Department of Chemistry, Technical University of Denmark, Kemitorvet 206, 2800 Kongens Lyngby, Denmark.
J Chem Phys. 2017 Jun 28;146(24):244311. doi: 10.1063/1.4990042.
The true global potential energy minimum configuration of the formaldehyde dimer (CHO), including the presence of a single or a double weak intermolecular CH⋯O hydrogen bond motif, has been a long-standing subject among both experimentalists and theoreticians as two different energy minima conformations of C and C symmetry have almost identical energies. The present work demonstrates how the class of large-amplitude hydrogen bond vibrational motion probed in the THz region provides excellent direct spectroscopic observables for these weak intermolecular CH⋯O hydrogen bond motifs. The combination of concentration dependency measurements, observed isotopic spectral shifts associated with H/D substitutions and dedicated annealing procedures, enables the unambiguous assignment of three large-amplitude infrared active hydrogen bond vibrational modes for the non-planar C configuration of (CHO) embedded in cryogenic neon and enriched para-hydrogen matrices. A (semi)-empirical value for the change of vibrational zero-point energy of 5.5 ± 0.3 kJ mol is proposed for the dimerization process. These THz spectroscopic observations are complemented by CCSD(T)-F12/aug-cc-pV5Z (electronic energies) and MP2/aug-cc-pVQZ (force fields) electronic structure calculations yielding a (semi)-empirical value of 13.7 ± 0.3 kJ mol for the dissociation energy D of this global potential energy minimum.
甲醛二聚体（CHO）的真正全球势能极小构型，包括存在单个或双弱分子间 CH⋯O 氢键模式，一直是实验家和理论家关注的长期课题，因为 C 和 C 对称性的两种不同能量极小构型的能量几乎相同。本工作展示了太赫兹区域探测的大振幅氢键振动运动类如何为这些弱分子间 CH⋯O 氢键模式提供极好的直接光谱可观测性。浓度依赖性测量、与 H/D 取代相关的观察到的同位素光谱位移以及专门的退火程序的结合，使（CHO）的非平面 C 构型在低温氖和富含 para 氢基质中嵌入的三个大振幅红外活性氢键振动模式的明确分配成为可能。提出了二聚化过程中振动零点能变化的（半）经验值为 5.5 ± 0.3 kJ mol。这些太赫兹光谱观察结果由 CCSD(T)-F12/aug-cc-pV5Z（电子能）和 MP2/aug-cc-pVQZ（力场）电子结构计算补充，得到此全局势能极小的离解能 D 的（半）经验值为 13.7 ± 0.3 kJ mol。
