1,721,076 research outputs found
A PROGRAM FOR INTERCONVERTING CONCENTRATION UNITS IN BINARY MIXTURES
No full textA 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
Acyl group vs nitrogen protonation of carboxylic and non-carboxylic amides in the gas phase and water
No full textThe 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
Ab initio calculation of NMR properties (shielding and electric field gradient) of S-33 in sulfur compounds
No full textChemical 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
Complete prediction of the H-1 NMR spectrum of organic molecules by DFT calculations of chemical shifts and spin-spin coupling constants
No full textH-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
The ab initio neon-water potential-energy surface and its relationship with the hydrophobic hydration shell
No full textThe 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
No full textThe 183W NMR spectra of the lacunary Keggin polyoxotungstate [PW11O39]7– 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
Computing the NMR spectra of the sponge metabolite arsenicin A: when simple becomes difficult
No full textDetermining 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
Probing the solvation shell of organic molecules by intermolecular 1H NOESY
No full textThe solvation of some neutral and charged organic molecules (phenol, nitroanilines, tetraalkylammonium) in binary solvent mixtures was investigated by means of intermolecular H-1-NOESY NMR spectroscopy. The solvation shell of the solute is, in most cases, selectively enriched in one of the cosolvents (preferential solvation). The origin of preferential solvation is discussed in terms of solute-solvent interactions and microheterogeneity in the solvent mixture
A PULSE SEQUENCE FOR T1 MEASUREMENT BY INVERSION RECOVERY IN THE PRESENCE OF ACOUSTIC RINGING
No full textA pulse sequence is presented which allows one to perform an inversion-recovery experiment for T1 determination on quadrupolar nuclei in the presence of acoustic ringing. The sequence is based on a modification of the reference baseline subtraction-90deg pulse sequence. Some examples of 17O and 14N relaxation are presented, including spectra with overlapping peaks from which T1 values cannot be simply extracted from line widths
Site of ionization of polyfunctional bases and acids .1. Ab initio proton affinities
No full textThe acid-base equilibria of a series of monofunctional and polyfunctional bases and acids have been investigated with ab initio theoretical methods; the structures and energies of the neutral form and of the forms ionized at all ionization sites have been determined. Predictions concerning the preferred site of ionization are made on the basis of the energetics. a) For bases of the type NHm-XHn (X = O,S) the preferred protonation site (by 20-30 kcal/mol) is the nitrogen, but the preference is less marked (deltaE = 7 kcal/mol) for X=P. (b) Amides deriving from weak acids (carboxylic, thiocarboxylic, cyanic, nitrous, sulfinic, etc.) protonate preferentially at the acid residue rather than at the nitrogen, with deltaE’s between 10 and 30 kcal/mol. (c) Amides deriving from strong acids (nitric, sulfonic) do not show a marked preference, deltaE forthe two tautomeric protonated forms being ca.2-3 kcal/mol. (d) Carboxylic acids and esters are protonated at the carbonyl oxygen, O-alkyl protonation being less favored by20 kcal/mol. (e) Dimethylsulfoxide is O-protonated. N-protonation of some non-carboxylic amides and O-alkyl protonation of carboxylic acids and esters leads to a substantial lengthening of the bond between N or O and the acid residue and to overall structural changes indicating dissociation. The gas-phase energetics are used for the estimation of relative solution basicities by means of the energies of solvation of simple models
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