NMR Chemical Shielding and Spin−Spin Coupling Constants of Liquid NH₃: A Systematic Investigation using the Sequential QM/MM Method

Abstract

The NMR spin coupling parameters, (1)J(N,H) and (2)J(H,H), and the chemical shielding, sigma((15)N), of liquid ammonia are studied from a combined and sequential QM/MM methodology. Monte Carlo simulations are performed to generate statistically uncorrelated configurations that are submitted to density functional theory calculations. Two different Lennard-Jones potentials are used in the liquid simulations. Electronic polarization is included in these two potentials via an iterative procedure with and without geometry relaxation, and the influence on the calculated properties are analyzed. B3LYP/aug-cc-pVTZ-J calculations were used to compute the (1)J(N,H) constants in the interval of -67.8 to -63.9 Hz, depending on the theoretical model used. These can be compared with the experimental results of -61.6 Hz. For the (2)J(H,H) coupling the theoretical results vary between -10.6 to -13.01 Hz. The indirect experimental result derived from partially deuterated liquid is -11.1 Hz. Inclusion of explicit hydrogen bonded molecules gives a small but important contribution. The vapor-to-liquid shifts are also considered. This shift is calculated to be negligible for (1)J(N,H) in agreement with experiment. This is rationalized as a cancellation of the geometry relaxation and pure solvent effects. For the chemical shielding, sigma((15)N) calculations at the B3LYP/aug-pcS-3 show that the vapor-to-liquid chemical shift requires the explicit use of solvent molecules. Considering only one ammonia molecule in an electrostatic embedding gives a wrong sign for the chemical shift that is corrected only with the use of explicit additional molecules. The best result calculated for the vapor to liquid chemical shift Delta sigma((15)N) is -25.2 ppm, in good agreement with the experimental value of -22.6 ppm.

Isomerization effects on chemical shifts and spin‐spin coupling constants of polyacetylene chains: A GIAO‐DFT study

“…Nuclear chemical shifts and indirect spin‐spin coupling constants are magnetic molecular properties that can be obtained from NMR spectroscopy. In recent years, substantial methodological efforts have been made to estimate such magnetic properties with high accuracy3–21. In particular, an efficient way to include the solvent effects on the shielding constants of solute molecules has been reported by Coutinho et al17.…”

Applications of nuclear shielding

“…369 The NMR spin coupling parameters, 1 J(N,H) and 2 J(H,H) and the chemical shielding, s( 15 N), of liquid ammonia were studied from a combined and sequential QM/MM methodology. 370 6 2 þ , Zn 2 þ , Cd 2 þ and Co(NH 3 ) 6 3 þ were used to probe the effect of flanking bases on metal binding sites in three different RNA motifs. 371 15 N backbone dynamics is reported in differentially enriched 1-73(U-13 C, 15 N)/74-108(U-15 N) reassembled thioredoxin on multiple time scales using a series of 2D and 3D MAS NMR experiments probing the backbone amide 15 N longitudinal relaxation, 1 H-15 N dipolar order parameters, 15 N chemical shift anisotropy (CSA), and signal intensities in the temperature-dependent and 1 H T 2 0 -filtered NCA experiments.…”

NMR spin–spin coupling constants in hydrogen‐bonded glycine clusters

“…Since H‐bonded interactions affect the electron density of the participating atoms, SSCCs can be useful in the detection and analysis of the nature of hydrogen bonds. A significant progress regarding the calculation of the SSCCs for H‐bonded molecular systems has been made in last few years …”