Volume 217, Issue 12

Several dinucleotide analogs, in which two of the nucleobases adenine, thymine and uracil are connected by a hexadecamethylene linker have been synthesized and studied for their base-base interaction in a chloroform solution. Using 1 H NMR spectroscopic techniques intramolecular base pair formation through hydrogen bonding is observed at ambient temperatures but found to strongly depend on the identity of paired bases. Thus, whereas cyclization through intramolecular base-base interactions predominate for adenine–(CH 2 ) 16 –thymine and adenine–(CH 2 ) 16 –uracil, no intramolecular adenine-adenine pairing was observed. Upon addition of acetic acid to adenine–(CH 2 ) 16 –thymine, strong adenine-acetic acid interactions result in the disruption of preformed intramolecular adenine-thymine base pairs.

Methoxycarbonyl urea (MCU), a potential long-term nitrogen fertilizer, is studied by 13 C and 15 N dipolar chemical shift NMR spectroscopy and ab initio calculations. Employing a combination of dipolar chemical shift NMR, selective isotope labeling and ab initio gas phase calculations, possible molecular structures and chemical shielding tensors of all 15 N nuclei and of two out of the three 13 C nuclei were revealed. Four possible stable configurations of the molecule with different energies were found in the calculations. The CSA tensors were calculated for these configurations. While the calculated 13 C(urea) CSA tensor orientation of the configuration with the lowest energy is in good agreement with the experimental tensor orientation, there are pronounced differences between calculated and experimental tensor eigenvalues. These differences are a clear indication of the presence of intermolecular hydrogen bonds in the experimental sample, which are neglected in the gas phase calculations. Four different possible orientations of the experimental 13 C(urea) CSA tensor exist, due to symmetry. This ambiguity is solved by comparison with results from GIAO calculations of the 13 C CSA tensor, employing the minimum energy configuration (EEZ). It is found that the orientation, where δ 11 points approximately in direction of N(imide), δ 22 approximately in direction of the C=O bond, and δ 33 is oriented perpendicular to the molecular frame, is adopted in the molecule.

The infrared spectrum of the O–H–O fragment of malonaldehyde is studied using a four-dimensional model. This comprises the OH stretching and the two OH bending vibrations as well an O–O ring deformation mode under the assumption of overall C s symmetry. The full anharmonic potential energy and dipole moment surfaces are calculated using density functional theory and the respective vibrational eigenvalue problem is solved by an iterative Lanczos method. Fundamental and combination transitions are discussed for the normal species and the symmetrically deuterated isotopomer. Special emphasis is paid to the OH/OD stretching region which reveals the signatures of strong mode mixing what renders a simple assignment in terms of fundamental transitions difficult. In addition results for hot transitions are presented which show a rather different OH/OD band due to the topology of the potential energy surface. The influence of H atom tunneling on the spectrum is briefly addressed employing an alternative three-dimensional model which takes into account the in-plane H atom motion as well as the O–O distance.

This review summarizes the spectral and theoretical results concerning different ways of proton transfer through hydrogen bonds (HB) to metal atoms (XH···M) and hydride ligands (XH···HM) leading to classical and nonclassical cationic hydrides. The spectral (NMR, IR, UV-Vis in the temperature range 190–290K) and theoretical studies of the structural and energetic characteristics of HB intermediates and proton transfer allow the representation of the experimental energy profiles. The problems concerning the influence of different factors on the processes and potential energy surfaces requiring active investigations in this new area are discussed

The decomposition of Ru(cod)(cot) ( 1 ; cod = 1,5-cyclooctadiene, cot = 1,3,5-cyclooctatriene) under an H 2 atmosphere (3 bars) in pure heptanol produces crystalline monodisperse nanoparticles of 3nm mean size characterized by TEM and WAXS techniques. Solution NMR studies demonstrate the presence of surface hydrogens due to the synthesis process and suggest a fast exchange between free and coordinated heptanol molecules.

Using ab initio calculations of 1 H, 19 F magnetic shielding tensors of hydrogen difluoride ion as the functions of three coordinates of symmetry, an attempt is made to estimate the contributions of different vibrational isotope effects to H/D NMR isotope shifts referred in the literature. It is shown that the contributions of the amplitudes of proton stretching and bending vibrations dominate whereas the contribution of the totally symmetric vibration can be neglected. Different signs of the H/D isotope effects on hydron and fluorine chemical shifts are caused by very strong angle dependence of the fluorine magnetic shielding. The agreement of the calculated and measured values is nearly quantitative. An unusually strong paramagnetic deshielding of hydrogen for the equilibrium geometry of the [FHF] − ion is noted.

