Department of Chemistry, 1-014 Center for Science and Technology, Syracuse University, Syracuse, New York 13244-4100, USA.
J Phys Chem A. 2010 Mar 18;114(10):3630-41. doi: 10.1021/jp9114095.
The incoherent inelastic neutron scattering spectrum of parabanic acid was measured and simulated using solid-state density functional theory (DFT). This molecule was previously the subject of low-temperature X-ray and neutron diffraction studies. While the simulated spectra from several density functionals account for relative intensities and factor group splitting regardless of functional choice, the hydrogen-bending vibrational energies for the out-of-plane modes are poorly described by all methods. The disagreement between calculated and observed out-of-plane hydrogen bending mode energies is examined along with geometry optimization differences of bond lengths, bond angles, and hydrogen-bonding interactions for different functionals. Neutron diffraction suggests nearly symmetric hydrogen atom positions in the crystalline solid for both heavy-atom and N-H bond distances but different hydrogen-bonding angles. The spectroscopic results suggest a significant factor group splitting for the out-of-plane bending motions associated with the hydrogen atoms (N-H) for both the symmetric and asymmetric bending modes, as is also supported by DFT simulations. The differences between the quality of the crystallographic and spectroscopic simulations by isolated-molecule DFT, cluster-based DFT (that account for only the hydrogen-bonding interactions around a single molecule), and solid-state DFT are considered in detail, with parabanic acid serving as an excellent case study due to its small size and the availability of high-quality structure data. These calculations show that hydrogen bonding results in a change in the bond distances and bond angles of parabanic acid from the free molecule values.
巴那酸的非相干非弹性中子散射谱通过固态密度泛函理论（DFT）进行了测量和模拟。该分子先前是低温 X 射线和中子衍射研究的对象。虽然几种密度泛函的模拟谱可以解释相对强度和因子群分裂，而不管功能选择如何，但所有方法都无法很好地描述面外模式的氢键弯曲振动能。本文研究了不同功能下的计算和观察到的面外氢弯曲模式能量之间的差异以及键长、键角和氢键相互作用的几何优化差异。中子衍射表明，对于重原子和 N-H 键距离，晶体固体中的氢原子位置几乎是对称的，但氢键角度不同。光谱结果表明，与对称和不对称弯曲模式相关的氢原子（N-H）的面外弯曲运动存在显著的因子群分裂，这也得到了 DFT 模拟的支持。孤立分子 DFT、基于簇的 DFT（仅考虑单个分子周围的氢键相互作用）和固态 DFT 对晶体学和光谱学模拟的质量差异进行了详细考虑，巴那酸是一个很好的案例研究，因为它的尺寸小，并且有高质量的结构数据。这些计算表明，氢键导致巴那酸的键距和键角从自由分子值发生变化。
