Hu Y J, Fu H B, Bernstein E R
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, USA.
J Chem Phys. 2006 Nov 14;125(18):184309. doi: 10.1063/1.2378628.
The vibrational spectrum of molecular propanoic acid, cooled in a supersonic expansion, in the region of 2500 to 7500 cm(-1) is obtained employing infrared plus vacuum ultraviolet nonresonant ionization detected spectroscopy. The fundamental and first overtone of the CH and OH stretch modes of cold propanoic acid molecules can be identified in the spectrum. Propanoic acid neutral and ionic clusters are also studied employing nonresonant ion dip and photodissociation spectroscopic techniques, respectively. For the neutral dimer, a sequence of features observed at ca. 2500-2700 cm(-1) can be assigned as combination bands of low frequency modes with the COH bending overtone; these features characterize the cyclic dimer ring structure. IR spectra of the larger neutral clusters n=3, 4, 5 indicate that they also have cyclic structures in which the OH groups are engaged in the cluster hydrogen bonding network. The CH groups are not involved in this hydrogen bonding structure. Free OH features are observed for the protonated ion clusters (C(2)H(5)COOH)(n)H(+), n=1,...,5, indicating that at least one OH group of these cluster ions is not involved in the cluster hydrogen bonding network. A comparison of the results for four hydrogen bonding neutral and ionic clusters (CH(3)OH, C(2)H(5)OH, CH(3)COOH, and C(2)H(5)COOH) is presented and discussed.
利用红外加真空紫外非共振电离探测光谱，获得了在超声速膨胀中冷却的分子丙酸在2500至7500厘米⁻¹区域的振动光谱。在该光谱中可以识别出冷丙酸分子的CH和OH伸缩模式的基频和第一泛音。还分别采用非共振离子偶极和光解离光谱技术研究了丙酸中性和离子簇。对于中性二聚体，在约2500 - 2700厘米⁻¹处观察到的一系列特征可归为低频模式与COH弯曲泛音的组合带；这些特征表征了环状二聚体环结构。较大的中性簇n = 3、4、5的红外光谱表明它们也具有环状结构，其中OH基团参与簇内氢键网络。CH基团不参与这种氢键结构。对于质子化离子簇（C₂H₅COOH）ₙH⁺，n = 1，...，5，观察到了游离OH特征，表明这些簇离子中至少有一个OH基团不参与簇内氢键网络。给出并讨论了四种氢键中性和离子簇（CH₃OH、C₂H₅OH、CH₃COOH和C₂H₅COOH）的结果比较。
