Department of Chemical Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005 (India), Fax: (+91) 22-2278-2106; Current address: Institute of Physical Chemistry, Georg-August University of Göttingen, 37077 Göttingen (Germany).
Chemphyschem. 2014 Jan 13;15(1):109-17. doi: 10.1002/cphc.201300731. Epub 2013 Nov 20.
The C-H···Y (Y=hydrogen-bond acceptor) interactions are somewhat unconventional in the context of hydrogen-bonding interactions. Typical C-H stretching frequency shifts in the hydrogen-bond donor C-H group are not only small, that is, of the order of a few tens of cm(-1) , but also bidirectional, that is, they can be red or blue shifted depending on the hydrogen-bond acceptor. In this work we examine the C-H···N interaction in complexes of 7-azaindole with CHCl3 and CHF3 that are prepared in the gas phase through supersonic jet expansion using the fluorescence depletion by infra-red (FDIR) method. Although the hydrogen-bond acceptor, 7-azaindole, has multiple sites of interaction, it is found that the C-H···N hydrogen-bonding interaction prevails over the others. The electronic excitation spectra suggest that both complexes are more stabilized in the S1 state than in the S0 state. The C-H stretching frequency is found to be red shifted by 82 cm(-1) in the CHCl3 complex, which is the largest redshift reported so far in gas-phase investigations of 1:1 haloform complexes with various substrates. In the CHF3 complex the observed C-H frequency is blue shifted by 4 cm(-1). This is at variance with the frequency shifts that are predicted using several computational methods; these predict at best a redshift of 8.5 cm(-1). This discrepancy is analogous to that reported for the pyridine-CHF3 complex [W. A. Herrebout, S. M. Melikova, S. N. Delanoye, K. S. Rutkowski, D. N. Shchepkin, B. J. van der Veken, J. Phys. Chem. A- 2005, 109, 3038], in which the blueshift is termed a pseudo blueshift and is shown to be due to the shifting of levels caused by Fermi resonance between the overtones of the C-H bending and stretching modes. The dissociation energies, (D0), of the CHCl3 and CHF3 complexes are computed (MP2/aug-cc-pVDZ level) as 6.46 and 5.06 kcal mol(-1), respectively.
C-H···Y（Y=氢键受体）相互作用在氢键相互作用的背景下有些非常规。典型的氢键供体 C-H 基团中 C-H 伸缩振动频率的位移不仅很小，即几 十厘米（-1），而且是双向的，即它们可以根据氢键受体红移或蓝移。在这项工作中，我们通过使用红外荧光猝灭（FDIR）方法，通过超音速射流膨胀，在气相中制备了 7-氮杂吲哚与 CHCl3 和 CHF3 的复合物，研究了 C-H···N 相互作用。尽管氢键受体 7-氮杂吲哚有多个相互作用位点，但发现 C-H···N 氢键相互作用占主导地位。电子激发光谱表明，两种复合物在 S1 态比在 S0 态更稳定。C-H 伸缩频率在 CHCl3 复合物中红移 82 厘米（-1），这是迄今为止在气相中对各种基质的卤仿 1:1 复合物进行研究中报告的最大红移。在 CHF3 复合物中，观察到的 C-H 频率蓝移 4 厘米（-1）。这与使用几种计算方法预测的频率位移不一致；这些方法预测的最大红移为 8.5 厘米（-1）。这种差异类似于报告的吡啶-CHF3 复合物[W. A. Herrebout, S. M. Melikova, S. N. Delanoye, K. S. Rutkowski, D. N. Shchepkin, B. J. van der Veken, J. Phys. Chem. A- 2005, 109, 3038]中的差异，其中蓝移被称为伪蓝移，并且表明这是由于 C-H 弯曲和伸缩模式的倍频之间的费米共振引起的能级移动所致。CHCl3 和 CHF3 复合物的离解能（D0）分别通过 MP2/aug-cc-pVDZ 水平计算为 6.46 和 5.06 kcal·mol（-1）。
