molecules Review Intramolecular Hydrogen Bonding Involving Organic Fluorine: NMR Investigations Corroborated by DFT-Based Theoretical Calculations Sandeep Kumar Mishra and N. Suryaprakash * NMR Research Centre, Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India; [email protected] * Correspondence: [email protected]; Tel.: +80-2293-7344 or +80-2293-3300 or +91-9845124802; Fax: +91-2360-1550 Academic Editor: Steve Scheiner Received: 31 January 2017; Accepted: 2 March 2017; Published: 7 March 2017 Abstract: The combined utility of many one and two dimensional NMR methodologies and DFT-based theoretical calculations have been exploited to detect the intramolecular hydrogen bond (HB) in number of different organic fluorine-containing derivatives of molecules, viz. benzanilides, hydrazides, imides, benzamides, and diphenyloxamides. The existence of two and three centered hydrogen bonds has been convincingly established in the investigated molecules. The NMR spectral parameters, viz., coupling mediated through hydrogen bond, one-bond NH scalar couplings, physical parameter dependent variation of chemical shifts of NH protons have paved the way for understanding the presence of hydrogen bond involving organic fluorine in all the investigated molecules. The experimental NMR findings are further corroborated by DFT-based theoretical calculations including NCI, QTAIM, MD simulations and NBO analysis. The monitoring of H/D exchange with NMR spectroscopy established the effect of intramolecular HB and the influence of electronegativity of various substituents on the chemical kinetics in the number of organic building blocks. The utility of DQ-SQ technique in determining the information about HB in various fluorine substituted molecules has been convincingly established. Keywords: NMR spectroscopy; intramolecular hydrogen bond; organic fluorine 1. Introduction 1.1. Weak Molecular Interactions The presence of molecular interactions in Nature cannot be ignored. The existence of various forms of matter, such as, solids and liquids is mainly due to the presence of intermolecular interactions. One can safely make a statement that the world would be a uniform ideal gas in the absence of intermolecular interactions. The existence of intermolecular interactions is reflected at the molecular level, viz., the thermodynamic non-ideal-gas behavior arising due to vapor pressure, viscosity, virial coefficients, absorption, and superficial tension [1]. The molecular interactions are non-covalent and are inherently electrostatic in nature. These forces could be attractive, repulsive or the combination of attractive as well as repulsive between or within the molecules. They could also be between non-bonded atoms. Intermolecular interactions play predominant role in many fields, such as, conformation of biomolecules, drug design, etc. Kaplan classified the intermolecular interactions on the basis of distance between the interacting objects [2]. In the classification, there are three main ranges for molecular forces depending on the Molecules 2017, 22, 423; doi:10.3390/molecules22030423 www.mdpi.com/journal/molecules Molecules 2017, 22, 423 2 of 44 Moleculesinteratomic 2017, 22 distances, 423 (R) for an interatomic potential (V). For range I the short distances are defined2 of 41 where the potential is repulsive in nature and the electronic exchange is due to the overlapped wheremolecular the potential electronic is repulsive shells. The in nature range and II representsthe electronic the exchange intermediate is due distancesto the overlapped with the molecular van der electronicWaals minimum, shells. The which range arises II represents due to the the balance intermediate between distances repulsive with and the attractive van der Waals forces. minimum, Range III whichrepresents arises the due large to the distances balance with between negligible repulsive electronic and attractive exchange forces. where Range the intermolecular III represents forcesthe large are distancespredominantly with negligible attractive. electronic The representation exchange ofwh rangeere the of molecularintermolecular forces forces is illustrated are predominantly in Figure1a. attractive.Depending The on representation the nature, the of various range of molecular molecular interactions forces is illustrated can also bein Figure classified 1a. asDepending given in theon theFigure nature,1b. the various molecular interactions can also be classified as given in the Figure 1b. FigureFigure 1. 