Structure of host–guest complexes between dibenzo-18-crown-6 and water, ammonia, methanol, and acetylene: Evidence of molecular recognition on the complexation

Abstract

Complexes of dibenzo-18-crown-6 (DB18C6, host) with water, ammonia, methanol, and acetylene (guest) in supersonic jets have been characterized by laser induced fluorescence (LIF), UV-UV hole-burning (UV-UV HB), and IR-UV double resonance (IR-UV DR) spectroscopy. Firstly, we reinvestigated the conformation of bare DB18C6 (species m1 and m2) and the structure of DB18C6-H(2)O (species a) [R. Kusaka, Y. Inokuchi, T. Ebata, Phys. Chem. Chem. Phys., 2008, 10, 6238] by measuring IR-UV DR spectra in the region of the methylene CH stretching vibrations. The IR spectral feature of the methylene CH stretch of DB18C6-H(2)O is clearly different from those of bare DB18C6 conformers, suggesting that DB18C6 changes its conformation when forming a complex with a water molecule. With the aid of Monte Carlo simulation for extensive conformational search and density functional calculations (M05-2X/6-31+G*), we reassigned species m1 and m2 to conformers having C(1) and C(2) symmetry, respectively. Also, we confirmed the DB18C6 part in species a of DB18C6-H(2)O to be "boat" conformation (C(2v)). Secondly, we identified nine, one, and two species for the DB18C6 complexes with ammonia, methanol, and acetylene, respectively, by the combination of LIF and UV-UV HB spectroscopy. From the IR spectroscopic measurement in the methylene CH stretching region, a similar conformational change was identified in the DB18C6-ammonia complexes, but not in the complexes with methanol or acetylene. The structures of all the complexes were determined by analyzing the electronic transition energies, exciton splitting, and IR spectra in the region of the OH, NH, and CH stretching vibrations. In DB18C6-ammonia complexes, an ammonia molecule is incorporated into the cavity of the boat conformation by forming "bifurcated" and "bidentate" hydrogen-bond (H-bond), similar to the case of the DB18C6-H(2)O complex. On the other hand, in the DB18C6-methanol and -acetylene complexes, methanol and acetylene molecules are simply attached to the C(1) and C(2) conformations, respectively. From the difference of the DB18C6 conformations depending on the type of the guest molecules, it is concluded that DB18C6 distinguishes water and ammonia from methanol and acetylene when it forms complexes, depending on whether guest molecules have an ability to form bidentate H-bonding.

Laser Spectroscopic Study of Cold Host–Guest Complexes of Crown Ethers in the Gas Phase

“…Figure 2 c shows the simulated IR spectra for the energy-optimized structures of DB18C6 and DB18C6-(H 2 O) 1 complex, the structures of which are shown in Figure 3. [51] It should be noted that the C i and C 2v conformers have rather high energies so they cannot be candidates for the observed ones. On the other hand, the IR spectrum of boat-DB18C6-(H 2 O) 1 complex is very similar to the observed one of band a.…”

“…Recently, it has been demonstrated that the laser spectroscopic study of cold gas-phase CEs and their complexes, not only for ionic species [32][33][34][35][36][37][38][39][40][41][42][43] but also the neutral species, [44][45][46][47][48][49][50][51][52] can provide A laser spectroscopic study on the structure and dynamics of cold host-guest inclusion complexes of crown ethers (CEs) with various neutral and ionic species in the gas phase is presented. The complexes with neutral guest species are formed by using supersonic free jets, and those with ionic species are generated with electrospray ionization combined with a cold 22-pole ion trap.…”

“…The CEs chosen are dibenzo-18-crown-6 (DB18C6), [46,47,50,51] benzo-18-crown-6 (B18C6), [46,48] dibenzo-24crown-8 (DB24C8), [49] and 3n-crown-n (n = 4, 5, 6, and 8). The CEs chosen are dibenzo-18-crown-6 (DB18C6), [46,47,50,51] benzo-18-crown-6 (B18C6), [46,48] dibenzo-24crown-8 (DB24C8), [49] and 3n-crown-n (n = 4, 5, 6, and 8).…”

UV photodissociation spectroscopy of cryogenically cooled gas phase host–guest complex ions of crown ethers

“…[18][19][20][21][22][23][24][25][26][27] The cold gas phase complexes are generated either by the supersonic expansion technique for neutral complexes [18][19][20][21][22][23][24] or by the electrospray ionization (ESI)/cold ion-trap method for ionic complexes. [18][19][20][21][22][23][24][25][26][27] The cold gas phase complexes are generated either by the supersonic expansion technique for neutral complexes [18][19][20][21][22][23][24] or by the electrospray ionization (ESI)/cold ion-trap method for ionic complexes.…”

“…[18][19][20][21][22][23][24][25][26][27] The cold gas phase complexes are generated either by the supersonic expansion technique for neutral complexes [18][19][20][21][22][23][24] or by the electrospray ionization (ESI)/cold ion-trap method for ionic complexes. 20,22,23 In the present work, we report a study on host-guest complexes of CEs with protonated aromatic amines. Based on these studies, we reported that the conformations of CEs in the inclusion complexes are generally different from the most stable conformer of bare forms, because CEs will change their structures so that they can include the different size and structure of guest species in their cavities.…”