Johan Smets

Correlations between ab initio and experimental data for isolated 1: 1 hydrogen-bonded complexes of pyridine and imidazole derivatives with water

Abstract Correlations between selected ab initio predicted and experimentally observed properties of 1:1 H-bonded complexes of a series of pyridine derivatives with water are investigated. Specifically, relationships are found between the experimentally observed properties of proton affinity, frequency shift, and dipole moments, and the ab initio calculated values of bond distance, interaction energy, frequency shift, IR absorption intensity and charge differences obtained from Mulliken population analysis. The examined correlations were then compared with data collected for pyridine–H 2 O and imidazole–H 2 O complexes. Similarities were found between the correlations in the theoretically determined properties and the correlations in experimentally observed parameters. It was also found that well-defined relations could be established between the calculated and observed properties for the pyridine derivatives, but these could not be reliably extended to the imidazole systems. These similarities demonstrate that the presently available ab initio calculations are useful predictors of experimental behavior of H-bonded systems but only for chemically closely related molecules.

Density functional theory and ab-initio computational study of the 2- hydroxypyridine/2-pyridone system: A comparison with FT-IR data from matrix isolation experiments

Abstract The tautomeric equilibrium 2-hydroxypyridine⇔2-pyridone was observed in a low-temperature Ar matrix. The infrared spectra of the tautomers were analysed and assigned using DFT/(B3-LYP and B3-PW91)/6-31++G** and RHF/6-31++G** methods. The molecular parameters (rotational constants and dipole moments) are calculated with different methods and compared with recent gas-phase microwave data. The rotational constants and dipole moments predicted using the DFT method agree much better with the experimental results than the parameters predicted at the HF level. The energy difference for the tautomeric equilibrium and the tautomerization constant were calculated and these parameters are compared with recent theoretical and experimental predictions.

AB INITIO THEORETICAL STUDY OF DIPOLE-BOUND ANIONS OF MOLECULAR COMPLEXES : WATER TETRAMER ANIONS

We present results of ab initio calculations of the (H2O)4/(H2O)4− system. The main conclusions of this work are as follows: The calculated results predict that water tetramer anions are metastable systems in agreement with weak spectral manifestation of these systems in gas-phase experiments of Bowen and co-workers; the excess electrons in all four structural isomers of water tetramer anions found in the calculations are attached to the clusters by the virtue of dipole-electron interaction; all four (H2O)4− anions found in the calculations are almost isoenergetic but have different vertical electron detachment energies (VDEs) ranging from 22 to 279 meV; the most stable cyclic structure of (H2O)4 has a null dipole moment and does not form a dipole–bound state with an excess electron; the water tetramer anions observed experimentally probably are formed as a result of hydration of the water dimer anion, (H2O)2−, by a neutral water dimer or by hydration of the water trimer anion, (H2O)3−, by a single water ...

Infrared-spectra and tautomerism of isocytosine - an ab-initio and matrix-isolation study

Abstract Prototropic tautomerism of isocytosine has been investigated using both theoretical ab initio and experimental matrix isolation IR methods. The coexistence of the amino-hydroxy and amino-oxo N(3)H forms, with a clear predominance of the hydroxy form, was observed. The tautomerization constant [oxo]/[hydroxy] obtained from experimental and calculated IR intensities was 0.11 at the micro-oven temperature of 400 K. The ab initio prediction of the relative energies of the tautomers is in reasonable agreement with the experimental estimation. The change of the tautomeric form oxo→hydroxy upon UV irradiation was used to separate the IR spectra of both tautomers. A theoretically assisted interpretation of the IR spectra of both observed tautomers is proposed.

Anions of the hydrogen-bonded guanine-cytosine dimer : theoretical study

Abstract The quantum mechanical calculations performed in this work revealed an interesting nature of the stationary states of an excess electron in the guanine–cytosine (G–C) anion. The electron can form either a dipole-bound (DB) state or a covalent state. The coexistence of the two states may enhance the ability of the G–C dimer to capture an excess electron.

