Ronald F. Evilia

QUANTITATIVE NMR SPECTROSCOPY

In this review the theoretical and practical factors that govern the suitability and accuracy of NMR measurements for quantitative analysis purposes are discussed. The review is limited to factors important for spectra acquired by modern Fourier Transform instruments.

Study of solvent-solute and solute-solute interaction effects on naphthalene in carbon dioxide by NMR measurement of rotational ang angular momentum correlation times in CO2 at sub- and supercritical temperatures

Abstract The effect of temperature variation from subcritical to supercritical conditions on the rotational and angular momentum correlation times of naphthalene dissolved in carbon dioxide is reported. The rotational correlation time, τ c is found to be only slightly dependent upon solution viscosity, unaffected by the CO 2 phase change at about 32°C and is essentially equal to its value in acetone at the same temperature. τ c is approximately twice its gas phase free rotor time. These results are interpreted to indicate that ground state solute-solute interactions are unimportant. The angular momentum correlation time, τ j , undergoes a dramatic increase at the temperature of the phase change indicating an approximately 3.2 fold solvent density augmentation around the naphthalene compared to the bulk gas phase CO 2 density under the experimental conditions beyond the critical temperature. These results indicate that a substantial solvent density augmentation occurs that does not cause measurable changes in the rotational properties of the solute. The solvent augmentation is found to disappear within approximately 2°C on either side of the critical temperature.

Bayesian Estimation of NMR Spectral Parameters Under Low Signal-to-Noise Conditions

Abstract The Bayesian statistical method of spectral estimation is applied to NMR free induction decay signals at various values of signal-to-noise ratio (SNR). The frequency and amplitude estimates from the Bayesian calculations are more accurate than those from the commonly used fast Fourier transformation (FFT) of the same data sets. Both real and synthetic data sets are examined with the Bayesian results being superior in all cases. In addition to the superior performance at low SNR the Bayesian derived amplitudes and frequency estimates were not as affected by signal decay as in Fourier Transformed spectra. Finally, the amplitudes obtained are equal to the FFT integrated intensities resulting in an apparent frequency domain signal-to-noise ratio (SNR) greater than the FFT SNR by a factor proportional to the FFT frequency domain linewidth. For typical high resolution spectra this improvement was approximately a factor of 2.5. Even greater improvement is obtained when rapidly decaying signals are analy...

Evidence for some pentadentate coordination in ethylenediamine-N,N'-diacetato-N,N'-di-3-propionatonickelate(II)

The 1H and 13C contact shifted NMR spectra of the nickel(Il) complex of EDDDA are reported as a function of temperature. The data indicate that the ligand acts as a hexadentate coordinator at least 98% of the time at room temperature with the six-membered propionate rings occupying the in-plane sites and the acetates occupying the out-of-plane sites. The acetate rings are almost planar with a time averaged puckering angle of approximately 7°. Puckering of the ethylenediamine ring is approximately the same as in EDTA and the propionate ring puckering is comparable to that in 1,3-diamines. The amount of pentadentate form of the ligand increases at high temperatures, reaching 22% pentadentate at 100 °C. ΔH and ΔS for the hexadentate ⇄ pentadentate equilibrium are +13 ± 8 kJ mol−1 and +17 ± 1 J mol−1 K−1 respectively. The conversion from hexadentate to pentadentate form involves removal of one of the out-of-plane acetates. No evidence for uncoordinated propionate was observed at any temperature.

Sample Input Modulation and Lock-in-Amplifier Detection for Signal-to-Noise Ratio Improvement in Inductively Coupled Plasma Atomic Emission Spectroscopy

Abstract The effect of modulation of the sample atomization, accomplished by pulsing the nebulizer pump, and lock-in-amplifier (LIA) detection of the modulated output signal of an ICP atomic emission spectrometer is studied. The time constant of the nebulizer spray chamber allows a maximum modulation frequency of approximately 2 Hz, but optimum performance for this system is found at a modulation frequency of approximately 1 Hz. A signal-to-noise (SNR) ratio improvement approaching a factor of three is found for the arrangement employed. It is proposed that more rapid modulation, achievable through the use of a nebulization system with a shorter time constant, should lead to even greater improvement in the SNR than was accomplished in this study. The dynamic range is improved, relative to the unmodulated system, as the detection limit is lowered without any loss of linearity at high concentrations. A linear dynamic range of greater than 4 orders of magnitude is found for the modulated system.

Deuteration of amino acids in acidic deuterium oxide solution at supercritical temperatures

Abstract The deuterations of a number of the common amino acids at 400 °C and 390 bars in aqueous strong acid are reported. Protium/deuterium exchange is found to be rapid at the α carbon in all examples. Decarboxylation occurred in most cases at a sufficiently rapid rate that the reaction could not be used for deuteration at other sites in most cases. Exchange was accompanied by racemization of the α chiral center as was previously observed for reaction in supercritical base. While rapid α deuteration was noted for all the amino acids, slower deuteration of alkyl chains was not observed because of decomposition. Electrophilic substitution of activated aromatic groups, however, was more rapid than decomposition leading to the production of some products deuterated at sites other than the α carbon. Several of the decomposition products are identified and a general scheme for the deuteration and decomposition reactions is suggested.

The effect of ortho methyl groups on cumyl systems

Abstract The reactions of 2-mesityl-2-propanol ( 2 ) and 2-mesityl-1-propene ( 6 ) have been investigated with the objective of evaluating the effect of ortho methyl groups on chemical reactivity. The 13 C NMR spectrum of 2-mesityl-2-propyl cation ( 3 ) indicates the expected steric inhibition to resonance.

Improved resolution of spectral splittings via bayesian spectral analysis

Recently, the application of Bayesian analysis to NMR spectroscopy has provided excellent predictions of spectral parameters, suggesting that this technique may offer advantages over typical FT methods ( 1) . When the correct model is used, the results of the Bayesian technique are superior to those of Fourier transformation in both spectral linewidth and apparent frequency-domain signal-to-noise ratio (2). In this Communication, the results of the application of Bayesian analysis to a complex spectrum in which the peaks are not clearly resolved by FFT are presented. In general the FID can be represented as

An NMR Spectroscopic Determination of an Equilibrium Isotope Effect on the Hydration of Cobalt(II)

Abstract A recently reported NMR method for the measurement of deuterium equilibrium isotope effects is applied to the hydration of the paramagnetic cobalt(II) ion. An isotope effect of about 1.3% is measured. A substantial difference between the intrinsic shift of H2O and D2O when coordinated to cobalt is also measured.

Coulometric Generation of Gradient Elution Programs: Separation of Amino Acids Without Buffer Mixing

Abstract A coulometric cell and buffer system is reported which allows control of the pH of the mobile phase by control of the electrolysis current passed through the cell. The electrolysis cell is capable of varying the pH over the range of 2.5 – 12 without gas evolution at flow rates up to 1 ml/min. The pH is flow rate dependent in the configuration reported, but appropriate pH monitoring and electronic feedback can eliminate that problem. The use of coulometric pH control for the separation of amino acids without any buffer mixing is illustrated.