F. Ramondo

MOLECULAR CONFORMATIONS AND HARMONIC FORCE FIELD OF 1,3,5-BENZENETRIOL MOLECULE FROM AB INITIO AND DENSITY FUNCTIONAL THEORY INVESTIGATIONS

Abstract The molecular conformers of the molecule 1,3,5-benzenetriol have been studied by ab initio and density functional methods to determine optimized equilibrium geometries, harmonic vibrational frequencies and relative stability. The results of the quantum-chemical calculations have been used to investigate the functional theory-infrared (FT-IR) spectrum of the 1,3,5-benzenetriol vapors trapped in Ar matrix at 12 K.

Ab initio SCF study on LiClO4 and Li2SO4 molecules: Geometries and vibrational frequencies

Abstract The coordination structures of the molecules LiClO4 and Li2SO4 were studied by ab initio SCF calculations carried out at the 3-21G*, 6-31G* and DZP basis sets level; the geometries of ClO4− and SO42− were determined at the 3-21G*, 6-31G*, 6-31 + G*, DZP and TZP levels. LiClO4 and Li2SO4 prefer at all the levels of the HF-SCF theory bidentate structures of C2v and D2d symmetries, respectively. The tridentate and monodentate C3v, symmetry structures of LiClO4, and the C2v and tridentate C3v ones of Li2SO4 were also considered in detail. Harmonic frequencies of vibration of the anions and of each coordination model of LiClO4 and Li2SO4 as well as the 18O spectra of LiClO4 (C2v symmetry) and Li2SO4 (D2d symmetry) are reported and commented on using the experimental IR spectroscopy results.

Structural and spectroscopical study of glutamic acid in the nonzwitterionic form

Abstract The molecular structure of glutamic acid in the non-zwitterionic form has been optimized by using the semiempirical MNDO and AMI methods and an ab initio calculation at the 4–31G level. The results were compared with previous reported data for other amino acids. The ab initio optimized structure was used as the starting point for a further force field calculation, and vibrational frequencies and theoretical assignments were thus obtained. In order to compare these results with experiments, the FT-IR spectrum of glutamic acid in an argon matrix was recorded and the measured frequencies were successfully compared with theoretical values, which were previously scaled by using a set of scaling factors.

Ab initio SCF study of the BO−2 ion and the NaBO2 and HBO2 molecules

Abstract The results of ab initio molecular orbital calculations on the BO − 2 anion and on the NaBO 2 and HBO 2 molecules are presented and discussed. NaBO 2 is a linear molecule of C ∞ v symmetry and its transition state corresponds to a cyclic structure with a bent BO − 2 group. Preference for an angular structure of C 8 symmetry is suggested for HBO 2 . The results of this study are compared with previous theoretical and vibrational spectroscopy investigations. Limited pseudo-potential calculations are also reported for BO − 2 and the linear structure of NaBO 2 .

Molecular structure of ethynylbenzene from electron diffraction and ab initio molecular orbital calculations

The molecular structure and benzene ring distortions of ethynylbenzene have been investigated by gas-phase electron diffraction and ab initio MO calculations at the HF/6-31G* and 6-3G** levels. Least-squares refinement of a model withC2v, symmetry, with constraints from the MO calculations, yielded the following important bond distances and angles:rg(Ci-Co)=1.407±0.003 Å,rg(Co-Cm)=1.397±0.003 Å,rg(Cm-Cp)=1.400±0.003 Å,rg(Cri-CCH)=1.436 ±0.004 Å,rg(C=C)=1.205±0.005 Å, ∠Co-Ci-Co=119.8±0.4°. The deformation of the benzene ring of ethynylbenzene given by the MO calculations, including <Co-Ci-Co=119.4°, is insensitive to the basis set used and agrees with that obtained by low-temperature X-ray crystallography for the phenylethynyl fragment, C6H5-C≡C-, in two different crystal environments. The partial substitution structure of ethynylbenzene from microwave spectroscopy is shown to be inaccurate in the ipso region of the benzene ring.

Effect of Intermolecular Hydrogen Bonding on the Molecular Structures of Imidazole and 1,2,4-Triazole: A Study by ab initio Molecular Orbital Calculations

The effect of intermolecular hydrogen bonding in the solid state on the molecular structures of imidazole and 1,2,4-triazole has been studied by SCF ab initio molecular orbital calculations at the HF/6-31G* level. The crystals of these species contain endless chains of molecules, connected by unusually strong N-H ⋯ N hydrogen bonds. Our simulation of the crystal field, based on two simple models, unequivocally shows that hydrogen bond formation not only lengthens the N-H bond but also causes a concerted change in the length of two N-C bonds. The change indicates that the contribution of a polar canonical form to the structure of the molecule increases in going from the gaseous phase to the crystal. This provides a rationale for the strong intermolecular hydrogen bond occurring in the solid state. We have also optimized the geometry of the free molecules at the MP2/6-31G* level, to investigate the effect that correcting for electron correlation has on the equilibrium structure of these systems.

