a prototypic open-shell diatomic system for investigating photodissociation involving multiple potential energy curves and nonadiabatic interactions. Previous theoretical and experimental studies have focused on electronic structures and spectroscopy of OH, especially the A2T,+-X2n band system and the predissociation of rovibrational levels of the M2S+ state,84-93 while there was no experimental work on the photodissociation dynamics to characterize the atomic products. The M2S+ state [asymptotically correlating with the excited-state products 0(1 D) + H(2S)] crosses with three repulsive states [4>J, 2E-, and 4n, correlating with the ground-state fragments 0(3Pj) + H(2S)[ in... 

The photolysis experiments have also received theoretical attention, with electronic structure methods used to calculate the nature of the excited states (44), as well as the potential energy curves for loss of CO (45). Theoretical models for the excited-state dynamics leading to dissociation have also been proposed (46). 

Besides a transition to a continuum level of an excited electronic state, dissociation can occur by another mechanism in electronic absorption spectroscopy. If the potential-energy curve of an excited electronic state A that has a minimum in UA(R) happens to be intersected by the U(R) curve of an unstable excited state B with no minimum in U, then a vibrational level of A whose energy lies near the point of intersection of UA and UB has a substantial probability to make a radiationless transition to state B, which then dissociates. This phenomenon is called predissociation. Predissociation shortens the lifetimes of those vibrational levels of A that are involved, and therefore by the uncertainty principle gives broad vibrational bands with rotational fine structure washed out. 

The general composition of this chapter is as follows. We first present a critical review of the current status of electronic-structure calculations for molecular systems. This is followed by a compilation of the potential-energy curves, derived spectroscopic analysis, and pertinent discussion of the selected atmospheric molecules mentioned in the preceding paragraph. 

Differential scattering experiments with Ne and other beams state selected with a tuneable dye laser are near realization. Differences in the potential-energy curves and reaction probabilities for the iP2 and iP0 states will provide valuable insight into the role of the core ion on the collision dynamics and electronic structure as well as clarify the relative importance of the two states in macroscopic processes. Experiments using a metal-atom crossed beam, also currently in progress at Freiburg, promise a revealing contrast to the weak van der Waals interactions thus far studied. 

The electronic structure of Ni(CO)4 is not as well defined as those of either Cr(CO)6 or Fe(CO)5. This makes the assignment of processes in the early development of the excited-state dynamics somewhat speculative. However there are a number of unique features to the photophysics of CO-loss from Ni(CO)4. Firstly, the CO loss is very slow compared to the other two systems outlined herein taking approximately 600 fs. In addition the Ni(CO)3 fragment is produced in its St state and this state persists because there is no facile deactivation process available based on molecular motions. Deactivation can be achieved only by further CO loss or by radiative processes of either fluorescence or phosphorescence. The overall scheme of potential energy curves and pathways for photoinduced loss of CO from Ni(CO)4 is represented in Fig. 29. 

The use of Walsh diagrams, based on one-electron molecular orbitals, shows that on n —> 7t excitation the azobenzene molecule is stretched, which is the beginning of inversion. All calculations and suggestions for an inversion mechanism agree that the potential energy curve for inversion has a relatively steep slope at the E- and the Z- geometries. This is corroborated by the experimental evidence of a continuous n tc absorption band in both isomers. In fact, a structured n band in an azo compound that can isomerize has never been observed. 

The potential energy curves shown in Figure 6.7 are among the most important concepts in the quantum picture of the chemical bond and molecular structure. It is important to see how these curves arise from the electron-nucleus attractions and the nuclear-nuclear repulsions in the molecule and how the chemical bond is formed. 

The present results show that the DV-DFS method is applicable to calculations of potential energy curves for such a heavy and complicated system as uranyl nitrate dihydrate. It may also be useful to derive first-principles potential energy curves for the MD simulations. The electronic structure and MD results will be valuable for understanding dynamical properties of actinide ions in solution and for molecular design of novel extractants for selective separation of actinides. 

Figure 3. (a) Valence bond representation of the electronic structure of the (HON)-CH-(NOH) radical, a prototype of nitronyl nitroxide. (b) Potential energy curve for the interaction of two (HON)-CH-(NOH) radicals placed one on top of the other (the coordinates differing by a displacement along the vertical coordinate z). Each fragment is a doublet state, and the curve was computed for the triplet state at the UB3LYP/6-31+G(d) level. 

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