Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
J Chem Phys. 2018 Jul 21;149(3):034306. doi: 10.1063/1.5034110.
Using the stimulated-emission-pumping/resonant 2-photon ionization (SEP-R2PI) method, we have determined accurate intermolecular dissociation energies D of supersonic jet-cooled intermolecular complexes of 1-naphthol (1NpOH) with alkanes, 1NpOH·S, with S = methane, ethane, propane, and n-butane. Experimentally, the smaller alkanes form a single minimum-energy structure, while 1-naphthol·n-butane forms three different isomers. The ground-state dissociation energies D(S) for the complexes with propane and n-butane (isomers A and B) were bracketed within ±0.5%, being 16.71 ± 0.08 kJ/mol for S = propane and 20.5 ± 0.1 kJ/mol for isomer A and 20.2 ± 0.1 kJ/mol for isomer B of n-butane. All 1NpOH·S complexes measured previously exhibit a clear dissociation threshold in their hot-band detected SEP-R2PI spectra, but weak SEP-R2PI bands are observed above the putative dissociation onset for the methane and ethane complexes. We attribute these bands to long-lived complexes that retain energy in rotation-type intermolecular vibrations, which couple only weakly to the dissociation coordinates. Accounting for this, we find dissociation energies of D(S) = 7.98 ± 0.55 kJ/mol (±7%) for S = methane and 14.5 ± 0.28 kJ/mol (±2%) for S = ethane. The D values increase by only 1% upon S → S excitation of 1-naphthol. The dispersion-corrected density functional theory methods B97-D3, B3LYP-D3, and ωB97X-D predict that the n-alkanes bind dispersively to the naphthalene "Face." The assignment of the complexes to Face structures is supported by the small spectral shifts of the S → S electronic origins, which range from +0.5 to -15 cm. Agreement with the calculated dissociation energies D(S) is quite uneven, the B97-D3 values agree within 5% for propane and n-butane, but differ by up to 20% for methane and ethane. The ωB97X-D method shows good agreement for methane and ethane but overestimates the D(S) values for the larger n-alkanes by up to 20%. The agreement of the B3LYP-D3 D values is intermediate between the other two methods.
利用受激发射抽运/共振双光子电离（SEP-R2PI）方法，我们确定了超音速射流冷却的 1-萘酚（1NpOH）与烷烃、1NpOH·S 之间的分子间复合物的准确离解能 D，其中 S = 甲烷、乙烷、丙烷和正丁烷。实验上，较小的烷烃形成单一的最低能量结构，而 1-萘酚·正丁烷形成三种不同的异构体。丙烷和正丁烷（异构体 A 和 B）复合物的基态离解能 D（S）在±0.5%范围内，丙烷为 16.71 ± 0.08 kJ/mol，异构体 A 为 20.5 ± 0.1 kJ/mol，异构体 B 为 20.2 ± 0.1 kJ/mol。以前测量的所有 1NpOH·S 复合物在其热带检测的 SEP-R2PI 光谱中都表现出明显的离解阈值，但在甲烷和乙烷复合物的假定离解起始处上方观察到较弱的 SEP-R2PI 带。我们将这些带归因于保留在旋转型分子间振动中的能量的长寿命复合物，这些振动仅与离解坐标弱耦合。考虑到这一点，我们发现 S = 甲烷的离解能 D（S）= 7.98 ± 0.55 kJ/mol（±7%），S = 乙烷的离解能 D（S）= 14.5 ± 0.28 kJ/mol（±2%）。当 1-萘酚的 S → S 激发时，D 值仅增加 1%。色散校正的密度泛函理论方法 B97-D3、B3LYP-D3 和 ωB97X-D 预测，正烷烃以分散的方式与萘“面”结合。S → S 电子起源的小光谱位移支持复合物的面结构的分配，其范围从+0.5 到-15 cm。与计算的离解能 D（S）的一致性相当不均匀，B97-D3 值对于丙烷和正丁烷在 5%以内一致，但对于甲烷和乙烷相差高达 20%。ωB97X-D 方法对于甲烷和乙烷表现出良好的一致性，但对于较大的正烷烃，D（S）值高估了高达 20%。B3LYP-D3 D 值的一致性介于其他两种方法之间。
