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Benchmark ab initio thermochemistry of the isomers of diimide, N_2H_2, using accurate computed structures and anharmonic force fields | | Jan M.L. Martin
; Peter R. Taylor
; | | Date: |
12 Aug 1998 | | Journal: | Molecular Physics 96, 681-692 (1999) [B. Liu memorial issue] | | Subject: | Chemical Physics | physics.chem-ph | | Abstract: | A benchmark ab initio study on the thermochemistry of the trans-HNNH, cis-HNNH, and H$_2$NN isomers of diazene has been carried out using the CCSD(T) coupled cluster method, basis sets as large as $[7s6p5d4f3g2h/5s4p3d2f1g]$, and extrapolations towards the 1-particle basis set limit. The effects on inner-shell correlation and of anharmonicity in the zero-point energy were taken into account: accurate geometries and anharmonic force fields were thus obtained as by-products. Our best computed $Delta H^circ_{f,0}$ for trans-HNNH, 49.2 pm 0.3 kcal/mol, is in very good agreement with a recent experimental lower limit of 48.8 pm 0.5 kcal/mol. CCSD(T) basis set limit values for the isomerization energies at 0 K are 5.2 pm 0.2 kcal/mol (cis-trans) and 24.1 pm 0.2 kcal/mol (iso-trans). Our best computed geometry for trans-HNNH, $r_e$(NN)=1.2468 AA, $r_e$(NH)=1.0283 AA, and $ heta_e$=106.17$^circ$, reproduces the precisely known ground-state rotational constants of trans-HNNH to within better than 0.1 %. The rotation-vibration spectra of both cis-HNNH and H$_2$NN are dominated by very strong Coriolis and Fermi resonances. In addition, the NH stretches in H$_2$NN are so strongly anharmonic that vibrational perturbation theory breaks down, and the molecule appears to be an excellent test case for variational treatments of the vibrational Schrödinger equation. | | Source: | arXiv, physics/9808014 | | Services: | Forum | Review | PDF | Favorites | |
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