Using AM1 and ZINDO molecular modeling, we calculated the excitations energies of four thiacyanine dyes DTC, DTCC, DTDC, DTTC to be 430nm, 485nm, 544nm, and 600nm. Using UV?Visible Absorption Spectroscopy, in a previous lab we determined that the absorption wavelengths were 422nm, 544nm, 655nm, and 757nm, respectively. Thus, only calculations for the smaller molecules were pretty accurate. Additionally, the 1-D and 2-D particle in a box models were used to calculated the excitation energies, but were not as accurate.
[...] ZINDO is another semi-empirical quantum chemistry method used in computational chemistry and it stands for Zerner's Intermediate Neglect of Differential Overlap. Similar to AM1 it uses zero-differential overlap for the two-electron integrals; however, it does not do this for integrals of orbitals that are centered on the same atom. Thus, ZINDO is more accurate for electronic structure calculations, like computing the UV visible excitation energies Complete the table below: Molecule # π electrons* # valence electrons for a positively charged molecule *Each double bond counts as 2 electrons and lone pairs on each sulfur atom count as Report the S0 S1 and S0 S2 transition energies obtained from your ZINDO calculations for DTC, DTCC, DTDC and DTTC in units of cm-1. [...]
[...] was further optimized using the semi-empirical electronic structure optimization AM1. Finally, the electron structure was further refined using the INDO/S method to calculate the HOMO and LUMO orbitals and the electron absorption spectrum with a high level of configuration interaction Results Figure 1. DTC Highest Occupied Molecular Orbital (HOMO) Figure 2. DTC Lowest Unoccupied Molecular Orbital (LUMO) Figure 3. DTCC Highest Occupied Molecular Orbital (HOMO) Figure 4. DTCC Lowest Unoccupied Molecular Orbital (LUMO) Figure 5. DTDC Highest Occupied Molecular Orbital (HOMO) Figure 6. [...]
[...] Use the distances you obtained in 4 and above to estimate the transition energies for your molecules for a 1-D and a 2-D particle in a box model. 1-D Particle in a Box Model Thiacyanine Length Number of pi Theoretical Predicted Dye electrons in Excitation Absorption Linkage Sample Calculation for DTC: Theoretical Excitation Energy: ΔEn=h2(2n+1)/8meL2=( 6.626 E-34 J 9.11 E-31kg)( 3.483 E-10m)2]/ ( 6.626 E-34 3.0 E10cm/s)= 124906.5 cm-1 Predicted Wavelength: λ=(1/ΔE)10^7nm/cm= 80.1 nm 2-D Particle in a Box Model Thiacyanine C-C Ring Perpendicula RMS Number of pi Theoretical Predicted Dye Length r Length Length electrons in Excitation Absorption total Energy Wavelength Sample Calculation for DTC: RMS Length 11.807 3.582 12.33 Å Theoretical Excitation Energy: ΔEn=h2(2n+1)/8meL2=( 6.626 E-34 J 9.11 E-31kg)( 12.33 E-10m)2]/ ( 6.626 E-34 3.0 E10cm/s)= 81729.6 cm-1 Predicted Wavelength: λ=(1/ΔE)10^7nm/cm= 122.4 nm 6. [...]
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