C–H···O Hydrogen Bonding in Pentamers of Isatin

Abstract

Self-assembled monolayers of 1H-indole-2,3-dione (isatin), 3-methyl 2-oxindole, and 7-fluoroisatin are observed on the Au(111) surface via scanning tunneling microscopy (STM). We observed that isatin forms pentamers with density functional theory providing support for a cyclic structure stabilized by both N−His made between the 7-position C−H acting as the hydrogen bond donor and the 3-position carbonyl as the hydrogen bond acceptor, and calculations show that the isatin pentamer structure is 12 kJ/mol more stable than the dimer. When the 3-position carbonyl is removed and replaced with a methyl group (3-methyl 2-oxindole), we observe a monolayer with a mixture of catemer chains and pentameric clusters that are qualitatively different from those of isatin. Pentamer formation is completely broken when the 7position hydrogen is removed and replaced with fluorine; the monolayer of 7-fluoroisatin is composed of a mixture of close packed ordered domains and hexamer clusters. The role of C−H•••O bonding in forming isatin pentamers is supported by electrospray ionization mass spectrometry measurements, which show a propensity for isatin cluster formation, including magic-number isatin pentamers, while 3-methyl 2-oxindole and 7-fluoroisatin show relatively little clustering under the same conditions.

Hydrogen-Bonded Tetramers of Carbamazepine

“…Tetramers are present both as anomalously large peaks in the mass spectrum and as the predominant building block of surface monolayers observed by STM. This follows a number of previous experiments from our laboratory which show agreement between the size of surface-adsorbed clusters with “magic-number” peak intensities in ESI–MS. , We suggest that the concordance between two very different experimental measurements is evidence that the CBZ tetramers are forming in solution and then (depending on the experiment) either delivered to the surface intact through solution deposition or ionized intact through electrospray.…”

In Situ Attachment of Acrylamido Sulfonic Acid-Based Monomer in Terpolymer Hydrogel Optimized by Response Surface Methodology for Individual and/or Simultaneous Removal(s) of M(III) and Cationic Dyes

“…Because STR is mostly populated with methine C–H carrying greater H-bond-forming ability than those of >CH 2 and −CH 3 , methine C–H of pyranose rings acted as the preferred H-bond donor in C–H···O H-bond. 39 However, such reduced broadening in STR- g -TerPol and the appearance of considerably sharp and intense peaks at 2926, 1385, and 2874 cm –1 (sh) (Figure 3a) were related to the facilitated C–H vibrations of free −CH 3 in STR- g -TerPol, via replacement of −CH 3 ···O H-bonds with stronger methine C–H···O H-bonds, leading to altered mutual C–H/O–H/N–H H-bonding in STR- g -TerPol. Alongside, in both hydrogels, broad peaks within 2010–2210 cm –1 could be assigned to C=NH + and >NH 2 + moieties of AM and AMPS, respectively, which might be ascribed to protein and phosphate monoesters as impurities in STR (Table S4).…”

“…Because STR is mostly populated with methine C−H carrying greater H-bond-forming ability than those of >CH 2 and −CH 3 , methine C−H of pyranose rings acted as the preferred H-bond donor in C−H•••O H-bond. 39 However, such reduced broadening in STR-g-TerPol and the appearance of considerably sharp and intense peaks at 2926, 1385, and 2874 cm −1 (sh) (Figure 3a + moieties of AM and AMPS, respectively, which might be ascribed to protein and phosphate monoesters as impurities in STR (Table S4). Moreover, the −CH 2 − and >CH− units resulted through freeradical polymerization of AMPS, AM, and MBA, which were confirmed from the characteristic peaks within 1.05−1.78/ 2.16−2.70 and 0.84−1.57/2.07−2.62 ppm (Figure 4a,b) in TerPol and STR-g-TerPol, respectively.…”

Methylisatin Structural Isomers Have Different Kinetic Pathways to Self-Assembly

“…All calculations were performed with the Q-Chem 5.0 software package. 23 As in our previous studies on this class of molecules, 22 all structures were optimized using the B3LYP exchange-correlation func- tional, along with the 6-311++G(d,p) basis set and Grimme's D3 dispersion correction. 24 The initial geometries of the multimolecular structures were generated to be cyclic n-mers, where n is the number of molecules in the cluster.…”

“…19−21 Previous work from our laboratory has demonstrated that metastable cyclic pentamers of isatin form on the Au bond. 22 The current study is of methylated isatin derivatives that are functionalized at the 5 and 6 positions, which are remote from the hydrogen bond contacts that form the isatin pentamer (structures shown in Figure 1). The substitution of a methyl group was chosen to test the effect of a non-hydrogenbonding functional group on the formation of the cyclic pentamers.…”