NORMAL-COORDINATE ANALYSIS AND VIBRATIONAL-SPECTRA OF 9-BETA-D-ARABINOFURANOSYLADENINE HYDROCHLORIDE (ARA-A.HCL)

The vibrational spectra of a synthetic purine nucleoside with known an tiviral activity, 9-beta-D-arabino-furanosyladenine hydrochloride (ara -A.HCl) are reported. The Fourier transform infrared (FT-IR) and Fouri er transform Raman (FT-Raman) spectra were recorded in the 4000-30 cm( -1) spectral region. The harmonic frequencies and potential energy dis tributions (PED) of the vibrational modes of ara-A.HCl were calculated by two different methods: a classical molecular mechanics method and a semiempirical molecular orbital (MO) method, PM3. The results of bot h computational methods, based on the Wilson GF method, are compared w ith observed spectra, and an assignment of the vibrational modes of ar a-A.HCl is proposed on the basis of the potential energy distributions (PED). It is found that the wavenumbers can be calculated with remark able accuracy (approximate to 1% deviation in most cases), with the cl assical mechanics method, by transferring a sufficiently large set of available harmonic force constants, thus permitting a reliable assignm ent. The semiempirical MO method, PM3, is found to be useful for the a ssignment of experimental frequencies although it is less accurate (ap proximate to 10% deviation). TR intensities calculated by this method did not coincide with the experimental values. Certain out-of-plane vi brations in the base, not reported in previous studies, have been obse rved. The performance of both methods was related to the crystallograp hic and ab initio data available. Previous normal coordinate calculati ons for the adenine base and the nucleoside 5'-dCMP are compared with our results and discussed, in relation to the crystal structure of Ara -A.HCl.