Rotational constants cytosine

Table 3 Rotational Constants (MHz) and Dipole Moment (D) of Cytosine Tautomers Calculated with 6-31G(d,p) Basis Set ...

As shown in Figure 7.7, cytosine has three major tautomeric forms which have different properties enol, keto, and keto-imine. In the gas phase, the keto-imine is the most unstable and the enol tautomer is shghtly more stable than the keto form by about 0.03 eV [59-61]. The keto form is the biologically important one, with Watson-Crick base-pairing in DNA, and predominant in solution. In matrix isolation, Szczesniak et al. [62] observed both keto and enol forms, with higher abundances for the latter and small contributions from the imino form. Brown et al. [63] have obtained the rotational constants for all three tautomeric forms by microwave spectroscopy. Schiedt et al. [64] also identified the existence of keto and enol tautomers of neutral cytosine in the gas phase by anion spectroscopy. 

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... 

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