Adolfo C. Fantoni

Rotational spectrum and ab initio calculations of N-methylformamide

The microwave spectrum of N-methylformamide has been investigated in the 18–40 GHz frequency range. The rotational spectrum of the conformer with the methyl group cis to the aldehydic oxygen has been assigned. Several rotational transitions of the A sublevel of the ground torsional state have been measured for both normal and N—D isotopic species. The V3 barrier to the methyl group internal rotation has been obtained from the effective pseudo-defect of inertia. The dipole moment and the 14N quadrupole coupling constants have been determined. The results of ab initio calculations performed at the HF and MP2 levels are in agreement with the experimental data

Intra- and intermolecular hydrogen bonding and conformation in 1-acyl thioureas: An experimental and theoretical approach on 1-(2-chlorobenzoyl)thiourea

The vibrational analysis (FT-IR and FT-Raman) for the new 1-(2-chlorobenzoyl)thiourea species suggests that strong intramolecular interactions affect the conformational properties. The X-ray structure determination corroborates that an intramolecular N-H⋯OC hydrogen bond occurs between the carbonyl (-CO) and thioamide (-NH2) groups. Moreover, periodic system electron density and topological analysis have been applied to characterize the intermolecular interactions in the crystal. Extended N-H⋯SC hydrogen-bonding networks between both the thioamide (N-H) and carbamide (NH2) groups and the thiocarbonyl bond (CS) determine the crystal packing. The Natural Bond Orbital (NBO) population analysis demonstrates that strong hyperconjugative remote interactions are responsible for both, intra and intermolecular interactions. The Atom in Molecule (AIM) results also show that the N-H⋯Cl intramolecular hydrogen bond between the 2-Cl-phenyl ring and the amide group characterized in the free molecule changes to an N⋯Cl interaction as a consequence of crystal packing.

On the roles of close shell interactions in the structure of acyl-substituted hydrazones: An experimental and theoretical approach

The 2-(phenyl-hydrazono)-succinic acid dimethyl ester compound was synthesized by reacting phenylhydrazine with dimethylacetylene dicarboxylate at room temperature and characterized by elemental analysis, infrared, Raman, (1)H and (13)C NMR spectroscopies and mass spectrometry. Its solid state structure was determined by X-ray diffraction methods. The X-ray structure determination corroborates that the molecule is present in the crystal as the hydrazone tautomer, probably favored by a strong intramolecular N-H···O=C hydrogen bond occurring between the carbonyl (-C=O) and the hydrazone -C=N-NH- groups. A substantial fragment of the molecular skeleton is planar due to an extended π-bonding delocalization. The topological analysis of the electron densities (Atom in Molecule, AIM) allows characterization of intramolecular N-H···O interaction, that can be classified as a resonant assisted hydrogen bond (RAHB). Moreover, the Natural Bond Orbital population analysis confirms that a strong hyperconjugative lpO1→σ*(N2-H) remote interaction between the C2=O1 and N2-H groups takes place. Periodic system electron density and topological analysis have been applied to characterize the intermolecular interactions in the crystal. Weak intermolecular interactions determine the crystal packing, and the prevalence of non-directional dispersive contributions are inferred on topological grounds. The IR spectrum of the crystalline compound was investigated by means of density functional theory calculations carried out with periodic boundary conditions on the crystal, showing excellent agreement between theory and the experiments. The vibrational assignment is complemented with the analysis of the Raman spectrum.

Microwave spectrum and conformational properties of methylfluorocarbonyl disulphide, FC(O)SSCH3

The microwave spectra of methylfluorocarbonyl disulfide (FC(O)SSCH3) in the ground state and four excited vibrational states are reported. The assigned transitions are consistent with a conformation having the SS bond in syn position with respect to the CO bond. Evidence about the stabilization effect of this double bond has been obtained from a partial structural refinement. A lower limit for the barrier to the methyl group internal rotation has been determined.

High resolution microwave spectrum and torsional potential energy surfaces of trans-crotyl alcohol

The high resolution microwave spectrum of trans crotyl alcohol, CH3-CH=CH-CH2-OH, is reported for the normal and -OD isotopic species. The assigned transitions are consistent with a hydrogen bonded conformation having the oxygen atom skew with respect to the C=C bond and the hydroxylic hydrogen gauche with respect to the C-C bond. The analysis of the A-E doublings in the ground and some torsionally excited states allowed the potential energy surfaces of the methyl and skeletal torsions to be inferred and the kinetic and potential energy interaction between the two motions, through the use of a two-dimensional flexible model, to the determined. The electric dipole moment has also been determined.

