Blue-shifting hydrogen bonds—are they improper or proper?

Bonding topology, hydrogen bond strength, and vibrational chemical shifts on hetero-ring hydrogen-bonded complexes — Theoretical insights revisited

This work presents a theoretical study about the interaction strength of the hydrogen-bonded complexes C2H4O···HF, C3H6O···HF, C2H4O···HCF3, and C3H6O···HCF3 at the B3LYP/6–311++G(d,p) level. The structures, hydrogen bond energies, charge transfers, and dipole moments of these complexes were analyzed in accordance with routine spectroscopy events, such as the red- and blue-shifts on the stretch frequencies of the proton donors (HF and HCF3). The ChelpG atomic charges were used to quantify the charge-transfer fluxes from electron donor (O) towards to acceptors (HF or HCF3). Moreover, the topological calculations on the basis of the quantum theory of atoms in molecules (QTAIM) approach were also used to unveil the hydrogen bond strength (O···H), mainly in the determination of their electronic densities and Laplacian shapes.

A comparative computational study of FKrCCH...Y, FCCKrH...Y, and FCCH...Y complexes (Y = BF, CO, N2, OH2, OH(CH3), O(CH3)2)

The structural and spectroscopic changes in complexes of FCCKrH...Y and FKrCCH...Y (Y = BF, CO, N(2), OH(2), OH(CH(3)), O(CH(3))(2)) were computed at the MP2∕6-31++G(d,p) level of theory and compared with the corresponding properties for FCCH...Y. The computed bond length changes and frequency shifts on complexation were rationalized by comparing with a perturbation model, which gives quantitative agreement with the standard ab initio results. A recently proposed model also gives a reasonable qualitative account of the observed trends in these complexes.

A theoretical study of hydrogen- and lithium-bonded complexes of F-H∕Li and Cl-H∕Li with NF3, NH3, and NH2(CH3)

Hydrogen- and lithium-bonded complexes of A-H∕Li (A = F, Cl) with the amine analogues NF(3), NH(3), and NH(2)(CH(3)) were studied at the MP2∕6-311++G(d,p) level of theory. Bond extensions and redshifts were obtained for the H-bonded complexes, while bond extensions and blueshifts were obtained for the Li-bonded species. The variation of these and other properties with the basicity of the amines was investigated and rationalized by comparing the ab initio results with predictions from a model derived from perturbation theory.

Theoretical aspects of binary and ternary complexes of aziridine···ammonia ruled by hydrogen bond strength

B3LYP calculations, ChelpG atomic charges, and quantum theory of atoms in molecules (QTAIM) integrations were used to investigate the binary (1:1) and ternary (1:2) hydrogen-bonded complexes formed by aziridine (1) and ammonia (2). In a series of analysis, geometry data, electronic parameters, vibrational oscillators, and topological descriptors were used to evaluate hydrogen bond strength, and additionally to determine the more prominent molecular deformations upon the formation of C(2)H(5)N···NH(3) (1:1) and C(2)H(5)N···2NH(3) (1:2) systems. Taking a spectroscopic viewpoint, results obtained from analysis of the harmonic infrared spectrum were examined. From these, new vibrational modes and red- and blue-shifts related to the stretch frequencies of either donors or acceptors of protons were identified. Furthermore, the molecular topology of the electronic density modeled in accord with QTAIM was absolutely critical in defining bond critical points (BCP) and ring critical points (RCP) on the heterocyclic structures. Taking all the results together allowed us to identify and characterize all the N···H hydrogen bonds, as well as the strain ring of the aziridine and its stability.

Computational study of proper and improper hydrogen bonding in methanol complexes

The bulk properties of alcohols, like those of aqueous solutions, are governed mostly by hydrogen bonding; however, in contrast with water molecules, the chemical structure of a simple alcohol such as methanol offers an opportunity to explore the effects of both proper and improper hydrogen bonding on a single hydrogen donor. The presence of the hydroxyl group generally gives rise to a strong proper hydrogen bond, while the methyl group of methanol is likely involved in the weaker improper hydrogen bond, among other weak non-covalent interactions. The effects of the two types of hydrogen bonds on the stability, geometric parameters, and properties of electron density of methanol complexes are examined while considering different geometrical arrangements of the methanol dimer and the binary complexes of methanol with water, acetonitrile, and chloromethane. Subsequently, potential conclusions about the nature of improper hydrogen bonding and the origin of the C–H bond contraction that results upon complex formation are discussed. Quantum theory of atoms in molecules and natural bond orbital methods were used in the analysis; all calculations were performed at the MP2(full)/6-311++G(d,p) level of theory.

