Hunt Sherri W, Higgins Kelly J, Craddock Matthew B, Brauer Carolyn S, Leopold Kenneth R
Department of Chemistry, University of Minnesota, 207 Pleasant Street, SE, Minneapolis, Minnesota 55455, USA.
J Am Chem Soc. 2003 Nov 12;125(45):13850-60. doi: 10.1021/ja030435x.
Rotational spectroscopy and ab initio calculations have been used to characterize the complexes H(3)N-HF and H(3)N-HF-HF in the gas phase. H(3)N-HF is a C(3v) symmetric, hydrogen bonded system with an NF distance of 2.640(21) A and an N...H hydrogen bond length of 1.693(42) A. The H(3)N-HF-HF complex, on the other hand, forms a six-membered HN-HF-HF ring, in which both the linear hydrogen bond in the H(3)N-HF moiety and the F-H-F angle of (HF)(2) are perturbed relative to those in the corresponding dimers. The N...F and F...F distances in the trimer are 2.4509(74) A and 2.651(11) A, respectively. The N...H hydrogen bond length in H(3)N-HF-HF is 1.488(12) A, a value which is 0.205(54) A shorter than that in H(3)N-HF. Similarly, the F...F distance, 2.651(11) A, is 0.13(2) A shorter than that in (HF)(2). Counterpoise-corrected geometry optimizations are presented, which are in good agreement with the experimental structures for both the dimer and trimer, and further characterize small, but significant, changes in the NH(3) and HF subunits upon complexation. Analysis of internal rotation in the spectrum of H(3)N-HF-HF gives the potential barrier for internal rotation of the NH(3) unit, V(3), to be 118(2) cm(-1). Ab initio calculations reproduce this number to within 10% if the monomer units and the molecular frame are allowed to fully relax as the internal rotation takes place. The binding energies of H(3)N-HF and H(3)N-HF-HF, calculated at the MP2/aug-cc-pVTZ level and corrected for basis set superposition error are 12.3 and 22.0 kcal/mol, respectively. Additional energy calculations have been performed to explore the lowest frequency vibration of H(3)N-HF-HF, a ring-opening motion that increases the NFF angle. The addition of one HF molecule to H(3)N-HF represents the first step of microsolvation of a hydrogen bonded complex and the results of this study demonstrate that a single, polar near-neighbor has a significant influence on the extent of proton transfer across the hydrogen bond. As measured using the proton-transfer parameter rho(PT), previously defined by Kurnig and Scheiner [Int. J. Quantum Chem., Quantum Biol. Symp. 1987, 14, 47], the degree of proton transfer in H(3)N-HF-HF is greater than that in either (CH(3))(3)N-HF or H(3)N-HCl but less than that in (CH(3))(3)N-HCl.
转动光谱学和从头算已被用于表征气相中的配合物H₃N - HF和H₃N - HF - HF。H₃N - HF是一个具有C₃v对称性的氢键体系，NF距离为2.640(21) Å，N…H氢键长度为1.693(42) Å。另一方面，H₃N - HF - HF配合物形成了一个六元HN - HF - HF环，其中H₃N - HF部分的线性氢键和(HF)₂的F - H - F角相对于相应二聚体中的都受到了扰动。三聚体中的N…F和F…F距离分别为2.4509(74) Å和2.651(11) Å。H₃N - HF - HF中N…H氢键长度为1.488(12) Å，比H₃N - HF中的短0.205(54) Å。同样，F…F距离2.651(11) Å比(HF)₂中的短0.13(2) Å。给出了经平衡校正的几何优化结果，其与二聚体和三聚体的实验结构吻合良好，并进一步表征了络合时NH₃和HF亚基中虽小但显著的变化。对H₃N - HF - HF光谱中内旋转的分析得出NH₃单元内旋转的势垒V₃为118(2) cm⁻¹。如果在发生内旋转时允许单体单元和分子框架充分弛豫，从头算计算能将这个数值精确到10%以内。在MP2/aug - cc - pVTZ水平计算并经基组叠加误差校正后，H₃N - HF和H₃N - HF - HF的结合能分别为12.3和22.0 kcal/mol。还进行了额外的能量计算以探究H₃N - HF - HF的最低频率振动，即一种增加NFF角的开环运动。向H₃N - HF添加一个HF分子代表了氢键配合物微溶剂化的第一步，本研究结果表明单个极性近邻对氢键上质子转移的程度有显著影响。如使用先前由库尔尼格和谢纳[《国际量子化学杂志，量子生物学专题》，1987年，第14卷，第47页]定义的质子转移参数ρ(PT)所测量，H₃N - HF - HF中的质子转移程度大于(CH₃)₃N - HF或H₃N - HCl中的，但小于(CH₃)₃N - HCl中的。
