Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2.
Phys Chem Chem Phys. 2010 Aug 1;12(29):8260-7. doi: 10.1039/c0cp00193g. Epub 2010 Jun 1.
High resolution microwave (a-type) and millimetre-wave (b-type) spectra of He(N)-(13)C(16)O, He(N)-(12)C(18)O, and He(N)-(13)C(18)O clusters (N<or= 10) were observed, significantly extending the initial spectroscopic observations on He(N)-(12)C(16)O by Surin et al. [Phys. Rev. Lett., 2008, 101, 233401]. The frequencies of the a-type series, which evolves from the end-over-end rotation of the dimer, decrease from N = 1 to 3, then increase smoothly to at least N = 9. This turnaround indicates a rapid evolution of the solvation character from classical to quantum. The b-type series, which evolves from the free molecule rotation of CO, increases from N = 0 to 6 and then decreases to at least N = 10. This is consistent with an initially increasing anisotropy of the helium environment, followed by a tendency of the solvation shell to become more isotropic. The shift of the vibrational frequency of CO as determined from the infrared [A. R. W. McKellar, J. Chem. Phys., 2004, 121, 6868; ibid., 2006, 125, 164328] and microwave data reveals an approximately linear decrease from N = 1 to at least 9. If the linear shift were to continue until completion of the first solvation shell (N approximately 14), the estimated helium nanodroplet shift will be well undershot [K. von Haeften, S. Rudolph, I. Simanovski, M. Havenith, R. E. Zillich and K. B. Whaley, Phys. Rev. B: Condens. Matter Mater. Phys., 2006, 73, 054502]. In this case, there must be an upturn in the vibrational shift beyond N = 14, which is not predicted by theory [T. Skrbić, S. Moroni and S. Baroni, J. Phys. Chem. A, 2007, 111, 7640]. By extrapolating the a-type series to N = 14 (assuming a linear vibrational shift), we estimate the rotational constant, B, of CO in the helium nanodroplet to be approximately 74% of its gas phase value. This is in reasonable agreement with simulations (76% at N = 14), which predict the limiting value to be approximately reached upon completion of the first solvation shell (73% at N = 100) [T. Skrbić, S. Moroni and S. Baroni, J. Phys. Chem. A, 2007, 111, 7640]. However, this value is significantly larger than that inferred from helium nanodroplet experiments (63%).
高分辨率微波（A型）和毫米波（B 型）光谱的 He(N)-(13)C(16)O、He(N)-(12)C(18)O 和 He(N)-(13)C(18)O 团簇（N<或=10）被观察到，这大大扩展了 Surin 等人对 He(N)-(12)C(16)O 的初始光谱观测。[物理评论快报，2008 年，101，233401]。A型系列的频率从二聚体的端到端旋转演化而来，从 N=1 到 3 降低，然后平滑增加到至少 N=9。这种逆转表明溶剂化特征从经典到量子的快速演变。B 型系列从 CO 的自由分子旋转演化而来，从 N=0 增加到 6，然后至少减少到 N=10。这与氦环境的各向异性最初增加一致，随后溶剂化壳趋于更加各向同性。从红外[ARWMcKellar，J.Chem.Phys.，2004，121，6868;同上，2006，125，164328]和微波数据确定的 CO 的振动频率的位移揭示了从 N=1 到至少 9 的近似线性减小。如果线性位移继续直到第一个溶剂化壳完成（N 约为 14），估计氦纳米液滴位移将大大低于[KHaeften，SRudolph，ISimanovski，MHavenith，REZillich 和 KBWhaley，物理评论 B：凝聚物质物理，2006，73，054502]。在这种情况下，在 N>14 处必须出现振动位移的上升，这是理论未预测的[TSkrbić，SMoroni 和 SBaroni，J.Phys.Chem.A，2007，111，7640]。通过将 A 型系列外推到 N=14（假设振动位移呈线性），我们估计氦纳米液滴中 CO 的转动常数 B 约为其气相值的 74%。这与模拟（N=14 时为 76%）相符，模拟预测当第一个溶剂化壳完成时（N=100 时为 73%），该限制值将大致达到[TSkrbić，SMoroni 和 SBaroni，J.Phys.Chem.A，2007，111，7640]。然而，这一值明显大于从氦纳米液滴实验推断的值（63%）。
