Infrared-induced rotational isomerization of 1,2-ethanediamine in argon matrices and conformational analysis by DFT calculation

Conformational analysis, vibrational and NMR spectroscopic study of the methanesulfonamide-N,N'-1,2-ethanediylbis

A conformational analysis of the methanesulfonamide-N,N'-1,2-ethanediylbis (msen) was performed by using vibrational and NMR spectroscopies as well as theoretical computations. The possible stable conformers of msen on its potential energy surface were investigated by semi-empirical PM5 method and appropriate structures were defined with B3LYP hybrid density functional theory (DFT) method along with the basis sets of different size and type. Six different rotational isomers were found as the result of DFT calculation. The two isomer, called trans-trans-gauche(+)-eclipsed, synperiplanar (ttg(+)-e,bis) and trans-gauche(+)-gauche(-)-staggered, antiplanar (tg(+)g(-)-s,anti), were considered in the vibrational spectral analysis. The infrared (4000-30 cm(-1)) and Raman (4000-60 cm(-1)) spectra of msen were measured in solid state. For a complete assignment of the vibrational spectra, DFT calculations at B3LYP/6311-G(d,p) theory level combined with scaled quantum mechanics force field (SQMFF) methodology was performed. Furthermore, (13)C and (1)H NMR analyses were performed for six conformers at B3LYP/6-311++G(d,p) level of theory and compared with the experimental findings. Results from experimental and theoretical data showed the ttg(+)-e,bis to be the most stable form of a msen molecule.

Rotational isomerization of small carboxylic acids isolated in argon matrices: tunnelling and quantum yields for the photoinduced processes

The quantum yields for internal rotation around the C-O bond induced by excitation of the first overtone of the hydroxyl stretching mode in formic, acetic, and propionic acids isolated in solid Ar are comparatively discussed. The tunnelling kinetics for isomerization from the higher energy arrangement of the carboxylic group (cis) to the lower energy arrangement (trans) in this series of compounds is also analysed. Finally, the quantum yield for the C(alpha)-C isomerization in propionic acid was investigated and, in contrast with the C-O isomerization, shown to be probably sensitive to the local matrix morphology.

Dynamics of Intramolecular Hydrogen-Atom Migrations in 2-Hydroxy-3-nitropyridine Studied by Low-Temperature Matrix-Isolation Infrared Spectroscopy and Density Functional Theory Calculation

Intramolecular hydrogen-atom migrations in 2-hydroxy-3-nitropyridine have been investigated by low-temperature matrix-isolation infrared (IR) spectroscopy with the aid of density functional theory (DFT) calculation. An IR spectrum measured after deposition was assigned to an enol isomer, the conformation of which is anti in relation to OH versus N in the pyridine ring. When the matrix sample was exposed to UV and visible light (lambda>350 nm), an IR spectrum consistent with a keto product was observed. During the irradiation, an IR spectrum of a transient species, a photoreaction intermediate between anti-enol and keto, was observed, which was assigned to syn-enol. The bands of syn-enol disappeared completely when the irradiation was stopped, while those of the original isomer, anti-enol, reappeared. No reverse isomerization was observable in the corresponding deuterated species. This led to the conclusion that the isomerization from syn to anti occurs through hydrogen-atom tunneling. On the other hand, an aci-nitro form was produced by UV irradiation (lambda=365+/-10 nm) without visible light. The conformation around the aci-nitro group was determined to be cis-cis by comparison with the spectral patterns obtained by the DFT/B3LYP/6-31++G** calculation. The dynamics of the hydrogen-atom migrations between anti- and syn-enols, syn-enol and keto, and anti-enol and aci-nitro are discussed in terms of the potential surfaces obtained by the DFT calculation.

pKa Calculations of Aliphatic Amines, Diamines, and Aminoamides via Density Functional Theory with a Poisson−Boltzmann Continuum Solvent Model

In order to make reliable predictions of the acid-base properties of macroligands with a large number of ionizable sites such as dendrimers, one needs to develop and validate computational methods that accurately estimate the acidity constants (pKa) of their chemical building blocks. In this article, we couple density functional theory (B3LYP) with a Poisson-Boltzmann continuum solvent model to calculate the aqueous pKa of aliphatic amines, diamines, and aminoamides, which are building blocks for several classes of dendrimers. No empirical correction terms were employed in the calculations except for the free energy of solvation of the proton (H+) adjusted to give the best match with experimental data. The use of solution-phase optimized geometries gives calculated pKa values in excellent agreement with experimental measurements. The mean absolute error is <0.5 pKa unit in all cases. Conversely, calculations for diamines and aminoamides based on gas-phase geometries lead to a mean absolute error >0.5 pKa unit compared to experimental measurements. We find that geometry optimization in solution is essential for making accurate pKa predictions for systems possessing intramolecular hydrogen bonds.

