Institut für Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstrasse 36, Berlin D-10623, Germany.
J Chem Phys. 2009 Nov 14;131(18):184307. doi: 10.1063/1.3262720.
The infrared (IR) spectrum of protonated azulene (AzuH(+), C(10)H(9)(+)) has been measured in the fingerprint range (600-1800 cm(-1)) by means of IR multiple photon dissociation (IRMPD) spectroscopy in a Fourier transform ion cyclotron resonance mass spectrometer equipped with an electrospray ionization source using a free electron laser. The potential energy surface of AzuH(+) has been characterized at the B3LYP/6-311G(**) level in order to determine the global and local minima and the corresponding transition states for interconversion. The energies of the local and global minima, the dissociation energies for the lowest-energy fragmentation pathways, and the proton affinity have been evaluated at the CBS-QB3 level. Comparison with calculated linear IR absorption spectra supports the assignment of the IRMPD spectrum to C4-protonated AzuH(+), the most stable of the six distinguishable C-protonated AzuH(+) isomers. Comparison between Azu and C4-AzuH(+) reveals the effects of protonation on the geometry, vibrational properties, and the charge distribution of these fundamental aromatic molecules. Calculations at the MP2 level indicate that this technique is not suitable to predict reliable IR spectra for this type of carbocations even for relatively large basis sets. The IRMPD spectrum of protonated azulene is compared to that of isomeric protonated naphthalene and to an astronomical spectrum of the unidentified IR emission bands.
质子化薁（AzuH(+)，C(10)H(9)(+)）的红外（IR）光谱已通过傅里叶变换离子回旋共振质谱仪中配备的电喷雾电离源的红外多光子解离（IRMPD）光谱法在指纹范围内（600-1800 cm(-1)) 进行了测量，该仪器采用自由电子激光。为了确定相互转换的全局和局部最小值以及相应的过渡态，在 B3LYP/6-311G(**) 水平上对 AzuH(+)的势能面进行了特征化。在 CBS-QB3 水平上评估了局部和全局最小值的能量、最低能量碎片途径的离解能和质子亲和力。与计算得出的线性 IR 吸收光谱的比较支持将 IRMPD 光谱分配给 C4-质子化 AzuH(+)，这是六种可区分的 C-质子化 AzuH(+)异构体中最稳定的异构体。与 Azu 和 C4-AzuH(+) 的比较揭示了质子化对这些基本芳香族分子的几何形状、振动性质和电荷分布的影响。MP2 水平的计算表明，即使对于较大的基组，该技术也不适合预测此类碳正离子的可靠 IR 光谱。将质子化薁的 IRMPD 光谱与异构质子化萘的光谱以及未识别的红外发射带的天文光谱进行了比较。
