The elusive ≡C-H⋯O complex in the hydrogen bonded systems of Phenylacetylene: A Matrix Isolation Infrared and Ab Initio Study

Abstract

Hydrogen-bonded complexes of phenylacetylene (PhAc) with methanol (MeOH) and diethylether (DEE) were studied using matrix isolation infrared spectroscopy. This study specifically searched for the ≡C-H⋯O hydrogen bonded complex in these systems, which manifest a n-σ* interaction and which is a local minimum on the PhAc-MeOH potential surface, as in the case of PhAc-H₂O heterodimer. This n-σ* local minimum eluded observation in gas phase studies and it was therefore thought interesting to look for this isomer in cryogenic matrices. While MeOH can interact with PhAc as both a proton donor (O-H⋯π complex) or a proton acceptor (n-σ* complex), DEE can only manifest the n-σ* isomer. A comparison of the spectral shifts observed in the features of PhAc-MeOH and PhAc-DEE would therefore independently confirm the existence or not of n-σ* complex in both these systems. In addition to the n-σ* complex observed in both the above systems, the O-H⋯π complex was also discerned in the PhAc-MeOH system. These complexes have stabilization energy in the range of 8-25 kJ /mol. The experimental results were corroborated by computations performed at MP2 and M06-2X, levels of theory, using 6-311++G(d,p) and aug-cc-pVDZ basis sets. Single point calculations at the CCSD level of theory were also performed. Atoms-in-molecules (AIM), NBO and LMOEDA analysis were also performed to understand the nature of the intermolecular interactions in these complexes

Antagonistic Interplay Between an Intermolecular CH···O and an Intramolecular OH···O Hydrogen Bond in a 1:1 Complex Between 1,2-Cyclohexanedione and Chloroform: A Combined Matrix Isolation Infrared and Quantum Chemistry Study

“…Weak hydrogen bonds (HBs) have been the subject of intense studies in recent years. − Of particular interest are the CH···O hydrogen bonds, − which have been recognized as vital for structural stability and functioning of many biological macromolecules. − A lot of effort has been devoted to understanding the physical forces responsible for these weak molecular interactions, particularly with respect to the ways they differ from the conventional X–H···Y type hydrogen bonds, ,,− where X and Y are two electronegative elements.…”

“…In earlier studies, the existence of CH···O hydrogen bonds was identified from crystallographic data, where shorter CH···O contacts in comparison to the sum of the van der Waals radii of respective CH groups and O atom were considered to be indicators of presence of such hydrogen bond. , In recent years, however, a variety of binary CH···O hydrogen bonded complexes have been identified in liquid and gas phases using vibrational spectroscopic methods, and infrared spectroscopy in particular has emerged as the method of choice for studying their spectroscopic properties. − A well-tested approach is to correlate the experimentally measured spectral shifts of suitable probe vibrational transitions at the hydrogen bonded sites with predictions of electronic structure calculations to understand the attributes of such bonding. Thus, quantum chemical methods have also established themselves as indispensable components in studies of weak hydrogen bonding. − An intriguing spectroscopic feature displayed by many of such complexes is the blue-shifting of the donor C–H stretching fundamental (ν C–H ). − ,− Electronic structure studies have suggested that the factors responsible for blue shifts are not characteristically different from those responsible for red-shifting of the donor groups (X–H) in conventional (X–H···Y) hydrogen bonds. A subtle balance of all the factors determines whether the net spectral shift, in a particular case, would be blue or red. − In this context, a pertinent question is whether it is possible to predict intuitively the preferred mode of binding of a weak donor (C–H group) when more than one apparently similar acceptor site is present in the system.…”

Phenylacetylene as a gas phase sliding balance for solvating alcohols

“…Phenylacetylene has a long research tradition as a multivalent binding partner towards molecules such as water, alcohols, diethylether, formic acid, ammonia, trifluoroethanol, methylamine and others, including itself. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] It has been termed as ''chameleon'' because of its versatile binding behaviour with different molecules. 4 It offers the acetylenic CH group (RCH) as a hydrogen bond donor and the p systems of the acetylene and benzene moieties as hydrogen bond acceptors.…”

“…All three docking sites have been postulated and observed experimentally, depending on the solvent molecule and the environment. 4,13,[16][17][18] Matrix isolation is able to stabilize metastable docking sites as observed for water, 12 methanol, 13 formic acid 14 and HCl, 16 where higher energy structures were also detected along with the global minima, whereas the low temperature supersonic jet studies (IR/UV double resonance and microwave experiments) so far have only revealed a single p binding site for each binding partner (e.g. acetylenic p and phenyl p for water and H 2 S, respectively).…”

Multiple Hydrogen Bond Tethers for Grazing Formic Acid in Its Complexes with Phenylacetylene

“…Techniques such as supersonic jet spectroscopy and matrix isolation have been employed to study noncovalent interactions. − Using such techniques, FA has been studied for its interactions with precursors such as H 2 O, C 2 H 2 , and C 6 H 6 , to name a few. − Lisa et al performed matrix isolation experiments in Ar to study the interactions of FA with C 2 H 2 , which is a nonaromatic π system . FA was shown to act as a bidentate hydrogen bonding ligand, forming two contacts.…”