Vacuum Ultraviolet Photoionization Mass Spectrometric Study of Ethylenediamine

The photoionization and dissociative photoionizations of ethylenediamine have been studied both experimentally and theoretically. In experiments, photoionization efficiency spectra for ions NH(2)CHCH(3)(+), NH(2)CH=CH(2)(+), CH(2)NH(2)(+), NH(3)(+), NH(2)CH(2)CHNH(2)(+) and NH(2)CH(2)CH(2)NH(2)(+) have been obtained. In addition, the energetics of the dissociative photoionization is investigated with ab initio Gaussian-3 (G3) calculations. The computational results are useful in analyzing the dissociation channels near the ionization thresholds. With the help of the G3 results, the dissociation channels for the formation of the aforementioned fragment ions have been established.

Spectroscopy and Structures of Copper Complexes with Ethylenediamine and Methyl-Substituted Derivatives

Copper complexes of ethylenediamine (en), N-methylethylenediamine (meen), N,N-dimethylethylenediamine (dmen), N,N,N'-trimethylethylenediamine (tren), and N,N,N',N'-tetramethylethylenediamine (tmen) are synthesized in laser-vaporization supersonic molecular beams and studied by pulsed-field ionization zero electron kinetic energy (ZEKE) and photoionization efficiency spectroscopies and second-order Moller-Plesset perturbation theory. Precise ionization energies and vibrational frequencies of Cu-en, -meen, and -dmen are measured from the ZEKE spectra, and ionization thresholds of Cu-tren and -tmen are estimated from the photoionization efficiency spectra. The measured vibrational modes span a frequency range of 35-1646 cm(-1) and include metal-ligand stretch and bend, hydrogen-bond stretch, and ligand-based torsion. A number of low-energy structures with Cu binding to one or two nitrogen atoms are predicted for each complex by the ab initio calculations. The combination of the spectroscopic measurements and ab initio calculations has identified a hydrogen-bond-stabilized monodentate structure for the Cu-en complex and bidentate cyclic structures for the methyl-substituted derivatives. The change of the Cu binding from the monodentate to the bidentate mode arises from the competition between copper coordination and hydrogen bonding.

Rotational isomerism of acetic acid isolated in rare-gas matrices: Effect of medium and isotopic substitution on IR-induced isomerization quantum yield and cis→trans tunneling rate

Rotational isomerization of acetic acid (CH3COOH) is studied in Ar, Kr, and Xe matrices. The light-induced trans-->cis reaction is promoted using resonant excitation of a number of modes in the 3500-7000 cm(-1) region, and the quantum yields for this process are measured for various acetic acid isotopologues and matrix materials. For excitation of acetic acid at energies above the predicted isomerization energy barrier (> or =4400 cm(-1)), the measured quantum yields are in average 2%-3%, and this is one order of magnitude smaller than the corresponding values known for formic acid (HCOOH). This difference is interpreted in terms of the presence of the methyl group in acetic acid, which enhances energy relaxation channels competing with the rotational isomerization. This picture is supported by the observed large effect of deuteration of the methyl group on the photoisomerization quantum yield. The trans-->cis reaction quantum yields are found to be similar for Ar, Kr, and Xe matrices, suggesting similar energy relaxation processes for this molecule in the various matrices. The IR-induced cis-->trans process, studied for acetic acid deuterated in the hydroxyl group, shows reliably larger quantum yields as compared with the trans-->cis process. For pumping of acetic acid at energies below the predicted isomerization barrier, the trans-->cis reaction quantum yields decrease strongly when the photon energy decreases, and tunneling is the most probable mechanism for this process. For the cis-->trans dark reaction, the observed temperature and medium effects indicate the participation of the lattice phonons in the tunneling-induced process.

Photoelectron spectroscopy and density functional theory of puckered ring structures of Group 13 metal-ethylenediamine

The ethylenediamine (en) complexes of Al, Ga, and In atoms were prepared in laser-vaporization supersonic molecular beams and studied with pulsed field ionization zero electron kinetic energy photoelectron spectroscopy and density functional theory. Several conformers of each metal complex are obtained by B3LYP calculations, and a five-membered cyclic structure is identified by combining the experimental measurements and theoretical calculations. Adiabatic ionization potentials, vibrational frequencies, and bond dissociation energies are determined for the ring structure. The ionization potentials of the Al, Ga, and In species are measured to be 32 784 (5), 33 324 (5), and 33 637 (7) cm(-1), respectively, and metal-ligand dissociation energies of the ionic and neutral complexes are calculated to be 60.2/16.2 (Al(+)/Al), 55.5/13.0 (Ga(+)/Ga), and 50.0/11.4 (In(+)/In) kcal mol(-1). Metal-ligand stretch and bend as well as a number of ligand-based vibrations are measured. Harmonic frequencies and anharmonicities of the M(+)-N (M=Al,Ga,In) stretch are determined for all three M(+)-en ions and the C-C-N bend of Ga(+)-en and In(+)-en. In comparison to monodentate methylamine, the bidentate binding of ethylenediamine leads to a significantly lower ionization potential and higher metal-ligand bond strength of the metal complexes.