ABSTRACT
Closed Shells ............................................................. 396 7.2.1 Lewis-Type NBOs of Carbonyl
Chromophores .................................................. 396
7.2.2 Non-Lewis Delocalization Corrections ........... 402 7.2.3 Natural Resonance Theory and Bond
Orders............................................................... 406 7.2.4 Alternative NBO-Based Bonding and
Reactivity Indices ............................................ 409 7.3 Extension of NBO Concepts to Open-Shell
Species......................................................................... 411 7.3.1 Theoretical and Practical Difficulties of
Open-Shell NBO Description.......................... 411 7.3.2 Necessary NBO Algorithm Modifications
for Excited States ............................................ 414 7.3.3 Lewis-Type NBOs of Vinoxy Radical ............. 416 7.3.4 Vinoxy Radical Hyperconjugation .................. 421 7.3.5 NRT Description of Vinoxy Resonance .......... 424
7.4 NBO/NRT Description of Photochemical Excited States............................................................. 426 7.4.1 Low-Lying Vertical Excitations of Vinoxy
Radical.............................................................. 426 7.4.1.1 First Excited Ã2 A¢ State ................... 429 7.4.1.2 Second Excited B˜ 2A¢¢ State .............. 433 7.4.1.3 Third Excited C˜ 2A¢ State.................. 437
7.4.2 Adiabatic Relaxation in Vinoxy Excited States .................................................. 440
7.4.3 Conical Intersections and Related Internal Conversion Pathways ...................................... 449
7.4.4 Complexation and Solvation Effects .............. 459 7.5 Summary: Future of NBO-Based Methods in
Computational Photochemistry................................. 469 Note Added after Manuscript Completion ........................ 471 Acknowledgments................................................................ 471 References............................................................................ 471
7.1 INTRODUCTION
The natural bond obital (NBO) method1 was developed as a bridge between the theorist's abstract mathematical wave function y and the localized bonding concepts of experimental chemists. Such a bridge rests on two anchor points: one in the deeply arcane ab initio domain of Schrödinger's equation, the other in the equally arcane domain of structural and mechanistic chemistry. These domains naturally evolved with disparate methodologies and languages. Growing awareness that y encompasses “the whole of chemistry”2 stimulated the search for improved lines of connection and communication to exploit this deeply mysterious commonality. NBO analysis is a general method for optimally expressing y in the language of structural chemists.
