Nuclear magnetic shielding in the acetylene isotopomers calculated from correlated shielding surfaces

Abstract

Articles you may be interested inCurrent density maps, magnetizability, and nuclear magnetic shielding tensors of bis-heteropentalenes. I. Dihydro-pyrrolo-pyrrole isomers Natural chemical shielding analysis of nuclear magnetic resonance shielding tensors from gauge-including atomic orbital calculations Ab initio, symmetry-coordinate and internal valence coordinate carbon and hydrogen nuclear shielding surfaces for the acetylene molecule are presented. Calculations were performed at the correlated level of theory using gauge-including atomic orbitals and a large basis set. The shielding was calculated at equilibrium and at 34 distinct geometries corresponding to 53 distinct sites for each nucleus. The results were fitted to fourth order in Taylor series expansions and are presented to second order in the coordinates. The carbon-13 shielding is sensitive to all geometrical parameters and displays some unexpected features; most significantly, the shielding at a carbon nucleus ͑C 1 , say͒ is six times more sensitive to change of the C 1 C 2 H 2 angle than it is to change of the H 1 C 1 C 2 angle. In addition, for small changes, ͑C 1 ͒ is more sensitive to the C 2 H 2 bond length than it is to the C 1 H 1 bond length. These, and other, examples of ''unexpected differential sensitivity'' are discussed. The proton shielding surface is much more as expected with ͑H 1 ͒ being most sensitive to the C 1 H 1 bond length, less so to the CC bond length and hardly at all to the C 2 H 2 bond length. The surfaces have been averaged over a very accurate force field to give values of ͑C͒, ͑H͒, and ͑D͒ for the ten isotopomers containing all possible combinations of 12 C, 13 C, 1 H, and 2 H nuclei at 0 K and at a number of selected temperatures in the range accessible to experiment. For the carbon shielding the dominant nuclear motion contribution comes from the bending at ''the other'' carbon atom with the combined stretching contributions being only 20% of those from bending. For the proton shielding it is the stretching of the CH bond containing the proton of interest which provides the major nuclear motion contribution. For ͑C͒ in H 13 C 13 CH at 300 K our best result is 117.59 ppm which is very close to the experimental value of 116.9 ͑Ϯ0.9͒ ppm. For ͑H͒ in H 13 C 13 CH at 300 K we obtain 29.511 ppm which is also in very close agreement with the experimental value of 29.277 ͑Ϯ0.001͒ ppm. Calculated values are also very close to recent, highly accurate carbon and proton isotope shifts in the ten isotopomers; carbon isotope shifts differ by no more than 10% from the measured values and proton isotope shifts are generally in even better agreement than this. The observed anomaly whereby the 13 C isotope shift in H 13 C 12 CD is greater than that in D 13 C 12 CH both with respect to H 13 C 12 CH is explained in terms of the bending contribution at ''the other'' carbon. The observed nonadditivity of deuterium isotope effects on the carbon shielding can be traced to a cross term involving second order bending.

The Effect of Substituents on Indirect Nuclear Spin-Spin Coupling Constants: Methan- and Ethanimine, Methanal- and Ethanaloxime

“…We have carried out state of the art ab initio calculations on the above mentioned compounds using the second order polarization propagator approximation with coupled cluster single and double amplitudes (SOPPA(CCSD)) [15,25] and our aug-cc-pVTZ-J basis sets [15,16]. Previous applications of this method gave results in very good agreement with experiment [15,16,[26][27][28][29][30][31][32][33].…”

Geometry Dependence of Spin–Spin Couplings in Cyanamide by DFT Analysis

“…The rovibrational correction on calculated 1 J(CH) couplings in acetylene and CH 4 was previously estimated to be of the order of + 5 Hz. [34,37] The influence of such factors on 1 J(NH) couplings in NH 2 groups is not well known. However, it is plausible that the contrasting physiochemical conditions are mainly responsible for the observed differences in the coupling magnitudes.…”

“…[34] Increases in the one-bond coupling constant with increasing atom-atom distances were also calculated for N 2 and CO. [39][40][41] For these molecules the one-bond coupling changes sign from a negative to a positive value with increasing atom-atom distance. However, stretching the atomatom bond leads to a decrease in the one-bond coupling in most other molecular systems, for example, He 2 , [42] stretching R(NH) in NH 4 + and non-amino NÀH···N H-bonded complexes, [5,13,14] stretching RA C H T U N G T R E N N U N G (CN) in HCN and HNC, [43] and increasing R(NH) in NÀH···O=C H bonds in peptides.…”

“…To provide an experimental reference point for our calculations, the 1 J(NH) coupling constant of cyanamide was determined by measuring the amino proton resonance splitting in a standard [34] HCN, [35] and a number of unsubstituted and substituted hydrocarbons. [36] The disparities between the experimental and computed 1 J(NH) couplings are reasonable considering that the calculations were performed at 0 K with an isolated cyanamide monomer, whereas the experimental couplings include influences from rovibrational averaging and solvent and solute interactions, in particular, between the strong H-bond acceptor DMSO and cyanamide.…”

A correlatedab initio study of Karplus relations for model peptides

“…21 Others have considered vibrational and thermal corrections. 22,23 In this paper, we present such a study of N-methylacetamide (NMA), 24 -26 by employing the recently developed equation-of-motion coupled cluster (EOM-CC) approach. To obtain A, B and C in Eqn (1), the most rigorous treatment requires optimization of all the degrees of freedom of the molecule for a given value of the dihedral angle.…”