Thermal averaging of the indirect nuclear spin-spin coupling constants of ammonia: the importance of the large amplitude inversion mode

TY - JOUR

T1 - Thermal averaging of the indirect nuclear spin-spin coupling constants of ammonia: the importance of the large amplitude inversion mode

AU - Yachmenev, Andrey

AU - Yurchenko, Sergei N.

AU - Paidarová, Ivana

AU - Jensen, Per

AU - Thiel, Walter

AU - Sauer, Stephan P. A.

PY - 2010

Y1 - 2010

N2 - Analytic internal-coordinate representations are reported for two accurate ab initio spin-spin coupling surfaces of the ammonia molecule, J1 (N 15,H) and J2 (H,H). Calculations were carried out at the level of the second-order polarization propagator approximation involving coupled-cluster singles and doubles amplitudes (CCSD) and using a large specialized basis set, for a total of 841 different geometries corresponding to 2523 distinct points on the J1 (N 15,H) and J2 (H,H) surfaces. The results were fitted to power series expansions truncated after the fourth-order terms. While the one-bond nitrogen-hydrogen coupling depends more on the internuclear distance, the geminal hydrogen-hydrogen coupling exhibits a pronounced dependence on the bond angle. The spin-spin parameters are first vibrationally averaged, using vibrational wave functions obtained variationally from the TROVE computer program with a CCSD(T) based potential energy surface, for ammonia and its various deuterated isotopologues. The vibrationally averaged quantities are then thermally averaged to give values of the couplings at absolute temperatures of 300 and 600 K. We find that the nuclear-motion corrections are rather small. The computed one-bond couplings and their minute isotope effects are in excellent agreement with the experimental values.

AB - Analytic internal-coordinate representations are reported for two accurate ab initio spin-spin coupling surfaces of the ammonia molecule, J1 (N 15,H) and J2 (H,H). Calculations were carried out at the level of the second-order polarization propagator approximation involving coupled-cluster singles and doubles amplitudes (CCSD) and using a large specialized basis set, for a total of 841 different geometries corresponding to 2523 distinct points on the J1 (N 15,H) and J2 (H,H) surfaces. The results were fitted to power series expansions truncated after the fourth-order terms. While the one-bond nitrogen-hydrogen coupling depends more on the internuclear distance, the geminal hydrogen-hydrogen coupling exhibits a pronounced dependence on the bond angle. The spin-spin parameters are first vibrationally averaged, using vibrational wave functions obtained variationally from the TROVE computer program with a CCSD(T) based potential energy surface, for ammonia and its various deuterated isotopologues. The vibrationally averaged quantities are then thermally averaged to give values of the couplings at absolute temperatures of 300 and 600 K. We find that the nuclear-motion corrections are rather small. The computed one-bond couplings and their minute isotope effects are in excellent agreement with the experimental values.

U2 - 10.1063/1.3359850

DO - 10.1063/1.3359850

M3 - Journal article

C2 - 20331295

SN - 0021-9606

VL - 132

SP - 114305

JO - The Journal of Chemical Physics

JF - The Journal of Chemical Physics

IS - 11

ER -