Isotope and temperature effects on nuclear magnetic shieldings and spin-rotation constants calculated at the coupled-cluster level

The temperature dependence of nuclear shieldings as well as isotope effects on shieldings and spin-rotation constants have been computationally investigated for H₂, HF, F₂, CO, and N₂ employing the coupled-cluster singles and doubles (CCSD) method augmented by a perturbative treatment of triple excitations (CCSD(T)) for the calculation of potential curves, shieldings and spin–rotation functions together with finite-element techniques for the solution of the rovibrational problem. Calculated and measured temperature dependence of the isotropic shieldings agrees for N₂, while for CO and F₂ the computed temperature dependence is smaller than the experimental result. Isotropic shieldings have been deduced on the basis of our calculations from the measured spin-rotation constants for four isotopomers of H₂ and agree, as required by theory. However, calculated and measured temperature dependence of the isotope shifts between HD and D₂ differ by up to 10% which is larger than the estimated error bars for the experimental values. For HF and CO, calculated and measured isotope shifts agree, while for N₂ no experimental data for comparison are available. In case of spin–rotation constants, the calculated dependence on the rotational angular momentum quantum number are for both H₂ and F₂ in good agreement with the dependence deduced from measurements, while for HF not enough experimental data are available for a comparison.     

SPECTROSCOPIC INVESTIGATION OF DIHYDRONICOTINAMIDES-I: CONFORMATION, ABSORPTION, AND FLUORESCENCE

Abstract— The 1(N)-(2,6-dichlorobenzyl)-1,4-dihydronicotinamide (I), N-methyl- and N,N-dimethyl-1(N)-(2,6-dichlorobenzyl)-1,4-dihydronicotinamide (II and III), respectively), and 1(N)-(2,6-dichloro-benzyl)-2-aminomethyl-1,4-dihydronicotinic acid lactame (IV) were synthesized as model compounds for natural coenzymes, and systematically studied by 1H NMR, UV/V1S absorption and fluorescence spectroscopy. The absorption at ∼ 340 nm argues for an effective conjugation between dihydropyridine and carboxamide π-system, and rules out any severely twisted conformation. For the natural coenzymes NADH and NMNH, as well as for I and II (with no or only one N-amide substituent), 1H NMR definitively establishes a transoid conformation in solution, with the carbonyl O close to 2-H of the dihydropyridine ring. N,N-dimethyl substitution effectively inverts the carboxamide orientation into the cisoid form. The 1H NMR data (as well as molar extinctions) for the fused-ring derivatives IV and V, with a fixed cisoid and transoid structure, respectively, provide final proof for the conformational assignment.
Absorption maxima are shifted to lower energies with increasing solvent polarity. In solvents which can act as hydrogen bond acceptors to the carboxamide N-H, absorption shows a general blue-shift of ∼ 10 nm. H-bond donor solvents do not affect absorption maxima but enhance molar extinction. Fluorescence maxima show a similar dependence on solvent polarity but no specific hydrogen-bonding effect. Fluorescence quantum yields appear increased tenfold in solvents donating H-bonds to the carboxamide C=O group. These results are interpreted in terms of the vinylogous amide resonance between C=O function and ring-N lone pair being the electronic interaction dominating in the ground state of dihydronicotinamides.