Gas‐Phase Infrared Spectroscopy of Substituted Cyanobutadiynes: Roles of the Bromine Atom and Methyl Group as Substituents

The IR spectra of 5‐bromo‐2,4‐pentadiynenitrile (Br?C≡C?C≡C?CN) and 2,4‐hexadiynenitrile (CH₃?C≡C?C≡C?CN), a compound of interstellar interest, have been recorded within the 4000–500 cm^?1 spectral region and calculated by means of high‐level ab initio and density functional calculations. Although the calculated structures of both compounds are rather similar, there are very subtle differences, mainly in the strength of the C≡C bond not directly bound to the substituent. These subtle bonding differences are reflected in small, but not negligible, differences in the electron density at the corresponding bond critical points, and, more importantly, are reflected in the IR spectra. Indeed, the IR spectrum for the bromine derivative presents two well‐differentiated strong bands around 2250 cm^?1, whereas for the methyl derivative both absorptions coalesce in a single band. These bands correspond in both cases to the coupling between C≡C and C≡N stretching displacements. A third, very weak, band also associated with C≡C and C≡N coupled stretches is observed for the bromine derivative, but not for the methyl one, owing to its extremely low intensity.     

Infrared and Raman Spectroscopy of Methylcyanodiacetylene (CH3C5N)

A spectroscopic study combining IR absorption and Raman scattering is presented for methylcyanodiacetylene (CH₃C₅N). Gas‐phase, cryogenic matrix‐isolated, and pure solid‐phase substance was analyzed. Out of 16 normal vibrational modes, 14 were directly observed. The analysis of the spectra was assisted by quantum chemical calculations of vibrational frequencies, IR absorption intensities, and Raman scattering activities at density functional theory and ab initio levels. Previous assignments of gas‐phase IR absorption bands were revisited and extended.     

Nuclear quantum effects on the thermal expansion coefficient of hexagonal boron nitride monolayer

This work examines the importance of vibrational delocalization on a basicthermomechanical property of a hexagonal boron nitride monolayer, namely its thermalexpansion coefficient (TEC). Using a recently parametrized bond-order potential of theTersoff type, the TEC was theoretically obtained from the thermal variations of thelattice parameter a(T) calculated using threedifferent methods: (i) the quasiharmonic approximation; (ii) its anharmonic improvementbased on self-consistent phonons; (iii) fully anharmonic Monte Carlo simulations possiblyenhanced within the path-integral framework to account for nuclear quantum effects. Theresults obtained with the three methods are generally consistent with one another and withother recently published data, and indicate that the TEC is negative at least up to ca.700 K, quantum mechanical effects leading to a significant expansion by about 50% relativeto the classical result. Comparison with experimental data on bulk hexagonal BN suggestssignificant differences, which originate from possible inaccuracies in the model that tendto underestimate the lattice parameter itself, and most likely from the 2D nature of themonolayer and the key contribution of out-of-plane modes. The effects of isotopic purityin the natural abundances of boron are found to be insignificant.     

Estimate of the vibrational contribution to the entropy change associated with the spin transition in the d4 systems [MnIII(pyrol)3tren] and [CrII(depe)2I2

The vibrational contribution to DeltaS of the low-spin ((3)T(1)) to high-spin ((5)E) spin transition in two 3d(4) octahedral systems [Mn(III)(pyrol)(3)tren] and [Cr(depe)(2)I(2)] have been estimated by means of DFT calculations (B3LYP/CEP-31G) of the vibrational normal-modes frequencies. The obtained value at the transition temperature for the Mn(iii) complex is DeltaS(vib)(44 K) = 6.3 J K(-1) mol(-1), which is comparable with the proposed Jahn-Teller contribution of R ln3 = 9.1 J K(-1) mol(-1) and which is approximately half of the experimentally determined 13.8 J K(-1) mol(-1). The corresponding value for the Cr(ii) complex is DeltaS(vib)(171.45 K) = 46.5 J K(-1) mol(-1), as compared to the experimental value of 39.45 J K(-1) mol(-1). The analysis of the vibrational normal modes reveals that for the d(4) systems under study, contrary to Fe(ii) d(6) systems, not all metal-ligand stretching vibrations make a contribution. For the Mn(iii) complex, the only vibration that contributes to DeltaS(vib) involve the nitrogens occupying the Jahn-Teller axis, while in the case of Cr(ii) the contributing vibrations involve the Cr-I bonds. Low-frequency modes due to ring vibrations, metal-ligand bending and movement of the molecule as a whole also contribute to the vibrational entropy associated with the spin transition.     

