Electric Quadrupole Moment of the ₁₄N Nucleus Determined by ab initio Cl Calculation on NH₃

Abstract

Based on the calculated electric field gradient (q) from ab initio Cl wavefunctions with large basis set (including / functions on N and d functions on H) and the experimental nuclear quadrupole coupling constant (e Q q) for nitrogen in N H 3, an improved value o f the quadrupole moment o f l4N is proposed as Q ('4N ) = 2.08 x 10~26 cm 2. The value calculated using a wavefunction near the Hartree-Fock lim it enlarged ~ 10% by the Cl treatment. IntroductionA direct determination of the electric quadrupole moment of the 14N nucleus (£H14N)) is very difficult since the electronic ground state of the nitrogen atom (4S) is almost spherically symmetrical, a slight deviation from the sphericity being only due to the spin-orbit coupling. The best available method of obtaining the (?(14N) value seems to be to compute the electric field gradient (q) in mole cules containing nitrogen, whose nuclear quadru pole coupling constants (eQq) are experimentally known. Among nitrogen-containing molecules, N H 3 is best because a very accurate experimental value obtained from the microwave study in the gaseous phase is available (e(?<7( 14N) = 4.0924 ± 0.0009 MHz) [1] and because the N H 3 molecule is small enough to obtain a very accurate molecular elec tronic wavefunction to compute the electric field gradient at the site of the nitrogen nucleus.A large number of ab initio self-consistent field (SCF) and configuration interaction (Cl) calcula tions have been made on N H 3 for this purpose, as summarized in [2], The available q values at nitrogen in N H 3 obtained from several levels of accuracy are, however, in the range from about -0 .6 to -2 .0 a.u. Most of these previous studies employ ab initio SCF wavefunctions with more or less extended basis sets.While one-electron properties such as the electric field gradient may be expected to be calculated with reasonable accuracy from the Hartree-Fock SCF wavefunction, the influence of electron corre lations on the Q value is of interest, especially since the discrepancy among various calculated values is large.As shown previously, the molecular quadrupole moment of benzene, also a one-electron property, could only be calculated very accurately from an extended Cl wavefunction [3], and the calculated value of -8.778 B which agreed excellently with the most recent experimental value (-8.69 ± 0.51 B) was at variance with the Hartree-Fock SCF value ( -9.662 B) by about 10%. Results and DiscussionIn the present study q at nitrogen in NH 3, calcu lated from ab initio Cl wavefunctions of different levels of accuracy, is reported and compared to the Hartree-Fock value. From a systematic comparison of the calculated q value an improved (?(!4N) value is proposed. The calculated q or £>(,4N) value is also at variance with the Hartree-Fock value by about 10%.The ab initio wavefunction which precedes the Cl study employs the gaussian basis set of Huzinaga M om ent o f the 14N N ucleus gaussian expansion of the hydrogen 1 s orbital with a scale factor 1.2 was similarly contracted to a (2 s) set fo...