Molecular Structure and Infrared Spectra of Adenine. Experimental Matrix Isolation and Density Functional Theory Study of Adenine¹⁵N Isotopomers

The infrared spectra of matrix-isolated adenine and its 15 N isotopomers with 15 N at the N(9) or N(7) positions have been studied. The experimentally observed IR spectra were compared with the spectra predicted at the DFT(B3-LYP)/6-31G(d,p) level. This method was also used to calculate molecular parameters (rotational constants, dipole moments) of both N(9)H and N(7)H tautomers of adenine. The agreement between experimental and theoretical spectral positions, intensities, and isotopic shifts of the IR bands is good. That allows for reliable assignment of the IR spectra and for conclusion that the amino-N(9)H tautomer of adenine strongly dominates in low-temperature matrices.