1. ((aa)) The The classification classification of intermolecular interactionsinteractions onon thethe basis basis of of interatomic interatomic distances distances for for a atypical typical interatomic interatomic potential. potential. Where WhereR Ris is thethe distancedistance betweenbetween the centers of masses masses of of the the molecules, molecules, V is is the the Lennard-Johns Lennard-Johns potential potential and and the the graph graph of V of as V the as function the function of R is of called R is Le callednnard-Johns Lennard-Johns model potentialmodel potential graph; graph;(b) Schematic (b) Schematic illustration illustration of some of some of known of known molecular molecular interactions. interactions. Some Some other other molecularmolecular interactions interactions that that are are not not listed listed in in the sch schemeeme may also be possible. This This review review is is mainly mainly focusedfocused on on the the intramolecular intramolecular HB HB hence hence hydr hydrogenogen bonding is highlighted in the scheme. The change in molecular interactions are reflected in physical, chemical and biological phenomenon, such as, phase transitions of water (ice to water to vapor or vice versa), protein folding and unfolding and separation of DNA strands, RNA unfolding, etc. Such processes do not fall under chemical reactions. All the weak molecular interactions have their specific significance and govern the properties of a substrate it may be pointed out that the intermolecular interactions cannot be measured directly by any experiment. Molecules 2017, 22, 423 3 of 44 The change in molecular interactions are reflected in physical, chemical and biological phenomenon, such as, phase transitions of water (ice to water to vapor or vice versa), protein folding and unfolding and separation of DNA strands, RNA unfolding, etc. Such processes do not fall under chemical reactions. All the weak molecular interactions have their specific significance and govern the properties of a substrate it may be pointed out that the intermolecular interactions cannot be measured Moleculesdirectly 2017 by any, 22, 423 experiment. 3 of 41 Among all the weak molecular interactions described in Figure1, except for the hydrogen bond (HB),Among usually all pertain the weak to molecular intermolecular interactions interactions. described Onin Figure the other 1, except hand, for thethe hydrogen HB can be bond detected (HB), usuallywithin thepertain molecule to intermolecular or between theinteractions. two or more On the molecules. other hand, This the review HB can is focusedbe detected on thewithin studies the moleculecarried out or by between authors’ the laboratory two or more on the molecules. rare type This of intramolecular review is focu hydrogensed on the bonding studies (HB)carried involving out by authors’organic fluorine.laboratory on the rare type of intramolecular hydrogen bonding (HB) involving organic fluorine. 1.2. Hydrogen Hydrogen Bond The idea of HB was first first suggested by Huggins [3–6] [3–6] in 1919, and was further described by Larimer and Rodebush in 1920 [7]. [7]. HB is an interaction which occurs between an atom containing a lone pair of of electrons electrons (a (a Lewis Lewis base) base) and and a a hydrogen hydrogen atom atom which which is is bonded bonded to to an an electronegative electronegative atom atom (e.g., N, O,O, SS oror F)F) throughthrough a a covalent covalent bond. bond. In In a a HB, HB, the the Lewis Lewis base base plays plays the the role role of aof HB a HB acceptor acceptor (A) (A)and and the the electronegative electronegative atom atom bonded bonded to to proton proton is is called called the the HB HB donor donor (D). (D). TheThe hydrogenhydrogen bonding interaction is schematically depicted in Figure2 2.. Figure 2. 2. TheThe pictorial pictorial illustration illustration of of hydrogen hydrogen bond bond inte interaction,raction, where where HB HB acceptor/donor acceptor/donor can canbe F, be O, F, N,O, N,or S or atom S atom in the in the molecule. molecule. Polarization Polarization of electron of electron and and exposure exposure of ofpositive positive proton proton on oneither either side side is shownis shown schematically. schematically. The figure and the associated text were adapted from [8]. Hydrogen is the only atom that easily participate in the HB. This is due to the fact that hydrogen Hydrogen is the only atom that easily participate in the HB. This is due to the fact that hydrogen atom can form covalent sigma bonds with electronegative atoms like F, N, O and S, etc., where the atom can form covalent sigma bonds with electronegative atoms like F, N, O and S, etc., where the electron of 1s shells participates in the covalent bond. Since the more electronegative atom has tendency electron of 1s shells participates in the covalent bond. Since the more electronegative atom has tendency to pull the shared electron pair towards it, in the covalent bond between hydrogen and electronegative