MP2/6-31G* structural study of tropone and tropolone molecules

Abstract The effects of electron correlation on the molecular structure and stability of tropone and tropolone molecules have been studied by MP2/6-31G* calculations. Geometry optimizations have been carried out for tropone, cis -tropolone and the non-hydrogen bonded orthogonal and trans rotamers of tropolone. The tropone seven-membered ring exhibits an appreciable triene-like character which is slightly smoothed upon OH substitution. The asymmetry of the hydrogen bonding of tropolone, emerging from previous HF/6-31G* studies, is reduced by inclusion of electron correlation, and the proton tunnelling barrier height drastically lowers from 70 kJ mol −1 (HF/6-31G*) to 25 kJ mol −1 (MP2/6-31G*). The structural changes induced by OH torsion, supported by additional MP2/6-31G* geometry optimizations of the analogous systems 1,4-pentadiene-3-one and 2-hydroxy-1,4-pentadiene-3-one indicate that intramolecular Cue5fbO⋯HO hydrogen bonding is favoured by π-delocalization through the cyclic carbon skeleton. SCF calculations on the centrosymmetrical tropolone dimer indicate that self-association enhances π-delocalization in the troponoid ring and causes vibrational frequency shifts which satisfactorily agree with the changes observed in the FT-IR matrix spectra of isolated tropolone molecules and of solid samples.

Coordination structures and vibrational frequencies of Li and Na metaphosphates and nitrates. An ab initio SCF study

Abstract Ab initio quantum-mechanical calculations, at the HF SCF level of theory, were performed on the planar C 2v and pyramidal C 3v coordination structures of LiPO 3 and NaPO 3 . Preference for bidentate binding is indicated for alkali-metal metaphosphates. Harmonic vibrational frequencies were computed for the free PO 3 − and all the C 2v and C 3v forms of LiPO 3 and NaPO 3 . The 18 isotopic patterns predicted for bidentate and monodentate C 2v structures are discussed. SCF MO calculations are also reported for LiNO 3 and NaNO 3 molecules in order to extend previous ab initio studies. The theoretical results were compared with gas phase electron diffraction and IR spectroscopy data.

Study of the hydrogen-bonded (NH2CONH2)(H2O)2 and (NH2CONH2)(HF)2 complexes and of the interaction of H2O with metal cations and anions

Abstract This paper reports the results of an ab initio study on the C2 and C2v symmetry hydrogen-bonded complexes (NH2CONH2)(HaO)2 and (NH2CONH2)(HF)2 and on the C2 symmetry complex of ethyleneurea with two H2O molecules. The relative stability of the C2 and C2v symmetry structures of the (NH2CONH2)(H2O)2 and (NH2CONH2)(HF)2 complexes was determined from single-point energy calculations performed at the HF/4-31G(d,p), HF/6-31+G(d,p), HF/DZ(d,p) and MP2(fc)/4-31G(d,p)//HF/4-31G(d) levels. The effect of hydrogen bonding on the vibrational frequencies of urea and ethyleneurea is studied on the basis of the calculated frequency shifts of the stretching modes of the molecules. Results of calculations on the C2 symmetry complexes (NH2CONH2)(H2O)2 and (NH2CONH2)(H2O)6 have also been included in the work. The last part of the paper reports the study of the hydration structures of Li+, Na+ , Be2+ , Mg2+, Ca+ and Al3+ metal cations and those of the F− and Cl− anions.

Geometries and vibrational frequencies of oxyacids and carboxylic acids. a study on structural and vibrational effects

Abstract The equilibrium geometries of the acids H 3 BO 3 , H 2 CO 3 , HNO 2 , HNO 3 , HPO 3 , HClO 4 , H 2 SO 4 , CH 3 SO 3 H, CF 3 SO 3 H, FSO 3 H, ClSO 3 H, H 2 SeO 4 , H 3 PO 4 . HCO 2 H, HCOOOH and C 6 H 5 CO 2 H, and of the anions H 2 BO − 3 , HBO - 3 , BO 3− 3 , HCO 3 − , HSO 4 − , CH 3 SO 3 − , CF 3 SO 3 − , FSO 3 − , CISO 3 − , HSeO 4 − , HPO 4 2− , H 2 PO 4 − , PO 4 3− , HCO 2 − , HCOOO − and C 6 H 5 CO 2 − were determined at the self-consistent field (SCF) level, employing polarized split-valence basis sets. The MP2/6-31G ∗∗ level was adopted to optimize the geometries of H 3 BO 3 , HBO 2 , H 2 CO 3 , HNO 3 , HPO 3 , HNO 2 , HCO 2 H and HCOOOH; those of the anions H 2 BO 3 − , HBO 3 2− , HCO 3 − , HCO 2 − and HCOOO − were studied at the MP2/6-31G ∗∗ level. The last section of the study reports the results of SCF calculations on the hydrogen-bonded complexes (HCO 2 − ) (H 2 O), (CO 3 2− ) (H 2 O) and (HCO 3 − ) (H 2 O).