Giuseppe Buemi

Is the intramolecular hydrogen bond energy valuable from internal rotation barriers

Abstract A method for evaluating the energy of intramolecular hydrogen bridges utilizing the rotation barriers of the donor and/or of the acceptor groups is proposed. The procedure has been tested successfully on various molecules containing O–H⋯O, O–H⋯Halogen, O–H⋯N, N–H⋯O, N–H⋯N and O–H⋯S bridges. Comparison between the obtained results and those coming from the classic method (ΔE between the open and chelate conformations) shows good agreement and reliability. Failure has been noted only for the S–H⋯O and S–H⋯S hydrogen bridges.

Molecular conformation of cyclenes. I. Cyclohexene, cycloheptene, cis- and trans-cyclooctene, cis- and trans-cyclononene

Abstract The strain energies of cyclenes with one double bond in the ring, ranging from C6 to C9, have been calculated as a function of various geometric parameters. The conformations of minimum energy are discussed and the energies compared with available spectroscopic and thermodynamic data: a fair agreement is found. The results imply a half-chair form of C2 symmetry for cyclohexene, a chair form of Cs symmetry for cycloheptene, an intermediate asymmetrical form for cis-cyclooctene and cis-cyclononene; in these most stable conformations the major part of the total strain energy is due to the torsional component.

Barbituric and thiobarbituric acids: a conformational and spectroscopic study

A conformational study on Barbituric (BA) and Thiobarbituric (TBA) acids was performed at ab initio MP2/6-31G** level on the neutral, protonated, mono- and di-anionic forms. Acid-base equilibria were studied by comparing the electronic transitions evaluated for the most stable conformations and the experimental spectra at different pH values. The electronic transitions were obtained through the ZINDO approach.

Hydrogen bonding and rotation barriers: A comparison between MNDO and AM1 results

Abstract Intra- and/or inter-molecular hydrogen bonding strengths, as well as rotation barriers, have been investigated for several molecular systems by means of MNDO and AM1 methods in order to test the reliability of the two semiempirical approaches. Comparison of the results show that although AM1 values do not satisfactorily match the experimental findings, they are far better than those of MNDO. In particular, the inability of MNDO to predict rotational barriers in conjugated molecules and hydrogen bonding energies has not been noticed in AM1.

Malondialdehyde and acetylacetone. An AM1 study of their molecular structures and keto–enol tautomerism

The semiempirical AM1 method has been used for a complete study of the molecular structures of all the possible conformations of malondialdehyde (MDA) and acetylacetone (ACAC) in order to obtain their order of stability and evaluated the hydrogen-bond strength, which here is expected to be stronger than in other organic compounds. Although the calculated lengths and bond angles agree fairly well with the experimental and ab initio values, when available, the degree of non-planarity of the diketo isomers is rather different from those reported in the literature. In particular, the most stable isomer of MDA is the W-trans one (the S-cis isomer shown no minimum) and the most stable isomer of ACAC has the CO groups rotated by ca. 93° in opposite directions. The enol tautomer is predicted to be 24.4 kJ mol–1(MDA) and 19.2 kJ mol–1(ACAC) more stable than the diketo tautomer. Although the diketo structures are found to be less polar than the hydrogenbonded ones, the solvation energies, calculated according to the Kirkwood formalism, show that they are the most stabilized on increasing the solvent dielectric constant. The hydrogen-bond strength is evaluated as 37.36 and 32.76 kJ mol–1 in MDA and ACAC, respectively, in good agreement with the value expected on the grounds of i.r. measurements, but the barrier to be overcome in the proton-transfer process between the two equivalent asymmetrical enol forms appears to be strongly overestimated.

Stereoselective synthesis of fused γ-lactams by intramolecular nitrone cycloaddition

Abstract A series of nitrones 5 joined by amides to olefines were prepared in situ from the related aldehydes with N-methylhydroxylamine. The nitrones added intramolecularly to the olefin, and the cycloadditions gave fused γ-lactams 6 stereoselectively. A stereocentre located in position α to the nitronic functionality 13 completely controls the stereochemical course of the intramolecular cycloadditions, which exclusively affords compound 14 with simultaneous introduction of four stereocentres. The formation of this latter compound was also supported by PM3 calculations. Furthermore, simple heating of unsaturated oxime 17 led to compound 19 via intramolecular oxime olefin cycloaddition.

Ab initio study of 2,4-dihalosubstituted malonaldehyde and 2-halo-phenols in gas phase and solution

Abstract All the possible enol and keto conformations of malonaldehyde and its 2,4-difluoro-, 2,4-dichloro-, and 2,4-dibromo-derivatives were studied at the ab initio level, using the 6-31G** basis set. The correlation and Zero Point Vibration energies were evaluated by means of the MP2 approach and the DFT-B3LYP functional. Calculations were performed also in water and CCl 4 solutions, following the SCRF–PCM method. Differently from malonaldehyde, the keto conformations of the halo-derivatives are more stable than the hydrogen-bonded enol ones and very close in energy to each other. The strength of the moderately strong OH⋯O (E-I rotamer) and of the weak OH⋯X (E-III rotamer) bridges, evaluated according to the classic method and from the OH internal rotation barriers, is discussed and compared with that of the corresponding bridges of malonaldehyde and ortho -halo-phenols, respectively. For these latter compounds the B3LYP/6-311++G(d,p) basis too was used.

Conformational energies of cyclenes

Abstract An universal function for non-bonded interactions, which takes into account the relative orientation of the bonds is considered in calculating the conformational energies of cycloalkenes and cycloalkadienes. A comparison is made with previous results obtained by using usual 6-exp functions for non-bonded interactions.

Molecular conformation of cyclenes

Abstract The strain energy of cycloalkadienes with six, seven and eight carbon atoms in the ring has been calculated as a function of various geometric parameters. The minimum-energy forms are discussed and compared with electron diffraction, spectroscopic and molecular polarisability data; good agreement is generally found except for the case of 1,3-cycloheptadiene.

Ab initio study of the potential-energy well of malondialdehyde on varying the O⋯O distance

The evolution of the proton-transfer potential-energy curves of malondialdehyde on decreasing the O⋯O distance until the barrier to Cs→C2v→Cs interconversion disappeared, was studied at the ab initio level (6-31G** basis sets including and excluding correlation energy). The frequencies of the stretching and in-plane-bending modes were also calculated for conformations with different rO⋯O values.The results obtained suggest that the energy cost when two oxygen atoms approach each other is not relevant and the remarkable shifts of the O—H stretching mode can be accounted for mainly by the modification of the potential-energy well accompanying the molecular geometry variations; only a moderate increase in the hydrogen-bridge strength is induced by the decrease in rO⋯O.