Conformation of the N(CH3)2 group in cytosine and in simple model pyrimidines and pyridines. Steric effects of ortho-methyl substitution on infrared spectra and molecular dipole moments.

Infrared spectra of amino and dimethylamino derivatives with and without an ortho-methyl group of 4- and 5-substituted pyrimidines, 4-substituted pyridine, benzene and of the respective cytosines were recorded in the region of skeletal ring vibrations. Integrated intensities of ring vibration(s) v8 at about 1600 cm-1 sensitive to the presence of electron-donating substituents were used for elucidation of the steric effects of ortho-methyl on the mesomeric interaction between the -N(CH3)2 group and the ring. Molecular dipole moments were also determined experimentally in benzene for simple pyrimidine and pyridine derivatives and analysed vectorially with the use of component group moments in terms of the N(CH3)2 group conformation. The data point to a progressive twist of the dimethylamino group in hindered derivatives in the order: pyrimidine-5 greater than pyridine-4 greater than pyrimidine-4. They are also in agreement with the essential planarity of sterically crowded m41,4,4,5cytosine.

Barrier to rotation and conformation of the -NR2 group in cytosine and its derivatives. Part II. Experimental and theoretical dipole moments of methylated cytosines.

The dipole moments of several cytosine, methylaminocytosine and dime-thylaminocytosine derivatives with and without an ortho methyl group were determined experimentally in dioxane and benzene. Calculations of total energies and dipole moments were performed by the CNDO/2 and INDO methods for sp2 and sp3 hybridization of exocyclic nitrogen for different values of rotational angle phiC-N. Comparison of the experimental dipole moments with those calculated for the energy minima suggests that the conformation of the dimethylamino group is not planar and differs from that found in cytosine. 1,5,7-Trimethylcytosine, with the dipole moment of 7 Deby units, was considered to be the model compound which closely reproduces the dipole moment of cytosine.