MATRIX-ISOLATION FT-IR STUDIES AND THEORETICAL CALCULATIONS OF HYDROGEN-BONDED COMPLEXES OF MOLECULES MODELING ADENINE TAUTOMERS - 1 - H-BONDING OF BENZIMIDAZOLES WITH H2O IN AR MATRICES

This work opens a series of studies on the water complexes of adenines . We use a similar approach as used in our earlier studies of cytosine -water complexes (i.e., first we investigate the LR spectral manifesta tions of hydrogen-bonding at selected interaction sites of the stable amino N9H tautomeric form of adenine by studying simpler model molecul es which have only a single or a very few selected hydrogen-bond inter action sites typical for adenine). The present study concerns the firs t two of such model molecules, benzimidazole and 1-CH3-benzimidazole. LR vibrational spectra of matrix-isolated benzimidazole, 1-CH3-benzimi dazole, and their complexes with water are analyzed and assigned by co mparing the experimental spectra with the IR frequencies and intensiti es computed with the use of ab initio and density functional theory (D FT) methods. When the DFT/B3LYP/6-31++G* monomer frequencies are scal ed with three different scaling factors, the mean differences between the experimental and calculated frequencies are only 10 and 8 cm(-1) f or benzimidazole and 1-CH3-benzimidazole, respectively. The calculated , MP2/6-31++G*//RHF/6-31++G** (MP2 denotes the second-order Moller-Pl esset Perturbation Theory, RHF denotes the restricted Hartree-Fock met hod, and notation MP2//RHF denotes that the molecular geometries were optimized at the RHF level and then used to calculate total energies u sing the MP2 method), H-bond interaction energies, with the basis set superposition error accounted for, are -22.6, -21.2, and -22.0 kJ/mol for the benzimidazole N-1-H ... OH2 and N-3... H-OH complexes and the 1-CH3-benzimidazole N-3... H-OH complex, respectively. The DFT/B3LYP/6 -31++G* method yields similar H-bond interaction energies. The freque ncy shifts of the vibrational modes directly involved in the H-bond in teractions are better predicted by the DFT method than by the RHF meth od. For other vibrational modes not directly involved in the H-bonds, the two methods provide a similar level of accuracy in predicting the shifts of the fundamental modes caused by H-bonding interactions. In t his work we also establish correlations between experimental and theor etical characteristics of the N-H ... OH2 H-bonding in water complexes of benzimidazole and 1-CH3-benzimidazole, and these correlations will be used in future elucidation of FT-IR spectra of water complexes of adenine.