Jacek Koput

Reconsideration of solvent effects calculated by semiempirical quantum chemical methods

AM1 and PM3 semiempirical calculations are reported for the solvent effects on the tautomeric equilibria of 2‐pyridone/2‐hydroxypyridine and 4‐pyridone/4‐hydroxypyridine in the gas phase and solution. The solvent effects on the tautomeric equilibria were investigated by self‐consistent reaction field (SCRF) theory implemented in the AMPAC and MOPAC program in two different ways: one in which all the solvent relaxation is included in the quantum mechanics and the total energy must be corrected for the solvent change in energy, method A; and a second in which the quantum mechanics directly includes this term, method B. The calculated (AM1, method A) tautomeric equilibrium constants (log K1) for 2‐pyridone in the gas phase, cyclohexane, chloroform, and acetonitrile are −0.3, 0.3, 0.8, and 1.3, respectively, in good agreement with the experimental data (−0.4, 0.24, 0.78, and 2.17, respectively). For 4‐pyridone/4‐hydroxypyridine differences between calculated log K1 for the gas phase, chloroform and acetonitrile (−6.0, −2.6, and −1.2, respectively) and experimental data (< −1, 0.11, and 0.66, respectively) are larger but the experimental values are also less certain. The experimental acetonitrile data are disturbed by specific interactions. An extension of the SCRF for aqueous solutions is reviewed.

Ab initio prediction of the vibrational-rotational energy levels of hydrogen peroxide and its isotopomers

The vibrational-rotational energy levels of the hydrogen peroxide isotopomers H2O2, D2O2, HOOD, and H218O2 have been predicted by the variational method using a high-quality ab initio six-dimensional potential energy surface. The calculated energy levels are found to be in good agreement with available experimental data. The predicted spectroscopic constants for various isotopomers can be useful in a future analysis of the rotational-torsional spectra and experimental determination of the equilibrium structure of hydrogen peroxide.

An ab initio study on the equilibrium structure and CCC bending energy levels of carbon suboxide

Abstract The molecular parameters of carbon suboxide, C 3 O 2 , have been determined in large-scale ab initio calculations using the coupled-cluster method, CCSD(T), and basis sets of double- through quadruple-zeta quality. The potential energy function for the large-amplitude CCC bending motion (the ν 7 mode) was determined to be strongly anharmonic. The equilibrium structure of the molecule was found to be bent, with a barrier to linearity of only 18 cm −1 . The rotation-bending energy levels were then calculated using a semirigid-bender Hamiltonian. The vibrational energy levels and effective rotational constants determined for various ν 7 states were found to be in good agreement with the experimental data.

Excited and ground state conformations of p-dimethylamino-benzaldehyde and p-dimethylaminoacetophenone

Abstract p-dimethylaminobenzaldehyde (DMABA) and p-dimethylaminoacetophenone (DMAA) were investigated in nonpolar and polar protic and aprotic solvents over a wide temperature range. The sequence of the lower lying excited states was established. The temperature dependence of the fluorescence yields was interpreted within the irreversible reaction kinetics scheme b∗→TICT (twisted internal charge transfer) state. The activation energies and the rate constants were evaluated. Differences between the low-temperature fluorescence or phosphorescence excitation spectra and the absorption spectrum were found to be due to the existence of different rotamers in the ground state. Quantum chemical (INDO) calculations of the energies of electronic transitions, oscillator strengths and dipole moments were performed for flat and twisted conformations of DMABA.

Spectroscopy and Photophysics of Iso- and Alloxazines: Experimental and Theoretical Study

We present a systematic study of the effect of methyl substitution on iso- and alloxazines in acetonitrile solutions. Substitution patterns have profound effects on both spectral and photophysical properties, with fluorescence quantum yields varying by more than an order of magnitude. TD-DFT calculation were used for the first time to correlate electronic structure changes with the substitution patterns, with good agreement between calculated and theoretical band positions and oscillator strengths. Both n−π* and π −π* states in these compounds are predicted, with the oscillator strengths indicating that only the π −π* states should be observable in the absorption spectra. Substitution patterns are shown to be responsible for energy order inversion between these states.