Ab initio studies of the vibrational spectra of some hydrogen-bonded complexes of fluoroacetylene

Ab initio molecular orbital theory has been used to compute the properties of a number of hydrogen-bonded complexes between fluoroacetylene as proton donor and ammonia, water, hydrogen fluoride, phosphine, hydrogen sulfide, and hydrogen chloride as proton acceptors. The properties considered were the vibrational spectra, the molecular structures, the hydrogen-bond energies, and the electron densities, and one of the aims of the study was to ascertain whether there was any evidence of blue-shifting hydrogen-bond character in the complexes formed. The adducts with NH3, H2O, PH3, and H2S were of the conventional CH···X kind (X = N, O, P, S), with hydrogen-bond energies decreasing in the order NH3 > H2O > PH3 ≈ H2S. Those formed with HF and HCl showed the presence of three alternative structures; in addition to the CH···F(Cl) complexes, adducts of the F(Cl)H···F and F(Cl)H···π type were also found to be stationary points on the potential energy surfaces, with stabilities in the order F(Cl)H···π > CH···F(Cl) > F(Cl)H···F. The hydrogen-bond energies of the CH···X series correlated with the gas-phase basicities of the proton acceptors; moreover, the CH bond-length changes, the wavenumber shifts, the complex–monomer infrared intensity ratios of the CH stretching modes, and the amounts of charge transferred on complex formation were all found to track with the hydrogen-bond energies. All those properties considered here are consistent with the formation of red-shifting hydrogen bonds, to the exclusion of the blue-shifting alternatives.

Divalent carbon atom as the proton acceptor in hydrogen bonding

Proton-accepting properties of the divalent carbon atom in carbodiphosphoranes and their simple derivatives as well as in carbenes have been investigated. Both these groups of chemical compounds may be characterized by the formula CL2, where L is a sigma electron donor. Therefore, the carbon atom within both these systems, being in its atomic state, can have one or two lone electron pairs and, as a result, it may form hydrogen bonds of the type D-H...CL2, where C acts as a proton acceptor. Complexes of C(NH3)2, C(PH3)2, C[P(CH3)3]2, CF2, CCl2, and imidazol-2-ylidene with such proton donors as H2O, HCF3, HCN and HCCH have been analyzed by means of high-level quantum chemical methods. Density functional theory (DFT) and second-order Møller-Plesset (MP2) approaches have been applied in conjunction with the aug-cc-pVTZ basis set. The electron density distribution calculated by means of the atoms in the molecules procedure has also been analyzed. Proton-accepting properties of the carbon atom are discussed in detail. It is shown that the divalent carbon atom in the group of chemical systems investigated should be treated as a normal proton acceptor, similar to the much more electronegative O or N atoms. Moreover, hydrogen bonds of the type D-H...CL2 within the complexes investigated have been found to be rather strong. The highest proton accepting ability of the carbon(0) atom found for the (NH3)2C derivative of carbodiphosphorane is explained on the basis of the Leffler-Hammond postulate. Within the group of carbenes, the strongest hydrogen bonds are formed by imidazol-2-ylidene. This is attributed to the significant aromatic character of the imidazol-2-ylidene ring that increases the proton-accepting properties of the carbene carbon atom.

Identification of spectroscopic patterns of CH...O H-bonds in proteins

Ab initio calculations are used to identify characteristics of vibrational and NMR spectra that signal the involvement of a protein backbone in a CH...O H-bond and that distinguish this sort of interaction from other H-bonds in which a protein might participate. Glycine and alanine dipeptides, in both their C7 and C5 minimum-energy structures, are paired with formamide in a number of different H-bonding arrangements. The CH...O H-bond is characterized by a small contraction of the C-H bond length, along with a blue shift in its stretching frequency, accompanied by an intensification of this vibrational band. In the context of NMR spectra, the bridging CH proton's chemical shift is moved downfield by 1-2 ppm. The aforementioned features are not produced by other H-bonds in which the protein backbone might participate, such as NH proton donation or accepting a proton via the peptide C=O.

NMR Study of Blue-Shifting Hydrogen Bonds Formed by Fluoroform in Solution

Low-temperature (193 K) 1H, 13C and 15N NMR spectra of blue- and red-shifting H-bonded complexes formed by fluoroform with various proton acceptors were measured. Experimental NMR parameters were plotted versus the ab initio calculated H-bond strength (MP2/6–31+G(d, p); interaction energy varies from ~5 up to 25 kJ · mol−1 in the series). We show that experimental 1H and 15N shieldings, as well as the H/D isotope effect on 13C shielding change monotonously with the calculated H-bond strengthening. The 13C chemical shift and the CH scalar coupling change non-monotonously and the extremum points are situated approximately in the region of transformation from blue- to red-shifting H-bonds. The most informative NMR feature is the H/D isotope effect on 15N shielding which changes its sign upon transformation from blue- to red-shifting H-bonds.