FT-IR spectroscopic study of uracil derivatives and their hydrogen bonded complexes with model proton donors Part 4. Self-association of uracils in Ar matrices: differentiation between open and closed dimers

Abstract The spectral perturbations induced by intermolecular self-association of several uracil derivatives in dilute and concentrated Ar matrices are reported and discussed. Matrix isolation is a unique method for discriminating between open and closed dimer structures which occur in variable amounts depending on deposition and annealing conditions. The CO and NH interaction sites found to participate in the H-bonds linking the molecules in the dimers are approximately the same as in the complexes with water in Ar. Comparison of spectral shifts of stretching and out-of-plane modes for the bonded NH groups in open and in cyclic dimers allows us to determine a H-bond cooperativity of about 40% in the latter structures.

COMPETITION BETWEEN H-BONDED AND STACKED DIMERS OF PYRIMIDINE: IR AND THEORETICAL AB-INITIO STUDY

A combined matrix isolation Fourier transform infrared experimental and ab-initio theoretical approach has been used to examine the vibrational spectra of pyrimidine dimers. It was found that the presence of pyrimidine dimers resulted in experimentally observed frequencies shifted relative to those of the pyrimidine monomer. Four stable pyrimidine dimer conformers were found via ab-initio calculations. Two of these conformers namely a stacked geometry and a H-bonded geometry in which pyrimidine monomer dipole moments orient antiparallel, had the lowest but very similar energies. ab-initio harmonic frequencies of these two dimers and their shifts relative to the monomers values were determined. The effect of Basis Set Superposition error on these calculated frequency shifts was also investigated. The comparison of the ab initio predicted and experimentally observed frequency shifts and an accompanying potential energy distribution analysis suggest that the lowest-energy H-bonded configuration is responsib...

Correlations between experimental matrix-isolation FT-IR and DFT(B3LYP) calculated data for isolated 1 : 1 H-bonded complexes of water and pyridine or imidazole derivatives

Abstract Relations between selected experimentally observed matrix isolation FT-IR and/or DFT(B3LYP)/6-31++G** predicted parameters are investigated for a series of 1:1 water complexes of pyridine and imidazole derivatives. Regular correlations are observed for: (i) the experimental and calculated OH stretching frequency shifts; (ii) the ratio between the observed and calculated IR stretching frequencies of the water OH group involved in the hydrogen bond and the proton affinity of the hydrogen bonding site (N atom) of the base; (iii) the H-bond interaction energy and the OH stretching frequency shift; (iv) the calculated OH and N⋯H distances and the interaction energy of the H-bond; and (v) the interaction energy of the H-bond and the square root of the intensity enhancement of the OH stretching mode. These correlations are restricted to structurally similar systems (i.e. to the pyridine or the imidazole analogs). These correlations demonstrate that the DFT(B3LYP) method is very suitable to predict trends in the parameters related to the H-bonding.

Ab initio theoretical study of dipole-bound anions of molecular complexes: (HF)3− and (HF)4− anions

Ab initio calculations have been performed to determine structures and vertical electron detachment energy (VDE) of the hydrogen fluoride trimer and tetramer anions, (HF)3− and (HF)4−. In these systems the excess electron is bound by the dipole field of the complex. It was determined that, unlike the neutral complexes which prefer the cyclic structures, the equilibrium geometries of the anions have “zig–zag” shapes. For both complexes the predicted VDEs are positive [210 meV and 363 meV for (HF)3− and (HF)4−, respectively], indicating that the anions are stable systems with respect to the vertical electron detachment. These results were obtained at the coupled-cluster level of theory with single, double and triple excitations [CCSD(T) method; the triple-excitation contribution in this method is calculated approximately using the perturbation approach] with the anion geometries obtained using the second-order Mo/ller–Plesset perturbation theory (MP2) method. The same approach was also used to determine the...

Matrix-isolation FT-IR study on the protomeric tautomerism 2-hydroxypyridine⇌2-pyridone

Abstract The vibrational spectrum of 2-hydroxypyridine isolated in an Ar matrix was studied using FT-IR spectroscopy. From experimental intensities of different pairs of bands assigned to the 2-oxo-(P 2 ) and the 2-hydroxy-(HP 2 ) tautomer respectively, a tautomerization constant of 0.40±0.10 was calculated at 303 K, in excellent agreement with recent theoretical predictions. However, the P 2 form is more abundant in a low temperature matrix than recently suggested by theoretical calculations.