Saddle-node bifurcations in the spectrum of HOCl

A detailed analysis of the bound-state spectrum of HOCl (hypoclorous acid) in the ground electronic state is presented. Exact quantum mechanical calculations (filter diagonalization) are performed employing an ab initio potential energy surface, which has been constructed using the multireference configuration-interaction method and a quintuple-zeta one-particle basis set. The wave functions of all bound states up to the HO+Cl dissociation threshold are visually inspected in order to assign the spectrum in a rigorous way and to elucidate how the spectrum develops with energy. The dominant features are (1) a 2:1 anharmonic resonance between the bending mode and the OCl stretching mode, which is gradually tuned in as the energy increases, and (2) a saddle-node bifurcation, i.e., the sudden birth of a new family of states. The bifurcation is further investigated in terms of the structure of the classical phase space (periodic orbits, continuation/bifurcation diagram). It is also discussed how the spectrum of...

Ab initio and DFT calculations of structure and vibrational spectra of pyridine and its isotopomers

Abstract The molecular parameters and vibrational spectra of pyridine, pyridine-d 5 and partially deuterated pyridines have been computed using the MP2, BLYP, BP86, BVWN, SVWN and B3LYP methodology and the cc-pVDZ, cc-pVTZ and 6-311++G(d,p) basis sets. The results are compared with the available experimental data in the gas phase. Bond distances, bond angles, dipole moment and harmonic frequencies computed by the MP2/cc-pVTZ and B3LYP/cc-pVTZ are in good agreement with the available experimental data. The calculations are found to be valuable in verifying a number of less certain experimental vibrational assignments. The results suggest that the DFT methods incorporate some anharmonic contributions, which improve the agreement with the experimental frequencies.

Unimolecular dissociation of HOCl: unexpectedly broad distribution of rate constants

Abstract The dissociation of HOCl in its electronic ground state is investigated by means of quantum dynamics calculations (filter diagonalization and harmonic inversion of the autocorrelation function) and an ab initio potential energy surface. At threshold, the state-resolved rate constants are scattered over seven orders of magnitude, which is significantly broader than the distribution predicted by random matrix theory. This remarkable state specificity is the fingerprint of the regular dynamics of HOCl even at high energies.

Ab initio prediction of the potential energy surface and vibration-rotation energy levels of BeH2.

The equilibrium structure and potential energy surface of beryllium dihydride BeH(2) in its ground electronic state have been determined from highly accurate ab initio calculations. The vibration-rotation energy levels of three isotopomers BeH(2), BeD(2), and BeHD were predicted using the variational method. The calculated spectroscopic constants are in remarkably good agreement with the existing experimental data (sub-cm(-1) accuracy) and should be useful in a further analysis of high-resolution vibration-rotation spectra of all three isotopomers.

X-ray, FT-IR and PM3 studies of hydrogen bonds in complexes of some pyridines with trifluoroacetic acid

Abstract The crystal structures of trifluoroacetic acid complexes with 4-NMe 2 -, 4-Me- and 4-CN-pyridines were determined by X-ray analysis; the NH⋯O bonds are 2.724(3), 2.702(4) and 2.587(5) A respectively. The H-bonds are nearly linear for 4-NMe 2 , 177(4)°; 4-Me, 177(4)°; and 4-CN, 174(6)°. IR spectra (Nujol) show continuous absorption, whose intensity decreases with elongation of the H-bond length. The continuous absorption is not observed in D 2 O spectra. The solid-state spectra in the 1700 cm −1 region are more complex than those in D 2 O; the characteristic overtones of the pyridine rings borrow intensity from the continuous absorption via Fermi resonance. The overtones indicate modified structure in the 1700 cm −1 region. The results of both diffraction and FT-IR experiments are comparable. The structural parameters of the complexes were also determined by quantum-mechanical calculations with the semiempirical MNDO—PM3 method. A solvent effect was taken into account using a self-consistent reaction field theory.