A PROGRAM FOR INTERCONVERTING CONCENTRATION UNITS IN BINARY MIXTURES

A Pascal program (CUC, Concentration Units Converter) is described. Given the concentration of the solute in a binary mixture in any of the following units (mole fraction, molar ratio, percent by weight, molality or molarity) it calculates the concentration expressed in the remaining scales

Ab initio calculation of NMR properties (shielding and electric field gradient) of S-33 in sulfur compounds

Chemical shieldings of the sulfur nucleus have been calculated with the GIAO method for a wide range of organic and inorganic sulfur compounds, using the 6-311++G(2d,2p) basis set. The resulting data, including calculated electric field gradients, are compared with other theoretical results and experimental S-33 NMR chemical shifts and line widths; with a few accountable exceptions (mostly charged species), calculated shieldings are well correlated with experimental data. The resulting relationship, joined to calculated electric field gradients at the nuclei, can be employed for predicting the chemical shift and line width of hitherto undetected signals. There is no general relationship between S-33 Shielding and electronic charge at sulfur

Acyl group vs nitrogen protonation of carboxylic and non-carboxylic amides in the gas phase and water

The site of protonation of carboxylic and non-carboxylic amides (whether the amido nitrogen or an atom in the acyl group, generally oxygen) was investigated through quantum chemical calculations and heteronuclear NMR measurements. The relative energies of the various ions deriving from protonation at each site were calculated both in the gas phase and in water, and NMR properties of the involved heteronuclei (nuclear shielding and electric field gradient) were also calculated and compared with chemical shins and relaxation rates experimentally measured in N-14, O-17 and P-31 spectra. It is shown that such a combination of theoretical and experimental tools allows the reliable prediction of spectral parameters and ultimately of the protonation site. In general, amides are protonated at the acyl group, with the exception of (a) when the parent acid is strong (for which the preference is not marked), (b) the protonation site of sulfinamides may easily shift from N to O as a result of slight structural changes and (c) sulfenamides behave as substituted amines and are nitrogen bases

Complete prediction of the H-1 NMR spectrum of organic molecules by DFT calculations of chemical shifts and spin-spin coupling constants

H-1 NMR chemical shifts and coupling constants for several aromatic and aliphatic organic molecules have been calculated with DFT methods. In some test cases (furan, o-dichlorobenzene and n-butyl chloride) the performance of several functionals and basis sets has been analyzed, and the various contributions to spin-spin coupling (Fermi-contact, diamagnetic and paramagnetic spin-orbit) have been evaluated. The latter two components cancel each other, so that the calculation of the contact term only is sufficient for an accurate evaluation of proton-proton couplings. Such calculated values are used to simulate the H-1 NMR spectra of organic molecules with complicated spin systems (e.g. naphthalene, o-bromochlorobenzene), obtaining a generally very good agreement with experimental spectra with no prior knowledge of the involved parameters

Computing the NMR spectra of the sponge metabolite arsenicin A: when simple becomes difficult

Determining the structure of the natural polyarsenical arsenicin A has proved difficult, but computational chemistry has played a decisive role in the process. This paper aims at tracing the winding path that, in time, led to the correct structure

The ab initio neon-water potential-energy surface and its relationship with the hydrophobic hydration shell

The neon-water system has been investigated by ab initio quantum-chemical calculations. The potential-energy surface (PES) far the 1:1 Ne-H2O interaction has been scanned at the MP2(full) level, with the 6-311 + + G(3df,2pd) basis set augmented with bond functions located midway between Ne and O. The most stable neon-water arrangement has r = 320 pm, theta = 120 degrees, with a counterpoise-corrected binding energy of -0.54 kJ mol(-1). The PES is otherwise quite flat (with binding energies of ca. -0.4 kJ mol(-1)) for all a values if r > ca. 310 pm. The off-plane, perpendicular approach is unfavourable. NMR shielding and the electric field gradient at all nuclei show only very small changes with respect to the isolated components

Counterion effects on the 183W NMR spectra of the lacunary Keggin polyoxotungstate [PW11O39]7-. Relativistic DFT calculations

The 183W NMR spectra of the lacunary Keggin polyoxotungstate [PW11O39]7\u2013 feature noticeable differences according to the counterion (Li or Na). Such differences are modeled by ion pairs where the lacuna is occupied by a Li(H2O)+ or Na(H2O)+ group through relativistic DFT calculation of the tungsten chemical shifts

Carbon and nitrogen basicity of aminothiophenes and anilines

The protonation site, aromaticity, charge distribution, and NMR properties of 3-aminothiophene, 3,4-diaminothiophene, aniline, and 1,2-benzenediamine have been investigated by means of quantum chemical calculations both for the isolated and solvated species tin water and DMSO). For the isolated species (G3-(MP2) level), the C-protonated form of aminothiophenes is more stable than the N-protonated form (by 5-9 kcal/mol), whereas the stability order of the protonated forms of anilines is reversed, with a closer energy balance (2-5 kcal/mol). In water or DMSO the stability of the C- and N-protonated forms of aminothiophenes is essentially the same las obtained by a combination of G3(MP2) and DFT-IPCM solution data), whereas for anilines a strong preference for N-protonation is borne out. However, a comparison of experimental and calculated C-13 NMR chemical shifts shows N-protonation to be the major process in solution. While the aromaticity of the two ring types (as probed by nucleus-independent chemical shifts) is very similar, the larger nucleophilicity of the C-2 atom of aminothiophenes as compared to anilines is shown to arise from a strong polarization of the C-2-C-3 bond

DFT study of the NMR properties of xenon in covalent compounds and van der Waals complexes-implications for the use of Xe-129 as a molecular probe

The NMR properties (chemical shift and spin-spin coupling constants) of Xe-129 in covalent compounds and weakly bound complexes have been investigated by DFT methods including relativistic effects. For covalent species, a good agreement between experimental and calculated results is achieved without scalar relativistic effects, but their inclusion (with a triple-zeta, doublepolarization basis set) leads to some improvement in the quality of the correlation. The spin-orbit coupling term has a significant effect on the shielding constant, but makes a small contribution to the chemical shift. Coupling constants contain substantial contributions from the Fermi contact and paramagnetic spin-orbit terms; unlike light nuclei the spin-dipole term is also large, whereas the diamagnetic spin-orbit term is negligible. For van der Waals dimers, the dependence of the xenon chemical shift and anisotropy is calculated as a function of the distance. Small (< 1 Hz) but non-negligible throughspace coupling constants between Xe-129 and C-13 or H-1 are predicted. Much larger couplings, of the order of few Hz, are calculated between xenon and O-17 in a model silicate residue