TY - JOUR

T1 - H/D isotope effects on NMR chemical shifts of nuclei involved in a hydrogen bridge of hydrogen isocyanide complexes with fluoride anion

AU - Golubev, Nikolai S.

AU - Detering, Carsten

AU - Smirnov, Sergei N.

AU - Shenderovich, Ilja G.

AU - Denisov, Gleb S.

AU - Limbach, Hans Heinrich

AU - Tolstoy, Peter M.

PY - 2009

Y1 - 2009

N2 - 1H, 2H, 19F and 15N NMR spectra of a strongly hydrogen-bonded anionic cluster, CNHF-, as an ion pair with a tetrabutylammonium cation dissolved in CDF3-CDF2Cl mixture were recorded in the slow exchange regime at temperatures down to 110 K. The fine structure due to spin-spin coupling of all nuclei involved in the hydrogen bridge was resolved. H/D isotope effects on the chemical shifts were measured. The results were compared with those obtained earlier for a similar anion, FHF-, and interpreted via ab initio calculations of magnetic shielding as functions of internal vibrational coordinates, namely an anti-symmetric proton stretching and a doubly-degenerate bending. The values of primary and secondary isotope effects on NMR chemical shifts were estimated using a power expansion of the shielding surface as a function of vibrational coordinates. A positive primary isotope effect was explained as a result of the decrease of the hydron stretching amplitude upon deuteration. We show that the proton shielding surface has a minimum close to the equilibrium geometry of the CNHF- anion, leading to the positive primary H/D isotope effect in a rather asymmetric hydrogen bond. We conclude that caution should be used when making geometric estimations on the basis of NMR data, since the shapes of the shielding functions of the internal vibrational coordinates can be rather exclusive for each complex.

AB - 1H, 2H, 19F and 15N NMR spectra of a strongly hydrogen-bonded anionic cluster, CNHF-, as an ion pair with a tetrabutylammonium cation dissolved in CDF3-CDF2Cl mixture were recorded in the slow exchange regime at temperatures down to 110 K. The fine structure due to spin-spin coupling of all nuclei involved in the hydrogen bridge was resolved. H/D isotope effects on the chemical shifts were measured. The results were compared with those obtained earlier for a similar anion, FHF-, and interpreted via ab initio calculations of magnetic shielding as functions of internal vibrational coordinates, namely an anti-symmetric proton stretching and a doubly-degenerate bending. The values of primary and secondary isotope effects on NMR chemical shifts were estimated using a power expansion of the shielding surface as a function of vibrational coordinates. A positive primary isotope effect was explained as a result of the decrease of the hydron stretching amplitude upon deuteration. We show that the proton shielding surface has a minimum close to the equilibrium geometry of the CNHF- anion, leading to the positive primary H/D isotope effect in a rather asymmetric hydrogen bond. We conclude that caution should be used when making geometric estimations on the basis of NMR data, since the shapes of the shielding functions of the internal vibrational coordinates can be rather exclusive for each complex.

UR - http://www.scopus.com/inward/record.url?scp=67650361630&partnerID=8YFLogxK

U2 - 10.1039/b900152b

DO - 10.1039/b900152b

M3 - Article

C2 - 19562148

AN - SCOPUS:67650361630

VL - 11

SP - 5154

EP - 5159

JO - Physical Chemistry Chemical Physics

JF - Physical Chemistry Chemical Physics

SN - 1463-9076

IS - 25

ER -