Cheok N. Tam

Measurement and Ab initio modeling of the inelastic neutron scattering of solid N-methylformamide.

Vibrational motions of solid N-methylformamide (NMF) and its N-deuterated analogue are investigated using the inelastic neutron scattering (INS) technique at 15 K. The force field for obtaining the normal vibrational modes of the crystal is based on a quantum chemical calculation and a subsequent transfer of a harmonic force field of a smaller pentameric segment to a fragment of 11 NMF molecules. Two types of hydrogen bonds present in crystalline NMF are also modeled with dimers. The distinct bonding leads to a splitting of the N-hydrogen wagging mode in the spectrum. Although the hydrogen bonding has a profound effect on vibrational frequencies, the results indicate that an occurrence of a double-well potential for bonded hydrogen proposed previously is unlikely. Instead, a limited electronic conjugation along the hydrogen bonds in crystalline NMF is observed. Unlike in previous models, we simulate the relative INS intensity of each vibrational transition separately, which leads to a substantial improvem...

Properties of water in calcium- and hexadecyltrimethylammonium-exchanged bentonite

Dynamical and thermodynamic properties of water at room temperature in Ca- and hexade-cyltrimethylammonium- (HDTMA) exchanged bentonite were determined for 4 different water contents (~0.03–0.55 g water g-1 clay). Incoherent quasi-elastic neutron scattering (QENS) was used to measure the translational and rotational mobility of water in the clays, while chilled mirror dewpoint psychrometry measured water activity of the samples, differential scanning calorimetry (DSC) provided information about the temperature of dehydration and X-ray diffraction (XRD) quantified layer spacings for the clays. The neutron scattering data were fit to a jump diffusion model that yielded mean jump lengths, jump diffusion residence times and rotational relaxation times for water in the clays. Mean jump lengths were quite similar for the 2 different cation saturations at equivalent water contents, and decreased with increasing water content. The fitted jump lengths ranged from 0.27–0.5 nm and were 2–4 times larger than that found for bulk water (0.13 nm). Jump diffusion residence times were 3–30 times longer than that for bulk water (1.2 ps) and also decreased with increasing water content. The residence times were somewhat shorter for HDTMA-clay as compared with Ca-clay at equivalent water contents. Rotational motion was less strongly influenced than translational motion by the presence of the clay surface. The energy state of water in the 2 cation saturations were quite different; dehydration temperatures for the HDTMA-clay were approximately 30 °C lower than the Ca-clay at equal water contents, while water activities, as PIP0, were up to 0.6 units higher. A linear relationship was found between water activity and the translational diffusion coefficient, although at the highest water content, the diffusion coefficient of water for the HDTMA-clay was approximately 30% higher than that measured for bulk water.

Magnetic vibrational circular dichroism of C60

The magnetic vibrational circular dichroism spectra of C 60 fullerene in solution are presented for excitation of the 1430 and 1183 cm −l t t1u -symmetry fundamental vibrations. These transitions give rise to A-term MVCD of opposite sign ( A 1 / D 0 =−3.8 and l.1 × 10 −4 , respectively) which are similar in magnitude to that of benzene. Using the MCD spectra for the two lowest-energy 1 T 1u electronic excitations of C 60 , the single excited-state vibronic coupling model correctly predicts the sign of one of the MVCD bands but underestimates both magnitudes.

Rotationally resolved magnetic vibrational circular dichroism: Experimental spectra and theoretical simulation for diamagnetic molecules

The rotationally resolved magnetic vibrational circular dichroism (RR MVCD) spectra of the diamagnetic molecules DCl and NH3 are analysed on the basis of theoretical simulation. Basic theoretical equations are derived for the RR MVCD dispersed intensity pattern from the conventional rovibrational energy level expression. This provides a convenient method for interpretation of MVCD spectra in terms of fundamental spectroscopic parameters. Good agreement was obtained between the simulated and the experimental data measured for DCl and NH3. Minor deviations from the theory occur in the dipole strength distribution for DCl, but this is corrected by measuring the A 1/D 0 ratio, from which the molecular g value can be determined for resolved transitions. Although magnetic properties of DCl and NH3 are known, this study demonstrates the ability of MVCD to provide an alternate method of determining molecular g-values as compared to classical microwave studies of the Zeeman effect. The data further show that exper...

A convenient synthesis of chiral succinic acid-d2 by catalytic asymmetric reduction using a ruthenium BINAP catalyst

Abstract The synthesis of (2R,3R) and (2S,3S) dideuteriosuccinic acid in 63 ± 10% enantiomeric excess by reduction of the half acid ester of ethyl fumarate from (R) and (S) BINAP ruthenium (II) diacetate complex, respectively, is reported. (2R,3R) and (2S,3S)-Dideuteriosuccinic acid has also been prepared from (R)-BINAP ruthenium (II) dicarboxylate complex directly in similar optical purity by reversing the sequence of introduction of the isotopic label.

Observations of rotational magnetic moments in the ground and some excited vibrational Σ states of C2H2, C2HD, and C2D2 by magnetic vibrational circular dichroism

Magnetic vibrational circular dichroism (MVCD) spectra of acetylene and its deuterated isotopomers have been recorded for the following Σ symmetry combination and overtone bands of C2H2: ν4+ν5; C2HD: ν4+ν5, 2ν4, 2ν5; C2D2: ν4+ν5, the ν3 fundamental for C2HD and C2D2; and the ν4→2ν4+ν5 and ν5→ν4+2ν5 hot bands for C2H2. For a Σg→Σu transition, the MVCD A terms observed must arise primarily from the rotational Zeeman effect. These negative A1/D0 values for low J″ transitions confirm that the sign of the rotational g‐value for acetylene is positive. The rotational magnetic moments in both the lower and upper vibrational states were determined by comparison of moment analyses of experimental and simulated MVCD spectra obtained with a model Hamiltonian for acetylene. The gJ values in all the excited bending combination and overtone vibrational levels observed are smaller than those in the ground and the first excited stretching vibrational levels. This observation has been confirmed by theoretical simulation of...

Direct measurement of the rotational g-value in the ground vibrational state of acetylene by magnetic vibrational circular dichroism

The magnetic vibrational circular dichroism (MVCD) spectra of the v 4 + v 5 combination band of acetylene was measured. from the result for the P(J″ = 1 → J′ = 0) rotational transition, the g J -value in the ground vibrational state for C 2 H 2 was determined to be +0.049 (±0.005). The sign of the g J -value measured by MVCD confirms that the correct sign is positive as found in a revised molecular beam magnetic resonance experiment and predicted on the basis of molecular Zeeman studies of other molecules and magnetic effects on heat flow in acetylene. MVCD offers an exceptionally simple method of determining the sign of molecular g -values for both the ground and excited states.

The regiochemistry of reduction of mono-ethyl fumarate and maleate using a ruthenium BINAP catalyst

Abstract The regiochemistry of the reduction of mono-ethyl fumarate using bis (carboxylato){2,2′- bis (diphenylphosphino)-[R]-1,1-binapthyl}-ruthenium(II) (Ru[BINAP]) in H 2 /CH 3 OD is reported and occurs opposite to the regiochemistry observed in the reduction of tiglic acid. Reduction of mono-ethyl maleate with D 2 /CH 3 OD is very sluggish but produces meso -dideuteriosuccinic acid with high stereoselectivity. Reduction of mono-ethyl maleate with Ru[BINAP](O 2 CR) 2 and H 2 /CH 3 OD results in mono-ethyl succinate-d 1 with nearly complete loss of regiochemistry.