Thymine rotational constants

Among the computer-efficient yet reliable methods for geometry optimization of NABs are the MP2 and DFT (B3LYP) approximations with the 6-31G(d,p) basis set. The latter level also proves to be extremely valuable for predicting the IR harmonic frequencies and intensities. Tables 2 and 3 present experimental and calculated rotational constants and dipole moments for the normal tautomers of uracil and thymine and three experimentally characterized forms of... 

Table 2 Rotational Constants (MHz) and Dipole Moment (D) of Uracil and Thymine Calculated with 6-3IG(d,p) Basis Sel ...

Our group published the first organometallic derivatives of PNA monomers [138]. Ferrocene carboxylic acid 3 and benzoic acid chromium tricarbonyl 50 were coupled to the amino group of PNA monomers with different nucleobases (Thymine (T) a and Z-protected Cytosine (Z-C) b) using HBTU as the coupling agent to give compounds 51 and 52, respectively (Scheme 5.27). The activation barrier AG for the rotation about the tertiary amide bond has been determined for the ferrocenylated T-PNA monomer 51a to be 75 0.5 kj moD. For the same compound 51a, a T-T self-association constant = 2.5 0.2 has been... 

In the case of thymine, the rotational spectrum was complicated not only by the hyperfine structure of two atoms but also by a further doubling arising from the coupling of the internal rotation of the methyl group to the overall rotation (see Fig. 25). The resulting structure was completely resolved and analysis of the spectrum yielded the quadrupole coupling constants for both nuclei and the barrier to internal rotation of the methyl group, U3 = 1.502(9) kcal/mol obtained from the A-E doublet (see Fig. 25). 

In Form III from acetonitrile (Type 2), both stacking of the thymine bases and crystallization of the alkyl chains are found. Acetonitrile (dielectric constant = 37.5 at 20°C, Dipole Moment = 3.44 D) is an aprotic and polar solvent. Also, in acetonitrile, thymines form hydrogen-bonded pairs at first. For the association of the hydrogen-bonded pairs, the stacking interaction of thymine bases and the van der Waals interaction of the long alkyl chains occur at the same time. In this crystal, acetonitrile interacts with C2-0 of thymine to give an inclusion crystal Rotation of the thymine bases in crystal is possible because included acetonitrile is mobile. The formation of the inclusion crystal may be kinetic control, but slow evaporation of acetonitrile gives thermodynamically stable plates. 

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