Diamides are important molecules in both biological and synthetic systems, and understanding their electronic transitions is important for my project at increasing the accuracy of numerical methods to predict a protein's CD spectrum. ### Understanding the Transitions In a diamide system, we have two amide groups (labeled C1 and C4) that can undergo similar types of transitions. Each amide group can participate in: 1. **n → π* transitions**: These involve the non-bonding (n) lone pair on the oxygen atom transitioning to the π* anti-bonding orbital. These transitions are: - Typically weak in intensity - Sensitive to hydrogen bonding - Important for CD spectroscopy as they can show strong Cotton effects 2. **π_nb → π* transitions**: These involve the non-bonding π orbital (localized above and below the N-O plane) transitioning to the π* orbital. These transitions are: - Generally stronger than n → π* transitions - More sensitive to the relative orientation of the amide groups - Crucial for understanding the electronic coupling between the two amide groups The C1 and C4 labels refer to the two different amide groups in the system, and we need to calculate transitions for both because: - They may have different energies due to different environments - They can couple with each other - Their relative orientations affect the overall CD spectrum In my project on calculating better electronic structure of diamides, I was looking for 4 different electronic transitions: - n C1 → π* C1 - n C4 → π* C4 - π_nb C1 → π* C1 - π_nb C4 → π* C4 ### Transition Dipole Properties The transition dipole moment is a crucial property that determines the intensity and nature of electronic transitions. In diamides: 1. **Local n→π* Transitions**: - Have small electric transition dipole moments (typically < 0.1 Debye) - Are often forbidden in the electric dipole approximation - Can gain intensity through vibronic coupling - Are particularly sensitive to hydrogen bonding 2. **Local π→π* Transitions**: - Have large electric transition dipole moments (typically ~2 Debye) - Are strongly allowed transitions - Show significant intensity in both absorption and CD spectra - Are less sensitive to hydrogen bonding than n→π* transitions ### Computational Considerations **How many roots do I need?** To capture all relevant transitions, I used RASSCF calculations with seven roots. Initial attempts with nine roots didn't provide additional useful information, suggesting that seven roots are sufficient to describe the important electronic transitions in the system. ### Orbital Localization The transitions involve several key orbitals: 1. **Non-bonding Orbitals**: - One localized on the oxygen atom of amide 1 (n1) - One localized on the oxygen atom of amide 2 (n2) 2. **π Non-bonding Orbitals**: - One localized above and below the plane of the nitrogen and oxygen of amide 1 (π-nb1) - One localized above and below the plane of the nitrogen and oxygen of amide 2 (π-nb2) 3. **π* Anti-bonding Orbitals**: - One localized above and below the plane of the carbon, nitrogen, and oxygen of amide 1 (π*1) - One localized above and below the plane of the carbon, nitrogen, and oxygen of amide 2 (π*2)