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130 Ralf Steudel et al. Table 7 Energy differences (kJ mol1) between the global minimum structures and the low- est-energy branched isomers of Sn molecules (n=4–10), calculated at the G3X(MP2) level of theory [53] S(n1)=S species DE0 S3=S 62 S4=S 119 S5=Seqa 104 S6=S 58 S7=S 33 S8=S 32 S9=S 54 DH298 DG298 62 61 120 117 106 95 58 58 33 28 32 32 54 52 a The S5=Sax species with the exocyclic atom in an axial position is not a stable equilibri- um structure at this level of theory but collapses to the prismatic isomer of D3h symme- try. On the other hand, S5=Seq with an equatorial exocyclic atom is a local energy mini- mum. All other species in this table have the exocyclic atom in the axial position Table 8 The three strongest UV absorptions (T1, T2, and T3, >200 nm) of various singlet S4 isomers, calculated at the CIS/6-311+G(2df)//B3LYP/6-31G(2df) level. The computed transi- tion energies are given in nm and the values of oscillator strength are in parenthesis [53] Species T1 C2v 212 (0.063) C2h 201 (0.486) Cs 203 (0.016) D2d 201 (0.018) D3h 213 (0.095) a Only two UV absorptions above 200 nm T2 272 (0.056) 227 (0.200) 211 (0.033) 203 (0.006) 320 (0.216) T3 566 (0.192) 634 (0.292) 241 (0.339) 433 (0.004) a [75]. Thus, our calculated electronic spectra clearly indicate that the assign- ment of S3=S as the red light absorbing isomer [23, 43] is incorrect. More importantly, the calculated transition energies of the C2v and C2h isomers (566 and 634 nm, respectively) match very well with the experimental re- sults. We therefore conclude that the observed green and red absorbing S4 isomers correspond to the C2v and C2h structures of S4, respectively. In Table 9 the calculated vibrational wavenumbers of the three most stable S4 isomers are listed together with their infrared and Raman intensities. These data, obtained at the B3LYP/6-31G(2df) level of theory allow the reli- able identification of two isomers observed in hot [23] and quenched [43] sulfur vapor. Our assignment of the infrared and Raman spectra of the S4 isomers is given in Table 9. The evidence for the cis-planar isomer is strong. In particu- lar, the two calculated Raman lines at 373 and 674 cm1 of the C2v isomer have also been observed in the spectrum of ultramarine red, the red color being caused by the 520 nm absorption band of S4 [76]. The observed IR ab- sorption at 642 cm1 and the Raman line at 635 cm1 can be assigned to thePDF Image | Topics in Current Chemistry
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