二酰亚胺异构体N_2H_2的从头算热化学研究
Benchmark ab initio thermochemistry of the isomers of diimide, N_2H_2, using accurate computed structures and anharmonic force fields
摘要：
采用CCSD（T）耦合团簇方法，基组为$[7s6p5d4f3g2h/5s4p3d2f1g]$，外推到单粒子基组极限，对二氮烯的trans-HNNH，cis-HNNH和H-$_2$ NN异构体的热化学进行了基准从头算研究.考虑了内壳层相关性和零点能中的非谐性的影响：因此获得了精确的几何形状和非谐力场作为副产品。我们计算的反式-HNNH的最佳$ΔH^\circ_{f,0}$为49.2 \pm 0.3 kcal/mol，与最近的实验下限48.8 \pm 0.5 kcal/mol非常一致。CCSD（T）基组在0 K时的异构化能极限值为5.2 \pm 0.2 kcal/mol（顺-反）和24.1 \pm 0.2 kcal/mol（异-反）。我们对trans-HNNH的最佳计算几何，$r_e$（NN）=1.2468，$r_e$（NH）=1.0283，和$θ_e$ =106.17 $^\circ$，再现了精确已知的trans-HNNH的基态转动常数，其误差小于0.1%。顺式-HNNH和H $_2$ NN的旋转振动光谱都由非常强的科里奥利和费米共振主导。此外，NH伸展H $_2$ NN是如此强烈的非谐振动微扰理论崩溃，和分子似乎是一个很好的测试案例的振动薛定谔方程的变分处理。
英文摘要：
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 $ΔH^\circ_{f,0}$ 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 Å, $r_e$(NH)=1.0283 Å, and $θ_e$=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.
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