Leszek Lapinski

Matrix isolation studies of cytosine: The separation of the infrared spectra of cytosine tautomers

Abstract The i.r. spectral studies of cytosine and its d3-deuterated derivative isolated in Ar and Ne matrices are reported. The amino-hydroxy and the amino-oxo tautomeric forms dominate in matrices. Separation of the i.r. spectra of the two tautomers was based on observation of spectral changes following u.v. irradiation of the matrix. Detailed analysis and interpretation of the spectra was performed.

The infrared spectra of matrix isolated uracil and thymine : an assignment based on new theoretical calculations

Abstract The IR spectra of uracil and thymine predicted theoretically at the ab initio Hartree-Fock level with 6-31G basis set are reported and compared with Ar matrix experimental spectra. The IR spectra computed at the SCF/6-31G level reproduce the experimental spectra with an accuracy which allows a reliable vibrational assignment. A split valence basis set, augmented with polarization functions on all atoms, was found to be quite sufficient for a reliable prediction and assignment of the IR absorption bands in the spectra of medium-sized molecules.

Experimental matrix isolation and theoretical ab initio HF/6-31G(d, p) studies of infrared spectra of purine, adenine and 2-chloroadenine

Abstract The results of ab initio calculations at the Hartree—Fock level with the 6-31G( d , p ) basis set are reported for the harmonic vibrational infrared spectra of purine, adenine and 2-chloroadenine. These were compared with the experimental IR spectra of the compounds isolated in low-temperature matrices. The experimental spectra of 2-chloroadenine are reported for the first time. In the case of purine this comparison resulted in the complete assignment of the experimental IR spectrum. The proposed assignments of both the adenines studied are still incomplete.

A Computational Study on the Mechanism of Intramolecular Oxo−Hydroxy Phototautomerism Driven by Repulsive πσ* State

Potential energy (PE) surfaces of the lowest excited states of the 4(3H)-pyrimidinone/4-hydroxypyrimidine system were investigated with the aid of the CC2 and CASSCF methods of the electronic structure theory. These studies resulted in identification of a low-lying pi sigma* state, which is dissociative with respect to the stretching of the N-H or O-H bonds in the oxo and hydroxy structures of the compound, respectively. After initial excitation to the lowest local n pi* and/or pi pi* singlet states, the system can access the PE surface of the pi sigma* state by crossing a low barrier. It was computationally demonstrated that the system should evolve on the PE surface of the repulsive pi sigma* state toward a broad seam of intersection with the PE surface of the ground state. At the intersection, the nonadiabatic transition to the ground electronic state takes place and the system can either evolve to a minimum of the initially excited tautomer or to the ground-state minimum of the other tautomer. The steps listed above provide a mechanism of photoinduced dissociation-association (PIDA) phototautomerism, experimentally observed for a number of monomeric molecules, structurally similar to 4(3H)-pyrimidinone/4-hydroxypyrimidine. This mechanism describes a new class of intramolecular phototautomeric reactions driven by a repulsive pi sigma* state.

INFRARED MATRIX ISOLATION SPECTRA OF 1-METHYLURACIL. REVISED ASSIGNMENT BASED ON THE HARTREE-FOCK AND POST-HARTREE-FOCK STUDIES

Abstract Ab initio calculations of the infrared spectra of 1-methyluracil have been carried out with the Hartree–Fock and the second order Moller–Plesset perturbation (MP2) methods using the D95V** basis set. Theoretical harmonic frequencies and absolute infrared intensities reproduce well the experimental infrared spectra of the Ar matrix isolated species. The revised assignment of the infrared spectra of monomeric 1-methyluracil isolated in Ar and N2 matrices is presented.

An infrared matrix isolation study of tautomerism in purine and adenine

Abstract Contrary to previous interpretations of the infrared spectra of purine and adenine isolated in low-temperature argon matrices suggesting that the molecules exist as a mixture of two tautomeric forms N(9)H and N(7)H of comparable weights, reinvestigation of the vibrational spectra in inert matrices (neon, argon, nitrogen) suggests that the observed IR absorption is due to one tautomeric form of either purine or adenine. This is in agreement with ab initio quantum-mechanical calculations of the relative internal energies of purine and adenine tautomers which predict the N(9)H tautomer to be the most stable form in both cases.

Vibrational spectrum and molecular structure of triphenylamine monomer: A combined matrix-isolation FTIR and theoretical study

Theoretical optimization of triphenylamine geometry, carried out at DFT(B3LYP) level using 6-31G** and aug-cc-pVDZ basis sets, predicted a propeller-like structure of the compound with D3 overall symmetry. In this structure, the central NCCC atoms are coplanar and the phenyl rings are symmetrically twisted from this plane by 41.5° (6-31G**) or 41.6° (aug-cc-pVDZ). The experimental FTIR spectrum of triphenylamine monomers isolated in an argon matrix was measured and interpreted by comparison with theoretical spectra calculated at the DFT(B3LYP) level with 6-31G** or aug-cc-pVDZ basis sets. The good agreement between the experimental and theoretical spectra allowed a positive assignment of the observed infrared absorption bands. Conformational flexibility of triphenylamine was investigated by carrying out a series of theoretical scans of the potential energy hypersurface of the system. Special attention was granted to the minimal energy pathway between the left-hand rotating and right-hand rotating symmetry identical structures of the compound. A route conserving a C2 symmetry axis was identified as implying an energy barrier of 20 kJ mol−1 only, whereas the calculated barrier for the concerted twist of all the phenyl rings (the route with conservation of the C3 symmetry axis) was as high as 54 kJ mol−1.

Infrared matrix isolation and ab initio quantum mechanical studies of purine and adenine

Abstract Results are presented from ab initio SCF (3-21G) calculations for the geometries of the N(9)H and N(7)H tautomers of purine and adenine and vibrational spectra (wavenumbers and intensities) of the N(9)H forms. All these results are compared with available geometries from crystallographic studies and with reported infrared spectra of the molecules isolated in inert gas low-temperature matrices. The N(9)⇌N(7)H tautomerism of the molecules in question is also briefly discussed.

A new theoretical prediction of the infrared spectra of cytosine tautomers

Abstract The infrared spectra of the cytosine amino-oxo and amino-hydroxy tautomers predicted theoretically at the ab initio Hartree—Fock level with a 6-31G** basis set are reported. These are compared with the experimental spectra obtained in an argon low-temperature matrix. The IR spectra computed at this level reproduce the experimental spectra closer than the previous predictions.

Molecular structure and infrared spectra of 2-hydroxy-1,4- naphthoquinone; Experimental matrix isolation and theoretical Hartree–Fock and post Hartree–Fock study

Abstract The infrared spectra of 2-hydroxy-1,4-naphthoquinone (HNQ) isolated in low-temperature Ar and N 2 matrices are reported. The calculations of electronic energies for possible tautomers of HNQ (performed at RHF/6-31++G** and MP2/6-31++G** levels of theory) and the comparison of the experimental spectra with those theoretically calculated (at the RHF/6-31++G** and DFT(B3-LYP)/6-31++G** levels) show that the molecules of the compound isolated in low-temperature matrices exist in the form with intramolecular hydrogen bond. Theoretical results reproduce well the experimental infrared spectra of the matrix isolated HNQ. The assignment of the infrared absorption bands due to monomers of the compound isolated in Ar and N 2 matrices to the theoretically predicted normal modes is presented.