Two-, three-, and four-bond 19 F- 19 F spin-spin coupling constants ( n J FF ) for a set of small fluoro-substituted organic molecules have been computed using ab initio equation-of-motion coupled cluster singles and doubles (EOM-CCSD) theory. The computed values reproduce the experimental signs and magnitudes of n J FF . The straight line that relates the experimental and computed coupling constants has a slope of approximately 1 and passes through the point (0,0 Hz) within the uncertainties of the fit. Hence, EOM-CCSD values of n J FF should be excellent predictors of experimental values when these are not available. All of the components of n J FF except for the diamagnetic spin-orbit term may be large and must be evaluated if agreement between theory and experiment is to be obtained.

Intense few-cycle femtosecond (fs) infrared (IR) laser pulses yield dynamical symmetry breaking of oriented strong symmetric hydrogen bonds A···H···A due to nearly coherent vibrational excitation. Consequently, the system evolves with alternate stretches or compressions of the competing bonds A···H and H···A. For a specific stretch of, say, the H···A bond, an ultrashort ultraviolet (UV) laser pulse may induce a nearly instantaneous electronic excitation and/or photo-detachment by means of a Franck–Condon(FC)-type transition to a dissociative potential energy surface of the excited state. The stretched bond, H····A, will then dissociate selectively, yielding preferably the products AH+A. A minor fraction of the other products A+HA may also be formed due to competing wavepacket dispersion. The corresponding isotope effects are investigated by means of the representative laser driven molecular wavepackets which are propagated on ab initio potential energy surfaces and with ab initio dipole coupling for the model systems, FHF − versus FDF − . The resulting quantum dynamics for both systems are nearly equivalent if the driving IR laser fields are scaled with decreasing carrier frequency and amplitudes and increasing durations for the corresponding increasing masses of the isotopomers.

The nucleation kinetics of n -alkane droplets with carbon numbers of 15 and 17 was observed in an electrodynamic balance. Changes in the elastic light scattering pattern of the single levitated microdroplets indicate the phase transition liquid to solid. Measurements of the nucleation rates gave information on the dynamics of the nucleation process and allow to propose a new mechanism for the nucleation. Large induction times observed for the C 15 H 32 droplets indicate that the nucleation starts after a solid layer has been built up at the surface.

The influence of cation exchange on the 27 Al-NMR spectra of NaA-zeolites has been studied by 27 Al-MAS- and MQ-MAS-Solid State-NMR. From the 27 Al-spectra a characterization of the different Al sites in the A zeolites according to their chemical environment and the structural changes on the aluminosilicate network caused by the cation exchange are obtained. It is found that the exchange with cations with smaller ion-radius cause stronger distortions of the 27 Al-NMR-spectra than exchange with larger cations like Ba 2+ . Employing MQ-MAS spectroscopy these distortions are revealed as second order quadrupolar effects for the smaller cations and as a combination of chemical shift and second order quadrupolar interaction for the Ba cation. These changes of the quadrupolar coupling are interpreted numerically via calculations of the lowering of the symmetry of the EFG tensor. Finally it is found that the exchange with divalent cations leads to distortions of the zeolitic framework and the formation of an extra-framework aluminum. To the best of our knowledge this is for the first time that evidence for the production of extra frame work aluminum by pure cation exchange without any thermal treatment has been found in type A zeolites.

The local structure of the fast solid ion conductor Cu 2 P 3 I 2 = (CuI) 8 P 12 is investigated using one- and two-dimensional 31 P and 65 Cu magic-angle spinning NMR techniques. Peak assignments are proposed on the basis of anisotropic and isotropic spin-spin interactions recovered via radio frequency driven dipolar recoupling (RFDR) and total through bond correlation spectroscopy (TOBSY) methods. Especially the previously proposed R30 6 14 pulse symmetry (R-TOBSY), which exploits scalar spin-spin interactions for coherence transfer, offers outstanding spectral selectivity with respect to 1 J coupling. Using sufficiently short mixing times it is thus possible to correlate selectively those 31 P spins that are directly bonded, thereby providing detailed insights into phosphorus-phosphorus connectivities in the network. The final assignment of the nine resolved 31 P resonances to the twelve crystallographically distinct phosphorus sites suggests a significant relationship between 31 P chemical shifts and the detailed pyramidal P(P 1/3 ) 3 bonding geometry.

A new, convenient method of analyzing the spin polarization of a non-equilibrium system of N coupled nuclei is described and applied to photo-reactions exhibiting chemically induced dynamic nuclear polarization (CIDNP). It is based on the Fourier analysis of the variation of NMR line intensities as a function of the radio frequency excitation pulse length. A relationship between the spectral components at various harmonic order and the alignment in the spin multiplet is established. In application to the Norrish type I photolysis of cyclodecanone we demonstrate that at low magnetic field the rate determining step in the reaction kinetics depends on the mutual orientation of at least four pairs of non-equivalent spins.