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The electronic ground states (X 1Σ+) of HSiN, HNSi, and the transition state connecting the two isomers were systematically studied using configuration interaction with single and double (CISD) excitations, coupled cluster with single and double (CCSD) excitations, CCSD with perturbative triple corrections [CCSD(T)], multireference complete active space self-consistent field (CASSCF), and internally contracted multireference configuration interaction (ICMRCI) methods. The correlation-consistent polarized valence (cc-pVXZ), augmented correlation-consistent polarized valence (aug-cc-pVXZ) (X=T,Q,5), correlation-consistent polarized core-valence (cc-pCVYZ), and augmented correlation-consistent polarized core-valence (aug-cc-pCVYZ) (Y=T,Q) basis sets were used. Via focal point analyses, we confirmed the HNSi isomer as the global minimum on the ground state HSiN[Single_Bond]HNSi zero-point vibrational energy corrected surface and is predicted to lie 64.7 kcal mol-1 (22 640 cm-1, 2.81 eV) below the HSiN isomer. The barrier height for the forward isomerization reaction (HSiN→HNSi) is predicted to be 9.7 kcal mol-1, while the barrier height for the reverse process (HNSi→HSiN) is determined to be 74.4 kcal mol-1. The dipole moments of the HSiN and HNSi isomers are predicted to be 4.36 and 0.26 D, respectively. The theoretical vibrational isotopic shifts for the HSiN/DSiN and HNSi/DNSi isotopomers are in strong agreement with the available experimental values. The dissociation energy for HSiN [HSiN(X 1Σ+)→H(2S)+SiN(X 2Σ+)] is predicted to be D0=59.6 kcal mol-1, whereas the dissociation energy for HNSi [HNSi(X 1Σ+)→H(2S)+NSi(X 2Σ+)] is predicted to be D0=125.0 kcal mol-1 at the CCSD(T)/aug-cc-pCVQZ level of theory. Anharmonic vibrational frequencies computed using second order vibrational perturbation theory are in good agreement with available matrix isolation experimental data for both HSiN and HNSi isomers root mean squared derivation (RMSD=9 cm-1). [ABSTRACT FROM AUTHOR]

The structures and energies of the deprotonated adenine-uracil base pairs are studied by using the B3LYP/DZP++ level of density functional theory. The findings provide insight into the driving force for the strong base pair interaction of proton transfer from the uracil N3 atom to the adenine N1 atom, accompanied by negative charge transfer from the adenine to the uracil unit and the electron affinities of the AU-H radicals.

A study was conducted to theoretically explore the energetic and structural properties of the nine possible neutral radicals formed via hydrogen abstraction from the adenine-thymine base pair. Except for hydrogen bond length changes for some of the radicals, the A-T base pair geometry is resilient to formation of radicals through hydrogen abstraction.