Rotational strengths theoretical calculation

The syn and anti conformations of pyrimidine ribonucleosides have an opposite sign for certain transitions (e.g., the B2u transition) (67B843 69JA831 71JA(93)1600 72JCP2736 72MI3). Such relations are empirical correlations, since the molecular structures are too complicated to allow theoretical calculations of the rotational strengths. A combination of DNMR, CD spectroscopy, and molecular mechanics calculations has been applied to derivatives of indole 77 and thiazoline-2-thione 78 substituted by chiral rotors (Scheme 58). In these molecules the rotor adopts one of the bisected... 

SAC-CI method was applied to calculate the electronic CD spectrum of uridine [43], Based on theoretical CD and absorption spectra, observed peaks in the experimental spectra were assigned. The rotational strength (R) in the length form [44] was calculated as imaginary part of the inner product of the electric transition dipole moment (ETDM) and magnetic transition dipole moment (MTDM). 

Figure 4-2. (a) Absorption and (b) CD spectra of uridine. In the theoretical CD spectrum, the calculated rotational strengths (solid vertical lines) were convoluted with die Gaussian envelopes... 

Two independent calculations of absorption and CD spectra based on the (unrefined) atomic coordinates of the Rps. viridis RC have now been published. Although based on quite different sets of theoretical assumptions, each claims suc-cess in explaining major features of optical spectra, including CD. However, one of them apparently still fails to explain the rotational strength of the long-wavelength Qy band, and the other is possibly too complicated to be completely satisfactory. 

The calculation involves optimization of the molecular structure, computation of vibrational modes (by far the most demanding part computationally), computation of atomic polar and axial tensors and of all the sums leading to dipole and rotational strengths. As the last step, the theoretical VCD curve is simulated by using the empirical values for bandwidths. The quantum chemical part of the calculation... 

A similar study of methyl 3-amino-2-nitrocrotonates 18b, 18d and 18f gave con-considerably lower barriers (11.8-15.1 kcal mol Table 5) because of ground-state strain. The barriers increase with increasing electron attraction by the N-substituent R. This may reflect an effect on the strength of the hydrogen bond, which is a component in the barrier. The authors favor a thermal process for the Z-E rotation, partly because the barriers are well reproduced by theoretical calculations by the AMI method, including solvent molecules vide infra, Section VTI.B). As expected, the C=C barriers for the acetyl derivatives 18h and 18i are lower than for the corresponding ester derivatives 18c and 18b. 

Theoretical studies of the rotational strength of the peptide transitions have appeared (Schellman, 1968 Bayley et al, 1969 Madison and Schellman, 1970a,b,c, 1972 Mayers and Urry, 1971 Nielsen and Schellman, 1971 Richardson et ai, 1973). The good agreement between the calculated and experimental curves indicates that the assumption that the rotatory properties of peptides are dominated by the n-7c and n-n transitions is valid in the wavelength region of 180-300 nm. 

IR, Raman, and VCD spectroscopy all excite the same vibrational fundamentals. The respective vibrational spectra are different because the mechanism by which light is absorbed is different in each case, the amount of absorption depending on changes in the dipole moment, the polarizability and the rotational strength, respectively. AH these quantities are amenable to computation, and modern ab initio theory can reliably predict the frequencies of a molecule s vibrational fundamentals as well as the intensity of the signal in IR, Raman, and VCD spectra. The latter in particular are greatly assisted by electronic structure calculations comparison of experimental and theoretical VCD spectra enable absolute molecular conformations to be determined. 

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