Excited State Energy Distribution and Redistribution and Chemical Reactivity; Mechanistic and Exploratory Organic Photochemistry^1,2

Abstract

Four decades ago one of us presented a means of relating, qualitatively, the structures of electronically excited states to many of the known organic reactions. 3 We now describe a method of predicting excited-state reactivity more generally. 4 A particularly intriguing but elusive problem is how excitation energy is distributed in electronic excited states, particularly those exhibiting photochemical reactivity. Using modern quantum mechanical wavefunctions, the present paper not only provides an answer to this question but also provides a method for predicting photochemical reactivity.In very early efforts 5 we reported the use of a "∆D Matrix" (also termed ∆p Matrix) which gives the change in electron densities and bond orders at different molecular sites as a result of electronic excitation. The promise was the ability to predict the molecular consequences of electronic excitation as well as being able to predict and understand photochemical reactions in general. These early studies were limited to the use of a truncated system of basis orbitals, these being in chromophores and bonds involved in a given reaction. However, now with the Weinhold Natural Hybrid Orbitals (NHOs) 6 being available in Gaussian98, 7 it is possible to determine the validity of the concept in virtually any electronically excited state of interest using any of the quantum mechanical methods capable of affording density matrices for ground and excited singlet and triplet electronic states. We define our ∆D matrix elements as in eq 1.Here, the D* rt refers to the excited-state density and D°r t refers to the ground state. The S rt terms are overlap integrals to adjust for distance effects and to maintain proper relative orbital signs.Thus, ∆D might be properly termed an "overlap density matrix"; r and t designate a pair of orbitals.The idea is that, where a ∆D element (e.g., ∆D rt ) is negative, that bond not only is weakened in the excitation process but also is generated by Franck-Condon vertical excitation in a nonminimum geometry with an accumulation of vibrational as well as electronic energy. 8 There may be some bonds which have positive ∆D elements and are stabilized and strengthened, but they will necessarily be fewer, since the molecular electronic energy has risen. 9 Also, the ∆D method gives changes in one-center electron densities, and this portion of our concept has been of value in the literature. 10 The ∆D method not only predicts the occurrence of photochemical reactions but also subtleties such as regioselectivity. Some reactions considered are (a) the Norrish Type I and its regioselectivity, 11a (b) the Yates ring-strained ketone to carbene ring expansion, 11b (c) the cyclopropyl ketone ring opening reactions and regioselectivity, 11c (d) the Norrish Type-II reaction, 11d (e) the butadiene to cyclobutene disrotatory transformation, 11e (f) Type-B bicyclic transformations, 11f (g) p y -orbital hydrogen abstraction, 11g (h) the R-expulsion reaction of R-substituted ketones, 11h (i) meta-electron transmission, 11i (j) ...

Frontier molecular orbital effects control the hole-catalyzed racemization of atropisomeric biaryls

“…The HOMO has anti-bonding overlap between these two atoms: removal of an electron therefore increases the local bond order in this region. This orbital could also be depopulated via photochemical means, bringing a similar change in the bond order as well 24. Two electrons remain in the HOMO–1, which has a bonding interaction here.…”

Internal Oriented Electric Fields as a Strategy for Selectively Modifying Photochemical Reactivity

“…As is well known, the 1 nπ* state is associated with a dipole with a significant charge-transfer contribution (Scheme ), and hence appropriately placed charged groups should be able to electrostatically stabilize this dipole. From Scheme it is clear that substituents in the para position should be best aligned with the dipole and hence most effective, and this alignment is poorer in the meta and, to a greater extent, the ortho position.…”

Photoelimination of Nitrogen from Diazoalkanes: Involvement of Higher Excited Singlet States in the Carbene Formation

“…To connect the nature of the initial excitation and the photochemical fate of the system, it is useful to consider the difference in the electron densities between the ground and the excited states. 46 Figure 3 shows that the electronic excitation of 1 to S 1 involves translocation of negative charge from the carbon to the nitrogen atoms in the xy-plane of the molecule. As the nuclei follow the electrons, one can expect that nuclear motion will initially take place in the plane of the molecule.…”

“…To connect the nature of the initial excitation and the photochemical fate of the system it is useful to consider the difference in the electron densities between the ground and the excited states. 46 As the nuclei follow the electrons, one can expect that nuclear motion will initially take place in the plane of the molecule. The S0 → S2 transition also increases the electron density at the middle N atom, but it does so perpendicularly to the xy-plane of the molecule, suggesting an initial out-of-plane motion of the nitrogen atom.…”