To rationalize these observations, ab initio calculations of 13C and 15N shieldings as functions of C···H and C···N distances were performed for complexes of CHF3 with acetonitrile (blue-shifting) and pyridine (red-shifting). The coupling of the vibrations of the covalent and hydrogen bonds has been accounted for by direct computation of the distance C···N = f(C···H) dependence. We demonstrate that the unusual sign of the H/D isotope effect on 15N chemical shift across a blue-shifting H-bond can be explained as a result of the inversion of the dynamic coupling of two vibrations.

Conformation dependence of the C(alpha)D(alpha) stretch mode in peptides. II. explicitly hydrated alanine peptide structures

Our previous studies of the potential utility of the C(alpha)D(alpha) stretch frequency, nu(CD), as a tool for determining conformation in peptide systems (Mirkin and Krimm, J Phys Chem A 2004, 108, 10923-10924; 2007, 111, 5300-5303) dealt with the spectroscopic characteristics of isolated alanine peptides with alpha(R), beta, and polyproline II structures. We have now extended these ab initio calculations to include various explicit-water environments interacting with such conformers. We find that the structure-discriminating feature of this technique is in fact enhanced as a result of the conformation-specific interactions of the bonding waters, in part due to our finding (Mirkin and Krimm, J Phys Chem B 2008, 112, 15268) that C(alpha)--D(alpha)...O(water) hydrogen bonds can be present in addition to those expected between water and the CO and NH of the peptide groups. In fact, nu(CD) is hardly affected by the latter bonding but can be shifted by up to 70 cm(-1) by the former hydrogen bonds. We also discuss the factors that will have to be considered in developing the molecular dynamics (MD) treatment needed to satisfactorily take account of the influence of outer water layers on the structure of the first-layer water molecules that hydrogen bond to the peptide backbone.

A theoretical study of red-shifting and blue-shifting hydrogen bonds occurring between imidazolidine derivatives and PEG/PVP polymers

A theoretical study is presented with the aim to investigate the molecular properties of intermolecular complexes formed by the monomeric units of polyvinylpyrrolidone (PVP) or polyethyleneglycol (PEG) polymers and a set of four imidazolidine (hydantoine) derivatives. The substitution of the carbonyl groups for thiocarbonyl in the hydantoin scaffold was taken into account when analyzing the effect of the hydrogen bonds on imidazolidine derivatives. B3LYP/6-31G(d,p) calculations and topological integrations derived from the quantum theory of atoms in molecules (QTAIM) were applied with the purpose of examining the N-H···O hydrogen bond strengths formed between the amide group of the hydantoine ring and the oxygen atoms of PVP and PEG polymers. The effects caused by the N-H···O interaction fit the typical evidence for hydrogen bonds, which includes a variation in the stretch frequencies of the N-H bonds. These frequencies were identified as being vibrational red-shifts because their values decreased. Although the values of such calculated interaction energies are between 12 and 33 kJ mol(-1), secondary intermolecular interactions were also identified. One of these secondary interactions is formed through the interaction of the benzyl hydrogen atoms with the oxygen atoms of the PVP and PEG structures. As such, we have analyzed the stretch frequencies on the C-H bonds of the benzyl groups, and blue-shifts were identified on these bonds. In this sense, the intermolecular systems formed by hydantoine derivatives and PVP/PEG monomers were characterized as a mix of red-shifting and blue-shifting hydrogen-bonded complexes.

Peptide C(alpha)D(alpha) stretch frequencies in a hydrated conformation are perturbed mainly by C(alpha)-D(alpha)...O hydrogen bonding

We have shown (J. Phys. Chem. A 2004, 108, 10923; 2007, 111, 5300) that the C(alpha)D(alpha) stretch frequency, nu(CD), can discriminate between uniform alpha(R), beta, and polyproline II conformations of isolated peptides. Similar results for such peptides to which explicit waters are hydrogen bonded exhibit shifts in nu(CD) from those of the isolated structures. We demonstrate that the main source of these frequency shifts is the formation of C(alpha)-D(alpha)...O hydrogen bonds to water. Taking into account C-H...O(water) hydrogen bonding, together with the traditional bonding of peptide groups to water, can be expected to increase our understanding of the interaction of proteins with their aqueous environment.

High resolution infrared spectroscopy of the asymmetric C-H stretch of 1,2,4,5-tetracyanobenzene (TCNB) and (TCNB)(2) in superfluid helium nanodroplets

We report the observation of the C-H stretch vibration of 1,2,4,5-tetracyanobenzene (TCNB) and (TCNB)(2) embedded in superfluid helium droplets. The asymmetric C-H stretch of TCNB was observed at 3053.488(3) cm(-1). The corresponding band of the (TCNB)(2) is slightly blueshifted and was observed at 3054.651(2) cm(-1). The intensity of the IR band is increased compared to the monomer absorption. Based on a comparison of our experimental results with predicted IR bands, we conclude that (TCNB)(2) is formed in a C(2h) structure in the droplet.