Dynamics and supramolecular organization of the 1D spin transition polymeric chain compound [Fe(NH2trz)3](NO3)2. Muon spin relaxation

The thermal spin transition that occurs in the polymeric chain compound [Fe(NH(2)trz)3](NO3)2 above room temperature has been investigated by zero-field muon spin relaxation (microSR) over the temperature range approximately 8-402 K. The depolarization curves are best described by a Lorentzian and a Gaussian line that represent fast and slow components, respectively. The spin transition is associated with a hysteresis loop of width DeltaT = 34 K (T1/2 upward arrow = 346 K and T1/2 downward arrow = 312 K) that has been delineated by the temperature variation of the initial asymmetry parameter, in good agreement with previously published magnetic measurements. Zero-field and applied field (20-2000 Oe) microSR measurements show the presence of diamagnetic muon species and paramagnetic muonium radical species (A = 753 +/- 77 MHz) over the entire temperature range. Fast dynamics have been revealed in the high-spin state of [Fe(NH(2)trz)3](NO3)2 with the presence of a Gaussian relaxation mode that is mostly due to the dipolar interaction with static nuclear moments. This situation, where the muonium radicals are totally decoupled and not able to sense paramagnetic fluctuations, implies that the high-spin dynamics fall outside the muon time scale. Insights to the origin of the cooperative effects associated with the spin transition of [Fe(NH(2)trz)3](NO3)2 through muon implantation are presented.     

Protein nanopores with covalently attached molecular adapters

Molecular adapters are crucial for the stochastic sensing of organic analytes with alpha-hemolysin (alphaHL) protein nanopores when direct interactions between analytes and the pore cannot readily be arranged by conventional protein engineering. In our earlier studies, cyclodextrin adapters were lodged noncovalently within the lumen of the alphaHL pore. In the present work, we have realized the controlled covalent attachment of a beta-cyclodextrin (betaCD) adapter in the two possible molecular orientations inside alphaHL pores prepared by genetic engineering. There are two advantages to such a covalent system. First, the adapter cannot dissociate, which means there are no gaps during stochastic detection, a crucial advance for single-molecule exonuclease DNA sequencing where the continuous presence of a molecular adapter will be essential for reading individual nucleotides. Second, the ability to orient the adapter allows analytes to bind through only one of the two entrances to the betaCD cavity. We demonstrate that the covalently attached adapters can be used to alter the ion selectivity of the alphaHL pore, examine binding events at elevated temperatures, and detect analytes with prolonged dwell times.     

Selective thin-layer chromatography of 4-R-1,2,4-triazoles

A convenient and selective TLC of 4-R-1,2,4-triazoles (Rtrz) and nitrogen containing aromatic rings is presented (molecules 1-11). This simple revelation method implies a complexation of heterocycles to Co(II) ions on a TLC plate, followed by a subsequent modification of the reduction potential which enables oxidation of Co(II) by permanganate.     

Reactivity study of 1H-thieno[3,2-d][1,3]oxazine-2,4-dione toward the synthesis of bicyclic 3,4-dihydro-1H-thieno[3,2-e][1,4]diazepine-2,5-dione analogues

A series of 10 optically pure 3,4-dihydro-1H-thieno[3,2-e][1,4]diazepine-2,5-dione derivatives has been synthesized in 41-75% yields on treatment of 1H-thieno[3,2-d][1,3]oxazine-2,4-dione with different natural alpha-amino acids.     

Phosphinoborane and Sulfidoborohydride as Chelating Ligands in Polyhydride Ruthenium Complexes: Agostic σ‐Borane versus Dihydroborate Coordination

A question of coordination mode : Two new borane compounds are prepared. They act as bifunctional ligands as illustrated by their reaction with ruthenium polyhydrides which leads to the formation of two complexes (see scheme) displaying either a δ‐agostic interaction of a η²‐B? H bond involving a trivalent boron atom or a dihydroborate ligation.