Electronic properties and dissociative photoionization of thiocyanates. Part II. Valence and shallow-core (sulfur and chlorine 2p) regions of chloromethyl thiocyanate, CH2ClSCN.

A combination of photoelectron spectroscopy and synchrotron based photoelectron photoion coincidence (PEPICO) spectra has been applied to investigate the electronic structure and the dissociative ionization of the CH(2)ClSCN molecule in the valence region. The PES is assigned with the electronic structure calculations at the outer-valence Greens function and symmetry adapted cluster/configuration interaction (SAC-CI) levels offer an explanation of our experimental results. Upon vacuum ultraviolet irradiation the low-lying radical cation, located at 10.39 eV is formed. The molecular ion is observed in the time-of-flight mass spectra, together with the CH(2)SCN(+) and CH(2)Cl(+) daughter ions. The total ion yield spectra have been measured in the S 2p and Cl 2p regions and several channels have been determined in dissociative photoionization events for the core-excited species. Thus, by using time-of-flight mass spectrometry and synchrotron radiation the relative abundances of the ionic fragments and their kinetic energy release values were obtained from both PEPICO and photoelectron photoion photoion coincidence spectra. Possible fragmentation processes are discussed and compared with that found for the related CH(3)SCN species.

Methyl and skeletal torsion interaction in normal propyl fluoride

The analysis of the microwave spectra of some excited states of the skeletal torsion in both antiperiplanar and synclinal forms of normal propyl fluoride has been extended in order to obtain the splittings (Δ0=EE -EA ) due to the methyl group internal rotation. The results have been used, together with the experimental data already available, to obtain information on the structural relaxation upon torsional excitation and conformation exchange, and to evaluate a simple set of potential energy parameters for the two torsions by applying one- and two-dimensional flexible models.

Microwave investigation and the ring puckering potential function of 3-methylthietan

Abstract The microwave spectrum of 3-methylthietan was investigated in the frequency range 26.5–40 GHz. The rotational spectra were measured for the ground state, five puckering states (including the lowest state of the second conformer), and two methyl torsion excited states. The conformer with the methyl group in the equatorial orientation (the ground state, v p = 0) is more stable than the axial counterpart ( v p = 1) by 100(17) cm −1 (1.2(2) kJ/mol). The electric dipole moment was determined for both conformers by measuring the Stark effect on several transitions. A V 3 barrier of 1470(30) cm −1 (17.6(4) kJ/mol) was obtained for the internal rotation of the axial methyl group from the A - E doubling of several rotational transitions in the first torsional excited state. Earlier far-infrared assignments were revised on the basis of microwave intensity measurements. The flexible model approach was used to reproduce the spacings between puckering levels simultaneously with the variation of the rotational constants with puckering excitation, allowing some structural relaxation accompanying the ring deformation. Comparisons are made with the results obtained when an asymmetric one-dimensional puckering Hamiltonian is used.

On the Electron Density Topology and Electrostatic Properties of Nitroanilines. A Theoretical Investigation on m-Nitroaniline and 2-Methyl-5-nitroaniline Crystals

On the basis of the AIM theory, intermolecular interactions have been characterized in the B3LYP/6-31G** periodic electron density of the title compounds. Although the set of bond paths identified in each system is not fully equivalent to its experimental counterpart, agreement is reasonable with regard to the nature and relative importance of the intermolecular interactions at play. Within the AIM partition scheme, the molecular dipole moment of the in-crystal molecule was determined for the title compounds and the two closely related crystals of 2-methyl-4-nitroaniline and p-nitroaniline. Using a method that relies only on molecular calculations and a mean electric field approximation, it was possible to reproduce within 6% the values of the molecular dipole moment modulus obtained directly from the periodic electron densities. This result reveals that, for this kind of molecular crystal, enhancement of the dipole moment in going from the isolated molecule to the in-crystal one is an almost exclusively inductive effect.

Conformation, hydrogen bonding and large amplitude motion investigation on N-methylethylediamine by microwave spectroscopy

Abstract The microwave spectrum of N -methylethylenediamine and several deuterated species has been investigated in the frequency range 26.5–40 GHz. The rotational spectra of two different conformers with a NH ⋯ N internal hydrogen bond have been assigned. Both conformers have the methyl group trans to the CC bond. The N atom connected to the methyl group acts as proton donor for a conformer ( T 1), and as acceptor for the second one ( T2g ). The former is more stable in energy by 0.65(15) kcal mol −1 . Rotational lines of several vibrational satellites have been assigned in order to investigate their large amplitude motions and interactions.