B3LYP/TZV

The effect of different heteroatoms on the ring opening of substituted three-membered rings was investigated at the B3LYP/TZV+P level of theory.115 Two specific water molecules were included in the calculations and the COSMO method was used to include the bulk solvent. With the methyl thiolate nucleophile, thiirane was... 

Figure 9.1 Chemical structure and spatial orientation of the three major isohexide isomers top-view (top) and side-view (bottom) are based on DPT calculations (B3LYP-TZV).

Table 3 Comparison of Calculated [TDDFT-B3LYP/TZV(d,p)] and Experimental 0-0 Excitation Energies (in eV) for the Lowest Singlet States (nn ) of Unsaturated Systems...

MRCI computations (d-aug-cc-pVDZ AO basis, B3LYP/TZV(d,p) ground-state geometry) are compared with experimental data in Table 5. A graphical comparison with an experimental gas-phase absorption spectrum is shown in Figure 12. 

Figure 13 Comparison of experimental and computed [B3LYP/TZV(d,p)] UV spectra of thioindigo.

Figure 16 Comparison of the computed [UTDDFT-B3LYP/TZV(d,p)] and experi-mental UV spectra of the phenoxyl radical. The theoretical A values are shifted by...

To account for this fact, the TDDFT-B3LYP calculations of the spectra have been carried out with a TZV(2df) basis set augmented with diffuse functions at the sulfur atoms as well as at the neighboring carbon atoms and a TZV(d,p) basis set for the remaining atoms. The B3LYP/TZV(d,p) ground-state geometry has been used for these calculations. The theoretical spectrum has been blue shifted by 0.39 eV to match the experimental band A. 

As a first example, let us consider the vibronic structure of the first absorption band in the UV spectrum of anthracene. In agreement with experiment, the TDDFT calculation gives an Si state with B2 symmetry and a vertical excitation energy of 3.23 eV [AE° °(exp.) = 3.43 eV]. This band can be described by a HOMO LUMO excitation and in the Platt nomenclature (perimeter model) it is denoted as the La state. Both states were optimized at the (TD)DFT-B3LYP/TZV(d,p) level in D21, symmetry. 

Figure 22 Comparison of the computed [TDDFT-B3LYP/TZV(d,p)] and experimental UV spectra for the state of anthracene. The 0-0 transition energy is set to zero.

All calculations were performed at the (TD)DFT-B3LYP/TZV(d,p) level employing Cih symmetry. In contrast to the experimental data, the TDDFT calculation yields an Si state with B symmetry (AE = = 4.05 eV) and the 2Ag state as the second one (AE = 4.89 eV). This error can be traced to the systematic underestimation of excitation energies for ionic states with TDDFT-B3LYP. Because of the adiabatic approximation used in the... 

Figure 25 Ground and first excited-state potential energy curves [TDDFT-B3LYP/ TZV(d,p)] along the pyramidalization coordinate of formaldehyde. For comparison, the dashed line shows the harmonic potential.

Figure 19 Comparison of the experimental and computed [TDDFT-B3LYP/aug-TZV(2df)] UV and CD spectra for 2,3-(5,5)-dithiadecalin. The theoretical spectra have been blue-shifted by 0.39 eV.

Hexahelicene is one of the most widely studied molecules in theoretical CD spectroscopy (see, e.g.. Refs. 47 and 136 and references cited therein). The system is rather large [120 valence electrons, 590 basis functions with a TZV(d,p) AO basis] and IJ-IO excited states are necessary to describe the experimental spectrum entirely. This example is presented here to show that correlated ab initio treatments are also applicable for such cases. The simplified coupled-cluster model CC2 together with the RI approximation is used and compared to the standard TDDFT-B3LYP approach. 

Figure 20 Comparison of the experimental and computed [TZV(d,p) AO basis, B3LYP optimized geometry] CD spectrafor ( M)-[6]helicene. The theoretical spectra have been shifted by 0.20 (B3LYP) and —0.22 eV (CC2), respectively. The filled circles/triangles indicate the two lowest states with small intensity obtained by CC2/TDDFT-B3LYP. The vertical lines correspond to results from the CC2 method.

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